This PDF file contains the front matter associated with SPIE Proceedings Volume 10721, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists, and Introduction (if applicable).

In this talk I will describe a new concept/application of high-Q semiconductor metasurfaces:P spectral shifting of the incident radiation through the phenomenon of photon acceleration. Semiconductor metasurfaces represent a unique platform for linear and nonlinear optics. Because semiconductors typically have a high refractive index, the resulting meta-molecules can support a wide selection of sharp Mie resonances that can be used for shaping their optical response to incident light. Mid-IR photonic has additional benefit: the optical response can be tuned through optical injection of free carriers because of the lambda^2 scaling of the refractive index change. I will describe how ultra-intense femtosecond laser pulses rapidly create free carriers in a spectrally-selective metasurface, thereby blue-shifting its resonant frequency during the lifetime of a trapped mid-IR photon. As the result, the photon is “accelerated”, and can contribute to spectrally blue-shifted and broadened harmonics generation. To our knowledge, this is the first experimental demonstration of a metasurface that is simultaneously time-dependent and nonlinear. The fundamental importance of photon acceleration is that it overcomes what was considered a key limitation of the resonant efficiency enhancement of nonlinear processes using spectrally-selective metasurfaces: that it must come at the expense of the bandwidth. Our results demonstrate that both the efficiency and the bandwidth can be simultaneously enhanced.

INVITED TALK

Conference Presentation for "Recent advances in linear and nonlinear all-dielectric metamaterials"

Keynote talk

We demonstrate a way to coherently control light at the nanoscale and achieve coherent perfect absorption (CPA) by using epsilon-near-zero (ENZ) plasmonic waveguides. The presented waveguides support an effective ENZ response at their cut-off frequency, combined with strong and homogeneous field enhancement along their nanochannels. The CPA conditions are perfectly satisfied at the ENZ frequency, surprisingly by a subwavelength plasmonic structure, resulting in strong CPA under the illumination of two counter-propagating plane waves with appropriate amplitudes and phases. In addition, we investigate the nonlinear response of the proposed ENZ plasmonic configuration as we increase the input intensity of the incident waves. We demonstrate that the CPA phenomenon can become both intensity- and phasedependent in this case leading to new tunable all-optical switching and absorption devices.

We discuss a dynamical model of harmonic generation that arises from surfaces that demarcate two different metals or conductors, or the electron cloud that spills outside a simple metal surface and the interior bulk, having different electron densities, for example a noble metal such as gold, and indium tin oxide (ITO). While in general two adjacent materials may contain free and bound charges that determine their respective dielectric constants, the transition region may be characterized by a large discontinuity in the free electron density, , epsilon-near-zero conditions, or multiple, nested plasmonic resonances. For example, , while the free-electron cloud that spills outside a noble metal surface decreases as a function of position from the hard ionic surface. These discrepancies lead to the prediction that the angular dependence of second harmonic generation (SHG) from a simple planar structure is direction-dependent, and highlights the sensitivity of both SHG and third harmonic generation to the makeup of the surface and what surrounds it. Our calculations also suggest that the nonlinear optical analysis of more complicated, hybrid structures, such as metal/oxide nanoantennas or metasurfaces, should always be performed by including effects that are generally overlooked, such as nonlocal effects (viscosity and pressure); the presence of linear and nonlinear quantum tunneling currents in the nano- and sub nano-gaps between the nanoantenna and embedded nanoparticles; linear and nonlinear contributions of bound (inner-core) electrons to the dielectric constant.

The processing of information encoded in frequency combs or spectral lattices has multiple applications for both classical and quantum states of light ranging from communications to spectroscopy. There is a strong interest in all-optical approaches for ultra-fast processing on integrated platforms. Here, we develop a concept and demonstrate experimentally all-optical flexible spectral comb reshaping in a nonlinear waveguide for two novel applications. First, we reveal that the evolution of an optical spectral comb can emulate wave dynamics in multi-dimensional lattices, which is a nontrivial generalization of previous theoretical proposals. In our experiment, a discrete signal spectrum is modulated by stronger pumps co-propagating in a nonlinear fiber with Kerr-type nonlinearity. Four-wave mixing Bragg scattering then induces coupling between many spectral lines, including nonlocal couplings between spectral lines which are further apart. We find that such a configuration can be exactly mapped to wave dynamics in complex multi-dimensional lattices, and as a representative example we realize a tube of triangular lattice. Importantly, the nontrivial phase of complex-valued couplings can give rise to synthetic gauge fields, and we directly measure corresponding asymmetric spectral reshaping. Our approach is scalable to higher-dimensional synthetic lattices. Second, we show that such a lattice with nonlocal couplings can enable the full reconstruction of the input spectra, including information on the phase and coherence, with a single-shot spectral intensity measurement. We demonstrate the reconstruction of input states composed of four frequency channels. Remarkably, the coherent nature of nonlinearly induced couplings is applicable for quantum states with spectral encoding.

Ultrashort optical pulse characterisation typically requires gating a reference pulse with an unknown pulse in a nonlinear medium. Since nonlinear crystals are typically produced to enable a single nonlinear process, only a single pulse can be characterised at any one time. In this work, we explore the use of gold nanoparticles to produce a range of nonlinear processes simultaneously. We show that a single nanoparticle produces a multiple nonlinear responses without relying on conventional phase matching. This allows us to exploit Four Wave Mixing and Sum Frequency Generation, which are simultaneously present in our nonlinear signal, to characterise two near IR ultrafast pulses separated by about an octave in wavelength. Remarkably, this "double-blind" method does not require the use of a known reference pulse, since the pulse retrieval problem is specified by the two nonlinear mixing processes.

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Student contribution: Plasmonic systems are efficient in converting optical energy into heat hence show technological significance in solar thermophotovoltaics, nanoparticle manipulation, and photocatalysis, etc. Conventional techniques to characterize plasmonic heaters are mostly thermal camera- and thermographic phosphor (TGP)- based. In this work, we present our results of characterizing a plasmonic heater using thermoreflectance imaging (TRI). The TRI technique presented here outperforms thermal camera-based technique in spatial resolution due to the visible light utilized for illumination, and does not require special sample preparation as in TGP-based technique. We chose to use a gap plasmon structure to maximize the optical absorption, and fabricated structures with various dimensions that exhibit varying optical absorptions at a fixed wavelength of 825 nm, which is the wavelength of pump light used in the TRI measurement. The TRI setup uses a millisecond-modulated continuous-wave pump laser to induce local temperature fluctuation on the sample surface, a 530 nm LED probe light then senses the change in the temperature-dependent material reflectance between high and low temperatures, which combined with a pre-calibrated thermoreflectance coefficient can be used to calculate the temperature rise on each image pixel. This technique grants us a resolution of ~200 nm. The experimentally obtained temperature rise on various gap plasmon structures correlates well with their optical absorption, and we compare the results against a finite element heat transfer model. Using a separate pump-probe thermoreflectance technique, we experimentally obtain the heat transfer dynamics of such gap plasmon structure under laser irradiation with picosecond resolution.

We demonstrate that a hybrid c-Si/Au nanocavity can serve as a multifunctional sensing platform for nanoscale (about 100 nm) thermometry with high accuracy (>0.4 K) and fast response (<0.1 second), controlled local optical heating up to 1200 K and also provide Raman scattering enhancement (>10^4 fold). The system has been tested in the experiment on thermally induced unfolding of BSA molecules, plased inside the hybrid nanocavity. Moreover, numerical modeling reveal, that two possible operation modes of the system: with and without considerable optical heating at the nanometer scale, while other functionalities (nanothermometry, RS enhancement, and tracing the events) are preserved. These regimes make the hybrid nanocavity more versatile sensing system than fully plasmonic counterparts. The simplicity and multifunctionality of the hybrid nanocavity make it a promising platform for photochemistry and photophysics applications.

This paper introduces tunable color reflectors for three primary colors (red, blue, and green) for use in low power display systems employing a phase change material (PCM) using interference resonance in an optical cavity. Optical index tunability of the PCM sitting on top of the cavity, results in tuning the device reflection spectrum and thus permits vivid color tuning. The phase change material used to achieve these results was Germanium Telluride (GeTe) due to its high stability. Specifically, ultra-thin films of GeTe was grown on top of a SiO₂ cavity with a bottom palladium reflector in a Fabery-Perot type design. This enhances the color tuning when a double-layer anti-reflection coating with high and low refractive indexes are used on top of the GeTe film. Low sensitivity to incident light angle and polarization without the need of sub-micron lithography, provide the potential for this device to be very useful for portable device applications. The devices with different thickness of GeTe were fabricated to demonstrate green, red, blue colors. Electrical pulses with different periods and duty cycles were used to switch the phase of the GeTe locally using joule heating method for several cycles. After transition, darker green, blue, and purple colors were shown for devices with 8 to 20 nm thickness of GeTe films.

We introduce a non-parity-time-symmetric three-layer structure, consisting of a gain medium layer sandwiched between two phase-change medium layers for switching of the direction of reflectionless light propagation. We show that for this structure unidirectional reflectionlessness in the forward direction can be switched to unidirectional reflectionlessness in the backward direction at the optical communication wavelength by switching the phase-change material Ge2Sb2Te5 (GST) from its amorphous to its crystalline phase. We also show that it is the existence of exceptional points for this structure with GST in both its amorphous and crystalline phases which leads to unidirectional reflectionless propagation in the forward direction for GST in its amorphous phase, and in the backward direction for GST in its crystalline phase. Our results could be potentially important for developing a new generation of compact active free-space optical devices. We also show that phase-change materials can be used to switch photonic nanostructures between cloaking and superscattering regimes at mid-infrared wavelengths. More specifically, we investigate the scattering properties of subwavelength three-layer cylindrical structures in which the material in the outer shell is the phase-change material GST. We first show that, when GST is switched between its amorphous and crystalline phases, properly designed electrically small structures can switch between resonant scattering and cloaking invisibility regimes. The contrast ratio between the scattering cross sections of the cloaking invisibility and resonant scattering regimes reaches almost unity. We then also show that larger, moderately small cylindrical structures can be designed to switch between superscattering and cloaking invisibility regimes, when GST is switched between its crystalline and amorphous phases. The contrast ratio between the scattering cross sections of cloaking invisibility and superscattering regimes can be as high as ~ 93%. Our results could be potentially important for developing a new generation of compact reconfigurable optical devices.

A new approach for realization of lateral heterostructures with lithography-defined junction profiles based on selective-area control of two-dimensional transition metal dichalcogenide (2D TMDC) alloys is presented. The unprecedented degree of control over the in-plane spatial profile and alloy composition of the 2D TMDC alloying process enables the precise tuning of the shape and optoelectronic properties of the resulting TMDC heterostructures and formation of quantum structures (e.g., quantum dots and quantum lines) for optoelectronic applications. The main challenges in realization of such high-quality lateral heterostructures, especially the effect of alloying-induced strain and defects on the structural and optoelectronic properties of TMDC alloys, and solutions to address these challenges will be discussed in depth. The application of these unique structures for light generation and detection will also be discussed.

The two-dimensionality of graphene and other layered materials can be exploited to simplify the theoretical description of their plasmonic and polaritonic modes. We present an analytical theory that allows us to simulate these excitations in laterally patterned structures in terms of plasmon wave functions (PWFs). Closed-form expressions are offered for their associated extinction spectra, involving only two real parameters for each plasmon mode and graphene morphology, which we calculate and tabulate once and for all. Classical and quantum-mechanical formulations of this PWF formalism are introduced, in excellent mutual agreement for armchaired islands with >10 nm characteristic size. Examples of application are presented to predict both plasmon-induced transparency in interacting nanoribbons and excellent sensing capabilities through the response to the dielectric environment. We argue that the PWF formalism has general applicability and allows us to analytically describe a wide range of 2D polaritonic behavior, thus providing a convenient tool for the design of actual devices.

MXenes are a recently discovered family of two-dimensional nanomaterials formed of transition metal carbides and carbon nitrides with the general chemical form Mn+1XnTx, where ‘M’ is a transitional metal, ‘X’ is either C or N, and ‘T’ represents a surface functional group (O, -OH or -F). MXenes are derived from layered ternary carbides and nitrides known as MAX (Mn+1AXn) phases by selective chemical etching of the ‘A’ layers and addition of functional groups ‘T’. In our work, we focus on one of the most well studied MXene, titanium carbide (Ti3C2Tx). Single to few layer flakes of Ti3C2Tx (in a solution dispersed form) are used to create a continuous film on a desired substrate by using spin coating technique. Losses inherent to the bulk MXene and existence of strong localized SP resonances in Ti3C2Tx disks/pillar-like nanostructures at near-IR frequencies are utilized to design an efficient broadband absorber. For Ti3C2Tx MXene disk array sitting on a bilayer stack of Au/Al2O3, high efficiency (>90%) absorption across visible to near-IR frequencies (bandwidth ~1.55 μm), is observed experimentally. We also experimentally study random lasing behavior in a metamaterial constructed by randomly dispersing single layer nanosheets of Ti3C2Tx into a gain medium (rhodamine 101, R101). Sharp lasing peaks are observed when the pump energy reaches the threshold value of ~ 0.70 μJ/pulse. This active metamaterial holds a great potential to achieve tunable random lasing by changing the optical properties of Ti3C2Tx flakes.

Monolayer two-dimensional transition metal dichalcogenides (2D-TMDCs) have gained immense attention for their desirable transport properties and direct bandgap that have led to a plethora of studies on modern nanoelectronic and optoelectronic applications. These properties are known to occur exclusively in TMDCs when thinned down to one or few monolayers. However reduced dimensionality poses a significant challenge for photonics and optoelectronics applications due to poor light absorption and emission dictated by the volume of semiconductor material. Plasmonic nanostructures have been widely studied for enhancing light-matter interactions in wide variety of material systems resulting in increased emission and absorption properties. 2D Materials provide the ultimate lower limit in terms of material thickness, therefore investigation of plasmon/2D Material hybrid material systems with a specific aim to enhance light-matter interactions is essential for practical optoelectronic applications. In this talk, I will discuss increased photoluminescence emission from MoS2 using both periodic plasmonic nanodisc arrays as well as a single plasmonic optical antenna. I will also describe a method for understanding and identifying the contributions of excitation and emission field enhancements to the overall photoluminescence enhancement using a tapered gold antenna. Additionally, I will describe a systematic study in which we have demonstrated increased light absorption in a monolayer WS2 film.

In this talk, we discuss our recent progress in the area of non-reciprocal photonics, discussing approaches to break reciprocity without magnetic bias, using temporal modulation schemes or nonlinearities. We will discuss our progress in the synthesis of non-reciprocal devices based on these principles, and arrays of them to enable topological bandgaps with non-reciprocal response.

Many technologically important properties of materials such as the optical diode effect depend on the interplay between the concepts of reciprocity, chirality and broken spatial and temporal symmetries. We illustrate these notions through a number of examples including non-reciprocal directional dichroism. Specifically, we discuss the conditions for non-reciprocity of ferro-rotational order in several materials and indicate the use of linear optical gyration and possibly vortex beams as a likely way to detect ferro-rotational domains. The concept of vector order parameters is then generalized to second- and higher-rank tensor order parameters. Finally, we elucidate how to achieve high-temperature optical diode effect. This work was performed in collaboration with S.-W. Cheong (Rutgers Univ.), D. Talbayev (Tulane Univ) and V. Kiryukhin (Rurgers Univ.).

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Invited talk to Professor Xiang Zhang (UC Berkeley) on the subject of topological photonics

There is a well-established link between the spin angular momentum of light, which is manifest in circular or elliptical polarizations, and chiral interactions with matter. However, any inference of such a simple connection between chirality and angular momentum in general is unwarranted. The pursuit of any parallels in chiroptical effects with vortex structured light, which convey orbital angular momentum, has until recently proved problematic, whilst the consideration of multipole aspects for the individual photon has elicited a variety of views. This Keynote review details the resolution of such issues based on a fundamental symmetry analysis of a generalized interaction between matter and structured light, revealing mechanisms that can support the long-sought connection indicated by the latest experiments.

Invited talk

Bound states in the continuum (BICs) are modes that, although energy and momentum conservation allow coupling to far-field radiation, do not show any radiation loss. As such, energy can theoretically be stored in the mode for infinite time. Such states have been shown to exist for e.g. photonic and acoustic waves, and show great promise for applications including lasing, (bio)sensing and filtering. Despite intense research, the mechanism behind these states and their robustness is still poorly understood. Recently it was proposed theoretically that BICs occur at points where the far-field polarization of the radiated waves shows a vortex, i.e. points where the polarization is undefined [1]. Due to the integer winding number associated to such vortices, the modes should be topologically protected against disorder. In this work, we verify this claim experimentally. We fabricate a SiN grating and use reflection measurements to show that it supports an optical BIC around 700 nm wavelength. We then perform polarimetry measurements in a Fourier reflection microscopy scheme to map the far-field polarization at every angle and wavelength, demonstrating the existence of a vortex at the BIC. We use a simple dipole model to characterize the BIC as a Friedrich-Wintgen type, arising from the interference between two electromagnetic dipoles induced in the grating. Our method can be used to characterize the polarization structure of any leaky photonic mode, including those supporting polarization vortices of arbitrary winding numbers. [1] Zhen, B., et al. (2014). Physical review letters, 113(25), 257401.

Twisted light has recently been recognised to provide added degrees of freedom for encoding information in optical quantum communication with light pulses. Here we propose and discuss plasmonic nanoantennas for twisted light allowing through plasmonic bright and dark modes to convert the information about the orbital angular momentum embedded in twisted light into spectral information. We also discuss orbital angular momentum dichroism in the interaction of twisted light with plasmonic nanostructures.

We discuss some of our recent works in exploring topological and non-Hermitian effects in photonic structures. Specifically, we discuss the existence of topological structures in scattering matrix. We also show that the band structure of a photonic crystal slab can be designed to achieve optical differentiation.

In recent years, topological insulator and topological semimetal have drawn much attention, due to the novel properties such as robust edge states, chiral anomaly, diverging local density of states and so on. However, the dimension of momentum space is less than three, which limits the exploration the physics in higher dimensions. Take advantage of the concept of synthetic dimension, we can construct higher dimensional spaces to realize more amazing phenomenon including Weyl surface in 5D space, 4D Quantum Hall effect and so on. Some works also focused on probing the topological physics of non-Hermitian systems, which leads to additional novel consequences such as the transformation of the Weyl point into an exceptional ring when introducing a non-Hermitian term. In this work, we give a flexible platform to investigate the PT-symmetric topological physics in 4D synthetic space. Here, based on 1D photonic crystal with a complex seven-layer unit cell, we construct a 4D synthetic space with two momentum vectors and two geometric parameters. Before introducing the non-Hermitian term, the system shows a 2D Nodal-hyperedge in 4D space. If we add gain and loss in to the PCs but preserving PT symmetry, the 2D hyperedge will be transformed into a 3D exceptional hypersurface, on which the Hamiltonian becomes defective. Such 3D exceptional hypersurface also guarantees the unidirectional reflectionless resonance in 1D PCs even with oblique incidence. Furthermore, we also find the existence of PT-symmetric interface states in our systems, which will give rise to the strong field enhancement, and is quite useful in nonlinear and quantum optics.

Topological insulator is a material in which helical conducting states exist on the surface of the bulk insulator. These states can transport electrons or photons at the boundary without any back scattering, even in presence of obstacles enabling to make topological cavities with arbitrary geometries that light can propagate in one direction. Here, we present the demonstration of the first experimental non-reciprocal topological laser that operates at telecommunication wavelengths. The unidirectional stimulated emission from edge states is coupled to a selected waveguide output port with an isolation ratio of 11 dB. Topological cavities are made of hybrid photonic crystals (i.e., two different photonic crystals) with distinct topological phase invariants, which are bonded on a magnetic material of yttrium iron garnet to break the time-reversal symmetry. Our experimental demonstration, paves the way to develop complex nonreciprocal topological devices of arbitrary geometries for integrated and robust generation and transport of light in classical and quantum regimes.

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Here we report on an electrically-driven, CMOS compatible, ring resonator coupling modulation mechanism based on tuning of free carriers in Indium Tin Oxide (ITO). The modulator device consists of two ITO layers separated by oxide fabricated on the coupling region of silicon ring resonator. Micro ring resonators are extensively used in in integrated photonic applications and are highly sensitive to electro-optical effects. Majority of available ring based modulators are interactivity modulation where active region changes the phase or absorption of the stored optical mode in the cavity. Such devices are essentially limited by the photon lifetime in the cavity. In contrast, coupling modulation devices can change the cross-coupling coefficient of the resonator and take advantage of the non-quasi-static modulation regime. We demonstrate an electrically-driven, CMOS compatible, ring resonator coupling modulation mechanism based on tuning of free carriers in Indium Tin Oxide (ITO). The modulator device consists of two ITO layers separated by oxide fabricated on the coupling region of silicon ring resonator. We are investigating modulation performance of such CMOS compatible coupling modulation devices. We have demonstrated the first reservoir coupling ITO modulator by leveraging critical coupling effects on a SOI ring resonator.

The ability to control all the important constitutive properties of light via interaction with nanoscale elements is a central concept in nanophotonics. In the last several years, metasurfaces have demonstrated promise as both flat optical elements to replace conventional three-dimensional components as well as to access functions that are unachievable in conventional optics. To date, the functional performance of metasurfaces has typically been encoded at the time of fabrication. However, active control of the properties of metasurfaces would enable dynamic holograms, focusing lenses with reconfigurable focal lengths, and beam steering, a key requirement for future chip-based light detection and ranging (LIDAR) systems. Here, we report the design and experimental demonstration of a continuous beam steering at telecommunication wavelengths using field-effect-tunable metasurfaces. The proposed beam steering device is actively controlled by incorporating indium tin oxide (ITO), as a material with voltage-tunable optical properties, into a metasurface. Using ITO as an active material and a composite hafnium-aluminum-oxide nanolaminate as the gate dielectric, we demonstrate a prototype tunable metasurface with a continuous phase shift from 0 to 300°. Our design enables independent control of each metasurface element via an individual application of DC voltage. This enables electrical control of the metasurface phase profile, which is an essential requirement for demonstration of continuous beam steering. By careful application of bias voltages to 96 biasing channels, we achieve a quasi-continuous beam steering with the steering angles of up to 75°.

Metasurfaces have been investigated for various applications ranging from beam steering, focusing, to polarization conversion. Along with passive metasurfaces, significant efforts are also being made to design metasurfaces with tunable optical response. Among various approaches, voltage tuning is of particular interest because it creates the possibility of integration with electronics. In this work, we demonstrate voltage tuning of reflectance from a complementary metasurface strongly coupled to an epsilon-near-zero (ENZ) mode in an ultrathin semiconductor layer. Our approach involves electrically controlling the carrier concentration of the ENZ layer to modulate the polaritonic coupling between the dipole resonances of the metasurface and the ENZ mode for modulating the reflectance of the metasurface. The hybrid structure we fabricate is similar to MOSCAP configuration where the complementary metasurface offers a continuous gold top layer for biasing and positive/negative bias to the metasurface leads to accumulation/depletion of carriers in the ENZ layer beneath it. We optimized our structure by using InGaAs as the ENZ material because of its high mobility and low effective mass. This allowed us to reduce the doping requirement and thereby reduce the ionized impurity scattering as well as the reverse bias required to deplete the ENZ layer. For low leakage and efficient modulation of carrier density, we used Hafnia as the gate dielectric. We further added a reflecting backplane below the ENZ layer to enhance the interaction and by applying bias, we achieved spectral shifts of 500 nm and amplitude modulation of 11% of one of the polariton branches at 14 µm.

All-optical nanophotonic switches, not bound by the inherent RC delays of electronic circuits, have the potential to push data-processing speeds beyond the limits of Moore’s Law. This has lead to the investigation of light-matter interactions in nanostructured materials in several all-optical data processing applications. To have a true impact on the field of ultrafast data-transfer, it is important to demonstrate switching in the telecom frequency range. We have designed a continuous layer gap plasmon metasurface, comprising a layer of gold nanodisk resonators on a 20 nm film of ZnO deposited on an optically thick gold layer. The performance of the metasurface has been investigated through numerical studies, using the optical properties of as-grown gold and zinc oxide, characterized by ellipsometry. An on-off ratio of 10.6 dB has been observed in simulations. Experimental studies are underway. The findings of this research work will pave the pathway to the design of ultra-compact and ultrafast optical switches employing ultrafast, dynamically tunable metasurfaces.

We describe some recent developments in the efficient modelling of light-matter interactions in plasmonics and nanophotonics. In particular, we describe an efficient finite-difference time-domain (FDTD) method to compute regularized quasinormal modes in resonant photonic structures, which have a wide range of applications in classical and quantum nanophotonics, and present methods to compute disorder-induced localization modes (Anderson localization) in slow light photonic crystal waveguides.

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Near-field nanophotonics offers the promise of orders-of-magnitude enhancements for phenomena ranging from spontaneous-emission engineering to Casimir forces via zero-point quantum fluctuations. An increasing variety of approaches — photonic crystals, metamaterials, metasurfaces, antennas, and more — has underscored our lack of understanding as to how large these effects can be. We provide a general answer to this question, deriving the first sum rule for near-field optical response as well as general upper bounds for any bandwidth, i.e. power–bandwidth limits. Within such a unified framework valid for structures of arbitrary shape and size, we approach single-frequency limits as bandwidth goes to zero and the sum rule as bandwidth goes to infinity. Power–bandwidth limits are derived from energy-conservation principles and depend on the susceptibility at the frequency of interest, and the sum rule arises from the requirement of causality and only depends on susceptibility at zero frequency. We explore to what extent power–bandwidth bounds can be attained for real materials and how the sum rule can be realized for canonical geometries. We further prove a "monotonicity" theorem that enables us to bound the integrated frequency response of any complicated structure in terms of the response of simple geometries. Our framework provides a universal measure of intrinsic optical-response characteristics that helps identify optimal nanophotonic materials for any combination of frequency and bandwidth, leading to wide-ranging applications in medical imaging and thermophotovoltaics.

CMOS-compatible light emitters are intensely investigated for integrated active silicon photonic circuits. One of the approaches to achieve on-chip light emitters is the epitaxial growth of Ge(Si) QDs on silicon. Their broad emission in the 1.3-1.5 um range is attractive for the telecom applications. We investigate optical properties of Ge(Si) QD multilayers, which are grown in a thin Si slab on an SOI wafer, by steady-state and time-resolved micro-photoluminescence. We identify Auger recombination as the governing mechanism of carrier dynamics in such heterostructures. Then we demonstrate the possibility of light manipulation at the nanoscale by resonant nanostructures investigating Si nanodisks with embedded Ge(Si) QDs. We show that the Mie resonances of the disks govern the enhancement of the photoluminescent signal from the embedded QDs due to a good spatial overlap of the emitter position with the electric field of Mie modes. Furthermore, we engineer collective Mie-resonances in a nanodisk trimer resulting in an increased Q-factor and an up to 10-fold enhancement of the luminescent signal due to the excitation of anti-symmetric magnetic and electric dipole modes. Using time-resolved measurements we show that the minima of the radiative lifetime coincide with the positions of the Mie resonances for a large variation of disk sizes confirming the impact of the Purcell effect on QD emission rate. Purcell factors at the different Mie-resonances are determined.

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Neuromorphic photonics is an emerging field at the intersection of photonics and neuromorphic engineering, with the goal of producing accelerated processors that combine the information processing capacity of neuromorphic processing architectures and the speed and bandwidth of photonics. It is motivated by the widening gap between current computing capabilities and computing needs that result from the limitations of conventional, microelectronic processors. Here, I will present these challenges, describe photonic neural-network approaches being developed by our lab and others, and offer a glimpse at this fields future.

Radiative emission experienced by a subwavelength particle near a resonant cavity is typically characterized by the well-known Purcell factor figure of merit. In recent work, we presented a generalization of Purcell enhancement that applies to situations involving exceptional points (EP)---spectral singularities in non-Hermitian systems where two or more eigenvectors and their corresponding complex eigenvalues coalesce, leading to a non-diagonalizable, defective Hamiltonian. EPs are attended by many intriguing physical effects and have been studied in various contexts, including lasers, atomic and molecular systems, photonic crystals, parity-time symmetric lattices, and optomechanical resonators. Thus far, the main focus of these works has been on analyzing the impact of second-order exceptional points on scattering from eternally incident light, e.g. for unidirectional transmission. An important but little explored property of EPs related to light-matter interactions is their ability to modify and enhance the local density of states (LDOS). Recently, we showed that EPs can modify the spontaneous emission rate or Purcell factor of narrow-band emitters embedded in resonant cavities. In this talk, we show that EPs can have an even greater impact on nonlinear optical processes like frequency conversion. In particular, we derive a general formula quantifying radiative emission from a subwavelength emitter in the vicinity of a triply resonant χ(2) cavity that supports an EP near the emission frequency and a bright mode at the second harmonic. We show that the resulting frequency up-conversion process can be enhanced by up to two orders of magnitude compared to nondegenerate scenarios and that, in contrast to the recently predicted spontaneous-emission enhancements, nonlinear EP enhancements can persist even when considering spatial distributions of broadband emitters, provided that the cavity satisfies special nonlinear selection rules. This is demonstrated via a two-dimensional proof-of-concept PhC designed to partially fulfill the various criteria needed to approach the derived bounds on the maximum achievable up-conversion efficiencies. Along these lines, we show that similar enhancements can arise in quantum systems consisting of single and multi-level atoms embedded in photonic cavities. Our predictions suggest an indirect but practically relevant route to experimentally observe the impact of EPs on spontaneous emission and related light–matter interactions, with implications to quantum information science.

Optical coherence is of fundamental importance for both classical and quantum applications. This motivates the development of approaches for increasing the degree of coherence, which can be quantified by a measure of purity. The purity is preserved in linear conservative systems, and accordingly the manipulation of coherence was realized with specially introduced loss in bulk optical setups or diffraction on metal films involving optical absorption and plasmon coupling. Here we suggest and show experimentally for the first time that manipulation and measurement of optical coherence and state purification can be efficiently realized in integrated non-Hermitian parity-time (PT) symmetric photonic structures composed of elements with different loss or gain. Specifically, we design and fabricate laser-written waveguide directional couplers that contain two sections. The first section realizes a PT-like coupler, where one of the two waveguides features extra radiative losses via modulation. The second section consists of straight coupled waveguides with specially detuned propagation constants, which are optimized to enable a full reconstruction of the purity and optical coherence by measuring the interference pattern in both waveguides through fluorescence imaging. In PT symmetric regime, we observe that the purity of an initially fully incoherent (mixed) state is increased followed by a revival of the input state. This constitutes an important experimental evidence of reversible manipulation of light coherence in PT coupled waveguides. We anticipate that this method can facilitate a wide range of applications from classical to quantum optics, including filtering out noise and optimizing the visibility of interferometric measurements.

INVITED TALK

Over the last decade, there has been tremendous success in developing optical metasurfaces with desired properties using innovative nanopatterned metal-dielectric composite films. However, major problems towards the mass production and use of these metafilms include expensive and poorly-scalable nanofabrication along with ohmic losses in their nanostructured metallic elements. More advanced approaches in this area include designs providing for (i) electrical or all-optical control of the metasurface responses to get their switchable or multi-functional performance, and (ii) ability to compensate loss and achieve lasing by adding gain inclusions. In all these cases, modeling only light propagation in nanostructured and optically dispersive media is not sufficient to fully understand, control and optimize the performance of a given metadevice. Instead, 3D full-wave time-domain electrodynamics should be coupled to additional nanoscale equations describing complex light-matter interactions at ab-initio level, thus providing a designer with an advanced multiphysics and possibly multiscale numerical modeling framework. Here, we present our multiphysics time-domain modeling framework for tunable and active photonics. First, we start with reviewing our efficient time-domain approach to modeling tunable graphene-based devices, where the integral multi-parametric surface conductivity is reformulated in time domain with physically interpretable and fast-to-compute integration-free terms. Then, we discuss a multiphysics approach to model optically tunable materials, where classical electrodynamics is coupled to non-equilibrium thermodynamics of electrons and lattice ions. Finally, we present our models of non-linear media built on carrier kinetics, including nanolasers and loss-compensated plasmonic metafilms, as well as metadevices with absorption saturation and reversed absorption saturation effects.

We present the main features of first principles numerical methods to describe plasmonic excitations in bulk and nanosized materials, and we apply these methods to a number of bulk and lower-dimensional nanosystems. Our main focus lies on graphene, which is an interesting numerical and experimental paradigm to study plasmonic excitations in a nanosystem with anisotropic and lossy dielectric functions. Beyond graphene we also discuss plasmonic excitations in similar two-dimensional nanosystems. In order to analyse more complex collective excitations of the electron gas in nanosystems, we take advantage of a fundamental relation between density fluctuations and the electron energy loss spectra (EELS), and suggest a general method to study noise in nanosystems.

Today it is possible to engineer the building blocks of artificial materials (meta-materials) with feature sizes smaller than the wavelength of light. The ability to design meta-atoms in a largely arbitrary fashion adds a new degree of freedom in material engineering, allowing to create artificial materials with unusual electromagnetic properties rare or absent in nature. Achieving tunable, switchable and non-linear functionalities of meta-materials at individual meta-atom level could potentially lead to additional flexibility in designing active photonic devices. These include among others, meta-materials based on phase-change materials, whose properties could be altered by thermal or photo-thermal means. In this presentation, our recent results on developing appropriate numerical methods to study hybrid meta-material structures containing phase-change materials will be discussed. Meta-atoms based on plasmon polaritonic materials are considered. We develop appropriate phenomenological models of phase transition and self-consistently couple them with the full wave electromagnetic and heat transfer solvers. Developed methods are used to design meta-surface based tunable components. We demonstrate an importance of the multiphysical modelling and discuss deficiencies of the commonly used purely electromagnetic simulations approaches.

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INVITED TALK

We present a new approach for building three-dimensional (3D) photonic crystals from periodic nanoparticle lattices that use spacers between plasmonic particles to control particle interactions, as compared to typical designs where dielectric materials are in contact. We delineate a set of simple yet general design principles that can be used to quickly derive the superlattice stopband features based on just two lattice parameters: nanoparticle-layer periodicity and volume fraction. By fixing the lattice parameters and comparing stopband properties from lattices composed of a variety of metallic and dielectric nanoparticles, we show that plasmonic nanoparticles are advantageous for optimizing the stopband features in photonic crystals made with nanoparticles and spacers.

Coupling of metasurfaces to intersubband transitions (ISTs) in semiconductor quantum wells (QWs) has been extensively studied for various applications ranging from generating giant nonlinear optical response to designing tunable metasurfaces for applications such as ultrafast spatial optical modulators and voltage tunable filters. In this work, we experimentally demonstrate a fundamentally new approach of actively controlling the coupling of ISTs in QWs to a metasurface for voltage tuning its optical response. Unlike previous approaches, we use voltage-controlled quantum tunneling to control the carrier concentration in the QWs for turning on/off the ISTs. We design a multi-quantum well structure consisting of four undoped InGaAs wells with AlInAs barriers grown on top of a highly doped InGaAs layer that acts as an electron reservoir. The heterostructure is optimized such that the first IST in all the wells is at 11µm. A complementary gold metasurface with dipole resonances at 11µm is fabricated on top of the QW structure. We designed the heterostructure such that by applying a bias of 1V, the energy bands of all the QWs get aligned simultaneously, leading to the occupation of the ground state of all the QWs via quantum tunneling of the electrons from the electron reservoir. The ISTs which were turned off due to negligible electron density gets turned on at 1V, and this leads to coupling between the ISTs and the dipoles resonances of the metasurface. The voltage induced coupling leads to reflectance modulation which we confirmed experimentally by rapid scan double modulation FTIR measurements.

2D materials capable of altering innate optical properties are highly demanded with broad photonic and optoelectronic applications, especially for ultrathin and ultracompact optical devices.We first demonstrated electrostatic doping driven structural phase transition in monolayer molybdenum ditelluride (MoTe2). The reversible structural phase transition between the hexagonal and monoclinic phases is directly controlled by electrostatic gating and verified through non-polar and polarized Raman spectra. An interesting hysteresis behavior has been observed in Raman and Second Harmonic Generation (SHG). And crystal orientation during phase change is found to be conserved, making such transition robust. We also discover out-of-plane 2D ferroelectricity in atomically thin In2Se3 crystal. Through Piezoresponse Force Microscopy (PFM) and SHG, we experimentally found that in-plane lattice asymmetry and out-of-plane polarization is dynamically locked during phase change. Such unique locking mechanism stabilizes the polar order and enables a robust 2D ferroelectricity at ambient conditions with a high transition temperature. The discovery is important to the atomically thin sensors and ultrahigh density nonvolatile memory devices.

INVITED talk

Metasurfaces have been used for light manipulation and control, from the point of view of planar optics or polarization control, or non-linear light extraction. The properties of metasurfaces is rooted in the presence of resonant basic elements which are responsible for strongly confined electromagnetic surface modes. We propose to use these modes to enhance light-matter interaction in the vicinity of the surface by considering hybrid structure made of a thin layer, such as a 2D quantum material or a thin film with embedded quantum dots. A semi-classical theory will be proposed along with some functionalities attainable with the device.

Mie theory describes how electromagnetic waves scatter at the interface between a homogeneous spherical dielectric particle surrounded by a material of a different optical index. Numerical improvements have allowed studying more complicated geometries with the multipole decomposition of the spherical harmonics. Hence, Mie theory is widely applied in theoretical and applied physics, to enable novel light manipulation, to model Fano resonances, nonlinear optics, or to design dielectric metamaterials. Recently, the anapole state has brought attention to the community as one of the most interesting phenomena. It can be interpreted as a destructive interference in the far field between the fields scattered by the toroidal and electrical dipoles at a given frequency. Such element is therefore transparent to any incoming plane wave. However, things are different if the element is excited in its near field, where it can be excited by an internal source. In this work, we experimentally demonstrate a semiconductor laser based on a single cylindrical resonator suspended in air. An epitaxially grown InGaAsP layer on an InP substrate is patterned by e-beam lithography. We study the shift of the Mie resonance as geometrical parameters are varied, and show how it affects the shift of the lasing frequency. Our investigation of Mie resonances from an active gain medium would is a rich platform to study nontrivial excitation of a complex field and paves the way to designing active devices exploiting Mie theory.

The intrinsic broadband and ultrafast photoresponse of graphene has been extensively studied in recent years, promising the new generation of photodetectors covering the unprecedentedly broad spectrum from THz to near-infrared. It has been demonstrated that the broadband and ultrafast photocurrent generation takes place at the graphene-metal interface with contribution from both photo-thermoelectric and photovoltaic effects, stemming from the efficient generation of hot carriers. Although the hot carrier lifetime is of key importance for their efficient extraction, the dynamics of carrier cooling is still far from being completely understood. So far, two fundamentally different scattering mechanisms have been suggested to dominate in graphene: the momentum-conserved collisions with the high-energy optical phonons, and the disorder-driven supercollisions with the acoustic phonons. However, the co-existing relaxation via both optical and acoustic phonons has not been considered, hindering the interpretation of different experiments within a single physical model. In our work, we discuss the non-uniform graphene properties in the graphene-metal photodetectors, and demonstrate that different cooling mechanisms equally contribute to the process due to the presence of the photocurrentgenerating interface defect. Noting the overlooked role of the metal contact in cooling dynamics, we show that the purity of graphene employed for photodetection is of less importance for the relaxation dynamics compared to the contact area in terms of introduced system disorder. Further, we show that the transient photo-thermoelectric response, so far attributed exclusively to supercollisions, can be predicted by considering the contribution from both relaxation pathways: normal and supercollision scattering of hot carriers.

The application of ultrashort laser fields to atomic, molecular and solid-state systems has become a subject of increasing interest in the last decades, with specific emphasis in the possibility for the coherent control of fundamental quantum processes like photoexcitation, photoionization and photofragmentation, among others. In the case of molecular systems, their photoreactivity involving excited states implies two ubiquitous type of transitions: because of radiative couplings due to the presence of the laser field and because of non-adiabatic couplings due to the breakdown of the Born-Oppenheimer approximation. We present a simple quantum model based on a particle in a box that includes both type of radiative and non-adiabatic couplings. Firstly, we show that non-adiabatic interactions may be as effective as the radiative ones to produce an equivalent dynamics. In addition, optimal control theory is applied to this model system to aid the excitation to a selected target and to understand the inner workings of the optimization method as applied to cases driven both by radiative and non-adiabatic interactions.

Evanescent coupling between the waveguides is the most basic and generic configuration in the design of nanophotonic devices. Various photonic devices for light switching, amplification and modulation are based on this phenomenon. Recent years have witnessed an increased attentions towards microresonators due to their compact size and the wide range of possible applications. The coupling between the resonator and waveguide is dependent on the losses inside the microresonator. A switching technique can be realized by bringing the resonator in and out of the critical coupling condition by controlling the losses. Graphene has emerged as a promising candidate for the wide variety of optoelectronic applications. The tunable conductivity of graphene via electrostatic doping makes it a possible choice for modulating the losses in a waveguide. In this paper, we analyze the effect of graphene integration on silicon on insulator (SOI) waveguide. In the telecommunication range, graphene induced losses have been measured that ranges from 0.35 to 0.05 dB/μm for two graphene layers embedded in the waveguide with oxide in between. The graphene coverage length in the ring is selected as a design parameter for waveguide coupled ring resonator system with graphene induced losses affecting the output transmission. The impact of graphene integration length on the Q factor and extinction ratio has been studied, that are key performance metrics for electro-optic applications. With a radius of 4 μm, the microresonator demonstrates an extinction ratio of approximately 15 dB with Q factor decreasing to almost three times due to graphene induced losses for quarter coverage in the ring. The finite-difference time-domain (FDTD) analysis has been used to study the behavior of waveguide and resonator.

Optical encryption is an open and prolific field with a continual growth. Several cryptosystems as the 4f encrypting system, joint transform correlator (JTC), joint free space cryptosystem (JFSC) and the fractional JTC (FrJTC) cryptosystem have demonstrated high-performance data protection. The experimental implementation of each architecture shows different performances, opening a wide area of research. In this work we experimentally implement an interferometric cryptosystem in which the encryption key is a ground glass diffuser (GGD) located in the reference arm. In the object arm, the information to be encoded is displayed in a spatial light modulator placed in contact with another GGD representing the input object plane. In the setup a lens brings the Fourier transform of the input object plane. In the cryptosystem, the encryption process is achieved by the interference between the Fresnel transform of the key and the Fourier transform of the input object plane. This interference pattern is registered by a CMOS camera. The free propagation distance z between the key plane and the output plane determines the correct Fresnel transform of the key in the register plane. Then, for the decryption process, both the encrypted data and the information of the Fresnel transform of the key in the correct distance are necessary. In order to test the capabilities of the cryptosystem, we implemented experimentally QR codes as an information container making possible a protocol for a noise-free information recovery. The experimental results show the viability and applicability of the proposal.