This PDF file contains the front matter associated with SPIE Proceedings Volume 10530 including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

At the quantum limit of nonlinear optics, where single photons interact with individual emitters, new phenomena are found. One-dimensional, subwavelength waveguides have recently emerged as a promising quantum optical platform capable of both guiding and confining photons, leading to efficient light-matter interactions and enhancing quantum optical nonlinearities. We explore this regime using both single organic molecules and epitaxially grown quantum dots coupled to nanoscale waveguides. In the first case, two-color experiments allow us to observe distinctly quantum nonlinearities such as Stark shifts of the molecular resonances or amplification without population inversion, although we are limited by relatively weak molecule-waveguide coupling. In contrast, quantum dots can be nearly perfectly coupled to waveguides, allowing us to demonstrate optical nonlinearities at the true single photon level. These, as we discuss, can be used as a basis for novel on-chip quantum technologies.

The response of a material to applied intense radiation is characterized by its nonlinear optical susceptibility. While the conventional microscopic picture of nonlinear optics of materials involves expressions using higher order perturbation theory, recent theoretical studies have established a link between nonlinear optics and geometrical properties of the electronic wavefunction. Weyl semimetals are a recently discovered class of materials with nontrivial band structure geometry. We use optical second harmonic generation to measure the second order nonlinear optical response of Weyl semimetals of the transition metal monopnictide (TMMP) family. We find that the TMMP compounds have the largest measured nonlinear optical susceptibility of any bulk crystalline materials, with a susceptibility nearly an order of magnitude higher than that of other nonlinear optical materials.

THG is used nowadays in many practical applications such as a substance diagnostics, and biological objects imaging, and etс. Therefore, THG features understanding are urgent problem and this problem attracts an attention of many researchers. In this paper we demonstrate a formation of sub-pulse comb with subfemtosecond duration at THG of femtosecond laser pulse propagating in a medium with cubic nonlinear response. This pulse comb has a regular structure in many cases of interacting waves. The sub-pulse duration is governed by the incident femtosecond pulse duration. We consider both incident chirped and un-chirped pulse.

Consideration is based on computer simulation of the laser pulse propagation with taking into account a self- and cross- modulation of interacting waves, and their SOD, and phase mismatching. Observed sub-pulse formation can be used for generation of sequence of attosecond pulses with soliton or soliton-like shape.

Terahertz (THz) spectroscopy offers unique insights into the electronic and vibrational degrees of freedom of materials. Because of the long wavelength of THz portion of the electromagnetic spectrum, it can be challenging to generate and detect THz radiation on nanometer scales. In this talk I will describe our efforts to develop a platform for generation and detection of broadband terahertz (~10 THz bandwidth) emission using 10-nm-scale LaAlO3/SrTiO3 nanostructures created by conductive atomic force microscope (c-AFM) lithography. The technique relies on the large inherent third-order optical susceptibility of SrTiO3. Inversion symmetry is locally broken at this interface at nanoscale junctions, enabling local optical rectification of light from ultrashort near-infrared optical pulses that is electrically detected by the same junction1. This system is capable of generating and detecting broadband THz emission on a scale set by the junction, which can be as small as 10 nanometers2. I will describe how these devices are created and used to probe nanoscale objects (e.g., Au nanorods3) that can be placed and co-located with these sensitive sources and detectors of THz emission. Our current focus involves graphene nanostructures that are integrated with the LaAlO3/SrTiO3 system, but a host of other materials can in principle be integrated with this platform. 1. Irvin, P.; Ma, Y. J.; Bogorin, D. F.; Cen, C.; Bark, C. W.; Folkman, C. M.; Eom, C. B.; Levy, J., Rewritable nanoscale oxide photodetector. Nat Photonics 2010, 4 (12), 849-852. 2. Ma, Y.; Huang, M.; Ryu, S.; Bark, C. W.; Eom, C.-B.; Irvin, P.; Levy, J., Broadband Terahertz generation and detection at 10 nm scale. Nano Lett 2013, 13 (6), 2884-2888. 3. Jnawali, G.; Chen, L.; Huang, M.; Lee, H.; Ryu, S.; Podkaminer, J. P.; Eom, C.-B.; Irvin, P.; Levy, J., Photoconductive response of a single Au nanorod coupled to LaAlO$_3$/SrTiO$_3$ nanowires. Applied Physics Letters 2015, 106 (21), 211101.

The mechanisms through which electrons are ejected from atoms (or molecules) and subsequently driven to high-energies in a strong oscillating electric field are generally well understood, enabling a variety of techniques from attosecond pulse generation to imaging via time-resolved electron scattering. Analogous processes are at play in strong-field emission and acceleration of electrons from solids and, for nano-scale targets, brief electron bursts from particles with dimensions much less than the wavelength of the drive field hold great promise for ultrafast imaging and other applications. For long wavelength drivers, the enhanced local field near nano-particles or nano-tips can fundamentally change the energy transfer process and allow for the creation of very high energy electrons. As an extreme example of the long-wavelength limit, we have used intense single-cycle THz pulses to drive electron emission from unbiased, nano-tipped tungsten wires. Energies easily exceeding 5 keV are observed, substantially greater than those previously attained at higher THz and infrared frequencies. The large electron energies reflect electric field enhancement factors as large as 3000 in the vicinity of the sharpest tips. Despite large differences in the magnitude of the respective local fields, we find that the maximum electron energies are only weakly dependent on the tip radius, for 10nm < R < 1μm. Due to the single-cycle nature of the field, the high-energy electron emission should be confined to a single burst, potentially enabling applications in ultrafast imaging.

Metasurfaces constructed from metal nanostructures can operate as efficient coupling structures for incident optical beams to surface plasmons. On a semiconductor, metallic metasurfaces can act simultaneously as a device electrode while ensuring strong plasmon field overlap with the active region. Additionally, plasmon fields can be confined to subwavelength dimensions and significantly enhanced relative to the exciting field. These features are very attractive for nanoscale optoelectronic device applications, such as photodetectors and modulators, as the excitation of surface plasmons alters conventional trade-offs between responsivity and speed, or modulation and speed, respectively. We discuss recent progress on optoelectronic metasurfaces, particularly device demonstrations for sub-bandgap hot-carrier photodetection and high-speed modulation.

A circular grating coupler on a single-crystalline Au platelet is used to create surface plasmon polariton waves with spherical phase fronts that form a focal point in the center of the circle. In a time-resolved photoemission microscopy experiment, the propagation and interaction of plasmon waves in the focusing structure is investigated with sub-femtosecond time-resolution. Several space-time signatures of propagating and transiently formed standing plasmon waves are identified. The method of contrast enhancement for the data-analysis is discussed, and all observed experimental features are explained. In addition to the known electron emission from the nonlinear superposition of light and plasmon field, we also observe plasmoemission signatures, i.e., emitted electrons that arise from a nonlinear emission process driven purely by the plasmonic field.

We discuss propagation behaviors of surface plasmons (SPs) in 2-dimensional (2D) and 1-dimensional (1D) materials, such as graphene and carbon nanotubes. Firstly, we introduce recent theoretical and experimental studies on the reflections of SPs at the edge of 2D material, and discuss corresponding anomalous phase shifts. Also, we extend our theoretical model to describe the tunneling of SPs through an abrupt nano-gap in 2D surface. Specifically, we examine gap-size dependent tunneling efficiency and the nature of phase shift. Finally, we apply our model to explain SP interaction with the end of 1D material, and discuss how the SP reflection in the 1D system can be described.

Surface plasmons in a DC current lead to an increase in scattering processes, resulting in a measurable increase in electrical resistance of a plasmonic nano-grating. This enables a purely electronic readout of plasmonically mediated optical absorption. We show that there is a time-dependence in these resistance changes on the order of 100ps that we attribute to electron-phonon and phonon-phonon scattering processes in the metal of the nano-gratings. Since plasmonic responses are strongly structurally dependent, an appropriately designed plasmoelectronic detector could potentially offer an extremely fast response at communication wavelengths in a fully CMOS compatible system.

We present results on photon echo spectroscopy for resonant excitation of localized charged exciton complexes (trions) in CdTe/CdMgTe semiconductor quantum wells. We demonstrate that the Zeeman splitting of resident electron spin levels in transverse magnetic field leads to quantum beats in the photon echoes with the Larmor precession frequency. This allows us to perform a coherent transfer of optical excitation into a spin ensemble and to observe long-lived photon echoes. Our approach can be used as a tool for remarkably high resolution spectroscopy of the ground state levels: We are able to resolve splittings between the spin levels with sub-µeV precision and to distinguish between different types of electrons in the ensemble, namely electrons either bound to donors or localized on quantum well potential fluctuations. To that end we show that stimulated step-like Raman processes in the two-pulse excitation scheme allow us to probe the electron spin ensemble with high selectivity and precision even for systems with broad optical transitions. Next, Rabi oscillations for exciton complexes with different degree of localization are detected by photon echo spectroscopy. We observe that an increase of the area of either the first or the second pulse leads to a significant decrease of the photon echo signal, which is strongest for the neutral excitons and less pronounced for the donor-bound exciton complex.

Minimizing the luminescence lifetime while maintaining a high emission quantum yield is paramount in optimizing the excitation cross-section, radiative decay rate, and brightness of quantum solid-state light sources, particularly at room temperature, where non-radiative processes can dominate. In that sense, plasmon-based optical nanoantennas can feature strongly enhanced and confined optical fields to enhance excitation probabilities and fluorescence decay rates. Their morphology and their coupling to luminescent emitters can be engineered to minimize non-radiative losses and optimize their overall brightness. We demonstrate here that short DNA strands are an excellent template to introduce individual fluorescent molecules in dimers of gold nanoparticles in order to achieve single photon emission with decay rates enhanced by more than two orders of magnitude (M. P. Busson et al, Nat. Commun. 3, 962 (2012)). The coupling between single dye molecules and plasmonic gap antennas can be further optimized by selecting nanostructures where the transition dipole of the emitter is aligned with the gold particle dimer axis (M. P. Busson & S. Bidault, Nano Lett. 14, 284 (2014)). Furthermore, by using dimers of 60 and 80 nm diameter gold particles, we demonstrate the assembly of nanostructures exhibiting single-photon emission with lifetimes that can fall below 10 ps and typical quantum yields in a 45−70% range (S. Bidault et al, ACS Nano 10, 4806 (2016)). These data are in excellent agreement with theoretical calculations and demonstrate that millions of bright fluorescent nanostructures, with radiative lifetimes below 100 ps, can be produced in parallel.

Through the effect of Purcell enhancement, nanoantennas strongly modify the local density of optical states and control the emission of coupled emitters. These antennas determine, in addition to the emission spectrum and polarization, also the angular distribution of the emitted photons, i.e., the radiation pattern. Nearly all directional nanoantennas reported so far, rely on external excitation schemes such as a laser or scanning tunneling microscope (STM) tip, which severely hamper on-chip integration. Here, we experimentally demonstrate for the first time, unidirectional light emission from electrically driven in-plane two-wire nanoantennas in the shape of the letter V. The antenna wires are connected with narrow electrical leads which support electrical currents while preserving the resonant properties of the antenna [1]. A nanoscopic tunneling gap is formed at the feed point of the antenna through a controlled electromigration procedure. Strong far-field interference between the spontaneous dipolar light emission of the tunnel junction and the fundamental quadrupolar resonance of the antenna gives rise to a directional radiation pattern [2]. We show that this directivity can be actively tuned with the applied voltage, and passively tuned with the antenna geometry. The experimental findings are analyzed in detail through electro-optical characterization and extensive numerical simulations. These fully configurable ultra-fast tunneling nanoantennas seamlessly exploit light-matter interactions at the nanoscale and set a new paradigm for directing optical energy on chip using an extremely small footprint. [1] Kern et al. Nature Photonics (2015) 9, 582–586; [2] Vercruysse et al. ACS Nano (2014) 8(8), 8232−8241.

Excitonic excitations play an important role in the optical response of low-dimension nanoscale semi-conducting materials. During its lifetime, excitons may diffuse or migrate in particular directions, thus constituting a form of excitation information transfer on the nanoscale. The details of the spatio-temporal evolution of excitons remain unclear, because it has been challenging to directly visualize this process with nanometer spatial resolution and femtosecond temporal resolution. Here we describe pump-probe measurements at the nanoscale, using the photo-induced force microscopy (PiFM) and near-field scanning optical microscopy (NSOM) at ambient conditions. We analyze the spatial and temporal characteristics of the excitons in quasi-1D semiconductor nanowires, and provide unprecedented views of their evolution.

The novel and versatile physical properties of nanowires have stimulated large fundamental and technological interests during the last decades. The appearance of novel boundary conditions, compared to bulk materials, drastically modifies phonon propagation and scattering in these materials. As a consequence nanowires has been intensively studied for the various implications of such modifications: control of thermal transport at the nanoscale, thermoelectricity or nano-sized acoustic transducer. Ultrafast pump probe spectroscopy is perfectly suited to study phonons in such structures. The absorption of a femtosecond pulse leads to the generation of high frequency coherent acoustic phonons and their evolution can be time resolved thanks to the high temporal resolution of this technique. Here, I will discuss some of our most recent studies of coherent acoustic phonons in semiconductor nanowires. I will first briefly describe the modifications of light and sound behaviour due to the 1D structure. I will then present observations of both confined and propagating acoustic phonons in nanowires. Finally, I will show how from the observation of the phonons, complete elastic characterization of nanowires can be achieved. After demonstrating the capabilities of this method on well-known materials, it is applied to the wurtzite phase of GaAs, a metastable phase in bulk, and measured for the first time its elasticity tensor. These results, in addition to the characterization capabilities, also provide an understanding of the thermal transport at the nanoscale and opens new possibilities for light control at the nanoscale.

We show time resolved spectroscopy measurements with femtosecond time resolution in LaVS3 crystal. Transient reflectivity reveals the existence of a preferential coupling between the photexcited electrons and the coherently excited A1g phonon mode. The time scale of this coupling is around 140 fs, whereas the time-scale of the interaction between the A1g phonon mode and the other phonons is around 400 fs. This phonon mode is damped in less then 2 ps. On longer time scale, the transient dynamics is characterized by a long-standing plateau of reflectivity, which can be due to either low inter-plane thermal conductivity or to a metastable state caused by carrier trapping in vanadium cluster.

FeSe is one of the simplest Fe-based unconventional superconductors yet the full impact of correlation effects remains subject of debate. Early DFT studies found negligible electron-phonon coupling and focused on spin fluctuations as mediator of pairing. However, more advanced DFT+DMFT calculations predicted that electronic correlations boost the electron-phonon coupling yet direct experimental evidence was lacking. Here, we combine time-resolved x-ray diffraction and time- and angle-resolved photoemission to study the coherent response of a A1g mode in FeSe. X-ray diffraction tracks the light-induced coherent lattice motion and photoemission monitors the coherent changes in the electronic band structure. Focusing on the coherent lattice vibrations is analogous to electronic lock-in measurements, where a weak electronic signal is extracted by locking-in to a reference signal at the same frequency. Similarly, the coherent phonon mode provides an internal reference for measuring the electron-phonon coupling. ‘Locking-in’ on the phonon frequency avoids low-frequency contributions from other dynamical processes such as acoustic mode coupling or heat transport, which inevitably accompany optical excitation. Without any a-priori assumptions we determine the deformation potential solely from experiment and find that correlation effects in FeSe enhance the electron-phonon coupling by an order of magnitude compared to DFT. Our results are in excellent agreement with DFT+DMFT calculations that explicitly account for electron-electron correlations and their impact on electron-phonon coupling. In the future, this combined time-domain approach may provide unambiguous results for more controversial materials like Cu- and Fe-based unconventional superconductors.

We have observed emission of terahertz radiation from photoexcited GeS nanosheets without external bias. We attribute the origin of terahertz pulse emission to the shift current resulting from inversion symmetry breaking in ferroelectric single- or few-layer GeS nanosheets. We find that the direction of the shift current, and the corresponding polarity of the emitted THz pulses is determined by the spontaneous polarization in the ferroelectric GeS nanosheets. Experimental observation of zero-bias photocurrents puts GeS nanosheets forth as a promising candidate material for applications in third generation photovoltaics based on shift current, or bulk photovoltaic effect.

To combine the advantages of ultrafast femtosecond optics with an on-chip communication scheme, optical signals with a frequency of several hundreds of THz need to be down-converted to coherent electronic signals of GHz or less. Here, we present an optoelectronic measurement scheme that allows for the direct read-out of ultrafast electronic nonequilibrium processes in nanoscale circuits. Particular, we demonstrate that photocurrents in single-walled carbon nanotubes (CNTs) under a resonant optical excitation of their subbands can be ballistic on subpicosecond timescales. The investigated semiconducting CNTs are integrated as functional parts of on-chip THz stripline circuits. In turn, the ballistic currents in the CNTs drive THz transients in the on-chip THz circuits with a bandwidth of up to 2 THz. The transients propagate within the striplines on a macroscopic, millimeter scale. Our results pave the way towards femtosecond on-chip electronics based on single-walled CNTs.

Control of electronic, optical and thermal properties in semiconductor nanostructures allows for design of electronic, optoelectronic and thermoelectric devices. For some applications, a prolonged excited-state carrier lifetime is desired without carrier thermalization or recombination. Engineering charge separation and indirect recombination pathways leads to hot electrons dominating the device response. In this work, transient absorption of terhertz (THz) probe pulses measure the recombination dynamics of photoexcited carriers in a type-II InAs/InAsSb multiple quantum well (MQW). THz measures free-photocarrier absorption and lattice expansions within the MQW as a result of phonons or polarons. The carriers are photoexcited close to the fundamental excited-state resonance of the MQW for a range of lattice temperatures between 5 K and 300. Excitation above the MQW resonance at low temperature shows fast (~15 ps), intermediate (~150 ps) and slow (~1500 ps) recombination times. As the lattice temperature is increased, fast recombination subsides and the slower recombination components grow. This switch of recombination components is almost conservative and is agreement with photoluminescence results suggesting that radiative recombination occurs strongly for the entire temperature range. Fast recombination results from direct recombination within the MQW, as conduction electrons combine with localized holes arising from alloy fluctuations that are frozen in low temperature. At higher temperatures, recombination processes are indirect, between the well’s conduction electrons and barrier’s valence holes. The identical temperature dependence of slower recombination contributions indicated this to be a two-step mechanism that is also reliant on the electron-phonon coupling. Type-II MQWs can enhance this recombination times to prolong hot carriers for optoelectronic devices.

Multiorbital correlated materials are often on the verge of multiple electronic phases (metallic, insulating, superconducting, charge and orbitally ordered), which can be explored and controlled by small changes of the external parameters. The use of ultrashort light pulses as a mean to transiently modify the band population is leading to fundamentally new results. In this paper we will review recent advances in the field and we will discuss the possibility of manipulating the orbital polarization in correlated multi-band solid state systems. This technique can provide new understanding of the ground state properties of many interesting classes of quantum materials and offers a new tool to induce transient emergent properties with no counterpart at equilibrium. We will address: the discovery of high-energy Mottness in superconducting copper oxides and its impact on our understanding of the cuprate phase diagram; the instability of the Mott insulating phase in photoexcited vanadium oxides; the manipulation of orbital-selective correlations in iron-based superconductors; the pumping of local electronic excitons and the consequent transient effective quasiparticle cooling in alkali-doped fullerides. Finally, we will discuss a novel route to manipulate the orbital polarization in a a k-resolved fashion.

Ultrastrong light matter coupling has raised high interest in recent years for the predicted unusual quantum properties of its ground state, which contains photons. We have investigated such physics in a system based on the cyclotron transition of a 2D confined electrons (or holes) gas in semiconductors coupled to the modes of highly subwavelength metallic resonators in the 200-1000 GHz range. The extreme reduction of the cavity volume and surface (Seff/λ0=3 x 10-7) led to the observation of ultrastrong coupling on a small (<100) number of electrons. Such extreme conditions reveal also a previously unobserved renormalization of the cyclotron effective mass, effectively breaking Kohn’s theorem. Kohn's theorem states the independence of the cyclotron resonance frequency from many-body effects in the case of a parabolic and translationally invariant system. For our resonator the translational invariance is clearly broken since the electric field is concentrated on a circular region of around r= 350 nm for a cyclotron radius of the order of 60 nm for a free space wavelength of 1 mm (300 GHz). In our case we can reveal many body effects on the cyclotron mass because we break the translational invariance of the system with the extreme photonic confinement provided by the cavity, observing an increase of the m*/m0 of 6% with respect to the uncoupled cyclotron mass. Experiments conduced on the same 2DEG with a standard split-ring resonator at the same frequency do not show any effective mass shift.

Negative electron affinity (NEA) photocathodes have attracted a lot of interest over the last two decades due to their high quantum efficiency and low dark emission, which are desirable for night vision and other low-light applications. Recently, gradient-doping technique has shown promise to significantly improve the quantum yield of GaAs/AlGaAs heterojunction photocathodes by assisting electron diffusion toward the surface. In the present work, femtosecond pumpprobe transient reflectivity measurement has been used to study the ultrafast carrier dynamics in NEA GaAs/AlGaAs photocathodes. The research focuses on the comparison between a traditional, uniform-doped structure (1.7 μm p-GaAs (1×10¹⁹ cm^-3) / 0.7 μm p-Al_0.57Ga_0.43As (3×10¹⁸ cm^-3) / si-GaAs substrate) and a gradient-doped structure (0.1 μm pGaAs (1×10¹⁸ cm^-3) / 1.2 μm p-Al_0.63Ga_0.37As (doping level gradually changes from 1×10¹⁸ cm^-3 to 1×10¹⁹ cm^-3) / 0.5 μm p-GaAlAs (1×10¹⁹ cm^-3) / si-GaAs substrate). Our result indicates that gradient doping not only leads to more efficient electron transportation but also results in better electron accumulation (i.e. higher concentration and longer lifetime) near device surface, a feature well-suited for photocathodes. Moreover, we have shown that pump-probe transient reflectivity measurement is able to offer a direct picture of electron diffusion inside NEA photocathodes, which can be of significant importance to device development.

When solids are exposed to intense optical fields, the intraband electron motion may influence interband transitions, potentially causing a transition of light-matter interaction from a quantum (photon-driven) regime to a semi-classical (field-driven) regime. We demonstrate this transition in monolayer graphene. We observe a carrier-envelope-phasedependent current in graphene irradiated with phase-stable two-cycle laser pulses, showing a striking reversal of the current direction as a function of the driving field amplitude at ~2 V/nm. This reversal indicates the transition into the field-driven (or strong-field) regime. We show furthermore that in this regime electron dynamics are governed by suboptical-cycle Landau-Zener-Stückelberg interference, comprised of coherent repeated Landau-Zener transitions. We expect these results to have direct ramifications for light-wave driven electronics in graphene.

The most intriguing property of organic semiconductors for optoelectronic applications is their chemical tunability which yields high potential to tailor and control their properties according to their functional purpose in the device. However, the device relevant properties of organic films such as the energy level alignment of the molecular transport levels or the optical generation of (free) charge carriers are not only determined by the intrinsic properties of the individual molecules. They are also severely influenced by dielectric screening mechanisms as well as by the polarizability of the organic material and hence by the collective properties of the organic solid. In this work, we present new insight into the ultrafast collective response of organic semiconductor films after optical excitation. The combination of time- and angle resolved photoemission with a fs-XUV light source allows us to simultaneously follow the transient evolution of the occupied as well as of the unoccupied band structure after fs-optical excitation with visible light. As model system, we investigate thin films of the prototypical organic molecule C60 on the Ag(111) surface. The unoccupied part of the C60 band structure reveals the well-known quasi-particle dynamics of the exciton formation and its decay in C60. Most interestingly, we observe transient changes of the linewidth of all occupied molecular orbitals upon optical excitation which can be directly linked to the characteristic timescales of the exciton decay of C60. These observations are attributed to a collective transient polarization of the molecular film caused by the exciton formation at distinct C60 sites.

Carbon nanotubes (CNTs) have many uses in energy storage, electron emission, molecular electronics, and optoelectronics. Understanding their light-matter interactions is crucial to their development. Here, we study a film of single-walled CNTs with a thickness of 1.67 μm and a 2D orientational order parameter of 0.51, measured by polarized Raman spectroscopy. The film is expected to have a work function of about 5.1 eV. In this study, ~100-fs pulses with 1.5 (ℏω) and 3 eV (2ℏω) photon energy are used to pump the CNT film while observing its electron emission in vacuum. Ultrafast pulses produce nonlinear phenomena in enhanced field emission, as the CNTs absorb strongly enough that thermally excited carriers can tunnel through the potential barrier. Through curve fitting of the power dependence for each pump energy, we find that the light at ℏω is absorbed via 5-photon absorption, and the light at 2ℏω is absorbed via a combination of 2- and 3-photon absorption. Further study reveals a space-charge limited regime with low applied bias, a photoemission regime with moderate bias, and a laser-assisted field emission regime when the bias is high enough that the photon pump is no longer important. Cross-correlation pumping with the two colors simultaneously shows 4x enhancement of the emission, with a FWHM that suggests a lifetime of ~190 fs, similar to the dephasing time of electrons in CNTs. These studies help illuminate the properties of CNTs as a nonlinear optical material and go towards a more thorough understanding of their optoelectronic properties.

Two-dimensional transition-metal dichalcogenides (TMDCs) have recently emerged as a promising class of materials. A fascinating aspect of these atomically thin crystals is the possibility of combining different TMDCs into heterostructures. For several TMDC combinations, a staggered band alignment occurs, so that optically excited electron-hole pairs are spatially separated into different layers and form interlayer excitons (IEX). Here, we report on time-resolved, low-temperature photoluminescence (PL) of these IEX in a MoSe₂-WSe₂ heterostructure. In the time-resolved measurements, we observe indications of IEX diffusion in an inhomogeneous potential landscape. Excitation-density-dependent measurements reveal a dipolar, repulsive exciton-exciton interaction. PL measurements in applied magnetic fields show a giant valley-selective splitting of the IEX luminescence, with an effective g factor of about -15. This large value stems from the alignment of K+ and K- valleys of the constituent monolayers in our heterostructure, making intervalley transitions optically bright, so that contributions to the field-induced splitting arising from electron and hole valley magnetic moments add up. This giant splitting enables us to generate a near-unity valley polarization of interlayer excitons even under linearly polarized excitation by applying sufficiently large magnetic fields.

When electronic excitations in a semiconductor interact with light, the relevant quasiparticles are hybrid lightmatter dressed states, or exciton-polaritons. In monolayer transition metal dichalcogenides, a class of 2D direct bandgap semiconductors, optical excitations selectively populate distinct momentum valleys with correlated spin projection. The combination of this spin-valley locking with photon dressed states can lead to new optical phenomena in these materials. We present spectroscopic measurements of valley-specific exciton-polaritons in monolayer 2D materials in distinct regimes. When a monolayer is embedded in a dielectric microcavity, strong coupling exciton-polaritons are achieved. Cavity-modified dynamics of the dressed states are inferred from emission. Polarization persists up to room temperature in monolayer MoS₂, in contrast with bare material. We also show that distinct regimes of valley-polarized exciton-polaritons can be accessed with microcavity engineering by tuning system parameters such as cavity decay rate and exciton-photon coupling strength. Further, we report results showing that polarization-sensitive ultrafast spectroscopy can enable sensitive measurements of the valley optical Stark shift, a light-induced dressed state energy shift, in monolayer semiconductors such as WSe₂ and MoS₂. These findings demonstrate distinct approaches to manipulating the picosecond dynamics of valleysensitive dressed states in monolayer semiconductors.

Subwavelength imaging and control of localized near-field distribution under off-resonant excitation within identical gold bowtie structure, and of dark mode distribution within nanoring were demonstrated. The near-field control was established by coherent control of two orthogonally polarized fs laser pulses in bowtie and by varying polarization direction and wavelength of single femtosecond laser beam in the nanoring structure. We found that the hot spot under off-resonant wavelength illumination mainly distributed along the edges of the nanoprism in the bowtie and quadruple mode formation in the nanoring. The obtained results show that the PEEM images correspond generally to the simulated patterns of the plasmonic modes for the both structures and difference exists between experimental and simulated images. The responsible reasons for difference are discussed in terms of band structure near Fermi level and of surface imperfects of the structure. Our finding for the near field control of the nanostructure provides a fundamental understanding of the non-radiative optical near field and will pave the ways for the applications such as sensing, SERS, biomedicine and plasmonic devices.

The estimation of electric field amplitudes of surface plasmon polaritons, or at least ratios of field amplitudes, can be cumbersome if they need to be determined from time-resolved photoemission electron microscopy measurements. We discuss a Fourier–based strategy for the data analysis of two-photon photoemission electron microscopy images that adresses this issue and allows distinguishing surface plasmon polariton pulses that overlap in space but propagate in different directions. The Fourier-based strategy is applied to determine the coefficients of transmission and reflection of plasmonic Bragg reflectors. The reflection coefficient increases with the number of reflector grooves. Our work demonstrates that 2PPE–PEEM can be routinely used to quantitatively analyze the performance of plasmonic Bragg reflectors.