This PDF file contains the front matter associated with SPIE Proceedings Volume 11348, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

The Terahertz (THz) technology has now reached a level of maturation, which allows its uses beyond its core domains of application (telecom and imaging for security or healthcare). Vibrational spectroscopy in the THz range is employed in various fields and is specifically promising in (μ)biology. Indeed, the probed vibrational states extend over several nanometers and give a signature of the sample 3D structure at the nanoscale. This is particularly salient for macromolecules (proteins, DNA and RNA strands etc.) since, on one hand, their 3D structure is very difficult to probe in physiological condition with other techniques, and on the other hand, this structure determines their function and is consequently of utmost importance for the living. A major hurdle still arises when applying THz spectroscopy on biological or macromolecular samples. The samples are generally smaller than the THz wavelength, which requires concentrating the THz field in the sample. Solutions aimed at tackling this challenge by using μ/nano technology of THz field concentration and a proper data analysis will be presented.

Layer-thickness measurement is one of the most promising and attractive fields of application for terahertz measurement systems, as they really provide benefits in comparison to competing techniques. In contrast to ultrasound systems, terahertz measurements can be carried out without a coupling medium and is therefore a truly contactless measurement. The possibility to measure individual layers in a multilayer stack is highly advantageous in contrast to established eddy current measurement devices. Unlike X-ray devices, terahertz radiation of common measurement systems is not harmful to biological tissue. Terahertz measurement systems have undergone a remarkable development in terms of the performance as well as in the evaluation algorithms. Increase of speed and enhancement of measurement robustness make these optically complex systems ready for industrial employment. In our contribution, we will cover the development of photonic terahertz measurement systems with a focus on terahertz layer thickness determination.

We have developed a thin THz-wave planar lens based on the phase-patterned Fresnel zone plate (FZP) concept to obtain a high transmittance and short focal length in free space. The FZP lens was designed for focusing THz waves at 1.0 THz (λ=300 μm) with a transmittance of more than 80% and a focal length of 24 mm (80λ). The developed FZP lens was made of polymer BCB as a flexible film substrate with concentric zones of metamaterial-based phase shifter patterns with a subwavelength thickness of approximately 48 μm (0.16λ). To obtain the THz-wave phase retardation of π/2 compared to the naked polymer substrate, we employed the metamaterial unit structure consisting of double-layer unsplit ring resonators (USRRs) with a 32-μm distance between the two layers. The experimental result confirms that the FZP lens creates a focus by constructive interference of incident THz waves through concentric zones of metamaterial-patterned and un-patterned regions. By using a narrowband THz-wave beam from an injection-seeded THz-wave parametric generator, the measured focus spot size of 0.57 mm at full width at half maximum was obtained at the designed frequency of 1.0 THz. Using this FZP lens, the THz-wave imaging test in transmission geometry has also been demonstrated.

In the evolution of ultrafast spectroscopy and time-resolved measurements, in particular terahertz time-domain systems (THz-TDS), the demand for high-speed delay engines increases. Applications in scientific and industrial environments involve material science, high-resolution spectroscopy and non-destructive testing. Where ultra-fast techniques become crucial to reduce the acquisition time, mechanical or acousto-optic delay lines are limiting factors. Optical sampling methods are able to overcome these restrictions, by eliminating downsides of mechanical delay lines, such as comparably low scan speeds of tens of Hz. Different technical approaches have been developed to obtain two synchronized, temporally delayed femtosecond pulse trains without using a conventional, mechanical delay line. Common optical sampling techniques employ either a single oscillator or come as twin oscillator systems. The asynchronous optical sampling technique (ASOPS) has proven to enable high scan speeds and high-resolution spectroscopy. Two femtosecond fiber lasers are synchronized by locking electronics and operate in a controlled repetition rate offset state. We have established such dual-laser based systems and integrated them into fully fiber-coupled THz-TDS systems for the scientific community already. Optical sampling by cavity tuning (OSCAT) addresses higher costs that come with dual-laser systems using a single oscillator albeit one with a variable pulse repetition rate. We present a new engine based on the electronically controlled optical sampling principle (ECOPS) - but 10 times faster than achievable with conventional piezo-electric (PZT) based systems. We introduce an optical sampling engine (OSE) bringing ultra-fast, time-resolved measurements to 10 kHz - with unprecedented compactness of 19” 3U.

Radiometry in the sub-millimeter and THz region is required for applications as imaging, spectroscopy, earth observation, planetary missions and radio astronomy. The recent advances in the development of high electron mobility transistors (HEMT) low noise amplifiers (LNAs) have pushed their operation frequency to the sub millimeter range, showing noise figures below 4dB inside cryocoolers. In the THz range detection has been achieved with Schottky mixers working either at room temperature or inside cryostats. They have shown noise temperatures comparable with those of the most recent LNAs. Superconducting devices such as SIS mixers can achieve near quantum limited noise performance while operating at 4K. Sensitive detection is also possible by adapting the principles of photonic detectors to THz frequencies. In this case, low noise operation implies cryogenic cooling of the detectors. Bolometer-based THz receivers operating in cryostats have shown photon counting sensitivity. In this work we study another approach proposed in the last two decades for high sensitivity THz detection at room temperature. The detection principle consists on taking advantage of the nonlinearity of crystals such as lithium niobate to enable a sum frequency generation (SFG) process that boosts the THz photon energy to the optical domain. This occurs via electro-optic modulation of the laser pump by the THz radiation when the waves are phase-matched inside the crystal. To significantly increase the conversion efficiency, an ultra high-Q whispering-gallery resonator (WGR) is used to confine and enhance the optical field. The WGR is also resonant in the THz domain, enhancing further the photon conversion efficiency. In this work we present optimized geometries for the doubly resonant WGR structure and coupling mechanisms. Then, theoretical models are used to predict the photon conversion efficiencies of such electro-optic modulators along with their thermal occupation levels and overall noise performance as THz radiometers.

This contribution addresses theoretically and experimentally the photomixing signal in a low-temperature-grown GaAs (LTG-GaAs) device with a planar antenna that is driven with an alternative electric field superposed on a bias electric field. The total electric field is applied on the optically-driven nonlinear photoconductance of LTG-GaAs. A theoretical framework based on unidimensional carrier transport is developed to calculate the amplitudes and the phases of different spectral components of the current density. Radiofrequency modulation of the photomixing signal in the microwave domain is experimentally demonstrated. Conversely, the photomixer is exploited as an optically-driven detector. Heterodyne detection with a photomixer is experimentally demonstrated with modulated THz-waves. The photomixing approach allows to extract THz modulation parameters by measurements with a modulated optical beat.

Recently, on-chip quantum-cascade-laser-based frequency combs are gaining increasing attention both in the Mid-IR and in the THz spectral regions. THz devices offer the possibility of filling the gap of comb sources in a spectral region were no table-top comb is available. I will discuss direct THz comb generation from both homogeneous and heterogeneous quantum cascade lasers. Octave spanning emission spectra and comb operation on bandwidth larger than 1 THz are reported for heterogeneous cascades. I will also report on a series of new structures with homogeneous cascade design that feature a very low threshold current density (< 100 A/cm2), a bandwidth of roughly 1 THz centered a 3 THz and an extremely wide bandwidth (>1.8 THz) when driven in the NDR region. This extremely broadband emission in the NDR is studied as well with NEGF simulation and is based on an interplay between strong photon assisted transport due to the highly diagonal transition and domain formation.These structures are also showing RF injection locking with extremely reduced microwave powers. We will discuss locking experiments as well as a method to finely control the repetition rate of the laser based on controlled optical feedback. Time resolved spectral measurements aimed to clarify the physics of field domains in the NDR will be also presented.

Radiation and detection of ultra-short terahertz pulses with picosecond duration advance a variety of applications, including imaging, spectroscopy, and wireless communication. Silicon-based integrated circuits can replace bulky, expensive femtosecond lasers with low-cost solutions to generate and detect THz pulses with GHz repetition rates. In this paper, we present laser-free fully electronic THz pulse sources and detectors to radiate and detect broadband frequency combs in mm-wave and terahertz regimes. A THz pulse radiator chip based on the reverse recovery of a PIN diode is presented. This chip radiates pulses with a tunable repetition rate that can go up to 10.5 GHz. In the frequency domain, the radiated pulses generate a frequency comb that extends up to 1.1 THz. The spacing between the THz tones can be tuned by changing the repetition rates of the pulses to cover the desired frequency range. In addition to the THz comb source, a broadband frequency comb detector chip is presented. The detector chip uses a tunable frequency comb as a reference to sense the spectrum over a wide bandwidth. Single-tone measurements were performed using the detector from 50 GHz to 280 GHz. The source and detector technologies are used to implement a dual-comb sensing system, in which the mm-wave/THz frequency components of the radiated combs are compressed to a small bandwidth in the RF regime.

An injection-seeded THz-wave parametric generator (is-TPG) uses a near-infrared (NIR) pump beam and seed beam input to the MgO:LiNbO3 crystal. In contrast, the THz parametric detector uses a THz wave as the seed beam instead of a NIR beam. In the detection configuration, when the pump beam and THz wave are input into the crystal, the THz wave is upconverted to the NIR idler beam by parametric processes in the crystal that are measured using a NIR detector. This THz generation and detection scheme has allowed us to develop a high dynamic range THz wave spectroscopic system that can be used for spectroscopic imaging of saccharides hidden in thick envelopes. Recently, we have improved the sensitivity of the THz parametric detector drastically using a multistage configuration in order to suppress the spontaneous THz emission and enhance the gain. In our new system, the THz parametric detectors were divided into two parts, i.e., for the up-conversion (pre-amplifier) and for the main amplifier. THz waves were upconverted to a NIR idler beam in the first part of the set-up. An iris positioned behind the upconverted region passed the idler beam and blocked the broadband spontaneous THz emission. Thus, in the amplification region, only the desired idler beam was clearly amplified. The amplified detection idler beam was then measured by a NIR beam profiler.

Ultrafast Terahertz sources with high average power are of increasing interest for various spectroscopic investigations, currently limited by signal-to-noise ratio. A straightforward path to increase the average power of THz sources is to make use of state-of-the-art femtosecond near infrared driving lasers with higher average power than the commonly used Ti:Sa lasers. Diode pumped solid state lasers based on Yb now reliably provide from hundreds of watts up to kilowatts of average power with sub-ps pulses, while THz generation with more than a few tens of watts driving power remains widely unexplored. Among these technologies, modelocked thin-disk oscillators are particularly attractive to drive high power, high repetition rate THz sources, providing hundreds of watts directly from a compact one-box oscillator without the need for any additional amplification stages, thereby reducing the overall system complexity. Here, we will present our recent results using optical rectification (OR) in GaP and Lithium Niobate (LN), driven by a home-built Yb:YAG femtosecond modelocked thin-disk oscillator with an average power of more than 100 W at 13 MHz repetition rate. Using GaP, we achieve milliwatt average power levels with a bandwidth extending > 6 THz making this an ideal tool for THz-TDS for example of absorptive samples. Using the tilted pulse front scheme in LN, we achieve THz powers exceeding 40 mW at 13 MHz repetition rate, which represents the highest average power of any THz sources with MHz repetition rate. Additionally to these results, we will present our ongoing investigation of thermal effects and further average power scaling of OR in this unusual excitation regime.

We propose an alignment strategy for millimeter spectroscopy of cornea that uses imaging to screen for sufficient alignment conditions. The performance of different corneal imaging objectives, in the presence of misalignment, is evaluated. The cornea is illuminated with a TEM00 Gaussian beam at 650 GHz and the beam is swept across the cornea. Images are generated by calculating the coupling between illumination and scattered beams for each illumination beam position and angle. The cornea is displaced at intervals of 500 microns in the transverse and axial directions and with new coupling coefficient maps generated at each misaligned position. Contrast in the misaligned cases are compared to the aligned case via zero normalized spatial cross correlation. The results show a maximum normalized cross correlation of 0.92 for a two-mirror scanning system and 0.74 for a one-mirror scanning objective. The analysis suggests that imaging contrast at 650 GHz can be used to screen for misalignment that would be difficult to detect with MMW.

Terahertz time-domain systems are known as a precise imaging tool. These systems make use of parabolic mirrors or lenses to illuminate a small spot of a sample under test. By moving the sample or the sensor head, an image can be recorded. This imaging technique guarantees a high signal-to-noise ratio and large bandwidth. However, using this method a priori knowledge of the sample shape is needed for the correct focusing of the system. This limits the performance and robustness of the imaging system as only specular reflections are considered for the image. Here, we propose a fast reflection-based broadband terahertz time-domain imaging method that overcomes these hurdles by making use of both the specular and diffuse reflections of a divergent terahertz beam. The proposed method employs no optical lenses or mirrors but uses signal processing and classical radar migration techniques. High-resolution imaging is achieved by focusing the divergent terahertz beam via post-processing. To compensate the inherent poor signal-to-noise ratio of the unfocused terahertz beam, calibration and post-processing methods are used. For the evaluation of the imaging method, geometrically complex samples are scanned by a fast terahertz time-domain spectroscopy system based on electronically controlled optical sampling. The bandwidth achieved with the divergent beam is 2.5 THz with a signal-to-noise ratio of around 30 dB. We demonstrate that this method is capable to generate high-resolution 2D terahertz images of objects with arbitrary size, shape, orientation and relative position to the emitter and detector antennas. Objects with sub-mm dimension can be clearly reconstructed for arbitrary positions and orientation achieving resolution in the µm region. Furthermore, the presented method can be applied for any reflection-based scenarios and antenna configuration.

Terahertz imaging system has aroused great attentions in recent years due to its unique applications in security screening, industrial inspection and biomedical evaluation. Most conventional terahertz PCA imaging setup is based on raster scan method, hence, image acquisition time is severely limited by the speed of mechanical movement. Typically, image acquisition time of terahertz PCA tomography systems costs hours to days depending on the size of observed objects, which severely limits their practical feasibility to real-world applications. Here, we propose a terahertz least absolute shrinkage and selection operator (LASSO) compressed sensing (CS) tomography system to reduce more than one order of magnitude of data acquisition time. Our approach replaces slow-moving mechanical raster scanning method by the highspeed spatial modulation of terahertz radiation with designed patterns, resulting in a compact, fast and noise-reduced way to obtain terahertz 3D image dataset. Based on the measured dataset, modified Hadamard and LASSO algorithms are designed to reconstruct high-quality 3D image (voxel number: 128x128x128) in 120s at 10% compressed rate. The reconstructed LASSO 3D terahertz image offers a less than 0.002 mean squared error and 80% structural similarity index compared with the ground truth image. This paves the way toward real-time terahertz 3D imaging in near future, which opens the door for varies of exciting applications in non-destructive sensing, imaging and material inspection.

We explore current-driven Dirac plasmon dynamics in monolayer graphene metasurfaces. DC-current-induced complete suppression of the graphene absorption is experimentally observed in a broad frequency range followed by a giant amplification (up to ∼ 9 % gain) of an incoming terahertz radiation at room temperature.

Graphene has exceptional nonlinear terahertz (THz) properties demonstrated by a strong THz absorption bleaching and a highly efficient frequency multiplication. This nonlinearity is attributed to the collective thermodynamic response of the background electron population of graphene to the exciting THz field, resulting in a temporal modulation (suppression) of the graphene conductivity (bleaching) and consequently leading to re-emission from graphene at higher-order harmonics. The revealed nonlinear coefficients of graphene are found to be several orders of magnitude larger than those of other solids. These findings pave the way for potential graphene-based technological applications including electronics and optoelectronics operating at THz rates.

Graphene/hBN heterostructures are very attractive materials for advanced optoelectronic devices at THz frequencies. The recombination dynamics of non-equilibrium carriers in graphene, which rely on carrier–carrier and carrier-optical phonon scattering, have shown to possess only sub-picosecond characteristic times in the case of large non-equilibrium carrier density at high energy. An additional channel has been recently demonstrated in graphene/hBN heterostructures by emission of hBN hyperbolic phonon polaritons (HPhPs) with <2 ps decay time. However, for the development of THz emitters and photodetectors based on interband transitions, long carrier lifetimes are needed. Here, using mid-infrared photoconductivity measurements we investigate carrier recombination processes for non-equilibrium carriers at low density and energy in graphene/hBN Zener-Klein transistors. We report on carrier lifetimes in excess of 30 ps, ultimately limited by interband Auger processes. We also unveil the possibility to switch on at finite dc bias or mid-infrared optical power the very efficient electron-hyperbolic phonon recombination channel. This allows the control of carrier lifetime which falls below few picoseconds upon ignition of HPhP relaxation. Furthermore, we have investigated the interplay between optical and electrical pumping and demonstrated the opto-electrical pumping of HPhPs in the hBN layer at high Joule power and high optical power. This works may promote graphene/hBN heterostructures as a platform for phonon polariton optics and nanoscale thermal management.

This presentation was first delivered at Photonics West 2020 on 3 February 2020 and has been included as part of this Digital Forum to enable scholarly dialogue. Please use the original citation when citing: Proceedings Volume 11278, Ultrafast Phenomena and Nanophotonics XXIV; 112780O (2020) https://doi.org/10.1117/12.2543969

In semiconductors and semimetals, strong THz electric fields can induce a controlled coherent motion of the electrons in the conduction band, via ballistic excitation. In the first picoseconds after THz excitation, the nonlinearities induced by this coherent excitation prevail before more incoherent high field effects start dominating the nonlinear response. Disentangling these different nonlinear contributions with 2D THz spectroscopy, we follow the trajectory of the out-of-equilibrium electron population in low-bandgap semiconductor InSb and. We then extract information on the conduction band curvature and evaluate its anharmonicity and its anisotropy, close to the Gamma-point.

In laser-excited ferromagnetic heterostructures, both ultrafast local magnetization dynamics and spin-transport processes can lead to a THz emission. Here, we demonstrate that the THz emission spectroscopy is a powerful tool to investigate ultrafast magnetization dynamics in laser-excited magnetic systems. The polarity of emitted THz can be used to distinguish which process, local or non-local, dominates the emission of THz in a ferromagnetic heterostructure. The measured THz radiation can be used for rigorous reconstruction of ultrafast magnetization process in the laser-excited magnetic material.

MXenes are 2D transition metal carbides and nitrides with electronic properties that can be tuned by their chemistry and structure. Three members of MXene family, Ti3C2Tz , Mo2Ti2C3Tz and Mo2TiC2Tz are all intrinsically metallic, with high intrinsic free carrier densities and high carrier mobility within individual nanosheets. However, they respond to photoexcitation in dramatically different ways: while photoexcitation suppresses conductivity in Ti3C2Tz, it results in a long-lived positive photoconductivity in both Mo2Ti2C3Tz and Mo2TiC2Tz. Those responses suggest applications of MXenes in a variety of electro-optical and THz devices.

We present design and simulation of spoof Surface Plasmon Polariton (sSPP) delay lines with same physical length to compose a 1-bit 180ᵒ phase shifter at 1 THz. The sSPP delay lines are based on single conductor waveguide, which has rectangular and identical corrugations on both sides and is attached to a dielectric. The delay lines are engineered by determining only the corrugation depths and keeping all the other parameters same as each other. The corrugation depths of the delay lines vary between 4.5 μm and 15.75 μm. The average percentage phase error, insertion and return losses of 207 μm delay lines are %2.6, -2.2 dB and -17.54 dB at 1 THz, respectively.

We report terahertz optical properties of 1% Nd-doped potassium gadolinium tungstate (KGW) monocrystals studied by terahertz time-domain spectroscopy in the range of 0.25–2.1 THz. KGW refractive index and absorption coefficient are measured for waves polarized parallel to all three axes of optical indicatrix Ng, Nm, Np. Typical values for the refractive indices are ng = 3.75, nm = 3.37, np = 3.40 at 1 THz corresponding to rather large birefringence of ng – nm ≈ 0.38. We find that KGW exhibits dispersion in the studied spectral range with the refractive index ng, for example, significantly increasing by the value of 0.5. We approximate the dispersion of the refractive index in the form of Sellmeier equations. Absorption coefficient behaves similarly for all three optical axes and it is less than 5 cm_-1 for the frequencies below 1 THz. It rises significantly up to 50 cm-1 at higher frequencies where we also observe dichroism of more than 10 cm^-1. Observed properties can be attributed to vibrational modes at 2.58 and 3.39 THz found by Raman and IR spectroscopy in previous studies. However, we find no particular features in the spectral range under investigation in contrast to Raman and IR spectroscopy results. Relatively high cubic nonlinearity of KGW crystal, its small low-frequency terahertz absorption and high birefringence indicate an opportunity for its usage for optical-to-optical and optical-to-terahertz conversion.

Measurements and modeling of the optical properties of adipose tissue and its components in the terahertz range with a change in tissue temperature were carried out. It was shown that the optical density (OD) of adipose tissue samples decreases with increasing temperature, which can be mainly associated with dehydration of the sample. We can also expect some contribution to the decrease in the OD of suppression of THz wave scattering when matching the refractive indices of scatterers and their environment due to the intake of free fatty acids secreted by adipocytes due to thermally induced cell lipolysis. It is shown that in the experimental model, the difference between the THz absorption spectra of water and oil allows us to estimate the water content in adipose tissue. A comparison of the measurement results and molecular modeling in the terahertz region confirmed the hypothesis about the reasons for the change in the optical properties of heated adipose tissue.

In recent years, the terahertz sources have attracted much attention for its special use compared with so for UV-visible range. Crystal with high quality and special properties is their indispensable part. Optical properties such as the absorption coefficient and refractive index at three basic crystal orientations (x, y, and z-direction) of the BIBO crystal have been studied by the Terahertz Time-Domain Spectroscopy at room temperature. A large birefringence was observed. The measured refractive index components were approximated in the form of Sellmeier equations. Phase-matching curves for collinear down-conversion of IR laser frequencies into the THz domain were preliminarily estimated. A prospect of BIBO crystals as THz generators are discussed, and comparison with popular crystals is given.

Polarizing beam splitter occupies a very important position in the generation and control of vector beam, the demand for it has been increasing. Metasurfaces provide a highly flexible platform for regulating the propagation of electromagnetic waves. In this paper, a planar polarization beam splitter based on dielectric metasurface at wavelength of 118.8 μm, is demonstrated and simulated. It is mainly achieved by the array composed of silicon cubes of different sizes placed periodically on a silicon dioxide substrate. The three-dimensional finite-difference time-domain method was used to numerically simulate the optical characteristics of the polarizing beam splitter. The polarizing beam splitter can transmit vertically incident polarized light into x polarization and y polarization propagating in different directions, and the efficiency is as high as 90%. As an example, to further demonstrate the performance of the polarization beam splitter, we also design a focusing lens based on the hyperboloidal profile theory at λ=118.8 μm. The lens can focus differently polarized light beams into one spot in different directions. With appropriate design, we expect this structure can be applied to many practical applications with high efficiency in transmitting mode in terahertz range, such as vector beam generators, polarizers and so on.

We propose a modified approach to more detailed study of gyrotropic materials using terahertz (THz) ellipsometry method based on the magneto-optical Kerr effect (MOKE). This approach allows to obtain polarization properties and to calculate the permittivity tensor of materials which are reflective or opaque in THz frequency range. The method allows to measure any values of the diagonal and off-diagonal components of the permittivity tensor and can be used for materials with a strong magneto-optical response.