This PDF file contains the front matter associated with SPIE Proceedings Volume 11499, including the Title Page, Copyright Information, and Table of Contents.

We introduce our developing photonics-based technologies for THz-wave generation/combination by arrayed photomixers. We demonstrated THz-wave power combination and beam steering at 300 GHz with phase tuning at optical delay lines. In addition, we made electrically-controlled arrayed phase shifters on a silica-based planar lightwave circuit, which showed a high-speed beam steering with 1-kHz repetition. For a future integrated THz-wave source, we also designed and fabricated arrayed lightwave sources to be coupled to the arrayed photomixers. Combination of these two arrays would enable to generate THz wave only with DC currents or voltages.

The recent emergence of spintronic terahertz (THz) emitters has provided a low-cost source of high field-strength, broadband THz radiation, exploiting the laser-induced electron spin properties in magnetic multi-layers to produce gap-free emission covering 1-30 THz and electric fields up to 300 kV/cm. An interesting property of spintronic emitters is that they have been shown to generate THz radiation polarized perpendicular to the applied magnetic field and independent of the pump laser polarization. We exploit this magnetic field-dependent emission process to create arbitrary terahertz polarization profiles. We show that by applying a specific magnetic field pattern to the source, it is possible to generate a quadrupole-like THz polarization profile. Measurements of the THz electric field at the focus of the beam revealed a polarity flip in the transverse profile of the quadrupole-like mode with a resulting strong, on-axis longitudinal component of 17.7 kV/cm.

We have developed injection-seeded terahertz (THz)-wave parametric generators (is-TPGs), which have potential properties to THz applications because of high output, widely tunability and room temperature operation with compactness. Recently, the remarkable results are a 100 kHz repetition-rate is-TPG which marks a thousandfold enhancement, and a rapidly frequency-switching is-TPG using a multi-furcated Nd:YAG microchip laser with 11-GHz frequency separation. In addition, we demonstrated a security application with a is-TPG, which is a real-time screening system as passengers gate based on THz-wave spectroscopic detection.

We present the overview of the results on the development of compact THz setups based on the quantum dot photoconductive antennas obtained during the past five years. We demonstrate the potential of the InAs/GaAs Quantum-Dot based setups to become an efficient approach to compact, room-temperature operating CW and pulsed terahertz setups for spectroscopy and imaging. We describe the photoelectronic processes in quantum dot substrates and reveal the role of quantum dots in free carrier lifetimes and the formation of the ultrafast photocurrent. We demonstrate the operation mode of the proposed antennas in pulsed and CW regimes under resonant (carriers are excited only inside the quantum dots) and off-resonant (carriers are excited in the bulk volume of the substrate) pumps with compact quantum dot semiconductor lasers. The results allow suggesting the quantum dot based setups as a new approach to field condition compact THz sources for imaging and spectroscopy.

We theoretically study the influence of asymmetric scattering processes on the high-frequency response of miniband electrons in a semiconductor superlattice (SSL) under the action of an AC electric field. We show that asymmetric current affects the spontaneous emission and can result in significant enhancement of even harmonics by tuning the interface quality. We model the system using the Boltzmann equation in the path integral form, treated non-perturbatively in the illuminating field by employing local boundary conditions which allow the inclusion of asymmetric relaxation times. Finally, we consider further the deviations from a completely anti-symmetric current-voltage characteristic and analyze the nonlinear response of SSL excited by a Gaussian optical pulse.

We report three approaches to development of electrically-pumped THz emitters based on III-Nitride structures. The first approach entails the investigation of two-dimensional (2D) plasmons in grating-gated AlGaN/GaN heterostructures performed at temperatures above liquid nitrogen by means of THz time-domain spectroscopy (TDS). Comparative analysis of the experimental data revealed the considerable phase shift of transmitted THz pulses at the resonant frequencies of collective oscillations of the grating-gated 2D electron gas (2DEG). The use of 2D plasmons is proposed for the development of tunable-frequency THz emitters with electrical control of the emission frequency, beam wave front and directivity. Another approach is based on the THz electroluminescence of shallow impurities such as oxygen and silicon in the standard AlGaN/GaN high electron mobility transistor (HEMT) structures. The surface plasmonphonon polaritons (SPPhP) in n-GaN grating are also considered for the development of electrically-pumped THz sources under thermal and electrical excitation of directive (coherent) radiation. We note that emission frequency can be governed either by gate voltage (2D plasmons) or structural design (SPPhP). All discussed methods are compared in terms of achieved quality factor, operating temperature and emitted power.

Quantum well intersubband polaritons are traditionally studied in large scale systems, over many wavelengths in size. In this presentation, we demonstrate that it is possible to detect and investigate intersubband polaritons in a single subwavelength nanoantenna in the IR frequency range. We observe polariton formation using a scattering-type near-field microscope and nano-FTIR spectroscopy. We will discuss near-field spectroscopic signatures of plasmonic antennae with and without coupling to the intersubband transition in quantum wells located underneath the antenna. Evanescent field amplitude spectra recorded on the antenna surface show a mode anti-crossing behavior in the strong coupling case. We also observe a corresponding strong-coupling signature in the phase of the detected field. We anticipate that this near-field approach will enable explorations of strong and ultrastrong light-matter coupling in the single nanoantenna regime, including investigations of the elusive effect of ISB polariton condensation.

In this work, we experimentally demonstrate a 3D printed compact THz spectral splitter based on a single planar broadband diffractive optical element, designed to disperse an incident collimated THz beam (0.5 THz to 0.7 THz) designed with the help of an inverse design technique namely gradient descent assisted binary search algorithm. Aside from the current challenges associated with 3D printing and better measurement facilities, preliminary experimental demonstration highlights the fact that these computationally optimized free space devices could be a significant step towards enabling new types of compact THz spectral splitters or even splitters operating beyond the THz regime.

Designing of diffractive optical elements (DOEs) requires knowledge about possible methods of calculating and simulating their performance, possible materials and characteristics of the particular range of radiation. The demand for compact and lightweight setups intuitively leads to the application of diffractive elements, which are characterised by both these features, having though one significant drawback – large chromatic aberration. As DOEs are meant to introduce specific phase shift, they are related to one particular design wavelength (DWL). However, thanks to different design approaches (e.g. kinoforms of higher order), elements functioning also for broader spectral ranges can be created. They are thicker, thus usage of appropriate material, having small attenuation coefficient or adjusting structure height during design process is required. Here, a simple method of designing diffractive lenses working in on- and off-axis regimes is presented. Using 3D printing for manufacturing is possible because different materials, polyamide, wax or chocolate, are relatively transparent below 0.5 THz. Each material has its own limitations like hardness, thermal resistance or ability for mechanical processing that have to be considered. Thus, using such simple methods of manufacturing for DOEs working for frequencies larger than 0.5 THz can be achieved using different design approach and ordinary devices with easily accessible materials (e.g. paraffin). It seems very important to create a method of producing diffractive elements that will be available in many laboratories to show the advantage of using such optical structures.

In THz-gated Scanning Tunneling Microscopy (THz-STM), the electric field of a single-cycle THz pulse acts as a transient bias modulating the STM-junction, enabling control of the tunneling current on femtosecond time scales. Optimal operation of a THz-STM requires exact knowledge and precise control of the THz near field waveform. In this regard, we demonstrate THz near field sampling via THz-induced modulation of ultrafast photocurrents in a metal-metal junction, and characterize in detail the coupling of broadband (1-30 THz) single-cycle THz pulses generated from a spintronic emitter to the STM tip. Specifically, we show that employing NIR laser pulses with a curved wavefront for THz generation allows for precise control of the phase, amplitude and bandwidth of the THz near field. Depending on the excitation conditions, THz near fields with frequencies up to 10 THz and peak voltages of several volts can be achieved at 1 MHz repetition rate.

Terahertz (THz) spectroscopy and imaging allow to differentiate between healthy tissues and tumors of different types, due to keen sensitivity to the water content. However, a high absorption of THz waves by water molecules limits application of THz technology for medical diagnosis. Among the existing methods for tissue clearing only immersive optical clearing (IOC) is acceptable for in vivo study. In order to choose optimal agents for IOC, we reconstructed THz dielectric properties of common agents using methods of THz pulsed spectroscopy and we measured diffusion coefficients of agents in rat brain analyzing collimated transmittance in visible range.

We have been developing an injection-seeded terahertz (THz) parametric generator (is-TPG), detector, and amplifier. This THz generation and detection scheme have allowed us to develop a wide-dynamic range THz wave spectrometer which can be used for nondestructive inspection of illicit drugs under thick envelopes. In this work, we have improved the sensitivity of the THz parametric detector drastically using a multistage configuration, which consists of the up-conversion (pre-amplifier) and the main amplifier parts. Using this new configuration, we were able to suppress the broadband noise, and only an upconverted detection beam can be measured using the NIR detector. As a result of this improvement, the minimum sensitivity reached 130 zJ (zJ=10-21J), which equals to about 90 photons at 1.05 THz. Moreover, the principle of this multistage detection can be used for high gain THz amplification which has more than 6 billion-fold amplification factor.

If a two-level atomic system characterized by inversion symmetry in space is coupled to a classical electromagnetic field, it undergoes Rabi oscillations where the population flips between the ground and excited levels. We investigate a scenario, where a system of broken inversion symmetry is used instead. In such case the dynamics is modified for the following reason: Eigenstates of an asymmetric system can be characterized with a permanent electric dipole moment originating from the polarization of charges, which plays a significant role of an additional source of dipole radiation. Its frequency corresponds to the Rabi frequency of population transfer between the eigenstates and therefore is optically tunable with the intensity of the driving field. We construct the numerical solver to simulate the medium of such asymmetric systems driven by the laser beam and by the Bloch-Maxwell equations calculate the generated radiation's propagation.

Intense Terahertz (THz) pulses offer a unique way to manipulate matter on a sub-picosecond timescale. Their strong electric and magnetic field components efficiently couple to low-energy degrees of freedom like optical phonons or magnons while we can simultaneously use ultrafast probe techniques to monitor changes in the sample with femtosecond resolution. This allows us to study novel nonequilibrium phases and the response of matter to strong transient fields. I will give a brief overview over THz experiments at the LCLS and how we can probe changes in properties like ferroelectricity, magnetism or orbital order with femtosecond x-ray pulses. I will specifically focus on nonlinear phonon dynamics and discuss recent results on SrTiO3 where we observed strong nonlinearity of the soft phonon mode and the transfer of energy to higher frequency vibrational modes.

In this contribution, we investigate the features of the two-beam interference of a set of wave trains, in which one of the beams contains an orbital angular momentum caused by the presence of inline topologically charged vortices in each spectral component. The infrared pump beam that generates terahertz radiation is first doubled in Mach-Zehnder interferometer with a specific time delay value to form the wave train with quasidiscrete temporal spectrum. Then, the two delayed terahertz pulses are split into two arms. One of the arms contains a delay line and geometric phase shaping elements for broadband uniformly topologically charged beam formation. The resulting structures are combined with a beamsplitter and detected with a terahertz holographic system upon their propagation. We analyze the features of the resulting spatio-frequency structures and discuss the possibility to implement the information encoding without spectral decomposition.

Electro-optic sampling (EOS) has established itself as one of the main techniques for coherent THz spectroscopy. In recent years, EOS has been pushed towards recording infrared or even optical waveforms, opening up new possibilities for spectroscopy in these spectral regions. In this contribution, we demonstrate the potential of electric-field-resolved detection via EOS for broadband, mid-infrared molecular vibrational spectroscopy. We show that field-resolved spectroscopy can achieve detection sensitivities orders of magnitude higher than state-of-the art Fourier-transform infrared spectroscopy. This is achieved by high-quantum-efficiency non-linear temporal “piercing” and, therefore, isolation of the molecular signal from the orders-of-magnitude stronger impulsive excitation. Thereby limitations due to detector dynamic range and source excitation noise can be avoided. This promises a new level of molecular sensitivity and molecular coverage for probing complex biological samples.

THz spectrometers in the frequency range from 120 GHz to 9.3 THz could be implemented using single Si MOSFETs, AlGaN/GaN HEMTs, AlGaAs/InGaAs HEMTs, and p-diamond FETs. The spectrometer detects the rectified voltage between the source and drain that is proportional to the sine of the phase shift between the voltages induced by the THz signal between gate-to-drain and gate-to-source terminals. This phase difference could be created by using different antennas for the source-to-gate and drain-to gate contacts or by using a delay line introducing a phase shift. The FET with the gate lengths from 20 nm to 130 nm could operate at room temperature as with different frequency ranges requiring different features sizes. The spectrometers are simulated using the multi segment unified charge control model implemented in SPICE and ADS.

Biological processes often take place at surfaces of proteins, where the dynamic and structural properties of aqueous solvents are modified. Information about solvent properties including hydration dynamics and structure, and protein collective motions can be obtained by measuring directly the dielectric response in the megahertz to terahertz frequencies of aqueous protein solutions. Due to the strong absorption of water in this frequency range, the experiment is challenging. Our home built dielectric spectrometer using a vector network analyzer together with frequency extenders allows us to perform the experiment in a wide range of frequency from megahertz to terahertz with a high dynamical range up to 120 dB. A detailed investigation of the dielectric response has revealed the hydration structure including the tightly, loosely bound layers and the number of water molecules in each hydration layer. These water molecules relax with different time constants at different temperatures. As a result, the dynamics of hydrated protein is also probed at different temperatures. Understanding the hydration structure and dynamics of lysozyme in biological conditions can explain the enzymatic activities of biomolecules.

Strong light-matter coupling in the terahertz (THz) region has strong potential for a new generation of THz devices based on polariton devices. In this work we investigate intersubband polaritons by time domain spectroscopy and characterize the coupled light-matter states at low k-vector for potential THz laser structures. A range of THz devices are studied to control the light-matter interaction and compared with the simulated polariton dispersion. The potential and investigations of THz stimulated emission via optical pumping at the so called “magic angle” for conservation of momentum and energy are presented.

In this work we focus separately on the fundamental interactions of intense single-cycle THz magnetic and electric fields with molecular liquids. We demonstrate that intense THz magnetic fields, from table-top sources, are able to induce Faraday rotation in liquids in ultrafast timescales. Analogously to the electric Hall effect in conducting materials, this observation is explained in terms of a transient molecular Hall effect, due to an instantaneous optically induced polarization in the presence of the THz magnetic field and opens a new avenue for successful disentanglement of electronic versus nuclear dynamics in complex molecular liquids. Moreover, we show that by tuning the frequency and phase of the electric field of intense THz pulses we can achieve different molecular rotational distributions in liquids, paving the way not only for better fundamental understanding of the intermolecular interactions, but also for achieving THz coherent control of chemical reactions.

It is well known that one of the major bottlenecks of MMW and Terahertz system technology is the lack of inexpensive but sensitive and fast detectors. One way of overcoming this problem is the use of commercial neon indicator plasma lamps or glow discharge detectors (GDD) as MMW and THz detectors. Prices are on the order of about half a dollar per lamp. In electronic mode of detection, the incident MMW or THz wave electric field gives rise to an increase in free electron energy above that provided by DC bias, and thus increases ionization collision rate with neutral Ne atoms and increases current. NEP on the order of 10^-10 W/sqrt Hz is easily obtainable, with rise time on the order of a microsecond. Noise is characterized by electron temperature. However, recently we have reported an optical means of detection which is manifested as an increase in light intensity emitted by the lamps caused by the incident MMW/THz wave. The detected signal is thus upconverted to the visible, and is measured using optical detectors such as avalanche photodiodes or CCD or CMOS cameras focused on the GDD to measure change in emitted light intensity. This gives rise to measured improvement in NEP by about 2 orders of magnitude and in speed by at least 3 orders of magnitude. The detection stems from increase in free electron recombination rate with positive Ne ions, especially in the cathode region, according to the increase in ionization collisions caused by the MMW/THz electric field. There, positive ions bombard the cathode and cause secondary emission of free electrons. There are thus high concentrations of positive Ne ions and free electrons in the cathode vicinity, giving rise to recombination rate increase and, thus, light intensity increase. The recombination rate increase there derives primarily from the role of positive Ne ions which also govern the secondary electron emission rate from the cathode. Consequently, noise temperature in upconversion derives primarily from the positive ion temperature, which is several orders of magnitude less than that of free electrons because of the much heavier mass of the positive ions, thus reducing NEP. Response speed in electronic detection is limited by parasitic capacitance and inductance deriving from the electrodes. However, in optical detection or upconversion, we do not use detection current in the GDD. The input is the MMW/THz wave. The output is the visible wave deriving from the change in GDD emitted light intensity. Unlike detection current, both waves propagate at the speed of light. Indeed, we report detected modulation frequencies on the order of 3 GHZ for 100 GHz wireless communication. This is limited not by the GDD but rather by external components such as optical detectors and amplifiers. We report also imaging experiment results with GDD focal plane arrays of 16x16 detectors and various techniques for increasing the number of focal plane array detectors. Super resolution techniques permit surpassing the diffraction resolution limit.

Topological materials often require detailed understanding of the band structure at very low energies. To get reliable access to a milli-electron-volt scale, we employ a combination of Landau level spectroscopy, infrared measurements at zero magnetic field, together with effective Hamiltonian models. In my talk, I will illustrate our combined approach on several Dirac and Weyl semimetals.

The field effect transistor structures with periodically changing width support excitations of the electron density (plasma waves) in the gated channels. We present the variable width design of such structures that support the plasma wave instabilities enabling terahertz (THz) emission and amplification. This design includes narrow protruding gated regions – plasmonic stubs - that enable the energy pumping of the plasma oscillations. Our calculations show that efficient THz radiation could be achieved in a wide sub-THz and THz band using feature sizes from 22 nm to 130 nm and materials such as Si, GaN, InGaAs, and p-diamond.

Fundamental optoelectronic excitations in two-dimensional van der Waals materials are determined by strong Coulomb interaction. The resulting exciton complexes govern the response of the materials to external fields and offer a flexible platform to study many-body interactions in solids. Both optical and terahertz spectroscopy offer direct approaches to investigate their fundamental properties and monitor their dynamics. Here, I will address the topics of exciton dynamics probed by optical and THz spectroscopy and focus on environmental energy fluctuations and exciton propagation in monolayer semiconductors. I will present an alternative, fundamental source of disorder based entirely on the local changes of the Coulomb interaction with major consequences for optics and transport. Time-resolved microscopy measurements of exciton propagation will be presented for both linear and non-linear regime with an outlook towards opportunities for time-resolved terahertz spectroscopy.

In this contribution, we present the direct comparison between Ophir Pyrocam IV and Terasense Tera-1024 cameras used for imaging of terahertz (THz) and sub-THz signals. We compare general properties, such as frequency dependent and polarisation dependent sensitivity, angle dependent sensitivity essential for holographic and noncollinear interferometric measurements, and draw a conclusion about the most suitable camera for the discussed imaging approaches. Both cameras show acceptable performance and sensitivity at imaging both 0.14 THz and 0.3 THz signals. The Terasense camera, expectedly, shows stronger polarisation dependent properties, however, is significantly more angle independent, showing an acceptable performance at all tested incident angles up to 50 degrees. At the same time, although the angle dependence is stronger for the Ophir camera, it has smaller pixel pitch and more extended post-processing features, thus making it somewhat better suited for noncollinear interferometric and holographic sub THz imaging.

Germanium is a nonpolar semiconductor with missing Reststrahlen band. In spite of other promising properties including low bandgap and small effective mass, its long, µs-scale recombination time has been prohibitive for applications as photoconductive THz emitters. Using Au-implantated Ge, with recombination times reduced to sub-ns values, we have demonstrated a broadband photoconductive THz emitter compatible with modelocked fibre lasers operating at wavelengths of 1.1 and 1.55 µm and with pulse repetition rates of 78 MHz. Reaching up to 70 THz bandwidth, this approach points towards the possibility of compact, high-bandwidth THz photonic devices compatible with Si CMOS technology.

Optically-driven photoconductive switches are one of the predominant sources currently used in terahertz imaging systems. However, owing to their low average powers, only raster-based images can be taken, resulting in slow acquisition times. In this work, we show that placing a photoconductive switch within a cavity, we are able to generate absolute THz powers of 181 µW. The cavity is based on a metal-insulator-metal structure that permits an enhancement of the average power by almost one order of magnitude whilst conserving a broadband response. We demonstrate real-time imaging using this source, with the broadband spectrum permitting to eliminate diffraction artefacts.

We report on a frequency-tunable plasmonic structure for multi-frequency terahertz (THz) analysis on a sub-wavelength scale. In spite of much interest in plasmon-based THz technologies, a main shortcoming of conventional devices is that they exhibit poor frequency-tunability in fixed structures. We here demonstrated that the frequency of THz plasmonic resonance is continuously tunable according to the angle of polarization of the incident THz wave, which enhanced the flexibility in near-field THz applications. Biological and medical applications with this method are presented.

We have developed a theory of the helicity-driven plasmonic dc response of the gated two-dimensional electron gas to the terahertz (THz) radiation. It was demonstrated that the phase difference between THz signals coupled to source and drain of the field effect transistor (TeraFET) induces a plasmon-assisted direct current (dc), which is dramatically enhanced in the vicinity of plasmonic resonances. We have also proposed an appropriate scheme for a phase-sensitive homodyne detector operating in this phase-asymmetry regime. As a key result, helicity and phase-sensitive conversion of circular polarized radiation into dc photovoltage induced by the plasmon- interference mechanism was observed: two plasma waves, excited at the source and the drain part of the transistor interfere inside of the channel. The helicity sensitive phase shift between these waves can be achieved by using an asymmetric antenna configuration. Suggested plasmonic detector is capable of measuring the phase difference between two arbitrary phase-shifted optical signals.

In this article, we studied new porous material, which is based on artificial opals, made of 300-nm-diameter nanoporous SiO₂ globules and annealed at different temperatures in the range of 200–1500^◦C, as a prospective terahertz (THz) optical material. It was demonstrated experimentally that the THz optical properties of such material can be varied in a wide range (e.g. its refractive index varies from 1.65 to 1.95) by annealing, being a function of the total material porosity. Additionally, this material has rather low THz absorption coefficient (by field), which decreases from 10 to 1 cm⁻¹ with increasing annealing temperature. Based on the Bruggeman effective medium theory, the practical model was introduced to predict the optical properties of the considered material as a function of the annealing temperature. A wide tunability of refractive index and a low-to-moderate THz-wave absorption, make the discussed nanoporous SiO₂ a promising THz optical material.