This PDF file contains the front matter associated with SPIE Proceedings Volume 10756 including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Understanding how spins move at pico- and femtosecond time scales is the focus of much of contemporary research in magnetism. I will go through some basic and more advanced concepts in the exciting emerging field of terahertz (THz) magnetism, where electromagnetic radiation in the 0.1-10 THz range, the so-called THz gap, is used to probe or to control spin dynamics at these time scales. I will give an overview of the current research in THz magnetism. As illustrating examples, I will briefly discuss how low-intensity THz radiation can be used to probe the fundamentals of spin- dependent transport in the linear regime [1]. I will then describe how intense THz fields can be used to drive coherent and incoherent ultrafast spin dynamics in nonlinear regimes, both with broadband [2] and narrowband radiation [3]. Finally, I will show some recent implementation of metamaterials [4] aimed at selectively enhancing the terahertz magnetic field in the near-field [5]. I will also illustrate the design of an anti-reflection coating that allows for table-top, femtosecond pump-probe experiments in generic nanostructures surrounded by highly reflective metamaterials [6]. [1] Z. Jin et al., Nature Physics 11, 761 (2015) [2] S. Bonetti et al, Physical Review Letters 117, 087205 (2016) [3] Z. Wang et al., Selective THz control of magnetic order: new opportunities from superradiant undulator sources, Journal of Physics D: Applied Physics, in press (2018) [4] Hou-Tong Chen et al., Terahertz Science and Technology 1, 42 (2008) [5] D. Polley, et al. Journal of Physics D: Applied Physics 51, 084001 (2018) [6] M. Pancaldi et al. Optics Letters 26, 2917 (2018)

TELBE is the first operational super-radiant quasi-cw SRF accelerator-based user facility for selective THz control experiments. Ultrafast dynamics, selectively driven by tunable narrow-band THz pulses, can be studied at unprecedented repetition rates. Research opportunities of this new type of photon facility are discussed based on early-stage experiments at TELBE.

The Smith-Purcell radiation occurs when electrons move above a metallic diffraction grating. Its mechanism can be explained by the radiation of a moving oscillating dipole. Based on this, increasing the number of the dipole oscillations will be an effective way to enhance Smith-Purcell radiation. Here, we use a rectangular metal above the grating ridge. When electrons pass through the gap between the ridge and rectangular metal, there are two image charges arising. The theory analysis and the numerical calculation results well agree with the computer simulation results. The enhancement has promising prospect in developing high-power and efficient THz sources.

As continuous wave (CW) terahertz (THz) sources, the differential-frequency-mixing (DFM) has an advantage for the frequency tunability by changing the energy separation of the two lasers. In particular, considering the inhomogeneous width in the quantum confinement systems, use of the exciton lines enables wide frequency tuning. The THz sources with the narrow bandwidth and wide frequency tunability will be applied to the high resolution THz spectroscopy. Recently, we realized the CW-THz wave generation by DFM under the exciton excitation conditions in a GaAs/AlAs multiple quantum well (MQW), which shows the wide frequency tuning range over 18 THz. Therefore, in this work, we report the polarization characteristics of a continuous THz electromagnetic wave generated by DFM due to excitation of two exciton states in the GaAs/AlAs multiple quantum well. As a sample, we used an undoped GaAs/AlAs MQW embedded in a p-i-n structure on a (001) n+-GaAs substrate. The thickness of GaAs and AlAs layer is 7.5 nm. The measurements of the THz wave were carried out at 296 K. As the laser sources, a semiconductor laser and a CW-mode Ti:sapphire laser to change the excitation energy were used. The two beams were focused on the sample surface. Comparing the polarization of the laser beams with that of the THz wave, the conversion process from the laser lights to the THz wave via the exciton states, such as the heavy hole and light hole excitons split by quantum confined effects, will be demonstrated.

Terahertz conductivity spectra of degenerate electron gas confined in various model 2D nanostructures were calculated using semi-classical Monte-Carlo calculations based on Kubo formula. We show theoretically that the conductivity of high-quality nanostructures containing a degenerate electron gas may exhibit pronounced spectral structure. This is in contrast with general featureless character of experimental terahertz conductivity spectra measured so far in most semiconductor nanostructures..

We use a Monte-Carlo model to simulate semi-classical photo-carrier dynamics on bulk InAs, InGaAs and GaAs that leads to terahertz emission after ultrafast photoexcitation. This detailed model has allowed us to understand various aspects of the THz emission process, including the near-field distribution which has been experimentally observed, the role of the excess excitation photon energy, and the relative importance of the surface field driven, diffusive (photo-Dember) and ballistic currents. In order to understand the near-field emission we coupled a finite-difference time-domain routine to the carrier dynamics simulation, by doing this, we were able to analyse the near terahertz field emission caused by the motion of such carriers even when the excitation is performed at normal incidence. We found that both the current parallel, which has traditionally been assumed not to take part in the emission, and normal to the interface take a relevant role in the terahertz generation. We performed another set of simulations for different bandgaps and excitation-photon energies in order to compare the emission power of all three semiconductors as function of excitation photon energy finding that the carrier excess excitation energy is more relevant to explain their performance difference than their motilities. We conclude that ballistic transport after photoexcitation is the dominant mechanism for terahertz emission instead of diffusion driven or surface field driven charge separation, which were traditionally considered the most relevant mechanisms.

Two-dimensional metamaterials - metasurfaces - offer tremendous opportunities in realizing exotic optical phenomena and functionalities to address the technological challenges encountered in the terahertz frequency regime. By tailoring the resonant response of basic building blocks as well as their mutual interactions, we are able to effectively control of amplitude, phase, and polarization state of terahertz waves. Here we report the realization of highly efficient polarization conversions including: (1) Reflective linear polarization rotation using an array of anisotropic resonators backed with a ground plane; (2) Transmissive linear polarization rotation using an array of anisotropic resonator array sandwiched by two orthogonal gratings; and (3) Reflective linear-to-circular polarization conversion using two cascading arrays of complementary resonators. They operate over a broad bandwidth more than one octave and approaching two octaves in some cases. We further show that the linear polarization rotation is accompanied by a tunable phase discontinuity, which allows us to demonstrate an ultrathin terahertz flat lens enabling diffraction-limited focusing. The broadband linear-to-circular polarization may also find applications including terahertz circular dichroism spectroscopy and excitation of valley polarization in 2D materials.

We present recent progress in understanding nonlinear phonon dynamics driven by intense THz field strengths using ultrafast x-ray diffraction. This technique allows us to characterize the motion of the atomic lattice inside the sample on a sub-cycle timescale while the driving field is still present. In the example of strontium titanate (SrTiO₃) we observe harmonic generation and phonon upconversion when the fundamental soft mode is driven into the nonlinear regime by a single-cycle THz pulse.

We present the recent development of high performance compact frequency comb sources based on mid-infrared quantum cascade lasers. Significant performance improvements of our frequency combs with respect to the continuous wave power output, spectral bandwidth, and beatnote linewidth are achieved by systematic optimization of the device's active region, group velocity dispersion, and waveguide design. To date, we have demonstrated the most efficient, high power frequency comb operation from a free-running room temperature continuous wave (RT CW) dispersion engineered QCL atλ ~5-9μm. In terms of bandwidth, the comb covered a broad spectral range of 120 cm^-1 with a radio-frequency intermode beatnote spectral linewidth of 40 Hz and a total power output of 880 mW at 8 μm and 1 W at λ~5.0μm. The developing characteristics show the potential for fast detection of various gas molecules. Furthermore, THz comb sources based on difference frequency generation in a mid-IR QCL combs could be potentially developed.

The application of transient terahertz (THz) pulses to excite and probe low-energy quantum and collective excitations in materials represents a powerful tool to study both intrinsic interactions and non-equilibrium phases. In the following, we discuss ultrafast multi-THz studies that resolve the dynamics of electronic itineracy and vibrational symmetries in a strongly-correlated nickelate. Many transition-metal oxides exhibit the emergence of “stripes,” corresponding to quasione- dimensional charge, spin and lattice modulations as a manifestation of strong correlations. In our experiments, optical excitation of a stripe-phase nickel oxide triggers the rapid melting of its atomic-scale charge order and results in dynamics that yields insight into the couplings underlying the stripes. The transient optical conductivity is sensitive to both charges and in-plane vibrations and reveals a succession of ultrafast processes, ranging from rapid delocalization and localization of charges, via a time-delayed reaction of vibrational distortions to the electronic quench, up to the multi-picosecond re-establishment of the symmetry-broken phase.

The anomalous velocity causes spin-polarized carriers to move perpendicular to an electric or parallel to a magnetic driving field. It is at the origin of the spin Hall and anomalous Hall effects. Here, we employ time-domain THz spectroscopy to study the anomalous velocity and the anomalous Hall effect, both of which are optically induced in GaAs, on a sub-picosecond time scale. Our results not only enable a distinction between intrinsic (Berry curvature) and extrinsic (scattering) effects but also demonstrate an inversion of anomalous Hall currents versus excitation photon energy.

A hetero-barrier rectifier, Fermi-level managed barrier (FMB) diode, was developed for broadband and low-noise THzwave detection at room temperature. The barrier height at the InP/InGaAs interface was controlled by the doping in an n-type InGaAs anode layer so that a very low height barrier could be attained for obtaining a small intrinsic differential resistance and a large output current density under a zero-biased condition. The fabricated module integrating a pre-amplifier could detect signals in a wide frequency range from 160 GHz to 1.4 THz with a very low noise equivalent power (NEP) of 3 × 10^-12 W/ √Hz at 300 GHz in the square-law detection mode. The NEP in the homodyne detection mode was even lower at 1.6 ×10^-17 W/Hz with a local oscillator power of only 5 ×10^-7 W at 300 GHz. A linear detector array consisting of 100 zero-biased FMB diodes was also developed for the THz imaging. A short-time imaging at 315 GHz was accomplished with a near-diffraction-limited resolution of about 0.7 mm.

I introduce recent studies on evaluation of interface and surface of semiconductor materials and devices with laser-induced terahertz emission spectroscopy and imaging that measure and visualize THz emissions from the materials and devices excited by femtosecond laser pulses. The waveforms of lase-induced THz emissions refrect the dinamics of photoexcited carriers at the area excited by the laser pulses, therefore we can extract various physical properties of the samples using this phenomena in principle. In this study, we have applied this technique to characterize local properties of semiconductor materials and devices such as solar cells, wide-gap semiconductors and Metal-Oxide-Semiconductor (MOS) devices. As a result, we demonstrated that it was possible to evaluate electric polarization, surface potentials, defects, damage, performance deterioration, which were difficult with conventional methods.

Spectra of complex dielectric constant Terahertz (THz) and sub-THz range, corresponding to picosecond dynamics of the hydrogen bond network, gives us information of water molecule dynamics: rotation mode and vibration of bulk water. HeLa cell cultured on a silicon prism was measured and then analyzed by two reflection interface model and Debye- Lorentz function. Finally fraction of hydration and dynamics of bulk water molecules were estimated.

Near field sub-THz array sensor in CMOS circuit for cell detection was developed as a tool to collect quantitative cellular property. More than 1400 number of LC oscillators designed at 60 GHz are arranged in 2 dimensions periodically, and resonance frequency of each oscillator shifts by dielectric target. In simulation and experiment, frequency shifts of the CMOS sensor agreed well with dielectric constant change of water and solution on the surface. Growth of bacterial colonies was monitored successfully. Moreover, significant difference of frequency shifts was demonstrated among NHDF and HeLa, normal and cancer human cell respectively, and liquid medium.

By using both linear and nonlinear terahertz spectroscopy on epitaxial Bi and Bi_1-xSb_x thin films, we systematically investigated the linear and nonlinear terahertz dynamics of Dirac electrons. The linear terahertz transmittance was analyzed by the Drude model up to 50 THz, and then the plasma frequency and the damping constant were evaluated as functions of the film thickness and Sb-concentration. We found surface metallic state for Bi ultra-thin films, while semimetal to semiconductor crossover for Bi_1-xSb_x thin films. In the nonlinear terahertz spectroscopy, the terahertz transmittance increases with increasing the field strength, which could be assigned to the carrier acceleration along the Dirac-like band dispersion at the L point in the Brillouin zone. In addition, we observed the terahertz-induced absorption in terahertz-pump and terahertz-probe spectroscopy, which could be assigned to carrier generation due to Zener tunneling in Dirac band structure. The results demonstrate that Bi-related materials are promising candidates for future nonlinear terahertz devices.

Terahertz wave generation from laser induced air plasma is widely used due to its high electric field and broad frequency bandwidth. For further understanding of the mechanism of the terahertz wave generation in laser-induced plasma as well as the terahertz modulator base on pre-formed air plasma, the generation of terahertz radiation using an effective wavelength scaling mechanism is examined when two-color laser fields are mixed in pre-formed plasma created by synchronized 800nm laser pulse. In our experiment, the effect of preformed plasma is investigated using an orthogonal pumping geometry. With a preformed plasma, both the modulation depth of terahertz radiation energy and the change of terahertz radiation polarization increases with increasing excitation laser wavelength. We found that the terahertz modulation depth and terahertz polarization changes increase as a function of the energy of the 800nm-prepulse. Some possible reasons are discussed. We attribute the terahertz polarization rotation to additional relative phase of the two-color fields introduced by the preformed air plasma. This provides a practical way to control the polarization and energy of terahertz pulses for potential applications.

Terahertz modulators based on frequency-tunable metamaterials are designed, produced and investigated. It is shown that optimization of the substrate thickness allows to increase the modulation depth and reduce the insertion loss of the modulator. Enhancement mechanism of higher-order resonant modes in densely packed resonator arrays is suggested. It is demonstrated that the resonant frequency of the fifth order plasmonic mode can be effectively tuned which makes the device attractive for practical applications.

Group-IV monochalcogenides belong to a family of 2D layered materials. Monolayers of group-IV monochalcogenides GeS, GeSe, SnS and SnSe have been theoretically predicted to exhibit a large shift current owing to a spontaneous electric polarization at room temperature. Using THz emission spectroscopy, we find that above band gap photoexcitation with ultrashort laser pulses results in emission of nearly single-cycle THz pulses due to a surface shift current in multi-layer, sub-μm to few- μm thick GeS and GeSe, as inversion symmetry breaking at the crystal surface enables THz emission by the shift current. Experimental demonstration of THz emission by the surface shift current puts this layered group-IV monochalcogenides forward as a candidate for next generation shift current photovoltaics, nonlinear photonic devices and THz sources.

Hybrid metamaterial/graphene amplitude and frequency modulators have been implemented as external optoelectronic mirrors in external cavity configurations with terahertz quantum cascade lasers (QCLs). These devices’ tunability is accomplished via the interplay between metamaterial resonant units, normally engineered in mm-size arrays, and graphene. The integration of these devices in external cavity QCLs offers unique emission features and realizes an unprecedented studied regime. The implementation of an external amplitude modulation allows the full switching of laser emission in single mode operation by electrostatically gating graphene. The introduction of more dispersive tunable architectures in frequency modulators yields additionally an all-electronic spectral laser bistability.

Electromagnetic metamaterials are typically comprised of subwavelength metal or dielectric resonators that, when fashioned as two or three-dimensional composites, result in novel optical and photonic functionalities. Importantly, the enhanced local electric and magnetic fields of these resonators are accessible leading to strong interactions upon integration with quantum materials. Ultimately, we seek to create emergent photonic composites where the whole is more than the sum of the parts. The possibilities are nearly endless with a host of quantum materials ranging from semiconductors to transition metal oxides to superconductors offering unique possibilities. This is especially true at terahertz frequencies where the electrodynamic response of quantum materials often manifest in dramatic fashion. In this talk, we will focus on terahertz quantum metamaterials (TQMs) highlighting recent examples and emphasizing that TQMs offer a two-way street to both create technologically relevant composites and to investigate fundamental condensed matter physics under extreme conditions.

In this work, the development of a technique for the early diagnosis of diabetic foot using terahertz spectroscopic images is presented. The degree of hydration of the skin on the sole of the foot of diabetic and non-diabetic subjects was obtained and related to the degree of deterioration. The hydration information was coded in three- color (red, yellow and greed) images which allow to easily identify areas in risk of ulceration. The hydration images together with the three-color images represent a quantitative indicator of the deterioration caused by the diabetic foot syndrome.