This PDF file contains the front matter associated with SPIE Proceedings Volume 10383, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

The progress in microfabrication techniques and three dimensional electromagnetic simulations have enabled the fabrication of vacuum electron devices up to 1 THz. In particular, the backward wave oscillator is a compact and powerful THz vacuum source, based on the transfer of energy from an electron beam to an electromagnetic (EM) wave propagating in a slow wave structure. The paper reports the design and fabrication challenges to realize a near-THz Backward Wave Oscillator for plasma diagnostics in nuclear fusion. In particular, the beam optics and confinement as well as the slow wave structure are described. Manufacturing as well as device implementation and demonstration considerations are discussed. The double corrugated waveguide is used with a cylindrical electron beam producing an output power on the order of 1 W.

This paper reviews the recent advances and future trends of terahertz (THz) emitters at CQD/NU, highlights the high-performance THz sources based on intracavity nonlinear frequency generation in mid-infrared quantum cascade lasers. Significant performance improvements of our THz sources in the power, wall plug efficiency are achieved by systematic optimizing the device's active region, waveguide, and chip bonding strategy. High THz power up to 1.9 mW and 0.014 mW for pulsed mode and continuous wave operations at room temperature are demonstrated, respectively. Even higher power and efficiency are envisioned based on enhancements in outcoupling efficiency and mid-IR performance. Our compact THz device with high power and wide tuning range is highly suitable for the imaging, sensing, spectroscopy, medical diagnosis, and many other applications.

Achieving high power in combination with high quality beam pattern is a ubiquitous challenge for semiconductor lasers. The demonstration of vertical-external-cavity surface-emitting lasers (VECSELs) in 1997 for visible and near-infrared semiconductor lasers has been a very successful approach. Terahertz (THz) quantum-cascade (QC) lasers, also have the challenge of combining high power and good beam pattern into one device – even more so because they typically use sub-wavelength metallic waveguides. The concept of VECSEL has been impossible to implement for QC lasers, since the optical gain is based on intersubband transitions of electrons, which only interact with the electric field polarized perpendicular to the quantum wells plane according to the "intersubband selection rule". To address this issue, we have developed an amplifying metasurface reflector that can couple the incident THz wave with the QC gain medium via metal-metal micro-cavity antenna reflectarray. Pairing the active metasurface with an output coupler, we demonstrated the first VECSEL in the THz regime in 2015. Based upon the prototype design, we have achieved a number of improvements to the QC-VECSEL including designing an inhomogeneous focusing metasurface to achieve a near-diffraction limited beam pattern with M2 = 1.3 and high brightness of 1.86×106 Wsr-1m-2, designing compact cavities and optimizing metasurface bias area to achieve continuous-wave operation above 77 K, achieving record high slope efficiency of 745 mW/A, as well as extending the VECSEL concept to cover a broad frequency range from 2.5 - 4.4 THz.

We demonstrate that an application of a III-V-on-silicon hybrid concept to terahertz (THz) Cherenkov difference frequency generation (DFG) quantum cascade laser (QCL) sources (THz DFG-QCLs) can dramatically improve THz output power and mid-infrared-to-THz conversion efficiency. Completely processed THz DFG-QCLs grown on a InP substrate are transfer-printed onto a 1-mm-thick high-resistive Si substrate using a 100-nm-thick SU-8 as an adhesive layer. Room temperature device performance of the reference InP and hybrid Si THz DFG-QCLs of the same ridge width (22 µm) and cavity length (4.2 mm) have been experimentally compared. The target THz frequency of 3.5 THz is selected for both devices using the dual-period first order surface gratings to select the mid-infrared pump wavelength of 994 cm-1 and 1110 cm-1. At the maximum bias current, the reference InP and hybrid Si devices produced THz power of 50 µW and 270 µW, respectively. The mid-infrared-to-THz conversion efficiency corresponds to 60 µW/W2 and 480 µW/W2, respectively, resulting in 5 times higher THz power and 8 times higher conversion efficiency from the best-performing hybrid devices. A hybrid Si device integrated in a Littrow external-cavity setup showed wavelength tuning in the 1-6 THz range without THz beam steering.

In this presentation we will review the principles and applications of THz generation based on spintronic phenomena: coherent spinwave generation [1], Mott scattering [2], and inverse spin-Hall effect [3]. [1] Z. Jin, Z. Mics, G. Ma, Z. Cheng, M. Bonn, and D. Turchinovich, "Single-pulse terahertz coherent control of spin resonance in the canted antiferromagnet YFeO3, mediated by dielectric anisotropy", Phys. Rev. B 87, 094422 (2013) [2] Z. Jin, A. Tkach, F. Casper, V. Spetter, H. Grimm, A. Thomas, T. Kampfrath, M. Bonn, M. Kläui, and D. Turchinovich, "Accessing the fundamentals of magnetotransport in metals with terahertz probes", Nature Physics 11, 761 (2015) [3] T. Seifert, S. Jaiswal, U. Martens, J. Hannegan, L. Braun, P. Maldonado, F. Freimuth, A. Kronenberg, J. Henrizi, I. Radu, F. Freimuth, E. Beaurepaire, Y. Mokrousov, P.M. Oppeneer, M. Jourdan, G. Jakob, D. Turchinovich, L. M. Hayden, M. Wolf, M. Münzenberg, M. Kläui, and T. Kampfrath, "Efficient metallic spintronic emitters of ultrabroadband terahertz radiation", Nature Photonics 10, 483 (2016)

Graphene is considered to be a promising candidate to replace indium tin oxide (ITO) as transparent conductive electrodes in optoelectronics applications. We use graphene films as transparent electrodes for THz applications such as phase shifters, reflector, and intensity modulators, leading to a low voltage operation and very small insertion loss. First, we experimentally demonstrate the excellent performance of THz modulators based on graphene/ionic liquid/graphene sandwich structures. The modulation covers a broadband frequency range from 0.1 to 2.5 THz with the modulation depth of up to 99% by applying a small gate voltage of 3 V. To our knowledge, this is the highest modulation ratio from graphene based THz devices to date. We also report a highly efficient tunable THz reflector in graphene. By applying a small gate voltage (up to ± 3 V), the reflectance of graphene is modulated from a minimum of 0.79% to a maximum of 33.4% using graphene/ionic liquid structures at room temperature, and the reflection tuning is uniform within a wide spectral range (0.1 – 1.5 THz). In addition, we design and fabricate terahertz phase shifters based on thin liquid crystal cells sandwiched by two graphene layers. A maximum 10.8 degree phase shift is obtained with 5 V voltage. The proposed phase shifters are fully electrical controllable, continuous tunable, and require very low DC voltages for operation. Finally, we show a high performance THz emitter based on ferromagnetic/nonmagnetic heterostructures. Our THz emitter based on nonmagnetic (NM) and ferromagnetic (FM) heterostructures has a peak intensity exceeding 500 μm thick ZnTe crystals (standard THz emitters). We have also fabricated the devices on flexible substrates with a great performance, and demonstrated that the devices can be driven by low power lasers.

A new scheme for generating high-energy terahertz (THz) pulses by optical rectification of tilted pulse front (TPF) femtosecond laser pulses in ZnTe crystal is proposed and analyzed. The TPF laser pulses are originated due to propagation through a multistep phase mask (MSPM) attached to the entrance surface of the nonlinear crystal. Similar to the case of contacting optical grating the necessity of the imaging optics is avoided. In addition, introduction of large amounts of angular dispersion is also eliminated. The operation principle is based on the fact that the MSPM splits a single input beam into many smaller time-delayed “beamlets”, which together form a discretely TPF in the nonlinear crystal. The dimensions of the mask’s steps required for high-energy THz-pulse generation in ZnTe and widely used lithium niobate (LN) crystals are calculated. The optimal number of steps is estimated taking into account individual beamlet’s spatial broadening and problems related to the mask fabrication. The THz field in no pump depletion approximation is analytically calculated using radiating antenna model. The analysis shows that application of ZnTe crystal allows obtaining higher THz-pulse energy than that of LN crystal, especially when long-wavelength pump sources are used. The proposed method is a promising way to develop high-energy, monolithic, and alignment-free THzpulse source.

HgCdTe compounds can be engineered to fabricate “gapped-at-will” structures. Therefore, 1D, 2D and even 3D massless particles can be observed in topological phase transitions driven by intrinsic and external physical parameters. In this work, we report on our experimental results, obtained by temperature-dependent Terahertz and Mid-Infrared magneto-spectroscopy, of topological phase transitions in HgCdTe-based quantum wells and bulk samples. These transitions are accompanied with the appearance of 2D and 3D massless particles called Dirac and Kane fermions, respectively.

Quantum cascade lasers (QCL) have become the most suitable laser sources from the mid-infrared to the THz range. This work examines the effects of external feedback in different high power mid infrared QCL structures and shows that different conditions of the feedback wave can produce complex dynamics hence stabilization, destabilization into strong mode-competition or undamping nonlinear oscillations. As a dynamical system, reinjection of light back into the cavity also can also provoke apparition of chaotic oscillations, which must be avoided for a stable operation both at mid-infrared and THz wavelengths.

Terahertz (THz) wave generation from laser induced air plasma is widely used due to its high electric field and broad frequency bandwidth. The most popular and efficient laser-plasma scheme used for THz generation is the two-color scheme, in which a laser pulse at the fundamental frequency is supplemented by its second harmonic that is obtained with the use of a nonlinear crystal. The type-I β-barium borate (BBO) crystal plays a very important role in second harmonic generation. In this research, we investigate the THz generation efficiency with changing the thicknesses of the BBO crystals. Moreover, the wavelength of the excitation laser is tunable from 1200 nm to 1600 nm. The THz generation efficiency is characterized by rotating the BBO crystal with the same tilt angle, changing laser wavelength with invariant pump power. And we also record the two orthogonal components of THz electric field by rotating the ZnTe crystal. We think that the thickness of BBO crystal affects the phase difference between the two components along the ordinary axis and extraordinary axis, resulting in the change of the polarization state of the fundamental wave. Meanwhile, the frequency doubling efficiency of BBO has an impact on the power ratio of the two-color laser. This provides a practical way to control the polarization of THz pulses for potential applications.

We discuss an effective method for the detection and identification of a substance, covered by a disordered structure, using the pulsed THz signal. The insufficiency of the standard THz-TDS method, based on the comparison of substance absorption spectra from database with the absorption spectrum of the substance under investigation, is demonstrated. To explain a physical mechanism of false absorption lines appearance in the signal we make a computer simulation on the base of 1D Maxwell's equations and density matrix formalism. For the detection and identification of substance we propose to utilize substance emission at high frequencies corresponding to the high energy levels excitation under the THz pulse action due to the cascade mechanism. In the case of a non-resonant medium we also discuss second harmonic generation and a possibility of its application to the substance detection and identification. The proposed method of the substance detection and identification is based on time-dependent integral correlation criteria calculated with the help of the spectral dynamics of medium response. A new type of the integral correlation criterion, which is less dependent on spectral characteristics of the noisy signal under investigation, is used for the identification.

Optical frequency combs are innovative tools for broadband spectroscopy because a series of comb modes can serve as frequency markers that are traceable to a microwave frequency standard. However, a mode distribution that is too discrete limits the spectral sampling interval to the mode frequency spacing even though individual mode linewidth is sufficiently narrow. Here, using a combination of a spectral interleaving and dual-comb spectroscopy in the terahertz (THz) region, we achieved a spectral sampling interval equal to the mode linewidth rather than the mode spacing. The spectrally interleaved THz comb was realized by sweeping the laser repetition frequency and interleaving additional frequency marks. In low-pressure gas spectroscopy, we achieved an improved spectral sampling density of 2.5 MHz and enhanced spectral accuracy of 8.39 × 10^-7 in the THz region. The proposed method is a powerful tool for simultaneously achieving high resolution, high accuracy, and broad spectral coverage in THz spectroscopy.

As applications such as heterodyne spectroscopy require only single mode operation, the selection, suppression and tuning of individual lasing modes in THz QCLs has received considerable attention over the last decade. By periodically patterning the QCL in one- or two dimensions (e.g. distributed feedback (DFB) or photonic crystal lasers), single mode emission can be enforced. An alternative approach which requires no modification of the QCL waveguide is based on injection seeding technique with tunable narrowband THz seeds. Using this technique, we will show how the same QCL can be operated in both multi-mode and single mode regimes. On the other hand, short pulses allow for time-resolved measurements and the generation of frequency combs. As the duration of a pulse is limited by its spectral bandwidth, a multimode operation of the QCL is highly desirable. By addition of a microwave modulation at the round-trip frequency, where the spacing and phase of the QCL modes is consequently fixed, results in active modelocking. This leads to laser emission of a train of THz pulses separated by the round-trip frequency. Coupled to coherent detection and a novel application of dispersion compensation, we demonstrate the generation of a stable 4 ps train pulse train. This opens up the possibility to reach sub-picosecond pulses and potentially the single cycle regime. To conclude, we will show two methods to control the THz QCL emission from single mode regimes to the generation of short THz pulses.

Terahertz(THz) wave modulator technology, due to its important value of imaging and detecting research. In the paper, we discuss the Polyethylene lens based the terahertz wave front modulation, which is benefit the terahertz wave image technology. Simulation results show that the optical system can extent the depth of imaging field of test objects based on continuous terahertz source. The way to get the image has significant meaning for detection and large image quality.

This paper reviews promising materials for compact sources and detectors for terahertz pulses via 1.55 μm photoconductive switches. We begin with a brief description of importance of effective terahertz generation and detection, then the mechanism of photoconductive switches will be introduced. Then, we provide a detailed discussion and comparison on a wide range of the state-of-the-art materials and their relevant properties for terahertz photoconductive switches including bandgap energy, dark resistance, carrier mobility, carrier lifetime, and etc.

We present a compact, all solid-state THz confocal microscope operating at 0.30 THz that achieves super-resolution by using the knife-edge scan approach. In the final reconstructed image, a lateral resolution of 60 μm ≈ λ/17 is demonstrated when the knife-edge is deep in the near-field of the sample surface. When the knife-edge is lifted up to λ/4 from the sample surface, a certain degree of super-resolution is maintained with a resolution of 0.4 mm, i.e. more than a factor 2 if compared to the diffraction-limited scheme. The present results open an interesting path towards super-resolved imaging with in-depth information that would be peculiar to THz microscopy systems.

Terahertz (THz) reflective imaging is applied to the stratigraphic and subsurface investigation of oil paintings, with a focus on the mid-20th century Italian painting, ‘After Fishing’, by Ausonio Tanda. THz frequency-wavelet domain deconvolution, which is an enhanced deconvolution technique combining frequency-domain filtering and stationary wavelet shrinkage, is utilized to resolve the optically thin paint layers or brush strokes. Based on the deconvolved terahertz data, the stratigraphy of the painting including the paint layers is reconstructed and subsurface features are clearly revealed. Specifically, THz C-scans and B-scans are analyzed based on different types of deconvolved signals to investigate the subsurface features of the painting, including the identification of regions with more than one paint layer, the refractive-index difference between paint layers, and the distribution of the paint-layer thickness. In addition, THz images are compared with X-ray images. The THz image of the thickness distribution of the paint exhibits a high degree of correlation with the X-ray transmission image, but THz images also reveal defects in the paperboard that cannot be identified in the X-ray image. Therefore, our results demonstrate that THz imaging can be considered as an effective tool for the stratigraphic and subsurface investigation of art paintings. They also open up the way for the use of non-ionizing THz imaging as a potential substitute for ionizing X-ray analysis in nondestructive evaluation of art paintings.

Convenience in use of room-temperature terahertz (THz) imaging systems, reduction of their dimensions and presence of on-chip solutions remains one of prime interests for direct implementation aims. Solid-state-based solutions in miniaturization of spectroscopic THz imaging systems including novel semiconductor nanostructures bias-free emitters, diffractive THz optics components and their on-chip integration with THz detectors are discussed. In particular, pulsed optoelectronic terahertz emitter based on a δ-doped p-i-n-i GaAs/Al_xGa_1−xAs heterostructure was studied and it is demonstrated that the heterostructure can serve as efficient antenna- and bias-free surface emitter. Diffractive optics elements – Fresnel zone plates –with integrated band-pass filters were simulated employing Finite-difference time domain method. Structures were fabricated using the laser direct writing and investigated using electronic THz sources and an optically pumped terahertz laser. Advantages of on-chip integration of diffractive optics and bow-tie-shaped InGaAs-based terahertz detectors are revealed via detection enhancement. Bow-tie diodes properties in frequency scale and detection sensitivity are considered and compared for different materials. Homodyne detection and imaging of low-absorbing objects at 0.6 THz are demonstrated and discussed.

We present a single pixel frequency division multiplexing imaging system with two metamaterial spatial light modulators (SLMs) for THz light field imaging. One SLM is used for slicing/modulating the 4D light field from various sub-apertures, while the second one together with a single pixel detector to implement 2D multiplexing measurement. This system is in essential a programming aperture in the frequency domain of carrier signals in our categorization of the traditional light field acquisition systems. We propose a prototype adaption design with available elements for THz imaging. Besides, we present the frequency selection method and reconstruction algorithm for square wave modulation/carrier signals for at most 4096-voxel light field dataset.