This PDF file contains the front matter associated with SPIE Proceedings Volume 9934, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

We present our recent results on CW and pulsed THz generation in quantum dot(QD) based photoconductive antennae(PCA) pumped by ultrafast and dual wavelength semiconductor lasers. QDPCA substrate incorporates InAs QDs in GaAs matrix, thus keeping semiconductor carrier mobility at higher levels that is typical for SI GaAs, while QDs themselves serve as lifetime shortening centres, allowing to achieve subpicosecond operation as in LT-GaAs. Thus, such substrates combine the advantages and lacking the disadvantages of GaAs and LT-GaAs, which are the most popular materials so far, and thus can be used for both CW and pulsed THz generation. Moreover, by changing QD size and mutual allocation, effective pump wavelengths can be tuned in the range between 0.9-1.3 μm, which is well beyond the GaAs energies, hence compact and relatively cheap ultrafast and narrow line double-wavelength semiconductor and fibre pump lasers can be used for pumping such antennae for both pulsed and CW THz generation. For double wavelength operation of semiconductor lasers, we implement either stacked double volume Bragg gratings, or double-Littrow configuration with two independent diffraction gratings to achieve tunability of the generated THz signal. High thermal tolerance of QD wafers allowed pumping single-gapped antennae with lasers producing up to 250 mW of CW optical power at simultaneous double wavelength operation and up to 1W average optical power in pulsed regime. We show these QD based antennae combined with such pump lasers to generate pulsed and CW THz radiation that is superlinearly proportional to pump power and bias applied to antenna.

Superlattices are artificial structures with a wide range of applications and open possibilities for controlling and study transport and optical [M.F. Pereira Jr., Phys. Rev. B 52, (1995)] properties of semiconductors. In this work, we start from the full Nonequilibrium Greens Functions approach [A. Wacker et a, IEEE Journal of Sel. Top. in Quantum Electron.,19 1200611, (2013),T. Schmielau and M.F. Pereira, Appl. Phys. Lett. 95 231111, (2009)] to obtain Voltage-Current curves and compare them with experiments. By adjusting the numerical solutions of the corresponding Dyson equations to a simple model, analytical solutions are given for the nonlinear response of a biased superlattice under sub-THz radiation. The frequency multiplication process leading to multiple harmonicgeneration is described. This hybrid approach leads to predictive simulations and may have important application for a new generation of devices where the superlattices are used as both sources and detectors and may be particular useful for high resolution transient spectroscopy [A.A. Yablokov et at, IEEE Transactions on THz Science and Technology 5, 845 (2015)].

Microscopic origin of THz emission from femtosecond laser excited narrow gap semiconductor surfaces is explained in terms of the photoelectron ballistic movement in non-parabolic and anisotropic conduction band. It has been shown that the azimuthal angle dependences of this emission are caused by the lateral photocurrent component resulting from that anisotropy. A strong THz radiation was observed from the lower symmetry crystal planes illuminated along their surface normal, which has allowed to demonstrate experimentally user-friendly line-of-sight THz emitters made from the bulk InAs and InSb, InAs pn diodes and p-i-n structures with GaInAs active layers.

Distinctive position of Terahertz (THz) frequencies (ν~0.3 -10 THz) in the electromagnetic spectrum with their lower quantum energy compared to IR and higher frequency compared to microwave range allows for many potential applications unique to them. Especially in the security side of the THz sensing applications, the distinct absorption spectra of explosives and related compounds in the range of 0.1–5 THz makes THz technology a competitive technique for detecting hidden explosives. A compact, high power, room temperature continuous wave terahertz source emitting in a wide frequency range will greatly boost the THz applications for the diagnosis and detection of explosives. Here we present a new strong-coupled strain-balanced quantum cascade laser design for efficient THz generation based intracavity DFG. Room temperature continuous wave operation with electrical frequency tuning range of 2.06-4.35 THz is demonstrated.

In this paper, we report on two different approaches that have been explored to realize tunable and reconfigurable THz devices for advanced imaging and adaptive wireless communication. The first approach makes use of electronically tunable varactor diodes. Frequency tunable THz antennas based on this approach have been successfully demonstrated for the first time in G-band, enabling the development of spectroscopic THz detectors and focal-plane imaging arrays. The second approach takes advantages of optical THz spatial modulation based on photo-induced free carriers in semiconductors. Using this approach, high-performance tunable THz modulators/attenuators, reconfigurable masks for THz coded aperture imaging, and photo-induced Fresnel-zone-plate antennas for dynamic THz beam steering and forming have been successfully demonstrated. Our recent study also shows that by employing the so-called mesa array technique, sub-wavelength spatial resolution and higher than 100 dB modulation depth can be achieved, making it possible to develop tunable THz devices (e.g., tunable filters) with performance and versatility far beyond those realized by conventional approaches. On the basis of the above investigation, the prospects of high-speed near-field THz imaging, real-time ultra-sensitive heterodyne imaging and prototype adaptive THz wireless communication links will be discussed.

The linewidth of a conventional laser is due to fluctuations in the laser field due to spontaneous emission and described by the Schalow-Townes formula. In addition to that, in a semiconductor laser there is a contribution arising from fluctuations in the refractive index induced by carrier density fluctuations. The later are quantitatively described by the linewidth enhancement or alpha factor [C. H. Henry, IEEE J. Quantum Electron. 18 (2), 259 (1982), W. W. Chow, S. W. Koch and M. Sargent III, Semiconductor-Laser Physics, Springer-Verlag (1994), M.F. Pereira Jr et al, J. Opt. Soc. Am. B10, 765 (1993). In this paper we investigate the alpha factor of quantum cascade lasers under actual operating conditions using the Nonequilibrium Greens Functions approach [A. Wacker et a, IEEE Journal of Sel. Top. in Quantum Electron.,19 1200611, (2013), T. Schmielau and M.F. Pereira, Appl. Phys. Lett. 95 231111, (2009)]. The simulations are compared with recent results obtained with different optical feedback techniques [L. Jumpertz et al, AIP ADVANCES 6, 015212 (2016)].

Remote sensing using the pulsed THz-TDS is of great interest because of its possible practical applications. Many ordinary materials (paper, for example) are transparent to THz radiation while the hazardous substances, which have to detect, possess fingerprints in this frequency range. However, covers of ordinary material can distort its spectrum in such a way that the spectrum of reflected THz pulse or transmitted THz pulse will contain absorption frequencies, which are inherent to dangerous substance (explosives, illistic drugs....), despite their absence in the material under consideration. On the other hand, it is well-known that the spectrum of medium response under the action of THz pulse is broader than the incident THz pulse spectrum. This is a result of a possibility of high energy level excitation due to cascade mechanism of their excitation. For practical point of view, it is very important to know about features of a covering substance influence on the medium response spectrum broadening. This problem is investigated in our report using the computer simulation.

We carry out computer simulation of a few-cycle electromagnetic pulse interaction with a substance exhibiting nonlinear non-instantaneous response. An influence of the pulse duration and polarization relaxation rate on the pulse interaction with an uncovered medium and with a medium, which is covered by a linear disordered structure, is investigated. We demonstrate that the spectra of reflected or transmitted pulses may comprise multiple additional spectral lines, caused by various transitions between energy levels of molecules. The spectral intensity of these energy level transitions is affected by the pulse duration and the effective time of interaction which depends on polarization relaxation rate as well as the medium layer thickness.

Regardless of all the success of Mid Infrared Quantum Cascade Lasers (QCLs), they still do not operate at room temperature in the THz range. The main temperature degrading mechanism for THz QCLs is not known in time of writing this abstract and it is still a topic of debate by the community [S. Khanal et al, J. Opt. 16 094001, 2014]. This is a challenge to theory and it is crucial to treat all possible scattering channels with the same mathematical footing. A summary of different methods for simulating these structures is found in [C. Jirauschek et al, Appl. Phys. Rev. 1 011307, 2014]. In this work we include and study the effects of electron-electron scattering via the Single Plasmon Pole Approximation (SPPA). In this approximation we capture both the static limit as well as dynamic effects. This gives an energy dependent (non-local in time) interaction beyond the Hartree-Fock approximation. This has been studied in a similar model with promising results [T. Schmielau and M.F. Pereira, Appl. Phys. Lett. 95 231111, 2009], and with this work we want to adapt the idea into the model described in Ref. [A. Wacker et a, IEEE Journal of Sel. Top. in Quantum Electron.,19 1200611, 2013]. We start by summarizing the theory underlying the SPPA and we show how it is implemented in the context of our formalism, by showing good agreement with the results for a four well quantum cascade laser [M. Amanti et al, New J. Phys. 11 125022, 2009].

We will report on new developments of quartz-enhanced photo-acoustic (QEPAS) sensor employing THz quantum cascade laser (QCLs). The extension of the QEPAS technique in the THz range was made possible by the realization of custom made QTFs. With the aim to improve the QTF acousto-electric transduction efficiency, we designed and realized new QTF designs. A detailed analysis of the quality factor, the resonance frequency and the electrical resistance of custom quartz tuning forks (QTFs) with different geometrical parameters is reported. The custom QTFs were employed in QEPAS sensors using THz QCLs as the laser excitation sources and targeting CH₃OH and H₂S. Minimum detection limits of a few tens of ppb and normalized noise equivalent absorption factors down to 3.75×10^-11 cm^-1W/Hz½ were achieved.

Nowadays the development of analytical spectroscopy with high performance, sensitivity and spectral resolution for exhaled breath research is attended. The method of two-frequency high precise THz spectroscopy and the method of high precise subTHz-THz-IR spectroscopy are presented. Development of a subTHz-THz-IR gas analyzer increases the number of gases that can be identified and the reliability of the detection by confirming the signature in both THz and MIR ranges. The testing measurements have testified this new direction of analytical spectroscopy to open widespread trends of its using for various problems of medicine and biology. First of all, there are laboratory investigations of the processes in exhaled breath and studying of their dynamics. Besides, the methods presented can be applied for detecting intermediate and short time living products of reactions in exhaled breath. The spectrometers have been employed for investigations of acetone, methanol and ethanol in the breath samples of healthy volunteers and diabetes patients. The results have demonstrated an increased concentration of acetone in breath of diabetes patients. The dynamic of changing the acetone concentration before and after taking the medicines is discovered. The potential markers of pre-cancer states and oncological diseases of gastrointestinal tract organs have been detected. The changes in the NO concentration in exhaled breath of cancer patients during radiotherapy as well as increase of the NH3 concentration at gastrointestinal diseases have been revealed. The preliminary investigations of biomarkers in three frequency ranges have demonstrated the advantages of the multifrequency high precise spectroscopy for noninvasive medical diagnostics.

The microwave-to-terahertz frequency range offers unique opportunities for the sensing of liquids based on the degree of molecular orientational and electronic polarization, Debye relaxation due to intermolecular forces between (semi-)polar molecules and collective vibrational modes within complex molecules. Methods for the fast dielectric characterization of (sub-)nanolitre volumes of mostly aqueous liquids and biological cell suspensions are discussed, with emphasis on labon- chip approaches aimed towards single-cell detection and label-free flow cytometry at microwave-to-terahertz frequencies. Among the most promising approaches, photonic crystal defect cavities made from high-resistivity silicon are compared with metallic split-ring resonant systems and high quality factor (Q-factor) whispering gallery-type resonances in dielectric resonators. Applications range from accurate haemoglobin measurements on nanolitre samples to label-free detection of circulating tumor cells.

Plasmonic Field Effect Transistor detectors (first proposed in 1996) have emerged as superior room temperature terahertz (THz) detectors. Recent theoretical and experimental results showed that such detectors are capable of subpicosecond resolution. Their sensitivity can be greatly enhanced by applying the DC drain-to-source current that increases the responsivity due to the enhanced non-linearity of the device but also adds 1/f noise. We now propose, and demonstrate a dramatic responsivity enhancement of these plasmonic THz pulse detectors by applying a femtosecond optical laser pulse superimposed on the THz pulse. The proposed physical mechanism links the enhanced detection to the superposition of the THz pulse field and the rectified optical field. A femtosecond pulse generates a large concentration of the electron-hole pairs shorting the drain and source contacts and, therefore, determining the moment of time when the THz induced charge starts discharging into the transmission line connecting the FET to an oscilloscope. This allows for scanning the THz pulse with the strongly enhanced sensitivity and/or for scanning the response waveform after the THz pulse is over. The experimental results obtained using AlGaAs/InGaAs deep submicron HEMTs are in good agreement with this mechanism. This new technique could find numerous imaging, sensing, and quality control applications.

Dual-independent-gate (DIG) silicon FinFETs were recently shown capable of operating with ultra-steep subthreshold slope of 3.4 mV/dec at room-temperature due to a weak impact ionization induced positive feedback. In this work we discuss the perspectives of these devices for room-temperature terahertz detector applications. Our analysis shows that DIG-FinFETs can enable room-temperature current-responsivities up to two orders of magnitude larger than those of regular FET and Schottky diode detectors at room-temperature. The device operation, detector configurations, and the sources of noise in the device are discussed and rigorously analyzed; moreover the device is also benchmarked against other present-day direct detector technologies.

Electron density oscillations in the transistor channels - plasma waves in the two-dimensional electron gas - determine the high frequency device response. Plasmonic field effect transistors have emerged as very sensitive, tunable, and extremely fast detectors of THz radiation. They have been implemented using silicon (CMOS), AlGaAs/InGaAs HEMTs, and AlGaAs/InGaAs HEMTs, with the HEMTs shown to operate more efficiently at higher THz frequencies. These HEMTs have both gated and ungated sections of the device channel between the source and drain, and the photovoltaic regime of operation requires an asymmetric gate placement in the device channel. The interactions of the plasma waves in the gated and ungated channel regions strongly affect the overall response and have been investigated in numerous publications. This work addresses a new aspect of such interaction - the effect of the relative position of the gated and ungated section. We show this previously unexplored effect plays a dominant role in determining the response. The results of the numerical simulation based on the solution of the complete system of the hydrodynamic equations describing the electron fluid in the device channel show that the inverse response frequency could be approximated by the sum of the gated plasmon transit time in the gated section of the device, the ungated plasmon transit time in the ungated section of the device between the gate and the drain, and the RC gate-to-source constant. Here R and C are the resistance and capacitance of the gate to source section. Hence, the highest speed is achieved when the gate is as close to the source as possible. This suggests a novel plasmonic detector design, where the gate and source electrode overlap, which is shown to have a superior frequency response for the same distance between the source and the drain.

This talk summarizes the achievements of COST ACTION MP1204 during the last four years. [M.F. Pereira, Opt Quant Electron 47, 815–820 (2015).]. TERA-MIR main objectives are to advance novel materials, concepts and device designs for generating and detecting THz and Mid Infrared radiation using semiconductor, superconductor, metamaterials and lasers and to beneficially exploit their common aspects within a synergetic approach. We used the unique networking and capacity-building capabilities provided by the COST framework to unify these two spectral domains from their common aspects of sources, detectors, materials and applications. We created a platform to investigate interdisciplinary topics in Physics, Electrical Engineering and Technology, Applied Chemistry, Materials Sciences and Biology and Radio Astronomy. The main emphasis has been on new fundamental material properties, concepts and device designs that are likely to open the way to new products or to the exploitation of new technologies in the fields of sensing, healthcare, biology, and industrial applications. End users are: research centres, academic, well-established and start-up Companies and hospitals. Results are presented along our main lines of research: Intersubband materials and devices with applications to fingerprint spectroscopy; Metamaterials, photonic crystals and new functionalities; Nonlinearities and interaction of radiation with matter including biomaterials; Generation and Detection based on Nitrides and Bismides. The talk is closed by indicating the future direction of the network that will remain active beyond the funding period and our expectations for future joint research.

We analyze the terahertz properties of complex oxide hetero-structures with record-high carrier concentration approaching 10¹⁵ cm^-2. Our results evidence a large room temperature terahertz conductivity, which corresponds to 3X to 6X larger mobility than what is extracted from electrical measurements. That is, in spite of a relatively lower mobility, when taking into account its ultra-large carrier concentration, the 2DEG in complex oxide hetero-structures can still attain a large terahertz conductivity, which is comparable with that in traditional high-mobility semiconductors or large-area CVVD graphene films. Moreover, we also discuss the perspectives off these hetero-structures for terahertz and high frequency electronic applications.

There are three major challenges in cancer imaging using terahertz electromagnetic waves, which are the limited penetration depth into wet tissues, the difficulty to observe terahertz fingerprints (resonance markers) of cancers, and the poor contrast between healthy and diseased tissues. This presentation addresses such issues in detail and explains the potential solutions to them with recent results.

In this work, a comparative research of biologically active organic molecules in its natural environment using the terahertz (THz) time domain spectroscopy (TDS) and Fourier transform spectroscopy (FTS) systems is carried out. Absorption coefficient and refractive index of Nicotiana tabacum L. leaves containing nicotine, Cannabis sativa L. leaves containing tetrahydrocannabinol, and Humulu lupulus L. leaves containing α-acids, active organic molecules that obtain in natural environment, were measured in broad frequency range from 0.1 to 13 THz at room temperature. In the spectra of absorption coefficient the features were found to be unique for N. tabacum, C. sativa and H. lupulus. Moreover, those features can be exploited for identification of C. sativa sex and N. tabacum origin. The refractive index can be also used to characterize different species.

Proteins solvated in their biologically milieu are expected to exhibit strong absorption in the terahertz frequencies, that contain information on their global and sub-global collective vibrational modes (conformational dynamics) and global dynamic correlations among solvent water and proteins. The dynamics play an important role in enzymatic activities of proteins, but obtaining an accurate and quantitative pictures of these activities, however, is challenging due to the strong absorption of water. In response, we have developed the world’s highest precision, highest sensitivity terahertz-frequency domain spectrometer and a standard terahertz-time domain system to probe the collective dynamics of proteins in aqueous solutions. Operating over the frequency range from 5 GHz up to 3 THz, our spectrometers provide an unparalleled ability to probe directly such questions as the hydration level, the dynamics of water and hydrated proteins over the 100 fs to 1 ns timescale. Employing an effective medium approximation to describe the complex dielectric response of the solvated proteins in solution we find that proteins are surrounded by a loosely and tightly held layers of water molecules that behave as if they are an integral part of the protein. The number of water molecules in the protein hydration shells varies with proteins, which can tell us the average surface structure of proteins. These measurements shed light on the macromolecular motions of proteins in their biologically relevant environment.

Sub-wavelength metamaterial structures are of great fundamental and practical interest because of their ability to manipulate the propagation of electromagnetic waves. Here we investigate the metamaterials composed of titanium and copper split-ring resonators for use in detection of living cells. Terahertz spectroscopy was utilized to detect a change in resonance frequencies of the bio-sensor in the presence of MDA-MB-231 breast cancer cells in culture in real time. The shift in frequency showed dependency upon cell density. We applied circuit model to interpret the resonance peak shift observed, and not only do we see shifts in resonance frequency but also in capacitance and resistance as time progresses.