This PDF file contains the front matter associated with SPIE Proceedings Volume 8496, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

The development of terahertz quantum cascade lasers (QCLs) has progressed significantly in the past ten years. Widely different types of QCLs have been demonstrated covering a frequency range from 1:2 THz to 5 THz (when operating without the existence of an external magnetic field). Improvement of operating temperatures of terahertz QCLs is one of the primary goals to make such devices viable for important terahertz applications. Some of the best techniques to obtain high operating temperatures have relied on electron-phonon scattering assisted depopulation. This paper reviews terahertz QCLs operating in a frequency range of 1:4 THz to 4:7 THz with such design schemes. Operation above a temperature of 160 K has been obtained across a broad range of frequencies from 1:8 THz - 4:3 THz. While the temperature degradation mechanisms are still not completely understood, it is speculated that collisional broadening of subbands may result in degradation of resonant tunneling transport at higher temperatures, which is critical to establishing population inversion in the QCL structure. The recently developed scattering-assisted injection techniques may mitigate subband broadening effects at higher temperatures, which is supported by experimental results. Further advances in the active region design as well as choice of different materials for growth and design of superlattices may result in even higher operating temperatures for terahertz QCLs.

GaInAs/AlInAs/InP quantum cascade lasers have established themselves as reliable laser sources in the mid-infrared region (3.8-10) μm, where they operate at room-temperature in continuous-wave with Watt-level output powers. However, wavelengths above this wavelength region are difficult to generate. At long wavelengths, devices suffer from increased free-carrier absorption and poor population inversion due to the short upper laser state lifetime, thus limiting their operation to cryogenic temperatures. An alternative way to generate new frequencies is the by means of nonlinear frequency mixing. For long-wavelengths, the process of difference frequency mixing is of particular interest, as it is possible to utilize the good performance of the mid-infrared QCLs, acting as pump sources, together with the giant nonlinear properties that can be realized in the intersubband transitions of the quantum wells. Moreover, the giant nonlinearity can be monolithically integrated with the pump sources, leading to a compact, electrically pumped room-temperature semiconductor laser source, emitting at terahertz frequencies. In our work, we present several different concepts of monolithic nonlinear quantum cascade laser sources, designed to emit in the THz range: devices with passive giant nonlinearities, active nonlinearities and, finally, devices with active nonlinearities, combined with novel THz waveguiding techniques. We will demonstrate how application of novel THz waveguiding techniques avoids the efficiency suppression the large free-carrier absorption at THz frequencies in the doped semiconductor layers enabling room-temperature operation up to 1.2 THz.

Room-temperature terahertz (THz) quantum cascade laser (QCL) sources based on intra-cavity difference-frequency generation (DFG) with record THz conversion efficiencies is reported. THz DFG QCLs reported previously are highly inefficient since THz radiation produced more than ~100 μm away from the exit facet is fully absorbed due to high THz losses in the QCL waveguide. Our lasers use a non-collinear Čerenkov DFG scheme to extract THz radiation from the active region. Dual-color mid-infrared quantum cascade lasers with integrated giant optical nonlinearity are grown on semi-insulating (S.I.) InP substrates. A lateral current extraction scheme is used. THz radiation is emitted at an angle into the substrate with respect to the mid-infrared pumps. Since S.I. InP is virtually lossless to THz radiation, this scheme allows for efficient extraction of THz radiation along the whole waveguide length. As a result, our sources demonstrate large mid-infrared-to-THz conversion efficiency and directional THz output. Experimentally, proof-of-principle devices demonstrate a conversion efficiency up to 70 μW/W2 and provide output across a 1.2 - 4.5 THz spectral range.

We investigate experimentally resonant-tunnelling-diode (RTD) oscillators, which are based on RTDs with heavily doped collector. We demonstrate that such RTD oscillators can work at frequencies, which are far beyond the limitations imposed by resonant-state lifetime and relaxation time. Exploiting further such RTDs, we have achieved the record operating frequency of 1.1 THz and show that substantially higher frequencies should be also achievable with RTD oscillators. RTD oscillators are extremely compact (less than a square millimeter) room-temperature sources of coherent cw THz radiation. Such sources should enable plenty of real-world THz applications.

Resonant tunneling diode high-frequency study on the base of proposed quantum models and numerical solution of timedependent Schrödinger equation with open-system boundary conditions in external electromagnetic field are performed for single-quantum well and double-quantum well resonant tunneling structures. As shown the presence of privileged additional level in double-quantum well structures breaks response symmetry, leads to narrow-band frequency selective amplification at frequencies equal to energy spacing between levels in neighbouring quantum wells and to selection of portion of emitter electrons that actively interact with external THz electromagnetic field. The phenomenon results in essentially increase of gain coefficient and opens the possibility of narrow-band amplification frequency tuning in TBRTS in THz range by variation of applied bias voltage.

THz has become a wide field of investigation opening new opportunities in a growing number of domains of physics, chemistry, and biology. Among the different techniques existing today to generate THz fields, heterodyning two optical frequencies is a useful approach when tunability is required. Moreover, to address high-resolution spectroscopy or metrology applications, a key point is the achievement of a narrow linewidth source. To this aim, two-propagation-axis dual-frequency lasers have been already shown to provide narrow linewidth tunable beat notes up to 2 THz. We report in this paper the demonstration of a narrow linewidth THz radiation source based upon this laser. Indeed the beat note provided by the laser is sent into a unitravelling carrier photodiode (UTC-PD), and radiated by a transverseelectromagnetic- horn antenna (TEM-HA). All components operate at room temperature. The emitted THz signal is detected by a subharmonic mixer coupled to an electrical spectrum analyzer. The THz signal is observed and analyzed thanks to a heterodyne detection. The measured dynamic range is 75 dB at 282 GHz, 50 dB at 500 GHz, 35 dB at 700 GHz and decreases to 20 dB at 1 THz. The decrease is due to the UTC-PD efficiency and conversion losses in the sub-harmonic mixer. The measured linewidth is better than 30 kHz at any frequency from DC to 1 THz.

Design and numerical simulations indicate the presented quasi-optical (QO) reflective circulator topology is a promising topology for next generation, ultrawideband terahertz systems. A side-by-side comparison between the reported 'state-of-the-art' waveguide technology versus the QO reflective circulator show significantly improved insertion loss and over two orders of magnitude improvement in power handling capability.

Compact, room temperature terahertz sources are much needed in the 1 to 3 THz band for developing multi-pixel heterodyne receivers for astrophysics and planetary science or for building short-range high spatial resolution THz imaging systems able to see through low water content and non metallic materials, smoke or dust for a variety of applications ranging from the inspection of art artifacts to the detection of masked or concealed objects. All solid-sate electronic sources based on a W-band synthesizer followed by a high-power W-band amplifier and a cascade of Schottky diode based THz frequency multipliers are now capable of producing more than 1 mW at 0.9THz, 50 μW at 2 THz and 18 μW at 2.6 THz without the need of any cryogenic system. These sources are frequency agile and have a relative bandwidth of 10 to 15%, limited by the high power W-band amplifiers. The paper will present the latest developments of this technology and its perspective in terms of frequency range, bandwidth and power.

A novel scheme based on opto-electronic down conversion is proposed and demonstrated to obtain an ultra-high spectral purity and tunable optical beat note in the GHz and millimeter wave range. An in-loop relative frequency stability better than 4 10^-12 is reported. This approach opens the way to THz metrology.

The concept of THz detection based on excitation of plasma waves in two-dimensional electron gas in Si FETs is one of the most attractive ones, as it makes possible the development of the large-scale integrated devices based on a conventional microelectronic technology including on-chip antennas and readout devices integration. In this work we report on investigations of Terahertz detectors based on low-cost silicon technology field effect transistors and asymmetric unit cell double grating gate field effect transistor. Double-grating-gate field-effect transistors have a great potential as terahertz detectors. This is because the double grating gate serves not only for carrier density tuning but also as an efficient THz radiation coupler. In this paper, we present characterization of these transistors using high magnetic fields. Low and high magnetic field data are used to determine the electron mobility and electron concentration, respectively, in different parts of the transistor channel. We show that detectors, consisting of a coupling antenna and a n-MOS field effect transistor as rectifying element, are efficient for THz detection and imaging. We demonstrate that in the atmospheric window around 300 GHz, these detectors can achieve a record noise equivalent power below 10 pW/Hz^0.5 and responsivity above 90 kV/W once integrated with on-chip amplifier. We show also that they can be used in a very wide frequency range: from ~0.2 THz up to 1.1 THz. THz detection by Si FETs paves the way towards high sensitivity silicon technology based focal plane arrays for THz imaging.

We present recent developments of two kinds of compact room-temperature detectors for terahertz radiation. These are asymmetrically-shaped bow-tie diodes, and field-effect transistors with sub-micrometer channel lengths. Both kinds of detectors exhibit fast response times which allow operating them as mixers, moreover they are suitable for the fabrication of large multi-pixel arrays. Here, we provide data on the experimental performance of detector arrays and show their applicability in coherent terahertz imaging systems. In addition, we demonstrate real-time operation of a 12×12-pixel CMOS camera in power-detection mode at 590 GHz.

A sub-millimeter(mm)-wave detector module implementing an InP-based zero-biased Schottky-barrier diode (SBD) has been fabricated. The SBD is monolithically integrated with a short-stub resonant matching circuit for increasing the detection sensitivity as well as providing a biasing circuit. The SBD chip is mounted in a compact J-band (WR-3) rectangular-waveguide-input module for practical use. The module exhibits a peak sensitivity at around 350 GHz due to the characteristics of the matching circuit, and a good linearity of the output voltage against the input sub-mm-wave power. A record sensitivity of 1460 V/W is obtained for the InP-based zero-biased SBD at 350 GHz.

We experimentally demonstrate an ultrathin polarization rotator (PR) capable of rotating linearly polarized radiation 90 degrees in the terahertz-gap region by integrating the polarization conversion property from asymmetric split-ring resonators (ASRRs) and the polarization selectivity from S-shape resonators (SRs). This ultrathin-PR possesses a nearly complete polarization conversion up to 97.7% at 1.04 THz and a high conversion transmission coefficient of 0.48, enhanced by a constructive Febry-Perot interference between the ASRRs and the SRs. Furthermore, the overall thickness of the ultrathin PR is about 50 μm, only one-sixth of the incident wavelength, so that the associated optical devices can be miniaturized. The simple metal-dielectric-metal trilayered structure of the ultrathin-PR allows the tolerance of translational misalignment between the ASRRs and SRs and hence significantly facilitates the fabrication process.

We report about fabrication and characterization of semiconductor nanowire-based field effect transistor devices which can act as detectors for electromagnetic radiation in the THz frequency range. The detection mechanism is based on the nonlinear transfer characteristic of the transistor, which is used to realize signal rectification; the small capacitance related to the nanowire small cross section is beneficial in allowing a good device sensitivity up to 1.5 THz at room temperature. Due to the extreme flexibility with which semiconductor nanowires can be grown, we discuss how the basic, homogeneous InAs or InSb nanowire FETs can be improved to realize smarter devices and functionalities.

It has long been known that shallow donors such as phosphorous and the other group-V elements, have a hydrogen-like optical spectrum. The main difference is that while the spectrum of atomic hydrogen lies in the visible band, the spectrum of shallow donors in silicon is downshifted to the THz frequency band. This is a direct consequence of the reduced Coulomb attraction seen by the loosely bound electron because the core electrons shield the positive donor atom nucleus, and because the electron is now moving in a dielectric material. While spectroscopy has already revealed much about the energy level structure, very little was known about the temporal dynamics of the system until now. We have used THz pulses from the FELIX free electron laser to probe these hydrogen-like levels. By exploiting the well-known pump-probe technique we have measured the characteristic lifetimes of the excited Rydberg states and found them to be of the order 200 ps. Then, by making subtle changes to the geometry of the pump-probe experimental setup we demonstrate the existence of a THz photon echo. The photon echo is a purely quantum phenomenon with no classical analogue, and it allows us to study the quantum state of the donor electron. We then show, using the photon echo, that it is possible to create a coherent superposition of the ground and excited state of the donor. Measuring the photon echo is important because it can also be used to measure a second important characteristic lifetime of the silicon-donor system, the phase decoherence time.

In storage rings, short electron bunches can produce an intense THz radiation called Coherent Synchrotron Radiation (CSR). The flux of this emission between 250 and 750 GHz is very advantageous for spectroscopy, but intensity fluctuations lead to artifacts in the FTIR spectra and, until now, prevented the use of CSR for high-resolution measurements. At SOLEIL, we found stable CSR conditions for which the signal-to-noise ratio (S/N) allows for measurements at high resolution. Moreover, we developed an artifact correction system, based on a simultaneous detection of the input and the output signals of the interferometer, which allows improving further the signal-to-noise ratio. The stable CSR combined with this ingenious technique allowed us to record for the first time high-resolution FTIR spectra in the sub-THz range, with an exceptional S/N of 100 in a few hours.

This paper describes a real-time transmission-type Terahertz (THz) microscope, with palm-size THz camera and compact quantum cascade laser (QCL). The THz camera contains 320x240 microbolometer focal plane array which operates at 30 Hz frame rate and has lock-in imaging function as well as integration functions such as frame integration and spatial filter. These functions are found very powerful in improving signal-to-noise ratio. QCL is installed in compact Stirling cycle cooler. A variety of QCLs covers frequency range from 1.5 to 5 THz and provides time-average power of 0.5~2 mW. The illumination area for sample is changed by adjusting one lens in the illumination optics. Performances of the THz microscope, such as signal-to-noise ratio and so on, were measured and are found consistent with the calculations. THz images taken with the THz microscope are finally presented.

Terahertz spectroscopy is able to probe several aspects of biological systems. Most well known is its sensitivity to water due to the strong water absorptions at terahertz frequencies. However an increasing number of studies have shown that it is not just water content that terahertz is sensitive to and that other factors such as tissue structure, molecular arrangement or even temperature can also affect the signal. Examples ranging from breast cancer spectroscopy to antibody protein spectroscopy will be presented and discussed.

Applications for terahertz (THz) medical imaging have proliferated over the past few years due to advancements in source/detector technology and vigorous application development. While considerable effort has been applied to improving source output power and detector sensitivity, significantly less work has been devoted to improving image acquisition method and time. The majority of THz medical imaging systems in the literature typically acquire pixels by translating the target of interest beneath a fixed illumination beam. While this single-pixel whiskbroom methodology is appropriate for in vitro models, it is unsuitable for in vivo large animal and patient imaging due to practical constraints. This paper presents a scanned beam imaging system based on prior work that enables for reduced image acquisition time while allowing the source, target and detector to remain stationary. The system employs a spinning polygonal mirror and a set of high-density polyethylene (HDPE) objective lenses, and operates at a center illumination frequency of 525GHz with ~125GHz of 3dB bandwidth. The system achieves a focused beam diameter of 1.66mm and a large depth of field of <25 mm. Images of characterization targets and ex vivo tissue samples are presented and compared to results obtained with conventional fixed beam scanning systems.

One of the problems arising in Time-Domain THz spectroscopy for the problem of security is the developing the criteria for assessment of probability for the detection and identification of the explosive and drugs. We analyze the efficiency of using the correlation function and another functional (more exactly, spectral norm) for this aim. These criteria are applied to spectral lines dynamics. For increasing the reliability of the assessment we subtract the averaged value of THz signal during time of analysis of the signal: it means deleting the constant from this part of the signal. Because of this, we can increase the contrast of assessment. We compare application of the Fourier-Gabor transform with unbounded (for example, Gaussian) window, which slides along the signal, for finding the spectral lines dynamics with application of the Fourier transform in short time interval (FTST), in which the Fourier transform is applied to parts of the signals, for the same aim. These methods are close each to other. Nevertheless, they differ by series of frequencies which they use. It is important for practice that the optimal window shape depends on chosen method for obtaining the spectral dynamics. The probability enhancements if we can find the train of pulses with different frequencies, which follow sequentially. We show that there is possibility to get pure spectral lines dynamics even under the condition of distorted spectrum of the substance response on the action of the THz pulse.

NASA and ESA are planning missions to Jupiter and its moons. There is strong interest in a submillimeter/Terahertz spectroscopic heterodyne instrument covering the bands 520 to 600 GHz and 1100 to 1300 GHz. Therefore, we are developing a prototype instrument incorporating unique features not previously developed for planetary instrumentation. These include (1) extremely wide, rapid tunability. The Herschel/HIFI astronomical instrument, is also wideband, but far larger. It incorporates a 3.5 meter telescope on a spacecraft massing over three tons orbiting near Earth, versus our 20 kg Jupiter spectrometer. Hence, we have developed a wideband low-phase-noise synthesizer pumping two Schottky diode LO multiplier chains outputting 520 to 600 and 550 to 650 GHz. Also based on Schottky diodes are (b) 550 and 1200 GHz room temperature mixers. The high frequency mixer is subharmonically pumped; the lower balanced fundamental. To analyze the IF signals from the mixers, (c) ASIC based digital polyphase spectrometers consuming only a few Watts each are being incorporated into the instrument. Finally, since signals for both receivers come from one telescope, we include a new (d) compact dual band low-loss optical bench. It uses the fact that each receiver accepts one polarization, making a polarizing beam splitter sufficient to split the beam with minimal loss.

Several applications of terahertz radiation pulses for characterizing semiconductor bulk materials and structures are described. Terahertz pulses emitted at the surfaces illuminated by femtosecond laser of a tunable wavelength are demonstrated to provide information on the electron energy spectrum in the conduction band as well as on the subsurface band bending. On the other hand, by sampling the conductivity of various structures with short electrical field transient photoexcited electron dynamics can be directly studied at its initial, subpicosecond time scale. Narrow gap semiconductors InSb and InAs as well as novel materials such as GaAsBi or self-assembled InAs quantum dots were characterized by using terahertz radiation pulses.

Some of the modern problems arising in the detection and identification of substances are an influence of water vapor and opaque coverings and surface of the explosive and an influence of ambient medium (for example, atmospheric gases) on the THz signal reflected from the sample. All influencing factors mentioned above distort the signal and its spectrum. Therefore, we get the signal with information about the ambient medium and coverings. We show in this paper that the method of the spectral dynamics analysis (SDA-method) is convenient tool for the detection and identification of the explosive and drugs at such conditions. For this aim we develop the method which allows us to get the dynamics of spectral lines of the explosive response distorted by action of various causes. We compare the new method of finding the spectral lines dynamics with previous method based on FTST (Fourier-Gabor transform). It is essential that we analyze the THz pulse with a few cycles. As consequence, this pulse has a broad spectrum. To remove the action of opaque coverings it is sufficient to analyze the sub-pulse which follows after reflected main pulse. To avoid an influence of water vapor it is sufficient to use the transparent windows of atmosphere. We show the possibility to get the dynamics of spectral lines without its distortion by neighboring frequencies under certain choice of both the order of measurements and time window for the computer processing of the signal reflected from the sample. Analysis of the spectral line dynamics allows us to avoid the influence of surface of the explosive on the detection and identification of forbidden substance as we believe. This problem requires additional investigations. Nevertheless, the preliminary results of investigations show the validity of such approach to get true way.

Terahertz (THz) imaging is an expanding area of research in the field of medical imaging due to its high sensitivity to changes in tissue water content. Previously reported in vivo rat studies demonstrate that spatially resolved hydration mapping with THz illumination can be used to rapidly and accurately detect fluid shifts following induction of burns and provide highly resolved spatial and temporal characterization of edematous tissue. THz imagery of partial and full thickness burn wounds acquired by our group correlate well with burn severity and suggest that hydration gradients are responsible for the observed contrast. This research aims to confirm the dominant contrast mechanism of THz burn imaging using a clinically accepted diagnostic method that relies on tissue water content for contrast generation to support the translation of this technology to clinical application. The hydration contrast sensing capabilities of magnetic resonance imaging (MRI), specifically T2 relaxation times and proton density values N(H), are well established and provide measures of mobile water content, lending MRI as a suitable method to validate hydration states of skin burns. This paper presents correlational studies performed with MR imaging of ex vivo porcine skin that confirm tissue hydration as the principal sensing mechanism in THz burn imaging. Insights from this preliminary research will be used to lay the groundwork for future, parallel MRI and THz imaging of in vivo rat models to further substantiate the clinical efficacy of reflective THz imaging in burn wound care.

We previously developed a high-speed terahertz spectroscopic imaging method based on electro-optic sampling with a noncollinear geometry of the THz beam and probe laser beam and using a multistep mirror in the path of the probe beam. We set the incident probe laser into MAST at a 45° angle, to prevent interference between adjacent beams. However, this produced beam vignetting, so imaging had to be performed twice, between sample movements, and this increased the imaging time accordingly. Thus, we improved the probe-laser imaging system after reflecting from the MAST to correct for the effects of diffraction. This prevents interference from adjacent beams and allows the angle of incidence on the MAST to be set to 0°, enabling the entire sample surface to be imaged in one measurement. As a result, we are able to perform measurements in 40 seconds, half the time of the previous method, and obtain a 28x28-pixel spectral image with spatial resolution of 1.07 mm. To verify the imaging performance, we also measured test samples, showing that the shape and thickness of items inside an opaque plastic case can be distinguished using amplitude and phase images, and metallic foreign objects can be detected. We also evaluated the method and were able to show the validity of the spectral imaging results by distinguishing the transmission or blocking of arbitrary frequency components.

We investigate subwavelength confinement of terahertz electromagnetic surface modes in a three-dimensional region with coupled slot structures. Two-dimensional resonance focusing on a subwavelength slot converts to three-dimensional subwavelength confinement, due to sharp edge confinement effect on asymmetric plasmonic structure, at the center position of the slot structures which consists of two or more slots. We also report on the polarization independent confinement of terahertz electromagnetic surface modes beyond diffraction limit. The structure which consists of radially arranged subwavelength slots located at a same center position shows the polarization-independent terahertz three-dimensional subwavelength confinement.

Noise characteristics of bow-tie InGaAs diodes in forward and backward directions were measured in frequency range from 10 Hz to 20 kHz at room temperature. It was found that the spectral density of voltage fluctuations changes with frequency approximately as 1/f, indicating that origin of noise is superposition of generation and recombination processes in defects of the structure. It was determined that the dependence of spectral density of current fluctuation on current in backward and forward directions at different frequencies exhibits asymmetry which reflects non-uniform electric field distribution within the structure. As both noise and sensitivity increase with bias current, optimal conditions for device operation are discussed.

Enhanced attention to solar cell development stimulates search of innovative solutions to their characterization and identification of possible technological defects in various steps of production in a contactless way. In the given work, investigation of solar cells structures by means of terahertz (THz) imaging is presented. Both continuous wave and pulsed THz imaging set-ups were employed in this study. Investigated objects included typical for various production stages test silicon structures – structured and unstructured surfaces, metalized and unmetalized contact areas, commercial silicon solar cells. We demonstrate that phase sensitive subTHz imaging can be employed for detecting manufacturing defects of the solder tabs, while images at higher frequencies reveal finer details of solar cells, such as cracks or different reflection coefficients for structured and unstructured surfaces with different doping.

Terahertz Time-Domain Spectroscopy (THz-TDS) is a new technique in studying the conformational state of molecules. Cell membranes are important structures in the interaction with extra cellular entities. Their principal building blocks are lipids, amphiphilic molecules that spontaneously self-assemble when in contact with water. In this work we report the use of THz-TDS in transmission mode to examine the behavior of supported phospholipid bilayers (SPBs) within the frequency range of 0.2 THz to 3 THz. SPBs were obtained by vesicle adsorption method involving the spread of a suspension (50-100 μl) of small unilamellar vesicles (SUVs) or multilamellar vesicles (MLVs) dissolved in PBS (phosphate buffer solution) on a support of silicon wafers. Both SUVs and MLVs were obtained from dipalmitoyl phosphatidylcholine (DPPC) and lecithin by using the thin-film hydration method. Broadband THz pulses are generated and detected using photoconductive antennas optically excited by a femtosecond laser pulse emitted from a self-mode locked fiber laser at a wavelength of 780 nm with a pulse widths of 150 fs. THz-TDS was proven to be a useful method in studying SPBs and their hydration states. The absorption coefficient and refractive index of the samples were calculated from THz measurements data. The THz absorption spectra for different lipids in SPBs indicate specific absorption frequency lines. A difference in the magnitude of the refractive index was also observed due to the different structure of supported lipid bilayers. The THz spectrum of DPPC was obtained by using theoretical simulations and then the experimental and theoretical THz spectra were compared.