This PDF file contains the front matter associated with SPIE Proceedings Volume 8846, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

We present the high performance THz sources based on intracavity difference-frequency generation from mid-infrared quantum cascade lasers. Room temperature single-mode operation in a wide THz spectral range of 1-4.6 THz is demonstrated from our Cerenkov phase-matched THz sources with dual-period DFB gratings. High THz power up to 215 μW at 3.5 THz is demonstrated via epi-down mounting of our THz device. The rapid development renders this type of THz sources promising local oscillators for many astronomical and medical applications.

The two main design schemes for Terahertz quantum cascade lasers, based on tunnelling and scattering injection, respectively, are theoretically compared. We apply our simulation package based on the non-equilibrium Green’s function technique. Our results provide a good description of the gain degradation with temperature. Thermal backfilling contributes to decrease of population inversion in both cases. However, the dropping inversion cannot account for the total reduction of gain.

We demonstrate a variety of gradient index (GRIN) devices for both freely propagating terahertz waves and terahertz surface waves. As a specific example of a static GRIN optics we experimentally investigated and measured the optical properties of a GRIN beam deflector. We measured a maximum deflection angle of 6 deg. The thickness of the beam deflector was only about 100 μm which is sub-wavelength at a frequency of 1.3 THz. Furthermore, we specifically designed meta-surfaces that support strongly confined terahertz surface waves. We show that the surface waves can be deliberately guided within the plane of propagation by use of GRIN structures. For example, we evidenced the focusing behavior of a meta-surface GRIN lens by near-field mapping of the terahertz field.

Scattering of THz radiation in inhomogeneous media is an important reason of reducing spectral information in received THz signal for different applications. Scattering of THz radiation in thin layers of dielectric material was studied, particularly, in fabric materials. A structure of different types of cloth fabric was studied, and mathematical model of fabric was suggested, which is a system of dielectric cylinders. Scattering of THz radiation was modeled numerically. Scattering cross-section and indicatrix were calculated, and then spectral transmittance was computed. During a set of experiments THz spectral transmittance of various fabric materials were obtained and numerical results were approved. The developed method for receiving scattering characteristics of different materials can be used for extracting useful information from received THz signal.

Our recent results of room-temperature THz oscillators using resonant tunneling diodes (RTDs) are reported. This oscillator is composed of a GaInAs/AlAs double-barrier RTD and a planar slot antenna. The maximum oscillation frequency in RTDs is limited by the electron delay time across the RTD layers, which consists of the dwell time in the resonant tunneling region and the transit time across the collector depletion region. The dwell time was reduced by a narrow quantum well, and a fundamental oscillation up to 1.31 THz with the output power of 10 μW was achieved at room temperature. Further increase in oscillation frequency is expected by optimized size and materials of the well and barriers for the dwell time and those of the collector depletion layer for the transit time. By these improvements, a fundamental oscillation up to around 2 THz is theoretically possible. For high output power, coherent power combining was demonstrated in a two-element array with offset slot antennas coupled with each other, and 610 μW at 620 GHz was obtained. Spectral characteristics were measured with a heterodyne detection, and the linewidth of less than 10 MHz was obtained. A frequency change of 1-5 % with bias voltage was also observed, which is attributed to the bias-dependent dwell time. Direct intensity modulation and wireless data transmission were demonstrated. A transmission rate of 3 Gbps with the bit error rate of 3×10^-5 was obtained at 540 GHz in a preliminary experiment, which is limited by the frequency characteristics of external modulation circuits at present.

In the last few years, InP HEMT maximum frequency of oscillation (fMAX) has pushed well beyond 1 THz (1000 GHz). This implies that solid state amplification is possible to frequencies approaching 1 THz. In this paper, we provide an overview of power amplifier and power generation work which has been done using InP HEMT technology. In particular, power generation has been demonstrated to 0.67 THz

Terahertz emitters are important for fundamental studies in an interesting frequency regime and for applications ranging from medical diagnostics to see-through imaging. A simple approach to THz emission from semiconductors is based on intracenter optical transitions in dopants and impurities in semiconductors. The centers can be excited either electrically or optically, and the THz emission occurs when carriers in the dopant upper energy states relax toward the ground state. Both n–type and p–type dopants as well as deep impurities can be used for THz emission from many host semiconductors including silicon, SiC, and GaN. Unlike with conventional p–n junction devices, the centers for THz emission must be occupied and not thermally ionized, which suggests the need for deep energy levels and/or low temperature operation. Significant center occupation at elevated temperatures favors the wide bandgap semiconductors such as SiC and GaN, in which the dopant ionization energy can greatly exceed the thermal energy k_BT at room temperature. For example, electrically pumped THz emitters fabricated from nitrogen-doped SiC can operate at temperatures to about 250 K in pulse mode. The SiC emission spectra had peaks from 5 to 12 THz (20 to 50 meV), and these surface-emitting devices produced a peak power density of 30 milliwatt-cm^-2 at 77 K, which is suitable for a wide range of high power THz applications. We report the characteristics and limitations of electrically pumped dopanttransition THz emitters, and their performance in several semiconductor systems.

The uni-traveling-carrier photodiode (UTC-PD) is a promising device for generating continuous terahertz (THz) waves by photomixing since it simultaneously operates at very high frequencies and generates relatively high output powers. These features are due to its unique operation mode utilizing only electrons as the active carriers. To utilize the performance of the UTC-PD fully, monolithic integration of an appropriate broadband antenna is essential. We fabricated devices integrating several types of planar antennas and compared their characteristics. Although all the modules exhibited broadband output characteristics at frequencies up to 1.6 THz, the behaviors at the lower frequency side as well as their polarization characteristics were quite different. Using a device integrating a novel planar antenna, we could detect output powers at frequencies from 30 GHz to 1.6 THz with almost constant polarization. Typical detected output powers were 120 μW at 200 GHz, 17 μW at 500 GHz, and 2.9 μW at 1 THz for a photocurrent of 10 mA with a bias voltage of only -0.6 V.

We demonstrate Terahertz (THz) signal sources based on photoconductive (PC) antenna devices comprising active layers of InAs semiconductor quantum dots (QDs) on GaAs. Antenna structures comprised of multiple active layers of InAs:GaAs PC materials are optically pumped using ultrashort pulses generated by a Ti:Sapphire laser and CW dualwavelength laser diodes. We also characterised THz output signals using a two-antenna coherent detection system. We discuss preliminary performance data from such InAs:GaAs THz devices which exhibit efficient emission of both pulsed and continuous wave (CW) THz signals and significant optical-to-THz conversion at both absorption wavelength ranges, ≤850 nm and ≤1300 nm.

We study the system with dipolaritons - mixed quasiparticles which are formed in the double quantum well heterostructure in the presence of strong light-matter interaction. These quasiparticles possess large dipole moment due to the resonant coupling between direct and indirect excitons via electronic tunnelling. Using the pulsed pumping of the cavity mode one can induce oscillations of the indirect exciton density. This corresponds to a harmonic change of the dipole moment in time and results in the classical electromagnetic wave emission with frequency being in the terahertz (THz) range. In the current paper we present a simple theory which describes this phenomenon and estimate possible output power of radiation.

In spite of rather impressive achievements in intersubband quantum cascade lasers their current parameters are still far from the needs of practical implementation. We compare theoretical prospects of THz gain for two cases: intersubband GaAs-based quantum cascade lasers and interband laser based on coupled quantum wells InAs-GaSb. Our methodology of such a comparison is reduced to following: The most typical design of GaAs-based QCL is compared with an InAs-GaSb coupled quantum well laser operating in the same frequency range. The detailed density matrix based calculation shows that the maximal possible gain for CQWL can be three orders of magnitude higher. We present details on LO-phonon emission rates in typical QCL structures. This calculation supports the statement that low depopulation selectivity might be an essential feature of QCL in few THz spectral range.

We report on tunable submillimeter-wave radiation sources based on micrometer-sized superconducting tunnel junction arrays optimized within a bandwidth of 350-520 GHz. The arrays consist of 10, 20 and 40 Superconductor-Insulator- Superconductor (SIS) parallel-connected Nb/AlO_x/Nb junctions embedded in superconducting microstrip lines. A SIS twin-junction is integrated along with each array to detect output signals. The pumped detector’s I-V characteristic exhibits clearly photon-assisted quasiparticule steps when the arrays are biased upon corresponding Josephson resonances ranging from 370 to 520 GHz.

Nanometer size field effect transistors can operate as efficient resonant or broadband terahertz detectors, mixers, phase shifters and frequency multipliers at frequencies far beyond their fundamental cut-of frequency. This work is an overview of some recent results concerning the low temperatures operation, linearity, and circular polarization studies of nanometer scale field effect transistors for the detection of terahertz radiation. Also first results on graphene transistors are discussed.

We use two antenna model to develop a theory of the recently observed helicity-sensitive detection of terahertz radiation by FETs. The effect is due to the mixing of the ac signals produced in the channel by the two antennas. We obtain the helicity-dependent part of the photoresponse and its dependence on the antenna impedance, gate length, and gate voltage.

CEA-Leti has developed a monolithic large focal plane array bolometric technology optimized for 2D real-time imaging in the terahertz range. Each pixel consists in a silicon microbolometer coupled to specific antennas and a resonant quarter-wavelength cavity. First prototypes of imaging arrays have been designed and manufactured for optimized sensing in the 1-3.5THz range where THz quantum cascade lasers are delivering high optical power. NEP in the order of 1 pW/sqrt(Hz) has been assessed at 2.5 THz. This paper reports the steps of this development, starting from the pixel level, to an array associated monolithically to its CMOS ROIC and finally a stand-alone camera. For each step, modeling, technological prototyping and experimental characterizations are presented.

Optimization routes to optimize the operation of InGaAs bow-tie diodes were demonstrated using simulations based on 3D finite-difference time-domain method. Calculations of the electromagnetic field amplitude distribution in a detector plane were performed. Three types of designs – bow-tie diode itself, log-periodic antenna and log-periodic antenna coupled with the bow-tie diode were analyzed; it was determined that frequency properties of such antennacoupled device can be tuned via variation of antenna shape and size. Multi-pixel InGaAs bow-tie diodes arrays of different designs – narrow band and broadband operation – were designed and fabricated. They were demonstrated to be well suited for continuous wave room temperature spectroscopic terahertz imaging at 0.58 THz and at 1.63 THz.

We present a spatial characterization of terahertz (THz) beams generated from a two-color air plasma under different conditions by measuring full 3D beam profiles using a commercial THz camera. We compare two THz beam profiles emitted from plasmas generated by 35 fs and 100 fs laser pulses, and show that the spatial properties of the two THz beams do not change significantly. For the THz beam profile generated by the 35 fs pulse, the spatial effect of eliminating the lower frequencies is investigated by implementing two crossed polarizers working as a high-pass filter. We show that this reduces the beam waist, and that the beam spot shape changes from Lorentzian to Gaussian. Finally, we observe a forward-propagating Gaussian THz beam by spatially filtering away the conical off-axis radiation with a 1 cm aperture.

In this paper, we present review and latest results obtained in the scope of terahertz holographic and other methods for phase retrieval in terahertz imaging. Not only accurate change of amplitude, but also rigorous phase retrieval is essential for precise calculation of optical parameters of the samples in terahertz range. Pulse terahertz holography introduced some years ago shows itself as perfect method for overall-object phase retrieval technique, but in the same time it allows measurement with low signal to noise that leads to less precise derivation of sample optical parameters. And certainly just point-by-point terahertz time-domain spectroscopy provides the most precise information of sample phase, but it is rather time consuming and has low spatial resolution as well. The other possible way assumes, in contrary to pulse terahertz holography and spectroscopy, using narrow-band continuous terahertz source, which tunability might also make the measurement process easier. And diffraction patterns registered with microbolometer array or any other terahertz intensity sensor placed at several different distances from the object and/or taken for several different terahertz frequencies are used for phase retrieval in this case. We present both numerical predictions and experimental results for the proposed methods, estimate the achievable spatial and other limits of the techniques and compare them to the others used in different spectral ranges.

Terahertz (THz) imaging has shown promise for nondestructive evaluation (NDE) of a wide variety of manufactured products including integrated circuits and pharmaceutical tablets. Its ability to penetrate many non-polar dielectrics allows tomographic imaging of an object's 3D structure. In NDE applications, the material properties of the target(s) and background media are often well-known a priori and the objective is to identify the presence and/or 3D location of structures or defects within. The authors' earlier work demonstrated the ability to produce accurate 3D images of conductive targets embedded within a high-density polyethylene (HDPE) background. That work assumed a priori knowledge of the refractive index of the HDPE as well as the physical location of the planar air-HDPE boundary. However, many objects of interest exhibit non-planar interfaces, such as varying degrees of curvature over the extent of the surface. Such irregular boundaries introduce refraction effects and other artifacts that distort 3D tomographic images. In this work, two reconstruction techniques are applied to THz synthetic aperture tomography; a holographic reconstruction method that accurately detects the 3D location of an object's irregular boundaries, and a split­-step Fourier algorithm that corrects the artifacts introduced by the surface irregularities. The methods are demonstrated with measurements from a THz time-domain imaging system.

One of the problems, arising in using of Time-Domain THz spectroscopy for the security, consists in developing of probability assessment criterion for the detection and identification of the explosive and drugs. Usually the detection and identification occur using the reflected THz signal. Therefore, the key problem is the detection of absorption features, belonging to the explosive and drugs, in the THz signal reflected from a substance under investigation. After the solution of this problem, the next step is a development of the criterion for the quantitative assessment of explosive presence in analyzing substance. In this paper, we discuss properties (advantages and disadvantages) of proposed criteria and their possibilities for the assessment of presence of required substance. We discuss also briefly the algorithm of various criteria using for the detection of substance in real-time on the base of spectral dynamics analysis of reflected THz pulse.

Field effect transistors are promising detectors of THz radiation. They operate at room temperatures have high responsivity, low noise equivalent power, and fast response time. However, their linearity (dynamic range) and possibility of their application in the domain of high power radiation has not been yet sufficiently studied. We have investigated room temperature field effect transistors, detection at frequencies from 0.3 to 3 THz with power up to 100 kW/cm². Several types of HEMTs and MOSFETs operating in the broadband non resonant detection regime, have been investigated. To provide a wide range of incident THz radiation intensities we used continuous-wave and pulsed sources: backward oscillators, CO₂ pumped methanol laser, free electron laser, NH₃, D₂O, and CH₃F lasers. We find that the photoresponse of HEMTs and MOSFETs is linear in radiation intensity up to a several kW/cm² and then it saturates. The onset of the saturation depends on the radiation frequency and the transistor type. The observed saturation behavior can not be explained by the existing theoretical model which predict a square root like dependence of the photoresponse. We tentatively attribute the unusual features of the photoresponse saturation observed at high intensities considering high electric field transport phenomena, e.g., electron heating and electron velocity saturation.

In this paper we measured the dependence of the signal of the third harmonic and terahertz radiation on the mutual polarization of the first and second harmonic to compare the mechanisms of their generation. For this purpose we used the output radiation of laser system Spitfire pro with the following parameters: pulse duration 35 fs, central wavelength 800nm, repetition rate 1kHz, energy of the first and second harmonic are 2mJ and 240mJ respectively. The radiation was focused by parabolic mirror with focal length 7,5 cm in air. The energy of terahertz and third harmonic radiation was measured by Holey cell and photomultiplier respectively.

In this work, we present for the first time, two dimensional low loss single mode optical waveguides fabricated with EPO materials. The waveguide geometry has a rectangular shape and it is buried between two EPOCLAD layers. Its refractive index distribution has a step-index profile with an index difference of 8.6×10^-3. Realized by polymer deposition, it consists mainly, of a stack of EPOCLAD/EPOCORE/EPOCLAD layers sandwiched between two thick glass substrates. The waveguide dimensions are: 3 ×2.2 μm². A cleaved single mode optical fiber, dedicated to the visible range (658 nm), is used to perform all the optical characterizations. The structure demonstrates an overall insertion loss of 2.6 dB and a numerical aperture of 0.19. Such waveguides will be used to create standing wave interference pattern using a high reflectance mirror embedded at the waveguide exit. Sampling of the pattern using an array of nano-antennas deposited on top of the waveguide yields information about the spatial distribution of the electrical field inside the waveguide. Hence, the spectral content of the signal can be deduced and the developed architecture demonstrates a strong potential for spectroscopic applications.

GaP is investigated for photoconductive terahertz (THz) generation. It is shown that the atypical bandstructure of GaP, with a central high-mobility valley and low-mobility sidevalleys, can be exploited to form a transient high-mobility state. The subsequent scattering and relaxation of hot electrons into and within the lower-mobility sidevalleys leaves the material in a relaxed low-conduction state. The experimental and theoretical study shows that ultrafast transient mobility, occurring over 800 fs, can create broadband THz pulses with reduced recovery times (and low leakage currents). The impacts of these findings are discussed for efficient and portable next-generation THz systems.