Far-IR electroluminescence is investigated in quantum cascade structures. Narrow luminescence peak with a full- width at half maximum of 0.7 meV are measured at low excitation currents and low temperature. The temperature dependence of the luminescence efficiency is studied systematically up to 120K, showing the interplay between electron-electron and optical phonon emission. Electroluminescence is also studied in a structure based on a diagonal transition and compared with the magnetotransport data. Wider luminescence peaks and intrinsic instabilities are observed.

High-power unipolar GaAs/AlGaAs lasers emitting in the 14-15 micrometers wavelength range under optical pumping by a pulsed CO₂ laser are investigated. Operation of edge lasers with side-facet pumping gas well as broad-area lasers with normal-incidence pumping is demonstrated. We show that record high optical powers can be obtained from these quantum fountain unipolar lasers. Optical powers per facet as high as 6.6 W for edge lasers and 7.8 W for broad-area lasers are achieved with TM₀₀ mode emission. Extended tunability of the lasing wavelength, (Delta) (lambda) /(lambda) approximately equals 2.5 percent, is observed by varying the pump wavelength. Operating temperatures as high as 137 K are presently achieved. Application of quantum fountain unipolar lasers to CO₂ gas detection is demonstrated.

Electron-longitudinal optic phonon and electron-electron intersubband scattering rates are calculated for a variety of quantum well systems. It is demonstrated that the internal quantum efficiency of a Terahertz radiative intersubband transition can be greater than in the mid-IR at 4K, however by room temperature has fallen to around 20 percent. A study of the internal quantum efficiency of a Terahertz energy intersubband transition in double and single quantum wells, has shown that the vertical intrawell transition is more efficient than the diagonal interwell transition.

Quantum cascade lasers are coherent light sources in the mid-IR spectral region. They are based on resonant tunneling and optical transitions between discrete energy levels in the conduction band arising form size quantization in semiconductor heterostructures. QCLs have been demonstrated on GaInAs/AlInAs/InP and GaAs/AlGaAs outperforming existing semiconductor laser technologies in the mid-IR spectral range. The present paper reports the realization of a QCL based on GaAs/AlGaAs material designed with an emission wavelength of 9.3 micrometers . Specific properties inherent to this material system and their influence on laser operation are discussed in detail. The paper concludes with the presentation of a new waveguide concept, which offers considerable performance improvements.

We report on the design, growth and characterization of electrically pumped quantum cascade lasers (QCL) and LEDs, based on the GaAs/AlGaAs material system. Intersubband and interminiband optical transitions within the conduction band of a heterostructure are used to achieve spontaneous emission and lasing action. Samples are grown using solid source molecular beam epitaxy. Transport measurements, IR photocurrent spectral response, transmission and emission measurements are performed. Laser wavelength above ten microns are achieved. Peak powers are in the 200 mW range. Laser operation up to 100K and threshold current densities below 15 kA/cm² are recorded. Ridge lasers exhibit multimode spectra, typical for Fabry-Perot resonators. Room temperature spontaneous emission is recorded, showing the feasibility of a room temperature operating QCL on the GaAs/AlGaAs material system.

The operating principles and experimental results concerning far-IR lasers based on the intersubband hot holes optical transitions in crossed electric and magnetic fields as well as on the optically excited intracenter shallow impurity states are reviewed and discussed. The analysis of the state of the art and the possible directions of the development of p-Ge hot hole intersubband transitions laser and the result of the recent theoretical and experimental investigations of new THz media based on impurity transitions in Si doped by phosphorus are presented.

The observation of spontaneous emission from phosphorus doped Si with a doping concentration of 0.9 X 10¹⁵ cm^-3 and 2 X 10¹⁵ cm^-3 is reported. Population inversion between the 2p₀ state and the 1s(T) and 1s(E) states was achieved by optically pumping with a CO₂ laser. The spontaneous emission increased with pump power. Electrons could be excited from the 1p₀ state into the conduction band due to the absorption of background radiation or radiation from a far-IR probe laser. The frequency dependence of the absorption is in agreement with the cross section for photon ionization of a transition from the 2p₀ state into the conduction band.

Wavelength dependent properties of the p-Ge THz laser are reported for pulsed as well as for mode locked operation. The original small mirror laser outcoupler has been replaced by a mesh outcoupler, resulting in clear improvements of laser action. The optical output has been analyzed using a grating spectrometer and fast Schottky diode detectors. FOr 0.25 <EQ B <EQ 0.6T, 170-185 micrometers emission occurs. Laser action starts at short wavelength; during the pulse, longer wavelength components gain intensity, until simultaneous emission across the whole band occurs. With the mesh outcoupler instead of a small mirror, the small signal gain is found to increase, for instance from 0.015 cm^-1 to 0.04 cm^-1 at 172 micrometers . With the rf field modulation applied, 770 MHz mode locking of the laser is achieved at 172 micrometers , yielding a train of 100 ps FWHM pulses. For 0.5 <EQ B <EQ 1.4T, 75-120 micrometers emission is observed, dependent on both B and E field. Time-and wavelength dependence is complicated; often an oscillatory behavior of spectral components is seen. Although this effect complicates the formation of stable pulse trains under mode locked conditions, 140 ps pulses have been produced.

The state-of-the-art radio frequency (RF) power levels at millimeter- and submillimeter-wave frequencies as well as the low-noise properties make GaAs tunnel injection transit- time (TUNNETT) diodes and InP Gunn devices prime candidates for fundamental solid-state terahertz oscillators. As examples, RF power levels of < 130 mW at 132 GHz, > 80 mW at 152 GHz, and > 1 mW at 315 GHz were measured with InP Gunn devices, whereas RF power levels of 100 mW at 107 GHz and > 10 mW at 202 GHz were measured with GaAs TUNNETT diodes. These experimental results as well as performance predictions for these devices at terahertz frequencies are reviewed and compared with the potential and the capabilities of other two-terminal devices such as impact avalanche transit-time diodes and resonant-tunneling diodes. The paper also summarizes advances in fabrication technologies as well as the application of more recent device principles and power-combining schemes.

Cw-power generation from fundamental mode GaAs-IMPATT didoes is presently limited to frequencies well below 200 GHz. This restriction follows mainly from the losses in diode and resonator which exceed the negative resistance of the diode at elevated frequencies. However, the strong non-linearities of IMPATT diodes can successfully be used for harmonic mode operation to achieve rf-output power well above 200 GHz. For harmonic mode operation,the active device must be incorporated in a resonator which is terminated reactively at the fundamental frequency and on the other hand should be power matched at the harmonic frequency. The reactive termination of the fundamental wave can easily be achieved by a waveguide section with appropriate cut-off frequency. Matching at the harmonic frequency is aspired by the resonator geometry and a sliding short. The initial material for the applied GaAs-IMPATT diodes is grown by MBE technique. The devices are mounted on diamond heat sinks with extremely low parasitics to avoid additional losses. Experimental results are given for GaAs double-drift IMPATT devices above 200 GHz with 2 mW at 232 GHz and 1 mW at 242 GHz as best results. The experimental data are compared to theoretical simulations of both active device and applied resonator.

Recent results of millimeter wave generation with doped GaAs/AlAs superlattices are reviewed. The wide-miniband superlattices show negative differential conductance caused by Bloch oscillations of the miniband electrons. The millimeter wave emission is due to current oscillations driven by traveling dipole domains. The oscillation frequency is given by the ratio of the domain velocity and the superlattice length. The experimental results show that the oscillation frequency increases with increasing peak drift velocity, which strongly depends on the miniband width. Current oscillations in GaAs/AlAs superlattices up to a frequency of 103 GHz at a power level of about 0.5 mW are reported. The superlattice oscillator will be compared with resonant tunneling diode oscillators and Gunn diode oscillators. The possibility of using different material systems for the superlattices will be discussed.

The recent advances in telecommunication and teledetection systems are creating a demand for system developments in the upper part of the millimeter wave spectrum and beyond that available in this frequency range. It is thought that novel device schemes, relying on quantum effect,s will play an increasing role in this development. At last, new micromachining technologies are currently being developed for these wavelengths not only for the fabrication of passive components but also for active devices. This paper gives an overview of the Terahertz components, currently fabricated at IEMN, which are aimed at operating in THz receivers. This concerns mixers and harmonic multipliers, engineered for ultra-fast electron dynamics and strongly reactive and resistive non linearities. Special attention has been paid to high performance InP-based Heterostructure Barrier Varactors for harmonic multiplication. Double Barrier Heterostructure Resonant Tunnelling Didoes for fundamental generation and T-gate Schottky's for sub- harmonic mixing. Novel ideas will be presented in order to control the particle and displacement currents and to overcome the intrinsic and extrinsic limitations.

In this communication, we report on the design and the fabrication of quantum well barrier varactor structures with state of the art results in terms of capacitance ratio over a narrow voltage range. Basically, the fact to consider is a barrier cladded by two quantum wells with respect to a single barrier heterostructure. It has several consequences for the non linear character of the device. The capacitance mechanism is governed at low voltage by the electron population rates off the quantum well rather than the conventional depletion mode process. A true band gap capacitance engineering is here demonstrated with thee kinds of structures either in the InP material system with a InGaAs/InAs/AlAs heterostructure or in the GaAs material system with GaAs/InGaAs/AlAs pseudomorphic epilayers and lattice matched AlGaAs/GaAs/AlAs heterojunctions. Self- consistent simulations, based on the solution of Poisson and Schroedinger coupled equations system, were performed in order to calculate the electron wave function and the conduction band bending. High capacitance ratios can be predicted depending on material parameters and structure geometry. Test samples were then fabricated and rf tested. The devices very high capacitance ratios is excess of 5 to 1 over a 1 Volt range.

Two low-temperature-grown GaAs photomixers were used to construct a transmit-and-receive module that is frequency agile over the band 25 GHz to 2 THz, or 6.3 octaves. A photomixer transmitter emits the THz difference frequency of two detuned diode lasers. A photomixer receiver then linearly detects the THz wave by homodyne down conversion. The concept was demonstrated using microwave and submillimeter-wave photomixers. Compared to time-domain photoconductive sampling, the photomixer transceiver offers improved frequency resolution, spectral brightness, system size, and cost.

Recently, we grew organic ionic-salt crystals of 4- dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST) with extremely large nonlinearity, and also realized dual wavelength oscillation of an electronically tuned Ti:Sapphire laser. In this report, the generation of a coherent THz-wave from DAST crystal was demonstrated for the first time by the difference frequency generation of a dual- wavelength oscillating Ti:Sapphire laser.

The prospects for using photonic bandgap effects in suitably fabricated 1D waveguide photonic microstructures to enhance the strength of the electrooptic interaction at terahertz frequencies are considered. Both ferroelectric and electrooptic semiconductor are assessed. More work will be required, both on measuring material properties and in the conception and fabrication of new device structures.

An idea to build a solid-state analog of vacuum traveling and backward wave tubes operating in the THz frequency range has long ben discussed in the literature. Experiments directed to the realization of this idea using the radiative decay of grating-coupled 2D plasmons in semiconductor heterostructures have been made since 1980, however the intensity of emitted radiation remains too low so far. A general theory presented here describes the main physical phenomena underlying the operation principles of this kind of devices, and answers the two most important practical questions: why the devices have not worked properly, and what should be done to improve their characteristics. Particular attention is given to recently proposed ideas of using the quantum-wire gratings, instead of commonly employed metal ones. The use of the new type of gratings is shown to lead to a substantial reduction of the threshold velocity of amplifications, and to a very large enhancement of the gain. Physically, this is a consequence of the resonant interaction of plasma waves in the 2D electron layer and in the quantum-wire grating. Specific recommendations on how to build a tunable semiconductor traveling wave tube are formulated, unsolved theoretical problems are discussed.

We propose her a class of quantum well structures designed to achieve a coherent generation of THz radiation through a plasma instability. This can be achieved, without population inversion, if a dynamical inhomogeneity is built into the active region of the structure. We show, through self- consistent calculation of the non-equilibrium steady state, that such structures can be inherently unstable against growing charge fluctuations under a variety of conditions, including lack of population inversion. Preliminary calculations of the I-V characteristics of such structures are in good agreement with experimental results.

A brief review is given of pump-probe carrier lifetime studies with the free electron laser at FOM-Rijnhuizen, emphasizing work on mid-IR interband recombination in narrow gap semiconductors. We compare results on the lead salt system with earlier work on HgCdTe and As-rich InAs/InAsSb strained layer superlattices, where we have studied the suppression of Auger recombination by band structure engineering. The Auger coefficient for either the lead slat structures or the As-rich strained layer superlattice is suppressed by some 2 orders of magnitude by comparison with HgCdTe of an equivalent bandgap composition. In addition we have made time resolved studied of local modes in ionic crystals, where non-radiative decay plays an important role in the optical pumping cycle of laser gain media. We have studied the local modes created upon the introduction of a light impurity, in particular the H-ion, in CaF₂ in the spectral region 700 to 1200 cm^-1. We have employed a two pulse photon echo experiment to remove the inhomogeneous broadening of the vibrational ensemble and measure the transverse relaxation time.

Normal dopant species in III-V semiconductors from shallow donors or acceptors whose atomic-like transitions have energies of the order of 3-20meV which corresponds to the Terahertz region of the spectrum. It has been suggested that these levels could be utilized in an impurity based THz laser system developing a solid-state THz source from such a technology will require engineering of the energy levels to favor radiative recombination. In this paper we report initial experiments to measure the 1s-2p scattering rate for holes bound to Beryllium acceptors in a bulk GaAs epilayer using the European free electron laser facility FELIX. Two absorption lines were studied the so-called D and C lines at 167 cm^-1 corresponding to 1s-2p transitions of the Beryllium acceptors. At high pump powers these lines were saturated and it was possible to perform Pump-probe measurements to observe the recovery of the absorption as a function of time. The temperature dependence of the decays was also measured. The D and C transitions were found to decay with lifetimes of 360ps and 440ps respectively. This represents the firs direct measurement of these transition lifetimes which are much longer than those reported for intersubband scattering. The result are highly encouraging and support the concept of an impurity based Terahertz device for room temperature operation.

We review recent work on the scattering of electrons in doped quantum wells in high magnetic fields. Picosecond time-resolved far-IR measurements have allowed the determination of scattering lifetime between Landau levels as a function of applied field in quantum wells and bulk material. The measurements show a clearer suppression of the cooling rate when the Landau level spacing is equal to the phonon energy, and thus the results provide unambiguous evidence for the phonon bottleneck.

We describe a range of techniques developed by the Oxford group for use in conjunction with the Millimeter-wave Vector Network Analyzer in measurements of magnetic resonance and high-frequency conductivity, at extremely low temperatures and high magnetic fields. Included are a variety of resonant cavity techniques. The cylindrical geometry is used to produce high-Q tunable cavities, ideally suited to measurements of the frequency and temperature dependence of, for example, cyclotron resonance of carriers in GaAs- (Ga,Al)As heterojunctions. A family of rectangular cavities has been designed specifically for measurements of the angle-dependent high-frequency conductivity of organic molecular metals; these systems allow us either to rotate the whole cavity in the external magnetic field, thus measuring the dependence of a particular component of the conductivity tensor on magnetic field orientation, or to rotate the sample within the cavity, thus measuring different components of the magneto-conductivity. We also describe a non-resonant measurement using a pressure cell with optical access permitting experiments at up to 1.8 GPa. Examples of data obtained from each technique are included.

A new technique is presented enabling the combination of highly transparent superconducting weak links with mesoscopic devices. These can serve as on-chip millimeter wave sources working at frequencies in the range of 10-100 GHz suitable for photon-assisted transport experiments. We use a modified FM tip to plough grooves into superconducting material, thus defining Josephson junctions. The weak links are easily integrated within mesoscopic structures such as quantum point contacts or quantum dots with high accuracy in alignment. In combination with a quantum point contact we observe photoconductance signal. Embedding these versatile millimeter wave sources in single and multiple quantum dot structures enables us to investigate photon-assisted transport phenomena and their modifications due to near- field effects.

In the beginning of this article we briefly outline the working principle of terahertz imaging. This relatively new technique is based on THz time-domain spectroscopy and has the potential to lead to the first portable far-IR imaging spectrometer. For such a spectrometer many applications can be foreseen in the fields of biology, medicine, chemistry and material science. Here we present two biological applications. First we show that THz-imaging is an ideal tool for dendrochronology as it allows us to obtain density profiles of wood specimen. Secondly, we monitor water take up in plants after plant water stress.

An imaging system has been developed based on pulses of Terahertz (THz) radiation generated and detected using all- optical effects accessed by irradiating semiconductors with ultrafast pulses of visible laser light. This technique, commonly referred to as T-Ray Imaging or THz Pulse Imaging (TPI), holds enormous promise for certain aspects of medical imaging. We have conducted an initial survey of possible medical applications of TPI and demonstrated that TPI images show good contrast between different animal tissue types. Moreover, the diagnostic power of TPI has been elicidated by the spectra available at each pixel in the image, which are markedly different for the different tissue types. This suggests that the spectral information inherent in TPI might be used to identify the type of soft and hard tissue at each pixel in an image and provide other diagnostic information not afforded by conventional imagin techniques. Preliminary TPI studies of pork skin show that 3D tomographic imaging of the skin surface and thickness is possible, and data from experiments on models of the human dermis are presented which demonstrate that different constituents of skin have different refractive indices. Lastly, we present the first THz image of human tissue, namely an extracted tooth. The time of flight of THz pulses through the tooth allows the thickness of the enamel to be determined, and is used to create an image showing the enamel and dentine regions. Absorption of THz pulses in the tooth allows the pulp cavity region to be identified. Initial evidence strongly suggests that TPI my be used to provide valuable diagnostic information pertaining to the enamel, dentine, and the pump cavity.

We have applied a newly developed transient terahertz time- domain spectrometer to study the temporal development of the dynamics of photogenerated carriers in semiconductor materials. The study presented here include semi-insulating (SI) and low-temperature-grown (LT) GaAs. By measuring the detailed shape of a subpicosecond electrical field pulse (THz pulse) transmitted through the sample at a time T after excitation with a femtosecond laser pulse, the absorption coefficient and refractive index in the region between 0.1 THz and 3 THz can be measured with high accuracy. By varying the time T, the transient absorption and index spectra can be measured with subpicosecond time resolution. Temporal and spectral behavior of the carrier dynamics in SI and LT GaAs, in dependence of intensity and wavelength of the excitation pulse, is measured. We directly observe carrier scattering to the sidevalleys and the subsequent return of the carriers to the central valley. The experimental data strongly suggest that the transmission of the THz pulse through the photoconducting surface layer of the semiconductor can be described as instantaneous tunneling of the electric field through a metal-like barrier.

State-of-the-art coherent THz radiation sources are reviewed and inversion-less amplification mechanisms are presented, which are applicable to a wide variety of optically impulsively excited THz emitters. The amplification schemes will be experimentally demonstrated and fundamental limitations and prerequisites discussed, stressing analogies and differences to standard amplification by stimulated emission.

We introduce tow modifications of the technology of optoelectronic continuous-wave THz spectroscopy. The first relates to the optoelectronic generation of cw THz radiation by heterodyne down conversion of two leaser beam. We present a cw Ti:Sapphire laser with a simultaneous output at two independently tunable colors which are used for the mixing process. Two alternative cavity designs, a linear (alpha) - shaped cavity and a ring cavity, have been realized. A comparison with respect to tunability, bandwidth, stability and experimental handling will be given. The second improvement relates to the detection of cw THz radiation. We discuss detection schemes which are based on the electro- optic effect in nonlinear media and employ the concept of quasi-phase matching for the enhancement of the sensitivity.

Impulsive optical excitation of the lowest two conduction or valence subbands of a GaAs/AlGaAs double quantum well can lead to coherent THz emission associated with quantum beating of subband states. We find that in the conduction band the emission arises from a time varying intersubband polarization generally dominated by the beating of continuum rather than bound exciton states. This is apparent in the electric field and excitation energy dependence of the frequency and amplitude of the THz radiation. Wavepackets made up of these continuum excitons have dephasing times of several picoseconds even for excitation an otpical phonon energy above the lowest subband edge. The long lived coherence in partly attributed to the small energy difference between the eigenstates, which substantially reduces the number of relevant scattering events, and partly to the very similar dispersion of the subbands which restricts dephasing by interference. The effect of interference is revealed in systems with significant dispersion of the intersubband gap. Two examples are presented: the valence band of a double well and the conduction band in the presence of an in-plane magnetic field.

Propagation of free-space femtosecond THz pulses (T rays) through and past metal structures with dimension on the order of a wavelength has been studied. In waveguides with diameters close to one wavelength, it is found that the phase velocity can become superluminal and even infinite or negative. T rays that propagate past a 100-micrometers metal wire are delayed when the polarization is perpendicular and advanced when it is parallel. In this case, it is also observed that the centroid velocity can become superluminal. Many of the result do not conform to simple waveguide theory, because of multiple reflections of the evanescent waves inside the waveguide. This 'Fabry-Perot' effect for evanescent waves is the cause of the negative phase velocities below the waveguide cutoff.

Monolithic mm-wave integrated circuits have experienced strong improvements in operation frequencies in the last years. However, reports of III-V semiconductor transistors to have maximum frequencies as high as 400 GHz often stem from extrapolated measurements made at lower frequencies, due to bandwidth-limitations of the electronic equipment. The extrapolation of measurements at lower frequencies is insufficient for an accurate determination of the characteristics of passive or active elements in this frequency range. Another frequent restriction of conventional measurement techniques is that the signal can only be probed at specially designed interfaces. Optical sampling techniques allow the detection of electric fields with a high temporal and spatial resolution of 150 fs and 10 micrometers , respectively, at any point within or outside the device. In addition to S-parameter measurements at passive devices we demonstrate the spatial field distribution of an ultra-short electric pulse propagating through a band-stop filter with a broad stop-band probed via electrooptic sampling. To demonstrate the potentially high bandwidth of the measurement system the geometry of the stubs has been designed to show significant attenuation around a frequency f₀ equals 350 GHz.

The communication among molecules in a liquid takes place through intermolecular interactions. Molecules talk to each other through dipole moments, quadrupole moments, etc. or sometimes more directly through weak intermolecular bonds, for example, hydrogen bonds. Understanding the structure and dynamics of these intermolecular interactions have proven to be crucial in the quest to obtain a better molecular description of chemical reactivity. If for example, a molecule is vibrationally excited, either as a consequence of a chemical transformation or excitation by a laser, then the molecule will initially relax by intramolecular vibrational relaxation where predominantly the low frequency vibrations are excited. Subsequently, these low frequency modes relax by coupling to the low frequency intermolecular solvent modes. The time scale for these intermolecular interactions is in the range from 0.1 to 10 ps, corresponding to a maximum spectral density at a few THz, thus matching the spectral coverage of THz-time domain spectrometers. This paper will describe recent THz-TDS results on both polar and non-dipolar liquids, in particular emphasizing the relation between result obtained with THz- TDS, depolarized Raman scattering, and OHD-RIKES, and the study of hydrogen bonds in polar liquids and crystals.

We report gas absorption spectra and energetic material reflection spectra measured with an all-electronic terahertz (THz) spectrometer. This instrument uses phase-locked microwave sources to drive picosecond GaAs nonlinear transmission lines, enabling measurement of both broadband spectra and single lines with hertz-level precision, a new mode of operation not readily available with optoelectronic THz techniques. We take two approaches to coherent measurements: (1) spatially combining the freely propagating beams from two coherent picosecond pulse generators, or (2) using a more conventional coherent source/detector arrangement with sampling detectors. The first method employs a dual-source interferometer modulating each harmonic of one source with a precisely-offset harmonic from the other source - both sources being driven with stable phase-locked synthesizers - the resultant beat frequency can be low enough for detection by a standard composite bolometer. Room-temperature detection possibilities for the DSI include antenna-coupled Schottky diodes. Finally, we have recently introduced a reflectometer based on serrodyne modulation of a linearized delay line, using a technique that is process-compatible with pulse generator circuits.

Effect of amplification of far-IR radiation on light hole cyclotron resonance in Ge and InSb under the optical pumping by CO₂ laser radiation has been calculated using the quantum mechanical model of valence band states in strong magnetic field. The model is based on 6 by 6 Luttinger Hamiltonian for valence band including split-off hole subband. We have found strong resonant dependence of pumping efficiency on magnetic field that is explained by quantum resonance of intersubband absorption of CO₂ laser radiation. It was shown that at the optimal magnetic fields the cross-section of the gain can reach 2 X 10^-14 cm² for pumping power density 2 divided by 4 MW/cm².

We report a low cost and material independent fabrication technique to reproduce sub-millimeter 3D photonic crystals. The crystal is made by stacking mechanically machine dielectric substrates. Interstitial defects can be introduced in the structures. This technique is illustrated by experimental studies with highly resistive silicon based crystals with and without defects. A terahertz time-domain spectroscopy set-up is used for broad band transmission characterization of the crystals. Systematic measurements of the transmission characteristics for different crystal thicknesses and incident angles were performed. A wide compete photonic band gap centered at 265 GHz with a 19 percent band-gap to mid-gap frequency ratio, and excellent filtering properties are observed with only six crystal periods. The influence of the defects was experimentally studied and an external control of their mode's transmission coefficients is demonstrated. The transmission coefficient of the defect modes is controlled by illuminating the interstitial silicon defects with a 330 mW laser beam. Numerical simulations reproduce this behavior by modeling the illumination as an increase of the defect absorption.

A compact tunable terahertz (THz)-wave source that operates at room temperature has been realized by introducing a novel configuration of a laser-pumped parametric oscillator using a trapezoidal LiMbO₃ crystal. We used total reflection for the pump and resonated idler waves under noncollinear phase-matching conditions by using a trapezoidal LiNbO₃ crystal, so that the interacting position was located at the THz-wave exit surface, and direct radiation could be produced without any coup;ling devices. Continuously tunable coherent THz-wave radiation was successfully demonstrated at wavelengths from 130-310 micrometers with a maximum output of 45pJ/pulse. In addition, the THz-wave had an excellent, circular Gaussian-like beam profile with a divergence of 2.4 degrees. Out parametric method has several advantages over other methods. This easy to use compact system has wide tunability, coherency, a relatively high peak power, and a single fixed wavelength pump source. These features will be useful for a wide range of applications.

We have used a novel millimeter-wave magneto-optical technique to study the angle-dependence of the high- frequency conductivity of the molecular superconductor (kappa) -(BEDT-TTF)₂Cu(NCS)₂. The data strongly suggests that the superconducting gap has nodes directed along the b and c directions of the crystal, in agreement with recent theoretical predictions. This supports the idea that the superconductivity in (kappa) -(BEDT-TTF)₂Cu(NCS)₂ is d-wave in nature, and is mediated by spin fluctuations.

The measurements of the spatial coherence of a macroscopic ensemble of carriers excited coherently by femtosecond laser pulses is presented. The spatial coherence of the excited ensemble is derived from time- and spatially resolved measurements of the far-field THz-emission pattern. The analysis concentrates on surface field emitters, which are widespread broadband sources of coherent THz-radiation. We find that these emitters are fully spatially coherent for emission frequencies up to 1.6 THz, above which frequency the spatial coherence starts to decrease. For frequencies above 2.5 THz the spatial coherence of the emitter is limited to one THz wavelength.

Time-resolved THz imaging for the incidence-angle dependent 3D tomographic characterization of layered structures is presented. We illustrate the capabilities of the developed system on multi-layer ceramic samples used for solid oxide fuel cells. Diverse methods for determining unknown refractive indices are discussed. The significant influence of the angle of incidence of a THz imaging system on the measured signal is demonstrated, which can be exploited especially in Brewster-angle configurations to enhance the capabilities of any THz tomography system.

In the context of very high frequency components, where quantum well devices have already shown promise, we report, in this communication, one the optimization, realization and characterization of GaAs and InP-based resonant tunneling diodes. A fully planar technology has been achieved for a GaAs triple-well structure, with the use of ion implantation techniques, whereas mesa-etched technology together with airbridge integration have been used for the InP-based devices. In terms of performance, both material system have shown excellent DC characteristics with a current density of 60 kA/cm² associated with a current contrast of 6:1 for the GaAs-based RTD. An even higher current density has been achieved for the InP-based devices with 215 kA/cm² while still preserving a peak-to-valley current ratio of 9:1. In addition, 1 micrometers ²-area InP-based RTD's have been found to be unconditionally stable without the necessity of a stabilizing network. These anticipated properties for very small area devices, which meet the stability criteria, enables us to perform small signal characteristics over the whole range of the negative differential resistance region. Analysis of the measured scattering parameters up to 50 GHz shows an increase in the capacitance-voltage characteristics.

In this paper we have characterized the refractive indexed and absorption coefficient of negative photoresist NANO XP SU-8 from 0.1-1.6 THz using THz time-domain spectroscopy. Over the measured frequency range it was found that the refractive index is a relatively flat function of frequency, decreasing from 1.8 to 1.7. The value of absorption coefficient is seen to increase in a line fashion over the given frequency range, being 25 cm^-1 at 1 THz. From this data we have extracted functions of dielectric constant and dielectric loss tangent versus frequency, quantities which will be of use for future THz circuit design. In addition to these measurements, we have demonstrated two novel applications of SU-8. First, we have fabricated membrane-like features by using a multi-exposure photolithographic technique on a single layer of SU-8, and second we have utilized a thin layer of SU-8 as a patternable adhesion layer as part of a semiconductor epitaxial lift-off process designed to transfer III-V semiconductor epitaxial layers onto lower loss host substrates.

Cyclotron resonance of 2D holes in high-mobility undoped multi-quantum-well Ge/GeSi heterostructure has been studied in both 'classical' and quantizing magnetic fields. Effects of hole heating on 2D hole cyclotron resonance has been investigated. The calculations of 2D hole Landau levels in rectangular quantum well have been performed allowing to interpret the evolution of CR spectra in going from 'classical' to 'quantum' range.

This paper describes a Gauss-Hermite beam-mode analysis of the far-field radiation pattern of a sectoral horn antenna with a tapered slot in its upper broadwall. This type of antenna is suitable for integration with micro-machined rectangular waveguide. Examples having been fabricated for frequencies as high as 1.6 THz. Microwave scale model radiation pattern measurement are presented. A procedure for carrying out a beam-mode analysis from the measured far- field pattern rather than the usual case of a theoretical near-field distribution is described. The beam-mode analysis technique is of general applicability to all types of antenna.

We analyze the precision of a quasi-optical null-balance bridge reflectometer in measuring waveguide characteristic impedance and attenuation using a one-port de-embedding after taking into account errors due to imperfect coupling of two fundamental Gaussian beam. In order to determine the desired precision, we present in-waveguide measurements of characteristic impedance and attenuation for a WR-8 adjustable precision short in the 75-110 GHz frequency range using a Hewlett-Packard HP 8510 vector network analyzer.

Fermi-surface traversal resonance (FTR) is caused by the periodic motion of carriers in a magnetic field across open sections of Fermi surface (FS). Owing to the warping of the FS, the real space velocities of the carries oscillate, generating resonances in the high frequency conductivity which may be described by a semiclassical model. A rectangular resonance cavity, oscillating at 70 GHz, which can rotate in the external magnetic field, has been used to confirm the existence of the effect in the organic metal (alpha) -(BEDT-TTF)₂KHg(SCN)₄. The data contain a great deal of information about the FS, including the direction and anharmonicity of warping components. A quantum mechanical model is presented which predicts all of the features of FTR appearing in the semiclassical model. This confirms that FTR is a fundamental property of low- dimensional systems, existing under a very wide range of conditions.

We have demonstrated a low power, optically-controllable, THz attenuator capable of high contrast ratios using a mixed type-I/type-II quantum well sample. When high free-carrier densities are optically excited in the quantum wells by a cw-laser, the transmitted THz intensity can be controllably reduced. Normally in quantum well samples high carrier densities cannot be achieved using low power excitation densities, because the carrier lifetime is so short. This is not the case for out sample which consists of 20 periods of a narrow and a wide GaAs well. After electron-hole pairs are created via optical excitation in the narrow well, they are separated in space, because the electronics are rapidly transferred into the wide well via an x-valley in the barrier material. The carrier lifetime at low sample temperatures i therefore extremely long, 0.48 ms, leading to high carrier densities. Using an optical excitation power of 2.1 mW from a cw-HeNe laser, the transmitted THz intensity can be reduced by 60 percent.

RF power generation of IMPATT diodes at mm-wave frequencies is limited mainly by thermal effects. The challenge to obtain output power at 200 GHz and above can be met only by extremely high DC current densities to push the characteristic avalanche frequency near to the oscillation frequency. The active devices have to be optimized with regard to pure IMPATT mode operation, low break-down voltage and efficient heat dissipation. Since these three conditions influence each other, a compromise is necessary. From theoretical simulations a double-drift Read IMPATT diode structure result which is capable to be operated at DC current densities up to 225 kA/cm² for short pulses of 50 ns with maximum device temperatures below 500 K. The initial material is grown by MBE technique taking special care to control the thickness and doping concentration of the different GaAs layers. The individual devices are fabricated by standard photo-resist technology with an integrated gold cone on top which adjusts the encapsulation height in the used full height inductive post waveguide resonator. Thereby, a low loss device mounting in the resonator without parasitic elements and an optimum impedance matching of diode and resonator can be realized. By means of different double-drift Read IMPATT structures the experimental results in terms of RF output power, conversion efficiency. At a frequency of 210 GHz 0.25 W output power and 1.8 percent conversion efficiency were realized at a DC current density of 225 kA/cm². The experimental result are in good agreement with calculations applying a pulsed oscillator model.

A frequency tripler from 70 GHz to 210 GHz semi-insulating GaAs substrate is characterized. Single-barrier varactors with symmetrical capacitance-voltage characteristics are chosen as non-linear devices which simplifies the total tripler circuit, i.e. no idler and bias circuits are necessary. The varactors are monolithically integrated and connected by an air bridge. The matching network is realized by a microstrip network next to the varactor and fine tuning is obtained by backshorts in input and output waveguides. The whole circuit including the varactor is computed at the fundamental and harmonic frequency using a simulation program for the device and a finite element program for the circuit, respectively. The aim of this work is to optimize the matching network to achieve maximum tripler performance. Two different matching networks have been calculated indicating the influence on output power and efficiency. Experimental results are presented and compared to theory.

In airborne heterodyne receivers for remote sensing of the OH radical in the stratosphere, optically pumped methanol gas lasers on the 2.523 THz line ar used as local oscillators. In order to optimize their performance, a new simulation concept has been developed and is described here. A standard design procedure for SMMW ring lasers using a focusing mirror for optical pump beam guiding is sketched briefly. To make computations easier, the pump radiation distribution is averaged along the resonator. Using a quantum mechanical approach, a SMMW laser process model which takes into account the ac Stark effect, the Maxwell distribution and the Doppler effect can be derived. Transversal profile so pump and SMMW intensity, absorption and gain coefficient, vibrational and physical temperature serve as a base for the determination of the SMMW laser output power. Based on this model, a computer program named FIRL has been developed. Computed result obtained with the program are presented and consequences for construction of SMMW lasers are discussed. Two laser prototypes have been built and demonstrate good agreement of simulated and measured results, indicating that the described program is a qualified CAD tool for methanol laser development.

We have investigated a phonon-cooled NbN hot electron bolometric (HEB) mixer in the frequency range from 0.7 THz to 5.2 THz. The device was a 3.5 nm thin film with an in- plane dimension of 1.7 X 0.2 micrometers ² integrated in a complementary logarithmic spiral antenna. The measured DSB receiver noise temperatures are 1500 K, 2200 K, 2600 K, 2900 K, 4000 K, 5600 K and 8800 K. The sensitivity fluctuation, the long term stability, and the antenna pattern were measured and the suitability of the mixer for a practical heterodyne receiver is discussed.

KASIMIR initiative is a development program started in early 1996 by the ESA in order to advance the mm-and sub-mm-wave sensor technology for satellite-based atmospheric observations. The initial goal of the project is to build integrated antenna/mixer frontends at 650 GHz which are qualifiable for the low Earth orbit environment. All of the frontends will make use of the so-called integrated quasi- vertical Schottky diodes developed at Technical University of Darmstadt in Germany.

The paper present both our theoretical and experimental findings regarding a sub-harmonically pumped Quantum Barrier Mixer. A qualitative treatment is given with examples to demonstrate some unique features of a Quantum Barrier Device mixer, which cannot be offered by Schottky junctions, and which would be particularly suited to efficient operation at terahertz frequencies.

In this paper we describe the realization and electrical performance of a micromachined E-plane filter for operation at a central frequency of 90 GHz. The micro-machining technique employed here for the filter fabrication is based on the use of an ultra-thick photoresist, the EPON SU-8, which gives precise control of 2D and 3D structures at the 1-100 micrometers level. In the work described her, the E-plane ladder was micromachined and mounted in a conventional metal waveguide for electrical characterization purposes. The results show that the performance of the micromachined filter is comparable to its metal counterpart, with the additional advantages of a much lighter structure, greater ease of fabrication and at lower cost.