THz spectroscopy of the atmosphere has been driven by the need to make remote sensing measurements of OH. While the THz region can be used for sensitive detection on many atmospheric molecules, the THz region is the best region for measuring the diurnal behavior of stratospheric OH by remote sensing.THe IR region near 3 micrometers requires solar illumination. The three techniques for OH emission measurements in the THz region include Fourier Transform interferometry, Fabry-Perot interferometry, and heterodyne radiometry. The first two use cryogenic direct detectors while the last technique uses a local oscillator and a mixer to down convert the THz signal to GHz frequencies. All techniques have been used to measure stratospheric OH from balloon platforms. OH result from the Fabry-Perot based FILOS instrument will be given. Heterodyne measurement of OH at 2.5 THz has been selected to be a component of the microwave limb sounder on the Earth Observing System CHEM-1 polar satellite. The design of this instrument will be described. A balloon-based prototype heterodyne 2.5 THz radiometer had its first flight on 24 May 1998. Results from this flight will be presented.

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. The photomixer transmitter emits the THz difference frequency of two detuned diode lasers. The photomixer receiver then linearly detects the THz wave by homodyne down conversion. The concept was demonstrated using microwave and quasioptical photomixers. Compared to time-domain photoconductive sampling, the photomixer transceiver offers improved frequency resolution, spectral brightness, system size, and cost.

We developed a tunable, cavity-locked diode laser source at 850 nm for difference-frequency generation of coherent THz- waves. The difference frequency is synthesized by three fiber-coupled external-cavity diode lasers, where tow of the lasers are locked to adjacent modes of an ultra-stable Fabry-Perot cavity and the third laser is offset-phase- locked to the second cavity-locked laser using a tunable microwave oscillator. The first cavity-locked laser and the offset-locked laser produces the difference frequency, whose value is precisely determined by sum of integer multiple of free spectral range of the Fabry-Perot cavity and the offset frequency. The difference-frequency signal is amplified to 500 mW by the master oscillator power amplifier technique, simultaneous two-frequency injection-seeding with a single semiconductor optical amplifier. Here we demonstrate the difference-frequency generation of THz waves with the low- temperature-grown GaAs photomixers and its application to high-resolution spectroscopy of simple molecules. An absolute frequency calibration was carried out with an accuracy of approximately 10^-7 using CO lines in the THz region.

The mesoscopic structure of water has long been a subject of discussion. We postulate that, on the mesoscopic scale, liquid water forms nm-size ice-like crystals and that his structure is responsible for absorption in the THz-frequency range. However, until the recent development of Thz-time domain spectroscopy (THz-TDS), it was difficult to determine the optical constants in this frequency range with a good signal-to-noise ratio and hence to study the absorption properties of water. Here we report on the optical properties of water in the frequency range 0.05-1.4 THz and discuss the mesoscopic structure of water. We use THz-TDS based on photoconductive dipole antennas gated by a 150 femtosecond laser pulses to generate and detect the THz- frequency pulses. A new theoretical approach is also presented which were use to explain the absorption behavior in the measured THz frequency range. In this theory, molecular plasma oscillations of H₃O₂ complexes, that are distinctly separate from the H₅O₂⁺ complexes which form an underlying crystalline lattice, are assumed to be responsible for absorption in the THz- frequency range. This model provides good agreement to our data.

THz time-domain spectroscopy is a powerful and fast technique for measuring the complex dielectric constant of materials over a wide range of frequencies. This technique can be applied to the characterization of anisotropic materials only if the polarization of the THz beam impinging on the sample is perfectly known. In this paper, we show that the polarization of the THz beam radiated by a standard THz antenna, composed of a photoconductive switch and an extended hemispherical lens, is frequency dependent. We experimentally observe that the polarization of the THz field is linear but its direction varies over more than 10 degrees in the 100 GHz - 1 THz range. This variation can be attributed to a slight misalignment of the antenna. Using a thick slide of a highly anisotropic material acting as a polarization temporal separator, the incident THz pulse is divided into two pulses with orthogonal linear polarizations. By temporal windowing, we keep only one of the two linear polarizations, and thus free ourselves from the response of emitting antenna in terms of polarization. Then, a precise characterization of any anisotropic sample along one of its optical axis becomes possible.

THz time-domain spectroscopy is a powerful and fast technique for measuring the optical constants of materials over a wide frequency range. In this paper, we analyze the weight of the different error terms on the optical constants determination. The different sources of noise are taken into account in a phenomenological way and the noise power can be expressed as a second order polynomial function of the complex transmission coefficient modulus (rho) of the sample under test. Then, for given experimental conditions, the accuracy on the optical constants determination depends on (rho) . In the case of optically thick sample, the echoes of the THz pulse, caused by multiple reflections into the sample, are temporally well separated and can be time- windowed. For each echo, (rho) can be measured and it depends on the echo number. We demonstrate that the precision on the optical constants is greatly enhanced using the p^th echo, the value of p depending on the material refractive index and absorption. Moreover, as the coefficients of the polynomial function can be easily experimentally evaluated, quantitative information on the sources of noise is deduced.

When a biased semiconductor is excited by femtosecond laser pulses, it generated terahertz radiation. The rapid change in the transport photocurrent, due to the ultrafast excitation of the carrier by laser pulses and biased electric fields, generate terahertz electromagnetic pulses. The change in the photocurrent arises from two processes: acceleration of the photogenerated carries under an electric field, and a rapid change in carrier density. These two processes contribute to the generation of the terahertz radiation. Our calculations show that the main part of the terahertz radiation result from the ultrafast change of the carrier density. We consider the terahertz radiation from biased photoconductive antennas pumped by femtosecond laser pulses. The calculations are based on the Drude-Lorentz theory of the carrier transport in semiconductors. Our calculations model includes the interaction between electrons and holes, trapping of carriers in mid-gap states, scattering of carries, and dynamical space-charge effects. Our calculations show that the local electric field will oscillate and induce electromagnetic radiation at high carrier generation density.

TUnable Antenna-Coupled Intersubband Terahertz (TACIT) detectors use semiconductor quantum well heterostructures to offer tunable detection of light at few-Terahertz frequencies. TACIT detectors have been predicted to have background-limited sensitivity for a 300 K blackbody when operating in either a bolometric or non-bolometric mode. The speed of detection is expected to be 1 ns to less than 10 ps depending on the operating electron temperature and device dimensions. A planar metal antenna couples the incident Terahertz radiation from free space to the quantum well heterostructure. Electrons in the quantum well absorb the radiation, exciting them from the first to the second energy subband. The absorption frequency of the intersubband transition can be tuned by applying a voltage across the device. The quantum well heterostructure is designed so that the subbands have different electron mobilities. Absorption changes the relative number of electrons in each subband, and the effective mobility of the device changes. A current is applied to the active area of the quantum well, and the change in effective mobility is detected as a change in the in-plane resistance of the device. TACIT detectors are being fabricated. Modeling and experimental progress will be discussed.

The fabrication of 3D helical antennas, designed for the low THz frequency range, will allow for the efficient transmission and reception of circularly polarized radiation. This paper presents a novel method of fabricating helical antennas, singular and arrayed, for the low THz frequency range. The THz antenna structures are fabricated by using Laser Chemical Vapor Deposition (LCVD) to form fibers that can be grown into complex 3D structures directly on semiconductor substrates. By focusing the laser through a diffractive optic, arrays of antennas can be fabricated at the same time. THz radiation detection devices can be realized by combining the LCVD antennas with MEMS micro- bolometers that convert received THz radiation into a change in resistance. Arrays of these antenna-bolometer pairs can be fabricated on the same substrate to realize a THz imaging device.

Superconductive hot-electron bolometer (HEB) mixers have been built and tested in the frequency range from 1.1 THz to 2.5 THz. The mixer device employs diffusion as a cooling mechanism for hot electrons. The double sideband receiver noise temperature was measured to be approximately equals 2750 K at 2.5 K at 2.5 THz; and mixer IF bandwidths as high as 9 GHz are achieved for 0.1 micrometers long devices. The local oscillator power dissipated in the HEB microbridge was in the range 20- 100 nW. Further reductions in LO power and mixer noise can be potentially achieved by using Al microbridges. The advantages and parameters of such devices are evaluated. A distributed-temperature model has been developed to properly describe the operation of the diffusion-cooled HEB mixer. The HEB mixer is a primary candidate for ground based, airborne and spaceborne heterodyne instruments at THz frequencies.

Sideband generation is a method for producing tunable sources in the far IR frequency range by mixing a tunable microwave source with a fixed laser source to produce tunable sidebands. A 36 element array of planar Schottky diodes was used to mix the output of a CO₂ pumped laser at 1.6 THz with a 1-20 GHz microwave source to generate 5.9 (mu) W of DSB power for a conversion efficiency of 28 dB. The array produces sidebands by modulating the amplitude of the laser with a low duty cycle and no matching network which is not the optimal condition. For unmatched conditions at 180 degree phase modulation by a square wave with a 50 percent duty cycle will provide 4 dB SB conversion efficiency. This can be implemented by resonating an inductor with a varactor to obtain a short circuit and then modulating away form resonance for an open circuit. A proof of principle demonstration was implemented in waveguide at 80 GHz which resulted in 9 dB conversion efficiency for sinusoidal phase modulation and about 3 GHz bandwidth. This technique will be attempted at 1.6 THz in waveguide.

The technique of terahertz imaging has created potential application in science, industry and medicine. In this article, we describe an all-optical THz imaging system based on the optically rectified THz generation and electro-optic sampling detection. A novel polarization modulation technique introduced in this imaging system has improved the dynamics range. The optimal working position for the electro-optic sampling geometry using the nearly zero optical bias point has been found. The theoretical analyses behind the above techniques are also included. As examples, the THz images of mammographic phantom and the watermarks of several common currencies are presented to demonstrate the possible applications of this THz imaging system.

Since terahertz electric fields can couple strongly to quantum well intersubband transitions we expect interband optical properties of a semiconductor heterostructure to change resonantly under a THz driving field. By driving the excitonic intersubband resonance of an asymmetric quantum well with intense THz electric fields from a free electron laser, we modulate the transmission of a near-IR (NIR) laser beam at terahertz frequencies. This process manifests itself as the emission of optical sidebands on the NIR probe. In previous THz electro-optical studies in semiconductors, only even sidebands of frequency (omega) _sideband equals (omega) _NIR + 2n(omega) _THz had been observed. BY breaking inversion symmetry we are able to generate a comb of even and odd sidebands. The sidebands obey both THz and near-IR polarization selection rules and are enhanced when the NIR energy is resonant with a peak in the excitonic density of states. The ability to generate THz optical sidebands of all orders is important for the future application of THz EO effects in nonlinear spectroscopy and in ultrafast optical phase and amplitude modulation.

We discuss the Cross-Reststrahlen band phasematching technique for narrow-band three-wave interactions. In GaP, assuming a maximum practical crystal length of 100 millimeters, the calculations predict that with a source wavelength of 0.965 micrometers, a tuning range of greater than 50 GHz is possible around the perfectly phasematched 3.0 THz center frequency. When GaP is pumped with a source wavelength of 1.000 micrometers, the coherence length is at least 100 mm for a frequency range of greater than 600 GHz around the perfectly phasematched 630 GHz center frequency. High Resistivity GaP is available in cylindrical boules that are larger than 50 millimeters in diameter and 75 millimeters in length. GaP has a bandgap cutoff wavelength in the Visible at 0.55 micrometers and a simple phonon absorption spectrum with a single fundamental absorption at approximately 27 micrometers, which indicates a potential for high transmission in both the NIR and FIR. Any remaining FIR absorption can be attributed to free electron absorption and two phonon absorption processes. In this paper, we report new measurements of GaP regarding FIR absorption, optical damage threshold and optical quality. These measurements indicate that undoped, high-resistivity GaP single crystal can be used to generate THz waves.

Terahertz-frequency intersubband transitions in semiconductor quantum wells are of interest due to the potential for making devices which operate at THz frequencies, and the importance of many body interactions on the intersubband dynamics. We present measurements of the linear absorption linewidth of ISB transitions in a single 40 nm delta-doped GaAs/Al_0.3Ga_0.7As square quantum well, with a transition energy of order 10 meV. Separate back- and front-gates allow independent control of charge density and DC bias. The absorption linewidth is proportional to the dephasing rate of the collective excitation. In order to examine the dephasing dynamics at THz frequencies, we have begun a detailed measurement of the ISB absorption versus charge density.

We have investigated the terahertz photoresponse of a single semiconductor quantum dot, electrostatically defined by a sharp conducing Atomic Force Microscope tip in contact with a resonant tunneling diode structure. The quantum dot is excited by radiation from a Free Electron Laser in experiments both at room temperature and at cryogenic temperatures. Pronounced resonant tunneling features and classical rectification at frequencies from 0.3 to 3THz are observed in the I-V curves of these devices. These results demonstrate a novel approach to achieving terahertz excitation and studying transport in quantum dots.

We explored harmonic generation by Bloch oscillation in miniband superlattices driven by intense THz radiation from the UCSB free electron lasers, as a function of both THz intensity and applied DC bias. To accomplish this we integrated micrometers size superlattice mesas in a quasi-optical array which amplified a plane wave incident normal to the array and coupled it into the growth direction of the superlattice. We were able to successfully measure both second and third harmonic generation quantitatively. The harmonics are compared to a quasi-classical picture of Bloch oscillation.

Subharmonic generation in quantum wells may have important technological applications. Potentially it can be used to produce 'squeezed' states of THz radiation. Galdrikian and Birnir, Phys. Rec. Lett 29, 3308, derived density matrix dynamical equations for a many -body two-level system of electrons confined in an asymmetric square well. They showed numerically that the electron current density would undergo period-doubling bifurcations and thus produce subharmonics of the laser frequency (omega) . Their equations are averaged to second order at twice the frequency of the bare two-level system, (omega) ₀. It is found that at (omega) equals 2(omega) ₀ the period doubling bifurcation is similar to the one of the Duffing's equation.

A source of high-intensity, ultra-short terahertz pulses has been developed. The operation and performance of a terahertz pulse-slicing system for use with the UCSB free-electron lasers are discussed. Short pulses are sliced from the microsecond long output of the free-electron laser using laser-activated semiconductor switches; the pulse length may be freely varied from a few picoseconds up to four nanoseconds. The temporal response of a heavily compensated gallium-doped germanium photoconductor has been investigated. At low excitation intensity, a recombination time of 2 +/- 0.1 ns is found. At higher THz pulse powers non-exponential relaxation is observed; the data is well modeled using a rate equation approach and including impact- ionization impact-ionization effects due to the terahertz- heated free holes.

A semi-confocal etalon has been sued as a quasi-optical cavity to explore the dynamical conductance of Bloch- oscillating superlattices at terahertz frequencies. To maintain both DC and irradiated field uniformity and to maximize the coverage of the cavity mode with the devices of interest, the tunneling structures have been photolithographically fabricated into micro-sized mesa- isolated devices forming a quasi-optical square array interconnected by a metal grid with a period which is less than the wavelength in the semiconductor of the IR probe radiation. At a given bias on the device array and scanning the cavity through a resonance, the loss and reactance of the tunneling devices embedded in the array is measured by detecting a change in the position and line shape of the cavity resonance. Transmission measurements of the cavity loading by the biased quasi-optical arrays at frequencies from 250GHz to 3.0THz will be presented and compared to theoretical predictions.

Electric control of the separation between two interleaved pulse trains from a far-IR p-Ge laser, which is actively mode-locked by rf gain modulation at the second harmonic of the roundtrip frequency, is demonstrated by changing the electric bias at the rf contacts. A suggested application is telemetry using pulse-separation modulation. Optimal operation of the laser on the light-to-heavy-hole transition requires strong, perfectly crossed electric and magnetic fields, but the experimental data reveal a electric-field component along the magnetic field caused by space-charge effects inside the laser crystal, even when the applied fields are perfectly orthogonal. Monte Carlo simulations together with a Poisson solver are used to discuss the various mechanisms behind these effects and to find the electric field inside the laser crystal. These calculations agree reasonably well with experimental data obtained so far, and show not only the significant impact that charging can have on the output of actively mode-locked p-Ge lasers, but also suggest that they strongly influence the average gain of p-Ge lasers in general.

The electron beam in a scanning electron microscope (SEM) and a diffraction grating placed in the focal regime of the SEM have been used to generate coherent tunable radiation in the FIR region of the spectrum. A brief survey of the basic theory governing the operation of the device and a summary of recent experimental results are presented.

We have performed time-resolved terahertz (THz) - near-IR (NIR) two-color spectroscopy on InSb, using the Stanford picosecond free-electron laser synchronized with a femtosecond NIR Ti:Sapphire laser. The initial NIR pulse excites non-equilibrium electron and holes, which absorb the picosecond THz pulse. The time profile of the photo-induced absorption is a sensitive probe of intraband carrier relaxation dynamics. Using these techniques we have made the first observation of time-resolved cyclotron resonance (TRCR) of photo-created electronics in InSb for time delays from a few picoseconds to several tens of nanoseconds. This TRCR data shows possible evidence of a magnetic-field- induced LO-photon bottleneck effect. Furthermore, we have detected very unusual multi-component relaxation and photo- induced transparency under certain conditions.