A. V. Muravjov

Multilayer silicon cavity mirrors for the far-infrared p-Ge laser

Multilayer mirrors capable of > 99.9% reflectivity in the far infrared (70-200 microm wavelengths) were constructed using thin silicon etalons separated by empty gaps. Calculations indicate that only three periods are required to produce 99.9% reflectivity because of the large difference between the index of refraction of silicon (3.384) and the vacuum (1). The mirror was assembled from high-purity silicon wafers, with resistivity over 4000 omega cm to reduce free-carrier absorption. Wafers were double-side polished with faces parallel within 10 arc sec. The multilayer mirror was demonstrated as a cavity mirror for the far-infrared p-Ge laser. Dependence of reflectivity on design accuracy was considered.

Terahertz gain on intersubband transitions in multilayer delta-doped p-Ge structures

A far-infrared laser concept based on intersubband transitions of holes in p-type periodically delta-doped semiconductor films is studied using numerical Monte Carlo simulation of hot-hole dynamics. The considered device consists of monocrystalline pure Ge layers periodically interleaved with delta-doped layers and operates with vertical hole transport in the presence of an in-plane magnetic field. Population inversion on intersubband transitions arises due to light-hole accumulation in E⊥B fields, as in the bulk p-Ge laser. However, the considered structure achieves spatial separation of hole accumulation regions from the doped layers, which reduces ionized-impurity and carrier-carrier scattering for the majority of light holes. This allows a remarkable increase of the gain in comparison with bulk p-Ge lasers. Population inversion and gain sufficient for laser operation are expected up to 77K. Test structures grown by chemical-vapor deposition demonstrate feasibility of producing the device with sufficie...

Piezo-controlled intracavity wavelength selector for the far-infrared p-Ge laser

An electrically-controlled tunable intracavity frequency selector is demonstrated for the far-infrared p-Ge laser. The tunable laser mirror was driven by a piezo-element inside a liquid helium bath. This design allows very small controllable displacements of the tunable mirror that were impossible in previous mechanical systems. High-resolution spectroscopy of the laser output reveals the nature of mode-hop tuning characteristic for the tuning-element construction used.

BROAD BAND P-GE OPTICAL AMPLIFIER OF TERAHERTZ RADIATION

A solid state broad band amplifier of terahertz radiation (1.5–4 THz), based on intersubband transitions of hot holes in p-Ge is demonstrated. The gain is investigated as a function of applied magnetic and electric fields by transmission measurements using a laser system with two p-Ge active crystals, when one operates as an oscillator and one as an amplifier. A peak gain higher than usually reported for p-Ge lasers has been achieved using time separated excitation of the oscillator and amplifier. Distinct differences in gain dependence on applied fields are noted between low- and high-frequency modes of p-Ge laser operation.

Far-infrared p-Ge laser with variable length cavity

Operation of a far-infrared (1.5–4.2 THz) p-Ge laser in an open quasi-optical resonator is demonstrated. This contrasts with previous designs where mirrors were fixed to surfaces of the active crystal. Enhanced stability and tuning of the laser cavity length are demonstrated, which are steps toward continuous tunability without mode-hops. 2002 Elsevier Science B.V. All rights reserved.

Toward hot-hole THz lasers in homoepitaxial Si and GaAs with layered doping

A recently proposed THz laser concept in homoepitaxially grown p-Ge with layered doping is reviewed. Prospects for realizing a similar design in Si or GaAs are considered.

Amplification of terahertz radiation in delta-doped germanium thin films

A new geometry for the intersubband THz laser on delta-doped multi-layer Ge thin films with in-plane transport of carriers in crossed electric and magnetic fields is proposed. A remarkable increase of the gain compared to existing bulk p-Ge lasers is based on spatial separation of light and heavy hole streams, which helps to eliminate scattering of light holes on ionized impurities and the majority of heavy holes. Inversion population and the gain have been studied using Monte-Carlo simulation. The terahertz transparency of a CVD-grown delta-doped Ge test structure has been experimentally studied by intracavity laser absorption spectroscopy using a bulk p-Ge laser. A practical goal of this study is development of a widely tunable (2-4 THz) laser based on intersubband hole transitions in thin germanium films with the gain sufficient to operate at liquid nitrogen temperatures.

Sensitivity of long-wave infrared intracavity laser absorption vapor detector

A quantum cascade laser at IR wavelengths with an open external cavity presents an opportunity for spectral sensing of molecular compounds that have low vapor pressure. The sensitivity of such a system is potentially very high due to extraordinarily long effective optical paths that can be achieved in an active cavity. We demonstrate here an external cavity mid-IR QCL molecular absorption sensor using a fixed Fabry-Perot etalon as the spectrum analyzer. The system is sensitive to the water vapor present in the laboratory air with an absorption coefficient of just 9.6 x 10-8 cm-1. The system is sensitive enough to detect the absorption coefficient of TNT vapor at room temperature.

Terahertz spectroscopy of TNT for explosive detection

Transmittance spectra of solid and vapor samples of trinitrotoluene (TNT) in the spectral range 0.6 to 10 THz at resolutions up to 1 GHz are reported. Uniform solid samples of ~100 &mgr;m thickness gave stronger absorption and more resolved structure than previous studies. New absorption lines for TNT solid below 100 cm-1 are reported. A heated 10 m multpass White cell was used for spectroscopy of the vapor. Strong absorption bands yield unexpectedly large absorption cross sections for the anticipated saturated vapor pressure at the cell temperature, leaving their assignment to TNT in doubt. These results indicate that path lengths exceeding 10 m and temperatures higher than 40 C, or significantly higher instrumental sensitivity, are needed for sensing of TNT vapor in the spectral range 0.6 to 10 THz.

Terahertz amplification in delta-doped germanium films with in-plane transport

Amplification of terahertz radiation on intersubband transitions has been analyzed by numerical Monte Carlo simulation for p-type delta-doped Ge films with in-plane transport configuration of applied electric and magnetic fields. A significant increase of the gain is found, compared to existing bulk p-Ge lasers, due to spatial separation of light and heavy hole streams, which reduces scattering of light holes on ionized impurities and heavy holes. The considered device has potential as a widely tunable (2–4THz) laser with high duty cycle and operating temperatures up to 50K.