Chris J. Fredricksen

High-resolution study of composite cavity effects for p-Ge lasers

The temporal dynamics, spectrum, and gain of the far-infrared p-Ge laser for composite cavities consisting of an active crystal and passive transparent elements have been studied with high temporal and spectral resolution. Results are relevant to improving the performance of mode-locked or tunable p-Ge lasers using intracavity modulators or wavelength selectors, respectively. It is shown that an interface between the active p-Ge crystal and a passive intracavity spacer causes partial frequency selection of the laser modes, characterized by a modulation of their relative intensities. Nevertheless, the longitudinal mode frequencies are determined by the entire optical length of the cavity and not by resonance frequencies of intracavity sub-components. Operation of the p-Ge laser with multiple interfaces between Ge, Si, and semi-insulating GaAs elements, or a gap, is demonstrated as a first step toward a p-Ge laser with an external quasioptical cavity and distributed active media.

Effects of Polymer Infusion and Characteristic Length Scale on Gold-Black Long-Wave and Far-Infrared Absorbance

Infrared absorbance is investigated in gold-black, a porous nano-structured conducting film. A two level full factorial optimization study with evaporation-chamber pressure, boat current, substrate temperature, and degree of polymer infusion (for hardening) was performed. Polymer infusion was found generally to reduce absorbance in the long wave IR but has little effect at THz wavelengths, although for samples with the highest absorbance there is a slight improvement in the absorbance figure of merit (FOM) in both wavelength regimes. The characteristic length scales of the structured films vary considerably as a function of deposition parameters, but the IR FOM is found to be weakly correlated with these distributions, which are determined by wavelet analysis of scanning electron micrographs.

SPECTRAL SIGNATURES OF ACETONE VAPOR FROM ULTRAVIOLET TO MILLIMETER WAVELENGTHS

This paper presents comparative analysis of different wavelength ranges for the spectroscopic detection of acetone vapor. We collected and analyzed original absorption line spectra arising from electronic transitions in the ultraviolet, near-infrared vibrational overtones, mid-infrared fundamentals, THz torsional modes, and mm-wave rotational transitions. Peak absorption cross sections of prominent spectral features are determined. The relative merit of each spectral range for sensing is considered, taking into account the absorption strength, available technology, and possible interferences.

Scanning Fabry-Perot filter for terahertz spectroscopy based on silicon dielectric mirrors

A scanning Fabry-Perot transmission filter composed of a pair of dielectric mirrors has been demonstrated at millimeter and sub-millimeter wavelengths. The mirrors are formed by alternating quarter-wave optical thicknesses of silicon and air in the usual Bragg configuration. Detailed theoretical considerations are presented for determining the optimum design. Characterization was performed at sub-mm wavelengths using a gas laser together with a Golay cell detector and at mm-wavelengths using a backward wave oscillator and microwave power meter. High resistivity in the silicon layers was found important for achieving high transmittance and finesse, especially at the longer wavelengths. A finesse value of 411 for a scanning Fabry-Perot cavity composed of three-period Bragg mirrors was experimentally demonstrated. Finesse values of several thousand are considered to be within reach. This suggests the possibility of a compact terahertz Fabry-Perot spectrometer that can operate in low resonance order to realize high free spectral range while simultaneously achieving a high spectral resolution. Such a device is directly suitable for airborne/satellite and man-portable sensing instrumentation.

Fixed wavelength selection for the far-infrared p-Ge laser using thin silicon intracavity etalon

A thin two-side polished silicon etalon is demonstrated as a fixed-wavelength intracavity selector for the far-infrared p-Ge laser. The active cavity finesse is ~ 0.1. The wavelength position and spectral purity are maintained over a wide range of laser operating fields. A p-Ge laser with such a selector may find application in chemical sensing, THz imaging, or non-destructive testing.

Quantum cascade laser intracavity absorption sensor

A mid-infrared intracavity laser absorption spectrometer based on an external cavity multi-mode quantum cascade laser is combined with a scanning Fabry-Perot interferometer is used as tunable narrow band transmission filter to analyze the laser emission spectrum. Sensitivity as a trace gas detector at 8.1 micron wavelengths has been demonstrated based on a weak water vapor line at an absorption coefficient of 1 x 10-5 cm-1. For molecules of reasonably strong absorption cross section (10-17 cm2), this corresponds to a detection limit of 40 ppb.

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 Acetone Vapor

Original terahertz spectra of acetone vapor-phase vibrational, torsional, and rotational transitions determine peak absorption cross sections for sensing applications.

Intracavity modulation of THz p-Ge laser gain by interband optical excitation

Optical quenching of the THz inter-sub-band p-Ge laser (tunable in the wavelength range 70-200 micron with ~1W output power) by Nd:YAG laser radiation has been investigated. YAG laser pulses were coupled into a p-Ge laser cavity through a SrTiO3 laser mirror, which is highly reflecting at cryogenic temperatures for THz frequencies and transparent for visible and near-IR light. Fast quenching of the p-Ge laser emission intensity was observed and attributed to free carrier absorption by optically generated electron-hole pairs in a thin layer of the active p-Ge crystal end surface. The effect also occurs when the interband absorption is confined to optically stimulated intracavity Si or GaAs spacers, which are transparent in the far-IR, placed between the SrTiO3 laser mirror and the active crystal end face. Such fast quenching of the p-Ge laser might be used to sharpen the trailing edge of the far-IR emission pulse for time-resolved or cavity-ring-down spectroscopic applications. Direct-gap semiconductor spacers might be used as fast, optically controlled intracavity modulators for active mode-locking.

Impact of temperature and gamma radiation on electron diffusion length and mobility in p-type InAs/GaSb superlattices

The minority carrier diffusion length was directly measured by the variable-temperature Electron Beam-Induced Current technique in InAs/GaSb type-II strain-layer-superlattice infrared-detector structures. The Molecular Beam Epitaxy-grown midwave infrared superlattices comprised 10 monolayers of InAs and 10 monolayers of GaSb to give a total absorber thickness of 4u2009μm. The diffusion length of minority electrons in the p-type absorber region of the p-type/barrier/n-type structure was found to increase from 1.08 to 2.24u2009μm with a thermal activation energy of 13.1u2009meV for temperatures ranging from 77 to 273u2009K. These lengths significantly exceed the individual 10-monolayer thicknesses of the InAs and GaSb, possibly indicating a low impact of interface scattering on the minority carrier diffusion length. The corresponding minority electron mobility varied from 48 to 65u2009cm2/V s. An absorbed gamma irradiation dose of 500u2009Gy halved the minority carrier diffusion length and increased the thermal activation energy to 18.6u2009meV, due to creation of radiation-induced defect recombination centers.The minority carrier diffusion length was directly measured by the variable-temperature Electron Beam-Induced Current technique in InAs/GaSb type-II strain-layer-superlattice infrared-detector structures. The Molecular Beam Epitaxy-grown midwave infrared superlattices comprised 10 monolayers of InAs and 10 monolayers of GaSb to give a total absorber thickness of 4u2009μm. The diffusion length of minority electrons in the p-type absorber region of the p-type/barrier/n-type structure was found to increase from 1.08 to 2.24u2009μm with a thermal activation energy of 13.1u2009meV for temperatures ranging from 77 to 273u2009K. These lengths significantly exceed the individual 10-monolayer thicknesses of the InAs and GaSb, possibly indicating a low impact of interface scattering on the minority carrier diffusion length. The corresponding minority electron mobility varied from 48 to 65u2009cm2/V s. An absorbed gamma irradiation dose of 500u2009Gy halved the minority carrier diffusion length and increased the thermal activation energy t...