Andrei V. Muravjov

Neutron transmutation doped far-infrared p-Ge laser

A far-infrared p-type germanium laser with active crystal prepared from ultra pure single-crystal Ge by neutron transmutation doping (NTD) is demonstrated. Calculations show that the high uniformity of Ga acceptor distribution achieved by NTD significantly improves average gain. The stronger ionized impurity scattering due to high compensation in NTD Ge is shown to have insignificant negative impact on the gain at the moderate doping concentrations sufficient for laser operation. Experimentally, this first NTD laser is found to have lower current-density lasing threshold than the best of a number of melt-doped laser crystals studied for comparison.

Active mode locking of a p-Ge hot hole laser

The generation of 200 picosecond pulses of far-infrared radiation from a p-Ge hot hole laser (50–140 cm−1) was achieved due to active mode locking by electrical intracavity modulation of the gain.

Mode fine structure of the p-Ge Intervalenceband laser measured by heterodyne mixing spectroscopy with an optically pumped ring gas laser

Abstract Heterodyne mixing of the far infrared p -Ge intervalenceband pulse laser and the optically pumped continuous wave ring gas laser at 117.7 μm (CH 2 F 2 ), 118.8 μm (CH 3 OH) and at 184.3 μm (CH 2 F 2 ) revealed the p -Ge laser mode fine structure with a resolution below 1 MHz, the absolute mode linewidth and a mode tunability of 25 MHz during the p -Ge laser pulse due to sample heating. For the first time an acousto-optical spectrometer has been used in such experiments which analysed the mode spectrum during a single pulse in a 1 GHz band. We have combined the methods of grating, homodyne mixing and heterodyne mixing spectroscopy for the investigation of the p -Ge laser mode fine structure in the broad band emission regime with different resonators including a high-Q tunable resonator for single mode operation.

Gain of the mode locked p-Ge laser in the low field region

Following the earlier observation of active mode locking in the high field region of the Voigt configured p-Ge intervalence band laser, presently mode locking in the low field region is also reported. The experimental results on the effective small signal gain for active- as well as for self-mode locked operation are given.

Single axial-mode selection in a far-infrared p-Ge laser

Single axial-mode operation of the pulsed far-infrared p-Ge laser with an intracavity Fabry–Perot type frequency selector has been observed by means of Fourier-transform spectroscopy. A spectral resolution better than 1 GHz has been achieved on an ordinary continuous-scan spectrometer using the event-locked technique for pulsed emission sources. A laser active-cavity finesse of at least unity has been directly confirmed from the measured emission spectral width. Analysis of the envelope of the corresponding interferogram suggests that the finesse exceeds 10.

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.

Evidence for self-mode-locking in p-Ge laser emission

Investigations of the dynamics of the far-infrared p-Ge laser emission reveal strong periodic soliton-like intensity spikes with less than 100 ps duration. We interpret these spikes as self-mode-locking of p-Ge laser modes. The effect becomes more pronounced when a GaAs/AlGaAs/InGaAs quantum well structure on a semi-insulating GaAs substrate is inserted into the laser cavity.

Grating-gate tunable plasmon absorption in InP and GaN based HEMTs

Gate-voltage tunable plasmon resonances in the two dimensional electron gas of high electron mobility transistors (HEMT) fabricated from the InGaAs/InP and AlGaN/GaN materials systems are reported. Gates were in the form of a grating to couple normally incident THz radiation into 2D plasmons. Narrow-band resonant absorption of THz radiation was observed in transmission for both systems in the frequency range 10 - 100 cm-1. The fundamental and harmonic resonances shift toward lower frequencies with negative gate bias. Calculated spectra based on the theory developed for MOSFETs by Schaich, Zheng, and McDonald (1990) agree well with the GaN results, but significant differences for the InGaAs/InP device suggest that modification of the theory may be required for HEMTs in some circumstances.

The p-Ge terahertz laser-properties under pulsed- and mode-locked operation

The results of a detailed study of the optical output of the p-Ge hot hole terahertz laser for pulsed-locked, as well as for mode-locked operation, is reported in this paper. The recently developed technique to achieve active mode locking is described. Results on the shape of the pulses in the small-signal gain, as well as in the saturated gain regime under mode-locked operation, are given. These will be discussed in the light of new results on time- and wavelength-resolved experiments for normal pulsed operation. Under favorable conditions, it is found that trains of pulses with a full width at half maximum pulsewidth of 100 ps can be produced.

Tunable hot hole FIR lasers and CR masers

Abstract Basic principles for arranging population inversion and stimulated emission in hot hole systems at low temperature are discussed. Recent experimental results on observation of emissions from intersubband lasers, CR NEMAGs and CR lasers made from p-Ge are given.