Eric W. Nelson

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.

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.

Dielectric selective mirror for intracavity wavelength selection in far-infrared p-Ge lasers

A robust metal-free intracavity fixed-wavelength selector for the cryogenically cooled far-infrared p-Ge laser is demonstrated. The device is a back mirror consisting of a thin silicon etalon and dielectric SrTiO3 flat. A laser line width of 0.2 cm−1 is achieved, which corresponds to an active cavity finesse of ∼0.15. The wavelength position and spectral purity are maintained over a wide range of laser operating fields. Use of SrTiO3 lowers the laser resonance line frequencies by ∼1 cm−1 compared with expectations for metal mirrors. The effect is due to phase shift, which is determined from far-infrared reflectivity measurements of SrTiO3. A p-Ge laser with such selector is free from danger of electrical breakdown and mirror oxidation during repeatable thermal cycling, which makes it more reliable than previous selection schemes for practical applications.

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.

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.

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.

High field p-Ge laser operation in permanent magnet assembly

Operation of the far-infrared gallium-doped p-type germanium laser in the broadband range above 70 cm � 1 , which requires relatively high magnetic fields, is demonstrated using novel permanent magnet assemblies. NdFeB and SmCo magnet assemblies were compared. The applied field from a NdFeB assembly was found to decrease strongly on cooling to the cryogenic temperature required for p-Ge laser operation, but that of a SmCo assembly changes insignificantly. 2002 Elsevier Science B.V. All rights reserved.

Uniform acceptor distribution in neutron-transmutation-doped far-infrared p-Ge lasers

A neutron transmutation doped (NTD) far-infrared p-Ge laser crystal and a melt-grown p-Ge laser are analyzed and compared. Though the doping level in the NTD active crystal is twice lower than optimal, the laser performance is comparable to that produced from high-quality melt-grown crystals because of superior dopant uniformity. Compensation was examined by comparing results of neutron activation analysis with majority carrier concentration. Study of impurity breakdown electric field reveals better crystal quality in NTD. The current saturation behavior confirms the expected higher doping uniformity over melt grown laser rods.

Gain improvement for the THz p-Ge laser using neutron transmutation doped active crystal

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 negative factor of stronger ionized impurity scattering due to high compensation in NTD Ge is shown to be unremarkable for the gain at 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.

Fixed wavelength selection for the far-infrared p-Ge laser using patterned silicon etched mirrors

An etched silicon gold plated lamellar mirror is demonstrated as a fixed-wavelength intracavity selector for the far-infrared p-Ge laser, facilitating spectroscopic applications. The depth of the selective mirror, which defines the laser operation wavelength, can be precisely controlled during the etching process. The third-order Fabry-Perot resonance of this selector yields an active cavity finesse of at least 0.06.