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
A concept for a terahertz laser in vapor-phase-grown homoepitaxial GaAs with spatially periodic doping profile was theoretically explored. Monte Carlo simulation of hole transport in multilayer delta-doped p-GaAs/GaAs structures in crossed electric and magnetic fields was performed to investigate possibilities of the terahertz amplification on intervalence-band light-to-heavy hole transitions. The results are compared to those calculated for uniformly doped bulk p-GaAs and recently proposed p-Ge/Ge structures. The improvement in the gain for delta-doped p-GaAs structures is about ~2-3 times over bulk p-GaAs. Terahertz laser generation in the considered GaAs device concept appears feasible, as is growth of structures with active thicknesses sufficient to support quasioptical cavity solutions at 100 μm vacuum wavelengths. Potential applications for the considered laser device include sensing of chem/bio agents and explosives, biomedical imaging, non-destructive testing, and communications.
Calculated terahertz gain for periodically delta-doped p-Ge films with vertical and in-plane transport and an orthogonal magnetic field are compared. Gain as a function of structure period, doping concentration, field strength, and temperature is calculated using distributions determined from Monte Carlo simulations. Both transport schemes achieve spatial separation of light holes from impurity layers and the majority of heavy holes, which significantly increases light hole lifetime and gain compared with bulk p-Ge lasers. For in-plane transport, an optimum doping period of 1-2 μm and a 10-fold increase in gain over bulk p-Ge are found. For vertical transport, the optimum period is 300-400 nm, and the gain increase found of 3-5 times bulk values is more modest. However, it is found that gain can persist to higher temperatures (up to 77 K) for vertical transport, while the in-plane transport scheme appears limited to 30-40 K.
A direct-write pulsed Nd:yttrium-aluminum-garnet laser treatment in an aluminum-containing gas was applied to the polished surface of an undoped Ge wafer. After KOH etching to remove metallic aluminum deposited on the surface, secondary ion mass spectroscopy (SIMS) revealed ~60-200 nm penetration for Al at a concentration of ~10<sup>17</sup> cm<sup>-3</sup>. Atomic force microscopy showed that surface roughness is much less than the measured penetration depth. Laser doping of Ge is a potential low cost, selective-area, and compact method, compared with ion-implantation, for production of high current ohmic contacts in Ge and SiGe opto-electronic devices.
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.
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.
Monte Carlo simulation of carrier dynamics and far-infrared absorption was performed to test the importance of electron-electron interaction in selectively doped multi-layer p-Ge laser at high doping concentration. The laser design exploits the known widely tunable mechanism of THz amplification on inter-sub-band transitions in p-Ge, but with spatial separation of carrier accumulation and relaxation regions, which allows remarkable enhancement of the gain. The structure consists of doped layers separated by 200 - 500 nm of pure-Ge. Vertical electric field (~ 1 - 2 kV/cm) and perpendicular magnetic field (~ 1 T) provide inversion population on direct intersubband light- to heavy-hole transitions. Heavy holes are found to transit the undoped layers quickly and to congregate mainly around the doped layers. Light holes, due to tighter magnetic confinement, are preferably accumulated within the undoped layers, whose reduced ionized impurity scattering rates allow higher total carrier concentrations, and therefore higher gain, in comparison to bulk p-Ge lasers. Preliminary results of the calculations show a possibility of laser operation at liquid nitrogen temperatures. Device design and diagnostics of CVD grown structure are presented. Combination of total internal reflection and quasi-optical cavity design provides high laser cavity Q.
Monte Carlo simulation of carrier dynamics and far-infrared absorption in a selectively-doped p-type multi-layer Ge structure with vertical transport was performed to test a novel terahertz laser concept. The design exploits the known mechanism of THz amplification on intersubband transitions in p-Ge, but with spatial separation of light hole accumulation regions from doped regions, which allows remarkable enhancement of the gain. The structure consists of doped layers separated by 300-500 nm gaps of pure-Ge. Vertical electric field (~ 1-2 kV/cm) and perpendicular magnetic field (~ 1T) provide inversion population on direct intersubband light- to heavy-hole transitions. Heavy holes are found to transit the undoped layers quickly and to congregate mainly around the doped layers. Light holes, due to tighter magnetic confinement, are preferably accumulated within the undoped layers. There the relatively small ionized impurity and electron-electron scattering rates allow higher total carrier concentrations, and therefore higher gain, than in bulk crystal p-Ge lasers. In contrast to GaAs-based THz quantum cascade lasers (QCL), the robust design and large structure period suggest that the proposed Ge structures might be grown by the technologically-advantageous chemical vapor deposition (CVD) method. The ability of CVD to grow relatively thick structures will simplify the electrodynamic cavity design and reduce electrodynamic losses in future THz lasers based on the presented scheme.
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.
Far-infrared p-Ge laser operation in an active crystal prepared by transmutation doping is demonstrated for the first time. Though saturated current density in the prepared active crystal is twice lower than optimal, the laser performance is comparable to that of good lasers made from commercially produced melt grown p-Ge. The current saturation behavior of this material confirms the expected higher doping uniformity over melt grown laser rods.