S. H. Withers

EXAFS study of rare-earth element coordination in calcite

Extended X-ray absorption fine-structure (EXAFS) spectroscopy is used to characterize the local coordination of selected rare-earth elements (Nd3+, Sm3+, Dy3+, Yb3+) coprecipitated with calcite in minor concentrations from room-temperature aqueous solutions. Fitting results confirm substitution in the Ca site, but first-shell Nd-O and Sm-O distances are longer than the Ca-O distance in calcite and longer than what is consistent with ionic radii sums for sixfold coordination in the octahedral Ca site. In contrast, first-shell Dy-O and Yb-O distances are shorter than the Ca-O distance and are consistent with ionic radii sums for sixfold coordination. Comparison of Nd-O and Sm-O bond lengths with those in lanthanide sesquioxides and with ionic radii trends across the lanthanide series suggests that Nd3+ and Sm3+ have sevenfold coordination in a modified Ca site in calcite. This would require some disruption of the local structure, with an expected decrease in stability, and possibly a different charge compensation mechanism between Nd and Sm vs. Yb and Dy. A possible explanation for the increased coordination for the larger rare-earth elements involves bidentate ligation from a CO3 group. Because trivalent actinides such as Am3+ and Cm3+ have ionic radii similar to Nd3+, their incorporation in calcite may result in a similar defect structure.

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.

Actively mode-locked p-Ge laser in Faraday configuration

Active mode locking of the far-infrared p-Ge laser has been achieved in the Faraday configuration of electric and magnetic fields applied to the laser crystal. The laser generates 200 ps pulses of 80–110 cm−1 radiation with a laser-cavity roundtrip frequency of 454 MHz. The mechanism of gain modulation by the external rf electric field is based on induced electric-field gradients inside the active crystal and requires less rf power than was found previously for Voigt geometry.

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.

Pulse separation control for mode-locked far-infrared p-Ge lasers

Active mode locking of the far-infrared p-Ge laser giving a train of 200 ps pulses is achieved via gain modulation by applying an rf electric field together with an additional bias at one end of the crystal parallel to the Voigt-configured magnetic field. Harmonic mode locking yields a train of pulse pairs with variable time separation from zero to half the roundtrip period, where pulse separation is electrically controlled by the external bias to the rf field.

Mode locking of far-infrared p-Ge lasers

Active mode locking of the THz p-Ge light-to-heavy-hole-band laser is reported yielding 200 ps FIR pulses with a few Watts peak power. It is achieved via radio-frequency (RF) gain modulation at one end of the laser crystal in Voigt configuration. Applying an additional bias voltage at the RF contacts allows one to optimize the gain and improve mode locking characteristics. This compensates an intrinsic bias onset caused by charging of the laser crystal due to the asymmetry of (warped-band) hole transport in crossed electric and magnetic fields for the orientation B/spl par/[112], E/spl par/[11~0] used in our experiments. A transition from single-pulse mode locking to second harmonic mode locking is observed for a crystal with roundtrip frequency equal to the RF frequency, and separation between the two pulses is tuned from zero to half the cavity roundtrip period by changing the external bias to the modulating RF field.

Actively mode-locked THz p-Ge hot-hole lasers with electric pulse-separation control and gain control

Electric control of the separation between two interleaved pulse trains from a far-IR p-Ge laser, which is actively mode-locked by rf gain modulation at the second harmonic of the roundtrip frequency, is demonstrated by changing the electric bias at the rf contacts. A suggested application is telemetry using pulse-separation modulation. Optimal operation of the laser on the light-to-heavy-hole transition requires strong, perfectly crossed electric and magnetic fields, but the experimental data reveal a electric-field component along the magnetic field caused by space-charge effects inside the laser crystal, even when the applied fields are perfectly orthogonal. Monte Carlo simulations together with a Poisson solver are used to discuss the various mechanisms behind these effects and to find the electric field inside the laser crystal. These calculations agree reasonably well with experimental data obtained so far, and show not only the significant impact that charging can have on the output of actively mode-locked p-Ge lasers, but also suggest that they strongly influence the average gain of p-Ge lasers in general.

Faraday-configured mode-locked p-Ge laser and p-Ge far-infrared amplifier

A solid-state broad-band amplifier of far-infrared radiation (1.5 - 4.2 THz) based on intersubband transitions of hot holes in p-Ge is demonstrated using two p-Ge active crystals, when one operates as an oscillator and one as an amplifier. A peak gain higher than usual for p-Ge lasers has been achieved using time separated excitation of the oscillator and amplifier. Active mode locking of the p-Ge laser has been achieved in the Faraday configuration of electric and magnetic fields with distinct advantages over Voigt geometry. The 200 ps pulses of 80-110 cm-1 radiation were achieved by local gain modulation from an applied rf electric field at the 454 MHz round trip frequency of the laser cavity.