Rene Eichholz

Frequency modulation spectroscopy with a THz quantum-cascade laser

We report on a terahertz spectrometer for high-resolution molecular spectroscopy based on a quantum-cascade laser. High-frequency modulation (up to 50 MHz) of the laser driving current produces a simultaneous modulation of the frequency and amplitude of the laser output. The modulation generates sidebands, which are symmetrically positioned with respect to the laser carrier frequency. The molecular transition is probed by scanning the sidebands across it. In this way, the absorption and the dispersion caused by the molecular transition are measured. The signals are modeled by taking into account the simultaneous modulation of the frequency and amplitude of the laser emission. This allows for the determination of the strength of the frequency as well as amplitude modulation of the laser and of molecular parameters such as pressure broadening.

Multi-channel terahertz grating spectrometer with quantum-cascade laser and microbolometer array

We report on a terahertz absorption spectrometer, which combines a grating monochromator, a quantum-cascade laser (QCL), and a microbolometer camera. The emission modes of the laser are spectrally resolved by the monochromator and imaged onto the camera. An absorption cell is placed between the QCL and the monochromator, and the absorption spectrum of methanol around 3.4 THz is measured by integrating simultaneously the signal of each of its Fabry-Perot modes as a function of the laser driving current. The frequency coverage of the spectrometer is about 20 GHz.

Terahertz lasing from silicon by infrared Raman scattering on bismuth centers

Stimulated emission at terahertz frequencies (4.5–5.8 THz) has been realized by electronic Raman scattering of infrared radiation on bismuth donor centers in silicon at low temperatures. The Stokes shift of the observed laser emission is 40.53 meV which corresponds to the bismuth intracenter transition between the 1s(A1) ground state and the excited 1s(E) state. The laser has a low optical threshold and the largest frequency coverage in comparison with other Raman silicon lasers based on shallow donor centers. Time-resolved pump spectra enable the separation of donor and Raman lasing.

Multifrequency terahertz lasing from codoped silicon crystals

Stimulated terahertz emission in the range from 4.5 to 6.4 THz has been realized from a single silicon crystal doped by two hydrogen-like donor centers, phosphorus and antimony, when pumped by midinfrared radiation from a free electron laser. Intracenter as well as Raman lasing has been observed. Simultaneous laser emission from both donors occurs when the pump photon energy is sufficient for photoionization of the antimony donors. The laser processes of both donors are not influenced by each other. Therefore the codoping approach can be extended to other group-V donors including more than two dopants in a single crystal.

Using terahertz cascade lasers for determination of optical losses in active medium of silicon intracenter lasers

Terahertz-range intracenter silicon lasers have demonstrated significant progress over past decade in expanding the frequency operation range, in manipulation of a silicon matrix as well as in lowering of optical pump threshold. While terahertz gain realized in silicon lasers has been already defined, optical losses in the active media have not been yet measured experimentally. We applied continuous wave terahertz quantum cascade lasers, operating at 2.5 THz and 3.1 THz for direct measurements of the light losses at conditions of optical pumping with pump intensities exceeding laser thresholds for corresponding media. These measurements allow estimates of THz losses as well as determination of physical mechanisms mainly contributing for total THz absorption in optically excited silicon. It was found that absorption by so called D¯-centers, dynamically induced in optically excited silicon, cause major loss of THz emission. These losses, however, can be significantly reduced by a proper procedure of crystal growth and doping.

Multi-frequency terahertz stimulated emission from optically pumped co-doped silicon crystals

Stimulated terahertz emission at different frequencies between 4.5 THz and 6.4 THz has been realized from a single silicon crystal doped simultaneously by two hydrogenlike donor centers, which were optically excited on intracenter transitions at low lattice temperature.

Terahertz-range stimulated emission due to electronic nonlinear frequency conversion in silicon

Silicon-based semiconductors offer optically low-loss and high-thermal-conducting lattice for the broad-band terahertz active media that can be used in the range of 5-7 THz. We report on realization of the terahertz-range stimulated emission from monocrystalline natural and isotopically enriched silicon crystals doped by group-V donor centers due to nonlinear frequency conversion. Lasing in the frequency bands of 1.2 - 1.8 THz; 2.5 - 3.4 THz has been achieved from silicon crystals doped by phosphorus and in the frequency band of 4.6 - 6.4 THz from different donors under optical pumping by radiation of mid-infrared free electron laser at cryogenic temperatures. Analysis of the data shows that the emission in high-frequency band corresponds to electronic Stokes-shifted Raman-type lasing. The low-frequency bands indicate on high-order nonlinear frequency conversion processes similar to four-wave mixing accompanied by highenergy intervalley g-phonons and f-phonons of host lattice. These lasers supplement terahertz silicon lasers operating on transitions between donor states.