Terahertz optically pumped silicon lasers

Stimulated terahertz (THz) emission from silicon single crystals doped by group-V donors has been obtained by optical excitation with pulsed infrared lasers. Pumping by a conventional TEA CO2 laser results in lasing on discrete lines between 1.3 and 7 THz (see figure). Laser thresholds can be as low as 10 kW/cm2. They depend on the donors species and the laser mechanism. Intracentre population inversion is realized between particular excited states which are large-spaced due to the chemical shift of the donor binding energy. The lifetime of an electron in an excited state (up to ~70 ps) is determined by the efficiency of phonon-assisted nonradiative relaxation. Optical excitation by the emission of a frequency-tunable free electron laser results in two different types of lasing. At relatively low pump intensities (~1 kW/cm2) the intracentre mechanism of lasing is dominating. At pump intensities above ~100 kW/cm2 stimulated scattering of pump photons on transverse acoustic intervalley phonons can occur in the vicinity of an impurity atom. This results in laser emission in the frequency range from 4.6 to 5.8 THz. In this case the laser frequency can be tuned proportionally to the pump frequency

THz lasing from donor centers in uniaxially stressed silicon

Recent results of experimental and theoretical studies of THz stimulated emission from optically excited group-V shallow donor centers (P, Sb, As, Bi) in uniaxially stressed silicon are reviewed and discussed. Low-temperature intracenter phonon-assisted relaxation responsible for the population inversion of bound excited states, the lifetime of principle states and small signal gain available on donor intracenter transitions are considered. The summary of the laser performance differed for differing donor centers is analyzed. Special attention is paid to the effects of essential enhancement of small signal gain, laser threshold diminution and switching of the laser transition in As donor under the strain

Frequency tunability of the terahertz silicon laser by a magnetic field

By applying a magnetic field to the terahertz intracenter silicon laser the degeneracy of the donor states is lifted. In the case of a bismuth doped silicon laser operating at 6 THz it is demonstrated that this effect can be used to tune the emission frequency. The frequency change depends on the relative orientation of the magnetic field and the crystallographic axis of the laser. The tuning rate is 40 - 60 GHz/T. The frequency tunability is explained by the linear Zeeman effect which splits the 2p/pm donor state acting as the upper laser level in the Si:Bi laser

Stress-controlled phonon-impurity resonant interactions in terahertz silicon lasers

Silicon terahertz lasers operate at frequencies between 1 THz and 7 THz under conditions of optical mid-infrared or far-infrared pumping and low lattice temperatures (‹30 K). The intracenter laser mechanism is based on optical transitions between excited states of group-V donors P, Sb, As and Bi, while Stokes stimulated light scattering is responsible for lasing in Si:Sb. In both cases population inversion is realized due to different rates of the phonon-assisted intracenter relaxation of captured electrons. The donor states, resonantly coupled by the interaction with intervalley phonons in the Si lattice, exhibit the shortest lifetimes that determines the specific laser schemes in n-Si. External stress applied to a Si crystal can release the resonant phonon-impurity coupling. By this it changes the laser operation transition and its efficiency

THz lasing of shallow donors in stressed silicon crystals

Results of experimental and theoretical study of terahertz stimulated emission from optically excited group-V donors (phosphor P, antimony Sb, arsenic As, bismuth Bi) in uniaxially stressed and liquid helium cooled silicon crystal are summarized and discussed. It is shown that compressive force of 1-1.5 kbar for P, Sb and of 2-3 kbar for As, Bi applied along {100} crystallographic orientations results in the remarkable enhancement of the laser gain as well as THz emission efficiency and laser threshold intensity is decreased by the order of magnitude or even more. For As and Bi donors it is accompanied by a switching of the emission line because of the upper laser state change. The effect of uniaxial stress on donor lasing originates from energy shift of the conduction band valleys of silicon which split donor states changing their eigen-values and eigenfunctions. According to the calculations of phonon-assisted relaxation rates appropriate stress-induced donor modification increases the lifetime and makes pump efficiency of the upper laser states better. Thus THz laser performance of donors in silicon can be substantially improved by host crystal deformation

THz amplification based on impurity-and transitions in Si/GeSi heterostructures

Terahertz stimulated emission based on impurity-band optical transitions of phosphor donor centers embedded in Si/GeSi heterostructures is reported. THz emission was measured from selectively doped Si/GeSi structures excited by CO2 laser radiation. Amplification of 8-9 THz emission with the coefficient of 2-3 1/cm is obtained for structures with gently strained selectively doped Si layers (Nd ≈ 10^17 cm^-3) under pump density of 200 kW/cm2. Corresponding net gain taking into account small overlapping of active layer with THz mode is estimated to be ~ 200-300 1/cm. Experimental data demonstrate the possibility to use impurity-band transitions for THz laser action. The capability of Si/GeSi quantum cascade scheme which can support inverted population on donor-continuum transitions of Si conduction band is also analyzed

Terahertz silicon lasers

Stimulated donor and Raman Stokes emission has been achieved under intracenter excitation and photoionization of shallow donor centers in silicon. A pulsed laser emission of up to a few mW of peak power in the 1 - 7 THz frequency range has been obtained at low temperatures ( < 30 K). Thorough laser characterization and further optimization of the laser threshold and efficiency has been carried out

Terahertz lasers based on nonlinear frequency conversion in silicon

New principals to generate stimulated emission in terahertz frequency range from silicon doped by shallow donor centers have been demonstrated. Lasing in the frequency bands of 1.2 – 1.8 THz; 2.5 – 3.4 THz, 4.6 – 5.8 THz and 6.1 – 6.4 THz, has been achieved from silicon crystals doped by phosphorus and antimony to around 10^15 cm^-3 under optical pumping by radiation of mid-infrared free electron laser at cryogenic temperatures. Analysis of the data shows that the emission from silicon in the high-frequency bands corresponds to Stokes-shifted Raman-type lasing, the shift is determined by the 1s(E)-1s(A1) donor electronic resonance. The low-frequency bands indicate on high-order nonlinear frequency conversion processes accompanied by highenergy intervalley phonons of host lattice

Relaxation of upper laser levels in terahertz silicon lasers

Relaxation of lower exited states of group-V donors in silicon which serve as upper working states of intracenter silicon lasers has been experimentally determined. The measurements show that decay times of 2p0 and 2p± states lie in the range of 4-90 ps

Raman terahertz Si:P and Si:Sb lasers

In the past few years significant progress has been made towards silicon-based lasers mainly due to new approaches in the infrared wavelength range, such as silicon nanocrystals, A-centre mediated direct recombination, Si/SiO2 and Si/SiGe superlattices, porous silicon, erbium-doped silicon, silicon light-emitting diodes, as well as 1.67 µm Raman silicon laser. Recently achieved optically pumped silicon terahertz lasers involve light scattering at donor centre states coupled by resonant interaction with an intervalley transverse acoustic g-TA phonon in silicon. Raman-type Stokes stimulated emission has been obtained from silicon crystals doped by antimony and phosphorus donors at low temperatures when optically excited by radiation from a tunable infrared free electron laser. The photon energy of the terahertz laser emission is equal to the pump photon energy reduced by the energy spacing between a singlet and a doublet donor states in silicon. The lasers emit a few tenths of mWs in a few ps pulse in the frequency ranges of 4.6 - 5.8 THz (Si:Sb) and 6.0 - 6.4 THz (Si:P) and have a gain of ~10^-3 1/cm at a pump intensity of ~100 kW/cm2. Raman-type silicon lasers based on a light scattering on donor centres can be potentially expanded onto mid-infrared lasing, where they could have higher gain and operation temperatures