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

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

Conditional outcome of ecosystem engineering: a case study on tussocks of the salt marsh pioneer Spartina anglica

The salt marsh grass Spartina anglica is an important habitat-modifying ecosystem engineering agent that facilitates large-scale salt marsh formation by enhancing sediment accretion. It dominates many European tidal environments and is invasive in many other parts of the world. We question (1) to what extent the ecosystem engineering ability of patchy Spartina vegetation depends on large-scale abiotic processes, and (2) whether tussock shape provides an indicator for future lateral salt marsh development. Analysing the topography of 83 individual tussocks in contrasting environments revealed that there are 6 clearly distinguishable tussock shapes, and that the classical example of a sediment-accumulating dome-shaped tussock only occurs under a limited set of abiotic conditions. The outcome of habitat modification by S. anglica is shown to be conditional, depending on large-scale morphodynamics and sediment grain size. Resulting tussock shape provides a clear indication for the long-term development of the pioneer zone. Understanding of the conditional outcome of ecosystem engineering is highly relevant in this era of climate change and ongoing anthropogenic influences on coastal ecosystems

Verification of polarization selection rules and implementation of selective coherent manipulations of hydrogenic transitions in n-GaAs

Electrons bound to shallow donors in GaAs have orbital energy levels analogous to those of the hydrogen atom. The polarization selection rules for optical transitions between the states analogous to the 1s and 2p states of hydrogen in a magnetic field are verified using Terahertz (THz) radiation from the UCSB Free Electron Laser. A polarization-selective coherent manipulation of the quantum states is demonstrated and the relevance to quantum information processing schemes is discussed.Comment: 4 pages, 3 figures, accepted to Phys. Rev. B: Rapid Communication

Silicon Stokes terahertz laser

A Raman-type silicon laser at terahertz frequencies has been realized. Stokes-shifted stimulated emission has been observed from silicon crystals doped by antimony donors when optically excited by an infrared free electron laser. The Raman lasing was obtained due to resonant scattering on electronic states of a donor atom. ©2007 American Institute of Physic

Silicon donor and Stokes terahertz lasers

Two types of lasers based on hydrogen-like impurity-related transitions in bulk silicon operate at frequencies between 1 and 7 THz (wavelength range of 50–230 μm). These lasers operate under mid-infrared optical pumping of n-doped silicon crystals at low temperatures (less than 30 K). Dipole-allowed optical transitions between particular excited states of group-V substitutional donors are utilized in the first type of terahertz silicon lasers. These lasers have a gain about 1–3 cm-1 above the laser thresholds (more than 1 kW/cm2) and provide 10ps – 1μs pulses with a few mW output power on discrete lines. Raman-type Stokes stimulated emission in the range 4.6–5.8 THz has been observed from silicon crystals doped by antimony and phosphorus donors when optically excited by radiation from a tunable infrared free electron laser. The scattering occurs on the 1s(E)→1s(A1) donor electronic transition accompanied by an emission of the intervalley transverse acoustic g-phonon. The Stokes lasing has a peak power of a few tenths of a mW and a pulse width of a few ns. The Raman optical gain is about 7.4 cm/GW and the optical threshold intensity is about 100 kW/cm2

Silicon Donor and Stokes terahertz lasers

In the past few years significant progress has been made towards silicon-based lasers. Numerous approaches in the infrared wavelength range were elaborated to overcome the indirect bandgap structure of silicon, such as nanocrystals, A-centre mediated direct recombination, Si/SiO2 and Si/SiGe superlattices, porous silicon, erbium-doped silicon, and silicon light-emitting diodes. Mid-infrared 1.67 µm Raman silicon laser has been achieved. Recently developed optically pumped silicon-based THz lasers involve direct optical transitions between donor excited states (Intracentre Si Donor laser) or the light scattering at donor centre states coupled by resonant interaction with an inter intervalley transverse acoustic g-phonon in silicon (Brillouin-type Si Stokes laser). Different features of the population inversion mechanisms in the silicon lasers allow covering the frequency range from 1 to 7 THz with a few tenths of mW output power under pumping by mid-infrared pulsed lasers. The laser mechanisms continue a development of basic silicon lasers following the terahertz Si intracentre laser and the infrared Raman Si laser

Stimulated Terahertz Stokes Emission of Silicon Crystals Doped with Antimony Donors

Stimulated Stokes emission has been observed from silicon crystals doped by antimony donors when optically excited by radiation from a tunable infrared free electron laser. The photon energy of the emission is equal to the pump photon energy reduced by the energy of the intervalley transverse acoustic (TA) g-phonon in silicon (2.92 THz). The emission frequency covers the range of 4.6–5.8 THz. The laser process occurs due to a resonant coupling of the 1s(E) and 1s(A1) donor states (separation 2.97 THz) via the g-TA phonon, which conserves momentum and energy within a single impurity center