Terahertz emission by diffusion of carriers and metal-mask dipole inhibition of radiation

Terahertz (THz) radiation can be generated by ultrafast photo-excitation of carriers in a semiconductor partly masked by a gold surface. A simulation of the effect taking into account the diffusion of carriers and the electric field shows that the total net current is approximately zero and cannot account for the THz radiation. Finite element modelling and analytic calculations indicate that the THz emission arises because the metal inhibits the radiation from part of the dipole population, thus creating an asymmetry and therefore a net current. Experimental investigations confirm the simulations and show that metal-mask dipole inhibition can be used to create THz emitters.Comment: 9 pages, 5 figures; Fixed figure

Ultrafast vertical-external-cavity surface-emitting semiconductor lasers

We describe recent advances in the development of optically pumped passively mode-locked semiconductor lasers; ultrashort pulse sources that begin to offer levels of pulse duration, beam quality, and average power that formerly belonged only to diode-pumped solid-state lasers (DPSSLs) based on impurity-doped dielectric gain media. Unlike dielectric gain media, however, III–V semiconductors exhibit immense spectral versatility, with alloy compositions allowing emission wavelengths spanning the spectrum from visible through to the mid-infrared. Within the past few years, it has been shown that strained InGaAs/GaAs quantum well lasers operating around 1 µm are capable of generating transform-limited pulse durations of 100 fs or less; and moreover, that sub-400-fs pulses with > 300 W peak power, and 1.5-ps pulses with ~ 500 W peak power can be generated. Very recently, material systems other than InGaAs quantum wells have been used to demonstrate femtosecond mode locking, with results reported for a self-assembled quantum dot laser, a 2-µm antimonide laser and a 1.5-µm indium phosphide device. The vertical-external-cavity surface-emitting semiconductor laser, or VECSEL, mode-locked under the influence of a semiconductor saturable absorber mirror in the external cavity, is thus capable of bridging the gap in performance between mode-locked edge-emitting diodes and DPSSLs. A particular advantage of VECSELs is that they operate easily at repetition frequencies in the 1–20 GHz range, where dielectric lasers tend toward Q-switching instability, whereas monolithic diodes become inconveniently large – the range addressed both by electronics, and by the optical resolution of simple grating devices

High peak power femtosecond pulse passively mode-locked vertical-external-cavity surface-emitting laser

We report a passively mode-locked vertical-external-cavity surface-emitting laser (VECSEL) producing 335-fs near-transform-limited pulses at a repetition rate of 1 GHz with an average output power of 120 mW at a center wavelength of 999 nm. The VECSEL was optically pumped with a power of 1.85 W at a wavelength of 830 nm. The peak power of the output pulses was 315 W.<br/

Variable repetition frequency femtosecond-pulse surface emitting semiconductor laser

We report a femtosecond-pulse vertical-external-cavity surface-emitting laser with a continuous repetition frequency tuning range of 8% near 1 GHz. A constant average output power of 56?±?1 mW and near-transform-limited pulse duration of 450?±?20 fs were observed across the entire tuning rang

Active stabilisation and timing jitter characterisation of sub-500 fs pulse passively modelocked VECSEL

An optical Stark modelocked vertical-external-cavity surface-emitting laser producing sub-500 fs pulses at a fundamental repetition rate of 1 GHz was actively stabilised by locking its output to a 10 MHz electrical oscillator using a programmable phase-locked-loop frequency synthesiser. The timing jitter was characterised by the von der Linde method and was found to be 190(12) fs in the bandwidth 300 Hz to 1.5 MHz

Wetting-layer-pumped continuous wave surface emitting quantum dot laser

We report a continuous wave 1 µm laser based on InAs Stranski-Krastanov quantum dots (SK-QD) which is optically pumped on a wetting layer absorption band at 915 nm. The slope efficiency of this laser relative to absorbed pump power was measured to be 56% with wetting layer pumping, 1.75 times larger than when pumped with 830 nm light absorbed into the barriers between the SK-QD layers. Compared to barrier pumping, wetting layer pumping benefits from a smaller quantum defect, with less heat deposited in the active region, at the expense of weaker pump absorption in the thin (~1 nm) wetting layer. When a 50 µm thick intracavity diamond heatspreader was contacted to the optically pumped gain structure, a 10-fold increase in output power, up to 2.25W, was obtained in the barrier pumped case. A much smaller 2-fold increase in power, to a maximum of 0.35 W, was seen for the wetting layer pumped case. The diamond heatspreader is more effective in removing heat from the active region, where it is deposited by barrier pumping, than from the substrate, which absorbs residual pump radiation in the barrier pumping case. A gain sample with a doubly periodic DBR to back reflect pump radiation, will allow the full potential of wetting layer pumping to be realised, both by increasing pump absorption due to the double pass through the active region, and by localising heat generation in the active region

High-repetition-rate subpicosecond source of fiber-amplified vertical-external-cavity surface-emitting semiconductor laser pulses

We report a 6GHz fundamental repetition-rate source of 545fs pulses with 1.5W average power at 1042.6nm, based on ytterbium-doped fiber amplification of a Stark mode-locked vertical-external-cavity surface-emitting semiconductor laser

Wetting-layer-pumped continuous-wave surface-emitting quantum-dot laser

Quantum-dot vertical-external-cavity surface-emitting lasers (QD VECSELs) are of interest due to their large inhomogeneous gain spectrum and the potential for temperature insensitivity and low threshold. We report a QD VECSEL which is wetting-layer-pumped using a 915-nm diode laser, reducing the quantum defect compared to barrier pumping. A slope efficiency of 56% relative to absorbed pump power was measured from an unprocessed sample with a heat-sink temperature of -30 °C, 1.75× higher than when barrier pumped using an 830-nm pump diode