Quantum dot materials for terahertz generation applications

Abstract

Compact and tunable semiconductor terahertz sources providing direct electrical control, efficient operation at room temperatures and device integration opportunities are of great interest at the present time. One of the most well-established techniques for terahertz generation utilises photoconductive antennas driven by ultrafast pulsed or dualwavelength continuous wave laser systems, though some limitations, such as confined optical wavelength pumping range and thermal breakdown, still exist. The use of quantum dotbased semiconductor materials, having unique carrier dynamics and material properties, can help to overcome limitations and enable efficient optical-to-terahertz signal conversion at room temperatures. Here we discuss the construction of novel and versatile terahertz transceiver systems based on quantum dot semiconductor devices. Configurable, energy-dependent optical and electronic characteristics of quantum-dot-based semiconductors are described, and the resonant response to optical pump wavelength is revealed. Terahertz signal generation and detection at energies that resonantly excite only the implanted quantum dots opens the potential for using compact quantum dot-based semiconductor lasers as pump sources. Proof-ofconcept experiments are demonstrated here that show quantum dot-based samples to have higher optical pump damage thresholds and reduced carrier lifetime with increasing pump power.

Compact All-Quantum-Dot-Based Tunable THz Laser Source

“…As it can be seen, the normalized (to the pump laser power) conductance peaks correspond to the ground state (GS) and the first ES revealed by the QD antenna structure. Although the laser power is significantly higher at QD GS energies, no CW THz signal was registered from PC antenna at these pump wavelengths, that perfectly agrees with the results previously demonstrated in the pulsed regime [25].…”

“…From our previous experiments [25], efficient THz radiation was expected under the pump in the vicinity of the QD excited states (ES), which for this very wafer, according to the photoluminescence spectrum (Fig.4), are around 1160 nm and at shorter wavelengths. The THz generation efficiency mostly depends on two main parameters of a semiconductor wafer: its photoconductance and carrier lifetimes.…”

“…As the aim of this work was to demonstrate broadly tunable THz generation in a photoconductive antenna pumped by a dual-wavelength tunable laser, the double-grating quasiLittrow configuration similar to that described in [20,25] was implemented. In this configuration, the radiation emitted from the back facet of the laser chip was collected with an ARcoated 40x aspheric lens (numerical aperture of 0.55) and then split by a non-polarizing beam splitter into two beams with each one of them coupled onto a diffraction grating (1200 grooves/mm).…”

“…An extra spacer layer of GaAs was grown under the active PC region on an AlAs/GaAs DBR of 30 layers. The need for the DBR is twofold: to reflect the pump beam thus reducing the IR power at the antenna output, and to allow the possibility for full optical cavity-type optimization of the structure [25]. A coplanar stripline design with a photoconductive gap of 50 µm and an overall contact thickness of 240 nm was used.…”

“…In this respect, semiconductor lasers, and InAs/GaAs quantum-dot (QD) lasers in particular, with their small size, high efficiency, reliability and low cost can offer broad near-infrared (1 -1.3 µm) wavelength coverage and wide tunability [23,24] T simultaneously [20]. At the same time, InAs/GaAs QD-based antennas capable of being pumped at very high optical intensities of higher than 1W optical power [25], i.e. about 50 times higher than the conventional low-temperature grown material based antennas, allow the development of an all-QD tunable THz laser system.…”