Natalia Bazieva

Continuous wave terahertz radiation from an InAs/GaAs quantum-dot photomixer device

Generation of continuous wave radiation at terahertz (THz) frequencies from a heterodyne source based on quantum-dot (QD) semiconductor materials is reported. The source comprises an active region characterised by multiple alternating photoconductive and QD carrier trapping layers and is pumped by two infrared optical signals with slightly offset wavelengths, allowing photoconductive device switching at the signals? difference frequency ~1 THz.(C) 2012 American Institute of Physics.

Multimodal spectral control of a quantum-dot diode laser for THz difference frequency generation

Generation of stable dual and/or multiple longitudinal modes emitted from a single quantum dot (QD) laser diode (LD) over a broad wavelength range by using volume Bragg gratings (VBGs) in an external cavity setup is reported. The LD operates in both the ground and excited states and the gratings give a dual-mode separation around each emission peak of 5 nm, which is suitable as a continuous wave (CW) optical pump signal for a terahertz (THz) photomixer device. The setup also generates dual modes around both 1180m and 1260 nm simultaneously, giving four simultaneous narrow linewidth modes comprising two simultaneous difference frequency pump signals

Efficient THz radiation from a nanocrystalline silicon-based multi-layer photomixer

In this paper we propose a novel type of multiple-layer photomixer based on amorphous/nano-crystalline-Si. Such a device implies that it could be possible to enhance the conversion efficiency from optical power to THz emission by increasing the absorption length and by reducing the device overheating through the use of substrates with higher thermal conductivity compared to GaAs. Our calculations show that the output power from a two-layer Si-based photomixer is at least ten times higher than that from conventional LT-GaAs photomixers at 1 THz.

Quantum-dot based ultrafast photoconductive antennae for efficient THz radiation

Here we overview our work on quantum dot based THz photoconductive antennae, capable of being pumped at very high optical intensities of higher than 1W optical mean power, i.e. about 50 times higher than the conventional LT-GaAs based antennae. Apart from high thermal tolerance, defect-free GaAs crystal layers in an InAs:GaAs quantum dot structure allow high carrier mobility and ultra-short photo carrier lifetimes simultaneously. Thus, they combine the advantages and lacking the disadvantages of GaAs and LT-GaAs, which are the most popular materials so far, and thus can be used for both CW and pulsed THz generation. By changing quantum dot size, composition, density of dots and number of quantum dot layers, the optoelectronic properties of the overall structure can be set over a reasonable range-compact semiconductor pump lasers that operate at wavelengths in the region of 1.0 μm to 1.3 μm can be used. InAs:GaAs quantum dot-based antennae samples show no saturation in pulsed THz generation for all average pump powers up to 1W focused into 30 μm spot. Generated THz power is super-linearly proportional to laser pump power. The generated THz spectrum depends on antenna design and can cover from 150 GHz up to 1.5 THz.

THz emission from quantum dot-based THz antennas pumped by a tunable quantum-dot laser diode

The THz optoelectronics field is now maturing and semiconductor-based THz antenna devices are becoming more widely implemented as analytical tools in spectroscopy and imaging. Photoconductive (PC) THz switches/antennas are driven optically typically using either an ultrashort-pulse laser or an optical signal composed of two simultaneous longitudinal wavelengths which are beat together in the PC material at a THz difference frequency. This allows the generation of (photo)carrier pairs which are then captured over ultrashort timescales usually by defects and trapping sites throughout the active material lattice. Defect-implanted PC materials with relatively high bandgap energy are typically used and many parameters such as carrier mobility and PC gain are greatly compromised. This paper demonstrates the implementation of low bandgap energy InAs quantum dots (QDs) embedded in standard crystalline GaAs as both the PC medium and the ultrafast capture mechanism in a PC THz antenna. This semiconductor structure is grown using standard MBE methods and allows the device to be optically driven efficiently at wavelengths up to ~1.3 μm, in this case by a single tunable dual-mode QD diode laser.

Progress in Compact Room Temperature THz Radiation Sources

There is currently a great deal of interest in the development of efficient compact sources of terahertz (THz) radiation. There are many factors of device and materials design which are still under ongoing improvement with the aim of producing an efficient, high power, room-temperature THz signal emission device which is tunable over a practical THz frequency range. The recent advances in production of various room-temperature THz radiation sources are reviewed with particular emphasis on photomixer antenna devices, and recent patents on technology and methods relating to: pulsed and continuous wave (CW) THz generation via nonlinear crystals; quantum cascade lasers (QCLs); ultrafast diodes and photodiodes; and heterodyne sources are discussed.

Efficient THz Radiation from Nanocrystalline Silicon-Based Multilayer Photomixer

In this paper we propose and model a novel multiple-layer photomixer based on amorphous/nano-crystalline-Si. The output power from such a photomixer is at least 10 times higher than conventional LTG-GaAs photomixers at 1 THz.

Fibre-optic probe for fluorescence diagnostics with blood influence compensation

To minimise the influence of blood content on the fluorescence measurements in vivo, a fibre optical probe combining fluorescence and diffuse reflectance measurements was developed. For the inverse solution of the blood content recovery, a set of neural networks trained by the Monte Carlo generated learning set was used. An approach of fluorescence measurements triggered by simultaneous real-time measurements of blood content in living tissue during moderate changes in contact pressure of the optic probe is proposed. The method allows one to decrease the necessary pressure on the probe as well as increase the repeatability of the measurements. The developed approach was verified in a series of experiments on volunteers with fluorescence excitation at 365 nm and 450 nm. The proposed technology is of particular interest in the development of new fluorescence-based optical biopsy systems.

Generating tunable terahertz radiation with a novel quantum dot photoconductive antenna

The terahertz (THz) range of electromagnetic waves (0.1– 10THz)—which lies between the microwave and optical regions—is of great interest. This is mainly because this band of the electromagnetic spectrum includes the frequencies of rotational and vibrational spectra of complex (e.g., biological) molecules. Most dielectric materials are transparent in the THz region, and THz waves are already used in many biomedical applications (e.g., for the detection of dangerous and illicit substances, as well as for the diagnosis and treatment of diseases). Photoconductive antennas are the most-developed room-temperature sources of THz radiation. However, ultrafast low-temperature-grown gallium arsenide (GaAs)—which is typically used as a substrate for such antennas—suffers (because of its large band gap) from low thermal efficiency, low carrier mobility, and a pump limit at a wavelength of about 850nm. An alternative substrate material is thus required so that efficient and tunable THz radiation can be generated. Quantum dots (QDs) were first discovered in the 1980s,1 i.e., around the same time as photoconductive antennas.2 The properties of these particles are controlled by both their size and the material from which they are made. QDs exhibit discrete energy levels and are highly configurable, and QD materials are successfully used in multiple applications, e.g., for lasers,3 saturable absorbers,4 photovoltaic devices,5 and biosensors.6 To generate pulsed and continuous wave (CW) THz radiation, photoeffective semiconductors that also possess a high carrier mobility and ultrashort carrier lifetime are necessary (i.e., QD wafers perfectly meet all the requirements). Figure 1. Left: Schematic diagram of a quantum-dot-based photoconductive antenna. The structure—grown over a distributed Bragg reflector (DBR)—contains indium arsenide (InAs) quantum dots (QDs) within a gallium arsenide (GaAs) lattice. Right: Transmission electron microscope image of one of the InAs QDs.

Towards efficient and tunable generation of THz radiation from quantum dot based ultrafast photoconductive antennae(Conference Presentation)

We present our recent results on CW and pulsed THz generation in quantum dot(QD) based photoconductive antennae(PCA) pumped by ultrafast and dual wavelength semiconductor lasers. QDPCA substrate incorporates InAs QDs in GaAs matrix, thus keeping semiconductor carrier mobility at higher levels that is typical for SI GaAs, while QDs themselves serve as lifetime shortening centres, allowing to achieve subpicosecond operation as in LT-GaAs. Thus, such substrates combine the advantages and lacking the disadvantages of GaAs and LT-GaAs, which are the most popular materials so far, and thus can be used for both CW and pulsed THz generation. Moreover, by changing QD size and mutual allocation, effective pump wavelengths can be tuned in the range between 0.9-1.3 μm, which is well beyond the GaAs energies, hence compact and relatively cheap ultrafast and narrow line double-wavelength semiconductor and fibre pump lasers can be used for pumping such antennae for both pulsed and CW THz generation. For double wavelength operation of semiconductor lasers, we implement either stacked double volume Bragg gratings, or double-Littrow configuration with two independent diffraction gratings to achieve tunability of the generated THz signal. High thermal tolerance of QD wafers allowed pumping single-gapped antennae with lasers producing up to 250 mW of CW optical power at simultaneous double wavelength operation and up to 1W average optical power in pulsed regime. We show these QD based antennae combined with such pump lasers to generate pulsed and CW THz radiation that is superlinearly proportional to pump power and bias applied to antenna.