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

Compact all-quantum-dot-based tunable THz laser source

We demonstrate an ultracompact, room temperature, tunable terahertz (THz) generating laser source based on difference-frequency-driven photomixing in a coplanar stripline InAs/GaAs quantum-dot (QD) antenna pumped by a broadly tunable, high power, continuous wave InAs/GaAs QD laser diode in the double-grating quasi-Littrow configuration. The dual-wavelength QD laser operating in the 1150- 1301 nm wavelength region with a maximum output power of 280 mW and with tunable difference-frequency (277 GHz to 30 THz) was used to achieve tunable THz generation in the QD antenna with a photoconductive gap of 50 μm. The best THz output performance was observed at pump wavelengths around the first excited state of the InAs/GaAs QDs (∼1160 nm), where a maximum output power of 0.6 nW at 0.83 THz was demonstrated

Towards novel compact laser sources for non-invasive diagnostics and treatment

An important field of application of lasers is biomedical optics. Here, they offer great utility for diagnosis, therapy and surgery. For the development of novel methods of laser-based biomedical diagnostics careful study of light propagation in biological tissues is necessary to enhance our understanding of the optical measurements undertaken, increase research and development capacity and the diagnostic reliability of optical technologies. Ultimately, fulfilling these requirements will increase uptake in clinical applications of laser based diagnostics and therapeutics. To address these challenges informative biomarkers relevant to the biological and physiological function or disease state of the organism must be selected. These indicators are the results of the analysis of tissues and cells, such as blood. For non-invasive diagnostics peripheral blood, cells and tissue can potentially provide comprehensive information on the condition of the human organism. A detailed study of the light scattering and absorption characteristics can quickly detect physiological and morphological changes in the cells due to thermal, chemical, antibiotic treatments, etc [1-5]. The selection of a laser source to study the structure of biological particles also benefits from the fact that gross pathological changes are not induced and diagnostics make effective use of the monochromatic directional coherence properties of laser radiation

Close relationship between Bessel–Gaussian and conical refraction beams

We demonstrate that the conical refraction of the input elegant Laguerre–Gaussian beams can be effectively described through generalized Bessel–Gaussian light beams. We performed numerical simulations and show good agreement between the exact solution and our proposed Bessel–Gaussian approximation model. Physical clarity of the proposed model has allowed us to explain the transition of the classical double-ring pattern of conical refraction in the Lloyd plane into a multi-ring one and predict new phenomenon such as the Raman spot shift and dependence of the conical refraction ring radius on the value of the orbital angular momentum

Operation of quantum dot based terahertz photoconductive antennas under extreme pumping conditions

Photoconductive antennas deposited onto GaAs substrates that incorporate InAs quantum dots have been recently shown to efficiently generate both pulsed and CW terahertz radiation. In this Letter, we determine the operational limits of these antennas and demonstrate their extreme thermal breakdown tolerance. Implanted quantum dots serve as free carrier capture sites, thus acting as lifetime shorteners, similar to defects in low-temperature grown substrates. However, unlike the latter, defect-free quantum-dot structures possess perfect lattice quality, thus not compromising high carrier mobility and pump intensity stealth. Single gap design quantum dot based photoconductive antennas are shown to operate under up to 1 W of average pump power (∼1.6 mJ cm−2 energy density), which is more than 20 times higher than the pumping limit of low-temperature grown GaAs based substrates. Conversion efficiency of the quantum dot based photoconductive antennas does not saturate up to 0.75 W of pump power (∼1.1 mJ cm−2 energy density). Such a thermal tolerance suggests a glowy prospect for the proposed antennas as a perspective candidate for intracavity optical-to-terahertz converters

Photoelectric Properties of InAs/GaAs Quantum Dot Photoconductive Antenna Wafers

In this paper, the study of the photoconductivity in self-assembled InAs/GaAs quantum dot photoconductive antenna in the wavelength region between 1140 nm and 1250 nm at temperatures ranging from 13 to 400 K is reported. These antennas are aimed to work in conjunction with quantum dot semiconductor lasers to effectively generate pulsed and continuous wave terahertz radiation. For the efficient operation, laser wavelengths providing the highest photocurrent should be determined. To study the interband photoconductivity of quantum dot photoconductive antennas, at room and cryogenic temperatures, we employed a broadly-tunable InAs/GaAs quantum dot based laser providing a coherent pump with power exceeding 20 mW over a 100 nm tunability range. The quantum dot antenna structure revealed sharp temperature-dependent photoconductivity peaks in the vicinity of wavelengths, corresponding to the ground and excited states of InAs/GaAs quantum dots. The ground state photoconductivity peak vanishes with a temperature drop, whereas the excited state peak persists. We associate this effect with different mechanisms of photoexcited carriers extraction from quantum dots

Computational model of bladder tissue based on its measured optical properties

Urinary bladder diseases are a common problem throughout the world and often difficult to accurately diagnose. Furthermore, they pose a heavy financial burden on health services. Urinary bladder tissue from male pigs was spectrophotometrically measured and the resulting data used to calculate the absorption, transmission, and reflectance parameters, along with the derived coefficients of scattering and absorption. These were employed to create a "generic" computational bladder model based on optical properties, simulating the propagation of photons through the tissue at different wavelengths. Using the Monte-Carlo method and fluorescence spectra of UV and blue excited wavelength, diagnostically important biomarkers were modeled. Additionally, the multifunctional noninvasive diagnostics system "LAKK-M" was used to gather fluorescence data to further provide essential comparisons. The ultimate goal of the study was to successfully simulate the effects of varying excited radiation wavelengths on bladder tissue to determine the effectiveness of photonics diagnostic devices. With increased accuracy, this model could be used to reliably aid in differentiating healthy and pathological tissues within the bladder and potentially other hollow organs

Stealth dicing of sapphire wafers with near infra-red femtosecond pulses

The quality of the reflecting faces after dicing is critical for the fabrication of efficient and stable laser diodes emitting in the green–violet region. However, high-quality faces can be difficult to achieve for devices grown on a sapphire substrate as this material is difficult to cleave cleanly. We have therefore investigated a technology known as “stealth dicing”. The technology uses a pulsed laser to damage a plane of material inside of the wafer due to multi-photon absorption instead of cutting through the wafer surface. If the damage is induced in a line of stress points, the sample can then be cleaved easily along the damaged plane to leave a high-quality surface. The use of this technique also reduces thermal damage and debris