We present the overview of the results on the development of compact THz setups based on the quantum dot photoconductive antennas obtained during the past five years. We demonstrate the potential of the InAs/GaAs Quantum-Dot based setups to become an efficient approach to compact, room-temperature operating CW and pulsed terahertz setups for spectroscopy and imaging. We describe the photoelectronic processes in quantum dot substrates and reveal the role of quantum dots in free carrier lifetimes and the formation of the ultrafast photocurrent. We demonstrate the operation mode of the proposed antennas in pulsed and CW regimes under resonant (carriers are excited only inside the quantum dots) and off-resonant (carriers are excited in the bulk volume of the substrate) pumps with compact quantum dot semiconductor lasers. The results allow suggesting the quantum dot based setups as a new approach to field condition compact THz sources for imaging and spectroscopy.
Over the past 20 years, research into Gallium Nitride (GaN) has evolved from LED lighting to Laser Diodes (LDs), with applications ranging from quantum to medical and into communications. Previously, off-the-shelf GaN LDs have been reported with a view on free space and underwater communications. However, there are applications where the ability to select a single emitted wavelength is highly desirable, namely in atomic clocks or in filtered free-space communications systems. To accomplish this, Distributed Feedback (DFB) geometries are utilised. Due to the complexity of overgrowth steps for buried gratings in III-Nitride material systems, GaN DFBs have a grating etched into the sidewall to ensure single mode operation, with wavelengths ranging from 405nm to 435nm achieved. The main motivation in developing these devices is for the cooling of strontium ions (Sr+ ) in atomic clock applications, but their feasibility for optical communications have also been investigated. Data transmission rates exceeding 1 Gbit/s have been observed in unfiltered systems, and work is currently ongoing to examine their viability for filtered communications. Ultimately, transmission through Wavelength Division Multiplexing (WDM) or Orthogonal Frequency Division Multiplexing (OFDM) is desired, to ensure that data is communicated more coherently and efficiently. We present results on the characterisation of GaN DFBs, and demonstrate their capability for use in filtered optical communications systems.
Laser diodes based on Gallium Nitride (GaN) are useful devices in a wide range of applications including atomic spectroscopy, data storage and optical communications. To fully exploit some of these application areas there is a need for a GaN laser diode with high spectral purity, e.g. in atomic clocks, where a narrow linewidth blue laser source can be used to target the atomic cooling transition. We report on the continuous wave, room temperature operation of a distributed feedback laser diode (DFB-LD) with high-order notched gratings. The design, fabrication and characterization of DFB devices based on the (Al,In) GaN material system is described. A single peak emission at 408.6 nm with an optical power of 20 mW at 225 mA and a side mode suppression ratio (SMSR) of 35 dB was achieved. Additionally, we demonstrate the use of a GaN DFB-LD as a transmitter in visible optical communications system. We also present results from a DFB-LD optimized for laser cooling of Sr+.
A pure Ce-doped silica fiber is fabricated using modified chemical vapor deposition (MCVD) technique. Fluorescence characteristics of a Ce-doped silica fiber are experimentally investigated with continuous wave pumping from 440 nm to 405 nm. Best pump absorption and broad fluorescence spectrum is observed for ~ 405 nm laser. Next, the detailed analysis of spectral response as a function of pump power and fiber length is performed. It is observed that a -10dB spectral width of ~ 280 mn can be easily achieved with different combinations of the fiber length and pump power. Lastly, we present, for the first time to the best of our knowledge, a broadband fluorescence spectrum with -10dB spectral width of 301 nm, spanning from ~ 517.36 nm to ~ 818 nm, from such fibers with non-UV pump lasers.
We have realised InGaN/GaN distributed feedback laser diodes emitting at a single wavelength in the 42X nm wavelength range. Laser diodes based on Gallium Nitride (GaN) are useful devices in a wide range of applications including atomic spectroscopy, data storage and optical communications. To fully exploit some of these application areas there is a need for a GaN laser diode with high spectral purity, e.g. in atomic clocks, where a narrow line width blue laser source can be used to target the atomic cooling transition. Previously, GaN DFB lasers have been realised using buried or surface gratings. Buried gratings require complex overgrowth steps which can introduce epi-defects. Surface gratings designs, can compromise the quality of the p-type contact due to dry etch damage and are prone to increased optical losses in the grating regions. In our approach the grating is etched into the sidewall of the ridge. Advantages include a simpler fabrication route and design freedom over the grating coupling strength.Our intended application for these devices is cooling of the Sr+ ion and for this objective the laser characteristics of SMSR, linewidth, and power are critical. We investigate how these characteristics are affected by adjusting laser design parameters such as grating coupling coefficient and cavity length.
Efficiency of commercial 620 nm AlGaInP Golden Dragon-cased high-power LEDs has been studied under extremely high pump current density up to 4.5 kA/cm2 and pulse duration from microsecond down to sub-nanosecond range. To understand the nature of LED efficiency decrease with current, pulse width variation is used. Analysis of the pulse-duration dependence of the LED efficiency and emission spectrum suggests the active region overheating to be the major factor controlling the LED efficiency reduction at CW and sub-microsecond pumping. The overheating can be effectively avoided by the use of sub-nanosecond current pulses. A direct correlation between the onset of the efficiency decrease and LED overheating is demonstrated.
Two blue (450 nm) light–emitting diodes (LED), which only differ in top p-GaN layer growth conditions, were
comparatively investigated. I-V, C-V, TLM, Electroluminescence (EL) and Photoluminescence (PL) techniques were
applied to clarify a correlation between MOCVD carrier gas and internal properties. The A-structure grown in the pure
N2 environment demonstrated better parameters than the B-structure grown in the N2/H2 (1:1) gas mixture. The mixed
growth atmosphere leaded to an increase of sheet resistances of p-GaN layer. EL and PL measurements confirmed the
advantage of the pure N2 utilization, and C(VR) measurement pointed the increase of static charge concentration near the
p-GaN interface in the B structure.