Louise Bradley

Size-selective photoluminescence excitation spectroscopy in CdTe quantum dots

We present a photoluminescence and photoluminescence excitation study of CdTe quantum dots, prepared via a novel organometallic approach. The global photoluminescence (excited at the energy above the absorption edge) showed a red shift of 75 meV with respect to the first absorption peak. This band edge emission was found to be strongly dependent on the excitation photon. Resonant emission spectra showed a pronounced spectral shift and line narrowing with decreasing excitation energy. The resonant Stokes shifts were extracted from photoluminescence and photoluminescence excitation data. The minimum magnitude of the resonant Stokes shift of 14 meV was obtained at room temperature.

Method to improve the noise figure and saturation power in multi-contact semiconductor optical amplifiers: simulation and experiment

The consequences of tailoring the longitudinal carrier density along the active layer of a multi-contact bulk semiconductor optical amplifier (SOA) are investigated using a rate equation model. It is shown that both the noise figure and output power saturation can be optimized for a fixed total injected bias current. The simulation results are validated by comparison with experiment using a multi-contact SOA. The inter-contact resistance is increased using a focused ion beam in order to optimize the carrier density control. A chip noise figure of 3.8 dB and a saturation output power of 9 dBm are measured experimentally for a total bias current of 150 mA.

The use of wide-bandgap CuCl on silicon for ultra-violet photonics

γ-CuCl is a wide-bandgap (Eg = 3.395eV), direct bandgap, semiconductor material with a cubic zincblende lattice structure. Its lattice constant, aCuCl = 0.541 nm, means that the lattice mismatch to Si (aSi = 0.543 nm) is <0.5%. γ-CuCl on Si-the growth of a wide-bandgap, direct bandgap, optoelectronics material on silicon substrates is a novel material system, with compatibility to current Si based electronic/optoelectronics technologies. The authors report on early investigations consisting of the growth of polycrystalline, CuCl thin films on Si (100), Si (111), and quartz substrates by physical vapour deposition. X-ray diffraction (XRD) studies indicate that CuCl grows preferentially in the <111> direction. Photoluminescence (PL) and Cathodoluminescence (CL) reveal a strong room temperature Z3 excitonic emission at ~387nm. A demonstration electroluminescent device (ELD) structure based on the deposition of CuCl on Si was developed. Preliminary electroluminescence measurements confirm UV light emission at wavelengths of ~380nm and ~387nm, due to excitonic behaviour. A further emission occurs in the bandgap region at ~360nm.

Short pulse transmission characteristics in multi-contact SOA

An experimental characterisation of a multi-contact semiconductor optical amplifier using ultrashort optical pulses is presented. The SOA in question allows the injection of bias current through multiple independent electrical contacts, allowing the direct control of the carrier density. Picosecond-scale optical pulses are transmitted through the SOA. The non-linear effects on the pulse shape and spectrum after transmission are determined. It is found that the bias current distribution in the SOA is a significant factor in determining the extent of the non-linearities affecting the pulses. Additionally, amplification of negatively chirped pulses in the saturated SOA is found to reduce the spectral width of the pulses.

Structural and optoelectronic properties of sputtered copper (I) chloride

Copper (I) Chloride is a wide band gap semiconductor with great potential for silicon-based optoelectronics due to the fact that is closely lattice matched with silicon. This work examines the deposition of CuCl thin films by magnetron sputtering on silicon and glass substrates. Film structural and morphological properties are studied with X-ray diffraction and atomic force microscopy. Optical absorbance and luminescence spectra of CuCl thin films are analysed in order to study the excitonic features. The influence of deposition process parameters and post annealing on the film properties are also reported.

Temporal and spectral dependence on polarization of the input signal in a semiconductor optical amplifier

The polarization dependency of the instantaneous frequency variation and pulse width in a semiconductor optical amplifier is studied using second-harmonic generation frequency resolved optical gating. A reduction of the time-bandwidth product is measured.

3.8dB Noise figure in bulk semiconductor optical amplifier

The present paper reports numerical and experimental investigation of noise figure of a multi-section semiconductor optical amplifier. The designed amplifier shows a 3.8dB noise figure, which seems to be the lowest figure reported.

THE USE OF POLARIZATION EFFECTS IN SEMICONDUCTOR OPTICAL AMPLIFIERS TO PERFORM ALL-OPTICAL SIGNAL PROCESSING

As the need for higher and higher bandwidths in telecommunication systems continues, it is widely predicted that at some point in the future optical processing will need to be performed all-optically. Several techniques have been proposed to perform such all-optical signal processing. In this paper a technique based on the nonlinear rotation in the state of polarization of an optical signal injected into a bulk semiconductor optical amplifier is discussed. An experiment to perform wavelength conversion at 2.5 Gbit/s based on this effect is presented. The performance of the wavelength converter is tested in both co-and counter-propagation, and wavelength independent wavelength conversion is found in the non-inverted co-propagation setup.

Wavelength tunable optical mode lifetimes in photonic molecules: new concept for multiplexing and delaying an optical signal

In this paper, we report on the space selective optical switching and detailed lineshape analysis of m-resonances in a PM formed from two dielectric spherical microcavities with CdTe nanocrystals, which allow us to propose a new concept of a multi-channel, wavelength tunable delaying optical device. A layer-by-layer deposition technique provides controllable coating of the latex microspheres (3 mum in diameter) with a shell of close-packed nanocrystals of approximately 4 nm in size

Optical design of GaN resonant cavity LEDs emitting at 510nm for use in plastic optical fiber applications

The optimized optical design of GaN resonant cavity light emitting didoes (RCLEDs) emitting at 510nm for maximum extraction efficiency into numerical apertures (NAs) of 1.0 (total emission) and 0.5 (typical plastic optical fiber NA) are determined using a modeling tool based on the simulation of dipole emission in a multilayer structure. The optimization is performed for a metal-AlGaN/GaN DBR cavity structure as functions of the aluminum fraction in the DBR and the internal quantum well (QW) emission linewidth. The optimum number of DBR pairs is shown to depend on both these parameters together and the emission NA, and varies between 3 and 14. The maximum calculated extraction efficiency for a metal-AlN/GaN cavity structure, assuming a QW emission linewidth of 30nm, is 0.18 (0.055) into an emission NA of 1.0 (0.5). The position of the QW relative to the metal mirror is shown to be the crucial device parameter in determining the extraction efficiency of the RCLED. Simulations show farfield measurements should provide information on the position of the QWs in the cavity. The reduction in the spectral emission linewidth of the RCLED due to the cavity is also modeled.