We report on the dynamics of micro-photoluminescence of single InP semiconductor nanowires trapped in a gradient
force optical tweezers. Nanowires studied were of zinc blende, wurtzite or mixed phase crystal poly-types and ranged in
length from one to ten micrometers. Our results show that the band-edge emission from trapped nanowires exhibits a
quenching of the initial intensity with a characteristic time scale of a few seconds and an associated spectral red shift is
also observed in the mixed phase nanowires.
Mesoporous silicon (PSi) photonic crystals have attracted interest as biosensing transducers owing to their high quality
optics and sensitivity in optical characteristics to changes in refractive index. We describe progress our group has made
derivatizing PSi towards devices for biology and medicine. PSi rugate filters display a high reflectivity resonant line in
the reflectance spectrum. As an example for biosensing, immobilization of peptides and biopolymers within the PSi is
demonstrated for detecting protease enzymes. Secretion of matrix metalloproteases from live cells was detected as a
blue shift in the photonic resonance within hours, demonstrating the promise of this biosensor.
The concept of photonic crystal (PC) was raised by E.Yablonovitch and S.John respectively in 1989. Since then on, photonic crystal made tremendous progress in the research and application. Photonic crystal is not some kind of new materials but periodic dielectric materials. We have designed, fabricated photonic crystal slab (PCS) based on porous silicon. These alternating dielectric porous silicon layers show a band gap in one direction. Incident light is reflected for colors of light in the band gap, so porous silicon photonic crystal slabs have high reflection in the range of band gap. By mean of electrochemical lift-off, the PCS could be removed from silicon substrate, and PC band gap could be observed clearly in the reflectivity spectra.
We describe a new method for doping high-quality porous silicon microcavities with erbium using ion implantation, where the erbium is confined to the spacer layer of the structure. This method involves fabricating porous silicon microcavities from a crystalline silicon wafer that has been implanted with erbium to a depth that coincides with a spacer layer of the microcavity. Using this technique erbium doped microcavities with Q-factors in excess of 1500 have been demonstrated. From low temperature photoluminescence measurements we observe a strong modification of the spontaneous emission spectrum of the erbium doped PSi, where the emission is enhanced 25 times at the resonance and suppressed elsewhere. Temperature dependent photoluminescence exhibited strong thermal quenching and excitation power dependent photoluminescence measurement showed saturation at high excitation powers. Both of these trends are characteristically similar to luminescent erbium centres in crystalline silicon. In addition we discuss the merits of localising the erbium in the crystalline part of the PSi and its potential for reducing the effects of Auger recombination and energy back-transfer, which limit the performance of the structures at room high temperatures.
We review a number of optical devices made from microporous silicon. Particular emphasis is placed on the fabrication method of porous silicon laser-mirrors, optical microcavities and one-dimensional photonic crystals with true photonic bandgap.
Our senior undergraduate laboratory offers 14 experiments in optics and photonics, including experiments on acousto- optics, properties of lasers, holography, optical fiber sensors and communications. Six of the experiments, and a mandatory assignment on laser safety, are individually completed by each student in a one semester course. A brief description of the experimental course and of each experiment is given, together with more detailed descriptions of the 'Fourier optics' and 'Photoluminescence of semiconductor quantum wells' experiments.
The effect of interdiffusion on the luminescence of InGaAs quantum dots grown by metal organic chemical vapor deposition with various In compositions was studied. The samples were subjected to thermal annealing with and without spin-on-glass. Up to 250 meV blueshifts and narrowing of the linewidths by up to 60 meV were observed in samples that were annealed without the dielectric cap. The effects of diffusion were seen at lower annealing temperatures for the samples with higher In content. The spin-on-glass created additional blueshift above that of simple thermal annealing. However, Ga-doped spin-on-glass suppressed significantly any additional intermixing other than that of simple thermal annealing. Strain is also a factor in the blueshift and narrowing of the photoluminescence. These results suggest that a range of bandgap energies could be achieved by selective area interdiffusion of the quantum dot samples.
We review an optical technique which is capable of detecting damage in semiconductors in a broad range of defect densities, and can be used to determine the distribution of damage on and below the sample surface. Using this contactless, room temperature technique, which is based on differential reflectance spectroscopy, we have been able to generate relief maps that show the spatial distribution of damage in a number of III-V compounds and Si, as well as the depth profile of damage in ion-implanted semiconductors.
A comparison has been made of the shifts induced in the photoluminescence (PL) emission wavelength of a GaAs/AlGaAs multiple quantum well (QW) structure following irradiation with H, He and As ions. Ions energies and fluences were chosen to produce matching numbers and distributions of lattice atom displacements across the structure. Samples were then annealed at 900 degrees C for 30s to intermix the QWs and low temperature photoluminescence as used to measure the shifts in the QW bandgap energies. At common concentration of atomic displacements, the PL blueshift increased with the mass of the implanted ion. For these anneal parameters, saturation of the blueshift from the narrowest QW was observed in all three irradiation at an average vacancy production concentration of approximately 1022 cm-3. No significant difference in PL shifts was found when the irradiations were performed at 200 degrees C sample temperature.
In this paper, we shall describe a modulated photoluminescence technique which, in semiconductors quantum wells, provides results similar to the conventional electromodulated absorption measurements. The advantage of this new technique is that it can be used on as grown samples or devices without the need for elaborate sample geometries or sample preparation which are necessary in the modulated absorption measurements.
We demonstrate the possibility of measuring the depth distribution of damage in GaAs using differential reflectance spectroscopy. Damage was intentionally generated by various ion- assisted processes, such as ion implantation and ion-assisted plasma etching. The high sensitivity of the techniques allowed us to measure damage profiles over a large range of ion energies and ion doses.
Differential reflectance (DR) spectroscopy is similar to other optical modulation techniques in so far as the resulting spectra exhibit sharp derivative-like lineshapes at photon energies corresponding to the critical point transitions. DR signals originate from inhomogeneities on or below the semiconductor surface. These inhomogeneities may be intrinsic, such as fluctuations in surface field, layer thickness, alloy composition, or externally induced, such as ion implantation, hydrogenation, etc. The DR spectra of semiconductor layer structures may be used to determine mole fraction, doping concentration, critical point energies, etc., much like photoreflectance (PR). In many cases, DR spectra have better signal-to-noise ratio than that of the PR spectra. In this report we discuss the application of DR in the study of doping inhomogeneities in GaAs as well as the use of DR to determine damage profiles in ion implanted GaAs.
Differential reflectance (DR) spectroscopy, applied to semiconductors, is shown to be
equivalent in some cases to a contactless electro-reflectance technique. DR spectra are achieved by
modifying one half of the sample surface or, in the case of semiconductor alloys, just relying on the
inhomogeneities present. Our DR spectra of GaAs reveal sharp critical point structures and are
comparable to the known electro-reflectance data. The DR spectra show a marked improvement in
signal to noise ratio over photoreflectance spectra of the same samples. This new technique has also
been used to characterize 111-V quantum well structures.