Optimisation of distributed feedback laser biosensors

A new integrated optical sensor chip is proposed, based on a modified distributed- feedback (DFB) semiconductor laser. The semiconductor layers of different refractive indices that comprise a laser form the basis of a waveguide sensor, where changes in the refractive index of material at the surface are sensed via changes in the evanescent field of the lasing mode. In DFB lasers, laser oscillation occurs at the Bragg wavelength. Since this is sensitive to the effective refractive index of the optical mode, the emission wavelength is sensitive to the index of a sample on the waveguide surface. Hence, lasers are modelled as planar waveguides and the effective index of the fundamental transverse electric mode is calculated as a function of index and thickness of a thin surface layer using the beam propagation method. We find that an optimised structure has a thin upper cladding layer of ~0.15 mum, which according to this model gives detection limits on test layer index and thickness resolution of 0.1 and 1.57 nm, respectively, a figure which may be further improved using two lasers in an interferometer-type configuration

Thermal characterization of electrically injected thin-film InGaAsP microdisk lasers on Si

Abstract—We have performed a numerical and experimental analysis of the thermal behavior of electrically injected microdisk lasers that are defined in an InGaAsP-based thin film bonded on top of a silicon wafer. Both the turn-on as well as the pulsed-regime temperature evolution in the lasing region was simulated using the finite-element method. The simulation results are in good agreement with experimental data, which was extracted from the broadening of the time-averaged emission spectra. Lasing at room temperature was only possible in pulsed regime due to the high thermal resistance (10 K/mW). Some strategies to decrease the thermal resistance of the microdisk lasers are proposed and discussed. Index Terms—Heterogeneous integration, InGaAsP, integrated optics, microdisk laser, Si, thermal characterization

Renormalization of the Optical Response of Semiconductors by Electron-Phonon Interaction

In the past five years enormous progress has been made in the ab initio calculations of the optical response of electrons in semiconductors. The calculations include the Coulomb interaction between the excited electron and the hole left behind, as well as local field effects. However, they are performed under the assumption that the atoms occupy fixed equilibrium positions and do not include effects of the interaction of the lattice vibrations with the electronic states (electron-phonon interaction). This interaction shifts and broadens the energies at which structure in the optical spectra is observed, the corresponding shifts being of the order of the accuracy claimed for the ab initio calculations. These shifts and broadenings can be calculated with various degrees of reliability using a number of semiempirical and ab initio techniques, but no full calculations of the optical spectra including electron-phonon interaction are available to date. This article discusses experimental and theoretical aspects of the renormalization of optical response functions by electron-phonon interaction, including both temperature and isotopic mass effects. Some of the theoretical techniques used can also be applied to analyze the renormalization of other response functions, such as the phonon spectral functions, the lattice parameters, and the elastic constants.Comment: Latex 2.09, 28 pages, 13 Figs., 2 Tables, submitted to Phys. Stat. Sol.

Vapour sorption, wavelength tracking and thermo-optic properties of dual slab waveguide interferometers

The dual slab waveguide interferometer is introduced as a device which has many applications in various research areas. Reported is its ability to provide details on the mechanism for the vapour sorption of thin polymer films, the development of the interferometer as a wavelength tracking device for the telecommunications industry and a method to characterise the thermo-optic properties of III-V semiconductor alloys. The vapour sorption mechanisms of thin films of polymers Polyisobutylene (PIB) and Polyvinylpyrrolidone (PVP) on exposure to several solvents are investigated. Coating interferometer chips with a thin layer of polymer and monitoring the interference fringe pattern for changes due to the exposure to a solvent vapour provides information on the mechanism for vapour sorption as one of swelling rather than void-filling. Interferometer sensitivities to vapour concentrations are linear and depend on refractive index differences between polymer and condensed vapour. An interferometer manufactured from III-V semiconductor compounds is developed to produce a device which can operate as a wavelength tracker. Sensitivities of over 6 mrad / pm and 7 mrad / pm for single and dual quaternary systems respectively indicate that a device of length around 5 mm would be capable of detecting picometer input wavelength changes including thermal background noise. The sensitivity to thermal changes provides a simple method for determining the thermo-optic coefficient of two Indium Gallium Arsenide Phosphide (InGaAsP) alloy compositions as (3.15±0.08)x 10(^-4)K(^-1) and (2.60±0.017)x10(^-4)k(6-1) forInGaAsP compounds with bandgap wavelengths around 1.3 μm and 1.15 μm respectively

Modelling and Optimisation of Single Junction Strain Balanced Quantum Well Solar Cells

In an attempt to find the optimum number of wells for maximum conversion efficiency a pair of otherwise identical strain balanced samples, one containing 50 wells and the other 65 wells have been characterised. The 65 well sample is found to possess a lower predicted efficiency than the 50 well sample, suggesting that the optimum well number lies between these values. Devices grown using tertiary butyl arsine (TBAs) are found to possess comparable conversion efficiencies to the control cells grown using arsine and slightly superior dark IV characteristics, indicating that TBAs may be substituted for arsine without loss of device efficiency and may even be beneficial to cell performance. Several fundamental refinements to the existing quantum efficiency model of are explored. Firstly, expressions for the strained band gaps are derived. A value for the conduction band offset is . determined using the difference in energy between the heavy and light hole exciton peaks in low temperature photo current scans and found to be 0.55??0.03. The magnitude of the el-hhl exciton binding energy is also estimated from these scans and found to be in excellent agreement with the value obtained from a simple, parameterized expression for the exciton binding energy. Finally, an absolute calculation for the absorption coefficient is incorporated into the quantum efficiency model and values for the heavy and light hole in-planes masses are obtained. The model is found to underestimate the level of absorption in the intrinsic region by an amount consistent with estimates of the magnitude of the reflection from the back surface. The conversion efficiency of a sample predicted using SOL is compared to an independently obtained value. Good agreement is observed between the two results (25.3% and 25.7% for 317 suns AM1.5D). Additionally, an optimum structure for illumination by the AM1.5D spectrum was found to be a 120A well ofIno.lGaAs.Imperial Users onl

The growth, processing and characterisation of II-VI semiconductor structures

The work contained in this thesis focuses on the growth, processing and characterization of II-VI semiconductors for use in opto-electronic devices. Included are efforts to develop both II-VI based distributed Bragg reflectors (DBRs) utilising ZnMgSSe and ZnSe and the epitaxial lift-off (ELO) process pioneered at Heriot-Watt University (HWU). The optical properties of a range of different II-VI compounds (inc. ZnSe, MgS, MnS and ZnMgSSe) are measured using a range of techniques including photoluminescence spectroscopy (PL), optical transmission measurement and spectroscopic ellipsometry. From these measurements, a more accurate value for the bowing parameter of ZnCdSe of 0.37±0.05eV is determined. The effect of lifting structures using an MgS sacrificial layer is investigated by optical microscopy, optical transmission measurement and PL, to allow any structural changes to be determined. The ELO process is also extended to allow structures grown on InP substrates to be lifted by using a magnesium selenide (MgSe) sacrificial layer. The μ-PL measurements of a series of CdSe QDs grown on ZnMgSSe barriers are also reported and compared to previous work on other barrier materials (ZnSe and MgS). The causes of the jitter (spectral diffusion) seen in these samples is also investigated and discussed