Laser haemocytometer

This thesis describes the work carried out to provide a proof of principle coupled-cavity laser measurement for blood cell analysis, using an integrated device with capillary fill microfluidics. The development of both light source and microfluidics on the same sensing platform provides complete integration and removes the dependence on external systems. In principle, InAsP quantum dot lasers, cover a wavelength range extending into the near infrared, where the response of biological matter can provide useful diagnostic information. The suitability and limitations, of both an InAsP quantum dot and GaInP quantum well active medium, are con-sidered for the coupled-cavity structure. A InAsP quantum dot structure with an 8 nm AlGaInP barrier between each dot layer was seen to have a slight improvement in device performance, but optical gain measurements indicated that this structure would not provide sufficient gain to over-come the high losses expected in the integrated device. Consequently, a GaInP quantum well was considered a sensible choice for a proof of principle coupled-cavity measurement. The efficiency of an etched facet is key to overall performance in the coupled-cavity device and has been quantified using the gain characteristics of the quantum well structure. A value of facet efficiency was found to be ηf = 0.37 ± 0.04, which is valid for all angles of etched facet. A very low facet reflectivity of 4.9x10−9 was measured for a laser with a 14.1o etched facet. Perturbation of the optical coupling between two laser/detector sections causes a change in the measured photo-voltage signal from the device. This effect has been employed to demonstrate detection of both 10 and 6 µm microbeads. In a coupled-cavity regime, a 22.6o angled facet coupled-cavity laser pair has been shown to have a lower threshold current density than either of its individual sections, indicating its potential for sensing applications

The limits to peak modal gain in p-modulation doped indium arsenide quantum dot laser diodes

A semi-empirical model is compared with measurements to establish limiting factors in the performance of p-modulation doped InAs quantum dot (QD) lasers. Fitted absorption spectra allow identification of supposed factors and comparison of multiple samples isolates their origin, providing insights for future laser design

Design and Characterisation of Multi-Mode Interference Reflector Lasers for Integrated Photonics

InAs quantum dot ridge waveguide lasers comprising single-port multi-mode-interference-reflectors (MMIR) and single-cleaved reflectors are designed, fabricated, and characterised, to demonstrate capability for optoelectronic-integrated-circuits. Simulations of an MMIR show high values of fundamental mode reflectivity ($> 80\% )$ and good selectivity against higher order modes. Deep-etched MMIR lasers fabricated with 0.5 mm long cavities have a threshold current of 24 mA, compared to 75 mA for standard Fabry–Perot cleaved–cleaved FP-RWG lasers of the same length, both at 25 °C, and 56 mA compared to 102 mA at 55 °C. MMIR lasers exhibit stable ground state operation up to 50 °C and show promise as small footprint sources for integrated photonics

Quick fabrication VCSELs for characterisation of epitaxial material

A systematic analysis of the performance of VCSELs, fabricated with a decreasing number of structural elements, is used to assess the complexity of fabrication (and therefore time) required to obtain sufficient information on epitaxial wafer suitability. Initially, sub-mA threshold current VCSEL devices are produced on AlGaAs-based material, designed for 940 nm emission, using processing methods widely employed in industry. From there, stripped-back Quick Fabrication (QF) devices, based on a bridge-mesa design, are fabricated and this negates the need for benzocyclcobutane (BCB) planarisation. Devices are produced with three variations on the QF design, to characterise the impact on laser performance from removing time-consuming process steps, including wet thermal oxidation and mechanical lapping used to reduce substrate thickness. An increase in threshold current of 1.5 mA for oxidised QF devices, relative to the standard VCSELs, and a further increase of 1.9 mA for unoxidised QF devices are observed, which is a result of leakage current. The tuning of the emission wavelength with current increases by ~0.1 nm/mA for a VCSEL with a 16 μm diameter mesa when the substrate is unlapped, which is ascribed to the increased thermal resistance. Generally, relative to the standard VCSELs, the QF methods employed do not significantly impact the threshold lasing wavelength and the differences in mean wavelengths of the device types that are observed are attributed to variation in cavity resonance with spatial position across the wafer, as determined by photovoltage spectroscopy measurements

Realisation of multi-mode reflector lasers for integrated photonics

The epitaxial growth of III-V materials on silicon is an alternative approach to combining silicon photonics with the active laser source. Substantial progress has been made to reduce the defects created at the III-V / Si interface to a level that has a negligible impact on laser operating current and lifetime, providing quantum dot gain materials are utilized [1], [2]. A number of issues remain for the integration of III-V structures with silicon, not least that of reducing the footprint and ensuring the fabrication required is as simple as possible. While the laser reflectors can be fabricated in the silicon here we focus on using the III-V material, which removes the need to have the III-V / Silicon interface and its associated losses within the laser cavity

Gain measurements on vertical cavity surface emitting laser material using segmented contact technique

We report direct measurements of the optical gain profile for a vertical cavity surface emitting laser (VCSEL) epitaxial structure, by characterising the transverse electric (TE) in-plane net modal gain using the segmented contact method

Novel coupled-cavity sensing mechanism for on-chip detection of microparticles (Conference Presentation)

Coupled-cavity lasers have attracted wide attention in the past, in particular for telecommunication applications where their wavelength tunability and ability for side mode suppression are desirable. The inherent sensitivity of these devices to changes in the optical coupling has also led to their proposed use in optical sensing systems. Small changes to the refractive index of the coupler section can lead to shifts in the resonance frequency of the laser. Here we present an alternative approach to coupled-cavity sensing that exploits changes to the imaginary part of the refractive index of the coupler. An optical loss, introduced to the cavity by the passage of micro-particles, influences the optical loss of the lasing mode and changes the threshold gain requirement of the laser. The sub-linear nature of the gain-current density characteristics of the quantum confined gain medium amplifies this effect, producing an even larger perturbation in output power. We demonstrate this sensing mechanism using a monolithic coupled-cavity particle detector with on-chip capillary fill microfluidics and an in-line photo-detector section for photo-voltage transduction. Both laser and detector are pulsed allowing for a time-resolved measurement to be taken

Effect of barrier layer width on the optical and spectral properties of InAsP/AlGaInP quantum dot lasers

The barrier layer thickness separating the dot-in-well structure in optoelectronic devices plays a crucial impact on the performance of these devices. Here, we report the effect of the barrier width in quantum dot (QD) laser. Three series of InAsP QD lasers were papered with different barrier widths (i.e., 8, 16, and 24 nm) of (AlGa)InP. The samples were prepared by MOVPE at 730 °C. Data of threshold current density, J th giving a lower J th for 8 nm barrier width sample and it becomes more temperature dependency evident by a low value of characteristic temperature, T o ≈ 77 k. Emission spectra of 2 mm long lasers showed a blue-shift by approximately 11 nm when the barrier width increased from 8 to 24 nm. The modal absorption study showed a decrease in the inhomogeneous broadening for 24 nm barrier width sample. Simulation results revealed an increase in the optical absorption cross-section, σ o with increasing barrier width. These results showed that significant improvements in the spectral properties of the InAsP QD lasers can be made by altering the (AlGa)InP barrier width