Roshan Makkar

A low temperature investigation of the optical properties of coupled InAs quantum dots with GaAsN/GaAs spacers

Epitaxially-grown 10-layer coupled InAs quantum dots with GaAsN/GaAs barrier layers have been investigated. The PL spectra was seen to be a complex convolution of bimodal distribution of QDs along with an asymmetric signature introduced by incorporation of nitrogen into the structures. Reducing the GaAsN/GaAs barrier thickness (from 2/16nm to 2/8nm) resulted in an improvement of PL linewidth as low as 20meV of the dominant PL peak for the sample with thinnest barrier layer. A blueshift in emission was observed due to higher indium intermixing as a result of an increase in overall strain in the multilayer structure. The highly asymmetric exponential tail signature evident from the PL spectra of as-grown samples indicated a higher presence of localized N-induced excitonic states near the conduction band edge. Samples with thicker barriers showed relatively lower asymmetry compared to samples with thinner barriers. Also, samples with thinner barriers showed an arrest in blueshift in the PL spectra with annealing temperature indicating thermal stability.

Signal processing of Spectral Domain Optical Coherence Tomography

Medical imaging techniques that are found in hospitals are X-ray imaging, MRI, ultrasound imaging, CT scan. Recently, a lot of scientific research work is being carried out with the aim of the improvement of these methods and the discovery of new non-invasive techniques. As a result, a new optical imaging technique, called Spectral Domain Optical Coherence Tomography (SD-OCT) was proposed in the early 2000s. Optical Coherence Tomography (OCT) is an emerging imaging technique having resolution in the μm range and depth of imaging in the mm range. In this paper we have described the basic principle, theory, types of OCT, signal processing steps and its applications in medical field. We have discussed some of the results related to signal processing that we got from simulation using MATLAB and Lab VIEW.

Single Mode SU8 Polymer Based Mach‐Zehnder Interferometer for Bio‐Sensing Application

This paper explains the influence of different parameters to the sensitivity of an optical waveguide Mach‐Zehnder Interferometer (MZI) for real time detection of biomolecules. The sensing principle is based on the interaction of evanescence field with the biomolecules that get immobilized on sensing arm. The sensitivity has been calculated by varying the sensing window length, wavelength and concentration of bio‐analyte. The maximum attainable sensitivity for the preferred design is the order of 10−8 RIU at 840 nm wavelength with a sensing window length of 1cm. All the simulation work has been carried out with Opti‐BPMCAD for the optimization of MZI device parameters. The SU8 polymers are used as a core and clad material to fabricate the waveguide. The refractive index of cladding layer is optimized by varying the curing temperature for a fixed time period and the achieved index difference between core and clad is Δn = 0.0151. The fabricated MZI device has been characterized with LASER beam profiler at 84...

Evanescent field absorption based photonic polymer waveguide biosensor

This paper details the design and fabrication of an integrated optical waveguide biosensor for antibody/antigen detection. SU-8 polymer is used as the core material to have a bi-conical tapered waveguide fabricated on a silicon substrate. PDMS is used as a buffer layer. The waist diameter of the biconical tapered waveguide has been optimized using Opti-BPM CAD software before fabrication. In addition, the fabrication technique employs simultaneous and single-step formation of the polymer waveguide structures for the guidance of light with V-grooves for low-cost passive alignment of glass optical fiber. The designed biosensor chip demonstrates sensing of FITC tagged goat anti human IgG (GaHIgG) and HIgG immobilized over the sensor surface was the bio receptor. The sensor uses the evanescent field that is present at the surface of the core for rapid and accurate sensing of antibody/antigen in the range of few micrograms per ml.

Design and development of portable fluorescence reader using silicon photo multiplier (SiPM) sensor

Fluorescent lateral flow assays (LFA) strips have gained popularity for medical diagnostics application by offering fast and reliable response in both qualitative and quantitative readout formats. The fluorescence emission is generated when excited with appropriate optical source, and is dependent on the analyte concentration in the sample spotted on the LFA strips. Quantitative detection requires a LFA reader to carry out accurate and precise measurements of the fluorescence emission. These readers can be of either benchtop or handheld type, and conventionally use either photo multiplier tube (PMT) or avalanche photodiodes (APD) or customized photo diodes. In addition to their proven benefits like high sensitivity, speed and gain, the availability of silicon photomultipliers (SiPM) in micro form factor makes them good choice for developing miniaturized LFA strip readers. In this study, a portable fluorescence reader using SiPM sensor has been designed, which records the fluorescence intensity of the spot on planar surfaces e.g. nitrocellulose membrane (NCM) packaged in the plastic cases. The design and operation details of the benchtop reader using SiPM sensor along with excitation source, focusing and collimating optics and power supplies integrated with general purpose microcontroller board in a mechanical housing are reported in this paper. The testing of the developed reader is done by using mercaptopropionic acid capped cadmium telluride quantum dots (MPA-CdTe QDs) as the fluorescent analytes spotted on NCM packaged plastic strips. The results obtained from the developed portable reader are compared with the standard fluorescence plate reader for QD concentration varying from 60 μg/mL to 420 μg/mL and are found to be in good accordance with the response and resolution of the conventional fluorescent plate reader. Further work is under development for testing the developed reader for disease diagnostic applications.

Low-temperature photoluminescence studies in epitaxially-grown GaAsN/InAs/GaAsN quantum-dot-in-well structures emitting at 1.31 μm

We report a single layer GaAsN/InAs/GaAsN quantum-dot-in-well (DWELL) structure with PL emission at 1.31μm important for applications in communication lasers. This extension has been achieved with a nitrogen composition of only 1.8% and QDs embedded within 1/6nm GaAsN which is higher compared to single layer QDs with GaAs and GaAsN capping layers as a result of confinement reduction on both sides of the QD energy levels. The structures remain as QDs till 800°C of annealing temperature alongwith a drastic enhancement in PL intensity as a result of annihilation of N-induced crystal defects which provide non-radiative recombination centers for carriers in the as-grown sample which is responsible for degraded luminescence. A typical highly asymmetric PL signature observed in dilute nitride structures is seen with a sharp cut-off at lower wavelengths and a large exponential tail at higher wavelengths in the as-grown and 650°C annealed samples. This is due to the presence of localized excitonic states extending into the bandgap close to the band edges. For higher annealing temperatures, this asymmetry disappears indicating an improvement in uniformity of nitrogen distribution and absence of localized states; which is also confirmed from a smaller blueshift in excitation intensity-dependent PL spectra of these samples. Well-resolved ground and first excited states in the PL spectrum of 700°C annealed sample indicates an improvement in QD confinement.

A low-temperature photoluminescence study of GaAs1-xNx/GaAs multiple quantum-wells

Five-period GaAs1−xNx/GaAs multiple quantum wells (MQWs) were grown on GaAs(001) substrates under different nitrogen background pressures through solid-source molecular beam epitaxy and the structural and optical properties at low temperature were investigated. High resolution x-ray diffraction revealed sharper satellite peaks observed for GaAs0.978N0.022/GaAs MQWs as compared to GaAs0.982N0.018/GaAs MQWs, indicating better interfaces. The MQWs with higher nitrogen content exhibited high photoluminescence (PL) intensity, whereas a degraded PL intensity was observed for the latter, attributed to reduction in surface recombination with high nitrogen incorporation. Moreover, the spectrum for the MQWs with higher nitrogen content was observed to be consisted of several Gaussian spectra, indicating thickness variation of QWs caused by randomness in distribution of N atoms. In the low energy regime of PL, a long asymmetric tail was observed because of nitrogen introduced potential fluctuations. Rapid thermal annealing enhanced PL intensity by multi-fold and substantially reduced the full width at maximum because of homogenization of MQWs. This investigation could enhance understandings of the MQWs-based optoelectronic devices.

Modelling for Spectral Domain Optical Coherence Tomography (SD-OCT) System

Optical Coherence Tomography (OCT), a revolutionary technology was initially developed for imaging retinas to investigate various eye diseases like glaucoma, diabetic retinopathy etc. It later found acceptance for its non-invasive nature in other bio medical applications like cardiology, dermatology etc. It is very similar to ultrasound in nature; the only difference is usage of light waves in place of sound waves and it offers higher axial resolution in comparison with most of other existing technologies. The technology behind SD-OCT is Michelson interferometry, which offers the usage of Super Luminescent Diode (SLD) source as low temporal coherent source with broad bandwidth to measure its absorption/scattering through the sample specimen. In the following paper, we depict the model of SD-OCT system. The spectral response of the source and the output generated waveforms show the response of the model in respect to the actual hardware. LabVIEW software could generate the graphical user interface. The samples were used on the basis of their refractive indices. The maximum depth obtained depends on the number of pixels. Depth of up to 0.5 mm can be obtained with 127 pixels and 1.7 mm with 400 pixels with axial resolution as 7.75 µm for 100 nm bandwidth 1310 nm SLD source. Simulation results at 632.8, 840 and 1310 nm are compared and discussed.

Impact of rapid thermal annealing on dilute nitride (GaAsN)-capped InAs/GaAs quantum dots exhibiting optical emission beyond ~1.5 μm

We report here self-assembled 2.6 ML InAs QDs capped with GaAsN0.021 on GaAs (001) substrate grown under high arsenic overpressure and high power by solid source molecular beam epitaxy. With variation in GaAsN0.021 layer thickness, InAs/GaAs QDs were studied by photoluminescence (PL) spectroscopy. It was found that with InAs dot density of 3 ×1010 cm-2 and 4 nm GaAsN capping layer, emission wavelength was possible to extend beyond 1.5 μm at 300K. Rapid thermal annealing was carried out in nitrogen ambient for 30 sec at temperatures ranging from 700°C to 800°C and a continuous blue-shift for the nitride-capped QDs was observed at 19 K PL spectra, and the sample annealed at 800°C exhibited highest intensity with narrowest full width at half maximum (FWHM). Both the as-grown and annealed samples exhibited asymmetric PL behavior in low energy region at low temperature, associated to the N-related states or cluster of N atoms. The peak emission wavelength at the annealing temperature domain of 750-800°C was remained constant, attributed to no In/Ga diffusion at the interface between the dot and the barrier. Hence, the InAs/GaAs dots capped with 4-nm GaAsN0.021 layer could be implemented in lasers in the temporal range of 750-800°C.

Spectral Domain Optical Coherence Tomography (SDOCT) at 1310 nm

Optical Coherence Tomography (OCT), a revolutionary technology was initially developed for imaging retinas to investigate various eye diseases like glaucoma, diabetic retinopathy etc. It later found acceptance for its non-invasive nature in other bio medical applications like cardiology, dermatology etc. It is very similar to ultrasound in nature; the only difference is usage of light waves in place of sound waves and it offers higher axial resolution in comparison with most of other existing technologies. The technology behind SD-OCT is Michelson interferometry, which offers the usage of Super Luminescent Diode (SLD) source as low temporal coherent source with broad bandwidth to measure its absorption/scattering through the sample specimen. In the following paper, we depict the experimental setup for A scan as well as B scan. The spectral response of the source and the output waveforms shows the response in LabVIEW software. Samples with different refractive indices are tested and depth of up to 0.5 mm can be obtained with 127 pixels.