Reducing active layer thickness of solar cell stresses on efficient light trapping mechanisms to keep the cell efficiency intact. Directional light scattering and promising refractive index of silicon nanoparticles make them encouraging scattering centers for thin-film silicon solar cells. Finite-difference time-domain simulations are used to study the optical properties of silicon nanospheres embedded in the top and bottom buffer layer of solar cells. Diameter of a silicon nanoparticle plays a crucial role in the forward and backward scattering of incident light into the cell. Silicon nanospheres outperform commonly used metallic and dielectric nanospheres and trapped the incident light over a broad spectrum. Silicon nanospheres require special attention when placed in both the buffer layers of the solar cell simultaneously, and lateral displacement of the silicon nanospheres at the top buffer layer with respect to nanospheres at the bottom buffer layer is beneficial. Lateral displacement of nanospheres provides a total quantum efficiency of 51.49% in comparison to 21.9% of the pristine cell. These exceptional scattering competencies of silicon nanospheres make them a promising candidate for photovoltaic applications. Silicon scatterers may be used with well-established fabrication techniques.
We present the theoretical results of the optimization of lossy mode resonance sensors at visible wavelengths. Both angular and spectral interrogations are carried out for absorbing indium tin oxide (ITO) films placed on glass prism. The inclusion of a low-index layer between the prism and the lossy (ITO) layer can produce an efficient refractive index sensor for bio/chemical applications. Further increase in sensitivity can be achieved by changing the index of the prism. It is shown that the sensitivity has strong dependence on the index of prism. Sensitivities as high as 4670 nm/RIU for spectral mode and 67 deg/RIU for angular mode with small values of full width at half maximum (FWHM) can be achieved. Dependence of sensitivity and FWHM on refractive index and thickness of low-index matching layer is also investigated.
Our present work proposes a systematic geometric model comprising vertical dual silver nanostrips placed on the top of a thin-film amorphous silicon solar cell. In the first layer, cylindrical silver nanostrips are embedded in the antireflection coating and the other one is placed just above it. Combining the two improves the absorption over the wide spectral range. A finite-difference time domain technique has been used to confirm that a vertical dual silver nanostructure improves absorption over a broad spectrum in comparison to a single layer. Size, shape, and interspacing of the nanostructures have been tuned to obtain the preeminent results. This optimized geometry gives a total quantum efficiency of 32.02% under AM1.5G.
We have investigated laser ablation in InSb in the fluence regime of 8.5 J/cm2 to 21 J/cm2 by studying the
crater morphology. Crater morphology shows a non linear change in depth, volume and roughness at fluence of 14.5
J/cm2 (Fcr) and above. These non linear variations with presence of several micro-cavities at the crater bottom (potential
bubble nucleation sites) suggest a different material removal mechanism at Fcr and above. The results have been
explained in light of various ablation theories which support explosive boiling as a possible mass removal mechanism at
Fcr and above. Thermal melting model for laser ablated InSb is in good agreement with the experimental results.
In this paper the crystalline and morphological properties of Pd thin films deposited on glass substrate by pulsed laser
deposition (PLD) technique at different substrate temperatures have been investigated. These films were deposited with
an excimer (XeCl) laser source (λ= 308 nm, pulse duration of 30 ns, repetition rate of 10 Hz).The fabricated films were
characterized by various methods such as X-ray diffraction (XRD) and atomic force microscopy (AFM). The thickness
and refractive index of samples were measured using ellipsometry. There was influence of substrate temperature on the
surface roughness of thin film. The rms roughness increases with increasing temperature. As the temperature increase the
crystallinity of the film also increases.
This work describes a homogeneous single layer model for surface roughness by polarized light. It has been shown that
the reflectance change in non-absorbing layer is directly proportional to the refractive index of the ambient and substrate
media for s polarization but inversely proportional to the p polarization and it is directly proportional to the square of the
thickness of the layer for both the polarization. The thickness of the film has been written in terms of surface roughness
to correlate the homogeneous model with the scattering theory. The consequence of the scattered light on the specular
reflectance and transmittance for oblique incidence shows that there is reduction in reflectance and transmittance, due to
roughness on the surface under the Drude effective-medium approximation.
The waveguiding properties of SC-450 photoresist films are presented. The basic waveguiding properties such as refractive index, propagation losses, thickness, and birefringence of photoresist film are optimized. The effect of dye doping and UV exposure on photoresist film is also observed. The film shows excellent transmission (above 90%) in the wavelength range of 500 to 1500 nm. The present results suggest that photoresist films may be the good option to be used as an optical component.
The entrance and exit surface damage has been observed in newly developed Neodymium doped phosphate laser glasses viz. Potassium Barium Aluminium Phosphate (KBAP), Lanthanum Potassium Barium Aluminium Phosphate (LKBAP), Yttrium Aluminium Phosphate (YAP) under Q switched Nd:YAG laser irradiation at 1.06 μm wavelength near normal incidence. The YAP glass is suitable for low repetition rate system such as range finders while KBAP and LKBAP find applications for high repetition rate systems which are used in material processing and electro-optic counter measures. The difference between the entrance and exit surface damage thresholds has been explained using electromagnetic effects. The comparison of the morphological features of the KBAP and LKBAP suggest the role of metallic inclusion in the samples whereas in the case of YAP sample, the damage is governed by the intrinsic physical properties of this glass.
Evolution of laser damage morphology has been studied in 112 oriented, mirror polished Indium Antimonide(InSb)samples as a function of increasing energy, pulse repetition rate and number of pulses using a Nd:Cr:GSGG laser of 1.06 μm wavelength having a pulse width of 20ns. Scanning Electron Microscope (SEM) investigations of the irradiated samples have been done to understand the evolution of damage morphology. Damage morphology is consistent with surface melting and solidification along with an evidence of subsurface overheating. Temperature profiles calculated at different fluence levels confirm substantial subsurface heating. Multiple pulse damage seen at 20Hz with increasing fluence levels is mainly thermal damage. Thermal modeling has been done to explain different morphological features.
Polyvinyl alcohol (PVA)-based thin film optical waveguides are fabricated and characterized. Various waveguide parameters, namely, refractive index, propagation losses and depth, birefringence, and polarization conversion properties of dip-coated thin film optical waveguides are evaluated. The effective refractive index of PVA film is found to be between 1.506 and 1.531 for various concentrations. The propagation losses are 2.35 to 0.30 dB/cm for various thicknesses. The propagation loss reduces when the thickness of the film is increased. The low propagation losses, birefringence insensitivity, high transmission in the aged and annealed waveguides, control over film refractive index, the good adhesiveness of the film and good compatibility with substrate show that PVA-based waveguides are promising candidates for the optoelectronic field.
Negative photoresist films (HNR-80 and HNR-120) are fabricated and characterized for waveguiding purpose. Various waveguiding parameters, namely, refractive index, thickness, propagation losses, and birefringence of the photoresist films are evaluated. Propagation losses are optimized to the value of 0.29 dB/cm for both materials. These photoresist materials show excellent transmission with almost no absorption in the wavelength range of 400 to 900 nm. The waveguides are polarization insensitive for various temperatures. Considering these properties, these materials offer a good option to use as waveguiding materials in integrated optics.
When sufficient intense laser pulse interacts with semi-conductors, permanent damage results due to formation of scattering centers in the semi-conductor surface. Reflectance of the damaged surface is reduced as compared to the smooth surface. Generally, damage threshold is defined as a value for which the reflectance is decreased by more than 10%. It is interesting that reflectance change can be seen below the damage threshold value. In the present work it has been shown that these changes occur at the rate of 0.5% per pulse. These changes are not due to any type of irreversible process in the material and slow but continuous decrease can be seen in the reflectance if the number of pulses is increased. Although when the fluence is increased, remarkable change can be seen in the reflectance decrease for the first pulse (in some cases, for 2-3 pulses), but if the number of incident pulses are further increased, the slow and continuous decrease is shown in reflectance value.
The present experiments were performed in the CdZnTe <111> single crystal surface exposed to Nd:YAG laser (pulse duration: 20ns, prs:1pps, wavelength: 1.064µm) in the ambient air.
In this paper homogeneous model for surface roughness in identical layer system has been presented. It has been shown that reflectance change in non-absorbing layer is directly proportional to the square of total thickness of the layers. The reflectance in the visible range of the wavelength changes (decreases) substantially when the roughness factor or the number of layers are increases. In the present model the reflectance of the double layer system can be explained with the help of only one parameter that is roughness factor σ.
Evolution of damage in mirror polished samples of HgCdTe, CdTe & CdZnTe has been studied for three fluence regimes-Damage Threshold (Fth), Five Times Damage Threshold (5Fth), Ten Times Damage Threshold (10Fth) with multiple pulses of a Q-switched 1.06 μm Nd:YAG laser of 20ns pulse duration. Damage morphology observed under Scanning Electron Microscope (SEM) seem to evolve almost in similar fashion with increasing number of pulses as well as incident energy in HgCdTe and CdZnTe on account of uniform heating through a significant depth through the sample surface whereas in case of CdTe, effect of subsurface superheating is evident. Thermal modeling has been done to explain the evolution of laser damage.
Pits formation during the laser induced damage with GaAs surface either for picosecond or for femtosecond laser pulses, shows that defects always play an important role in damage morphology of GaAs surface, as previously these pits formation have also been seen for micro and nanosecond laser pulses. The nature of these pits gives an important information about the morphological features of damaged GaAs surface in picosecond and femtosecond regime. In this paper we report a comparative study of laser induced damage morphology of GaAs surface based on the nature of these pits formation.
Polymeric Optical Waveguides have attracted a lot of attention with a view to their flexibility for fabrication and their cost effectiveness. In the search for new materials recently the study of Styrene Acrylonitrile (SAN) thin film optical waveguides has been reported. SAN exhibits a considerable transmission (80-90%) in the visible and infrared regions, which is important for integrated optical devices. But the propagation loss is a major drawback of SAN based waveguides. Here we report 4-layer waveguides with Glass plus SAN plus Polystyrene plus air, structure. With the help of this multilayer structure we were able to reduce propagation loss by 30% in comparison to single layer SAN based waveguides. We also present the other basic properties like index profile, guide thickness, birefringence.
Polymer thin film devices are fast emerging as efficient active elements for processing of optical signals for using civil, space and military applications. The requirements for a thin film material to be used in optical processing and communication are the high second and third order non-linear susceptibilities, low propagation loss and variable depth, propagation to allow the same device operate at different wavelength.
A systematic study of the fabrication and characterization of planar optical waveguides in Z-LiNbO3, fabricated by the proton exchange technique using orthophosphoric acid is reported. The guides are fabricated by immersing lithium niobate substrates in orthophosphoric acid for different durations. The results are compared with those for guides fabricated using benzoic acid under similar conditions. The data are analyzed to calculate the value of the diffusion coefficients. The surface index and the depth produced using the orthophosphoric acid proton exchange process is marginally higher.
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