Low Optical Absorption Research Articles

Anisotropic growth of long isolated graphene ribbons on the C face of graphite-capped6H-SiC

We present an investigation of large, isolated, graphene ribbons grown on the C-face of on-axis semi-insulating 6H-SiC wafers. Using a graphite cap to cover the SiC sample, we modify the desorption of the Si species during the Si sublimation process. This results in a better control of the growth kinetics, yielding very long (about 300 microns long, 5 microns wide), homogeneous monolayer graphene ribbons. These ribbons fully occupy unusually large terraces on the step bunched SiC surface, as shown by AFM, optical microscopy and SEM. Raman spectrometry indicates that the thermal stress has been partially relaxed by wrinkles formation, visible in AFM images. In addition, we show that despite the low optical absorption of graphene, optical differential transmission can be successfully used to prove the monolayer character of the ribbons.

Optical and mechanical properties of AlF3 films produced by pulse magnetron sputtering of Al targets with CF4/O2 gas

In this study, AlF3 thin films were deposited by pulse magnetron sputtering of Al targets with different ratios of CF4/O2 gas and at different sputtering powers. The optical and mechanical properties of the AlF3 thin films were analyzed. The transmittance spectra showed no obvious negative inhomogeneous refractive indices. Denser films with a low optical absorption were obtained when high sputtering powers were used (larger than 30 W). The lowest extinction coefficient (7.3 × 10−4 at 193 nm) of the films can be reached with 12 sccm O2 flow rate and at 160W sputtering power. All of the residual stresses were compressive and their trends were consistent with the refractive indices. The lowest compressive stress (0.068 GPa) was obtained when the AlF3 films were prepared at 160W sputtering power.

AlF 3 film deposited by IAD with end-Hall ion source using SF 6 as working gas

A novel and effective process to fabricate high quality fluoride thin films was presented. Aluminum fluoride films deposited by a conventional thermal evaporation with an ion-assisted deposition (IAD) using SF 6 as a working gas at around room temperature were investigated. In this study, the optimal voltage and current, 50 V and 0.25 A, were found according to the optical properties of the films: high refractive index (1.489 at 193 nm), low optical absorption and extinction coefficient (<10 −4 at 193 nm) in the UV range. The physical properties of the film are high packing density and amorphous without columnar structure. It was proved that using SF 6 working gas in IAD process is a good choice and significantly improves the quality of AlF 3 films.

Cold laser machining of nickel-yttrium stabilised zirconia cermets: Composition dependence

Cold laser micromachining efficiency in nickel-yttrium stabilised zirconia cermets was studied as a function of cermet composition. Nickel oxide-yttrium stabilised zirconia ceramic plates obtained via tape casting technique were machined using 8–25 ns pulses of a Nd: YAG laser at the fixed wavelength of 1.064 μm and a frequency of 1 kHz. The morphology of the holes, etched volume, drill diameter, shape and depth were evaluated as a function of the processing parameters such as pulse irradiance and of the initial composition. The laser drilling mechanism was evaluated in terms of laser-material interaction parameters such as beam absorptivity, material spallation and the impact on the overall process discussed. By varying the nickel oxide content of the composite the optical absorption (-value is greatly modified and significantly affected the drilling efficiency of the green state ceramic substrates and the morphology of the holes. Higher depth values and improved drilled volume upto 0.2 mm 3 per pulse were obtained for substrates with higher optical transparency (lower optical absorption value). In addition, a laser beam self-focussing effect is observed for the compositions with less nickel oxide content. Holes with average diameter from 60 μm to 110 μm and upto 1 mm in depth were drilled with a high rate of 40 ms per hole while the final microstructure of the cermet obtained by reduction of the nickel oxide-yttrium stabilised zirconia composites remained unchanged.

Silicon waveguide infrared photodiodes with &gt;35 GHz bandwidth and phototransistors with 50 AW-1 response

SOI CMOS compatible Si waveguide photodetectors are made responsive from 1100 to 1750 nm by Si+ implantation and annealing. Photodiodes have a bandwidth of >35 GHz, an internal quantum efficiency of 0.5 to 10 AW-1, and leakage currents of 0.5 nA to 0.5 microA. Phototransistors have an optical response of 50 AW-1 with a bandwidth of 0.2 GHz. These properties are related to carrier mobilities in the implanted Si waveguide. These detectors exhibit low optical absorption requiring lengths from <0.3 mm to 3 mm to absorb 50% of the incoming light. However, the high bandwidth, high quantum efficiency, low leakage current, and potentially high fabrication yields, make these devices very competitive when compared to other detector technologies.

Preparation and characterization of new fluorotellurite glasses for photonics application

Glasses based on (85 − x)TeO2–xZnF2–12PbO–3Nb2O5 (x = 0–40) system have been studied for the first time for fabricating mid-infrared optical fiber lasers. The thermal and optical properties including UV–Vis, Raman as well as FTIR spectra are reported. It is demonstrated that increasing the ZnF2 concentration to 30 mol% significantly increased the thermal stability of the glass. Adding ZnF2 also reduced the hydroxyl (OH) content of the glass resulting in lower optical absorption in the mid-infrared region, which is crucial for infrared laser applications. The glass absorption cut-off edge near 400 nm blue-shifts with increasing ZnF2 addition. Raman spectra show a depolymerization of the glass network with increasing transformation of TeO3+1 to TeO3 structures.

A New AlGaInP Multiple-Quantum-Well Light-Emitting Diode With a Thin Carbon-Doped GaP Contact Layer Structure

A new AlGaInP multiple quantum-well light-emitting diode (LED) with a thin carbon-doped GaP contact layer and a transparent conducting indium tin oxide film is fabricated and studied. For comparison, the LEDs with different contact layer structures are also included in this work. Experimental results indicate that the LED with a carbon-doped GaP contact layer exhibits a higher output power of 31.4 mW and a higher external quantum efficiency of 9%. The light-output power, under dc 20-mA operation, of this LED is increased by a factor of 18% as compared with that of conventional LEDs. These results are mainly attributed to the significantly lower series resistance and lower optical absorption effect. Moreover, the new device shows the reduced wavelength shift with 1.7-nm variation between 10 and 200 mA in electroluminescence spectrum.

Time-resolved thermal mirror method: A theoretical study

A general and complete theoretical model of the time-resolved thermal mirror method for the measurement of thermo-optical-mechanical properties of solid materials is developed. The laser-induced temperature profile in a sample and its thermoelastic surface displacement are derived. The center intensity of a probe beam at the detector plane is calculated using the Fresnel diffraction theory. Additionally, simplified models for high and low optical absorption samples are presented, and the suitability of the simplified models is also analyzed. The influence of experimental parameters on the sensitivity of the thermal mirror method is discussed for the optimization of the experimental apparatus. The presented model and the experimental technique can be used to quantitatively determine the physical properties of transparent and opaque solids.

Site selective spectroscopy of Yb3+ in NaT(WO4)2, T = Bi, Gd, Y, Lu, laser crystals: Assessment with simulated crystal field effects

The sequences of the energy levels of Yb3+ for the two different 2b and 2d crystal sites in tetragonal I4¯ disordered double tungstate NaT(WO4)2, T=Bi, Gd, Y, Lu, laser crystals have been determined by 6K site selective emission and excitation of the photoluminescence spectra. The results were assessed with predictive simulations of crystal field interactions for these Yb3+ sites in current NaT(WO4)2 hosts. The method allows the interpretation of low temperature optical absorption spectra containing unresolved information of all Yb centres in such disordered laser crystal hosts.

Design of MEMS devices with optical apertures for the detection of transparent biological cells

This paper provides a novel technique to detect transparent biological living cells trapped in a microfluidic MEMS device by optical diffraction. The device essentially consists of an optical aperture or an aperture array patterned in metal layer and a microfluidic chamber positioned above the center of the aperture. When the cells in the chamber are illuminated through the aperture, the far-field diffraction pattern can be recorded by a CCD camera or a photodetector array. This diffraction pattern uniquely corresponds to the sizes, positions, and intrinsic optical properties of the aperture, cells, and the microfluidic chamber materials, so any unknown but relevant parameter is able to be extrapolated when all other parameters are fixed or identified. This paper describes in detail the designs of various microfluidic chambers and apertures for this application, and the development of a complete set of software for the analysis of the cells' optical properties. Compared with other currently available methods for the detection of transparent living cells, this method has the advantages of simple device structure, easy to manipulate, able to simultaneously detect several cells of different species, as well as providing accurate and sensitive results. Besides the detection of living cells, this technique can also be used to detect or characterize other transparent or low optical absorption particles, such as polymer spheres or insoluble droplets, inside an aqueous solution.

Synthesis and Fabrication of Novel Polymeric/Organic Materials for Micro/Nano-Scale Optical Printed Circuit Board and VLSI Photonic Integrated Circuit Application

We report on the synthesis and fabrication of novel polymer/organic materials for application for what we call optical printed circuit board (O-PCB) for VLSI photonic circuit (VLSI-PIC). We synthesize copolymers containing cross-linkable pendant groups suitable for embossing and novel device fabrication. We also synthesize organic-inorganic hybrid materials of low optical absorption, good adhesion, low dielectric constant, high dielectric strength, high thermal stability, excellent photo-patterning possibility and variability of refractive index. Steps of synthesis and fabrication of these materials are described along with their optical, material, and performance characteristics. O-PCB and VLSI photonic circuit design and fabrication processes are also presented.

Fluoride thin film formation with low optical absorption by gas cluster ion beam assisted deposition

Gas cluster ion beam (GCIB) shows characteristics of low-energy irradiation effect and dense energy deposition. GCIB-assisted deposition is expected to be used for high-quality fluoride film formation, although these films are sensitive to damage by ion irradiation. In this study, LaF3 and MgF2 films were deposited by SF6-GCIB-assisted deposition. When the acceleration voltage was 3–7kV, the deposited LaF3 film had a high packing density, small columnar structure, and very flat surface. The average roughness of the LaF3 film surface was 0.25nm by the surface smoothing effect of the GCIB. The LaF3 film density showed 5.90g/cm3, which was very close to the theoretical density of crystalline LaF3. The refractive index (n) of the LaF3 film increased from 1.62 to 1.72 with an increase in the ion current density. The extinction coefficient (k) was 1×10−3 at a wavelength of 193nm. By the deposition of a thin layer of Al2O3(extinction coefficient: 5×10−3) on the quartz substrate as an interfacial film, a strong adhesive fluoride film was obtained.

Correlations between optical properties, microstructure, and processing conditions of Aluminum nitride thin films fabricated by pulsed laser deposition

Aluminum nitride (AlN) films were deposited using pulsed laser deposition (PLD) onto sapphire (0001) substrates with varying processing conditions (temperature, pressure, and laser fluence). We have studied the dependence of optical properties, structural properties and their correlations for these AlN films. The optical transmission spectra of the produced films were measured, and a numerical procedure was applied to accurately determine the optical constants for films of non-uniform thickness. The microstructure and texture of the films were studied using various X-ray diffraction techniques. The real part of the refractive index was found to not vary significantly with processing parameters, but absorption was found to be strongly dependent on the deposition temperature and the nitrogen pressure in the deposition chamber. We report that low optical absorption, textured polycrystalline AlN films can be produced by PLD on sapphire substrates at both low and high laser fluence using a background nitrogen pressure of 6.0 × 10 − 2 Pa (4.5 × 10 − 4 Torr) of 99.9% purity.

3D Photonic Crystals as Diffractive and Energy Selective Filter for Tandem Thin Film Solar Cells

AbstractWe suggest a photonic structure with energy selective and diffractive properties to be incorporated in thin-film tandem solar cells. Our device enhances the pathway of incident light within a amorphous silicon photovoltaic (PV) top cell in its spectral region of low optical absorption. This leads to an increase in the short-circuit current of the top cell. For a conductive inverted opal structure as intermiediate layer, we numerically determine an current increase of 1.44mA/cm2 for an a-Si:H / &amp;#61549;c-Si thin-film tandem cell corresponding to an increase in the absolute efficiency from 11.1% to 12.4%.

Single phase formation of Co-implanted ZnO thin films by swift heavy ion irradiation: Optical studies

Low temperature photoluminescence and optical absorption studies on 200MeV Ag+15 ion irradiated Co-implanted ZnO thin films were studied. The Co clusters present in as implanted samples were observed to be dissolved using 200MeV Ag+15 ion irradiation with a fluence of 1×1012ions∕cm2. The photoluminescence spectrum of pure ZnO thin film was characterized by the I4 peak due to the neutral donor bound excitons and the broad green emission. The Co-doped ZnO films show three sharp levels and two shoulders corresponding to 3t2g and 2eg levels of crystal field splitted Co d orbitals, respectively. The ultraviolet-visible absorption spectroscopy also shows the systematic variation of band gap after 200MeV Ag+15 ion irradiation.

Light extraction enhancement of InGaN-based green LEDs with a composite omnidirectional reflector

We have designed, fabricated and measured the electroluminescence of InGaN-based green light-emitting diodes (LEDs) having composite omni-directional reflectors (ODRs) deposited on their backside. The composite ODR is composed of a stack of two individual ODRs, each of which is made of alternating layers of TiO2 and SiO2 with a thickness ratio that gives the largest possible 1D photonic bandgap. The lattice constants of these individual ODRs are chosen so that the effective bandgap of the resulting ODR completely covers the emission spectrum of the LEDs. The effective bandgap of our ODR extends from 498 nm to 548 nm. At a driving current of 300 mA, and with the LED emission peak at about 525 nm and a FWHM of about 35 nm, the light output powers of the LED with the composite ODR and the LED with a conventional Ti/Al metal reflector are found to be 52.9 mW and 40.7 mW, respectively. This 30% light extraction enhancement can be attributed to our composite ODR which has a higher reflectance, a lower optical absorption and a wider reflection angle compared with the Ti/Al reflector.

Formation of diffractive microrelief on diamond film surface

The fabrication of diamond-based optical elements for high-power CO 2 lasers is of particular interest because of low optical absorption, high thermal conductivity and weak temperature dependence of the refractive index of diamond. In earlier publications, we presented the results of the implementation of ion-chemical etching technology for generating a diffraction grating on the surface of a diamond film. The results on generating diamond diffractive optical elements and a sub-wavelength antireflection structure by plasma etching were also presented. In this paper, we study the generation of the grating on the diamond surface by ion-chemical etching. The choice of (Ar+O 2) mixture as a working gas and use of niobium as a masking layer are substantiated. The results of studying a diffraction microrelief generated on the diamond-film surface through ion-chemical etching are discussed.

Synthesis and characterization of a photochromic sol–gel material functionalized with azo dye

An azo dye functionalized sol–gel material has been designed and synthesized to possess photochromism under a green laser beam illumination control. 10-mW laser illumination with a spot size of 1mm2 at 532nm wavelength on the film material was found to result in 6×10−4 change in material refractive index. The response time of the material photochromism is around 50ms (3-dB modulation bandwidth of 20Hz). Further optical transmission measurements reveal low optical absorption by the film material at 1.55μm wavelength window. The investigation of a tunable waveguide Bragg grating device with the sol–gel material suggests a new promising platform candidate for the fabrication of all-optical controlled photonic devices.

Analysis of the nonlinear transmission properties of some naphthalocyanines

In our search for the definition of new molecular compounds combining low linear absorption with high nonlinear optical absorption in the same wavelength range, we have prepared a series of vanadyl, chloroindium and lead naphthalocyaninato complexes with a relatively low linear optical absorption within the visible range when compared to phthalocyanines, but with similar chemical stability. The transmission properties of these complexes for nanosecond pulses at 532 nm have been evaluated and confronted in both linear and nonlinear regimes. Halogenated naphthalocyanines displayed the best optical-limiting performances. It was also found that an increase in concentration generally results in a reduction of the limiting efficiency of the systems, due to the phenomena of intermolecular aggregation and consequent excited-state quenching.

Electrical Design Optimization of Single-Mode Tunnel-Junction-Based Long-Wavelength VCSELs

We present principles for tunnel-junction (TJ) design optimization for use in intracavity contacted long-wavelength vertical-cavity surface-emitting lasers (LW-VCSELs). Using the WKB approximation, we find that layer thicknesses of 10 nm on the n++ side and 10 nm on the p++ side are large enough to maximize quantum tunneling probability and small enough to yield low optical free-carrier absorption loss. We also conjecture that our experimental test structures and actual devices have far lower active acceptor concentration than we expect based on an analytical model. Finally, we calculate the necessary doping levels to enable single-mode operation of LW-VCSELs and incorporate these conditions into a complete optimized model of our VCSELs. Based on optimal I-V curves, we can expect an increase in single-mode output power from 2 to 3.5 mW