Effective Optical Index Research Articles

(Invited) Applications of Electrochemistry Toward Blue/Green and SWIR-Wavelength VCSELs

Unlike the more established edge-emitting lasers (EELs) with a horizontal Fabry-Perot (FB) cavity, VCSELs have a vertical cavity with highly reflective top and bottom mirrors sandwiching the active region. This design enables wafer-level testing, one- and two-dimensional arrayed integration, and scalability for low- and high-power applications. VCSELs are poised to surpass EELs as a more versatile and intelligent option for semiconductor lasers in the future.Currently commercially available VCSELs only cover the red to near-infrared (NIR) range of 700-1100nm, whereas EELs offer products across a wide spectral range. The optimal active regions for blue and short-wavelength infrared (SWIR) spectra are only supported on GaN and InP substrates, respectively. Lattice-matched, epitaxial mirrors compatible with these two substrates remain major challenges. As a solution, we propose the use of electrochemistry to engineer the refractive index of the material. The innovation is based on the recognition that nanoporous (NP) semiconductor layers can be considered a nanocomposite with nanoscale voids of air uniformly dispersed in a single-crystalline semiconductor, thus rendering a unique way to control the effective optical index with a tunability that is typically un-attainable through epitaxy.We have studied the electrochemistry of semiconductors for applications in photonics, beginning with GaN and most recently with InP. In both material systems, we have discovered a selective porosification process based on the doping concentration. Due to the doping selectivity in EC etching, layers with high doping concentration will be converted to low index nanoporous material with an unprecedented tunability in the refractive index depending on the porosity. Meanwhile, the lightly doped layers remain intact. As a result, highly reflective mirrors were fabricated on GaN and InP substrates with wide stopbands. In this talk, we will provide latest updates in the VCSEL performances in the blue/green and long-wavelength region.

Electrically tunable optical refractive index of a WS2 film on a SiC substrate

For applications in portable electronic devices and machine vision, a small sized focus-tunable lens or lens system is highly desired. Materials with electrically tunable optical refractive indices are required in these fields. A functional material consisting of a WS2 thin film on a SiC substrate was synthesized and reported in our investigation. A 15-nm-thick WS2 film was deposited on an n-doped SiC substrate (7.37 × 1019 cm − 3) by pulsed laser deposition. The optical refractive index of the WS2 / SiC material was found to be electrically tunable. The value of the effective optical refractive index ranged from 4.03 to 5.84, which implies a 44.9% variation in the refractive index change, at a wavelength of 460 nm under an applied external DC voltage modulated in the range of 0 to 7 V. We also found experimentally that the focal length of the plane lens was tunable from 184 to 156 mm at a wavelength of 650 nm with the applied voltage ranging from 0 to 7 V. Our investigation presents a new way to modulate the refractive index and make the compact focus-tunable lens design convenient.

Effective medium description of plasmonic couplings in disordered polymer and gold nanoparticle composites

Disordered gold nanocomposite films were obtained by introducing spherical gold nanoparticles into a polymer matrix. These films were structurally characterized using microscopy techniques, and then analyzed for their effective optical indices using spectroscopic ellipsometry. It is found that even for a low volume fraction of gold nanoparticles (f~1−5%), the observed plasmonic resonance is affected by electromagnetic coupling between particles, related to the disorder of the particles. We show that the classical Maxwell Garnett Effective Medium Approximation (EMA) fails to predict the measured indices. Since couplings create deformations of the polarizability tensor of individual particles, we propose to take them into account phenomenologically using a modified Maxwell Garnett EMA based on a distribution of ellipsoidal polarizabilities. This modified model, albeit simple, appears appropriate: in simple cases, a unimodal distribution of ellipsoidal polarizabilities is used, allowing for good fits of the experimental data with only two free parameters. Bimodal distributions make it possible to handle more complex cases where the resonance presents a shoulder, suggesting that particles can then be categorized into weakly vs. strongly coupled resonators. Such modified Maxwell Garnett EMAs present the advantage of relying on physically meaningful parameters and could be a general tool for the phenomenological description of plasmonic couplings in different disordered nanocomposites.

About the Heisenberg's uncertainty principle and the determination of effective optical indices in integrated photonics at high sub-wavelength regime

Within the Heisenberg's uncertainty principle it is explicitly discussed the impact of these inequalities on the theory of integrated photonics at sub-wavelength regime. More especially, the uncertainty of the effective index values in nanophotonics at sub-wavelength regime, which is defined as the eigenvalue of the overall opto-geometric problems in integrated photonics, appears directly stemming from Heisenberg's uncertainty. An apt formula is obtained allowing us to assume that the incertitude and the notion of eigenvalue called effective optical index or propagation constant is inversely proportional to the spatial dimensions of a given nanostructure yielding a transfer of the fuzziness on relevant senses of eigenvalues below a specific limit's volume.

Numerical ellipsometry: Examination of growing nickel and rhenium thin films using n–k plane analysis and effective numerical substrates

Abstract Useful thin film information may be obtained using ellipsometry data recorded during film growth. In cases in which the growing film overlies a substrate already coated with other films, it is possible to use a model which includes prior knowledge of the optical properties and layer thicknesses of all underlying films and the substrate. However, accuracy of the analysis is limited by imprecision in knowledge of the underlying structure and is not possible if the underlying material structure is unknown. These problems may be addressed by substituting a four-variable “numerical substrate” in place of the underlying materials. The “numerical substrate” exactly mathematically replaces a model of the physical substrate. It is represented by two effective optical indices which are independent of the growing film and dependent only upon light incidence angle and wavelength. The method may be implemented with a set of five measurements at a single wavelength and incidence angle or a set of three measurements at each of two incidence angles. Measurements may be made in-situ during film growth or ex-situ using “step deposited” films of different thicknesses. The work presented here is to demonstrate the method for nickel and rhenium films deposited on a BK7 substrate by Yamamoto using ion sputtering.

InGaN Light-Emitting Diodes with Multiple-Porous GaN Structures Fabricated through a Photoelectrochemical Etching Process

Multiple-porous GaN structures were formed in InGaN light-emitting diodes (LEDs) structures through a selective photoelectrochemical (PEC) etching process. A 1.3 μm-thick PEC-treated n+-GaN:Si layer consisting of nanoporous, whisker, fiber-shaped, networking, and air-gap structures located at the bottom of the LED structure was fabricated. Multiple-porous GaN structures were formed caused by the non-uniformly lateral etching rate on the n+-GaN:Si layer under a high PEC-biased voltage. An 80 μm lateral etching width and a 70% bottom roughened area ratio were measured on the treated LED structure without affecting the electrical properties. The light output power of the treated LED had an approximate enhancement of 57% compared with the non-treated LED structure. Fabry–Pérot interferences of the treated LED structures were observed in angle-resolved electroluminescence spectra indicating that the effective optical refractive index of the bottom multiple-porous structure was reduced.

Statistical characterization of near-wall radiative properties of a statistically non-homogeneous and anisotropic porous medium

The radiative properties statistical characterization method of Tancrez and Taine has been extended to statistically anisotropic and non-homogeneous porous media. When a homogenized phase of such a medium does not follow Beer’s law on extinction, extinction and scattering coefficients have no physical meaning: the phase is completely characterised by an extinction cumulative distribution function, a scattering cumulative probability, a general phase function and an effective optical index. A simple expression of the emission source term, in local thermal equilibrium conditions, is given for any non-homogeneous and anisotropic porous medium. The radiative transfer can be modelled by a Generalized Radiative Transfer Equation that is also extended to non-homogeneous porous media. This characterization method has been applied to the radiative properties of a packed bed of opaque and diffuse spherical particles bounded by a wall: the local porosity strongly varies near the wall and the homogenized phase is non Beerian. In the bulk of the medium, Beer’s law becomes valid and extinction, absorption and scattering coefficients have been determined.

Comparative study of total absorption of light by two-dimensional channel and hole array gratings

A detailed study of light absorption by silver gratings having two-dimensional periodicity is presented for structures constructed either of channels or of holes with subwavelength dimensions. Rigorous numerical modelling shows a systematic difference between the two structures: hole (cavity) gratings can strongly absorb light provided the cavity is sufficiently deep, when compared to the wavelength, whereas very thin channel gratings can induce total absorption. A detailed analysis is given in the limit when the period tends towards zero, and an explanation of the differences in behavior is presented using the properties of effective optical index of the metamaterial layer that substitutes the periodical structure in the limit when the period tend to zero.

Confined coherent acoustic modes in a tubular nanoporous alumina film probed by picosecond acoustics methods

Coherent GHz acoustic phonon dynamics in a tubular nanoporous alumina film is studied with picosecond acoustic methods. This study has allowed the determination of the out-of-plane sound velocity and the effective optical index through a time-resolved Brillouin analysis. Several acoustic eigenmodes of the tubular nanoporous alumina film were detected. These GHz eigenmodes exhibit unusual frequency-dependent long lifetimes. Their strong confinement has been explained by the presence of a thin alumina layer at the film/substrate interface which plays the role of an acoustic mirror. Finally, we show that the optical detection process of these long-living eigenmodes can be either enhanced or reduced to zero following a selection rule. In particular, we demonstrate that detection configurations exist where the coherent acoustic phonon and probe light sensitivity functions are orthogonal, leading to cancellation of the photoelastic light scattering mechanism. This has never been reported so far in the physics of the interaction of coherent acoustic phonons with light.

Focusing the electromagnetic wave with a magnetic field

We examine manipulating the electromagnetic (EM) wave with an external static magnetic field (ESMF) taking advantage of the versatility of the magnetic photonic crystal (PC). The effect of a nonuniform ESMF on the permeability of the constituent magnetic material in the PC is demonstrated to create a gradient of the effective optical index in the crystal, leading to the focusing of the EM wave, with a magnetically tunable focal length, focused waist radius, and the intensity at the focus.

Towards -1 effective index with one-dimensional metal-dielectric metamaterial: a quantitative analysis of the role of absorption losses

We propose a theoretical study of the optimization of one dimensional metal-dielectric metamaterials in order to approach -1 effective optical index. Taking into account actual values of dielectric constants of metal (silver) and dielectrics (HfO(2), GaP), and taking advantage of the dispersion relation of Bloch modes, we get a silver/HfO(2)metamaterial with suitable parameters that possesses a near -1 effective optical index for all angles of incidence at a visible wavelength for H-polarized light (i.e. the magnetic field is parallel to the interfaces). The absorption losses of materials appear to be a crucial factor that affects the effective properties of the metamaterial. We show that the losses not only decrease the transmission of the stack, but also change the negative refraction effect. Then, we propose another silver/GaP structure design that is less sensitive to losses. When considering finite thickness structures, and with adequate thickness for the terminating layers, it is possible to achieve a high transmittance of the structure. A near -1 effective index and high transmittance metal-dielectric metamaterial may pave the way to the realization of negative refraction in the visible or ultraviolet wavelength range.

Large optical Kerr effect in matrix-embedded metal nanoparticles

We report an experimental and theoretical study on the third-order nonlinear optical properties of gold particles embedded in a dielectric host. Thin films of various metal concentrations are elaborated by the sputtering technique. Several complementary techniques provide information about the metal concentration, the film thickness and the size of the particles. The effective complex optical index of the film is determined by spectrophotometry and ellipsometry. The third-order nonlinear susceptibility, χ (3), is measured using the z-scan technique in the nanosecond regime. Experimental results reveal very large nonlinearities, increasing with the metal concentration. Theoretical calculations of the amplification factor, based on a recursive transfer method, allow a better understanding of these results.

Optical characterisation of 6t and 4t single crystals by ellipsometry; anisotropy and crystalline structure

Ellipsometry and absorption spectroscopies have been carried out on sexithiophene (α-6T), and quaterthiophene (α-4T) single crystals grown at High Temperature (HT) or Low Temperature (LT). Effective Optical Indices (EOI) neff and keff have been obtained in two positions of the crystals: b axis being either perpendicular to the yz incidence plane (α=0 °) or parallel to Oy (α=90 °).The spectra contain only a strong absorption feature around 3.4eV and 3.7eV for 6T and 4T respectively. We have compared experimental data and those obtained by simulation of EOI considering a biaxial semi-infinite medium with its three orthogonal principal axis parallel to Ox, Oy, Oz directions. The strong anisotropy of the 4T and 6T LT forms induces an anomalous dispersion curve of neff and a blue shift of keff (~0.1–0.3eV) regardless of the crystal set. The weak anisotropy of the HT form, leads to an anomalous dispersion and a blue shift for keff (~0.1–0.3eV) only for the crystal set α=0 °. The absorption anisotropy or (kz-kx,y) is related to the angle between the L molecular axis and the crystal plane (b, c).

Fabrication of sidelobe-suppressed InP-InGaAsP vertical coupler optical filter using pair grating structure

In this letter, we report -19-dB suppression of sidelobes in a grating-assisted codirectional coupler (GACC) optical filter, which is the lowest to the best of our knowledge. Pair grating structures has been employed to control the coupling strength with a raised cosine functional form. A unit pair grating is composed of two unit conventional gratings and the fundamental period of pair gratings is fixed at 2/spl Lambda/, where /spl Lambda/ is the grating period of conventional GACC filter. The distance between the two gratings inside a pair grating is linearly shifted to control the coupling strength. This structure provides a spatially weighted coupling strength without any change of averaged effective optical index or propagation constant.

Increasing channel capacity and range on urban optical analog AM-VSB CATV systems

It is often desirable to increase the capacity and the covered distance in lightwave TV systems. For doing so, the conventional CATV channel set-usually placed within the 50 to 550 MHz frequency range-may be up-shifted, by means of a subcarrier, to a low microwave frequency range, typically located at 2 GHz. Next, this shifted spectrum directly modulates a laser diode, in the analog AM mode. It is shown that this up-shift procedure may dislodge a number of nonlinear distortion components from the useful TV band. Consequently, the system effective optical modulation index may be increased, allowing for an expansion in the channel capacity or covered distance. This enlarged capacity or distance may compensate for the modest price increase concerning the up-shift equipment. All the performance evaluations presented here were carried out with a rigorous numerical model, and assuming the lightwave systems used the economical AM-VSB format.

In situ control of the growth of GaAs / GaAlAs structures in a metalorganic vapour phase epitaxy reactor by laser reflectometry

Metalorganic vapour phase epitaxy (MOVPE) growth of GaAs and GaAlAs has been studied by laser reflectometry using two wavelengths. At 1.32 μm, GaAs and GaAlAs are totally transparent for our growth temperature (750°C) so that layers up to a few micrometres thick can be monitored. On the other hand, 0.514 μm proves to be an optimum wavelength for the control of Bragg mirror, with layer thickness kin the range 60 to 90 nm. Effective optical indices have been determined for both wavelenghts by comparing in-situ and ex-situ measurements. By 1.32 μm reflectivity, in-situ determination of the composition of GaAlAs thick layer as well as the kperiod of a GaAs/GaAlAs multi quantum well (MQW) have been determined. Finally, by 0.514 nm reflectivity, Bragg reflectors centred at 980 nm have been grown. X-ray diffraction and reflectivity measurements performed on the reflectors confirm a 1% reproducibility and accuracy of the stop band centre wavelength.

In-situ thickness measurement of MOVPE grown GaAs GaAlAs by laser reflectometry

In-situ laser reflectometry was used at four different test wavelengths to monitor the thickness of GaAs/GaAlAs layers grown by metal organic vapor phase epitaxy. The studied wavelengths are 1.32 μm, 633 nm, 514 nm, and 442 nm. The effective optical indices have been calibrated at the growth temperature for different compositions of GaAlAs. According to these measured indices and under the requirements of an empirical thickness criterion, the range of in-situ thickness monitoring has been determined at the different wavelengths for each studied materials. Finally, as a demonstration, in-situ laser reflectometry has been successfully applied to the growth of some devices, showing a significative improvement of the reproducibility and the accurancy of the caracteristics of these devices.

Fluctuational crosstalk in Ti:LiNbO/sub 3/ directional coupler switches

It has been found that the fluctuations in the Ti concentration along waveguides of Ti:LiNbO/sub 3/ directional coupler switches degrade the switch extinction ratio and introduce an unwanted crosstalk between both channels. We discuss analytically the effect of effective optical index fluctuations on the transfer matrix of the optical switch. We demonstrate the sensitivity of the mismatch to the statistics of the fluctuations of the Ti concentration. >

In situ metalorganic vapor phase epitaxy control of GaAs/AlAs Bragg reflectors by laser reflectometry at 514 nm

In situ reflectometry with a 514-nm laser beam was used to monitor AlAs and GaAs layer thicknesses grown by metalorganic vapor phase epitaxy. The effective optical indices of these materials have been calibrated at the growth temperature by using an original method based on ex situ double crystal x-ray diffraction measurement. According to these measured indices, the in situ laser reflectometry at 514 nm appears to be well suited for a real-time thickness control of the GaAs/AlAs based Bragg reflectors. Finally, Bragg reflectors centered at 980 nm have been grown using the reflectometry at 514 nm. X-ray diffraction and reflectivity measurements performed on these reflectors confirm a 1% reproducibility and accuracy of the wavelength stop band center.

Distortion and cnr characteristics in fm‐fdm optical catv transmission

AbstractIn the multichannel frequency division multiplex (FDM) optical transmission, the intermodulation distortion generated by the nonlinearity of the LD is a problem. In this paper, it is analyzed that for the number of transmission channel, N, and the optical modulation index per channel, m, the CSO and CTB are constant for a different value of N if the effective optical modulation index, is maintained. The results are compared with experiments. In the present analysis, two types of nonlinearity of the current‐to‐optical conversion characteristics are considered. One of them is the nonlinearity near the bias current of the LD and is predominant in the region of a small effective optical modulation index. Another is the nonlinearity at the threshold value of the LD and is dominant when the effective optical modulation index is large. It is shown by means of the probability density function that the CSO and CTB degrade suddenly if the effect of the nonlinearity of the threshold value appears. In the FM‐FDM optical transmission, many distortions in the transmission bandwidth behave as the white noise due to frequency modulation when the carrier is frequency modulated by a video signal. Hence, the CN ratio is effectively degraded. In this paper, it is further presented that the CN ratio degraded by the distortions can be derived from the CSO and CTB by taking into consideration the frequency allocation of the FM signal.