Depth profiling of the refractive index from probe beam deflections induced by a serrated pump illumination on GaAs

We demonstrate a method to experimentally quantify the distribution with depth of photo-induced refractive index generated by a pump illumination, which is spatially modulated by a serrated mask on the surface of bulk GaAs. This photo-induced refractive index change distributes inhomogeneously with depth thus deflecting the probe beam transversely passing through the depth direction. The refractive index distribution with depth is deduced from the set of deflections of probe beam with respect to the propagating distances, which is collected experimentally in a pump probe arrangement. The obtained set of deflections of probe beam is theoretically proved to be reliable.     

Comparison of Kretschmann-Raether angular regimes for measuring changes in bulk refractive index

We compare two angular regimes for the measurement of changes in the real refractive index of bulk fluid analytes. The measurements are based on the use of the Kretschmann-Raether configuration to sense a change in reflectivity with index. Specifically, we numerically simulate the relative sensitivities of the total internal reflection (TIR) and the surface-plasmon resonance (SPR) regimes. For a fixed-angle apparatus, the method that gives the greatest change in reflectivity varies with metal film thickness. For films thicker than the skin depth, the SPR regime is the most sensitive to index changes. For thinner films, however, the TIR angle is then dominant, with increases in sensitivity on the order of 75% for 10-nm gold or silver media.     

Thermal nonlinear optical response of meso-tetraphenylporphyrin under aggregation conditions versus that in the absence of aggregation

In this work, measurement of thermally induced nonlinear refractive index of meso-tetraphenylporphyrin (H₂TPP) at different concentrations in 1,2-dicoloroethane using a double-grating interferometer set-up in a pump–probe configuration is reported. The formation of aggregates of H₂TPP at concentrations greater than ca. 5 × 10^?5 M was evident by deviation from Beer’s law. An almost focused pump beam passes through the solution. A part of the pump beam energy is absorbed by the sample and therefore a thermal lens is generated in the sample. An expanded probe beam propagates through the sample and indicates the sample refractive index changes. Just after the sample a band-pass filter cuts off the pump beam from the path but the distorted probe beam passes through a double-grating interferometer consisting of two similar diffraction gratings with a few centimetres distance. A CCD camera is installed after the interferometer in which on its sensitive area two diffraction orders of the gratings are overlying and producing interference pattern. The refractive index changes of the sample are obtained from the phase distribution of the successive interference patterns recorded at different times after turning on of the pump beam using Fourier transform method. In this study, for different concentrations of H₂TPP in 1,2-dichloroethane solution the thermal nonlinear refractive index is determined. Also, we present the measurement of the temperature changes induced by the pump beam in the solution. We found that value of nonlinear refractive index increased by increasing the concentration up to a concentration of 5 × 10^?4 M and then decreased at higher concentrations. In addition, we have investigated the stability of the observed thermal nonlinearity after a period of two weeks from the sample preparation.     

Optimisation of distributed feedback laser biosensors

A new integrated optical sensor chip is proposed, based on a modified distributed- feedback (DFB) semiconductor laser. The semiconductor layers of different refractive indices that comprise a laser form the basis of a waveguide sensor, where changes in the refractive index of material at the surface are sensed via changes in the evanescent field of the lasing mode. In DFB lasers, laser oscillation occurs at the Bragg wavelength. Since this is sensitive to the effective refractive index of the optical mode, the emission wavelength is sensitive to the index of a sample on the waveguide surface. Hence, lasers are modelled as planar waveguides and the effective index of the fundamental transverse electric mode is calculated as a function of index and thickness of a thin surface layer using the beam propagation method. We find that an optimised structure has a thin upper cladding layer of ~0.15 mum, which according to this model gives detection limits on test layer index and thickness resolution of 0.1 and 1.57 nm, respectively, a figure which may be further improved using two lasers in an interferometer-type configuration.     

Nonlinear optical behavior of ocular tissue during laser irradiation

A pump (cw Ho-YAG laser) and probe (He-Ne laser) system was used to study the dynamics of the optical behavior of ocular tissue during laser heating. The nonlinear optical behavior of porcine corneal and vitreous-humor tissue was characterized in vitro by means of measurements of the radial profile of a He-Ne laser beam transmitted through the tissue. Temperature gradients in the tissue created by the absorption of pump radiation caused the probe beam to diverge. For constant laser power, the rate of divergence was made dependent on the spot size of the pump beam. The profile of the transmitted probe beam returned to its original magnitude and shape after the tissue was permitted to cool. This reversible change in optical behavior was attributed to the formation of a negative lens owing to thermally induced local gradients in the refractive index of the tissue.     

Refractive-index patterning of tellurite glass surfaces by laser spot heating

A dot pattern of a refractive-index change was formed by spot heating with laser-beam irradiation on sodium tellurite glasses. The 15Na(2)O . 85TeO(2) (mol. %) glass doped with 2 mol. % of CoO was irradiated by a green light-beam spot (532 nm) ~800 mum in diameter from the second-harmonic generator of a Q-switched pulsed YAG laser. The map of the refractive index of the glass was determined with an He-Ne laser beam by a scanning ellipsometric technique at a resolution of 100 mum x 50 mum, indicating that the spots possessing a refractive index lower by ~0.05 were formed at the region irradiated by the laser beam.     

Effect of interfacial refractive index on optical molecular orientation measurements

The sensitivity of optical molecular orientation measurements to assumptions regarding thin film refractive index was investigated. Specifically, the influence of the interfacial refractive index on second harmonic generation (SHG) and linear dichroism measurements made in a total internal reflection (TIR) geometry was probed for five distinct molecular systems. The five molecular thin films ranged from weakly adsorbed species in equilibrium with solution to covalently bound molecules. Polarization data from the two techniques were fit using a range of assumed interfacial refractive indices. Surprisingly, a linear relationship between the difference in calculated apparent orientation angle and the difference in solvent-prism refractive index was observed. The trend indicates that for a TIR geometry, the error introduced by the thin film refractive index is negligible when the difference in solvent and prism refractive indices is less than approximately 0.08. However, there are clearly cases, such as a glass/air interface, in which assumptions regarding the thin film refractive index can result in significant error in the extracted orientation angle.     

Enhancing sensitivity and selectivity of long-period grating sensors using structure-switching aptamers bound to gold-doped macroporous silica coatings

High surface area, sol-gel derived macroporous silica films doped with gold nanoparticles (AuNP) are used as a platform for high-density affinity-based immobilization of functional structure-switching DNA aptamer molecules onto Michelson interferometer long-period grating (LPG) fiber sensors, allowing for label-free detection of small molecular weight analytes such as adenosine triphosphate (ATP). The high surface area afforded by the sol-gel derived material allowed high loading of DNA aptamers, while the inclusion of gold nanoparticles within the silica film provided a high refractive index (RI) overlay, which is required to enhance the sensitivity of the LPG sensor according to our numerical simulations. By using a structure-switching aptamer construct that could release an oligonucleotide upon binding of ATP, the effective change in RI was both enhanced and inverted (i.e., binding of ATP caused a net reduction in molecular weight and refractive index), resulting in a system that prevented signals originating from nonspecific binding. This is the first report on the coupling of aptamers to LPG fiber sensors and the first use of high RI AuNP/silica films as supports to immobilize biomolecules onto the LPG sensor surface. The dual functionality of such films to both improve binding density and LPG sensor cladding refractive index results in a substantial enhancement in the sensitivity of such sensors for small molecule detection.     

Optical nonlinearities of Fe2O3 nanoparticles investigated by Z-scan technique

The single beam Z-scan technique has been used to measure the third-order nonlinear refractive index, γ, the two-photon absorption coefficient, ß, and the variation of refractive index per unit photoexcited carrier density σ_r, and the free carrier absorption cross section σ_ab for quantum-confined Fe₂O₃ nanoparticles coated by a layer of organic molecular in toluene and bare particles in hydrosol. For coated Fe₂O₃ nanoparticles, the ratio γ/ß is enhanced by a factor of five compared to the bare sample. Their figures of merit related to linear and two-photon absorption for ultrafast all-optical devices application were determined.     

Photothermal measurements with a Jamin interferometer

We present a numerical study of the behavior of the signal in a photothermal experiment combining the mirage effect and a Jamin interferometer. Our analysis is limited to a square-pulse excitation by a weakly absorbed Gaussian pump laser beam with a large radius. We investigate the influence of three parameters: the time of illumination of the sample, the time of extinction of the pump beam, and the height of the probe-beam arm of the Jamin interferometer above the sample. We show that the path difference, which induces the variation of intensity at the output of the Jamin interferometer, is caused by both probe-beam deflection and temperature variation of the refractive index of air above the sample. The first effect is dominant for short times, and interferometry is a sensitive tool to monitor it.     

Highly efficient all-optical control of surface-plasmon-polariton generation based on a compact asymmetric single slit

By engaging a compact asymmetric single slit coated with a photorefractive polymer, surface-plasmon-polariton (SPP) generation was efficiently controlled by a pump beam. In the structure, the nonlinear light-matter interaction is enhanced because of the cavity effect, which increases the sensitivity of SPPs to the surrounding dielectric. By variation of the real part of the refractive index together with an interferometric configuration, high on/off switching ratios are achieved. Moreover, the SPP generation and modulation processes are integrated in the same asymmetric single slit, which makes the device ultracompact. Experimentally, a high on/off switching ratio of >20 dB and phase variation of >π were observed with the device lateral dimension of only about 2 μm.     

Tests of a silica aerogel Cherenkov counter

A Cherenkov counter with silica aerogel of refractive index 1.06 as radiator has been constructed and tested. The Cherenkov light produced in 9 cm thick silica aerogel was detected with four 5″-photomultipliers through a 162 cm long air light guide. The photoelectron yield for a 1 GeV/c pion beam was found to be 6 and uniform over a sensitive area of 40 × 30 cm².     

Pulse shaping by the electro-optic effect in chirped periodically poled lithium niobate

We propose an ultrafast pulse shaping method by modulating the pulse phase and amplitude by the electro-optic effect and Bragg diffraction in the aperiodically optical superlattice. Linear-chirped periodically poled lithium niobate is used. The input pulse can be shaped, for example, by compressing it through the extraordinary refractive index change of the crystal by applying and changing the external electric field.     

Quantitative methods for spatially resolved adsorption/desorption measurements in real time by surface plasmon resonance microscopy

A simple method for converting local reflectivity changes measured in surface plasmon resonance (SPR) microscopy to effective adlayer thicknesses and absolute surface coverages of adsorbed species is presented. For a range of high-contrast angles near the SPR resonance where the local metal surface's reflectivity changes linearly with angle, the change in reflectivity at fixed angle is proportional to the change in effective refractive index (eta(eff)) near the surface. This change in eta(eff) can be converted to absolute adsorbate coverage using methods developed for quantitative SPR spectroscopy. A measurement of the change in reflectivity due to changes in refractive index of bulk solutions, i.e., percent reflectivity change per refractive index unit (RIU), is the only calibration required. Application of this method is demonstrated for protein adsorption onto protein/DNA arrays on gold from aqueous solution using an SPR microscope operating at 633 nm. A detection limit of 0.072% change in absolute reflectivity is found for simultaneous measurements of all 200 microm x 200 microm areas within the 24-mm(2) light beam with 1-s time averaging. This corresponds to a change in effective refractive index of 1.8 x 10(-5) and a detection limit for protein adsorption of 1.2 ng/cm(2) (approximately 0.5 pg in a 200-microm spot). The linear dynamic range is Deltaeta(eff) = approximately 0.011 RIU or approximately 720 ng/cm(2) of adsorbed protein. Using a nearby spot as a reference channel, one can correct for instrumental drift and changes in refractive index of the solutions in the flow cell.     

Measurement of the complex refractive index of isotropic materials with mueller matrix polarimetry

The complex refractive index of materials at infrared wavelengths is often determined when absorption measurements are made at selected wavelengths, and then the Kramers-Kronig relationship is used to calculate the real part of the index. Because many organic materials are highly absorbing in the infrared, absorption measurements require a short path length. We report on the use of an attenuated total internal reflection (TIR) method in combination with an infrared Mueller matrix spectropolarimeter to measure the Mueller matrix spectrum of samples from 3 to 14 mum. From the elements of the Mueller matrix the complex refractive index is determined for materials whose TIR interfaces are eigenstates of s and p polarization. The calculated index for water compares well with data found in the literature.     

Controlled frustration of total internal reflection by electrophoresis of pigment particles

A model based on geometrical optics has been developed to describe the photometric observations associated with a novel method to control the reflectance of a surface. In this new reflectance modulation approach, electrophoresis of pigment particles is used to absorb light reflected by total internal reflection (TIR). The pigment particles are sufficiently small that they substantially do not scatter light, but rather they modify the effective refractive index at the reflection interface. An incident light ray interacting with this modified effective index is attenuated in a spectrally selective manner. Although frustrated TIR has been understood and used in various applications for some time, in this case it is used to substantially modify the color of the reflected light, which to our knowledge has not been previously reported. A numerical model of the pigment particle distribution has been developed to describe the observations.     

Tunable ultrafast infrared/visible laser to probe vibrational dynamics

A tunable, ultrafast (approximately 100 fs-approximately 1 ps) laser system generating mid-IR (3-10 microm) and UV/visible (392-417 nm, 785-835 nm) radiation is described and its output characterized. The system is designed to explore vibrational dynamics in the condensed phase in a direct, two-pulse, time-resolved manner, using Raman spectroscopy as the probe. To produce vibrational resolution, probe pulses are spectrally narrowed by use of a long doubling crystal. Frequency-resolved optical gating is used to evaluate beam characteristics. An effective method for determining the temporal overlap of the pump and probe pulses for a one-color, 400 nm configuration is illustrated. Representative results from studies of heme and paranitroaniline vibrational dynamics illustrate the effectiveness of the visible pump-visible probe portion of the system in illuminating fast structure and energy dynamics.     

Porous silica xerogel films as antireflective coatings – Fabrication and characterization

The paper presents a simple fabrication method of porous silica xerogel films. By adding a surface active agent Triton X-100™ to the starting solution, we can considerably reduce the surface tension, which, in turn, allows to fabricate silica films of high porosity. The paper presents the influence of surfactant content and the influence of heating temperature on the refractive index and thickness of the fabricated films. We fabricated silica films of the minimum refractive index below 1.3 and corresponding porosity ∼50%. Due to low refractive index, the elaborated porous silica xerogel films can be applied to reduce the light reflection coefficient in optical systems. In this work the spectral characteristics of the refractive index, extinction coefficients, the reflection and transmission coefficients and also depolarization factor are presented. The paper also provides results of surface morphology of produced layers, obtained using an atomic force microscope.     

Bistatic coherent laser radar signal-to-noise ratio

We investigate the signal-to-noise ratio (SNR) for a bistatic coherent laser radar (CLR) system. With a bistatic configuration, the spatial resolution is determined by the overlap of the transmit beam and the virtual backpropagated local oscillator beam. This eliminates the trade-off between range resolution and the bandwidth of the transmitted pulse inherent in monostatic systems. The presented analysis is completely general in that the expressions can be applied to both monostatic and bistatic CLR systems. The heterodyne SNR is computed under the assumption of untruncated Gaussian optics and untruncated Gaussian beam profiles. The analysis also includes the effects of refractive turbulence. The results show that, for maximum SNR, small transmit and local oscillator beam profiles (e-1 intensity radius) are desired.     

Optically tunable guided-mode resonance filter

We have demonstrated a guided-mode resonance filter (GMRF) whose properties are tunable with laser illumination through the incorporation of a nonlinear dye. Laser illumination causes a change in the refractive index of the dye-doped portion of the structure, leading to controlled tuning of the GMRF reflectance spectrum. Changes in the refractive index of dye-doped regions are proportional to the intensity of the incident laser beam and are as high as Delta n = 0.09. The reflectance tuning effect occurs on a time scale of many seconds and is completely reversible upon termination of the laser illumination.