The effects of deposition parameters on the optical performance of thin dielectric films

For certain applications of multilayer optical systems the need for minimum scattering losses is paramount. From the polycrystalline nature of most optical films, some scattering is unavoidable. Its extent depends on the conditions of deposition, especially the rate of deposition and substrate temperature. There is however considerable disagreement in the literature on the effects of these parameters and it is suspected that observed effects of these parameters have been masked by unrecognized extraneous effects. In this paper results are given for scattering losses in MgF₂ and ZnS films measured by three methods, namely (a) lateral waves, (b) waveguide modes and (c) integrating sphere. In addition, ellipsometric studies have been made of ZnS films. Variations of scattering loss with the deposition parameters mentioned have been studied, leading to the establishment of conditions of preparation which produce minimal scattering. The results, together with electron microscope studies of the films, provide some insight into the scattering process.     

Optical waveguides for imaging applications: Design by an inverse scattering approach

An inverse scattering theory is used to design optical waveguides capable of transmitting spatial images without degradation. The propagation characteristics of N modes each carrying a pixel of the image are specified by a transverse rational reflection coefficient. The Gel'fand-Levitan-Marchenko inverse scattering theory is used to obtain the unique solution of the permittivity profile of the waveguide from the reflection coefficient     

A procedure to obtain the complete electromagnetic scattering properties at discontinuities between integrated optical waveguides

Abstract

A method to obtain the complete electromagnetic scattering properties of discontinuities between arbitrary integrated optical waveguides is presented. The method involves a new generalized scattering matrix concept, together with the generalized telegraphist equations formulism and the modal matching technique. Radiation losses, as well as reflection and transmission coefficients between proper modes, can be obtained. Single and multiple discontinuities in planar and channel optical waveguides have been analysed. Numerical results of complex scattering coefficients are given. The possibilities of the method for analysing waveguide photonic crystals, as well as optical devices in waveguide periodic waveguide structures, are demonstrated.     

Squeezing from Raman Scattering in Optical Parametric Oscillators

Abstract

The squeezing properties of the Raman scattering process which takes place when the lower output frequency from a non-degenerate optical parametric interaction approaches a resonance of the nonlinear medium placed inside an optical resonant cavity are presented. The linear stability analysis is performed and the spectrum for the output fields is given in terms of the quadrature phase components in the Wigner representation. Perfect squeezing for the amplitude difference between the output Stokes and anti-Stokes modes at the Hopf bifurcation point is obtained.     

Nanoscale all-optical plasmonic switching using electromagnetically induced transparency

An all-optical switch composed of two interacting nanoparticles in front of an optical dielectric slab waveguide is proposed. An incident optical signal is coupled to the optical waveguide after scattering by the two nanoparticles. The scattered fields interfere constructively or destructively depending on the degree of optical transparency the nanoparticles induced by an optical control signal. The considered nanoparticles have a metallic core coated by an outer shell with three-level clusters such that the nanoparticles can exhibit electromagnetically induced transparency. A dipole-approximation model-based analysis reveals that a high rejection ratio can be achieved using the proposed configuration.     

Magneto-optical stokes polarimetry and nanostructured magnetic materials

Stokes parameters fully characterize the polarization state of light in an experimentally accessible manner. Photoelastic modulator (PEM) based Stokes polarimetry offers a very high sensitivity which is particularly suitable for the investigation of the magneto-optical properties of nanostructured magnetic materials. In this paper, we shall describe a robust methodology recently developed by us that utilizes a dual PEM setup. As an example of its application, we report on the magneto-optical characteristics of focused Ga ion beam patterned Fe films. We have investigated Ga ion irradiation of single-layer polycrystalline Fe films deposited on Si3N4 substrates, which allows us to study the effects of ion implantation with minimum added complications. Complemented by structural and other characterization techniques, the absolute measurement of magneto-optical effects through the determination of Stokes parameters has enabled us to effectively separate the various contributions from film thinning due to sputtering, structural modifications and compositional changes caused by Ga incorporation. A comparison is also made between the magneto-optical behavior of patterned thin films and that of anodic aluminum oxide embedded magnetic nanowire arrays.     

Analysis of optical waveguide structures by use of a combined finite-difference/finite-difference time-domain method

We present a method for full-wave characterization of optical waveguide structures. The method computes mode-propagation constants and cross-sectional field profiles from a straight forward discretization of Maxwell's equations. These modes are directly excited in a three-dimensional finite-difference time-domain simulation to obtain optical field transmission and reflection coefficients for arbitrary waveguide discontinuities. The implementation uses the perfectly-matched-layer technique to absorb both guided modes and radiated fields. A scattered-field formulation is also utilized to allow accurate determination of weak scattered-field strengths. Individual three-dimensional waveguide sections are cascaded by S-parameter analysis. A complete 10(4)-section Bragg resonator is successfully simulated with the method.     

Estimation of ultrasonic guided wave mode conversion in a plate with thickness variation

The hybrid boundary element method aimed at analyzing Lamb wave scattering from defects can provide us with an excellent numerical tool for tackling complicated mode conversion phenomena under waveguide thickness variation. In this paper, utilization of hybrid boundary element modeling for specific Lamb wave mode incidence situations with special energy distributions along the structural cross section is proposed for estimating reflection and transmission from various scatterers, such as a step discontinuity and tapered parts of a waveguide, etc. Interaction of individual Lamb wave modes with scatterers that represent arbitrary thickness variation along the direction of guided wave propagation is investigated by calculating the scattered fields for varying incident modes, frequency, and scatterer shape. The mode conversion phenomena through step discontinuity in a plate are also experimentally explored. The theoretical predictions of reflection and transmission by boundary element methods and the utility of dispersion curves are compared with experiments for specific modes. Results in this paper can be used to improve inspection sensitivity and penetration power for a variety of practical NDE applications, notably those in which thickness variation is found. In addition, the feasibility of inspecting sections located behind a waveguide thickness variation region and subsequent mode control will also be discussed.     

Statistical Characterization and Process Control for Improved Growth of La2−x Sr x CuO4 Films

We have used combinatorial molecular beam epitaxy (COMBE) technique to deposit thin cuprate films with continuous spread in chemical composition, as well as nominally uniform films. We have patterned them into linear pixel arrays and measured the transport properties of each pixel. We applied detailed statistical analysis to differentiate between various possible sources of random pixel-to-pixel variations, and utilized this knowledge to considerably tighten the process parameters and significantly reduce such variations. The density and quality of data points is high enough to allow detection of quantum phase transitions induced by tuning the chemical composition.     

Quantum Statistical Properties of Photon and Phonon Fields in Brillouin Scattering with Intense Pumping

The quantum statistical properties of Brillouin scattering of intense laser light are derived including the coupling of Stokes, anti-Stokes and phonon modes, if the anti-Stokes interaction prevails. Making use of the coherent-state technique, the Heisenberg equations of this process are solved neglecting the loss mechanism, and the normal quantum characteristic function and the normal generating function are derived. The time dependences of the photon distribution and its factorial moments are demonstrated if the phonon, Stokes and anti-Stokes modes are initially in a coherent state and periodical anti-bunching of the field is found when the phases of the incident fields fulfil certain phase conditions; the field can also return to the coherent state again.     

Intracavity Waveguide Spectroscopy of Thin Films

The method of intracavity waveguide spectroscopy for measuring low optical losses in thin films is proposed. The method also allows one to distinguish transverse and longitudinal modes in low-gain lasers without introducing considerable losses into the cavity.     

Direct fabrication of surface relief gratings in chalcogenide glasses by excimer laser interference lithography

In this paper we study the possibility to realise surface relief gratings in thin chalcogenide glass films by holographic exposure using a pulsed KrF excimer laser. Gratings with a period of 540 nm and depths of 100–300 nm were patterned at the surface of 1 μm thick films. Due to coupling of an incident near-infrared laser beam to waveguide modes a resonance-like polarisation dependent decline of transmission was observed at specific incidence angles. Just one laser pulse with a fluence of 12 mJ/cm² per beam was sufficient to achieve the required grating parameters in sulphide glasses with low T _g.     

Coherent population trapping in an rf-optical double resonance experiment in a neon discharge

We observed radio-frequency (rf) induced transparency for a probe optical field in an rf-optical double resonance experiment in a neon discharge. The origin of the transparency is coherent population trapping at Zeeman sublevels of the lower level of an optical transition of neon. Stokes and anti-Stokes optical fields were generated due to stimulated Raman scattering of both the radio-frequency field on the probe-induced optical coherence and the probe field on the Zeeman coherence.     

Raman spectra of intrinsic and doped hydrogenated nanocrystalline silicon films

Raman scattering characteristics of intrinsic and doped hydrogenated nanocrystalline silicon films which prepared by a plasma-enhanced chemical vapor deposition system are investigated. Results indicate that Raman spectra depend intensively on microstructure and impurity in the films. Taking into account phonon confinement effect and tensile strain effect in Si nanocrystals, peak redshift of measured transverse optical modes in Raman spectra of intrinsic films can be well interpreted. With respect to Raman scattering from doped samples, besides phonon confinement effect, the peak of experimental transverse optical mode further downshifts with heightening doping level, which can be primarily assigned to impurity effect from doping. In addition, the increase in relative integral intensity ratio of transverse acoustic branch to transverse optical mode and that of longitudinal acoustic branch to transverse optical mode with decreasing mean dimension of nanocrystals and heightening doping ratio, respectively, can be ascribed to disorder. Furthermore, at the same doping level, incorporation of boron can induce higher disorder than incorporation of phosphorus in nc-Si:H films.     

Quantum Theory of Hyper-Raman Scattering

Abstract

We develop the quantum theory of stimulated hyper-Raman scattering (HRS) and discuss the quantum statistics of pump and Stokes fields. We show that the initially coherent pump field acquires a strong squeezing that exceeds the similar squeezing in other nonlinear processes also involving two-photon absorption. Stokes field statistics are analysed in detail as a function of the initial statistics of both modes. Bunching effects are shown to be amplified in the short-time region, while in asymptotics the antibunching of Stokes photons becomes dominant, which in many cases results in sub-Poissonian statistics of Stokes mode. We also find a number of correlation effects between the photons of the two modes.     

Graphene‐Assisted Growth of Patterned Perovskite Films for Sensitive Light Detector and Optical Image Sensor Application

Controlled growth of high‐quality patterned perovskite films on a large scale is essentially required for the application of this class of materials in functional integrated devices and systems. Herein, graphene‐assisted hydrophilic–hydrophobic surface‐induced growth of Cs‐doped FAPbI₃ perovskite films with well‐patterned shapes by a one‐step spin‐coating process is developed. Such a facile fabrication technique is compatible with a range of spin‐coated perovskite materials, perovskite manufacturing processes, and substrates. By employing this growing method, controllable perovskite photodetector arrays are realized, which have not only prominent photoresponse properties with a responsivity and specific detectivity of 4.8 AW^?1 and 4.2 × 10¹² Jones, respectively, but also relatively small pixel‐to‐pixel variation. Moreover, the photodetectors array can function as an effective visible light image sensor with a decent spatial resolution. Holding the above merits, the proposed technique provides a convenient and effective pathway for large‐scale preparation of patterned perovskite films for multifunctional application purposes.     

Optical properties of zinc sulphide thin films prepared by sublimation in ultrahigh vacuum and by R.F. sputtering in argon

Thin films of zinc sulphide, in single-crystal and in polycrystalline form, were grown by vacuum deposition methods onto silicon and gallium arsenide substrates. The thin film optical constants are determined from a computer solution of the optical waveguide dispersion relationship for measured effective indices and loss of guided modes excited by prism coupling of laser light at discrete wavelenghts between 465.8 and 632.8 nm. Sputtered films exhibit much higher levels of absorption than films prepared in ultrahigh vacuum from a molecular beam source, yet both preparation techniques give consistent values of the refractive index close to those measured for bulk material. High absorption also occurs in films prepared from heavily doped sources. The lowest absorption coefficient obtained was 1.4 cm^-1 at 632.8 nm in an ultrahigh-vacuum-deposited film. At the same wavelenght, bulk absorption is 0.7 cm^-1 and sputtered film absorption is 10.6 cm^-1.     

Elastic light scattering with a LiNbO3 waveguide

We present the measurements of hemispherical elastic scattering of light guided by different modes of a planar LiNbO3 waveguide. It is shown that the fundamental and the lowest-order modes are more sensitive to the scattering properties of the air-core interface, whereas higher-order modes are more sensitive to the optical inhomogeneities of the core-substrate interface. We also demonstrate that because of static polarization of LiNbO3 crystal, the air-core interface is sensitive to the presence of dust particles in air, which causes a change in the scattered light based on time of observation. This sensitivity could be used to elaborate compact sensors of air contamination.     

Image properties of polycrystalline storage films

Polycrystalline storage films are applied in various fields of x-ray radiography. Depending on the application, different requirements exist for the sensitivity and spatial resolution of the film, and these are related to light scattering in the film. Scattering is controlled by optical properties of the storage medium, e.g., the scattering length by the grain size, the absorbance by the choice of the binder, and the geometric boundary conditions by the film thickness. A method treating the scattering of light as a diffusionlike process is presented. The sensitivity, resolution, and other image properties are determined from the optical properties. The influence of the scattering length, absorbance, and thickness on image parameters is calculated and discussed. The results indicate that optimizing the image properties of the film can be achieved by a reduction of the absorbance and an increase in film thickness to the penetration depth of the x rays. Furthermore, it is shown that the resolution of the film can be set strictly according to the requirements of an individual application if the scattering length in the medium is adjusted.     

Multiplicative and subtractive focal volume engineering in coherent Raman microscopy

Rigorous calculations are performed to study the effective reduction of the nonlinear excitation volumes when using phase-only masks to condition the pump and Stokes driving fields. Focal volume reduction was achieved using both a multiplicative operation of the excitation fields as well as a subtractive operation. Using a tunable optical bottle beam for the Stokes field, an effective reduction of the width of the excitation volume by a factor of 1.5 can be achieved in the focal plane. Further reduction of the focal volume introduces a rapid growth of sidelobes, which renders such volumes unsuitable for imaging applications. In addition, phase sensitive detection was found to provide information from selective sub-divisions of the engineered coherent anti-Stokes Raman scattering excitation volume. In the case of isolated nanoparticles, an apparent resolution improvement by a factor of 3 is demonstrated, and it is shown that the size of sub-diffraction-limited particles can be accurately determined using phase sensitive detection.