Rough Dielectric Film Research Articles

Performance Enhanced Thermopile With Rough Dielectric Film Black

In this letter, we report a performance enhanced thermopile with rough dielectric film black. The pyramidally-textured structure is in-situ formed in the thin dielectric film as an infrared absorber by hard-molding method. Compared to flat dielectric film-based thermopile with the same size, the output voltage, responsivity, and detectivity of the thermopile with rough dielectric film black can be improved by approximately 38%, while the response time constant remains less than 10 ms. The pyramidally- textured dielectric film provides a new opportunity to enhance the performance of thermopiles with the simple process as well as low cost.

Differentiation of roughness and surface defect impact on dielectric strength of polymeric thin films

Increasing the dielectric strength of polymer films has been a key theme as it is directly responsible for increasing energy density of relevant components such as film capacitors and insulation tapes. Dielectric films with higher roughness and surface defects are subject to the formation of an air gap at the interface between dielectric film and metallised polymer electrodes, which results in inaccurate dielectric strength. The air gap due to roughness was found to result in dielectric strength of 25% higher than that using depositing metal on dielectric films (integral electrode). The integral electrode method is proven to be a better way to test the genuine dielectric strength of thin and rough dielectric films. Surface defects, on the other hand, were revealed to cause lowering of dielectric strength because of their contribution to the localised electric field and charge injection. The detrimental effect of surface defects can be suppressed by submerging the film in oil or coating the film with an oxide layer.

Selective enhancement of Selényi rings induced by the cross-correlation between the interfaces of a two-dimensional randomly rough dielectric film

By the use of both perturbative and non-perturbative solutions of the reduced Rayleigh equation, we present a detailed study of the scattering of light from two-dimensional weakly rough dielectric films. It is shown that for several rough film configurations, Selényi interference rings exist in the diffusely scattered light. For film systems supported by dielectric substrates where only one of the two interfaces of the film is weakly rough and the other planar, Selényi interference rings are observed at angular positions that can be determined from simple phase arguments. For such single-rough-interface films, we find and explain by a single scattering model that the contrast in the interference patterns is better when the top interface of the film (the interface facing the incident light) is rough than when the bottom interface is rough. When both film interfaces are rough, Selényi interference rings exist but a potential cross-correlation of the two rough interfaces of the film can be used to selectively enhance some of the interference rings while others are attenuated and might even disappear. This feature may in principle be used in determining the correlation properties of interfaces of films that otherwise would be difficult to access.

Backscattering of light from a dielectric layer on a reflecting substrate.

Scattering of light from rough dielectric films is studied experimentally. It is shown that the interference pattern of the scattered field depends critically on the power spectrum of the roughness, especially on its long-scale component. When the height of roughness is small compared with the wavelength, the backscattering peak (if it exists) is due to the interference of the singly scattered fields; hence the properties of the backscattered peak are rather unusual. In particular, the width of the peak is determined by the thickness of the film and is independent of the parameters of disorder. The intensity of the peak increases with an increase of the rms height of the surface roughness and becomes independent of the rms when the roughness is of the order of the wavelength.

High-order correlations from rough-surface scattering.

It is proposed to apply an optical setup of a randomly weak rough dielectric film on a reflecting metal substrate for the measurement of high-order correlations from rough-surface scattering. The angular amplitude and intensity correlations are measured. Because of multiple scattering, when the input laser beam size is comparatively small or close to the travel pass length inside the film, C(2) and C(3) are measured by subtraction of the amplitude correlation from the intensity correlation.

Grazing angle enhanced backscattering from a dielectric film on a reflecting metal substrate

We have recently observed several features from a randomly rough dielectric film on a reflecting metal substrate including a change in the spectrum of light at the satellite peaks, the high-order correlation, and enhanced backscattering from the grazing angle. In this paper, we focus on the enhanced backscattering phenomena. The backscattering signal at small grazing angles is very important for vehicle re-entrance and subsurface radar sensing applications. Recently, we performed an experimental study of far-field scattering at small grazing angles, espe- cially enhanced backscattering at grazing angles. For a randomly weak, rough dielectric film on a reflecting metal substrate, a much larger en- hanced backscattering peak is measured. Experimental results are com- pared with theoretical predictions based on a two-scale surface rough- ness scattering model. © 2004 Society of Photo-Optical Instrumentation Engineers.

Enhanced backscattering at grazing angles

Backscattering signals at small grazing angles are important for space vehicle atmospheric reentrance and subsurface radar sensing applications. They are also useful in Fourier-transform infrared grazing-angle microscopy. Recently we performed an experimental study of far-field scattering at small grazing angles, in particular, of enhanced backscattering at grazing angles. For a randomly weak rough dielectric film upon a reflecting metal substrate, a large enhanced backscattering peak was measured. Experimental results are compared with small perturbation theoretical predictions.

Light scattering from an amplifying medium bounded by a randomly rough surface: A numerical study

We study by numerical simulations the scattering of $s$-polarized light from a rough dielectric film deposited on the planar surface of a semi-infinite perfect conductor. The dielectric film is allowed to be either active or passive, situations that we model by assigning negative and positive values, respectively, to the imaginary part $\epsilon_2$ of the dielectric constant of the film. We study the reflectance ${\cal R}$ and the total scattered energy ${\cal U}$ for the system as functions of both $\epsilon_2$ and the angle of incidence of the light. Furthermore, the positions and widths of the enhanced backscattering and satellite peaks are discussed. It is found that these peaks become narrower and higher when the amplification of the system is increased, and that their widths scale linearly with $\epsilon_2$. The positions of the backscattering peaks are found to be independent of $\epsilon_2$, while we find a weak dependence on this quantity in the positions of the satellite peaks.

Near-field and far-field changes in the spectrum of light scattered from a randomly rough surface

We consider the scattering of monochromatic and polychromatic light from a randomly rough dielectric film deposited on a planar, perfectly conducting surface. Setting up scattering integral equations with the image Green's-function technique, we develop improved Monte Carlo simulations and use them to investigate the as yet unstudied evolution of frequency and angular spectra of the scattered light on propagation from the near to the far zone. In the monochromatic case, we illustrate the disappearance of the evanescent component and the formation of coherent enhanced backscattering and satellite peaks with increasing distance from the surface. In the polychromatic case, we find that the scattering of light from a rough surface produces significant spectral changes, which tend to increase on propagation from the near to the far zone. Coherent features in the intensity of the scattered light (such as satellite peaks), whose angular positions are frequency dependent, are found to cause very pronounced spectral changes in the scattered radiation. Since the existence and intensity of these coherent features are determined by the shape of the power spectrum of the surface roughness, and by the spectrum of guided or surface waves in the scattering system, we conclude that the frequency spectrum of the scattered light in different zones provides rich and physically meaningful information about the scattering system.

Light scattering from slightly rough dielectric films

It is shown that the long-scale (smooth) component of the roughness spectrum of a slightly rough dielectric layer critically affects the angular distribution of radiation scattered from the surface. The interference pattern obtained from a sample with only small-scale roughness differs drastically from a sample with the same small-scale roughness but possessing slight (of the order λ/10) variation of the thickness of the dielectric layer. It is shown that when interference phenomena are significant and the dielectric film has long-scale roughness, conventional perturbation theory is invalid, even if the rms of roughness is much smaller than the wavelength. A model is presented that correctly predicts the measured angular intensity distributions in the scattered-light field for samples that possess arbitrary scales of roughness.

Scattering by randomly rough dielectric surfaces and rough dielectric films: influence of the height distribution

Scattering of electromagnetic waves by randomly rough surfaces can be modelled efficiently by perturbation theory if the root mean square (rms) height is smaller than 0.06. In this regime, the angular scattering pattern depends strongly on the correlation length a. It will be shown that for penetrable media, the height distribution strongly influences the angular scattering pattern. This property is illustrated with two different systems: a randomly rough dielectric surface and a rough dielectric film deposited on a perfectly conducting substrate. A model recently introduced (mean-field theory) yields a physical explanation of this behaviour. This approach is used to analyse the fringes produced by a randomly rough dielectric film deposited on a flat reflecting susbtrate. It is shown that the features of the fringes can be explained when accounting for the height distribution.

Scattering of light from a rough dielectric film on a reflecting substrate: diffuse fringes

The scattering of light from a one-dimensional randomly rough dielectric film deposited on a flat reflecting substrate is studied by means of a small-amplitude perturbation theory and rigorous numerical simulations. In particular, we study the appearance of well-defined fringes in the angular distribution of the diffusely scattered intensity and their dependence on the angle of incidence, the roughness of the film, and the film’s mean thickness. We find that, for slightly rough films, the angle of incidence modulates the intensity of the fringes but has no effect on their angular position. For rougher films the contrast of the pattern decreases, and the fringes move with the angle of incidence in such a way that there are always bright fringes in the specular and backscattering directions. Eventually, for very rough films, the fringe pattern disappears, and a well-defined backscattering peak emerges in the retroreflection direction.

Angular correlation function of speckle patterns scattered from a one-dimensional rough dielectric film on a glass substrate

We present the experimental results of the angular correlation function of far-field speckle patterns scattered by a one-dimensionally random rough surface of a thin dielectric film on a glass substrate when a polarized beam of light is incident upon the rough surface from vacuum. This surface, which separates the vacuum and the dielectric, is rough enough that only diffused speckles are observed. The experiment for the correlation measurement was set up to make use of a CCD camera to obtain the image of the speckle pattern in the specular direction for each given angle of incidence; the cross-correlation function is then calculated from the digitized images. It is found that the intensity correlation functions exhibit two distinct maxima: one arises from the autocorrelation and the other from the reciprocity condition. It is also found that different scattering processes give rise to quite different correlation functions: multiple-scattering processes produce narrow peaks with secondary maxima and single-scattering processes produce relatively broad peaks.

Numerical solutions of the reduced Rayleigh equation for the scattering of electromagnetic waves from rough dielectric films on perfectly conducting substrates

In the scattering of electromagnetic waves from a one-dimensional surface, the reduced Rayleigh equation is the single integral equation satisfied by the scattering amplitude that is obtained when the electromagnetic field in the scattering medium has been eliminated from the problem by the use of Green's second integral identity. We have solved the reduced Rayleigh equation numerically for the scattering of p- and s-polarized light from a one-dimensional, randomly rough dielectric film on a planar, perfectly conducting substrate, when the thickness of the film is such that it supports two or more guided waves of each polarization. The solution has been carried out by generating a segment of rough vacuum-dielectric interface of length L, where L is much larger than λ, the wavelength of the incident electromagnetic field, and then replicating this segment periodically to obtain a classical grating of period L. The integral equation for the scattering amplitude is converted into a matrix equation for the amplitudes of the propagating and evanescent Bragg beams diffracted by this grating. By solving this matrix equation for an ensemble of random surfaces, and averaging the results over this ensemble, we obtain the cross section for diffuse scattering. The power spectrum of the surface roughness is nonzero only in the ranges of wave numbers that contain the wave numbers of these guided waves, but for comparison purposes we also present some results obtained by the use of a Gaussian power spectrum. Well-defined satellite peaks are present in the diffuse scattering cross section, in addition to the enhanced backscattering peak.

Satellite peaks in the scattering of p-polarized light from a randomly rough film on a perfectly conducting substrate

Abstract The diffuse scattering of p-polarized light from a one-dimensional, randomly rough dielectric film deposited on a planar, perfectly conducting surface is studied by means of small-amplitude perturbation theory. The dielectric constant ed and the thickness d of the dielectric film are chosen in such a way that in the absence of roughness the scattering system supports N (≥ 2) guided modes whose wavenumbers are q 1 (ω), …,q N(ω) at the frequency ω of the light incident at an angle θo. We investigate the occurrence of satellite peaks, in addition to the enhanced back-scattering peak, in the angular distribution of the intensity of the diffusely scattered light, at angles θs(n, m) given by sin θs(n, m)= − sin θ0± (c/ω) [Qn(ω) − qm(ω)] for n, m=1, …, N(n ≠ m). These satellite peaks are multiple-scattering effects due to degenerate timereversal symmetry.

Observation of giant enhanced backscattering of light from weakly rough dielectric films on reflecting metal substrates

The enhanced backscattering of light from a randomly rough surface, which is manifested as a narrow peak in the retroreflection di- rection in the angular distribution of the intensity of the light that is scat- tered diffusely, has been extensively studied. Both theoretical and ex- perimental investigations have shown that the height of the peak is never more than twice the height of the background at the position of the peak. We report the observation of a giant enhanced backscattering of light from a randomly weak rough dielectric film on a reflecting metal sub- strate, in which the ratio of the height of the peak to the height of the background at its position is greater than 10. It is found that this giant enhanced backscattering peak is accompanied by concentric circular in- terference fringes, whose axis is normal to the mean scattering surface, with both the specular and backscattering peak on the same ring. A possible mechanism for the giant backscattering is suggested. © 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: enhanced backscattering; wave scattering; opposition effect; lo- calization; thin films; physical optics; lidar and remote sensing.

Polarization of light scattered from slightly rough dielectric film

We found that a slightly rough thin dielectric film polarizes diffusely scattered light. There is a discrete set of angles of incidence and observation for which the intensity of a nonspecular diffuse P-polarized component is significantly greater than that for S polarization.

Enhanced backscattering from a one-dimensional rough dielectric film on a glass substrate

We have observed the enhanced backscattering of light from a characterized dielectric film deposited upon a glass substrate when the light illuminates the rough surface from the vacuum. The vacuum-dielectric interface is one dimensional, randomly rough, while the dielectric-glass interface is approximately planar. Numerical and experimental studies reveal that the main mechanism responsible for the enhanced backscattering is the constructive interference between two waves that follow reciprocal scattering paths through the dielectric film, which is also strengthened by the multiple scattering from the rough vacuum-dielectric interface.

Enhanced backscattering from a rough dielectric film on a glass substrate

We have observed the enhanced backscattering of light from a characterized random rough dielectric film deposited onto a glass substrate when the light-illuminates the rough surface from the dielectric side, entering from the glass substrate. The vacuum/dielectric interface of the film is a two-dimensional random rough surface, while the dielectric/glass interface is approximately flat. Our numerical simulations for a one-dimensional, random rough surface not only predict similar phenomena but also reveal some information about the main mechanism responsible for the enhanced backscattering peak.

Enhanced backscattering from a rough dielectric film on a reflecting substrate

We carry out computer simulations of the scattering of a beam of light with p or s polarization from a transparent dielectric film (BaSO4) on a perfectly conducting substrate. The dielectric–vacuum interface is assumed to be a random grating; the dielectric–conductor interface is assumed to be planar. It is found that for incident light of either polarization the angular dependence of the incoherent contribution to the mean intensity of the scattered light has a well-defined peak when the scattering angle corresponds to the retroreflection direction (enhanced backscattering). The existence of enhanced backscattering of p-polarized light in this geometry contrasts with its absence in the scattering of p-polarized light from a random grating on a semi-infinite dielectric medium that is characterized by the same roughness parameters and index of refraction. The enhanced backscattering that is observed in the scattering of s-polarized light from a random grating on a semi-infinite dielectric medium is stronger when the scattering takes place from the structure studied. For both polarizations the enhancement is most pronounced when the distance of the perfectly conducting substrate from the mean dielectric–vacuum interface is several (2–10) times the distance of the focusing plane of the rough interface from the mean interface.