Vincent Brissonneau

Sand-castle biperiodic pattern for spectral and angular broadening of antireflective properties

This Letter deals with the antireflective properties of top-patterned pyramids, looking like sand castles, bi-periodically repeated on a silicon surface. It is demonstrated numerically that such an original pattern allows a dramatic spectral and angular broadening of the antireflective efficiency. Design examples are given for wavelengths ranging from 0.5 microm to 5 microm and incidence angles of 30 degrees and 45 degrees. Applications of such antireflective surfaces on photodetectors and solar cells are soon expected.

Inverted cones grating for flexible metafilter at optical and infrared frequencies

By combining the antireflective properties from gradual changes in the effective refractive index and cavity coupling from cone gratings and the efficient optical behavior of a tungsten film, a flexible filter showing very broad antireflective properties from the visible to short wavelength infrared region and, simultaneously, a mirror-like behavior in the mid-infrared wavelength region and long-infrared wavelength region has been conceived. Nanoimprint technology has permitted the replication of inverted cone patterns on a large scale on a flexible polymer, afterwards coated with a thin tungsten film. This optical metafilter is of great interest in the stealth domain where optical signature reduction from the optical to short wavelength infrared region is an important matter. As it also acts as selective thermal emitter offering a good solar-absorption/infrared-emissivity ratio, interests are found as well for solar heating applications.

Statistically modified surfaces: Experimental solutions for controlled scattered light

An experimental process is proposed to fabricate random rough surfaces which statistical properties can be controlled and tuned. The theoretical approach is developed opening the way to simple, low cost and large surface patterning method. A photoresist is exposed to a speckle pattern created from an argon ion laser. The laser beam has previously been shaped using a digital micromirror device, allowing controlling the correlation function of the speckle. The patterned resist is then transfer by reactive ion etching onto silicon, allowing use for optoelectronics applications. Examples of surfaces, fabricated with Gaussian and non-Gaussian (modified) correlation functions are shown and demonstrate an agreement with theoretical predictions.

Flexible optical-infrared metafilter

By combining the antireflective properties from gradual changes in the effective refractive index and cavity coupling from cone gratings, and the efficient optical behavior of a tungsten film, we have conceived a flexible filter showing very broad antireflective (AR) properties from the visible to short wavelength infrared region (SWIR: 0.7-1.5 μm) and simultaneously a mirror-like behavior in the mid-infrared wavelength region (MWIR: 3-5 μm) and long-infrared wavelength region (LWIR: 8 to 15 μm). Nanoimprint technology has permitted us to replicate inverted cone patterns on a large scale on a flexible polymer, afterwards coated with a thin tungsten film. This optical metafilter is of great interest in the stealth domain where optical signature reduction from the optical to SWIR region is an important matter. As it also acts as selective thermal emitter offering a good solar-absorption/ infrared-emissivity ratio, interests are found as well for solar heating applications.

Innovative guided mode method for measuring the penetration depth in partially exposed photo-resist layers

The “m-lines” guided mode method has been employed as a new approach to measure the penetration depth of UV light in partially exposed thin film photoresist layers. This non-destructive method presents the advantage that the penetration depth can be measured before developing the sample, allowing for fine tuning of exposure parameters. Results are presented for a positive photoresist (Shipley S1813) deposited by spin coating onto glass slides, forming layers approximately 2.2μm thick. Such films are exposed to varying degrees with a programmable UV exposure tool. Using the “m-lines” technique, light is coupled into the photoresist samples using a prism coupler in close proximity to the sample surface. This coupling occurs for specific incident angles, known as synchronous angles, which depend on the sample structure. By measuring two such incident angles, one can calculate the thickness and refractive index of a homogeneous film. We propose a two layer model which allows us to extract the thickness and the refractive index of the upper exposed layer from the synchronous angles provided by the “m-lines” technique.

Application of DMD: laser speckle control for rough surface photofabrication

Digital Micromirror Devices are mainly known for display in video projectors. More and more, they are used in industrial or research applications. DMD is a versatile tool for lithography allowing photoresist exposure with easy-changing masking step in direct-patterning. Any application where light shaping is necessary can be considered by using DMD. We here show photofabrication of random rough surfaces using an indirect modified beam exposure. A laser beam is enlarged and scattered by a diffusing element. The scattering from this diffusing surface allows the creation of a speckle pattern with a random light distribution. The intensity is then recorded on a photoresist coated substrate. The patterned photoresist is next developed and an etching step enables the transfer on the silicon. It can be shown that the statistical properties of the speckle pattern can be controlled. The intensity distribution is modified by the number of exposures and the correlation function is linked to the spatial distribution of the laser beam. Some examples of such photofabrication can be found using a Gaussian unmodified beam leading to Gaussian correlation photofabricated surfaces. In order to design random rough surfaces having a non Gaussian correlation we need to modify the laser beam shape. This modification is achieved using a DMD. The experimental processes from photoresist deposition to modified exposure are discussed in this paper.

Tuning of random rough surface statistics for optoelectronics

Optical surface structuration is of primary interest for applications such as photovoltaics or photodetectors. Over last years, periodical patterns allowing antireflective effects with efficient properties have been designed and fabricated. Some specific issues such as diffraction of undesired high energy orders are a direct consequence of the periodical nature of this kind of pattern. Random rough surfaces allow the antireflective effect without these undesired diffraction effects. By tuning their statistics, random rough surfaces offer new degrees of freedom for antireflection but also for controlling the scattering (polarization, spatial distribution). The two main parameters of such surfaces are the height probability density function and the autocorrelation function. The height probability density function carries information about height of the structures. The autocorrelation function is a representation of the lateral distribution of the surface. Our photofabrication method uses a speckle pattern recorded on a photoresist. By controlling the exposure parameters, such as the number of exposure and the beam intensity distribution, one is able to control the statistics of the speckle, and so of the photofabricated surfaces. Using a chromatic confocal sensor, height mapping of these surfaces are performed. From these mappings, the height probability density and the correlation function are calculated. The experimental statistics are compared with the predicted theoretical ones showing a good agreement. Results are presented showing a significant modification of the statistics of the photofabricated surfaces.

Random rough surface photofabrication

Random rough surfaces are of primary interest for their optical properties: reducing reflection at the interface or obtaining specific scattering diagram for example. Thus controlling surface statistics during the fabrication process paves the way to original and specific behaviors of reflected optical waves. We detail an experimental method allowing the fabrication of random rough surfaces showing tuned statistical properties. A two-step photoresist exposure process was developed. In order to initiate photoresist polymerization, an energy threshold needs to be reached by light exposure. This energy is brought by a uniform exposure equipment comprising UV-LEDs. This pre-exposure is studied by varying parameters such as optical power and exposure time. The second step consists in an exposure based on the Gray method.1 The speckle pattern of an enlarged scattered laser beam is used to insolate the photoresist. A specific photofabrication bench using an argon ion laser was implemented. Parameters such as exposure time and distances between optical components are discussed. Then, we describe how we modify the speckle-based exposure bench to include a spatial light modulator (SLM). The SLM used is a micromirror matrix known as Digital Micromirror Device (DMD) which allows spatial modulation by displaying binary images. Thus, the spatial beam shape can be tuned and so the speckle pattern on the photoresist is modified. As the photoresist photofabricated surface is correlated to the speckle pattern used to insolate, the roughness parameters can be adjusted.