Fabien Lemarchand

Experimental demonstration of a narrowband, angular tolerant, polarization independent, doubly periodic resonant grating filter

Resonant grating filters are promising components for free-space narrowband filtering. Unfortunately, due to their weak angular tolerance, their performances are strongly deteriorated when they are illuminated with a standard collimated beam. Yet this problem can be overcome by resorting to a complex periodic pattern known as the doubly periodic grating [Lemarchand et al., Opt. Lett.23, 1149 (1998)]. We report what we believe to be the first experimental fabrication and characterization of a bidimensional doubly periodic grating filter. We obtained a 0.5 nm bandpass polarization independent reflection filter for telecom wavelengths (1520-1570 nm) that presents a transmittivity minimum of 18% with a standard incident collimated beam.

Dipole radiation into grating structures

We present a detailed electromagnetic analysis for the radiation of an electric source located inside grating structures. Our analysis is based on the differential method and uses the scattering-matrix algorithm. We show that gratings that exhibit periodic modulations along two spatial directions (crossed gratings) enable one to couple out the totality of the light emitted by the source into the guided modes of the structure. This property is investigated through the computation of the far-field radiation patterns for crossed gratings with various etching depths. One key result is the possibility to confine the emitted light in a direction about the sample normal, a property that is of interest in the context of spontaneous emission control by microcavity structures.

Study of the resonant behaviour of waveguide gratings: increasing the angular tolerance of guided-mode filters

This paper is devoted to the study of resonant waveguide grating filters. It is shown that doubly periodic structures permit one to increase the angular tolerance of the filters without modifying the frequency linewidth. A theoretical study of such structures is presented and their response to an incident Gaussian beam is analysed. It is also shown that doubly periodic structures can be used to reduce absorption losses.

Interest of broadband optical monitoring for thin-film filter manufacturing

Broadband optical monitoring for thin-film filter manufacturing is more and more developed thanks to better performances of spectrometers with array detectors. We compare this optical monitoring with turning point monitoring and quartz monitoring of different designs. The sensitivity to thickness errors and to refractive index errors is evaluated. We show that real time determination of deposited thickness is a valuable criterion. We also present our experimental setup of transmittance and reflectance broadband optical monitoring. The use of a 400-1000 nm range combined with a signal-to-noise ratio of ~2500 in transmittance and 1000 in reflectance permits us to expect the manufacturing of high-performance non-quarter-wave designs. A first manufacturing of an 18-layer non-quarter-wave high-pass filter is provided.

A high accuracy femto-/picosecond laser damage test facility dedicated to the study of optical thin films

A laser damage test facility delivering pulses from 100 fs to 3 ps and designed to operate at 1030 nm is presented. The different details of its implementation and performances are given. The originality of this system relies the online damage detection system based on Nomarski microscopy and the use of a non-conventional energy detection method based on the utilization of a cooled CCD that offers the possibility to obtain the laser induced damage threshold (LIDT) with high accuracy. Applications of this instrument to study thin films under laser irradiation are presented. Particularly the deterministic behavior of the sub-picosecond damage is investigated in the case of fused silica and oxide films. It is demonstrated that the transition of 0-1 damage probability is very sharp and the LIDT is perfectly deterministic at few hundreds of femtoseconds. The damage process in dielectric materials being the results of electronic processes, specific information such as the material bandgap is needed for the interpretation of results and applications of scaling laws. A review of the different approaches for the estimation of the absorption gap of optical dielectric coatings is conducted and the results given by the different methods are compared and discussed. The LIDT and gap of several oxide materials are then measured with the presented instrument: Al(2)O(3), Nb(2)O(5), HfO(2), SiO(2), Ta(2)O(5), and ZrO(2). The obtained relation between the LIDT and gap at 1030 nm confirms the linear evolution of the threshold with the bandgap that exists at 800 nm, and our work expands the number of tested materials.

Impact of electron-beam lithography irregularities across millimeter-scale resonant grating filter performances

We investigated the impact of electron-beam lithography writing imperfections on the performance of two-dimensional resonant grating notch filters. This large area photonic device provides an interesting benchmark to assess the acceptable limits of unavoidable fabrication errors. We found that field stitching errors up to 100 nm have no detrimental effect on the filter linewidth, whereas a 2.5 nm electron-beam writing resolution, responsible for digitization disorder, is tolerable only for high-index contrast filter designs. Such an electron-beam writing strategy could also be beneficial for photonic crystal guiding structures or any periodic nanopatterned device with which the optical mode interacts with a large number of periodic elementary units.

Manufacturing of an absorbing filter controlled by a broadband optical monitoring

This paper deals with a broadband optical monitoring set up useful for the manufacturing of absorbing coatings. The monitoring strategy consists in simultaneous measurements of transmittance and reflectance over a large spectral range. The resulting analysis allows then to determine the real time deposited thickness. A stage of design correction is possible after the deposition and analysis of each layer. This method has potential for thin metallic layers coatings. We then describe layer after layer the strategy for the control and manufacturing of a filter with given colorimetric properties.

Solid-spaced filters: an alternative for narrow-bandpass applications

Solid-spaced filters are composed of one or several thin wafers of excellent optical quality acting as Fabry-Perot spacer layers. We study the different steps of the design and the manufacture of filters following dense-wavelength-division-multiplexing specifications. The design method of such filters requires a tight synergy between numerical simulations and experimental characterizations to correct possible thickness errors. Experimental results of the manufacture and characterization of a three-cavity narrow-bandpass filter and of an interleaver filter are given.

Experimental demonstration of ultrasharp unpolarized filtering by resonant gratings at oblique incidence

The peaks in the reflectivity spectrum of waveguide gratings observed when the incident beam couples to a mode of the structure are promising features for many applications. However their weak angular tolerance and their strong polarization sensitivity, especially under oblique incidence, limit their interest in practice. These problems can be overcome by forming slow degenerate modes outside the usual high symmetry points of the Brillouin zone with a complex periodic pattern [Fehrembach, Appl. Phys. Lett. 86, 121105 (2005)]. We show experimentally that spectrally sharp, lambda/Deltalambda approximately 4000, polarization-independent, angularly tolerant optical resonances can be obtained by exciting these modes under oblique incidence.

Light emission from europium chelates located in crossed grating structures

We investigate experimentally the emission of luminescent molecules located in corrugated waveguides. Both shallow and deep gratings are considered and comparison with a simple slab waveguide is discussed, in terms of extracted power. The key result is the demonstration that a corrugated waveguide allows one to couple out the guided mode power into arc-shaped paths which can be close to the sample normal. We believe that this type of structure is of interest in, for example, LED and single-photon-source applications.