Bruno Bêche

A GaAlAs-GaAs integrated coherence modulator

This paper presents a novel integrated III-V semiconductor waveguide modulator specially designed to generate optical delays of several hundred micrometers. This is achieved by simultaneous propagation of transverse electric (TE) and transverse magnetic (TM) lowest order modes in an original layered waveguide exhibiting a group birefringence greater than 0.02 at 1.3 /spl mu/m. The device has 40% contrast, a switching voltage of 7 V with a 10-mm long electrode and is suitable for transmission and multiplexing of signals by coherence modulation of light in an optical fiber network powered by a short coherence source such as a superluminescent diode.

A tunable filter with collinear acoustooptical TE-TM mode conversion in a GaAs-AlAs multiquantum-well waveguide

We investigate theoretically an acoustooptic tunable filter (AOTF) based on TE-TM mode conversion by collinear acoustooptic interaction, using the high form birefringence that occurs in a GaAs-AlAs multiquantum-well (MQW) rib waveguide. Mode conversion is achieved using a transverse piezoelectric surface acoustic wave. We propose to use a piezoelectric layer (ZnO or AlN) of 6-mm symmetry, which can be deposited by plasma-enhanced chemical vapor deposition. The c axis is then parallel to the plane of the layer. Under this condition, a transverse piezoelectric surface acoustic wave excited by inderdigital electrodes can propagate within the GaAs-AlAs MQW optical waveguide. TE-TM conversion is then possible with a relatively low acoustic frequency (some tens of megahertz), yielding a high diffraction efficiency and a figure of merit 14 times higher than that of conventional LiNbO/sub 3/ filters based on AOTFs.

Light propagation in single mode polymer nanotubes integrated on photonic circuits

We report the theoretical and experimental study of photonic propagation in organic dielectric nanotubes elaborated by a wetting template method and showing off an aspect ratio as high as 200. Single mode behaviour is theoretically demonstrated without any cut-off conditions. Efficient evanescent coupling between polymer microstructures and nanotubes dispersed on a photonic chip as well as the high confinement and propagation in a single nanotube have been demonstrated. These results show the potential of well-defined one-dimensional nanostructures as building blocks for integrated organic photonic devices. Applications such as sensing and high speed communication are envisaged.

Marcatili’s extended approach: comparison to semi-vectorial methods applied to pedestal waveguide design

This paper deals with a theoretical study of pedestal waveguides. An extension of the Marcatili method has been developed in order to adapt this analytical method to pedestal structures. Simulations are performed for two different T-pedestal waveguide (T-PW) configurations corresponding respectively to a high and a lower core to pedestal widths ratio (T-PW I and T-PW II). Each configuration is simulated considering two core widths (2 and 4 µm) and a core height ranging from 1 to 2 µm at a 670 nm wavelength. Then, this extended Marcatili method has been compared with a semi-vectorial finite difference method (SVFD) and a spectral method developed by Galerkin, both based on a numerical approach. The simulation of the T-PW structure with these three methods shows a good congruence since the relative differences between Marcatilis method and the numerical methods remain below 6%. Then, the three approaches are applied to study the modal birefringence minimization in the case of pedestal structures. Simulations are typically performed for waveguide height and width values ranging, respectively, around (1.6–2.6) µm and (1.8–6) µm, with pedestal widths ranging around (0.4–0.8) µm, at a 670 nm wavelength. The authors stress a specific property of pedestal configurations: by judiciously adjusting the dimensional parameters (core and pedestal width and core height), the birefringence can be completely screened out.

Fabrication and optical characterization of sub-micronic waveguide structures on UV210 polymer

In this paper, we report on the interest in a new polymer, UV210, in the field of integrated optics. As 400 nm wide and 1 µm high UV210 rib waveguides have been obtained by deep ultraviolet (DUV) lithography and controlled by scanning electron micrographs (SEMs), they allow us to realize sub-wavelength structures by way of easy and cheap processes. Structural and optical investigations have been carried out on UV210 resist as a function of exposure to DUV light. On the one hand, structural properties through ellipsometric measurements have been investigated: the UV210 refractive index increases with the DUV exposure dose, yielding a large index contrast at 980 nm between areas exposed or not (Δn = 2 × 10 − 2). It is expected that we shall soon be able to photo-print nanometric patterns onto UV210 films. On the other hand, optical studies of propagation losses measured in both irradiated and unexposed single-mode UV210 waveguides have been performed by a cut-back method. Concerning as-deposited rib waveguides, optical losses for TE00 and TM00 optical modes have been evaluated to 3.4 ± 0.4 dB cm − 1 and 6.2 ± 0.5 dB cm − 1, respectively. Hence, the UV210 polymer appears to be a promising candidate for the development of low-cost nanometric structures for miniaturized optical chips, with numerous applications in telecommunication and sensor technology.

Investigation of fabrication and resonant optical coupling in various 2D micro-resonator structures in a UV210 polymer

In this paper, we report on the design and the overall realization of micro-resonators based on the development of adequate processes on a UV210 polymer. These micro-optical structures are developed by deep ultraviolet lithography allowing fabrication of nano-structured devices by means of low cost and reproducible processes. Two families of resonant micro-structures shaped on disk and stadium with various sizes are investigated. Structural and optical imaging characterizations have been carried out to ensure their ability to act as resonant integrated micro-structures. At first, scanning electron microscopy and Nomarsky microscopy studies confirm the UV-light process resolution down to 450 nm developed on a UV210 polymer. Then, optical characterizations have been performed as regards intensity and spectral properties of such micro-resonators. Field intensity measurements in visible and infrared ranges have been realized and validate light propagation by evanescent coupling between waveguides and micro-resonators. Finally, spectral analyses on TE modes demonstrate the presence of optical resonances with 1.45 nm and 2.19 nm free spectral range values for respectively disk and stadium micro-structures. The UV210 polymer appears appropriate for the realization of micro-structures requiring a few hundred nanometers gap-scale while maintaining adequate spectral properties for versatile applications in telecommunication and metrology.

Theoretical formulation to shape versatile propagation characteristics of three-layer-tubular waveguides: sub-wavelength and asymptotic study

This theoretical study introduces an analytical formalism to shape the quantification of the photonic modes taking place in three-layer-tubular waveguides, including hollow cylindrical structures. Solving Maxwells equations in such tubular structure families allowed us to determine the electromagnetic field components in each media of the tube as well as the analytical eigenvalue equations. Thereby we have established an overall frame to be implemented for bound modes focusing on two studies: (1) the internal radius a of the tube is fixed and the external radius b ranges around]a, 10a], with a = [λ/10λ/2λ], encompassing then the sub-wavelength dimensional aspect; (2) the external radius b is fixed and the internal radius a ranges around]0, b[, with b = [λ/2λ10λ] at λ = 670 nm. The asymptotic behaviour of such optical mode quantification schemes in particular is discussed and demonstrated: indeed, as the internal radius a tends to zero, our prior tubular formalism predicts propagation characteristics identical to those of the well-known fibre theory by way of analytical and numerical strong evidence. We also focus on the cutoff limits of optical bound modes: a map of the different areas related to both monomode and multimode aspects has been dressed as a function of the internal and external radii normalized to the wavelength for versatile dimensional tubular structures. Finally, we discuss the propagation below cutoff in such tubular void structures and propose, for a future investigation, two methods to quantify leaky modes.

Injection and waveguiding properties in SU8 nanotubes for sub-wavelength regime propagation and nanophotonics integration

We report photonic concepts related to injection and sub-wavelength propagation in nanotubes, an unusual but promising geometry for highly integrated photonic devices. Theoretical simulation by the finite domain time-dependent (FDTD) method was first used to determine the features of the direct light injection and sub-wavelength propagation regime within nanotubes. Then, the injection into nanotubes of SU8, a photoresist used for integrated photonics, was successfully achieved by using polymer microlensed fibers with a sub-micronic radius of curvature, as theoretically expected from FDTD simulations. The propagation losses in a single SU8 nanotube were determined by using a comprehensive set-up and a protocol for optical characterization. The attenuation coefficient has been evaluated at 1.25 dB mm(-1) by a cut-back method transposed to such nanostructures. The mechanisms responsible for losses in nanotubes were identified with FDTD theoretical support. Both injection and cut-back methods developed here are compatible with any sub-micronic structures. This work on SU8 nanotubes suggests broader perspectives for future nanophotonics.

Improvement of efficient coupling and optical resonances by using taper-waveguides coupled to cascade of UV210 polymer micro-resonators

In this paper, we report the overall design, fabrication and optical characterization of single and multiple resonant micro-structures patterned on the UV210 polymer and shaped by adequate deep-UV lithography procedures. Various families of ring and racetrack forms are investigated with different geometrical dimensions linked to the micro-resonators and the specific taper-waveguides and gaps allowing the optimized coupling. Well defined photonic structures families in the sub-micrometer range obtained by this deep UV-light process are clearly confirmed through scanning electron microscopy. In order to evaluate and quantify the efficiency of the sub-micrometer coupling, the recirculation of the light and the quality of the optical resonance aspects, a global study including top view intensity imaging, spectral measurements and Fast Fourier Transform analysis is performed for all these devices based on single and multiple family resonators. The experimental TE-mode resonance transmissions reveal a complete agreement with the period of the theoretically expected resonances. A maximum value of the quality factor Q = 3.5 x 10^3 at 1035 nm with a 3.2 times higher resonance contrast is assessed for cascade of triple micro-resonators respect to the photonic devices based on only one micro-resonator. In addition, these UV210 circuits made of specific tapers coupling to cascade loops act directly on the improvement of the evanescent coupling and resonances in terms of quality factor and extinction rate, by selecting successively and more precisely the optical mode resonance. All these designs and low cost technological reproducible steps, and furthermore the devices and protocol measurements are markedly suitable for mass fabrication and metrology applications.

Efficient active waveguiding properties of Mo6 nano-cluster-doped polymer nanotubes

We investigate 1D nanostructures based on a Mo6@SU8 hybrid nanocomposite in which photoluminescent Mo6 clusters are embedded in the photosensitive SU8 resist. Tens of micrometers long Mo6@SU8-based tubular nanostructures were fabricated by the wetting template method, enabling the control of the inner and outer diameter to about 190 nm and 240 nm respectively, as supported by structural and optical characterizations. The image plane optical study of these nanotubes under optical pumping highlights the efficient waveguiding phenomenon of the red luminescence emitted by the clusters. Moreover, the wave vector distribution in the Fourier plane determined by leakage radiation microscopy gives additional features of the emission and waveguiding. First, the anisotropic red luminescence of the whole system can be attributed to the guided mode along the nanotube. Then, a low-loss propagation behavior is evidenced in the Mo6@SU8-based nanotubes. This result contrasts with the weaker waveguiding signature in the case of UV210-based nanotubes embedding PFO (poly(9,9-di-n-octylfluorenyl-2,7-diyl)). It is attributed to the strong reabsorption phenomenon, owing to overlapping between absorption and emission bands in the semi-conducting conjugated polymer PFO. These results make this Mo6@SU8 original class of nanocomposite a promising candidate as nanosources for submicronic photonic integration.