N. Le Thomas

Self-collimating photonic crystal polarization beam splitter

We present theoretical and experimental results of a polarization splitter device that consists of a photonic crystal (PhC) slab, which exhibits a large reflection coefficient for TE and a high transmission coefficient for TM polarization. The slab is embedded in a PhC tile operating in the self-collimation mode. Embedding the polarization-discriminating slab in a PhC with identical lattice symmetry suppresses the in-plane diffraction losses at the PhC-non-PhC interface. The optimization of the PhC-non-PhC interface is thereby decoupled from the optimization of the polarizing function. Transmissions as high as 35% for TM- and 30% for TE-polarized light are reported.

Dispersion properties of silicon nanophotonic waveguides investigated with Fourier optics

We experimentally investigate the dispersion relation of silicon-on-insulator waveguides in the 1.5 microm wavelength range by using a technique based on far-field Fourier-space imaging. The phase information of the propagating modes is transferred into the far field either by linear probe gratings positioned 1 microm away from the waveguide core or by residual gratings located on the sidewalls of the waveguide. As a result, the dispersion curve of rectangular and slot waveguides as well as the group index dispersion are accurately determined.

Cointegration of Gate-All-Around MOSFETs and Local Silicon-on-Insulator Optical Waveguides on Bulk Silicon

In this work we present a bulk silicon technology platform able to cointegrate gate-all-around (GAA) MOSFETs and local SOI waveguides with pentagonal cross section. Wire diagonals of 100-800 nm are obtained using a lithographic resolution of 0.8 mum. Well-functioning triangular multigate MOSFETs are reported, and tested up to 150 degC. A significant increase is observed in the low-field mobility mu0 for small devices (Weffles500 nm), which is attributed to local volume inversion in the corners. Preliminary characterization of the optical waveguides is carried out, showing optical losses of a few dB/cm. The processing is entirely CMOS compatible, does not require access to advanced lithography equipment, and is based on a silicon bulk substrate. Thus, this technology might serve as the basis for a low-cost, high-performance optical signaling platform

Statistical analysis of subnanometer residual disorder in photonic crystal waveguides: Correlation between slow light properties and structural properties

The authors present a statistical study of residual disorder in nominally identical planar photonic crystal waveguides operating in the slow light regime. The focus is on the role played by the subnanometer scaled residual disorder inherent to state-of-the-art electron-beam (EB) lithography systems, in particular, on the impact of the nature of the residual disorder on the maximum value of the guided mode group index. The authors analyze the statistical properties of the surface area, the position, and the shape of the air holes that define the photonic crystal with optimized scanning electron microscope micrographs. The authors identify the hole-area fluctuation as the main source of degradation of the dispersive slow light regime by correlating such a microscopic analysis of the structural disorder with large field-of-view optical characterizations based on a Fourier space imaging technique. The structure with the largest group index (ng = 40) exhibits a standard deviation σ of the radius of the hole as...

Radiation loss of photonic crystal coupled-cavity waveguides

We experimentally investigate the out-of-plane radiation losses of photonic crystal coupled-cavity waveguides. We observe a strong variation in the losses along the dispersion curve and show that such variation is closely linked with the specific far-field radiation pattern of a single cavity constituent. A simple theoretical model based on tight-binding approximation is used to describe this behavior.

Coupling length of silicon-on-insulator directional couplers probed by Fourier-space imaging

We use a Fourier-space imaging technique relying on outcoupling grating probes to study the coupled mode interaction and dispersion properties of guided modes in silicon-on-insulator codirectional couplers. Our approach allows us to measure the mode splitting inherent to coupled systems, determine the mode symmetry, and locally probe the coupling length with an accuracy of +/- 50 nm. A systematic study of directional couplers with different waveguide widths, coupling gaps, and e-beam exposure doses is reported in order to verify the results across a wider parameter space

Refractive index gas sensing in a hollow photonic crystal cavity

We investigate hollow photonic crystal structures consisting in air-slot photonic crystal nanocavities. The high quality factor ∼ 2.6×104 of such cavities along with a strong overlap between the resonant mode and the hollow core region allows for a strong interaction with an optically active medium in the air slot. This is illustrated in achieving highly sensitive refractive index sensing of a gas infiltrated in the slot. An experimental sensitivity of 610 nm per refractive index unit (RUI) and a detection limit below 1×10-5 RUI are demonstrated. The device has a remarkably low sensing volume of 40 attoliters, which at atmospheric pressure and room temperature contains as little as 1×106 molecules.

Group velocity and energy transport velocity near the band edge of a disordered coupled cavity waveguide: an analytical approach

We develop an analytical approach to theoretically investigate light speed propagation near the band edge of a coupled cavity waveguide in the presence of residual disorder. This approach that is based on a mean field theory allows us to define the domains of validity of the group velocity and the energy transport velocity concepts as well as a guideline to minimize the role of the residual disorder. Inspired by an analogy with the theory of multiple scattering of classical wave, we derive an analytical formula for the energy transport velocity in periodic photonic structures. Whereas the group velocity diverges near the band edge in the presence of any amount of residual disorder, we show that the energy transport velocity mainly follows the ideal group velocity of the unperturbed structure except for very strong disturbances out of the scope of the presented model.

Inhibition of the emission of electromagnetic modes of photonic crystal cavities with a top mirror

By imaging the angular spectrum of optical modes confined within subwavelength volumes, we investigate the impact of metallic and dielectric surrounding layers on the optical emission properties of planar silicon photonic crystal cavity. By scanning a mirror close to the surface of the cavity, we experimentally show an enhancement of the cavity quality factor Q based on the annihilation of the amplitude of the cavity leaky modes. The experimental observations are in agreement with a model based on transfer matrix formalism.

Light transport and limits of slow light in real photonic crystal structures in the presence of residual disorder

We experimentally investigate the effect of residual disorder on the dispersion curve of light near a band edge. A Fourier space imaging technique accurately distinguishes the propagating regime from diffusive transport and evanescent regime in planar photonic crystal. We highlight and quantify the impact of the disorder on the group velocity near a band edge. We stress that the achievement and determination of group indexes larger than 100 requires not only state of the art processing but also accurate techniques able to discriminate between propagating, evanescent and localized modes.