Ph. Lalanne

Theory of surface plasmon generation at nanoslit apertures

In this Letter, we study the scattering of light by a single subwavelength slit in a metal screen. In contrast with previous theoretical works, we provide a microscopic description of the scattering process by emphasizing the generation of surface plasmons at the slit apertures. The analysis is supported by a rigorous formalism based on a normal-mode-decomposition technique and by a semianalytical model that provides accurate formulas for the plasmonic generation strengths. The generation is shown to be fairly efficient for metals with a low conductivity, such as gold in the visible regime. Verification of the theory is also shown by comparison with recent experimental data [H. F. Schouten, Phys. Rev. Lett. 94, 053901 (2005)].

Light transmission through metallic channels much smaller than the wavelength

Abstract A resonant cavity-enhanced light transmission mechanism in metallic gratings with subwavelength apertures is theoretically interpreted for operation with visible light. It is shown that under appropriate boundary conditions, the apertures behave as open Fabry–Perot resonant cavities delivering a high photon flux, and that the coupling between the incident light and the fundamental mode supported by the aperture is strongly controlled by surface waves. Compared to the well-known perfect metallic case, we show that the effective index of the fundamental mode strongly depends on the aperture dimension, especially when aperture widths much smaller than the wavelength are considered. Consequently, it is predicted that small variations of aperture shape or dimensions have a huge effect on the transmission properties of real metallic gratings.

Photonic crystal waveguides: Out-of-plane losses and adiabatic modal conversion

An accurate model for the out-of-plane radiation losses occurring when a guided wave propagating in a conventional waveguide impinges on a photonic crystal waveguide is presented. The model makes clear that the losses originate from insertion losses resulting from a mode mismatch. A generic taper structure realizing an adiabatic modal conversion is proposed and validated through numerical computations for cavities with large Q’s and large peak transmission.

One-mode model and Airy-like formulae for one-dimensional metallic gratings

We present a semi-analytical model that quantitatively predicts the transmission, the absorption and the resonance locations of one-dimensional lamellar metallic gratings with subwavelength slit apertures. The model relies on the fact that subwavelength apertures perforated in metallic films behave like monomode waveguides and provide Airy-like formulae for the transmission and reflection coefficients. Limitations are outlined for too shallow grating depths.

Low-reflection photonic-crystal taper for efficient coupling between guide sections of arbitrary widths

We design and fabricate a new taper structure for adiabatic mode transformation in two-dimensional photonic-crystal waveguides patterned into a GaInAsP confining layer. The taper efficiency is validated by measurement of a reduction of the reflection between an access ridge and a photonic-crystal guide with one missing row from 6% to less than 1%. This taper is then incorporated into a 60 degrees bend; simulations demonstrate a 90% transmission between multimode ports.

Bloch-wave engineering for high-Q, small-V microcavities

The overall decay rate of the mode in an optical microcavity formed by a defect surrounded by two Bragg mirrors in a monomode waveguide is driven by two mechanisms, the desired coupling to a guided mode and the detrimental coupling to radiation modes. We propose two approaches fully compatible with planar fabrication, which allow to increase the cavity Qs by several orders of magnitude while keeping constant the mode volume V of the cavity. The first approach consists of engineering the mirror to taper the guided mode into the mirror Bloch mode, thus decreasing losses. The second approach is less intuitive and relies on a recycling mechanism of the radiation losses. The study is supported by computational results obtained for two-dimensional silicon-on-insulator geometries, but the results apply as well to other related geometries like three-dimensional photonic-wire cavities.

Two physical mechanisms for boosting the quality factor to cavity volume ratio of photonic crystal microcavities

We identify two physical mechanisms which drastically increase the Q/V factor of photonic crystal microcavities. Both mechanisms rely on a fine tuning the geometry of the holes around the cavity defect. The first mechanism relies on engineering the mirrors in order to reduce the out-of-plane far field radiation. The second mechanism is less intuitive and relies on a pure electromagnetism effect based on transient fields at the sub-wavelength scale, namely a recycling of the mirror losses by radiation modes. The recycling mechanism enables the design of high-performance microresonators with moderate requirements on the mirror reflectivity. Once the geometry around the defect is optimised, both mechanisms are shown to strongly impact the Q and the Purcell factors of the microcavity.

Out-of-plane losses of two-dimensional photonic crystals waveguides: Electromagnetic analysis

A method for the electromagnetic analysis of photonic crystal waveguides is described. It is tested against experimental transmission data obtained for AlGaAs slab-waveguide structures perforated by a two-dimensional hexagonal lattice. A good quantitative agreement is obtained for the band edge locations, for the ripples in the transmission windows, and more importantly, for the out-of-plane losses induced by the finite hole depth. The ultimate performance of the structure for deep etched holes is predicted.

Finite-depth and intrinsic losses in vertically etched two-dimensional photonic crystals

We address the issue of out-of-plane losses in two-dimensional (2D) photonic crystals (PC) etched through a GaAs monomode waveguide clad with standard GaAlAs alloys. We correlate experimental transmission of PCs with two kinds of loss simulation results. The first kind is 2D and introduces an ad hoc imaginary index in the air holes to account for the losses [see (Benisty et al. Appl. Phys. Lett. 76, 532, 2000)]. The second kind is a novel exact three-dimensional calculation inspired by grating-Fourier analysis that provides quantitatively unprecedented agreement with experimental measurements taking into account hole depth as a limiting parameter. We conclude that, in revision to the conclusions of the above reference, the experimental losses are not the intrinsic ones, being larger by a factor of 5 to 10 due to insufficient hole depth. The transition occurs at a critical etch depth shown to be here around 700 nm. We thus predict, for holes deeper than 700 nm, much improved crystals with very low transmission losses and microresonators with ultra-high quality factors.

Short Bragg mirrors with adiabatic modal conversion

A silicon-on-insulator waveguide-based microcavity with short tapers has been realized by incorporating two identical eight-groove tapers at the two reflector extremities. The microstructure has an overall length of 14 μm and consists of two first-order Bragg mirrors with narrow slits (90 nm) and of four series of slits with progressively varying widths (tapers). A comparison with a nontapered cavity evidences the beneficial effect of the taper, a lowering of the radiation losses.