A. Lupu

Recent Progress in High-Speed Silicon-Based Optical Modulators

The evolution of silicon optical modulators is recalled, from the first effect demonstrations to the characterization of high-performance devices integrated in optical waveguides. Among possibilities to achieve optical modulation in silicon-based materials, the carrier depletion effect has demonstrated good capacities. Carrier depletion in Si and SiGe/Si structures has been theoretically and experimentally investigated. Large phase modulation efficiency, low optical loss, and large cutoff frequency are obtained by considering simultaneously optical and electrical structure performances. Integrated Mach-Zehnder interferometers and resonators are compared to convert phase modulation into intensity modulation. Finally, recent results on high-speed and low-loss silicon optical modulator using an asymmetric Mach-Zehnder interferometer are presented. It is based on a p-doped slit embedded in the intrinsic region of a lateral pin diode integrated in a silicon-on-insulator waveguide. This design allows a good overlap between the optical mode and carrier density variations. An insertion loss of 5 dB has been measured with a -3 dB bandwidth of 15 GHz.

Metal-dielectric metamaterials for guided wave silicon photonics.

The aim of the present paper is to investigate the potential of metallic metamaterials for building optical functions in guided wave optics at 1.5 µm. A significant part of this work is focused on the optimization of the refractive index variation associated with localized plasmon resonances. The minimization of metal related losses is specifically addressed as well as the engineering of the resonance frequency of the localized plasmons. Our numerical modeling results show that a periodic chain of gold cut wires placed on the top of a 100 nm silicon waveguide makes it possible to achieve a significant index variation in the vicinity of the metamaterial resonance and serve as building blocks for implementing optical functions. The considered solutions are compatible with current nano-fabrication technologies.

Efficient control of a 3D optical mode using a thin sheet of transformation optical medium

We report on a mode adapter that transitions light between two SOI waveguides having different widths. The device has been designed using a two-dimensional embedded coordinate transformation and consists of a thin sheet of a controlled anisotropic material directly placed on top of the Si slab. We demonstrate that this layer effectively controls the flow of energy propagating in the Si slab by performing three-dimensional full-wave simulations. The proposed geometry can be implemented with planar optical metamaterials for various applications in guided optics.

Electroabsorption and refractive index modulation induced by intersubband transitions in GaN/AlN multiple quantum wells.

The aim of the present paper was to determine the index variation in the GaN/AlN heterostructures related to the population/depletion of the quantum well fundamental state leading to the absorption variation in the spectral domain around 1.5 µm. The variation of the refractive index was deduced from the shift of the position of the beating interference maxima of different order modes in a guided wave configuration. The obtained index variation with bias from complete depletion to full population of the quantum wells is around -5 × 10(-3). This value is similar to the typical index variation achieved in InP and is an order of magnitude higher than the index variation obtained in silicon.

Metal-dielectric metamaterials for guided wave optics applications

We address the experimental validation of the technological feasibility and operation of the metamaterials in a guided wave configuration in the spectral domain around 1.5μm. For our experiments we considered a 2D array of 200×50×50nm gold cut wires placed on the top of a 10μm wide and 200nm thick silicon waveguide. The transmission spectral measurements performed in the spectral range between 1.25 and 1.64μm using an end-fire coupling setup, revealed a marked dip for TE polarized light, corresponding to the cut wires resonance frequency obtained by numerical modeling. No such a dip in transmission was observed for TM polarized light, i.e. when the electric filed is perpendicular to the layers interface and the orientation of the cut wires. The scanning near field optical microscopy experiments (SNOM), performed in the same spectral range, revealed for TE polarized light a strong enhancement of the electric field confined in the region between the ends of the adjacent cut wires. These results confirm the efficient excitation of the cut wires resonance in a guided wave configuration for the TE polarization. The ability for local engineering of the field interaction with the metamaterial layer and thus the control in such a way of the light flow in a guiding slab, paves the way to a novel class of photonic devices.

Narrow band forward coupling using Bragg reflectors

In this work, we revisit the operation principles of Bragg reflectors assisted directional couplers. We show that an efficient narrow band forward coupling operation can be achieved by an appropriate engineering of the Bragg grating waveguides dispersion properties. Our theoretical analysis reveals the existence of a minimum Bragg grating coupling strength for co-directional phase matching. This threshold coupling condition is an essentially new aspect of Bragg grating asymmetric directional couplers as compared to conventional co-directional couplers and Bragg reflectors. The threshold condition is analytically determined, and a coupled mode theory four-wave model is successfully applied to describe the behavior of the investigated device. It is shown that the optimal operation is achieved with only one Bragg grating distributed along one of the two waveguides. A numerical validation of the results of coupled mode theory is performed for the case of shallow-etched Silicon on Insulator (SOI) ridge waveguides with Bragg grating assisted coupling. The selectivity is a factor of 5.5 higher than that obtained in the conventional approach of asymmetric directional couplers where III-V waveguides with different alloy compositions are coupled vertically. The proposed design is shown to be compatible with existing micronano- fabrication technology.

Nanoscale engineering of the waveguide local effective index by metamaterial resonances: Toward transformation optics applications

In this paper we address the potential of metallic metamaterials in guided wave configurations to explore the possibility of their use for transformation optics applications in the near-infrared domain (λ = 1.5 μm). The experimentally investigated hybrid metamaterial structure is made of 2D array of 200×50×50 nm gold cut wires located on the top of high index silicon on insulator (SOI) waveguide. The experimental and modelling results show that composite waveguide effective index and propagation losses can be accurately controlled through the engineering of metamaterial properties. The possibility for controlling at the nanoscale the local effective index can be used in transformation optics applications. The hybrid metamaterial guided wave configuration may become a promising alternative to the bulk multi-layers metamaterial structures in the near infrared domain.

Electroabsorption and refractive index modulation induced by intersubband transitions in GaN/AlN heterostructure waveguides

We present the determination of the index variation in the GaN/AlN heterostructures related to the population/depletion of the quantum wells fundamental state leading to the intersubband (ISB) absorption variation in the spectral domain around 1.5μm. The experiments were performed using wide-strip waveguide structure. It is shown that the determination of the refraction index in a wide-strip structure is possible when the waveguide is multimode in the vertical direction with a small number of higher order modes. The variation of the refractive index is then deduced from the shift of the position of the beating interference maxima of different order modes. The obtained index variation with bias from complete depletion to full population of the quantum wells is around -5×10-3. This value is similar to the typical index variation achieved in InP and is an order of magnitude higher than the index variation obtained in silicon. The remarkable feature is that maximum index variation is obtained at the wings of the ISB transition line where absorption is reduced with respect to the peak value. This index variation mechanism opens prospects for the realization of ISB phase modulators by inserting the active region in a Mach-Zehnder interferometer.

Slow-fast light evanescent coupling interactions in guided-wave optics

We address the problematic of co-propagative evanescent coupling interactions between fast and slow modes in guided optics. The analysis is performed in the case of an asymmetric directional coupler assisted by a Bragg grating.