Bob Bellini

Nematic Liquid Crystal Optical Channel Waveguides on Silicon

We demonstrate the first channel waveguides made of E7 nematic liquid crystal (LC) in SiO2-Si V-grooves. The grooves have been obtained by wet etching n-Si substrates first and then by thermally growing an approximately 2-mum-thick SiO2 cladding layer. Propagation of infrared light at a wavelength of 1550 nm shows a good optical confinement in 10-mum-wide LC waveguides. Modal analysis and beam propagation simulations predict single mode propagation. This is experimentally confirmed by the acquired near field images. The optical waveguide acts as an integrated optic polarizer, since only vertical polarization can propagate due to the orientation of the LC molecules. The horizontal polarization state is suppressed by more than 25 dB

A Switchable Liquid-Crystal Optical Channel Waveguide on Silicon

An integrated optical switch based on liquid crystal on silicon is reported. The switch consists of an optical waveguide, whose core is made of nematic liquid crystal E7 infiltrated in a SiO2/Si V-groove. The electrooptic effect allows us to control the waveguide propagation condition by means of ITO and Si electrodes. A voltage as low as 2 V applied to the driving electrodes turns the waveguide on. Since the output intensity increases with voltage, the waveguide behaves also as a variable optical attenuator. A maximum ON-OFF extinction ratio in excess of 44 dB was measured.

Modelling, design and analysis of liquid crystal waveguides in preferentially etched silicon grooves

This paper presents a fully consistent theoretical framework for liquid crystal (LC) channel waveguides that have been experimentally demonstrated in previous publications. We revise the optical design of the LC waveguides in silicon grooves and implement here a vectorial, fully consistent model of the LC waveguide electro-optical behaviour, based on the finite element method. The numerical investigation shows that LC waveguides demonstrate properties of propagation control and switching. They switch on and off with a low applied voltage. We discuss the major design parameters of the device and the effect of loss-inducing control electrodes.

Tunable one-dimensional photonic crystal slabs based on preferential etching of silicon-on-insulator

We design and assess a one-dimensional photonic crystal slab fabricated by preferential etching of a silicon-on-insulator substrate. The etched grooves are considered to be infiltrated by a highly-birefringent nematic liquid crystalline material. A detailed analysis of the nematic director response within the grooves is presented. We investigate different configurations and demonstrate large band gap shifting when switching the liquid crystal with an applied voltage. Furthermore, we assess this type of device as an efficient alternative for compact refractometric optical sensing applications.

Photonic devices based on preferential etching

We introduce a design concept of optical waveguides characterized by a practical and reproducible process based on preferential etching of crystalline silicon substrates. Low-loss waveguides, spot-size converters, and power dividers have been obtained with polymers. We have also aligned liquid crystals in the waveguides and demonstrated guided propagation. Therefore this technology is a suitable platform for soft-matter photonics and heterogeneous integration.

Realization of a Liquid Crystal Electrically Controlled Optical Waveguide on Micromachined Silicon

In this paper we report the major fabrication steps and preliminary electro-optical characterization of a novel integrated liquid crystal optical waveguide on silicon substrate. The optical waveguide is made of a SiO2/Si V-groove with a triangular cross-section filled with nematic liquid crystal E7. The optical propagation in the channel waveguide is controlled by an external voltage which induces a refractive index variation by means of molecular director reorientation. For TM polarized light at a wavelength of 1.55 μm, a threshold voltage of only about 2 V and an extinction ratio higher than 35 dB were measured with 6.5 V applied.

Integrated optics using smectic and nematic liquid crystals

In this paper two practical technological approaches are presented to demonstrate that liquid crystals can be employed to make low cost integrated optic devices with characteristics required for optical communication systems. In a first approach ferroelectric and nematic liquid crystals are combined with ion-exchanged glass waveguides to produce high performing optical switches. A polarization independent configuration of such switches is also shown. In a second approach nematic liquid crystals are embedded in SiO2/Si V-grooves to produce channel waveguides. Modal and polarization properties of such novel liquid crystal waveguides are experimentally demonstrated.

Integrated optics devices based on liquid crystals

We describe the technology behind a platform for integrated optics based on liquid crystals. Design and technological issues are addressed and effective technological solutions are presented. The physical implementation of optical waveguides characterized by a practical and reproducible process based on preferential etching of crystalline silicon substrates are presented. Devices are manufactured by wet etching a Si substrates first and then by thermally growing thick SiO2 cladding layer. A nematic liquid crystal is used as core of the waveguiding devices. Experimental results on polarizing properties and single mode propagation of infrared light are presented and discussed.

Tunable one-dimensional photonic crystal slabs

A 1D photonic crystal slab based on preferential etching of commercially available silicon-on-insulator wafers is presented. Compared to dry etching, anisotropic wet etching is more tolerant to errors as it is self-stopping on crystallographic {111} planes and it produces a more precise geometry with symmetries and homothetic properties, with surface roughness close to 1 nm. The resulting grooves are infiltrated by low viscosity liquid crystal having large positive optical anisotropy. The use of slanted grooves provides advantages: first of all the complete filling of slanted grooves is simplified when compared to vertical walls structures. Furthermore alignment is significantly facilitated. Indeed the liquid crystal molecules tend to align with their long axis along the submicron grooves. Therefore by forcing reorientation out of a rest position, the liquid crystal presents a choice of refractive indices to the propagating optical field. The liquid crystal behavior is simulated by a finite element method, and coupled to a finite difference time domain method. We investigate different photonic crystal configurations. Large tunability of bandgap edge for TE polarization is demonstrated when switching the liquid crystal with an applied voltage. We have also studied the use of the same device geometry as a very compact microfluidic refractometric sensor.

A Tuneable Waveguided Optical Filter Made of Polymer and Liquid Crystal Slices Operating in C-band: Analysis of Transmission and Reflection Properties

ABSTRACT We present the modelisation and simulation at wavelengths about 1550 nm of a waveguide grating made up of polymer and liquid crystal slices as the core layer and glass as cladding layers. The model combines a matrix-transfer method with a Modal Analysis. The pre-polymerised mixture considered in our calculations is made of NOA61 and 5CB. We derive that the device can behave as either an electrically tuneable narrow notch filter, or a switchable bandpass filter. A tuning range of 7 nm can be obtained for the notch filter with an optical bandwidth at −20 dB less than 1 nm.