Tuning of silicon-on-insulator ring resonators with liquid crystal cladding using the longitudinal field component

Integrated visible-light liquid-crystal-based phase modulators

In this work, an integrated liquid-crystal-based phase modulator operating at visible wavelengths was developed and experimentally demonstrated. A visible-light silicon-nitride-based 300-mm-wafer foundry platform and a liquid-crystal integration process were developed to leverage the birefringence of liquid crystal to actively tune the effective index of a section of silicon-nitride waveguide and induce a phase shift over its length. The device was experimentally shown to achieve a 41π phase shift within 4.8 V_pp for a 500-µm-long modulator, which means that a 2π phase shifter would need to be only 24.4 µm long. This device is a compact and low-power solution to the challenge of integrated phase modulation in silicon nitride and paves the way for future low-power small-form-factor integrated systems at visible wavelengths.

Initial alignment control technique using on-chip groove arrays for liquid crystal hybrid silicon optical phase shifters

Flexible and localized initial alignment control of liquid crystal (LC) is important to enhance the performance and functionality of LC hybrid silicon photonic devices. This work proposes an initial LC alignment control technique based on integration of a nanometer-scale groove array in the buried oxide layer near a Si waveguide. We achieved control of the initial angle of LC director around the Si waveguide by selecting the required integrated groove direction and reduced the driving voltage by introducing a vertical groove array into the phase shifter. We then used the local and flexible LC initial alignment controllability to develop a Mach-Zehnder optical switch and ring-resonator wavelength filter. This approach will be helpful when integrating LC-loaded devices with various characteristics and functionalities into optical integrated circuits.

Electrically tuneable lateral leakage loss in liquid crystal clad shallow-etched silicon waveguides

We demonstrate electrical tuning of the lateral leakage loss of TM-like modes in nematic liquid crystal (LC) clad shallow-etched Silicon-on-Insulator (SOI) waveguides. The refractive index of the LC layer can be modulated by applying a voltage over it. This results in a modulation of the effective index of the SOI waveguide modes. Since the leakage loss is linked to these effective indices, tunable leakage loss of the waveguides is achieved. We switch the wavelength at which the minimum in leakage loss occurs by 39.5nm (from 1564nm to 1524.5nm) in a 785nm wide waveguide. We show that the leakage loss in this waveguide can either be increased or decreased by modulating the refractive index of the LC cladding at a fixed wavelength.

Radially realigning nematic liquid crystal for efficient tuning of microring resonators

The efficient tuning of microring resonator with the radially realigning nematic liquid crystal (NLC) cladding is presented. By applying the voltage on the in-plane annular electrodes, the produced electric field realigns the homeotropically-aligned NLC in the radial direction. Under the voltage sufficient for 90° NLC reorientation, the guided mode senses the consistent cladding index distribution along the microring waveguide with the maximal index change equal to the optical anisotropy of NLC. The resultant tuning of the resonant wavelength has a blue shift of 23.1nm for the TM mode and a red shift of 10.1nm for the TE mode. The tuning rates for the TM and TE modes are -1.95nm/V and 0.90nm/V. The proposed microring resonator owns the excellent features of wide tuning ranges and high tuning rates for the TM and TE modes.

Experimental study on the performance of a variable optical attenuator using polymer dispersed liquid crystal

We applied polymer dispersed liquid crystal (PDLC) as the cladding material in a polymer-based variable optical attenuator. Three polymer inverted channel waveguides were fabricated, two with PDLC upper cladding (aligned PDLC and nonaligned PDLC) and one with aligned liquid crystal upper cladding. Upon operation, the waveguides with aligned upper claddings show relatively lower threshold and cutoff voltages compared to those with nonaligned PDLC cladding. But the waveguide with nonaligned PDLC upper cladding shows lower polarization dependence and a higher attenuation range of 39 and 41.37 dB for TM and TE modes, respectively, over a tuning field strength of 0.9 V/μm.

Optical tuning of silicon photonic structures with nematic liquid crystal claddings

An analysis of and experimental demonstration of active optical tuning of silicon strip waveguides with methyl red doped nematic liquid crystal claddings is presented. Under low-power irradiation by polarized light, the reorientation of the nematic, the resulting index change, and phase shift produce a tuning range of 5.6 nm for the microresonator resonances.

Wide tuning of SiN microring resonators by auto-realigning nematic liquid crystal

We present wide electrical tuning of microring resonators with auto-realigned nematic liquid crystal (NLC) cladding. By applying electric field, homeotropically-aligned negative Δε NLC with non-rubbed alignment layers is auto-realigned along the microring waveguide due to the protruding of the ridge structure. The consistent cladding index distribution along the microring waveguide produces effective tuning of the resonant wavelength. It achieves a large tuning range of 13nm for TM mode and 2.1nm for TE mode. The NLC reorientation characteristics are investigated by minimizing Oseen-Frank energy. The proposed microring resonator owns the features of large tuning range and bi-polarization wavelength tuning.

Trimming of silicon-on-insulator ring resonators with a polymerizable liquid crystal cladding

We demonstrate the trimming of silicon-on-insulator ring resonators with a cladding layer of polymerizable liquid crystal. An electric field is applied over the cladding layer to tune the resonance of the ring resonators, which is then fixed by UV illumination of the polymerizable liquid crystal. A range of 0.56 nm is obtained. We provide the material properties of the polymerizable liquid crystal, give a description of the tuning mechanism and present experimental results. This method opens up possibilities in the field of low-cost trimming of photonic devices.

Light emission from dye-doped cholesteric liquid crystals at oblique angles: Simulation and experiment

Dye-doped cholesteric liquid crystals with a helical pitch of the order of a wavelength have a strong effect on the fluorescence properties of dye molecules. This is a promising system for realizing tunable lasers at low cost. We apply a plane wave model to simulate the spontaneous emission from a layer of cholesteric liquid crystal. We simulate the spectral and angle dependence and the polarization of the emitted light as a function of the order parameter of the dye in the liquid crystal. Measurements of the angle dependent emission spectra and polarization are in good agreement with the simulations.

Wide tuning of silicon-on-insulator ring resonators with a liquid crystal cladding

Wide electrical tuning of silicon-on-insulator ring resonators is demonstrated using a top cladding layer of nematic liquid crystals. A tuning range of 31 nm is demonstrated for ring resonators guiding the TM mode, covering nearly the entire C-band of optical communications. Ring resonators guiding the TE mode can be tuned over 4.5 nm. The combination of a liquid crystal director calculation and a fully anisotropic mode solver confirms the interpretation of these experimental results. The realization of broad and low-power tuning in silicon-on-insulator opens up new opportunities in the field of tunable lasers, filters, and detectors.

Dipole radiation within one-dimensional anisotropic microcavities: a simulation method

We present a simulation method for light emitted in uniaxially anisotropic light-emitting thin film devices. The simulation is based on the radiation of dipole antennas inside a one-dimensional microcavity. Any layer in the microcaviy can be uniaxially anisotropic with an arbitrary orientation of the optical axis. A plane wave expansion for the field of an elementary dipole inside an anisotropic medium is derived from Maxwell's equations. We employ the scattering matrix method to calculate the emission by dipoles inside an anisotropic microcavity. The simulation method is applied to calculate the emission of dipole antennas in a number of cases: a dipole antenna in an infinite medium, emission into anisotropic slab waveguides and waveguides in liquid crystals. The dependency of the intensity and the polarization on the direction of emission is illustrated for a number of anisotropic microcavities.

Integrated chalcogenide waveguide resonators for mid-IR sensing: leveraging material properties to meet fabrication challenges

In this paper, attributes of chalcogenide glass (ChG) based integrated devices are discussed in detail, including origins of optical loss and processing steps used to reduce their contributions to optical component performance. Specifically, efforts to reduce loss and tailor optical characteristics of planar devices utilizing solution-based glass processing and thermal reflow techniques are presented and their results quantified. Post-fabrication trimming techniques based on the intrinsic photosensitivity of the chalcogenide glass are exploited to compensate for fabrication imperfections of ring resonators. Process parameters and implications on enhancement of device fabrication flexibility are presented.

Rolled-up optical microcavities with subwavelength wall thicknesses for enhanced liquid sensing applications

Microtubular optical microcavities from rolled-up ring resonators with subwavelength wall thicknesses have been fabricated by releasing prestressed SiO/SiO(2) bilayer nanomembranes from photoresist sacrificial layers. Whispering gallery modes are observed in the photoluminescence spectra from the rolled-up nanomembranes, and their spectral peak positions shift significantly when measurements are carried out in different surrounding liquids, thus indicating excellent sensing functionality of these optofluidic microcavities. Analytical calculations as well as finite-difference time-domain simulations are performed to investigate the light confinement in the optical microcavities numerically and to describe the experimental mode shifts very well. A maximum sensitivity of 425 nm/refractive index unit is achieved for the microtube ring resonators, which is caused by the pronounced propagation of the evanescent field in the surrounding media due to the subwavelength wall thickness design of the microcavity. Our optofluidic sensors show high potential for lab-on-a-chip applications, such as real-time bioanalytic systems.