Short-Wave Infrared Colloidal QD Photodetector with Nanosecond Response Times Enabled by Ultrathin Absorber Layers

Ultrafast short-wavelength infrared (SWIR) photodetection is of great interest for emerging automated vision and spatial mapping technologies. Colloidal quantum dots (QDs) stand out for SWIR photodetection compared to epitaxial (In,Ga)As or (Hg,Cd)Te semiconductors by their combining a size-tunable bandgap and a suitability for cost-effective, solution-based processing. However, achieving ultrafast, nanosecond-level response time has remained an outstanding challenge for QD-based SWIR photodiodes (QDPDs). Here, record 4 ns response time in PbS-based QDPDs that operate at SWIR wavelengths is reported, a result reaching the requirement of SWIR light detection and ranging based on colloidal QDs. These ultrafast QDPDs combine a thin active layer to reduce the carrier transport time and a small area to inhibit slow capacitive discharging. By implementing a concentration gradient ligand exchange method, high-quality p-n junctions are fabricated in these ultrathin QDPDs. Moreover, these ultrathin QDPDs attain an external quantum efficiency of 42% at 1330 nm, due to a 2.5-fold enhanced light absorption through the formation of a Fabry-Perot cavity within the QDPD and the highly efficient extraction (98%) of photogenerated charge carriers. Based on these results, it is estimated that a further increase of the charge-carrier mobility can lead to PbS QDPDs with sub-nanosecond response time.

Topical Application of Deglycating Enzymes as an Alternative Non-Invasive Treatment for Presbyopia

Presbyopia is an age-related vision disorder that is a global public health problem. Up to 85% of people aged ≥40 years develop presbyopia. In 2015, 1.8 billion people globally had presbyopia. Of those with significant near vision disabilities due to uncorrected presbyopia, 94% live in developing countries. Presbyopia is undercorrected in many countries, with reading glasses available for only 6-45% of patients living in developing countries. The high prevalence of uncorrected presbyopia in these parts of the world is due to the lack of adequate diagnosis and affordable treatment. The formation of advanced glycation end products (AGEs) is a non-enzymatic process known as the Maillard reaction. The accumulation of AGEs in the lens contributes to lens aging (leading to presbyopia and cataract formation). Non-enzymatic lens protein glycation induces the gradual accumulation of AGEs in aging lenses. AGE-reducing compounds may be effective at preventing and treating AGE-related processes. Fructosyl-amino acid oxidase (FAOD) is active on both fructosyl lysine and fructosyl valine. As the crosslinks encountered in presbyopia are mainly non-disulfide bridges, and based on the positive results of deglycating enzymes in cataracts (another disease caused by glycation of lens proteins), we studied the ex vivo effects of topical FAOD treatment on the power of human lenses as a new potential non-invasive treatment for presbyopia. This study demonstrated that topical FAOD treatment resulted in an increase in lens power, which is approximately equivalent to the correction obtained by most reading glasses. The best results were obtained for the newer lenses. Simultaneously, a decrease in lens opacity was observed, which improved lens quality. We also demonstrated that topical FAOD treatment results in a breakdown of AGEs, as evidenced by gel permeation chromatography and a marked reduction in autofluorescence. This study demonstrated the therapeutic potential of topical FAOD treatment in presbyopia.

Chiral liquid crystal based holographic reflective lens for spectral detection

Flat optics based on chiral liquid crystal (CLC) can be achieved using holographic polarization recording with the help of a photoalignment technique to vary the orientation of the optical axis in a thin CLC layer. A variety of reflective diffractive optical components with high efficiency and polarization selectivity can be realized employing this technique. In this work we discuss the use of CLC diffractive lenses in a spectrometer. The functionalities of two mirrors and a linear grating used in a traditional spectrometer are combined into a single holographic CLC component. Circularly polarized light entering through the slit can be reflected and projected onto a linear detector by the CLC component, with over 90% efficiency. This excellent optical functionality can be achieved with a micrometer thin CLC layer, offering the opportunity for device integration.

Quantum dot lasing from a waterproof and stretchable polymer film

Colloidal quantum dots (QDs) are excellent optical gain materials that combine high material gain, a strong absorption of pump light, stability under strong light exposure and a suitability for solution-based processing. The integration of QDs in laser cavities that fully exploit the potential of these emerging optical materials remains, however, a challenge. In this work, we report on a vertical cavity surface emitting laser, which consists of a thin film of QDs embedded between two layers of polymerized chiral liquid crystal. Forward directed, circularly polarized defect mode lasing under nanosecond-pulsed excitation is demonstrated within the photonic band gap of the chiral liquid crystal. Stable and long-term narrow-linewidth lasing of an exfoliated free-standing, flexible film under water is obtained at room temperature. Moreover, we show that the lasing wavelength of this flexible cavity shifts under influence of pressure, strain or temperature. As such, the combination of solution processable and stable inorganic QDs with high chiral liquid crystal reflectivity and effective polymer encapsulation leads to a flexible device with long operational lifetime, that can be immersed in different protic solvents to act as a sensor.

Light Modulation in Silicon Photonics by PZT Actuated Acoustic Waves

Tailoring the interaction between light and sound has opened new possibilities in photonic integrated circuits (PICs) that range from achieving quantum control of light to high-speed information processing. However, the actuation of sound waves in Si PICs usually requires integration of a piezoelectric thin film. Lead zirconate titanate (PZT) is a promising material due to its strong piezoelectric and electromechanical coupling coefficient. Unfortunately, the traditional methods to grow PZT on silicon are detrimental for photonic applications due to the presence of an optical lossy intermediate layer. In this work, we report integration of a high quality PZT thin film on a silicon-on-insulator (SOI) photonic chip using an optically transparent buffer layer. We demonstrate acousto-optic modulation in silicon waveguides with the PZT actuated acoustic waves. We fabricate interdigital transducers (IDTs) on the PZT film with a contact photolithography and electron-beam lithography to generate the acoustic waves in MHz and GHz ranges, respectively. We obtain a V _π L ∼ 3.35 V·cm at 576 MHz from a 350 nm thick gold (Au) IDT with 20 finger-pairs. After taking the effect of mass-loading and grating reflection into account, we measured a V _π L ∼ 3.60 V·cm at 2 GHz from a 100 nm thick aluminum (Al) IDT consisting of only four finger-pairs. Thus, without patterning the PZT film nor suspending the device, we obtained figures-of-merit comparable to state-of-the-art modulators based on SOI, making it a promising candidate for a broadband and efficient acousto-optic modulator for future integration.

Influence of period and surface anchoring strength in liquid crystal optical axis gratings

Liquid crystal (LC) based geometric phase optical elements are widely used to effectively change the wavefront or propagation direction of light. Using photoalignment, the liquid crystal can be aligned according to the designed pattern, leading to components such as gratings, lenses or general wavefront shaping devices. The functionality and efficiency of the component is strongly influenced by how well the LC follows the imposed alignment pattern. Next to a considerable tilting of the LC at the air interface, we report on the observation of symmetry breaking in polymerized LC polarization gratings. By carefully analyzing the experimental and numerical data for gratings with different periods, we conclude that the non-negligible homeotropic anchoring strength at the air interface is responsible for the tilt angle and the symmetry breaking. The role of anchoring strength at the photoaligned and air interface and other parameters are investigated.

Ring-shaped liquid crystal structures through patterned planar photo-alignment

Patterned liquid crystal (LC) configurations find widespread applications in functional devices such as lenses, gratings, displays and soft-robots. In combination with external stimuli such as an applied electric field, photo-alignment at the surfaces offers an attractive way to stabilize different LC structures in the bulk of a device. Herein, a planar LC cell is developed using a photo-alignment layer at the bottom substrate and a rubbed nylon film at the top substrate. Patterned planar photo-alignment is achieved by modulating the linear polarization with a spatial light modulator (SLM) and projecting the pattern onto the bottom substrate. A ring pattern is written into the photo-alignment layer with a continuous rotation between an inner radius and an outer radius. In the other regions the alignment is parallel to the rubbing direction at the top substrate. Four different LC configurations are observed: structure A in which a ring-shaped region is formed with an out of plane (vertical) orientation perpendicular to the substrate, structure B which has a single disclination loop and a 180° twist at the inner region of the photo-patterned ring (r < r_in), structure C which has no discontinuities but a 360° twist in the inner region of the photo-patterned ring (r < r_in) and structure D with 2 disclination loops. The LC director configuration for all 4 structures is simulated through finite element (FE) Q-tensor simulations and the optical transmission for each structure is simulated using a generalized beam propagation method.

Design, fabrication and characterization of a distributed Bragg reflector for reducing the étendue of a wavelength converting system

In this work, the design, fabrication and characterization are reported for a distributed Bragg reflector (DBR) filter with a specific wavelength and angular dependency, which aims to improve the light collection from a wavelength-converter-based light source into a smaller angle than the full angle Lambertian emission. The desired design is obtained by optimizing the transmission characteristics of a multi-layer structure. Titania (TiO₂) and silica (SiO₂) are used as high and low refractive index materials, respectively. The deposition is made by electron beam evaporation without substrate heating, followed by a post-annealing procedure. The optical properties of the evaporated layers are analyzed by ellipsometer and spectrometer measurements. The angular and wavelength dependency of the fabricated DBR is in good agreement with simulations for the designed structure.

Improvement of liquid crystal tunable lenses with weakly conductive layers using multifrequency driving

A common technique to realize the gradient electric field profile that is required in liquid crystal tunable lenses is the use of a weakly conductive layer. Thanks to this layer, an applied voltage with a certain frequency allows us to obtain a refractive index profile that is required for the lens operation. Due to the limited degrees of freedom, however, it is not possible to avoid aberrations in a weakly conductive layer-based tunable lens for a continuously tunable focal length. In this work, we discuss the use of additional higher frequency components in the voltage signal to reduce the lens aberrations drastically.

Voltage-controlled formation of short pitch chiral liquid crystal structures based on high-resolution surface topography

Chiral nematic liquid crystals (CLCs) offer interesting perspectives for device applications and are fascinating materials to study because of their ability to self-assemble into complex structures. This work demonstrates that narrow lines of electron-beam resist on top of an ITO coated glass surface can dramatically influence the formation and growth of short pitch chiral superstructures in the bulk. By applying a voltage to the cell, directional growth of CLC structures along the corrugated surface can be controlled. Below the electric unwinding threshold, chiral structures start to grow along the grating lines with their helical axis parallel to the substrates. This results in a uniform lying helix-like structure at intermediate voltages and a chiral configuration with periodic undulations of the helical axis at low voltages.

Mode coupling by scattering in chiral nematic liquid crystal ring lasing

Lasing in dye-doped chiral nematic liquid crystal can be realized with low pump energy and relatively high efficiency, thanks to the high reflectivity of the periodic structure. When the helical axis is oriented perpendicular to the substrates, the main lasing peak is normal to the substrates. In some cases, ring lasing of a particular wavelength is observed into an emission cone with axial symmetry. In this paper we explain how scattering of light in the liquid crystal layer leads to optical coupling between normal modes and inclined modes. Based on a numerical model that takes into account spontaneous emission, gain and scattering we show that scattering leads to emission characteristics that are similar to experimental results.

Generation of multiple solitons using competing nonlocal nonlinearities

We discuss the dynamics of fundamental Gaussian beams launched in saturable and nonlocal nonlinear media. Solely in the presence of a self-focusing saturable nonlinearity, the breathing solitons undergo strong deformation. The addition of a defocusing nonlinearity leads to the generation of couples of solitons. Experimentally, we demonstrate in nematic liquid crystals the formation of multiple spatial solitons starting from a bell-shaped input, with both direction and the number of filaments depending on the input power, confirming the theoretical predictions.

Complex liquid crystal superstructures induced by periodic photo-alignment at top and bottom substrates

The formation of nematic liquid crystal (LC) superstructures in cells with non-uniform photo-alignment at the confining substrates is studied experimentally and by simulations. An interference pattern of left- and right-handed circularly polarized light is used to define the alignment at both substrates separately, so that the alignment varies along the x-coordinate on one substrate and along the y-coordinate on the other substrate. The interplay between the complex surface alignment and the liquid crystalline soft matter leads to the formation of interesting 3D configurations. The periodic LC structures that are formed in the bulk of the cell are analyzed experimentally by polarizing optical microscopy (POM) for different applied voltages. In the region with strong photo-alignment at both substrates, a 2D LC polarization grating (PG) with a complex 3D director configuration is formed. Distinct periodic structures with different symmetry properties are observed in the regions with weak illumination at the top and/or bottom substrate. The director configuration in the different regions was successfully simulated with the help of finite element (FE) Q-tensor simulations. The agreement between the simulations and the experiments was verified by comparing the POM images with simulated results for the transmission between crossed polarizers.

Nanophotonic Pockels modulators on a silicon nitride platform

Silicon nitride (SiN) is emerging as a competitive platform for CMOS-compatible integrated photonics. However, active devices such as modulators are scarce and still lack in performance. Ideally, such a modulator should have a high bandwidth, good modulation efficiency, low loss, and cover a wide wavelength range. Here, we demonstrate the first electro-optic modulators based on ferroelectric lead zirconate titanate (PZT) films on SiN, in both the O-band and C-band. Bias-free operation, bandwidths beyond 33 GHz and data rates of 40 Gbps are shown, as well as low propagation losses (α ≈ 1 dB cm⁻¹). A half-wave voltage-length product of 3.2 V cm is measured. Simulations indicate that further improvement is possible. This approach offers a much-anticipated route towards high-performance phase modulators on SiN.

Active devices such as modulators made of silicon nitride still lack performance. Here, the authors demonstrate electro-optic modulators based on ferroelectric lead zirconate titanate films on silicon nitride, in both the O- and the C-band with a modulation bandwidth beyond 33 GHz and with data rates of 40 Gbps.

Interplay between multiple scattering and optical nonlinearity in liquid crystals

We discuss the role played by time-dependent scattering on light propagation in liquid crystals. In the linear regime, the effects of the molecular disorder accumulate in propagation, yielding a monotonic decrease in the beam spatial coherence. In the nonlinear case, despite the disorder-imposed Brownian-like motion to the self-guided waves, self-focusing increases the spatial coherence of the beam by inducing spatial localization. Eventually, a strong enhancement in the beam oscillations occurs when power is strong enough to induce self-steering, i.e., in the non-perturbative regime.

Multi-electrode tunable liquid crystal lenses with one lithography step

Electrically tunable lenses offer the possibility to control the focal distance by applying an electric field. Different liquid crystal tunable lenses have been demonstrated. In order to minimize lens aberrations, multi-electrode designs allow us to fine-tune the applied voltages for every possible focal distance. In this Letter, we provide a novel multi-electrode design in which only one lithography step is necessary, thereby offering a greatly simplified fabrication procedure compared to earlier proposed designs. The key factor is the use of a high-permittivity layer, in combination with floating electrodes.

Reflective liquid crystal hybrid beam-steerer

We report on efficient optical beam-steering using a hot-embossed reflective blazed grating in combination with liquid crystal. A numerical simulation of the electrical switching characteristics of the liquid crystal is performed and the results are used in an FDTD optical simulator to analyze the beam deflection. The corresponding experiment on the realized device is performed and is found to be in good agreement. Beam deflection angles of 4.4° upon perpendicular incidence are found with low applied voltages of 3.4 V. By tilting the device with respect to the incoming optical beam it can be electronically switched such that the beam undergoes either total internal reflection or reflection with a tunable angle.

Nematicon-driven injection of amplified spontaneous emission into an optical fiber

We investigate experimentally the interaction between amplified spontaneous emission (ASE) and a soliton, which are both generated in a dye-doped nematic liquid crystal (LC) cell. A light beam is injected through an optical fiber slid into the cell to form a soliton beam. ASE is then automatically collected by this self-induced waveguide and efficiently coupled into the same optical fiber, in the backward direction. We demonstrate that the presence of the soliton improves the ASE collection by one order of magnitude. We also show that the ASE is highly polarized in the plane of the LC cell and that the ASE spectrum depends on the pump stripe orientation with respect to the LC director. The origin of the spectral anisotropy of the gain curves is determined with the help of femtosecond pump-probe spectroscopy.

Ferroelectric thin films with liquid crystal for gradient index applications

We report on the first ever combination of a thin film of lead zirconate titanate (PZT) with a liquid crystal (LC) layer. Many liquid crystal applications use a transparent conductive oxide to switch the liquid crystal. Our proposed processing does not, instead relying on the extremely high dielectric constant of the ferroelectric layer to extend the electric field from widely spaced electrodes over the liquid crystal. It eliminates almost entirely the fringe field problems that arise in nearly all the liquid crystal devices that use multiple addressing electrodes. We show, both via rigorous simulations as well as experiments, that the addition of a PZT layer over the addressing electrodes leads to a markedly improved LC switching performance at distances of up to 30 μm from the addressing electrodes with the current PZT-layer thickness of 0.84 μm. This improvement in switching is used to tune the focal length of the microlens with electrodes spaced at 30 μm.

Switchable 3D liquid crystal grating generated by periodic photo-alignment on both substrates

A planar liquid crystal (LC) cell is developed in which two photo-alignment layers have been illuminated with respectively a horizontal and a vertical diffraction pattern of interfering left- and right-handed circularly polarized light. In the bulk of the cell, a complex LC configuration is obtained with periodicity in two dimensions. Remarkably, the period of the structure is larger than the period of the interference pattern, indicating that lowering of the symmetry allows a reduction in the elastic energy. The liquid crystal configuration depends on the periodicity of the alignment but also on the thickness of the cell. By applying a voltage over the electrodes, the power going into the different diffracted orders can be tuned. Finite element (FE) simulations based on Q-tensor theory are used to find the 3D equilibrium director distribution, which is used to simulate the near-field transmission profile based on the Jones calculus. A 2D Fourier transform is performed for both the x- and y-component of the transmitted wave to find the diffraction efficiency.

Lanthanide-Assisted Deposition of Strongly Electro-optic PZT Thin Films on Silicon: Toward Integrated Active Nanophotonic Devices

The electro-optical properties of lead zirconate titanate (PZT) thin films depend strongly on the quality and crystallographic orientation of the thin films. We demonstrate a novel method to grow highly textured PZT thin films on silicon using the chemical solution deposition (CSD) process. We report the use of ultrathin (5-15 nm) lanthanide (La, Pr, Nd, Sm) based intermediate layers for obtaining preferentially (100) oriented PZT thin films. X-ray diffraction measurements indicate preferentially oriented intermediate Ln2O2CO3 layers providing an excellent lattice match with the PZT thin films grown on top. The XRD and scanning electron microscopy measurements reveal that the annealed layers are dense, uniform, crack-free and highly oriented (>99.8%) without apparent defects or secondary phases. The EDX and HRTEM characterization confirm that the template layers act as an efficient diffusion barrier and form a sharp interface between the substrate and the PZT. The electrical measurements indicate a dielectric constant of ∼650, low dielectric loss of ∼0.02, coercive field of 70 kV/cm, remnant polarization of 25 μC/cm(2), and large breakdown electric field of 1000 kV/cm. Finally, the effective electro-optic coefficients of the films are estimated with a spectroscopic ellipsometer measurement, considering the electric field induced variations in the phase reflectance ratio. The electro-optic measurements reveal excellent linear effective pockels coefficients of 110 to 240 pm/V, which makes the CSD deposited PZT thin film an ideal candidate for Si-based active integrated nanophotonic devices.

Optimization of electrically tunable VCSEL with intracavity nematic liquid crystal

We optimize the wavelength tuning range of a Vertical-Cavity Surface-Emitting Laser with an intracavity layer of nematic Liquid Crystal (LC-VCSEL) lasing around 1.3 μm. The tunability is obtained by applying voltage to the liquid crystal layer, which esentially is to vary the refractive index from the extraordinary to the ordinary. We achieve 71.6 nm continuous tuning (without mode hopping) with liquid crystal thickness of about 3.2 μm. We investigate the impact of ambient temperature on the LC-VCSEL tuning range and show that mode-hop tuning can be achieved in the temperature range from -10°C to 50°C where the LC is in nematic phase.

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.

Optical gain from polyfluorene keto defects in a liquid crystal mixture

Confocal photoluminescence measurements and fs pump–probe spectroscopy to observe a polarized gain region from keto defects in polyfluorene isolated chains.

Vertical-cavity surface-emitting laser with a liquid crystal external cavity

A tuneable external cavity consisting of a thin layer of nematic liquid crystal (LC) and a dielectric reflector is placed on the top of a vertical-cavity surface-emitting laser (VCSEL). By changing the voltage across the LC layer, the optical path length of the external cavity can be tuned. As a result, the emitting properties of the LC-VCSEL, including polarization state and emission wavelength, can be controlled by the voltage applied over the LC layer. Stable polarization switching with high contrast is obtained by voltage driving. This device can be integrated in applications which require electrically tuneable VCSEL emission.

Active liquid crystal tuning of metallic nanoantenna enhanced light emission from colloidal quantum dots

A system comprising an aluminum nanoantenna array on top of a luminescent colloidal quantum dot waveguide and covered by a thermotropic liquid crystal (LC) is introduced. By heating the LC above its critical temperature, we demonstrate that the concomitant refractive index change modifies the hybrid plasmonic-photonic resonances in the system. This enables active control of the spectrum and directionality of the narrow-band (∼6 nm) enhancement of quantum dot photoluminescence by the metallic nanoantennas.

Widely tunable chiral nematic liquid crystal optical filter with microsecond switching time

A wavelength shift of the photonic band gap of 141 nm is obtained by electric switching of a partly polymerized chiral liquid crystal. The devices feature high reflectivity in the photonic band gap without any noticeable degradation or disruption and have response times of 50 µs and 20 µs for switching on and off. The device consists of a mixture of photo-polymerizable liquid crystal, non-reactive nematic liquid crystal and a chiral dopant that has been polymerized with UV light. We investigate the influence of the amplitude of the applied voltage on the width and the depth of the reflection band.

Tuning the lateral leakage loss of TM-like modes in shallow-etched waveguides using liquid crystals

We examine the tuning effect a liquid crystal (LC) cladding has on the lateral leakage loss of TM-like modes in shallow-etched waveguides. For such waveguides with an air cladding, the guided TM-like mode and the unguided cladding TE-like mode can only be phase matched at precisely one angle. We find that for an anisotropic cladding such as an LC, this phase matching is now possible for a range of angles. Each of these angles corresponds to a given orientation of the molecules in the LC cladding. We show that the waveguide width at which the minimum in leakage loss occurs can be changed by varying the orientation of the LC cladding. We find different tuning regimes, identify a suitable tuning range, and discuss the feasibility of tunable leakage loss experiments.

Preferentially oriented BaTiO3 thin films deposited on silicon with thin intermediate buffer layers

Barium titanate (BaTiO3) thin films are prepared by conventional 2-methoxy ethanol-based chemical solution deposition. We report highly c-axis-oriented BaTiO3 thin films grown on silicon substrates, coated with a lanthanum oxynitrate buffer layer of 8.9 nm. The influence of the intermediate buffer layer on the crystallization of BaTiO3 film is investigated. The annealing temperature and buffer layer sintering conditions are optimized to obtain good crystal growth. X-ray diffraction measurements show the growth of highly oriented BaTiO3 thin films having a single perovskite phase with tetragonal geometry. The scanning electron microscopy and atomic force microscopy studies indicate the presence of smooth, crack-free, uniform layers, with densely packed crystal grains on the silicon surface. A BaTiO3 film of 150-nm thickness, deposited on a buffer layer of 7.2 nm, shows a dielectric constant of 270, remnant polarization (2Pr) of 5 μC/cm2, and coercive field (Ec) of 60 kV/cm.

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.

Multicasting optical interconnects using liquid crystal over silicon devices

This work presents the characteristics and expected capabilities of an optical interconnect that uses a diffractive liquid crystal over silicon (LCOS) device as a routing element. Such an interconnect may be used in a neighborhood's optical network to distribute high definition television, thus avoiding an electronic or optical transmitter for each user. The optimal characteristics of the LCOS device are calculated in terms of pixel number and silicon area and found to be feasible with today's technology. Finally, its performance in terms of optical efficiency and number of output ports is evaluated and found suitable for a neighborhood with hundreds of households.

Finding exact spatial soliton profiles in nematic liquid crystals

Finding exact analytical soliton profile solutions is only possible for certain types of non-linear media. In most cases one must resort to numerical techniques to find the soliton profile. In this work we present numerical calculations of spatial soliton profiles in nematic liquid crystals. The nonlinearity is governed by the optical-field-induced liquid crystal director reorientation, which is described by a system of coupled nonlinear partial differential equations. The soliton profile is found using an iterative scheme whereby the induced waveguide and mode profiles are calculated alternatively until convergence is achieved. In this way it is also possible to find higher order solitons. The results in this work can be used to accurately design all-optical interconnections with soliton beams.

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

We show tuning of the resonance wavelength of silicon-on-insulator microring resonators with liquid crystal cladding. The electro-optic effect of the liquid crystal causes a decrease in effective refractive index for the TE-polarized light in the waveguides. Tuning of the liquid crystal birefringence affects primarily the longitudinal component of the electric field. We achieve a tuning range of 0.6 nm. Through simulation and experiment we perform a thorough study of this phenomenon.

A finite element beam propagation method for simulation of liquid crystal devices

An efficient full-vectorial finite element beam propagation method is presented that uses higher order vector elements to calculate the wide angle propagation of an optical field through inhomogeneous, anisotropic optical materials such as liquid crystals. The full dielectric permittivity tensor is considered in solving Maxwell's equations. The wide applicability of the method is illustrated with different examples: the propagation of a laser beam in a uniaxial medium, the tunability of a directional coupler based on liquid crystals and the near-field diffraction of a plane wave in a structure containing micrometer scale variations in the transverse refractive index, similar to the pixels of a spatial light modulator.

Countering spatial soliton breakdown in nematic liquid crystals

Spatial optical soliton propagation in any material is limited by the losses of the optical beam, which results in beam broadening. In nematic liquid crystals it is possible to tune the magnitude of the nonlinearity by means of a bias voltage. In this work we present the idea of increasing the nonlinearity along the propagation distance by changing the bias voltage. Next to a theoretical analysis, we present experimental proof of the validity of our method. The use of this technique offers a major advantage for optical interconnects because the beam broadening can be reduced over much longer propagation distances.

Dynamics of charge transport in planar devices

The Poisson-Nernst-Planck equations describe the dynamics of charge transport in an electric field. Although they are relevant in many applications, a general solution is not known and several aspects are not well understood. In many situations nonlinear effects arise for which no analytical description is available. In this work, we investigate charge transport in a planar device on application of a voltage step. We derive analytical expressions for the dynamical behavior in four extreme cases. In the "geometry limited" regime, applicable at high voltages and low charge contents, we neglect diffusion and the electric field induced by the charges. This leads to a uniform movement of all charges until the bulk is completely depleted. In the "space charge limited" regime, for high voltages and high charge contents, diffusion is still neglected but the electric field is almost completely screened over transient space charge layers. Eventually, however, the bulk becomes depleted of charges and the field becomes homogeneous again. This regime is solved under the assumption of a homogeneous current density, and is characterized by a typical t -3/4 behavior. In the "diffusion limited" regime, valid for low voltages and low charge contents, diffusion is the dominant transport mechanism and prevents the charges from separating. This results in only very small deviations from a homogeneous charge distribution throughout the device. In the "double layer limited" regime, for low voltages and high charge contents, the combination of dominant diffusion and screening of the electric field results in large variations occurring only in thin double layers near the electrodes. Numerical simulations confirm the validity of the derived analytical expressions for each of the four regimes, and allow us to investigate the parameter values for which they are applicable. We present transient current measurements on a nonpolar liquid with surfactant and compare them with the external current predicted by the theoretical description. The agreement of the analytical expressions with the experiments allows us to obtain values for a number of properties of the charges in the liquid, which are consistent with results in other works. The confirmation by simulations and measurements of the derived theoretical expressions gives confidence about their usefulness to understand various aspects of the Poisson-Nernst-Planck equations and the effects they represent in the dynamics of charge transport.

Induced modulation instability and recurrence in nematic liquid crystals

We study induced modulation instability in a nematic liquid crystal cell. Two broad elliptical beams along one direction are launched into the cell. The two beams have slightly different angle in order to create a sinusoidally varying intensity at the entrance of the cell. In this way, the gain of perturbations with different spatial frequency is investigated. The evolution of the optical pattern, for certain conditions, shows a recurrence of the signal. We believe that this is the manifestation of the Fermi-Pasta- Ulam recurrence and to the best of our knowledge, the first experimental observation of this phenomenon in the spatial optical domain. Numerical simulations show a good agreement with the experimental findings.

Simulations and experiments on self-focusing conditions in nematic liquid-crystal planar cells

Owing to the nonlinear effect of optical field-induced director reorientation, self-focusing of an optical beam can occur in nematic liquid crystals and an almost diffraction-compensated propagation can be observed with milliwatts of light power and propagation lengths of several millimeters. This opens the way for applications in all-optical signal handling and reconfigurable optical interconnections. Self-focusing of an optical beam in nematic liquid-crystal cells has been studied experimentally and by means of numerical simulation. The relationships between bias voltage, cell thickness and required optical power have been examined, thus allowing the determination of the most favorable conditions for soliton-like beam propagation.