Phase modulators are key building blocks for Photonic Integrated Circuits (PICs). Si modulators based on plasma dispersion suffer from spurious amplitude modulation and high insertion losses. Pockels effect has been explored for more efficient phase modulators. However, Si doesn’t exhibit Pockels effect due to its centrosymmetric structure. Co-integration of thin-film electro-optic materials possessing a strong linear electro-optic coefficient on Si has therefore been proposed as an ideal alternative for more efficient phase modulators. Strongly electro-optic thin films of ferro-electric Lead Zirconate Titanate (PZT) grown on Si waveguides allow for Hybrid PZT/Si phase modulators. We present here a TE/TM electro-optic modulator with bias-free operation, bandwidths beyond 10Ghz, and negligible spurious amplitude modulation. The modulator is a phase shifter which comprises of straight Si waveguides and thin films of PZT spin-coated on the waveguides. The phase shifters were experimentally characterized by beating the modulated signal with an external acousto-optic modulator and evaluating the ratio between both signals. This experiment enables fast and easy characterization of phase modulators as proof of concept.
Silicon (Si) photonics is a highly promising platform for integrated optoelectronics. The ability to leverage existing CMOS infrastructure and the high index contrast between silicon and its native oxide have enabled compact, low-loss, and relatively low-cost photonic integrated circuits. Recently, there has been a drive towards integrating other CMOS-compatible materials in the silicon photonics platform, with silicon nitride (SiN) on the forefront. SiN offers a wider transparency range, lower temperature sensitivity and lower linear and nonlinear losses than Si. Many potential applications would benefit from having efficient, low-loss and fast phase modulators in these platforms. However, current solutions still lack in performance, plasma dispersion-based modulators in silicon are intrinsically lossy and suffer from spurious amplitude modulation. On SiN, simply no viable solutions with bandwidths beyond the MHz range exist. We tackle these issues by co-integration of thin films of lead zirconate titanate (PZT) onto integrated photonic structures. PZT is known to exhibit a strong Pockels effect, as well as related second order optical nonlinearities. This enables us to harness these effects for electro-optic modulation and other applications on existing Si and SiN platforms. In this work, we summarize our recent results. Including the demonstration of phase modulation using PZT-on-SiN waveguides in both the C- and the O-band, with bias-free operation, data rates of at least 40 Gbps and bandwidths beyond 33 GHz. We moreover demonstrate efficient phase modulation with a half-wave voltage length product of 3.2 Vcm and low propagation losses. Simulations indicate that further improvements are possible.
We report the first experimental realization of spatial soliton formation by the Gaussian beam at 632.8 nm in the azobenzene liquid crystal (LC) layer with planar orientation of LC director. By appropriate anti-parallel rubbing of alignment layers on the upper and lower substrates of the cell LC molecules were oriented along the glass substrates nearly perpendicular to the input window of the cell with a small pre-tilt angle of ~2.60 relative to the beam propagation Z direction. The strong self-focusing effect and soliton formation for laser beam with vertical Y-polarization and beam diffraction for horizontal X-polarization have been observed in the absence of an external electric field. The physical model is considered which implies that the interaction of azobenzene molecules with a laser field is much stronger due to a larger coefficient of orientation nonlinearity compared to other LCs, as well as they are not rigidly anchored to the cell boundary. Thus the molecule alignment can be readily varied by a low-power laser field even for a small pre-tilt angle of molecules which leads to the refractive index change and beam self-focusing regime. The numerical integration of the propagation equation for spatial solitons describes the experimental data very well.
By illuminating a photo-alignment layer with two interfering UV laser beams with opposite circular polarization, a periodic alignment pattern is obtained for nematic liquid crystal. In a liquid crystal cell two substrates are used with periodic photo-alignment, with the periodicities perpendicular to each other. After filling with nematic liquid crystal the director obtains a 3D pattern with period twice that of the photo-alignment. The resulting 3D structure depends on the cell thickness (between 3 and 20 m) and the periodicity of the photo-alignment (also between 3 and 20 m). The director pattern can be estimated by performing numerical simulation with a Q-tensor method. The simulation results can be verified by polarization optical microscopy or by observing the diffraction properties of the structure. For a long range periodicity combined with a small period, the light is efficiently distributed over a small number of diffraction orders. The director pattern can be reoriented by applying a potential difference between the two substrate electrodes. Optical and electrical steering is investigated for devices with different dimensions.
In this presentation we will report on our recent work on new materials that can be monolithically integrated on high-index contrast silicon or silicon nitride photonic ICs to enhance their functionality. This includes graphene and other 2D-materials for realizing compact electro-absorption modulators and non-linear devices, ferroelectric materials for realizing phase modulators and adiabatic couplers for realizing bistable switches.
Most liquid crystal devices use transparent conductive electrodes such as indium tin oxide (ITO) to apply a potential difference in order to achieve electro-optic switching. As an alternative, we study a device with narrow metallic electrodes in combination with dielectric layers with large dielectric permittivity. In this approach the applied voltage can be a continuous function of the lateral distance from the electrode line. Simulations for a one-dimensional beam-steering device show that the switching of the liquid crystal (LC) director depends indeed on the distance from the addressing electrodes and on the value of the relative permittivity. We show that in a device with electrodes spaced 60 µm apart, the LC director halfway between the electrodes shows a considerable reorientation, when a dielectric layer with permittivity of Epsilonr = 550 is used, whereas no reorientation is observed for the uncoated reference sample at the same voltage. An added advantage is that the proposed configuration only contains dielectric materials, without resistive losses, which means that almost no heat is dissipated. This indicates that this technology could be used in low-power LC devices. The results show that using dielectric thin films with high relative permittivity in liquid crystal devices could form a cost-efficient and low-power alternative to many LC technologies where a gradient electric field is desirable.
Liquid crystal (LC) lasers have gained a lot of research interest in the last decade. Especially out-of-plane emitting chiral nematic liquid crystal (CLC) lasers have been studied extensively. These regular CLC lasers have a one-dimensional (1D) structure and the active cavity length is inherently limited. By using CLCs in two- and three-dimensional structures, the flexibility and applicability of the laser structures can be strongly enhanced. In this paper we focus on 2D in-plane emitting CLC lasers with a lying helix structure. We elaborate further on different techniques to obtain the lying helix structure and we analyze the lasing properties and compare these to regular 1D out-of-plane emitting CLC and NLC lasers. Both differences in emission spectrum, laser threshold, slope efficiency and maximal output energy are discussed.
We present a method to fabricate a thin film color filter based on a mixture of photo-polymerizable liquid crystal and chiral dopant. A chiral nematic liquid crystal layer reflects light for a certain wavelength interval Δλ (= Δn.P) with the period and Δn the birefringence of the liquid crystal. The reflection band is determined by the chiral dopant concentration. The bandwidth is limited to 80nm and the reflectance is at most 50% for unpolarized incident light. The thin color filter is interesting for innovative applications like polarizer-free reflective displays, polarization-independent devices, stealth technologies, or smart switchable reflective windows to control solar light and heat. The reflected light has strong color saturation without absorption because of the sharp band edges. A thin film polarizer is developed by using a mixture of photo-polymerizable liquid crystal and color-neutral dye. The fabricated thin film absorbs light that is polarized parallel to the c axis of the LC. The obtained polarization ratio is 80% for a film of only 12 μm. The thin film polarizer and the color filter feature excellent film characteristics without domains and can be detached from the substrate which is useful for e.g. flexible substrates.
We study theoretically and experimentally spectral and polarization characteristics of hybrid systems of VCSELs integrated within liquid crystal (LC) cells. Three cases are considered: Nematic or cholesteric LC on top of VCSEL, coupled-cavity system with the second cavity next to the VCSEL’s one filled in with nematic LC and a system with a nematic LC inside the VCSEL cavity. For the case of nematic liquid crystal - VCSEL coupled cavity system we demonstrate selection between two orthogonal directions of linear polarization of the fundamental mode by changing the LC length or by electro-optical tuning of the LC director. For the case of cholesteric liquid crystal-VCSEL system we demonstrate lasing on circularly polarized (CP) modes due to the LC band gap for CP light. The transition from nematic to isotropic phase of the LC when increasing temperature leads to a drastic change of the polarization of the generated light from left-handed circular to linear polarization. Finally, we investigate the possibility of efficient wavelength tuning by utilizing electrooptical effect in nematic LC layer integrated next to the active region in a VCSEL cavity.
We have developed a technology to integrate a thin layer of liquid crystal (LC) on top of a Vertical-Cavity Surface- Emitting Laser (VCSEL). Based on this technology, we demonstrate VCSELs with a chiral liquid crystal (CLC) layer, which acts as a tuneable mirror. The reflection properties of the CLC layer are controlled by temperature. Next we demonstrate VCSEL devices with tuneable external cavity using a nematic LC layer incorporated with an additional dielectric mirror (SiO2/Ta2O5). The VCSEL and the LC layer can be electrically driven independently and the optical length in the external cavity can be tuned by the applied voltage on the LC layer. In both configurations we show that the emission properties of the VCSEL can be changed, in terms of emission wavelength, polarization state and/or lasing threshold.
Chiral nematic liquid crystals have attracted substantial interest. They spontaneously self-organize to form a helical structure with no complex fabrication procedure required and exhibit a reflection band for a certain wavelength interval. Since the photonic band gap can be tuned by applying external factors (heat, voltage, light, elasticity) chiral nematic liquid crystals are potentially interesting for large area optical filters and shutters, reflective displays and tunable lasers. In this work, a device which consists of a mixture of photo-polymerizable liquid crystal, non-reactive nematic liquid crystal and a chiral dopant is fabricated. By selecting the appropriate chiral dopant concentration, it is possible to make devices for different operation wavelengths. The influence of UV illumination on a partially polymerized chiral liquid crystal is investigated. A blue-wavelength shift of the photonic band gap is obtained as a function of power, duration time of UV illumination and the thickness of the cells. Interestingly the width and depth of the photonic band gap is unaffected by the change in UV curing conditions, which indicates that there is no degradation by the UV light.
Lasing in liquid crystals has been demonstrated in numerous con gurations and material systems. In most reports the laser light is emitted perpendicular to the liquid crystal layer, using a chiral liquid crystal layer which exhibits a helical structure with a periodicity that gives rise to a stop band in the visible spectrum. The emission of light can then be modeled with one-dimensional models with reasonable accuracy. In the last few years also in-plane lasers have been demonstrated, for example by using a lying helix arrangement. The accurate optical modeling of the light generation in such systems is complex because the materials are optically anisotropic and the con guration should be modeled as two-dimensional. Advanced optical methods are necessary. For these simulations we rely on nite-element calculations of the optical modes in periodic two-dimensional structures including full position dependent anisotropy. The optical modes in a lying helix con guration are calculated as a proof-of-principle for this simulation method. Several interesting features of the optical modes in these structures are found.
A technological platform for a vertical cavity surface emitting laser (VCSEL) with tunable polarization is presented. It is realized by integrating an 850nm VCSEL chip in a liquid crystal (LC) cell that uses photo-alignment (PA) to orient the LC. Two kinds of LC are filled in and form a thin layer over the emitter of the VCSEL: nematic LC or chiral nematic LC (cLC). The VCSEL and the nematic LC layer can be electrically driven with separate electrodes. The polarization state of the laser emission can be controlled by applying an appropriate voltage over the nematic LC layer. The chiral nematic LC has a reflection band that contains the VCSEL emission wavelength, so that one circular polarized mode of the laser emission is reflected as a feedback into the VCSEL. We found that the emission from the VCSEL with cLC overlay is circularly polarized.
We study theoretically the spectral and polarization threshold characteristics of Vertical-Cavity Surface-Emitting Lasers with nematic and cholesteric liquid crystal overlay: LC-VCSELs. In the first case, we demonstrate the possibility of selecting between two orthogonal directions of linear polarization (LP) of the fundamental mode (x or y LP) by choosing appropriate NLC length and to achieve strong polarization discrimination: threshold gain difference as large as several times the threshold gain. We also demonstrate an active control of light polarization by electro-optically tuning the LC director and show that either polarization switching between x and y LP modes or continuous change of the LP direction is possible. For cholesteric LC-VCSEL we show that it becomes a coupled system with different spectral, threshold and polarization characteristics than the ones of the stand-alone VCSEL. Due to the existence of a band gap for circularly polarized light in the liquid crystal, lasing occurs in almost circularly polarized modes at the LC side.
We developed an in- house technology to overlay liquid crystal (LC) on top of a 850nm Vertical Cavity Surface Emitting
Laser (VCSEL) creating a so-called LC-VCSEL. Prior to this, the effect of the cell thickness on the planar alignment of
the E7 LC is investigated. It is observed that the LC orientation is planar, uniformly aligned over the whole cell with an
average pre-tilt of 22.50 in a thin a cell of 13μm thickness; such alignment uniformity is not observed in a thick cell of
125μm. Nevertheless, several domains of good uniformity are still present. Further, the polarization resolved LI
characteristics of LC-VCSEL are investigated with and without the insertion of LC in a cell glued directly onto VCSEL
package. Before filling in the LC, the VCSEL emits linearly polarized light and this linear polarization is lost after LC
filling. The output intensity as a function of polarizer angle shows partial planar alignment of the E7 LC, which is very
important for the further advancement of the LC-VCSEL integrated system.
Fast optical self-focusing has been observed in a homeotropic nematic liquid crystal cell. This nonlinearity is induced by
an intensity modulated infrared laser having a peak power of 160mW, a pulse repetition rate of 150Hz, and a duty cycle
of 0.05 and launched with extraordinary polarization. During these experiments the illumination time is kept at 0.3msec
and the ambient temperature is controlled. We have observed that self-focusing propagation depends on ambient
temperature, laser power and duty cycle. Notably, when illuminating with a continuous beam having the same
corresponding average power, only diffraction can be observed. These results suggest that the nonlinearity is produced
by a combination of thermal effects and molecular reorientation that leads to changes in the order parameter. Further
optical experiments and thermal calculations have been conducted to identify the responsible mechanism for the self-focusing
of the laser beam. It has been found that soliton formation can be achieved if the parameters as ambient
temperature, pulse repetition rate and duty cycle of the laser are set to optimal conditions. Although, this nonlinearity in
a liquid crystal cell has been already demonstrated for transverse illumination, the presence of beam propagation with
self-focusing has not been reported yet. The fast nonlinearity reported in this work has the potential to generate a number
of new applications of liquid crystals.
Nematic liquid crystals can switch the orientation of the director under influence of an electric field. Liquid crystals can
be combined with waveguides in many different ways: the liquid crystal can be in the core, in the cladding or in both. In
the recent past liquid crystals have been combined with glass fibers and with silicon-on-insulator waveguides. Important
progress has been achieved in the modeling of liquid crystals near inhomogeneous boundaries and the modeling of
optical waveguides with anisotropic materials. This paper discusses these recent advancements and illustrates how
waveguides with voltage tuned cutoff may be designed.
Liquid crystals are nowadays widely used in all types of display applications. However their unique electro-optic properties also make them a suitable material for nondisplay applications. We will focus on the use of liquid crystals in different photonic components: optical filters and switches, beam-steering devices, spatial light modulators, integrated devices based on optical waveguiding, lasers, and optical nonlinear components. Both the basic operating principles as well as the recent state-of-the art are discussed.
A finite element modesolver and beam propagation (BPM) algorithm are applied to the optical analysis of liquid crystal waveguides. Both approaches are used in combination with advanced liquid crystal calculations and include a full dielectric tensor in solving the Helmholtz equation to model the liquid crystal behavior properly. Simulation of the beam propagation in a waveguide with tunable liquid crystal cladding layer illustrates the coupling of a Gaussian beam to the fundamental waveguide mode obtained with the modesolver. Excellent quantitative agreement between both approaches illustrates the potential of these methods for the design of advanced devices. The high accuracy of the BPM algorithm for wide angle propagation, essential in the analysis of high index contrast waveguides, is illustrated for angles up to 40 deg.
A finite element framework is presented to combine advanced three-dimensional liquid crystal director calculations
with a full-vector beam propagation analysis. This approach becomes especially valuable to analyze and design
structures in which disclinations or diffraction effects play an important role. The wide applicability of the
approach is illustrated in our overview from several examples including small pixel LCOS microdisplays with
In this work we present numerical results on the propagation of high power laser beams in nematic liquid crystals. The
optical nonlinearity of the liquid crystal gives rise to
self-focusing and the generation of spatial optical solitons. We
consider only configurations in which no bias voltage is necessary for the generation of the spatial optical soliton. One of
the configurations considered here is one where the liquid crystal twists along the thickness of the layer over an angle of
180°. This configuration leads to spiraling solitons when the beam is injected with a certain offset with respect to the
middle of the liquid crystal layer. The sign of the initial angle of the beam is depending on the sign of the offset.
We demonstrate tuning of the resonance wavelength of
silicon-on-insulator optical ring resonators. The devices
are clad with a layer of nematic liquid crystal. The electrooptic effect of the anisotropic liquid crystal allows us to
change the effective index of the TE waveguide mode with an externally applied voltage. The electric field will
reorient the liquid crystal director which alters the refractive index of the cladding layer. The evanescent tails of
the waveguide mode feel this change. The change in effective index has a direct effect on the resonance
wavelength. In our setup, the director tilts from an orientation parallel to the waveguides to an orientation
perpendicular to the substrate. This way, it is the longitudinal component of the electric field of the light that
experiences the largest change in refractive index. Starting from this principle, we show experimental tuning of
the resonance wavelength over 0.6nm towards shorter wavelengths. Theoretical considerations and simulations
with a finite element modesolver capable of handling full anisotropy confirm the experimental results and provide
insights in the tuning mechanism.
Liquid crystals can switch under influence of an electric field or under influence of incident light. In this paper we
provide a mathematical description including electrical, optical and elastic torques. Depending on the applied voltage
and the incident light, bistability in the director orientation may be possible. Under certain conditions, the sequence of
applying incident TM polarized light and a static voltage allows to access different states.
Waveguiding in liquid crystals can be achieved by controlling the molecular orientation by means of external
fields or by shaping the geometry of the substrates that contain the liquid crystal material. The creation of these
waveguides in liquid crystals can also be achieved by using the optical nonlinear properties of the material. For a
sufficient optical power (in the order of a few mW), the beam can induce its own optical waveguide. This is a selfinduced
waveguide and the resulting beam is referred to as a soliton beam. In the last few years, the properties
of these soliton beams have been studied thoroughly, revealing some interesting phenomena. In this article,
simulations are reported on two common configurations in which solitons have been generated experimentally.
The soliton beam, for certain configurations, displays an undulative behavior inside the cell, which may be used
for large angle steering of the optical beam.
Optical waveguides are widely used in the telecom industry for long distance data transport. Glass fibers are designed to
have minimal losses. Different functionalities have been integrated in waveguides, such as wavelength filtering,
amplitude modulation and routing. Liquid crystals are promising, because their optical properties can be modified by
applying a small voltage or by illumination with light. The variation in optical properties can be exploited in different
kinds of waveguide systems.
It is possible to generate a wave guide in bulk liquid crystal by modulating the director orientation in an appropriate
pattern. Some guided modes in such pure liquid crystals are discussed. Because the liquid crystals are anisotropic, the
modes have some unusual properties. The influence of light can lead to director reorientation and modify the
waveguiding properties. This optical non-linearity determines largely the light propagation. In hybrid waveguides, liquid
crystal are used in combination with a material with higher (in the core) or lower (in the cladding) refractive index.
Silicon on insulator waveguides are convenient components to study the tuning possibilities in combination with liquid
Spatial solitons in liquid crystals can be observed with milliwatts of light power due to the nonlocal saturable nonlinear effect of field-induced director reorientation. In novel generations of all-optical switching circuits and optical networks spatial solitons show some promising possibilities. Director orientation in an LC-layer is simulated for a dc-voltage over the layer and an optical field injected lateral to the layer. Due to a torque induced by the electric field the molecules will tilt and the index of refraction in the layer will rise. The simulation of the soliton behavior is based on the Euler-Lagrange variational formula for the distortion free energy of the liquid crystal. The raise in the refractive index causes a self-focusing effect. When the self-focusing balances the diffraction, a spatial soliton can occur. A BPM-algorithm is used to simulate the light propagation in the layer. Without a dc-field, a large optical field for soliton propagation is required to reach the threshold to initiate the molecular reorientation of the liquid crystal molecules. A dc-field can be used to overcome the Fréederickz-transition so that a lower optical field is required. The relations between optical field profiles, dc-voltage and layer thickness which enable soliton propagation are discussed.