Optical Beam Deflector Research Articles

Nanoelectromechanical Tuning of High-Q Slot Metasurfaces.

Nanoelectromechanical devices have been used widely in many applications across photonics, electronics, and acoustics. Their incorporation into metasurface systems could be beneficial in designing new types of active photonic devices. Here, we propose a design of active metasurfaces using a nanoelectromechanical system (NEMS) composed of silicon bars which operates under CMOS-level voltage and achieves phase modulation with wavelength-scale pixel pitch. By introducing a perturbation to the slot mode propagating between the silicon bars, the device operates in a high-Q regime, making the optical mode highly sensitive to mechanical movement. An over 12 dB reflection modulation is observed by full-wave simulation, and over 10% is achieved in the proof-of-concept experiment under CMOS-level voltage. We also simulate a device with 1.8π phase response using a bottom gold mirror. Based on this device, a 3-pixel optical beam deflector is shown to have 75% diffraction efficiency.

Intuitive understanding of the connection between Pancharatnam-Berry optical beam deflectors and polarization volume holograms.

An approximate beam propagation method is proposed as an intuitive simulation of the optics of Pancharatnam-Berry phase and polarization volume hologram devices. Using this method, the connection between and polarization properties of these two types of devices are made clear.

Lithium niobate optical beam deflectors operating at both electro-optic and thermo-optic modes

A novel optical beam deflector based on a Lithium niobate (LN) waveguide is demonstrated, which can operate at both electro-optic and thermo-optic modes. At the electro-optic mode, the microstructured serrated electrodes induce inhomogeneous change in the refraction index of LN waveguide via the Pockels effect when a voltage is applied. The incident beam is thus deflected with the deflected distance changing linearly with the applied voltage. A high deflection speed up to 2.5 GHz is observed experimentally. The optical beam deflector can operate at the thermo-optic mode, where the deflected distance depends on the applied electric power; thus it varies quadratically with the applied voltage. The deflection capacity at the thermo-optic mode is much higher than electro-optic mode, since the deflection distances are respectively 27.19 and 2.53 μm for the thermo-optic and electro-optic modes at a 4 V applied voltage. Therefore, the distinct electro-optic and thermo-optic operating modes enable the LN optical beam deflector to be qualified for various tasks, such as optical scanning, optical modulator.

Reduction of beam-divergence fluctuation for KTN optical beam deflector

We report a new technique to suppress beam-divergence fluctuations in a DC-biased KTa1-xNbxO3 (KTN) optical beam deflector. In our new configuration, we introduced a temperature gradient in the direction of the electric field applied to the KTN crystal, to produce a permittivity gradient in the direction required to cancel out the effect of the asymmetry of the electron injection. We demonstrated experimentally that this temperature gradient successfully reduced the beam-divergence fluctuation.

High-Speed Time-Domain En Face Optical Coherence Tomography System Using KTN Optical Beam Deflector

We developed high-speed time-domain (TD) en face optical coherence tomography (OCT) system using KTN optical beam deflector. The KTN optical beam deflector operates at a high repetition rate of 200 kHz with a fairly large beam deflection angle. We proposed a high-speed en face OCT system that used a KTN optical deflector as the sample beam scanning. In the experiment, we obtained en face OCT images of human fingerprint with a frame rate of 800 fps, which is the fastest speed obtained by a TD-OCT imaging. Furthermore, a 3D-OCT image was also obtained at 0.2 s (=5 volumes/s) by our imaging system.

Subwavelength population density gratings in resonant medium created by few-cycle pulses

We consider theoretically recently proposed a new possibility of creation, erasing and ultrafast control of population density grating. Such grating can be created in resonant medium when ultrashort pulses with duration smaller than relaxation times in the resonant medium (coherent light matter interactions) propagate without overlapping in this medium. Possible applications in the ultrafast optics such as optical switcher and laser beam deflector are discussed.

Analyses of optical rays in KTN optical beam deflectors for device design.

The potassium tantalate niobate (KTN) optical beam deflector is an electro-optic deflector without any moving parts that works at frequencies higher than 200kHz. In this paper, we discuss the performance parameters of this deflector. Optical beams are bent by the spatial distribution of the refractive index in the KTN crystal block used in this deflector. In addition to the deflection function, the index distribution operates as a cylindrical convex lens. Therefore, the deflector is often used with glass cylindrical lenses to cancel out the lens function. We analyzed optical rays curving in the block based on the graded index lens theory. We describe the way in which performance parameters, such as the deflection angle, change, depending on both the choice of the compensating lenses and the parameters of the KTN block, namely its size and charge density. We concentrate especially on methods designed to improve the resolvable spot number, which is the most important figure of merit for optical deflectors. One way to achieve improvement is to input a collimated beam or a slightly converging beam into the KTN block.

Fast switching cholesteric liquid crystal optical beam deflector with polarization independence

Optical beam deflectors based on the combination of cholesteric liquid crystals and polymer micro gratings are reported. Dual frequency cholesteric liquid crystal (DFCh-LC) is adopted to accelerate the switching from the homeotropic state back to the planar state. Polarization independent beam steering components are realized whose transmission versus the polarizing angle only varies 4.4% and 2.6% for the planar state and the homeotropic state, respectively. A response time of 451 ms is achieved for DFCh-LC-grating beam deflectors, which is fast compared to other nematic LC beam steerers with similar LC thickness.

High-efficiency large-angle Pancharatnam phase deflector based on dual-twist design

We have previously shown through simulation that an optical beam deflector based on the Pancharatnam (geometric) phase can provide high efficiency with up to 80° deflection using a dual-twist structure for polarization-state control [Appl. Opt.54, 10035 (2015)]. In this report, we demonstrate that its optical performance is as predicted and far beyond what could be expected for a conventional diffractive optical device. We provide details about construction and characterization of a ± 40° beam-steering device with 90% diffraction efficiency based on our dual-twist design at a 633nm wavelength.

Development of High-Speed Time-Domain En face OCT Using a KTN Optical Beam Deflector

We developed a high-speed Time-Domain en face OCT with a KTa1‒xNbxO3 (KTN) optical beam deflector. A KTN optical beam deflector illuminates large area with fast scanning. In our imaging system, fast scanning was performed at 200 kHz by the KTN optical beam deflector, while slow scanning at 800 Hz was simultaneously performed by the galvanometer. In a preliminary experiment, we obtained en face OCT images of a human fingerprint at 800 fps, which is the fastest speed obtained by a TD-OCT. Furthermore, a 3D-OCT image was also obtained at 0.2 s by our imaging system.

Generic software tool for counteracting the dynamics effects of optical beam delivery systems

Laser micro-machining is a promising manufacturing solution for fabricating complex micro-engineering products in wide range of materials that incorporate different multi-scale functional features. Optical beam deflector systems are key components in laser micro-machining systems, and they are one of the main factors determining the processing speed and hence machining throughput. However, their performance is speed dependent and the negative dynamics effects have a direct impact on the laser micro-machining accuracy, repeatability and reproducibility. This article presents a generic software solution for minimising these negative dynamics effects, thus improving significantly the laser machining performance across the full range of available processing speeds. In particular, these improvements are achieved by introducing machine-specific compensations in machining vectors to counteract beam deflectors’ inertia regardless of their directions, length and set process speed. An empirical model was developed to obtain data about the actual dynamic response of the beam deflection system across the full range of available processing speeds, and then based on these data, the proposed generic software was implemented into a stand-alone ‘adaptive’ postprocessor. The generation of machine executable part programs is automated, and it is only necessary for the user to enter the selected scanning speeds and beam diameters. Experimental validation was conducted to demonstrate the capability of the proposed software tool. The results demonstrate that substantial improvements can be obtained in machining quality by maintaining a constant pulse distance throughout the machining operations, while the dimensional accuracy is maintained across the available processing speeds without sacrificing the machining efficiency.

Piezoelectric laser beam deflector for space applications

Optical beam deflectors are used to control direction of the optical beam therefore accuracy of the optical system strongly depends on dynamic characteristic of the deflector. Three degrees of freedom (3DOF) piezoelectric micro-actuator for optical beam deflector is investigated in the paper. Piezoelectric micro-actuator consists of piezoelectric cylinder and ferromagnetic hemisphere that contacts the cylinder in the three points. Electrodes of the outer surface of the cylinder are divided into three sections. Three harmonic electric signals are used for piezoelectric cylinder excitation. Numerical and experimental study was performed to analyse dynamic characteristics of the piezoelectric actuator. Results of the studies are compared and discussed.

Anomalous index modulations in electrooptic KTa(1-x)Nb(x)O(3) single crystals in relation to electrostrictive effect.

KTa(1-x)Nb(x)O(3) is known for its huge Kerr effect, which is a second order electrooptic (EO) effect. By utilizing the large refractive index change Δn of this EO effect, a fast optical beam deflector has been realized. However, anomalous spatial distributions of Δn were observed with this beam deflector. This anomaly is ascribed to distortions caused by the electrostrictive effect that occurs when voltage is applied. We assumed a spheric distortion and used a variational method to deduce an analytic solution for the strains that accompany this distortion. The analytic solution coincides with numerical results obtained with the finite element method. In addition, the solution agrees well with the experimentally obtained Δn distribution.

Development of novel high-speed en face optical coherence tomography system using KTN optical beam deflector

We developed a novel high-speed en face optical coherence tomography (OCT) system using a KTa1–xNbxO3 (KTN) optical beam deflector. Using the imaging system, fast scanning was performed at 200 kHz by the KTN beam deflector, while slow scanning was performed at 400 Hz by the galvanometer mirror. In a preliminary experiment, we obtained en face OCT images of a human fingerprint at 400 fps. This is the highest speed reported in time-domain en face OCT imaging and is comparable to the speed of swept-source OCT. A 3D-OCT image of a sweat gland was also obtained by our imaging system.

A proposed method of efficient laser trapping of neutral radium atoms

We report theoretical studies of an efficient accumulation of 225Ra atoms in a magneto-optical trap (MOT) from an atomic beam. The number of atoms in the trap is limited due to the large leakage into metastable D states from the cooling transition and the loss mechanism from collisions with the background gases and with other elements in the atomic beam. We propose a setup consisting of an efficient Zeeman slower, an optical atomic beam deflector, and an Egg-MOT, which provides a large capture velocity, e.g. the Zeeman slower capture velocity of 581 ms−1 and the MOT capture velocity of 46 ms−1. Furthermore, the pure slow atomic beam produced by the optical atomic beam deflector can maximize the number of trapped atoms and their lifetime in a ultra high vacuum chamber by minimizing the collisional loss. Our proposed setup allows for the search for a permanent electric dipole moment based on laser-cooled and trapped radium atoms with improved sensitivity.

Permittivity changes induced by injected electrons and field-induced phase transition in KTa1−xNbxO3 optical beam deflectors

A space-charge-controlled optical beam deflector made of a KTa1−xNbxO3 (KTN) single crystal utilizes electrons that are injected through the cathode by applying voltage. With the deflector made of lithium-doped KTN (K0.95Li0.05Ta0.73Nb0.27O3, KLTN/0.05/0.27), we observed large increases in the capacitance of the deflector when we injected electrons. The increases were not caused by changes in the electrode interface but by changes in the permittivity of the bulk crystal. In the paraelectric phase, the KLTN/0.05/0.27 crystal exhibited nonlinearity in the dielectric response with double hysteresis loops in the D–E curves. We ascribed the permittivity change to this nonlinear phenomenon. We also discuss this nonlinearity in terms of the Landau–Devonshire phenomenological theory. The coefficient g4 of the fourth power term in the expanded free energy was negative in the paraelectric phase near the phase transition temperature as it is for other materials that exhibit a first-order phase transition. However, g4 depended on the temperature and its sign became positive about 15 °C above the phase transition temperature.

Changes in permittivity of KTa1−xNbxO3 crystals induced by electron injection in relation to nonlinear dielectric response

We observed large increases in the capacitance of a KTa1−xNbxO3 (KTN) optical beam deflector that were induced by electron injection. The increases were not caused by changes in the electrode interface but rather by changes in the permittivity of the bulk crystal. In the paraelectric phase, the KTN crystal exhibited a nonlinear dielectric response with double hysteresis loops in the D–E curves. We ascribed the permittivity change to this nonlinear phenomenon.

Charge distributions in KTa₁₋xNbxO₃ optical beam deflectors formed by voltage application.

Controlling the space charge distributions in a crystal is indispensable for controlling a KTa₁₋xNbxO₃(KTN) optical beam deflector. The space charge is built up by applying a voltage and injecting electrons into the KTN crystal. Although a homogeneous distribution is preferable, we observed experimentally that the injected electrons concentrated in the vicinity of the cathode and for some samples the concentration was much lower around the anode. We investigated the electron dynamics theoretically and found that such inhomogeneity was caused by a freezing effect where the motion was very slow considering the duration of the practical voltage application. The depth of the space charge spread or the electron penetration depth from the cathode was proportional to the applied voltage and the permittivity, and inversely proportional to the density of traps or localized states that bind electrons. We believe that the trap density was too large for the samples with inhomogeneous charge distributions.

Measurement of charge density distributions in KTa_1-xNb_xO_3 optical beam deflectors

In a KTa1-xNbxO3 optical beam deflector, light rays are bent by a space charge formed by electrons that are injected through an electrode and captured by crystal defects. For complete device control, it is important to evaluate the space charge. We propose an optical method for measuring charge density distributions that utilizes the electrooptic (EO) effect of the material. With this method, electron accumulation caused by a screening effect was commonly observed near the cathode. The electron accumulation region extended toward the anode as the applied voltage and permittivity increased. This method can be applied to any EO materials that exhibit space charge distributions.

Evanescent-wave-coupled optical light beam deflector with symmetrical structure

We purpose a waveguide optical deflector using a Y-branch waveguide coupled to a slab waveguide via gaps between them. Narrow stripe electrodes placed on the Y-branch waveguide enables a fast deflection. A symmetrical light beam with low side lobes is generated by a simple structure.