Phase Modulation Device Research Articles

Design terahertz polarizers and vector polarized vortex terahertz wave generators based on the effective dielectric constant of metal gratings

Polarization and phase devices for terahertz waves have important applications in terahertz detection, imaging, communication, etc. Spatially variable metal gratings can be used for broad-spectrum, miniaturized, and low-cost terahertz polarization and phase modulation devices. Based on the effective dielectric constant and the theory of light propagation in multilayer media, we obtain the relationship between the transmittance and extinction ratio and the parameters such as the duty cycle of the metal grating, the frequency of the incident terahertz wave, the angle of incidence, the thickness of the metal grating, the refractive index of the substrate, and the thickness of the substrate. We propose a method of designing a spatially variable metal grating located on a transparent substrate. The designed spatially variable metal grating is also used to modulate the terahertz spatial polarization and phase to generate terahertz optical fields whose polarization and phase change simultaneously in space, such as azimuthally vector vortex terahertz optical fields, radially vector vortex terahertz optical fields, and so on. This will have important applications in terahertz time-domain spectroscopic detection, terahertz time-domain spectroscopic imaging, terahertz time-domain near-field microscopic imaging, terahertz communication, and so on.

Nanometer resolution of 2D single-shot low-coherence & comb-based profilometry based on the effect of discrete height of spatial phase modulator surface

A 2D single-shot comb-based interferometer is well known as a powerful tool for inline inspection owing to its high speed, extensive measurement range, and anti-vibration. However, nanometer resolution is still a milestone that has not been achieved in comb-based interferometers owing to the precision limitation caused by the discrete profile of the spatial phase modulator (SPM) device, typically in the range of several micrometers. Therefore, this study confirms 2D single-shot comb-based profilometry at sub-micron and nanometer-scale resolutions. The system first investigates the effect of the discrete limitation of the SPM surface profile and then verifies the submicron detection ability of the optical frequency comb-based setup. It was quantitatively shown that the highest resolution can be obtained by properly setting the step size of the spatial phase modulator and the SPM surface resolution of the imaging optics. The highest profilometry resolution was recorded at 26.4 nm in a short measurement range, while the setup using the optical comb source with millimeters measurement range for industrial application also can achieve a resolution of approximately 259.2 nm. The results demonstrate the system feasibility in overcoming the discrete limitations of the SPM profile. It also demonstrates for the first time that the so-called 2D single-shot comb-based interferometer can achieve nanometer-scale profilometry resolution covering the measurement range from tens of millimeters to sub-micrometer detection.

Robust polarization state generation for long-range quantum key distribution.

We present a new compact and robust polarization state transmitter designed to execute the BB84 quantum key distribution protocol. Our transmitter prepares polarization states using a single commercial-off-the-shelf phase modulator. Our scheme does not require global biasing to compensate thermal and mechanical drifts, as both of the system's two time-demultiplexed polarization modes share a single optical path. Furthermore, the transmitter's optical path entails a double-pass through the phase modulation device for each polarization mode, allowing multiple phase rotations to be impinged on each light pulse. We present a proof-of-concept prototype of this transmitter topology and demonstrate a mean intrinsic quantum bit error rate below 0.2% over a 5 hour measurement.

P‐12.12: Development and Application of High Birefringence Liquid Crystal Materials

The development of new liquid crystal optoelectronic devices, including laser phase modulation, microwave and terahertz devices, requires liquid crystal materials with high birefringence. In this paper, three new molecular design strategies are proposed, including the introduction of olefin chain, lateral fluorine substitution and new ring core. Four new series of isothiocyanate liquid crystal compounds with high birefringence, low melting point and wide nematic temperature range were synthesized. Based on these new liquid crystal compounds, a liquid crystal mixture with nematic temperature range of ‐40~146 ºC and birefringence of 0.42 was developed. At 19 GHz, the new liquid crystal mixture shows excellent dielectric properties, with dielectric anisotropy of 1.28 and dielectric loss of 0.0087. The new liquid crystal mixture also has very low viscoelastic coefficient. The results at 1550nm and 60 ºC show that the response speed can reach 3.6 milliseconds with 2π phase modulation in the reflection mode.

Non-mechanical self-alignment system for free-space optical communications based on a cascaded liquid crystal optical antenna.

Free-space optical (FSO) communication has attracted extensive attention in recent years. To maintain a reliable FSO link, two main issues need to be addressed: beam drift and vibration. In this paper, we demonstrate a non-mechanical self-alignment system based on a cascaded liquid crystal optical antenna, in which a frequency decoupled hybrid integration Kalman filter (FDHI-KF) method is proposed to achieve predictive beam drift tracking and vibration mitigation. By leveraging the integrated control on our lab-made liquid crystal phase modulation devices, and implementing the adaptive algorithm on a heterogeneous field programmable gate array (FPGA), this system is capable of realizing precise self-alignment without any moving parts. Experiments are conducted to verify its performance in practical applications. We envision it to set a benchmark for future liquid crystal non-mechanical beam-steering systems in FSO communications.

A one-step pattern transferring process for freestanding diffractive spiral photon sieves in extreme ultraviolet

Spiral zone plates in the form of photon sieves are one of the important phase modulation devices, being well developed in both visible lights and X-rays for mode conversion from Gaussian beam to vortex one. However, in extreme ultraviolet (EUV) region as a diffractive focusing lens, spiral photon sieves in freestanding SiNx membrane with through-holes to avoid significant absorptions of EUV light by the membrane material, still remain a big challenge in manufacturing. In this work, a simplified nanofabrication process using electron beam lithography directly followed by reactive ion etch, was developed. The fabricated spiral photon sieves involve two layers of SiNx/ZEP520A with the focusing efficiency estimated to be 23%. Optical characterization of the converted vortex beam by using a typical imprint technique was carried out using a 46.9 nm EUV laser. Donut-shaped ablation pattern by EUV laser was observed on the exposed PMMA (polymethyl methacrylate), indirectly proving the phase modulation functionality of the fabricated freestanding spiral photon sieves. The developed fabrication process greatly simplifies the manufacturing of freestanding zone plates/photon sieves as well as other freestanding nanometer structures, and should be applicable in the production in large volume.

Spatial self-phase modulation of a Gaussian beam transmitted through a ferrofluid

We report on the experimental observation of the diffraction pattern formed in the far-field region when a high-power continuous-wave laser convergent or divergent Gaussian beam passes through a cuvette with ferrofluid. Two different types of diffraction rings with opposite light-intensity distribution are shown in the far field. The difference between the diffractive patterns is attributed to the interaction of the strong spatial self-phase modulation caused by the refractive index change of the medium with wavefront curvature of the input Gaussian beam. The observed behavior of the diffraction pattern dynamics is interpreted theoretically based on the Fresnel-Kirchhoff integral. The negative polarity of nonlinear refraction can be identified by the central interference profiles and the diffraction pattern. At the same time, the self-defocusing phenomena of the ferrofluid can be determined by the type of pattern. The nonlinear refraction coefficients of the ferrofluid were estimated to be ∼-2.89×10-5cm2/W (convergent Gaussian beam) and ∼-3.53×10-5cm2/W (divergent Gaussian beam). In addition, the corresponding third-order nonlinear optical susceptibility of the sample was also estimated as ∼1.43×10-5esu and ∼1.75×10-5esu, respectively. The experimental results imply a novel potential application of ferrofluid in nonlinear phase modulation devices.

High Precision Self-Mixing Interferometer Based on Reflective Phase Modulation Method

In this paper, a novel self-mixing interferometer based on reflective phase modulation (RPM) method has been developed to perform micro-displacement reconstruction with nanometer accuracy. Broaden harmonic components spectrum of the self-mixing signal is produced by employing a high-frequency vibrating reflective mirror as the phase modulation device. Phase demodulation is implemented applying the orthogonal demodulation algorithm subject to the signal spectrum, in which orthogonal signal can be extracted from the harmonic components of the expanded Bessel function. The principle and signal processing approach are introduced in detail, and the simulation results indicate that the reconstruction error can be reduced as the number of reflections increases. A series of experiments at different vibration amplitudes show that the reconstructed errors are all less than 10 nm with modulation frequency of 1 kHz. And the minimum error of 3 nm has been achieved at the measured amplitude of 229 nm, which demonstrates the technical-superiority and high-performance of the method.

High precision beam steering using a liquid crystal spatial light modulator

Liquid crystal spatial light modulators (LCSLMs) are novel phase modulation devices. Due to the dielectric properties of the orientation layer, the orientation layer has a voltage division effect on the driving voltage. A more effective analysis of the relationship between the driving voltage and phase modulation properties of LCSLMs is needed, as well as improved beam steering accuracy. In this paper, we establish an electrical model of an LCSLM, and propose a method for quantitatively calculating the liquid crystal (LC) layer voltage. This differential iterative method based on nonlinear least squares is used to analyze the LC molecular directors. Considering the thickness of the orientation layer, we calculate the LC director distribution and the voltage division effect of the LC layer, obtaining a more accurate electro-optic characteristic curve of the LCSLM, and verify the calculations by experiment. Experimentally, the average beam pointing error of the LCSLM was 0.0098° at 532 nm, representing a significant improvement of the beam steering accuracy.

Generating ultrashort flat-top optical pulses with a fiber-loop time-lens system

For generating short flat-top optical pulses, traditional methods are either bandwidth limited or have low controllability (in pulse width and shape). We propose a fiber-loop based time-lens system to realize ps-level flat-top pulses, and theoretically study their features under various parameter circumstances. This time-lens system, functioning as many stacked time-lenses, introduces great controllability for the generated pulse’s characteristics through tuning both the phase modulation device and the dispersion-compensating setup. To achieve high-quality narrow flat-top pulses (with steep pulse edges), several factors including phase modulation function, modulation strength and dispersion compensating status must be optimized simultaneously. In simulation, after 50 loops of parabolic phase modulation and chirp compensation, the flat-top pulse’s duration can be narrowed to 2.8 ps with edge widths of 1.7 ps. This kind of ps-scale flat-top pulse is very desirable for application in various areas.

Ultra-broadband tunable continuous phase masks using optical aberrations.

We show that through the use of optical aberrations, a reflective cylindrical beam expander can be properly adjusted to perform as a tunable ultra-broadband continuous phase modulation device. We demonstrate the effectiveness of such a device for the case of a net cubic phase modulation that is used to generate ultra-broadband white light 2D Airy beams. In this case, the nature of our device enables all spectral components to copropagate, following the same accelerating trajectory. The scalability of our approach, both in respect of input power and bandwidth, makes it possible to be used in any part of the electromagnetic spectrum.

Nitrogen Doping Effect for Improving Operation Reliability of Phase Modulator Using Ge₂Sb₂Te5 Thin Film for Hologram Image Implementation.

A phase modulation device was proposed for the implementation of hologram image for display applications. A Ge2Sb2Te5 (GST) film as thin as 7 nm was prepared between the ITO films to form the cavities corresponding a unit pixel. Nitrogen was incorporated into the GST for improving the thermal stability of the GST active region. The effects of the nitrogen doping on the physical properties of GST was investigated with the variations in doping amounts. The nitrogen incorporation was found to reduce the surface micro-roughness and to improve the thermal stability of the GST even after the crystallization by effectively suppressing the excessive grain growth. As results, the number of repeatable operations for the fabricated phase modulation device was evidently improved from 10 to 69 cycles when a 2.7-at% nitrogen was doped into the GST.

Molecular tilting effect on Smectic liquid crystal sub-phase stability from its retardation switching behavior

Molecular tilting effect to the Smectic layer normal at Smectic C phase liquid crystal is investigated in terms of its retardation switching effect. This investigation clarified that in some specific initial molecular alignment case, Smectic C phase liquid crystal molecular behavior has in-plane only movement, resulting in almost complete suppression of out-of-plane retardation switching. Such behavior indicates that Smectic layer stability is also conceivable with initial surface anchoring influence in terms of artificial influence of surface anchoring to govern tilted Smectic liquid crystal molecular movement behavior. Possible control in in-plane only retardation switching would provide new phase modulation devices.

Refractive index and temperature sensing in anisotropic silver nanostructures with stable photo-physical properties

In this report, we have demonstrated the refractive index and temperature-sensing abilities of polyvinylpyrrolidone (PVP)-protected silver nanostructures of triangular, connected and plate-like shapes. Interestingly, these nanostructures even after 2 and ½ years of syntheses showed plasmonic-sensing ability of temperature in the temperature range of 283–333 K. Also, refractive index (R.I.) sensing has been demonstrated in the aged samples and obtained the highest R.I. sensitivity of 306 nm/RIU in one of the sample. The synthesized samples have been kept in dark (inside desiccators) intentionally for the extended period of 2 and ½ years after synthesis and monitored intermittently their UV–Vis absorption and photoluminescence (PL) emission characteristics to check the functionally of the aged silver nanostructures. It has been found the samples remain well dispersed in different solvents and can forbid agglomeration even in 0.25 M NaCl solution. We have also demonstrated here fabrication of a flexible and transparent thin film of the synthesized samples in polyvinyl alcohol (PVA) matrix and investigated its low power continuous-wave (CW) nonlinear optical properties using spatial self-phase modulation (SSPM) technique. The nonlinear refractive index (n 2) value of the film has been determined to be 5.6 × 10− 6 cm2/W at the He–Ne laser wavelength of 632.8 nm. In this report we have demonstrated temperature and R.I. sensing and also it has been demonstrated that the synthesized samples remain functional even after 2 and ½ years of synthesis. Also, samples may find potential applications in nonlinear optical phase modulation devices.

Acoustic phase hologram with labyrinthine metamaterials

Acoustic metamaterials offer large degree of design freedom and precise control over amplitude and phase at subwavelength scales. In the past, we have demonstrated a family of labyrinthine metamaterial unit cells as premium building blocks for phase modulation devices, as well as several 1D wavefront shaping devices based on these unit cells. Here we extend the complex modulations to 2D by demonstrating a computer generated metamaterial-based phase hologram. Through spatially modulating the phase of the wavefront, the hologram projects the incident wave to a designed three-dimensional amplitude pattern. The hologram is designed with a two-step process: first, an iterative holographic reconstruction algorithm is used to obtain the optimal phase pattern of the hologram; second, unit cells with desired phase modulations are designed and fabricated. Ray tracing and full-wave simulations have been performed to verify the design. The measurements of reconstruction in an anechoic chamber will be taken for a hologram designed to operate around 4 kHz. The metamaterial-based hologram creates a three-dimensional acoustic illusion with only passive structures. The designing process can also be extended to devices such as multi-focal lenses and wave-based analog processing/computing interfaces.

Tunable nonlinear refractive index of two-dimensional MoS_2, WS_2, and MoSe_2 nanosheet dispersions [Invited

Liquid-phase-exfoliation technology was utilized to prepare layered MoS2, WS2, and MoSe2 nanosheets in cyclohexylpyrrolidone. The nonlinear optical response of these nanosheets in dispersions was investigated by observing spatial self-phase modulation (SSPM) using a 488 nm continuous wave laser beam. The diffraction ring patterns of SSPM were found to be distorted along the vertical direction right after the laser traversing the nanosheet dispersions. The nonlinear refractive index of the three transition metal dichalcogenides dispersions n2 was measured to be ∼10−7 cm2 W−1, and the third-order nonlinear susceptibility χ(3)∼10−9 esu. The relative change of effective nonlinear refractive index Δn2e/n2e of the MoS2, WS2, and MoSe2 dispersions can be modulated 0.012–0.240, 0.029–0.154, and 0.091–0.304, respectively, by changing the incident intensities. Our experimental results imply novel potential application of two-dimensional transition metal dichalcogenides in nonlinear phase modulation devices.

Prospects for versatile phase manipulation in the TEM: Beyond aberration correction

In this paper we explore the desirability of a transmission electron microscope in which the phase of the electron wave can be freely controlled. We discuss different existing methods to manipulate the phase of the electron wave and their limitations. We show how with the help of current techniques the electron wave can already be crafted into specific classes of waves each having their own peculiar properties. Assuming a versatile phase modulation device is feasible, we explore possible benefits and methods that could come into existence borrowing from light optics where the so-called spatial light modulators provide programmable phase plates for quite some time now. We demonstrate that a fully controllable phase plate building on Harald Rose׳s legacy in aberration correction and electron optics in general would open an exciting field of research and applications.

An Improvement of the Characteristic of the Electronic Holography Using a Space Phase Modulation Device

An Improvement of the Characteristic of the Electronic Holography Using a Space Phase Modulation Device

High-Speed InAs Quantum-Dot Electrooptic Phase Modulators

In this letter, we report results of direct current (dc) and radio-frequency (RF) characteristics of self-assembled InAs quantum-dot (QD) electrooptic modulators. Although dc modulation results have been reported in the literature, these are the first high-frequency designs and measurements. These QD modulator wafers were grown by molecular beam epitaxy (MBE) and phase modulator devices were fabricated with coplanar traveling-wave electrodes. For the 800-μm-long modulators, the half-wave voltage V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">π</sub> was measured as 11.4 V at 1.55 μm and 7.34 V at 1.32 μm. The phase variation is 21.9°/(mm·V). The linear electrooptic coefficient T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">41</sub> is calculated to be 35.4 pm/V for QDs. These modulators have a 3-dB bandwidth of 10 GHz. A signal-to-noise ratio of 9 dB was measured at 20 GHz.

Dual frequency operation of a blue phase liquid crystal

The authors demonstrate dual frequency switching in a cubic blue phase liquid crystal. The dual frequency blue phase was prepared by adding a strongly twisting chiral dopant to a dual-frequency nematic liquid crystal mixture. The crossover frequency appeared at few hundred kHz in the blue phase, and sub-millisecond response was obtained for dual-frequency operation, which was comparable to conventional voltage operation. This new class of material is advantageous over conventional blue phases in polarization-independent phase modulation devices, since a larger lattice deformation can be obtained.