Nanoimprinting Solution-Derived Barium Titanate for Electro-Optic Metasurfaces.

Electro-optic metasurfaces have demonstrated significant potential in enhancing the modulation speed and efficiency for fast and large-scale free-space optical devices. Barium titanate has a strong electro-optic Pockels coefficient, but its availability in thin-film form is restricted due to costly growth processes or low thickness. Here, we fabricated active metasurfaces using an etch-free bottom-up process with sol-gel-based polycrystalline barium titanate with a large electro-optic coefficient similar to bulk lithium niobate. We achieve strong hybrid Mie/surface lattice resonances with a quality-factor of 200 at 633 nm wavelength, enhancing the light-matter interaction and therefore the Pockels effect. The metasurface transmission is electro-optically modulated with up to 5 MHz driving frequency at low voltages of less than 1 V thanks to resonant enhancement of the modulation amplitude by 2 orders of magnitude. This successful demonstration of electro-optic modulation in nanoimprinted barium titanate structures paves the way for low-cost and large-scale free-space modulators or tunable metalenses.

Nonvolatile Electro-optic Response of Graphene Driven by Ferroelectric Polarization.

Two-dimensional materials (2DMs) have exhibited remarkably tunable optical characteristics, which have been applied for significant applications in communications, sensing, and computing. However, the reported tunable optical properties of 2DMs are almost volatile, impeding them in the applications of multifarious emerging frameworks such as programmable operation and neuromorphic computing. In this work, nonvolatile electro-optic response is developed by the graphene-Al2O3-In2Se3 heterostructure integrating with microring resonators (MRRs). In such compact devices, the optical absorption coefficient of graphene is substantially tuned by the out-of-plane ferroelectric polarization in α-In2Se3, resulting in a nonvolatile optical transmission in MRRs. This work demonstrates that integrating graphene with ferroelectric materials paves the way to develop nonvolatile devices in photonic circuits for emerging applications such as optical neural networks.

Diagnostics of Internal Defects in Composite Overhead Insulators Using an Optic E-Field Sensor.

Composite insulators for high-voltage overhead lines have better performances and are lighter than traditional designs, especially in heavily polluted areas. However, since it is a relatively recent technology, reliable methods to perform live-line diagnostics are still under development, especially with regard to internal defects, which provide few external symptoms. Thermal cameras can be employed, but their use is not always straightforward as the sun radiation can hide the thermal footprint of internal degenerative effects. In this work, an optical E-field sensor has been used to diagnose the internal defects of a set of composite insulators (bandwidth 200 mHz-50 MHz, min. detectable E-field 100 V/m). Moreover, a modelling activity using finite elements has been carried out to identify the possible nature of the defects by comparing experimental E-field profiles with those simulated assuming a specific defect geometry. The results show that the sensor can detect the presence of an internal defect, since its presence distorts the E-field profile when compared to the profile of a sound insulator. Moreover, the measured E-field profiles are compatible with the corresponding simulated ones when a conductive defect is considered. However, it was observed that a defect whose conductivity is not at least two orders of magnitude greater than the conductivity of the surroundings remains undetected.

The Status of Environmental Electric Field Detection Technologies: Progress and Perspectives.

The detection of electric fields in the environment has great importance for understanding various natural phenomena, environmental monitoring, and ensuring human safety. This review paper provides an overview of the current state-of-the-art technologies utilized for sensing electric fields in the environment, the challenges encountered, and the diverse applications of this sensing technology. The technology is divided into three categories according to the differences in the physical mechanism: the electro-optic effect-based measurement system, the MEMS-based sensor, and the newly reported quantum effect-based sensors. The principles of the underlying methods are comprehensively introduced, and the tentative applications for each type are discussed. Detailed comparisons of the three different techniques are identified and discussed with regard to the instrument, its sensitivity, and bandwidth. Additionally, the challenges faced in environmental electric field sensing, the potential solutions, and future development directions are addressed.

Exposure Time Control Method for Higher Intermediate Frequency in Optical Heterodyne Imaging and Its Application to Electric-Field Imaging Based on Electro-Optic Effect.

We propose and demonstrate a method for equivalent time sampling using image sensors to selectively detect only the target frequency. Shortening the exposure time of the image sensor and using equivalent time sampling allows for the detection of frequency components that are higher than the frame rate. However, the imaging system in our previous work was also sensitive to the frequency component at 1/4 of the frame rate. In this study, we control the phase relationship between the exposure time and observed signal by inserting an additional interval once every four frames to detect the target frequency selectively. With this technique, we conducted electric field imaging based on the electro-optic effect under high noise conditions in the low-frequency band to which the conventional method is sensitive. The results demonstrated that the proposed method improved the signal-to-noise ratio.

Millimeter-Wave Band Electro-Optical Imaging System Using Polarization CMOS Image Sensor and Amplified Optical Local Oscillator Source.

In this study, we developed and demonstrated a millimeter-wave electric field imaging system using an electro-optic crystal and a highly sensitive polarization measurement technique using a polarization image sensor, which was fabricated using a 0.35-µm standard CMOS process. The polarization image sensor was equipped with differential amplifiers that amplified the difference between the 0° and 90° pixels. With the amplifier, the signal-to-noise ratio at low incident light levels was improved. Also, an optical modulator and a semiconductor optical amplifier were used to generate an optical local oscillator (LO) signal with a high modulation accuracy and sufficient optical intensity. By combining the amplified LO signal and a highly sensitive polarization imaging system, we successfully performed millimeter-wave electric field imaging with a spatial resolution of 30×60 µm at a rate of 1 FPS, corresponding to 2400 pixels/s.

Organic Electro-Optic Materials with High Electro-Optic Coefficients and Strong Stability.

The preparation of high-performance electro-optical materials is one of the key factors determining the application of optoelectronic communication technology such as 5G communication, radar detection, terahertz, and electro-optic modulators. Organic electro-optic materials have the advantage of a high electro-optic coefficient (~1000 pm/V) and could allow the utilization of photonic devices for the chip-scale integration of electronics and photonics, as compared to inorganic electro-optic materials. However, the application of organic nonlinear optical materials to commercial electro-optic modulators and other fields is also facing technical bottlenecks. Obtaining an organic electro-optic chromophore with a large electro-optic coefficient (r33 value), thermal stability, and long-term stability is still a difficulty in the industry. This brief review summarizes recent great progress and the strategies to obtain high-performance OEO materials with a high electro-optic coefficient and/or strong long-term stability. The configuration of D-π-A structure, the types of materials, and the effects of molecular engineering on the electro-optical coefficient and glass transition temperature of chromophores were summarized in detail. The difficulties and future development trends in the practical application of organic electro-optic materials was also discussed.

Systematic Study on Nonlinear Optical Chromophores with Improved Electro-Optic Activity by Introducing 3,5-Bis(trifluoromethyl)benzene Derivative Isolation Groups into the Bridge.

A series of novel chromophores A, B, C, and D, based on the julolidinyl donor and the tricyanofuran (TCF) and CF3-tricyanofuran (CF3-Ph-TCF) acceptors, have been synthesized and systematically investigated. The 3,5-bis(trifluoromethyl)benzene derivative isolation group was introduced into the bridge in the chromophores C and D. These nonlinear optical chromophores showed good thermal stability, and their decomposition temperatures were all above 220 °C. Density functional theory (DFT) was used to calculate the energy gaps and first-order hyperpolarizability (ß). The macroscopic electro-optic (EO) activity was measured using a simple reflection method. The highest EO coefficient of poled films containing 35 wt% of chromophore D doped in amorphous polycarbonate afforded values of 54 pm/V at 1310 nm. The results indicate that the 3,5-bis(trifluoromethyl)benzene isolation group can suppress the dipole-dipole interaction of chromophores. The moderate r33 value, good thermal stability, and good yield of chromophores suggest their potential use in the nonlinear optical area.

Real-Time Megapixel Electro-Optical Imaging of THz Beams with Probe Power Normalization.

In this work, we present a simple method to improve the spatial uniformity of two-dimensional electro-optical imaging of terahertz (THz) beams. In this system, near-field THz images are captured by fully illuminating a sample using conventional optical microscope objectives. Unfortunately, due to the linear relationship between the optical probe power and the measured THz electric field, any spatial variation in probe intensity translates directly into a variation of the recorded THz electric field. Using a single normalized background frame information map as a calibration tool prior to recording a sequence of THz images, we show a full recovery of a two-dimensional flat field for various combinations of magnification factors. Our results suggest that the implementation of dynamic intensity profile correction is a promising avenue for real-time electro-optical imaging of THz beams.

High-Performance Pockels Effect Modulation and Switching in Silicon-Based GaP/Si, AlP/Si, ZnS/Si, AlN/3C-SiC, GaAs/Ge, ZnSe/GaAs, and ZnSe/Ge Superlattice-On-Insulator Integrated Circuits.

We propose new a Si-based waveguided Superlattice-on-Insulator (SLOI) platforms for high-performance electro-optical (EO) 2 × 2 and N × M switching and 1 × 1 modulation, including broad spectrum and resonant. We present a theoretical investigation based on the tight-binding Hamiltonian of the Pockels EO effect in the lattice-matched undoped (GaP)N/(Si2)M, (AlP)N/(Si2)M, (ZnS)N/(Si2)M, (AlN)N/(3C-SiC)M, (GaAs)N/(Ge2)M, (ZnSe)N/(GaAs)M, and (ZnSe)N/(Ge2)M wafer-scale short-period superlattices that are etched into waveguided networks of small-footprint Mach-Zehnder interferometers and micro-ring resonators to yield opto-electronic chips. The spectra of the Pockels r33 coefficient have been simulated as a function of the number of the atomic monolayers for "non-relaxed" heterointerfaces. The large obtained r33 values enable the SLOI circuit platforms to offer a very favorable combination of monolithic construction, cost-effective manufacturability, high modulation/switching speed, high information bandwidth, tiny footprint, low energy per bit, low switching voltage, near-IR-and-telecom wavelength coverage, and push-pull operation. By optimizing waveguide, clad, and electrode dimensions, we obtained very desirable values of the VπL performance metric, in the range of 0.062 to 0.275 V·cm, portending a bright future for a variety of applications, such as sensor networks or Internet of Things (IoT).

High-Speed Plasmonic Electro-Optic Beam Deflectors.

Highly integrated active nanophotonics addressing both device footprint and operation speed demands is a key enabling technology for the next generation optical networks. Plasmonic systems have proven to be a serious contender to alleviate current performance limitations in electro-optic devices. Here, we demonstrate a plasmonic optical phased array (OPA) consisting of two 10 µm long plasmonic phase shifters, utilized to control the far-field radiation pattern of two subwavelength-separated emitters for aliasing-free beam steering with an angular range of ±5° and flat frequency response up to 18 GHz (with the potential bandwidth of 1.2 THz). Extreme optical and electrostatic field confinement with great spatial overlap results in high phase modulation efficiency (VπL = 0.24 Vcm). The demonstrated approach of using plasmonic lithium niobate technology for optical beam manipulation offers inertia-free, robust, ultracompact, and high-speed beam steering.

Broadband Metallic Fiber-to-Chip Couplers and a Low-Complexity Integrated Plasmonic Platform.

We present a plasmonic platform featuring efficient, broadband metallic fiber-to-chip couplers that directly interface plasmonic slot waveguides, such as compact and high-speed electro-optic modulators. The metallic gratings exhibit an experimental fiber-to-slot coupling efficiency of -2.7 dB with -1.4 dB in simulations with the same coupling principle. Further, they offer a huge spectral window with a 3 dB passband of 350 nm. The technology relies on a vertically arranged layer stack, metal-insulator-metal waveguides, and fiber-to-slot couplers and is formed in only one lithography step with a minimum feature size of 250 nm. As an application example, we fabricate new modulator devices with an electro-optic organic material in the slot waveguide and reach 50 and 100 Gbit/s data modulation in the O- and C-bands within the same device. The devices' broad spectral bandwidth and their relaxed fabrication may render them suitable for experiments and applications in the scope of sensing, nonlinear optics, or telecommunications.

A Highly Sensitive and Miniature Optical Fiber Sensor for Electromagnetic Pulse Fields.

The detection of an electromagnetic pulse (EMP) field is of great significance in determining the field environment of tested equipment in small spaces. Finger-shaped miniature optical fiber sensors for electromagnetic pulse field measurement were designed. The antenna of a weak field sensor was integrated with a shielding shell, and the wire welded at the direct electro-optic converting circuit connected to an optical fiber through special structure and circuit design was taken as the antenna of a strong field sensor. Measurements in the time domain and frequency domain had been carried out for the two sensors. Experiment results demonstrate that the weak field sensor and the strong field sensor have flat responses from 100 kHz to 1 GHz with a variation of 2.3 dB and 2.9 dB, respectively, and the EMP waveform detected by the sensors agrees well with the applied standard square wave. Moreover, the strong field sensor exhibits linear responses from 645 V/m to 83 kV/m. The resolution of the weak field sensor is as low as 13 V/m. The result indicated that the designed sensors had good performance.

Anisotropic Quantum Well Electro-Optics in Few-Layer Black Phosphorus.

The incorporation of electrically tunable materials into photonic structures such as waveguides and metasurfaces enables dynamic, electrical control of light propagation at the nanoscale. Few-layer black phosphorus is a promising material for these applications due to its in-plane anisotropic, quantum well band structure, with a direct band gap that can be tuned from 0.3 to 2 eV with a number of layers and subbands that manifest as additional optical transitions across a wide range of energies. In this Letter, we report an experimental investigation of three different, anisotropic electro-optic mechanisms that allow electrical control of the complex refractive index in few-layer black phosphorus from the mid-infrared to the visible: Pauli-blocking of intersubband optical transitions (the Burstein-Moss effect); the quantum-confined Stark effect; and the modification of quantum well selection rules by a symmetry-breaking, applied electric field. These effects generate near-unity tuning of the BP oscillator strength for some material thicknesses and photon energies, along a single in-plane crystal axis, transforming absorption from highly anisotropic to nearly isotropic. Lastly, the anisotropy of these electro-optical phenomena results in dynamic control of linear dichroism and birefringence, a promising concept for active control of the complex polarization state of light, or propagation direction of surface waves.

Anomalous Photothermoelectric Transport Due to Anisotropic Energy Dispersion in WTe2.

Band structures are vital in determining the electronic properties of materials. Recently, the two-dimensional (2D) semimetallic transition metal tellurides (WTe2 and MoTe2) have sparked broad research interest because of their elliptical or open Fermi surface, making distinct from the conventional 2D materials. In this study, we demonstrate a centrosymmetric photothermoelectric voltage distribution in WTe2 nanoflakes, which has not been observed in common 2D materials such as graphene and MoS2. Our theoretical model shows the anomalous photothermoelectric effect arises from an anisotropic energy dispersion and micrometer-scale hot carrier diffusion length of WTe2. Further, our results are more consistent with the anisotropic tilt direction of energy dispersion being aligned to the b-axis rather than the a-axis of the WTe2 crystal, which is consistent with the previous first-principle calculations as well as magneto-transport experiments. Our work shows the photothermoelectric current is strongly confined to the anisotropic direction of the energy dispersion in WTe2, which opens an avenue for interesting electro-optic applications such as electron beam collimation and electron lenses.

Amorphous Mn3 O4 Nanocages with High-Efficiency Charge Transfer for Enhancing Electro-Optic Properties of Liquid Crystals.

Improving electro-optic properties is essential for fabricating high-quality liquid crystal displays. Herein, by doping amorphous Mn3 O4 octahedral nanocages (a-Mn3 O4 ONCs) into a nematic liquid crystal (NLC) matrix E7, outstanding electro-optic properties of the blend are successfully obtained. At a doping concentration of 0.03 wt%, the maximum decreases of threshold voltage (Vth ) and saturation voltage (Vsat ) are 34% and 31%, respectively, and the increase of contrast (Con ) is 160%. This remarkable electro-optic activity can be attributed to high-efficiency charge transfer within the a-Mn3 O4 ONCs NLC system, caused by metastable electronic states of a-Mn3 O4 ONCs. To the best of our knowledge, such remarkable decreased electro-optic activity is observed for the first time from doping amorphous semiconductors, which could provide a new pathway to develop excellent energy-saving amorphous materials and improve their potential applications in electro-optical devices.

Photon diagnostics at the FLASH THz beamline.

The THz beamline at FLASH, DESY, provides both tunable (1-300 THz) narrow-bandwidth (∼10%) and broad-bandwidth intense (up to 150 uJ) THz pulses delivered in 1 MHz bursts and naturally synchronized with free-electron laser X-ray pulses. Combination of these pulses, along with the auxiliary NIR and VIS ultrashort lasers, supports a plethora of dynamic investigations in physics, material science and biology. The unique features of the FLASH THz pulses and the accelerator source, however, bring along a set of challenges in the diagnostics of their key parameters: pulse energy, spectral, temporal and spatial profiles. Here, these challenges are discussed and the pulse diagnostic tools developed at FLASH are presented. In particular, a radiometric power measurement is presented that enables the derivation of the average pulse energy within a pulse burst across the spectral range, jitter-corrected electro-optical sampling for the full spectro-temporal pulse characterization, spatial beam profiling along the beam transport line and at the sample, and a lamellar grating based Fourier transform infrared spectrometer for the on-line assessment of the average THz pulse spectra. Corresponding measurement results provide a comprehensive insight into the THz beamline capabilities.

Micrometer Scale Resolution Limit of a Fiber-Coupled Electro-Optic Probe.

We present the practical resolution limit of a fine electrical structure based on a fiber-coupled electro-optic probing system. The spatial resolution limit was experimentally evaluated on the sub-millimeter to micrometer scale of planar electrical transmission lines. The electrical lines were fabricated to have various potential differences depending on the dimensions and geometry. The electric field between the lines was measured through an electro-optic probe, which was miniaturized up to the optical bare fiber scale so as to investigate the spatial limit of electrical signals with minimal invasiveness. The experimental results show that the technical resolution limitation of a fiber-coupled probe can reasonably approach a fraction of the mode field diameter (~10 µm) of the fiber in use.

Recent Progress on Electromagnetic Field Measurement Based on Optical Sensors.

Electromagnetic field sensors are widely used in various areas. In recent years, great progress has been made in the optical sensing technique for electromagnetic field measurement, and varieties of corresponding sensors have been proposed. Types of magnetic field optical sensors were presented, including probes-based Faraday effect, magnetostrictive materials, and magnetic fluid. The sensing system-based Faraday effect is complex, and the sensors are mostly used in intensive magnetic field measurement. Magnetic field optical sensors based on magnetic fluid have high sensitivity compared to that based on magnetostrictive materials. Three types of electric field optical sensors are presented, including the sensor probes based on electric-optic crystal, piezoelectric materials, and electrostatic attraction. The majority of sensors are developed using the sensing scheme of combining the LiNbO3 crystal and optical fiber interferometer due to the good electro-optic properties of the crystal. The piezoelectric materials-based electric field sensors have simple structure and easy fabrication, but it is not suitable for weak electric field measurement. The sensing principle based on electrostatic attraction is less commonly-used sensing methods. This review aims at presenting the advances in optical sensing technology for electromagnetic field measurement, analyzing the principles of different types of sensors and discussing each advantage and disadvantage, as well as the future outlook on the performance improvement of sensors.

Multifunctional Microelectro-Opto-mechanical Platform Based on Phase-Transition Materials.

Along with the rapid development of hybrid electronic-photonic systems, multifunctional devices with dynamic responses have been widely investigated for improving many optoelectronic applications. For years, microelectro-opto-mechanical systems (MEOMS), one of the major approaches to realizing multifunctionality, have demonstrated profound reconfigurability and great reliability. However, modern MEOMS still suffer from limitations in modulation depth, actuation voltage, or miniaturization. Here, we demonstrate a new MEOMS multifunctional platform with greater than 50% optical modulation depth over a broad wavelength range. This platform is realized by a specially designed cantilever array, with each cantilever consisting of vanadium dioxide, chromium, and gold nanolayers. The abrupt structural phase transition of the embedded vanadium dioxide enables the reconfigurability of the platform. Diverse stimuli, such as temperature variation or electric current, can be utilized to control the platform, promising CMOS-compatible operating voltage. Multiple functionalities, including an active enhanced absorber and a reprogrammable electro-optic logic gate, are experimentally demonstrated to address the versatile applications of the MEOMS platform in fields such as communication, energy harvesting, and optical computing.