Electro-optic Research Articles

Towards higher electro-optic response in AlScN

Novel materials with large electro-optic (EO) coefficients are essential for developing ultra-compact broadband modulators and enabling effective quantum transduction. Compared to lithium niobate, the most widely used nonlinear optical material, wurtzite AlScN, offers advantages in nano-photonic devices due to its compatibility with integrated circuits. We perform detailed first-principles calculations to investigate the electro-optic effect in Al1−xScxN alloys and superlattices. At elevated Sc concentrations in alloys, the EO coefficients increase; importantly, we find that cation ordering along the c axis leads to enhanced EO response. Strain engineering can be used to further manipulate the EO coefficients of AlScN films. With applied in-plane strains, the piezoelectric contributions to the EO coefficients increase dramatically, even exceeding 250 pm/V. We also explore the possibility of EO enhancement through superlattice engineering, finding that nonpolar a-plane (AlN)m/(ScN)n superlattices increase EO coefficients beyond 40 pm/V. Our findings provide design principles to enhance the electro-optic effect through alloy engineering and heterostructure architecture.

Ultrabroadband integrated electro-optic frequency comb in lithium tantalate.

The integrated frequency comb generator based on Kerr parametric oscillation1 has led to chip-scale, gigahertz-spaced combs with new applications spanning hyperscale telecommunications, low-noise microwave synthesis, light detection and ranging, and astrophysical spectrometer calibration2-6. Recent progress in lithium niobate (LiNbO3) photonic integrated circuits (PICs) has resulted in chip-scale, electro-optic (EO) frequency combs7,8, offering precise comb-line positioning and simple operation without relying on the formation of dissipative Kerr solitons. However, current integrated EO combs face limited spectral coverage due to the large microwave power required to drive the non-resonant capacitive electrodes and the strong intrinsic birefringence of LiNbO3. Here we overcome both challenges with an integrated triply resonant architecture, combining monolithic microwave integrated circuits with PICs based on the recently emerged thin-film lithium tantalate (LiTaO3)9. With resonantly enhanced EO interaction and reduced birefringence in LiTaO3, we achieve a fourfold comb span extension and a 16-fold power reduction compared to the conventional, non-resonant microwave design. Driven by a hybrid integrated laser diode, the comb spans over 450 nm (more than 60 THz) with more than 2,000 lines, and the generator fits within a compact 1-cm2 footprint. We additionally observe that the strong EO coupling leads to an increased comb existence range approaching the full free spectral range of the optical microresonator. The ultra-broadband comb generator, combined with detuning-agnostic operation, could advance chip-scale spectrometry and ultra-low-noise millimetre wave synthesis10-13 and unlock octave-spanning EO combs. The methodology of co-designing microwave and photonics can be extended to a wide range of integrated EOs applications14-16.

Metallic Electro-optic Effect in Gapped Bilayer Graphene.

Electro-optic (EO) modulation is a critical device action in photonics. Recently, the non-Drude dynamics induced by the Berry curvature dipole (BCD) in metals have attracted attention as a potential candidate for terahertz EO modulation. However, such BCD-induced EO effects can be challenging to realize, often requiring flat bands and complex materials such as a strained magic-angle twisted bilayer graphene on hexagonal boron nitride. Here, we argue that metallic EO can be achieved with a much simpler material, gapped bilayer graphene, with EO coefficients comparable to that in flat-band materials in the terahertz range. In particular, we find metallic EO can be realized without a Berry curvature dipole; we identify skew-scattering and a "snap" (third-order derivative of velocity) can readily produce pronounced Pockels and Kerr EO effects in clean metals. These yield nonreciprocal and field-activated birefringence and field-induced modulations to transmission and reflection, essential components for terahertz EO modulators.

High-Energy Burst-Mode 3.5 μm MIR KTA-OPO

In this paper, a high energy 3.5 μm mid-infrared (MIR) burst-mode KTA optical parametric oscillator (OPO) was demonstrated. Utilizing a quasi-continuous wave (QCW) laser diode (LD) side-pump module and electro-optic (EO) Q-switching technique, a high beam quality 1064 nm burst-mode laser was achieved as the fundamental source, generating 30 mJ high-energy pulses at burst repetition rates of 100 Hz and 200 Hz with sub-burst repetition rates of 20 kHz, 40 kHz, and 50 kHz. The KTA-OPO produced a 3.5 μm MIR burst-mode laser output with 4 to 11 sub-pulses per pulse envelope. The output energies were 2.9 mJ, 2.81 mJ, and 2.79 mJ at 100 Hz, as well as 2.8 mJ, 2.75 mJ, and 2.72 mJ at 200 Hz, with corresponding conversion efficiencies of 9.6%, 9.3%, and 9.3% at 100 Hz, as well as 9.3%, 9.2%, and 9.1% at 200 Hz, respectively. Our results pave a new way for generating burst-mode MIR lasers.

Toward Large-Scale Photonic Chips Using Low-Anisotropy Thin-Film Lithium-Tantalate.

Photonic manipulation of large-capacity data with the advantages of high speed and low power consumption is a promising solution for explosive growth demands in the era of post-Moore. A well-developed lithium-niobate-on-insulator (LNOI) platform has been widely explored for high-performance electro-optic (EO) modulators to bridge electrical and optical signals. However, the photonic waveguides on the x-cut LNOI platform suffer serious polarization-mode conversion/coupling issues because of strong birefringence, making it hard to realize large-scale integration. Here, low-birefringence photonic integrated circuits (PICs) based on lithium-tantalate-on-insulator (LTOI) are proposed and demonstrated, which enables high-performance passive photonic devices as well as EO modulators, showing great potential for large-scale photonic chips. Analysis of mode conversion and evolution behaviors with both low- and high-birefringence shows undesired mode hybridizations can be effectively suppressed. A simple and universal fabrication process is developed and various representative passive photonic devices are demonstrated with impressive performances. Finally, a wavelength-division-multiplexed optical transmitter is developed with a data rate of 1.6 Tbps by monolithically integrating 8 EO modulators and an 8-channel arrayed waveguide grating. Therefore, the demonstrated low-birefringence LTOI platform shows strong ability in both passively and actively controlling photon behaviors on a chip, indicating great potential for ultrafast processing and communicating large-capacity data.

Polarization-insensitive silicon intensity modulator with a maximum speed of 224 Gb/s

Polarization-insensitive optical modulators allow an external laser to be remotely interconnected by single-mode optical fibers while avoiding polarization controllers, which would be convenient and cost-effective for co-packaged optics, 5G, and future 6G applications. In this article, a polarization-insensitive silicon intensity modulator is proposed and experimentally demonstrated based on two-dimensional centrally symmetric gratings, featuring a low polarization-dependent loss of 0.15 dB in minimum and polarization insensitivity of eye diagrams. The device exhibits a low fiber-to-fiber insertion loss of 9 dB and an electro-optic (EO) bandwidth of 49.8 GHz. A modulation speed of up to 224 Gb/s is also demonstrated.

Ferroelectric Optical Memristors Enabled by Non-Volatile Electro-Optic Effect.

Memristors enable non-volatile memory and neuromorphic computing. Optical memristors are the fundamental element for programmable photonic integrated circuits due to their high-bandwidth computing, low crosstalk, and minimal power consumption. Here, an optical memristor enabled by a non-volatile electro-optic (EO) effect, where refractive index modulation under zero field is realized by deliberate control of domain alignment in the ferroelectric material Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT) is proposed. The non-volatile EO memristor is designed exclusively for the modulation of the optical phase without degrading the optical transparency, and it allows the support for deterministic and repeated non-volatile multilevel EO states. A non-volatile tunable waveplate composed of the optical memrisor for free-space optics, which allows for deterministic multilevel, and non-volatile phase shifts from 0 to π/2 is presented. The state switching rate of the memristor is less than 100ms, with a switching energy consumption of 234nJ, and the states can be retained for up to 12h without requiring static power consumption. These results demonstrate a novel approach to fully realizing non-volatile optical memristors, where only optical phase modulation is involved, providing unprecedented opportunities for the development of new ferroelectric memristors.

A wide-spectrum mid-infrared electro-optic intensity modulator employing a two-point coupled lithium niobate racetrack resonator

Optical intensity modulators (OIMs) are essential for mid-infrared (mid-IR) photonics, enabling applications such as bond-selective molecular sensing, and free-space communications via atmospheric windows. Integrated photonics offers a compact and cost-effective solution, yet on-chip mid-IR OIMs significantly underperform compared to their near-IR counterparts. Furthermore, despite the potential benefits for system reconfiguration in accessing various communication frequencies and molecular absorption bands, developing a single OIM capable of operating across a broad spectral range remains a challenge. In this study, we introduce an on-chip OIM that operates over a wide wavelength range in the mid-IR, implemented using a racetrack resonator structure in thin film lithium niobate (TFLN). The modulator employs a two-point coupling scheme, allowing active control of the coupling strength to maintain critical coupling and thereby ensuring high modulation extinction across a wide spectral region. This approach not only achieves high modulation performance but also relaxes the design constraints and fabrication precision typically associated with resonator-based modulators, as confirmed through an analytic model. Implemented in TFLN having a wide transmission spectrum and strong electro-optic coefficient, the OIM demonstrates a modulation extinction ratio exceeding 20 dB with an electro-optic efficiency of 7.7 V cm over the wavelength range of 3.3–3.8 μm, which falls within the first atmospheric transmission widow in the mid-IR. This approach can be adapted to other spectral regions, providing a versatile solution for diverse photonic applications.

Design of silicon traveling-wave Mach-Zehnder modulators with transparent electrodes.

High-speed silicon traveling-wave Mach-Zehnder modulators (MZMs) are key components to support optical fiber communication. However, one major challenge with all-silicon MZMs is to achieve efficient high-speed electro-optic (EO) modulation. The reported 3 dB bandwidth of silicon MZMs are generally below 70 GHz, with half-wave voltage (V π) around 5 V or larger, which can not support future 200 Gbaud data transmission. Here we break the voltage-bandwidth trade-off limit in silicon MZMs by replacing the doped silicon slab with CMOS compatible transparent electrodes. Benefit from the measured high conductivity, low extinction coefficient, and low refractive index of indium tin oxide (ITO) materials, the microwave dielectric loss of the traveling wave electrode can be greatly reduced. The bandwidth would potentially increase from 40 GHz to 168 GHz, while V π and optical insertion loss remains almost unchanged. According to Si/ITO interface contact states, three operating mode were found, corresponding to Si/ITO ohmic contact, schottky contact and hybrid schottky/ohmic contact, respectively. We comprehensively analysis the Si/ITO interface characteristic, establish a complete high frequency equivalent circuit model. Our proposed transparent electrodes will open a new window for the high-speed silicon photonics platform.

Tunable Beam Steering Metasurface Based on a PMN-PT Crystal with a High Electro-Optic Coefficient

Existing tunable optical metasurfaces based on the electro-optic effect are either complex in structure or have a limited phase modulation range. In this paper, a simple rectangular metasurface structure based on a Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) crystal with high electro-optic coefficient of 120 pm/V was designed to demonstrate its electrically tunable performance in the optical communication band through simulations. By optimizing the structure parameters, a tunable metasurface was generated that can induce a complete 2π phase shift for beam deflection while maintaining relatively uniform transmittance. Simulations further demonstrated the electrical tunability of the beam deflection direction and operating wavelength of the metasurface. This tunable optical metasurface, with its simple and easily fabricated structure, can promote the development and application of multifunctional and controllable metasurfaces. Its adjustable beam deflection direction and operating wavelength may find applications in fields such as optical communication systems and imaging.

Heterogeneously integrated silicon-conductive oxide MOSCAP microring modulator array

In pursuit of energy-efficient optical interconnect, the silicon microring modulator (Si-MRM) has emerged as a pivotal device offering an ultra-compact footprint and capability of on-chip wavelength division multiplexing (WDM). This paper presents a 1×4 metal-oxide-semiconductor capacitor (MOSCAP) Si-MRM array gated by high-mobility titanium-doped indium oxide (ITiO), which was fabricated by combining Intel’s high-volume manufacturing process and the transparent conductive oxide (TCO) patterning with the university facility. The 1×4 Si-MRM array exhibits a high electro-optic (E-O) efficiency with V π ·L of 0.12 V·cm and achieves a modulation rate of (3×25+1×15) Gb/s with a measured bandwidth of 14 GHz. Additionally, it can perform on-chip WDM modulation at four equally spaced wavelengths without using thermal heaters. The process compatibility between silicon photonics and TCO materials is verified by such an industry-university co-fabrication approach for the MOSCAP Si-MRM array and demonstrated enhanced performance from heterogeneous integration.

Deep Learning Based Panchromatic2RGB Image Generation from VHR Images

Image colorization is the process of obtaining colored images by assigning RGB color values to a grayscale or panchromatic image. This technique has an important place in the field of computer vision because colored images provide a better visual experience and are widely used in areas such as image recognition and object detection. It also has many practical applications such as coloring historical photographs, adding colors to be used in the analysis of medical images, and improving the analysis of satellite images. Colorization methods are divided into two main categories: brush coloring and sample-based coloring. Both methods have certain limitations. The performance of these methods depends on the selected reference images and may sometimes contain false colors or significant errors. While these methods require operator intervention or pre-defined rules, deep learning based methods are largely automated and uses neural networks to understand the global and local context of an image, leading to more realistic and contextually accurate colorizations. The presented study uses the Denoising Diffusion Null-Space Model (DDNM) architecture. DDNM is a method that aims to obtain more efficient and high-quality results compared to the coloring approaches available in literature. In the study, the weight data of the DDNM architecture was used to predict colored images from panchromatic images using the SpaceNet 6 open access dataset. The SpaceNet 6 dataset includes a combination of Capella Space 0.5m Synthetic Aperture Radar (SAR) imagery and Maxar's 0.5m electro-optical (EO) imagery. In order to assess the results, Peak Signal-to-Noise Ratio (PSNR) and Structural Similarity Index Measure (SSIM) accuracy metrics are calculated.

Synthesis of fluorescent dicyanomethylenevinyl-1,3-dicoumarin compounds with donor-acceptor-π-donor (D-A-π-D) system and investigation of their photophysical, NLO, and chemosensor properties: Part 1.

Coumarin compounds have heterocyclic core with different properties such as high quantum yields, broad Stokes shifts, and superior photophysical and biological activity. It is known that fluorescence properties increase with increased intramolecular charge transfer in systems where electron-withdrawing or donor groups are attached to different positions of the coumarin compound. When these compounds interact with analytes in the environment, the analytes in the environment can be detected by quenching or increasing fluorescence. For this purpose, dicyanomethylenevinyl-1,3-dicoumarin compounds were obtained and 1H NMR, 13C NMR, FT-IR, HR-MS elucidated their structures. To determine the photophysical properties of the synthesized compounds, absorption, and emission spectra were examined in solvents with different polarities, and also quantum yields and Stokes shifts were calculated. Additionally, the sensitivity/selectivity properties of the compounds towards various anions were investigated by spectrophotometric, spectrofluorometric, and 1H NMR titration methods. The limit of detection (LOD) of the sensors to sense cyanide anion was considered based on absorption titration. The pKa value of compound that could be pH sensor candidate was determined. Thermogravimetric analysis was performed as an important parameter for compounds in electro-optical (EO) systems. Additionally, nonlinear optical (NLO) properties of the compounds were calculated experimentally and theoretically. The some experimental results were explained by Density Functional Theory (DFT) and time-dependent DFT (TDDFT) calculations.

Self-balanced and self-phase-corrected electro-optic sampling in a birefringent crystal.

Mid-to-far-infrared (IR) spectral content is recorded using the novel self-balanced and self-phase-corrected electro-optical (EO) sampling arrangement. Self-balancing guarantees that the electric field emerging from the EO crystal yields a signal of zero via a Wollaston prism and balanced photodetector (i.e., without a wave plate following the EO crystal) in the absence of the electric field being sampled. Moreover, self-phase-correction ensures a nearly frequency-independent phase difference between the probe electric field pulse and the electric field pulse generated within the EO crystal via second-order nonlinear optical interactions. The self-balanced and self-phase corrected arrangement has the potential to yield enhancement of EO signals recorded using previously investigated birefringent crystals (implemented within traditional EO sampling geometries) and deliver optimum EO signal strengths when considering unexplored birefringent crystals.

Electron-Lattice Synergistic Coordination for Boosting the Electro-Optic Response of the Crystal.

The electro-optic (E-O) response is fundamental where the refractive index of materials can be modulated by the external electric field. Up to now, E-O modulation has become a critical technology in photonics, quantum optics, modern communication, etc. However, the design of E-O crystals with broadband availabilities and large E-O effect is still challenging since the E-O response is contributed by both electrons and phonons. Here, an electron-lattice synergistic coordination strategy is proposed for boosting the E-O response, and with the langasite as the subject, a novel crystal La3Zr0.5Ga5Si0.5O14 (LZGS) is designed. By the rational introduction of the Zr4+ ion in the octahedral site, it is found that the E-O response can be obviously improved attributed to the active 4d orbitals of the Zr4+ ion and the optical phonons of the flexible (ZrO6) unit. This crystal also has advantages in transparency, laser damage threshold and E-O coefficient compared with the well-known E-O crystals. Moreover, broadband E-O modulation is also demonstrated in the lasers at wavelengths from 639 to 1991nm. These findings not only demonstrate an excellent E-O crystal for the development of modern devices in classical and quantum fields but also provide a design strategy for boosting the E-O response.

Unraveling the structure-property relationship of novel thiophene and furan-fused cyclopentadienyl chromophores for nonlinear optical applications.

Development of organic nonlinear optical materials has become progressively more important due to their emerging applications in new-generation photonic devices. A novel series of chromophores based on innovative thiophene and furan-fused cyclopentadienyl bridge with various powerful donor and acceptor moieties were designed and theoretically investigated for applications in nonlinear optics. To unravel the structure-property relationship between this new push-pull conjugated systems and their nonlinear optical property, multiple methods, including density of states analysis, coupled perturbed Kohn-Sham (CPKS) method, sum-over-states (SOS) model, the two-level model (TSM), hyperpolarizability density analysis, and the (hyper)polarizability contribution decomposition, were performed to comprehensively investigated the nonlinear optical and electronic properties of this new π-system. Due to excellent charge transfer ability of new bridge and distinctive structure of donor and acceptor, the designed chromophores exhibit deep HOMO levels, low excitation energy, high dipole moment difference and large hyperpolarizability, indicating the appealing air-stable property and remarkable electrooptic performance of them. Importantly, THQ-CS-A3 and PA-CS-A3 shows outstanding NLO response properties with βtot value of 6953.9 × 10-30 and 5066.0 × 10-30 esu in AN, respectively. The influence of the push-pull strength, the heterocycle and the π-conjugation of new bridge on the nonlinear optical properties of this novel powerful systems are clarified. This new series of chromophores exhibit remarkable electro-optical Pockels and optical rectification effect. More interestingly, PA-CS-A3 and THQ-CS-A2 also show appealing SHG effect. This study will help people understand the nature of nonlinear optical properties of innovative heteroarene-fused based cyclopentadienyl chromophores and offer guidance for the rational design of chromophores with outstanding electrooptic (EO) performance in the future.

On-Chip Photonic Simulating Band Structures toward Arbitrary-Range Coupled Frequency Lattices.

Photonic simulators are increasingly used to study physical systems for their affluent manipulable degrees of freedom. The advent of photonic chips offers a promising path towards compact and configurable simulators. Thin-film lithium niobate chips are particularly well suited for this purpose due to the high electro-optic coefficient, which allows for the creation of lattices in the frequency domain. Here, we fabricate and periodically modulate an on-chip resonator to observe band structures. The employed modulation rates are lower than the resonator linewidth, resulting in the inclusion of multiple lattice points within one resonant peak. This alleviates the difficulty of applying and detecting multiharmonic signals which are conventionally of ultrahigh frequency on chips and enables us to simulate structures with arbitrary-range coupling. As examples, we showcase the simulation of nanotubes along several directions where the required frequencies are reduced by more than 3 orders of magnitude (up to reduce near 100GHz to around 10MHz in our examples). Encompassing various models equipped with a gauge potential, our experiments demonstrate an effective and technically feasible scenario which may bolster the development of on-chip photonic simulators complementing existing techniques.

High-efficiency single-mode erbium-doped lithium niobate microring laser with milliwatt output power.

Erbium-doped thin-film lithium niobate (TFLN) lasers have attracted great interest in recent years due to their compatibility with high-speed electro-optic (EO) modulation on the same platform. In this work, high-efficiency single-mode erbium-doped microring lasers with milliwatt output powers were demonstrated. Monolithic lithium niobate microring resonators using pulley-waveguide-coupling were fabricated by the photolithography assisted chemo-mechanical etching (PLACE) technique. The maximum single-mode laser power of 1.26 mW with the side-mode suppression ratio (SMSR) of 50 dB was achieved around the wavelength of 1562 nm, as well as the maximum laser slope efficiency of 2.51% and the minimum laser linewidth of 30 kHz. Besides, the lasing band was easily switched by the pulley-coupler with variable waveguide widths. The demonstrated milliwatt-level on-chip microlasers hold great promise as bright light sources for various integrated devices on the TFLN platform such as EO modulators and combs.

Optimization Across-The-Board: Centro-Arylated Push-Pull Tetraene Chromophores and Guest-Host Polymers for Electro-Optics.

The research and development of push-pull tetraene chromophores (PPT-phores) have contributed greatly to the field of organic electro-optic (EO) materials and devices since the inauguration of CLD-1 in 2001. This study is thus a systematic contribution to synthesize and characterize a series of centro-arylated PPT-phores based on strong electron-donating tetrahydroquinolinyl groups and variable strong electron-accepting tricyanofuran derivatives. In particular, we report the crystallographic data to show various packing modes of these PPT-phores with detailed information about bond length alternation and intermolecular interactions, the optical absorption edges of guest-host polymers by the Tauc model, and the anisotropy and dispersion of Pockels tensors for the poled polymers by attenuated total reflection spectroscopy. Such analyses have not been addressed to any significant extent previously and are fundamentally important to the future development of PPT-phore-based EO materials and devices. The poled films of several centro-arylated PPT-phores in polycarbonates exhibited large EO activities, excellent thermal stability, and tunable optical transparency at the telecom O- and C-band. The study demonstrates the effectiveness of π-bridge centro-arylation enabled by molecular shape modification and rigidity enhancement, over the relatively flexible and labile thioether or alkoxy groups, in rational design of hyperpolarizable PPT-phores for high-performance EO polymers.

Integrated lithium niobate Vernier micro-ring filter with wide and fast tuning capabilities

Tunable optical micro-ring filters play significant roles in optical communications, microwave photonics, and photonic neural networks. Typical micro-ring filters are based on either a thermo-optic (TO) effect with microsecond timescales or an electro-optic (EO) effect with a limited tuning range. Here, we report a continuously tunable lithium niobate on insulator (LNOI) Vernier cascaded micro-ring filter with wire-bonded packaging integrated with both TO and EO tuning electrodes, featuring a 40-nm free spectral range (FSR), 2.3 GHz EO bandwidth, and a high sidelobe suppression ratio of 21.7 dB, simultaneously. Our high-performance optical micro-ring filter could become an important element in future LNOI photonic circuits with applications in high-capacity wavelength-division multiplexing (WDM) systems, broadband microwave photonics, fast-tunable external-cavity lasers, and high-speed photonic neural networks.