Pockels Effect Research Articles

Hybrid lithium tantalite-silicon integrated photonics platform for electro-optic modulation.

Integrated electro-optic modulators are key components in photonic integrated circuits. Silicon photonic technology is considered to be promising for large-scale and low-cost integration. However, silicon does not exhibit any Pockels effect, and the electro-optic modulator based on free-carrier dispersion suffers from challenges such as high-power consumption, limited bandwidth, and large optical propagation loss. Here, a new, to the best of our knowledge, hybrid lithium tantalite-silicon platform is proposed for electro-optic modulators based on the Pockels effect. Benefiting from the strong Pockels coefficients of a thin-film lithium tantalite, a hybrid microring-based modulator is demonstrated. The quality factor and the extinction ratio of the hybrid microring are 1.7 × 104 and 10 dB, respectively. The linear bidirectional wavelength tuning efficiency is measured as 12.8 pm/V. The measured 3-dB bandwidth is > 20 GHz. High-quality eye diagrams of 20 Gbps non-return-to-zero signal and 20 Gbps four-level pulse amplitude modulation signals are generated experimentally. The proposed platform extends the toolbox of silicon photonics technology, which paves the way for high-speed modulators and phase shifters in optical communication and optical phased array.

Investigation Nonlinear Properties (First and Second Hyperpolarizabilities) of The Nitro-thieno [3,2-b] thiophene -fullerene (C20) Molecule

In this study, nonlinear optical (NLO) properties of nitro-thieno [3,2-b] thiophene-fullerene (C20) molecule was systematically investigated using density functional theory (DFT) methods at the B3LYP level with a 6-31 + G(d,p) basis set. Fullerene (electron-donor) is associated with thieno [3,2-b] thiophene (π-conjugated bridge), forming a charge-transfer framework, and nitro is a strong electron-acceptor. The dynamic properties of the molecule, including first and second hyperpolarizability, resulting from second harmonic generation β(-2ω; ω,ω), third harmonic generation γ(– 2ω; ω, ω, ω), Pockels effect β(-ω; ω,0), and Kerr effect γ(–ω; ω, 0, 0), were investigated, which are essential evaluation indexes for creating nonlinear materials. The molecule exhibits excellent nonlinear responses, where it was found that the highest linear response of the coefficients above at the wavelength 455.6 nm. Absorption spectra reveal that these molecules have infrared transparent regions and are novel nonlinear molecules. Therefore, linking nitro-thieno[3,2-b]thiophene with fullerene (C20) efficiently designs high-performance nonlinear molecules.

Measurement of the voltage evolution on a load of X-pinch plasma system using the Pockels effect

A diagnostic system using the Pockels effect (linear electro-optic effect) has been developed to measure a voltage on a load of the SNU X-pinch device [Ryu et al., Rev. Sci. Instrum. 92, 053533 (2021)]. The sensor component of the diagnostic system comprises of a lithium niobate (LN) crystal and its mount. When the LN crystal is subjected to an external electric field, the refractive indices of the LN crystal change due to the Pockels effect, leading to a change in the polarization state of the propagating laser beam through the crystal. This concept allows the separation and shielding of electronic devices from the intense electric pulses generated by the X-pinch system. Furthermore, the compact size and high electric field tolerance of the LN crystal enable us to position the sensor in a close proximity to the load of the X-pinch, facilitating the measurement of voltage evolution on the load. In contrast to circuit-based voltage probes, our sensor possesses low inductance, allowing us to measure resistive-dominant voltage even in the presence of rapid current changes within the X-pinch system. In this paper, we outline the configuration of our voltage measurement system using the LN crystal within the X-pinch device and provide measurement results of the load voltage. Additionally, we explore the temporal evolution of the voltage in relation to the phase transition of the load.

Dielectric exposed on DC-pulsed helium plasma jet: Hollow distribution of the surface electric field

In this study, the electric field distribution on the dielectric surface exposed to helium atmospheric pressure plasma jets with varying operating parameters was investigated. The study considered the effect of applied voltage (7–10 kV), pulse width (0.9–150 μs), and rate (400–2000 sccm), with the investigation based on the Pockels effect. The results suggest that the surface electric field arises from the charge transfer by ionization waves and the expansion of surface discharges. The evolution of the electric field can be segmented into three phases: establishment, maintenance, and dissipation. The gas flow rate determines the mixing “boundary” between helium and air, and consequently affects the shape of the surface electric field distribution. At 400 sccm, the field structure exhibits a solid, elongated shape. At 800–1200 sccm, the field takes on a hollow-ring appearance, and at 1600–2000 sccm, it is presented as two channels. As the applied voltage increases, the surface discharge can be extended further, even surpassing the designated boundary. At 10 kV, the maximum surface electric field and total deposited charge are 5.9 kV/cm and 4.3 nC, respectively. At a fixed frequency of 5 kHz, changing the pulse width affects charging time. The maximum value of the surface electric field and the total deposited charge exhibits an initial increase followed by a decrease. Short pulses (0.9–2 μs) result in incomplete charge transfer, while long pulses (100–150 μs) form a “dark channel” of predominantly negative charge at falling edge of the pulse, which neutralizes the positive charge in the subsequent pulse. A pulse width of 20 μs allows complete charge transfer with less influence from the dark channel.

AC zero crossing compensation for thermal stress linear birefringence in optical voltage transducer

The optical voltage transducer (OVT) based on the Pockels effect has a perennial issue of the thermal stress linear birefringence (TSLB) in the electro-optic crystal being overlapped with the electro-optic phase delay, which is difficult to be eliminated and seriously affects the measurement accuracy and reliability. In this paper an AC zero crossing compensation method for TSLB is proposed. It is based on that when the steady-state AC voltage crosses zero points the electric field is zero, and the electro-optic crystal does not have any electro-optic effect, so the electro-optic phase delay is zero. In this case, the output of the OVT is an additional phase delay caused by the TSLB, which can be separated and compensated by the linear demodulation OVT and the AC zero crossing detection circuit. The experimental results show that in the temperature range of −40 °C to 70 °C, the measurement accuracy of the OVT after TSLB compensation is improved from 0.5 class to 0.2 class. This method is helpful for the practicality of the OVT.

Enhanced BaTiO3/Si3N4 integrated photonic platform with VO2 technology for large-scale neuromorphic computing [Invited

The hybrid barium titanate (BaTiO3 or BTO) – silicon nitride (Si3N4 or SiN) platform integrated on silicon has been established as a promising candidate for implementing photonic integrated circuits with unique features in terms of high modulation speeds and low transmission loss. However, despite the high Pockels effect in BTO, switching device footprints are relatively large, which could compromise their use in applications with large scalability requirements, such as neuromorphic computing hardware. To address this limitation, we propose the integration of vanadium dioxide (VO2) in the SiN/BTO platform to enable ultra-compact amplitude switching devices offering a scalar multiplication functionality with multilevel operation. More concretely, an electronically reprogrammable switching device with a 5-bit amplitude encoding capability and an insertion loss of only 0.5 dB is developed. The proposed device is built with a 9-µm-long VO2/SiN/BTO waveguide structure integrated with an efficient microheater using a transparent conducting oxide. Such a vital building block would offer significant potential for developing more complex photonic integrated circuits, including dot-product or matrix-vector multiplication engines. The combination of high speed, low loss, and reduced footprint makes the proposed enhanced platform an attractive solution for application in scalable and energy-efficient neuromorphic computing hardware.

DFT molecular simulations for static, dynamic and solvent-dependent nonlinear optical properties of triphenylamine-carbazole-based organic dyes with D-D-A framework

Nonlinear optical (NLO) materials are vital for advancing cutting-edge optoelectronic technology. Herein, triphenylamine-carbazole-based organic dyes with a D-D-A framework are quantum chemically designed with acceptor modifications, and their geometric, electronic, static, dynamic, and solvent-dependent NLO properties are explored using density functional theory (DFT) simulations. The IEFPCM model is employed to investigate the solvent effect on the NLO properties of proposed compounds (BSA-1 to BSA-9). Interestingly, the dipole moment (μtot), average linear polarizability (αtot), and first hyperpolarizability (βtot) were determined for all proposed compounds in gas phase and water (ε = 78.36), DMSO (ε = 46.83), methanol (ε = 32.61), tetrahydrofuran (ε = 7.43), chloroform (ε = 4.71), and benzene (ε = 2.27) solvents. Natural population analysis (NPA), transition density matrix (TDM), UV–Vis investigation, frontier molecular orbital (FMO), and topological analysis (MEP, ELF, and LOL) were carried out to explain the NLO findings. The topological study suggests that BSA and BSA-1 to BSA-9 are chemically active and may have NLO implications. Moreover, the findings demonstrate that BSA-9 is the most promising second-order NLO candidate because of the highest λmax (418.60 nm) in dichloromethane (DMF) and the lowest energy gap (1.49 eV) among all the derivatives. It was found that BSA-2 and BSA-9 demonstrated the giant static αo (7.26 × 102) and βo (9.91 × 104) values among the proposed compounds for use in a wide range of solvent-dependent NLO applications. The solvent effect on NLO in all proposed compounds is noticeable, especially in BSA-9, which exhibited βo = 1.89 × 105 in benzene, 2.80 × 105 in THF, 3.21 × 105 in methanol, 3.25 × 105 in DMSO and 3.28 × 105 in water. Furthermore, Electro-optical Pockels effect (EOPE) β(-ω,ω,0) and the second harmonic generation (SHG) β(-2ω,ω,ω) results computed at ω = 532 nm (0.0856 au) and ω = 1064 nm (0.0428 au) frequencies confirm that all proposed compounds are excellent NLO candidates and should be targeted for future static, frequency-dependent and solvent-dependent NLO applications.

Resource-efficient low-loss four-channel active demultiplexer for single photons

We report a design and implementation of a resource-efficient spatial demultiplexer which produces four indistinguishable photons with efficiency of 39.7% per channel. Our scheme is based on a free-space storage/delay line which accumulates four photons and releases them by a controlled polarization rotation using a single Pockels cell.

Resource-efficient low-loss four-channel active demultiplexer for single photons

We report a design and implementation of a resource-efficient spatial demultiplexer which produces four indistinguishable photons with efficiency of 39.7% per channel. Our scheme is based on a free-space storage/delay line which accumulates four photons and releases them by a controlled polarization rotation using a single Pockels cell.

Investigation of internal electric fields in graphene/6H-SiC under illumination by the Pockels effect.

In this paper, we introduce a method for mapping profiles of internal electric fields in birefringent crystals based on the electro-optic Pockels effect and measuring phase differences of low-intensity polarized light. In the case of the studied 6H-SiC crystal with graphene electrodes, the experiment is significantly affected by birefringence at zero bias voltage applied to the crystal and a strong thermo-optical effect. We dealt with these phenomena by adding a Soleil-Babinet compensator and using considerations based on measurements of crystal heating under laser illumination. The method can be generalized and adapted to any Pockels crystal that can withstand sufficiently high voltages. We demonstrate the significant formation of space charge in semi-insulating 6H-SiC under illumination by above-bandgap light.

Vibrational contributions to the electro-optic effect of BBO, KTP and RTP crystals

Barium borate (Ba3(B3O6)2, low temperature phase, BBO), potassium titanyl phosphate (KTiOPO4, KTP) and rubidium titanyl phosphate (RbTiOPO4, RTP) are not only best performing nonlinear optical materials but also show preferable electro-optic (EO) Pockels effect that serve as practical EO modulators widely applied in laser technology. Disparity to the repeatedly measurements of their properties related to the EO effects, the mechanism leads to their large EO coefficients have never been fully studied theoretically from first-principles calculations. In this paper, using an ab initio program CRYSTAL based on a linear combination of atomic orbitals approach, the vibrational spectra of the title crystals were firstly obtained. Then the electronic and vibrational contributions to the EO effect were calculated through coupled perturbed Kohn–Sham approximation and variations of atomic displacements. The results show that the EO coefficients, as well as other properties of dielectric constants, SHG coefficients and IR and Raman spectra of the three crystals are in excellent agreement with the experimental values. It is also found that the EO coefficients of all the crystals studies are dominantly contributed from few vibrational modes at low frequencies and the vibrational contributions amount to about 60% in BBO, 90% in KTP and RTP. General rules of finding crystals with large EO effects are finally proposed based on present study and previous studies on LiNbO3 crystals.

Pockels Effect at the Interface between Water and Ti Electrode

The Pockels coefficient of interfacial water between bulk water and a Ti electrode was estimated from the electroreflectance spectra (ΔR/R) to be r13≈−150 pm/V as the maximum value of magnitude, which is comparable in magnitude to the largest coefficient for electrode interfacial water, i.e., 200 pm/V for interfacial water on a transparent oxide electrode. This Pockels signal increased by a factor of about ±3 by applying a DC bias voltage of ±1 V. The reflectance (R) of the Ti electrode had a dip structure in the UV region (3.5–4.5 eV) due to the interference of a 14 nm thick surface oxide film, and the ΔR/R spectra in aqueous electrolyte solution showed a large reflectance change in the UV region with a dispersive shape due to the contribution of the TiO2 film. The reproducibility of the electroreflectance experiment was high, suggesting that the surface oxide film contributes to the large Pockels effect of interfacial water and the robustness of the electrode.

External field intensity and wavelength dependency from IR to deep UV of linear and nonlinear optical properties of 1-butyl-3-methylimidazolium dicyanamide ionic liquid

The electronic structure, inter-ionic interactions and charge transfer, polarizability, and first-order and second-order hyperpolarizabilities of 1-butyl-3-methylimidazolium dicyanamide ionic liquid have been explored via modeling at M06-2X/6-311++G(d,p) level of theory. External field intensity dependence of electronic properties was investigated by utilization of field intensity from 0.0001 to 0.002 au applied in the direction of anion-cation. Interionic interactions have been characterized through the reduced density gradient method to reveal and distinguish hydrogen bonding and other non-covalent interactions. Extraordinary bonding interactions were observed between the anion and ring system of the cation. Electronic excitation characteristics were analyzed based on the inter-fragment charge transfer method to distinguish charge transfer contributions from local excitations. Static and dynamic nonlinear optical parameters namely the total dipole moment μ, the mean polarizability α(0,0) and α(−ω; ω), the anisotropy of polarizability Δα(0,0) and Δα(−ω; ω) the first hyperpolarizability in the direction of dipole moment β(0,0,0), β(−ω; ω,0), and β (−2ω; ω,ω), the average second hyperpolarizability γ(0;0,0,0), γ(−ω; ω,0,0) and γ(−2ω; ω,ω,0) have been calculated for specific laser wavelengths corresponding to the UV, visible, and infrared region. Thus, a novel way of mapping the field intensity and wavelength dependence of NLO properties was achieved. The hyperpolarizability calculations indicate that 1-butyl-3-methylimidazolium dicyanamide has a significant potential to be used in electro-optics Pockels effect, second-harmonic generation, and electro-optical Kerr effect applications. NLO property maps indicate that the wavelength dependence in the IR and visible region is almost ignorable when compared with that of the UV region where the first hyperpolarizabilities can increase by ten thousand and the second hyperpolarizabilities can reach up to ten billion times.

Review of Surface Charge Accumulation on Insulators in DC Gas-Insulated Power Transmission Lines: Measurement and Suppression Measures

Gas-insulated power transmission lines (GILs) can replace cables and overhead transmission lines, playing an important role in DC transmission systems. However, the influence of surface charge accumulation on insulation reliability cannot be ignored as the operational voltage of the DC GIL increases. In this paper, the measurement methods for the insulator surface potential are summarized, including, dust maps, the Pockels effect method, and the electrostatic probe method. Then, a typical surface charge inversion algorithm is introduced. The main influencing factors of surface charge accumulation are analyzed, such as the applied voltage, insulation gas, insulator shape, and temperature. The charge accumulation pathway is revealed. Furthermore, methods for inhibiting the accumulation of surface charges and promoting the dissipation of accumulated charges are introduced to reduce the surface charges on insulators. Finally, the development direction of DC GIL insulators is predicted. We anticipate that the online monitoring of surface charge distribution, clarifying the percentage of charge accumulation pathways, and optimizing the insulator casting process will be the research directions for the insulator surface charge topic in the future. This article provides a comprehensive understanding of the surface charges of GIL insulators and a reference for the insulation design of DC GILs.

RF-sputtered Z-cut electro-optic barium titanate modulator on silicon photonic platform

Epitaxial BaTiO3 integrated on Si or Si-on-insulator using off-axis radio frequency sputtering is a promising material platform for building electro-optic modulators based on the Pockels effect. Barium titanate thin films with c-axis orientation have been epitaxially integrated on silicon-on-insulator wafers. They exhibit excellent structural quality with Pockels coefficient (r33) > 130 pm/V and propagation loss <2 dB/cm. Our results show that off-axis sputtered BaTiO3 films yield electro-optic modulation similar to that of high-quality films grown by molecular beam epitaxy and that the material is suitable for implementation of low-power Mach–Zehnder interferometer electro-optic modulators integrated on silicon in a Z-cut configuration.

High density lithium niobate photonic integrated circuits

Photonic integrated circuits have the potential to pervade into multiple applications traditionally limited to bulk optics. Of particular interest for new applications are ferroelectrics such as Lithium Niobate, which exhibit a large Pockels effect, but are difficult to process via dry etching. Here we demonstrate that diamond-like carbon (DLC) is a superior material for the manufacturing of photonic integrated circuits based on ferroelectrics, specifically LiNbO3. Using DLC as a hard mask, we demonstrate the fabrication of deeply etched, tightly confining, low loss waveguides with losses as low as 4 dB/m. In contrast to widely employed ridge waveguides, this approach benefits from a more than one order of magnitude higher area integration density while maintaining efficient electro-optical modulation, low loss, and offering a route for efficient optical fiber interfaces. As a proof of concept, we demonstrate a III-V/LiNbO3 based laser with sub-kHz intrinsic linewidth and tuning rate of 0.7 PHz/s with excellent linearity and CMOS-compatible driving voltage. We also demonstrated a MZM modulator with a 1.73 cm length and a halfwave voltage of 1.94 V.

Engineering the electro-optic effect in HfO2 and ZrO2 through strain and polarization control

The ability to control the optical properties of a material with an electric field has led to optical memory devices, communication systems, optical signal processing, or quantum cryptography. Understanding electro-optic effects, especially in thin films, would improve the efficiency of these applications. In particular, the influence of epitaxial strains is of prime importance. In addition, the active control of these effects would be of great interest to tailor the material to the desired performance. Here, we demonstrate through first-principle calculations that the linear electro-optic response (Pockels effect) of two silicon-compatible ferroelectrics is stable with respect to bi-axial strain and that the electro-optic response can be strongly enhanced through the electrical control of the polarization. We attribute the former to the lack of optical phonon softening and a weak elasto-optic response and the latter to the externally induced softening of a phonon of symmetry A1. Our results are readily applicable to other polar materials and show that the electro-optic effect can be efficiently engineered to meet the performance criteria of future technologies.

Heterogeneous integration of a high-speed lithium niobate modulator on silicon nitride using micro-transfer printing

Integrated photonic systems require fast modulators to keep up with demanding operation speeds and increasing data rates. The silicon nitride integrated photonic platform is of particular interest for applications such as datacom, light detection and ranging (LIDAR), quantum photonics, and computing owing to its low losses and CMOS compatibility. Yet, this platform inherently lacks high-speed modulators. Heterogeneous integration of lithium niobate on silicon nitride waveguides can address this drawback with its strong Pockels effect. We demonstrate the first high-speed lithium niobate modulator heterogeneously integrated on silicon nitride using micro-transfer printing. The device is 2 mm long with a half-wave voltage Vπ of 14.8 V. The insertion loss and extinction ratio are 3.3 and 39 dB, respectively. Operation beyond 50 GHz has been demonstrated with the generation of open eye diagrams up to 70 Gb/s. This proof-of-principle demonstration opens up possibilities for more scalable fabrication of these trusted and performant devices.

A Novel PWM Signal-Generation Strategy for Pockels Cell Drivers

A Pockels cell driver (PCD) can be viewed as a high-voltage pulse width generator for controlling the bi-refringence effect of electro-optical crystals. The main features of a PCD include a high repetition rate, fast on and off switching, variable pulse duration, and a true square pulse shape. The most commonly used PCD has a narrow pulse width tuning range, typically within a few microseconds. In this paper, we propose a PCD based on a novel pulse width modulation (PWM) signal-generation strategy that can continuously adjust its pulse width with a minimum step size of 10 ns and no restriction on the maximum width. Therefore, it is easily compatible with both “On-type” and “Off-type” applications of the electro-optic crystal quarter-wave voltage. The experimental results show that the rising and falling times of the proposed PCD are approximately 7.3 ns and 7.8 ns, respectively, with a maximum repetition rate of 1 MHz and a maximum voltage of approximately 2.0 kV. Finally, the functionality of the PCD is demonstrated in a home-built slab laser.

1.9 µJ external-cavity dumped ultra-broad-area semiconductor nanosecond laser.

An external-cavity dumped nanosecond (ns) ultra-broad-area laser diode (UBALD) at around 966 nm with high pulse energy is demonstrated. A 1 mm UBALD is used to produce high output power and high pulse energy. A Pockels cell (PC) combines with two polarization beam splitters (PBSs) and is employed to cavity-dump a UBALD operating at 10 kHz repetition rate. At a pump current of 23 A, 11.4 ns pulses with a maximum pulse energy of ≈1.9 µJ and a maximum peak power of ≈166 W are achieved. The beam quality factor is measured to be M x 2=19.5 in the slow axis direction and M y 2=2.17 in the fast axis direction. Moreover, maximum average output power stability is confirmed, with a power fluctuation of less than 0.8% rms over 60 min. To the best of our knowledge, this is the first high-energy external-cavity dumped demonstration from an UBALD.