Pockels Effect Research Articles

Ultrafast laser frequency tuning based on equivalent optical phase sampling and accumulation

Ultrafast laser frequency tuning is a crucial function in numerous applications, including light detection and ranging (LiDAR), optical coherence tomography (OCT), and spectroscopy. In general, laser frequency tuning is realized via the motion of mechanical structures, frequency-swept optical filters, or the Pockels effect in the laser cavity. Nevertheless, the maximum frequency tuning rate and sweep rate are smaller than 107 THz/s and 1 GHz, respectively, due to the inherent speed limitation of the existing tuning mechanisms. Here, we propose and demonstrate a brand-new concept for ultrafast laser frequency tuning, which is realized based on equivalent optical phase sampling and accumulation in the laser cavity. By inserting an electro-optic phase modulator (PM) into the laser cavity and properly setting the period of the driving signal applied to the PM to make its integer multiple slightly deviate from the round trip time of the laser cavity, a linear mapping from the voltage of the driving signal to the output laser frequency is realized via equivalent optical phase sampling and accumulation. This remarkable feature provides a rapid way to manipulate the laser frequency, breaking the frequency tuning speed limitation of the existing approaches. In the experiment, a record-breaking frequency tuning rate of 1.522 × 109 THz/s and a sweep rate of 2223.97 MHz are simultaneously realized. Moreover, the center wavelength and the frequency tuning range can be easily reconfigured. This work paves the way for ultrafast laser frequency tuning in a customized wavelength range.

Electro-optic effect in hexagonal compounds RFeO<sub>3</sub>

The linear electro-optic effect is investigated in films of RFeO3 compounds (R = Y, Lu, Yb) possessing hexagonal symmetry. A general equation for the optical indicatrix of the film at arbitrary orientation of an external electric field is derived. The equation of the indicatrix in the principal axes, as well as the principal refractive indices, are determined for the particular case of an external electric field orientation along the [010] axis. Two novel optical axes are identified in the presence of an electric field. It was determined that two isonormal modes propagate in the film when light propagates along the [100] direction. The refractive indices associated with these modes were also found. The phase shift associated with the propagation of these modes and caused by the presence of an electric field is calculated. The shift is linear along the electric field, thereby demonstrating the Pockels effect.

Research on the Modulation Characteristics of LiNbO3 Crystals Based on the Three-Dimensional Ray Tracing Method

To further study the electro-optical modulation characteristics of LiNbO3 crystals and analyze their modulation performance, a method for studying the modulation characteristics of LiNbO3 crystals, based on the three-dimensional ray tracing method, is introduced. With the help of the refractive index ellipsoidal theory, the optical properties of LiNbO3 crystals under the influence of the Pockels effect are systematically investigated. The research results show that the optical properties of LiNbO3 crystals under the action of an external electric field can be divided into two cases: the crystal optical axis is parallel to the clear light direction, and the crystal optical axis is perpendicular to the clear light direction. Subsequently, starting from Maxwell’s equation and the matter equation, the analytical expressions of optical parameters such as refractive index, wave vector, light vector, optical path, and phase delay in electro-optical crystals are derived. Finally, the propagation law of LiNbO3 crystals when the light is incident in any direction, i.e., when the optical axis of the crystal is parallel to the clear direction and perpendicular to the clear direction, and the light intensity and field of view of the LiNbO3 crystal for electro-optical modulation are discussed.

Modeling and optimization of photonic integrated Silicon-LiNbO3 hybrid interferometer for E-S-C-L wavelength band

Abstract The Silicon on Insulator (SOI) is a potential technology for thin-film optical waveguide, enabling the design of optical interconnects, including modulators and interferometers. The manuscript presents the broadband modulator for phase and intensity modulation. The nanoscale optimized interferometric modulator utilizes Silicon (Si) and Electro-optic material Lithium Niobate (LiNbO3) as the guiding material to work in terahertz regime, without any bends in the structure. Optimization of the structure is validated by the evaluation of the confinement factor at the operational wavelength of 1550 nm using Finite Element Method (FEM) based simulation. To excite the SOI interferometric modulator, an external RF signal with a maximum amplitude of 5 V is used. The hybrid structure of Si and LiNbO3-based modulators shows the electro-optic phenomenon caused by nonlinearities, namely the Pockels effect and Kerr effect, which in turn changes the effective mode index, providing π phase shift along with −0.92 dB/cm absorption loss, 117 μm electrode length at V π Volts and 265 nm 3dB optical bandwidth covering the E-S-C-L optical bands. The device footprint calculated is 667 μm2, whereas the active region footprint is ∼46.8 μm2.

An alexandrite laser system for positronium laser cooling

We report on a Q-switched alexandrite based ∼100ns long pulse duration ultra-violet laser system. The central wavelength of the fundamental pulse is set by a Volume Bragg Grating in reflection and can be tuned between 728nm and 742nm. The spectral bandwidth is ∼130GHz. This laser system was designed in view of Doppler cooling of a cloud of a near room temperature positronium by strongly saturating the 13S–23P transition. In addition, we report on the development of a KD*P Pockels cell driver designed to both Q-switch the cavity and induce a sharp falling edge of the laser pulse so that the end of the positronium-laser interaction time can be controlled with nanosecond precision.

Ultralarge Hyperpolarizability, Novel Ladder‐Type Heteroarenes Electro‐Optic Chromophores: Influence of Fused Heterocyclic π‐System and Push–Pull Effect on Nonlinear Optical Properties

ABSTRACTA new series of ladder‐type heteroarenes electro‐optic (EO) chromophores based on unique fused oligothiophene, oligofuran, and oligopyrrole bridge with a strong push–pull effect were designed, and their distinctive electric structure and nonlinear optical (NLO) properties were investigated by multiple methods, including DOS analysis, CPKS method, SOS model, TSM model, and hyperpolarizability density analysis. Interestingly, it is found that the introduction of different strengths of donor and acceptor moieties, the nature of ladder‐type heteroarenes bridge, and the medium polarity play critical roles in achieving ultra‐large hyperpolarizabilities. Encouragingly, the DN1‐6Fu‐AS1 and DN1‐6Py‐AS1 both exhibit ultra‐large βprj values (up to 12965.5 × 10−30 esu) and βtot value (up to 13238.2 × 10−30 esu), owing to their exceptionally low transitions energy and large variation of dipole moment. The DO2‐6Fu‐AO2, DO2‐6Th‐AO2, DO2‐6Fu‐AS1, and DO2‐6Th‐AS1, exhibit very deep HOMO levels, which are expected to have superior air stability and effectively prevent large leakage currents. Hirshfeld surface analysis and interaction region indicator analysis were employed to explore the noncovalent interactions in the dimer. The DO2‐6Fu‐AS1 and DO2‐6Py‐AS1 show the outstanding electro‐optical Pockels effect, and DN1‐6Fu‐AO2 exhibit large SHG responses. These unique and brand‐new push‐pull ladder‐type heteroarenes chromophores are promising candidates for chip‐scale EO modulators, and electronic and photonic devices in emerging communication, energy, analog, and digital technologies.

Unveiling the Pockels coefficient of ferroelectric nitride ScAlN

Nitride ferroelectrics have recently emerged as promising alternatives to oxide ferroelectrics due to their compatibility with mainstream semiconductor processing. ScAlN, in particular, has exhibited remarkable piezoelectric coupling strength (K2) comparable to that of lithium niobate, making it a valuable choice for RF filters in wireless communications. Recently, ScAlN has sparked interest in its use for nanophotonic devices, chiefly due to its large bandgap facilitating operation in blue wavelengths coupled with promises of enhanced nonlinear optical properties such as a large second-order susceptibility (χ(2)). It is still an open question whether ScAlN can outperform oxide ferroelectrics concerning the Pockels effect—an electro-optic coupling extensively utilized in optical communications devices. In this paper, we present a comprehensive theoretical analysis and experimental demonstration of ScAlN’s Pockels effect. Our findings reveal that the electro-optic coupling of ScAlN, despite being weak at low Sc concentration, may be significantly enhanced and exceed LiNbO3 at high levels of Sc doping, which points the direction of continued research efforts to unlock the full potential of ScAlN.

Integrated Pockels Modulators on Silicon Photonics Platform

AbstractElectro‐optic (EO) modulators are essential components in various fields, including optical communication, free‐space communication, microwave photonics, sensing, and light detection and ranging. The EO modulation enables the fast conversion of electric signals into optical signals, facilitating the precise manipulation of light. With advancements in fabrication processing techniques, next‐generation integrated EO modulators have demonstrated substantial improvements in modulation efficiency, bandwidth, and footprint. Here, the latest research progress in integrated EO modulation, focusing on the principle of the Pockels effect, key modulation metrics, novel EO thin‐film material platforms, and innovative device architectures is overviewed. Finally, it is evaluated different schemes and provide perspectives on future trends in developing integrated EO modulators, highlighting both the advantages and challenges of integrated EO modulation, including waveguide and electrode engineering, integrated methods, and other applications for large‐scale photonic integrated circuits.

Surface charge characteristics in a three-electrode surface dielectric barrier discharge

The surface charge characteristics in a three-electrode surface dielectric barrier discharge (SDBD) are experimentally investigated based on the Pockels effect of an electro-optical crystal. The actuator is based on the most commonly used SDBD structure for airflow control, with an exposed electrode supplied with sinusoidal AC high voltage, a grounded encapsulated electrode and an additional exposed electrode downstream supplied with DC voltage. The ionic wind velocity and thrust can be significantly improved by increasing DC voltage although the plasma discharge characteristics are virtually unaffected. It is found that the negative charges generated by the discharge of the three-electrode structure accumulate on the dielectric surface significantly further downstream in an AC period compared to the actuator with a two-electrode structure. The negative charges in the downstream region increase as the DC voltage increases. In addition, the DC voltage affects the time required for the positive charge filaments to decay. The positive DC voltage expands the ionic acceleration zone downstream to produce a greater EHD force. The amplitude of the DC voltage affects the electric field on the dielectric surface and is therefore a key factor in the formation of the EHD force. Further research on the surface charge characteristics of a three-electrode structure has been conducted using a pulse power to drive the discharge, and the same conclusions are drawn. This work demonstrates a link between surface charge characteristics and EHD performance of a three-electrode SDBD actuator.

Double rectangular-grooves metasurface for highly efficient electric modulation

With the rapid development of optical communication, how to achieve efficient modulation (fast response speed and high modulation depth) of optical signals has attracted more and more attention from researchers. Among all electro-optical modulator (EOM) designs, the electro-optical metasurface is undoubtedly a competitive solution for optical signal modulation in free space. Although current research on electro-optical metasurfaces has realized improving response speed owing to the Pockels effect, there are still difficulties in achieving high modulation depth under CMOS-compatible voltage and developing rational designs of metasurfaces to achieve voltage application that trigger electro-optical effects. In this work, an ultrahigh-Q factor BaTiO3 (BTO) electro-optical metasurface, which consists of a periodic array of rectangular grooves, was designed to provide a feasible solution to address these shortcomings. Based on bound states in the continuum (BIC) theory, ultrahigh-Q factor (2.87 × 105) quasi-BIC (Q-BIC) was obtained around 1550 nm by breaking the in-plane symmetry of the two rectangular grooves in a unit cell, which could significantly deepen the modulation depth. The concave and continuous structure of rectangular grooves made the application of voltage more efficient. The simulation results show that an optical signal modulation in free space with a modulation depth of 100% could be achieved. Multipole decomposition indicated that toroidal dipole (TD) was dominant in this Q-BIC. Our work may further promote the development of electro-optical modulation towards faster and deeper modulation.

Exploring the impact of heterocyclic bridges for tuning the amplitudes of Nonlinear optical properties under Gas, IEFPCM and COSMO solvent models

Nonlinear optical (NLO) materials are a distinctive class of materials due to their NLO response and wide range of applications. We present a systematic quantum chemical study for designing triphenylamine based derivatives (1Ph to 3PhDTDZ) by modifying central bridge with various size and types of heterocyclic rings. Density functional theory (DFT) method is used to calculate the NLO properties including linear and third-order NLO polarizabilities (α and γ) by using M06-2X functional coupled with the 6-311G** basis set. The calculation results predict that among the designed compounds, 1PhDTDZ has the highest third-order NLO polarizability of 2347.7 x 10−36 esu and lowest transition energy of 1.828 eV. On the other hand, 3PhDTDZ has a greater amplitude (108.5 x 10−24 esu) of static linear polarizability (αiso). We also computed frequency-dependent third-order NLO polarizability values at various laser wavelengths (1500 nm to 1970 nm). This provides valuable information about the complex interaction between light and matter. Dynamic second hyperpolarizability of electro-optic Pockels effect (EOPE effect, (γ (−ω; ω,0,0)) is calculated at 1500 nm laser wavelength with a larger γ-amplitude of 36708.4 x 10−36 esu. The effect of solvent on α and γ has also been the primary focus of this research. An extensive solvation impact has been studied using integral equation formalism Polarizable Continuum Model (IEFPCM) and Conductor-like Screening Model (COSMO). Magnitudes of αiso and 〈γ〉 have shown variation from gas to solvent methods. While among solvents, amplitudes are seen 3.87 % to 11.29 % greater under the effect of COSMO-methanol than PCM-methanol. The Frontier molecular orbital (FMO) studies found a significant decrease in energy band gaps from 5.75 eV to 2.98 eV. Since our systems exhibit remarkable photovoltaic properties, they can be used as dye-sensitized solar cells (DSSCs). Their open circuit voltage (VOC) ranging from 1 eV to 4 eV. Thus, our designed triphenylamine based derivatives with distinct central heterocyclic bridges showcase remarkable efficiency as NLO materials, with additional benefits of exhibiting good photovoltaic properties.

Cavity-dumped femtosecond Yb:CALYO laser based on SESAM assisted Kerr-lens mode locking.

In this paper, we present the demonstration of a cavity-dumped Kerr-lens mode-locked femtosecond oscillator based on an Yb:CALYO crystal for the first time. With the assistance of an SESAM, the Kerr-lens mode-locked Yb:CALYO laser delivered pulses with an average power of 2.8 W at a repetition rate of 61.3 MHz, corresponding to a pulse energy of 45.6 nJ. By employing a Pockels Cell consisting of dual β-BBO crystals and a thin film polarizer for cavity dumping, laser pulses of 280 nJ were achieved at both 1 MHz and 10 kHz repetition rates. The shortest pulse duration measured was 233 fs without external compression, resulting in a peak power of 1.06 MW.

Octave-spanning Kerr soliton frequency combs in dispersion- and dissipation-engineered lithium niobate microresonators

Dissipative Kerr solitons from optical microresonators, commonly referred to as soliton microcombs, have been developed for a broad range of applications, including precision measurement, optical frequency synthesis, and ultra-stable microwave and millimeter wave generation, all on a chip. An important goal for microcombs is self-referencing, which requires octave-spanning bandwidths to detect and stabilize the comb carrier envelope offset frequency. Further, detection and locking of the comb spacings are often achieved using frequency division by electro-optic modulation. The thin-film lithium niobate photonic platform, with its low loss, strong second- and third-order nonlinearities, as well as large Pockels effect, is ideally suited for these tasks. However, octave-spanning soliton microcombs are challenging to demonstrate on this platform, largely complicated by strong Raman effects hindering reliable fabrication of soliton devices. Here, we demonstrate entirely connected and octave-spanning soliton microcombs on thin-film lithium niobate. With appropriate control over microresonator free spectral range and dissipation spectrum, we show that soliton-inhibiting Raman effects are suppressed, and soliton devices are fabricated with near-unity yield. Our work offers an unambiguous method for soliton generation on strongly Raman-active materials. Further, it anticipates monolithically integrated, self-referenced frequency standards in conjunction with established technologies, such as periodically poled waveguides and electro-optic modulators, on thin-film lithium niobate.

An optical measurement method of DC total electric field with space charges based on probe self-rotation modulation

Researching the measurement of DC total electric field with space charges (Poisson field) can help establish a monitoring system for the electromagnetic environment of UHVDC transmission lines and protect the health of the ecological environment. This paper proposes a DC total electric field optical measurement method based on the Pockels effect and probe self-rotation modulation. It can address the issues of internal charge drift and external charge accumulation in traditional optical probes when measuring DC electric fields with space charges. Theoretical analysis was conducted on measurement under probe rotation, and the optical modulation system was integrated into a probe with dimensions of 60 mm × 30 mm × 20 mm. A self-rotating platform for the probe was designed, and the influence of rotation mode on the results was analyzed. It was found that the sensitivity was higher when using the horizontal-horizontal mode. The signal processing system was tested, showing that the probe retained around 50 % of its original output when rotating at low speeds (frequency 1–3 Hz), fully meeting the measurement requirements. The test platform has been constructed, and the probe’s sensitivity was assessed under an AC electric field and validated under a DC electric field. Finally, the optical method proposed in this paper was used to measure the DC total electric field with space charges under a small model of a unipolar HVDC transmission line. Five locations under the line were selected as measurement points, and the results were compared with those obtained from a field mill probe and numerical simulation. This comparison indicates that the proposed method agrees with experimental and simulation results. The average sensitivity of the self-rotating optical probe was 3.71 mV/(kV/m). The error did not exceed 5 % in high-field strength areas (above 20 kV/m) and was also controlled within 15 % in low-field strength areas (below 20 kV/m).

Exploring the ON/OFF nonlinear optical (NLO) response in tetracene-based chromophores: A DFT study on solvent and frequency-dependent NLO properties for switching applications

Tetracene-based nonlinear optical (NLO) materials are an ambient field of interest in optoelectronic technology. Inspired by this material, we theoretically designed D-D-A framework molecules (ZO and ZO-1 to ZO-16) with auxiliary donor and acceptor screening of solo tetracene linear surface and investigated their reactivity, electrical properties, solvent-dependent optical and ON/OFF NLO properties utilizing DFT simulations. The CPCM and IEFPCM model is used to examine how entire spectrum of medium polarity, ranging from less polar to more polar solvents (chloroform (ε = 4.71) < tetrahydrofuran (ε = 7.43) < DMSO (ε = 46.83 < water (ε = 78.36)) affect the optical absorption and NLO characteristics (βtot, αtot, and μtot) of suggested compounds (ZO and ZO-1 to ZO-16). Furthermore, natural bond orbital analysis (NBO) and topological analysis (MEP, ELF, and LOL) were used to explain the NLO results. Regarding the charge transfer robustness, ZO-9, a promising tetracene derivative, is identified as an appealing second-order NLO candidate, with the lowest energy gap (1.642 eV) and largest λmax (546.692 nm) among all investigated derivatives. Moreover, the solvent-dependent optical absorption spectrum shows that polar solvent (THF) alleviates the λmax from 273.081 nm to 707.511 nm in ZO-11. Similarly, solvent also influences the NLO properties in all designed compounds, especially in ZO-9, which displayed giant βtotal = 7.244 × 105 in chloroform, 8.640 × 105 in THF, 1.153 × 106 in DMSO, and 1.180 × 106 in water as well as in gas (1.993 × 105). Additionally, the results of the second harmonic generation (SHG) β(−2ω, ω, ω) and the electro-optical Pockels effect (EOPE) β(−ω,ω,0) computed at the frequencies of ω = 1064 nm (0.0428 au) and ω = 532 nm (0.0856 au) validates that all tetracene based derivatives are excellent NLO candidates for laser working condition and switching applications.

Phenyl-urea chromophores: DFT insights into nonlinear optical enhancement

The present investigation has examined the nonlinear optical (NLO) responses of molecules based on phenyl-urea. The optimized geometries of complexes: (4-nitrophenyl) urea (PU1), 1-methyl-3-(4-nitrophenyl) urea (PU2), and 1,1-dimethyl-3-(4-nitrophenyl) urea (PU3) are extracted using M06–2X/6–311++ g (d, p) level of theory. The estimated nonlinear optical responses (NLO) of the compounds given are based on static and dynamic hyperpolarizabilities, which are reinforced by the dipole moment, polarizabilities, global reactivity parameters, HOMO-LUMO gap, molecular electrostatic potentials (MEP), and βvec. Comparing PU1, PU2, and PU3 to reference substances like urea and p-nitroaniline, a considerably higher NLO activity is estimated. Out of all the compounds, PU3 has the largest NLO response. Such a significant NLO reaction can be attributed to a small HOMO-LUMO gap and high charge transfer. Second-harmonic generation (SHG) and electro-optic Pockel's effect (EOPE) are being explored for dynamic NLO responses. These compounds that are bases of phenyl-urea exhibit a very significant quadratic nonlinear optical response.

Strong terahertz pulse induced Pockels and Kerr effect in crystalline quartz for ultrafast pulse switching

Ultrafast pulse switching is one of the key elements for ultrahigh speed communication technology. We study the terahertz (THz) induced birefringence response on the laser pulse through the quartz with different THz electric field strength. The magnitude of the observed Pockels signals scales linearly with the THz field amplitude, while the Kerr signals scale quadratically with the THz field amplitude. We demonstrate that the quartz is a good candidate for polarization modulation of 800 nm laser pulse, which has the advantages of low-cost, large bandgap, and negligible dispersion. Furthermore, our investigation finds application beyond ultrafast polarization switching, and the THz-induced polarization gating technique works as a tool for intense THz pulse detection.

Mode-folded thin-film lithium niobate phase shifter for channel scalable optical interconnect with high switching efficiency.

Paralleled optical interconnection has been widely used in optical transceivers. Reconfigurable photonic integration with scalable channel numbers is thus highly useful in timely adjusting the link capacity to the changing traffic patterns in data centers or high-performance computing (HPC) systems. In this paper, a 1×8 Mach-Zehnder switch (MZS) over thin-film lithium niobate is proposed and experimentally demonstrated for 1-to-8 channel scalable optical interconnects, with high switching efficiency through utilizing the mode-folded phase shifter. The three-mode phase shifter recirculates the light three times, undergoing phase changing with different waveguide modes. Simultaneously, the multimode waveguiding within the phase shifter results in a pronounced enhancement of the optical field confinement, further improving the Pockels effect for all modes. A 3.5-time improvement of the switching efficiency is experimentally demonstrated, exhibiting a low Vπ·L of 0.6 V⋅cm. The proposed MZS also features low channel crosstalk (below -20 dB over 1530-1565 nm) and nanosecond-order switching time, paving the way towards channel scalable and versatile MSA-compatible optical interconnect.

Low-frequency electric field sensing via superposition orbital angular momentum light.

The polarization and orbital angular momentum (OAM) degrees of freedom carried by light have important applications in precision optical measurement and optical sensing. Here we show that the electro-optic Pockels effect of a magnesium-doped lithium niobate (MgO:LiNbO3) crystal can be used to measure a low-frequency electric field. By exploiting the rotation property of superposition OAM light, we experimentally observe that the minimum measured precision of electric field intensity is about 0.18 V/m. This study offers a method to perform low-frequency electric field sensing.

Voltage-induced modulation of interfacial ionic liquids measured using surface plasmon resonant grating nanostructures.

We have used surface plasmon resonant metal gratings to induce and probe the dielectric response (i.e., electro-optic modulation) of ionic liquids (ILs) at electrode interfaces. Here, the cross-plane electric field at the electrode surface modulates the refractive index of the IL due to the Pockels effect. This is observed as a shift in the resonant angle of the grating (i.e., Δϕ), which can be related to the change in the local index of refraction of the electrolyte (i.e., Δnlocal). The reflection modulation of the IL is compared against a polar (D2O) and a non-polar solvent (benzene) to confirm the electro-optic origin of resonance shift. The electrostatic accumulation of ions from the IL induces local index changes to the gratings over the extent of electrical double layer (EDL) thickness. Finite difference time domain simulations are used to relate the observed shifts in the plasmon resonance and change in reflection to the change in the local index of refraction of the electrolyte and the thickness of the EDL. Simultaneously using the wavelength and intensity shift of the resonance enables us to determine both the effective thickness and Δn of the double layer. We believe that this technique can be used more broadly, allowing the dynamics associated with the potential-induced ordering and rearrangement of ionic species in electrode-solution interfaces.