Lithium Niobate Slab Research Articles

Heterogeneous optomechanical crystal cavity coupled by a wavelength-scale mechanical waveguide

Integrated optomechanical crystal (OMC) cavities provide a vital device prototype for highly efficient microwave to optical conversion in quantum information processing. In this work, we propose a novel heterogeneous OMC cavity consisting of a thin-film lithium niobate (TFLN) slab and chalcogenide (ChG) photonic crystal nanobeam coupled by a wavelength-scale mechanical waveguide. The optomechanical coupling rate of the heterogeneous OMC cavity is optimized up to 340 kHz at 1.1197 GHz. Combined with phononic band and power decomposition, 17.38% energy from the loaded RF power is converted into dominant fundamental horizontal shear mode (SH0) in the narrow LN mechanical waveguide. Based on this fraction, as a result, 3.51% power relative to the loaded RF energy is scattered into the fundamental longitudinal mode (L0) facing the TFLN-ChG heterogeneous waveguide. The acoustic breathing mode of the heterogeneous OMC is successfully excited under the driving of the propagating L0 mode in the heterogeneous waveguide, demonstrating the great potentials of the heterogeneous piezo-optomechanical transducer in high-performance photon–phonon interaction fields.

Unidirectional scattering of surface plasmon polaritons at the interface of indium-tin-oxide coated Fe-doped LiNbO3

We demonstrated, both experimentally and theoretically, surface plasmon polaritons (SPPs) can be unidirectionally scattered at the interface of indium-tin-oxide (ITO) coated iron doped Y-cut lithium niobate (Fe:LiNbO3, Fe:LN) slab. The multipole analysis referring to the spontaneous polarization was utilized to introduce an asymmetry to the Fe:LN/ITO system. Furthermore, with strong bulk photovoltaic effect of Fe:LN, the diffusion mechanism of space charge transport brought local and nonlocal responses to the scattering process, leading to the unidirectional scattering of SPPs. This work opens an opportunity in designing ultracompact integrated plasmonic circuits without needs to resort to lossy metals.

Modulating optical properties of Lithium Niobate through acoustic stress

Optical index modulation in Z-X Lithium Niobate slab through acoustic stress is theoretically studied. To generate acoustic stress, fundamental symmetric Lamb mode is excited in the slab. A periodic disturbance of period 858 nm in 500 nm thick slab is noticed when 5 GHz frequency Lamb wave mode is used. The displacement field pattern caused by 5 GHz acoustic disturbance is then estimated using potential techniques and applied to photo-elastic relations. Theoretical calculation reveals that the disturbance modulates optical index sinusoidally, with alternate decrement and increment in optical index around the unperturbed value. The maximum observed variation was found along the direction of acoustic wave propagation is of order of 1.5 percent.

Second-harmonic generation and manipulation in lithium niobate slab waveguides by grating metasurfaces

Nonlinear optical processes in waveguides play important roles in compact integrated photonics, while efficient coupling and manipulations inside the waveguides still remain challenging. In this work, we propose a new scheme for second-harmonic generation as well as beam shaping in lithium niobate slab waveguides with the assistance of well-designed grating metasurfaces at λ = 1064 nm . By encoding the amplitude and phase into the holographic gratings, we further demonstrate strong functionalities of nonlinear beam shaping by the metasurface design, including dual focusing and Airy beam generation. Our approach would inspire new designs in the miniaturization and integration of compact multifunctional nonlinear light sources on chip.

Inverse Design of Photonic Crystals through Automatic Differentiation

Gradient-based inverse design in photonics has already achieved remarkable results in designing small-footprint, high-performance optical devices. The adjoint variable method, which allows for the efficient computation of gradients, has played a major role in this success. However, gradient-based optimization has not yet been applied to the mode-expansion methods that are the most common approach to studying periodic optical structures like photonic crystals. This is because, in such simulations, the adjoint variable method cannot be defined as explicitly as in standard finite-difference or finite-element time- or frequency-domain methods. Here, we overcome this through the use of automatic differentiation, which is a generalization of the adjoint variable method to arbitrary computational graphs. We implement the plane-wave expansion and the guided-mode expansion methods using an automatic differentiation library, and show that the gradient of any simulation output can be computed efficiently and in parallel with respect to all input parameters. We then use this implementation to optimize the dispersion of a photonic crystal waveguide, and the quality factor of an ultra-small cavity in a lithium niobate slab. This extends photonic inverse design to a whole new class of simulations, and more broadly highlights the importance that automatic differentiation could play in the future for tracking and optimizing complicated physical models.

Polarization and angle dependent subwavelength coupling and dramatic reflection enhancement/reduction from an ITO–LiNbO3 interface

Two-dimensional electron gases (2DEGs) were demonstrated in indium–tin–oxide (ITO)-coated lithium niobate (LN) slabs by monitoring the very first reflection (VFR). Being dictated by light–matter interaction within the half-a-wavelength range, two-fold enhancement of the VFR was observed at one incidence angle, while two-and-half-fold reduction was obtained at another angle, when illuminating a Z-cut ITO–LN slab with two laser beams at 532 nm from the same side. In terms of exponential gain coefficient, such VFR enhancement/reduction corresponds to a range from −7.416 × 104 cm−1 to +4.68 × 104 cm−1, three orders of magnitude higher than that expected from the conventional photorefractive (PR) theory. The nanometer-thick 2DEG and the coupling role played by the evanescent field of the surface plasmon polaritons (SPPs) impose a much looser coherency requirement for the interacting light beams, which was verified by the solid energy coupling observed while rotating the polarization state of one laser beam. As much as 18.4 mW of the side diffraction mode associated with the light track formed on the slab surface, which sheds some light on the dramatic coupling dynamics. All these findings above were far beyond the reach of conventional PR theory, yet well consistent with the physical picture of 2DEG-supported SPPs and their interactions at the ITO–LN interface, offering a good opportunity to conceive and design future photonic/electronic devices for optical modulators.

Generation of midinfrared and visible radiation in a multiband phase-matched subwavelength LiNbO3 slab waveguide

An on-chip lithium niobate (LiNbO3) slab waveguide platform is proposed that simultaneously produces electric field pulses through sum and difference frequency generation. By controlling the geometry of the slab waveguide, phase-matching is achieved that produces radiation in the visible and midinfrared spectral bands. The slab waveguide confines the sum frequency generation wavelengths to the waveguide core and allows the difference frequency generation wavelengths to be emitted as Cherenkov waves. A finite-difference time-domain analysis is performed to study this multiband generation, where the LiNbO3 slab waveguide produces electric field pulses at a conversion efficiency of 14×10−5 and 0.5×10−5 through sum and difference frequency generation, respectively. This slab waveguide platform provides suitable design flexibility because the emission wavelengths are adjustable through proper choice of the waveguide core width and the excitation wavelengths. This source allows for better utilization of on-chip space by reducing the total number of devices necessary to achieve multiband light generation.

Subwavelength coupling and ultra-high exponential gain coefficient originating from 2D electron gas at ITO/LiNbO3 interface

Two dimensional electron gases (2DEGs) formed at interfaces between two oxides have been drawing growing attention for their intriguing magnetic, superconducting, and optical properties. Remarkable anisotropic transmission was observed from an indium-tin-oxide (ITO) coated lithium niobate (LN) slab, implying that a 2DEG is formed at one of the ITO/LN interfaces, and this is seemingly behind the anisotropic transmission. To optically probe 2DEG formation at one of the ITO/LN interfaces, the first reflected beam was monitored with one (two) laser beam(s). Reflective dynamics as large as 5%-15% were observed, pointing unambiguously to a subwavelength coupling and corresponding to, conservatively, exponential gain coefficients of −26 800 to +2700 cm−1 with half a wavelength as the coupling range. All observations are far beyond the reach of conventional bulk photorefractive effects, but align well with a picture of surface plasmon polariton excitation based on 2DEG formation. The 2DEG proximal to the LN substrate is a promising candidate for designing nonlinear plasmonics based nanometric waveguides, rectifiers, modulators, and sensors, which are compatible with current photonic circuits.

All-optical logic gates and a half-adder based on lithium niobate photonic crystal micro-cavities

All-optical logic gates including AND, XOR, and NOT gates, as well as a half-adder, are realized based on two-dimensional lithium niobate photonic crystal (PhC) circuits with PhC micro-cavities. The proposed all-optical devices have an extinction ratio as high as 23 dB due to the effective all-optical switch function induced by two-missing-hole micro-cavities. These proposed devices can have potential implementation of complex integrated optical functionalities including all-optical computing in a lithium niobate slab or thin film.

Ultra-fast tuning of refractive index in Lithium Niobate slab by GHz acoustic wave

Refractive index modulation in optical materials facilitates dynamic control on electromagnetic waves. Ultra fast tuning of both refractive index and its spatial distribution in Y−Z Lithium Niobate slab via strain employed through GHz acoustic perturbation has been simulated and studied. To create acoustic disturbance in taken 500 nm thick slab, the fundamental symmetric Lamb mode at 5 GHz acoustic frequency is excited. Displacement field patterns generated by longitudinal and transverse disturbances of perturbing fundamental symmetric Lamb mode are analyzed under reinstate limits of optical material to create elastic strain. Modification in optical index of Lithium Niobate due to strain generated by longitudinal and transverse acoustic disturbance is then examined. The sinusoidal modulation in refractive index with maximum change of 0.070 along nz and 0.036 along ny is observed. The modulated refractive index has a periodic distribution over space with period equals to wavelength of acoustic disturbance created by fundamental Lamb mode. This acoustically induced ultra-fast change in refractive index and in its spatial distribution depends on perturbing acoustic frequency and thickness of slab. Thus, the slab can optimize dynamically, to control electromagnetic wave for various optical applications.

Tunable photonic band gaps in Lithium Niobate slab waveguide through Lamb waves

Tunable photonic band gap generation in Y–Z Lithium Niobate slab through Lamb wave is theoretically studied and presented. The applied 5.5 GHz frequency of Lamb wave able to provide a periodic sinusoidal variation of refractive index through photo-elastic effect in considered 600 nm thick Lithium Niobate slab with period 615 nm. The achieved maximum and minimum change in refractive index along z axis is 0.026 with respect to unperturbed refractive index 2.2 of considered Lithium Niobate slab. The induced photonic band gap in optically modified structure is calculated using Bloch’s theorem and the transmittance of the structure is obtained by transfer matrix method. The obtained photonic band gap at 5.5 GHz Lamb wave frequency is 7.8 THz with central angular frequency is 695.9 THz. Since, the unit cell dimension and central frequency of photonic band gap are highly depend on the applied Lamb wave frequency therefore the possible application of the proposed structure is a tunable photonic crystal.

Direct evidence of visible surface plasmon excitation in ITO film coated on LiNbO3 slabs.

An iron-doped Y-cut lithium niobate (Fe:LN) slab was coated with indium-tin-oxide (ITO) thin films by magnetron sputtering. The electron confinement in a sub-nanometer region at ITO/LN interfaces is resulted from electric screening effect. Consequently, the local plasma frequency in the sub-nanometer metallic-like layer is shifted to the UV regime. This makes it possible to excite surface plasmon polaritons (SPPs) in the visible region with photorefractive phase gratings in the LN slab and to transport SPPs much energy-efficiently. Direct evidence of the excitation of SPPs was demonstrated by the presence of deep transmission spectral valleys in the transmission spectra and striking dark bands in the 2D diffraction patterns while using a white reading beam. Theoretical arguments and confirmation experiments are presented to elucidate all the related findings.

Experimental and theoretical analysis of THz-frequency, direction-dependent, phonon polariton modes in a subwavelength, anisotropic slab waveguide

Femtosecond optical pulses were used to generate THz-frequency phonon polariton waves in a 50 micrometer lithium niobate slab, which acts as a subwavelength, anisotropic planar waveguide. The spatial and temporal electric field profiles of the THz waves were recorded for different propagation directions using a polarization gating imaging system, and experimental dispersion curves were determined via a two-dimensional Fourier transform. Dispersion relations for an anisotropic slab waveguide were derived via analytical analysis and found to be in excellent agreement with all observed experimental modes. From the dispersion relations, we analyze the propagation-direction-dependent behavior, effective refractive index values, and generation efficiencies for THz-frequency modes in the subwavelength, anisotropic slab waveguide.

Nonlinear optical terahertz wave sources

We developed a Cherenkov phase-matching method for monochromatic THz-wave generation using the DFG, process with a lithium niobate crystal, which resulted in both high conversion efficiency and wide tunability. Although THz-wave generation by Cherenkov phase matching has been demonstrated using femtosecond pumping pulses, producing very high peak power, these THz-wave sources are not monochromatic. Our method generates monochromatic and tunable THz, waves using a nanosecond pulsed laser source. We also show that Cherenkov radiation with waveguide structure is an effective strategy for achieving extremely wide tunable THz-wave source. We fabricated MgO-doped lithium niobate slab waveguide and demonstrated difference frequency generation of THz-wave generation with Cherenkov phase matching. Extremely frequency-widened THz-wave generation, from 0.1 to 7 THz was observed.

Extremely frequency-widened terahertz wave generation using Cherenkov-type radiation

Terahertz (THz) wave generation based on nonlinear frequency conversion is promising way for realizing a tunable monochromatic bright THz-wave source. Such a development of efficient and wide tunable THz-wave source depends on discovery of novel brilliant nonlinear crystal. Important factors of a nonlinear crystal for THz-wave generation are, 1. High nonlinearity and 2. Good transparency at THz frequency region. Unfortunately, many nonlinear crystals have strong absorption at THz frequency region. The fact limits efficient and wide tunable THz-wave generation. Here, we show that Cherenkov radiation with waveguide structure is an effective strategy for achieving efficient and extremely wide tunable THz-wave source. We fabricated MgO-doped lithium niobate slab waveguide with 3.8 microm of thickness and demonstrated difference frequency generation of THz-wave generation with Cherenkov phase matching. Extremely frequency-widened THz-wave generation, from 0.1 to 7.2 THz, without no structural dips successfully obtained. The tuning frequency range of waveguided Cherenkov radiation source was extremely widened compare to that of injection seeded-Terahertz Parametric Generator. The tuning range obtained in this work for THz-wave generation using lithium niobate crystal was the widest value in our knowledge. The highest THz-wave energy obtained was about 3.2 pJ, and the energy conversion efficiency was about 10(-5) %. The method can be easily applied for many conventional nonlinear crystals, results in realizing simple, reasonable, compact, high efficient and ultra broad band THz-wave sources.

Theoretical study of lithium niobate slab waveguides for integrated optics applications

We report on the theoretical study of lithium niobate slab and wire waveguides with different kinds of cladding (silicon dioxide, sapphire and air). The mode propagation, the light confinement and radiation losses are simulated using a software based on a beam propagation method. We propose from those results lithium niobate waveguide geometries for optical integrated applications.

Optical and electro-optical properties of submicrometer lithium niobate slab waveguides prepared by crystal ion slicing and wafer bonding

LiNbO 3 thin film crystals have been produced using crystal ion slicing and wafer bonding. Films with a thickness of 680nm are produced on a thin layer of SiO2, which is deposited on a substrate with electrodes. The crystalline and optical qualities of the fabricated thin films are investigated and are comparable to bulk LiNbO3 single crystals. The effect of thermal annealing is studied using Rutherford backscattering. The refractive indices and the electro-optical (EO) coefficient of the fabricated films are measured using prism coupling dark line spectroscopy and modified attenuated total reflection. The EO coefficient is equal to r33=31pm∕V at λ=633nm.

One-dimensional spatial soliton families in optimally engineered quasi-phase-matched lithium niobate waveguides.

The advantage for quadratic soliton generation of engineering the quasi-phase-matching period near the input of lithium niobate slab waveguides is demonstrated. This approach allows members of one-dimensional quadratic soliton families with different values of the wave-vector mismatch to be cleanly excited and to be characterized by quantitative intensity-profile measurements of both the fundamental and the second-harmonic soliton components.

Second-harmonic generation in hexagonally-poled lithium niobate slab waveguides

Slab nonlinear waveguides have been fabricated by annealing and reverse proton-exchange in hexagonally-poled lithium niobate (HexLN). The technology allowed the first demonstration of guided-wave quadratic interactions in 2D nonlinear photonic crystals.

Spatial modulational instability in one-dimensional lithium niobate slab waveguides

We report an experimental study of the breakup through modulational instability of broad fundamental beams near the phase-matching condition for second-harmonic generation in lithium niobate slab waveguides. Two mechanisms for initiating modulational instability, waveguide imperfections and noise on the input beam, are identified.