Analytical solution to incident angle quasi-phase-matching engineering for second harmonic generation in a periodic-poled lithium niobate crystal

Phase matching or quasi-phase matching(QPM)is of significant importance to the conversion efficiency of second harmonic generation(SHG)in artificial nonlinear crystals like lithium niobate(LN)crystal or microstructured nonlinear crystals like periodic-poled lithium niobate(PPLN)crystals.In this paper,we propose and show that the incident angle of pump laser light can be harnessed as an alternative versatile tool to engineer QPM for high-efficiency SHG in a PPLN crystal,in addition to conventional means of period adjusting or temperature tuning.A rigorous model is established and analytical solution of the nonlinear conversion efficiency under the small and large signal approximation theory is obtained at different incident angles.The variation of phase mismatching and walk-off length with incident angle or incident wavelength are also explored.Numerical simulations for a PPLN crystal with first order QPM structure are used to confirm our theoretical predictions based on the exact analytical solution of the general large-signal theory.The results show that the narrow-band tunable SHG output covers a range of 532 nm–552.8 nm at the ideal incident angle from 0°to 90°.This theoretical scheme,fully considering the reflection and transmission at the air-crystal interface,would offer an efficient theoretical system to evaluate the nonlinear frequency conversion and help to obtain the maximum SHG conversion efficiency by selecting an optimum incident wavelength and incident angle in a specially designed PPLN crystal,which would be very helpful for the design of tunable narrow-band pulse nanosecond,picosecond,and femtosecond laser devices via PPLN and other microstructured LN crystals.

Generation of broadband polarization-orthogonal photon pairs via the dispersion-engineered thin-film lithium niobate waveguide

Broadband photon pairs are highly desirable for quantum metrology,quantum sensing,and quantum communication.Such sources are usually designed through type-0 phase-matching spontaneous parametric down-conversion(SPDC)that makes the photon pairs hard to separate in the frequency-degenerate case and thus limits their applications.In this paper,we design a broadband frequency-degenerate telecom-band photon pair source via the type-II SPDC in a dispersion-engineered thin-film lithium niobate waveguide,where the polarization modes of photon pairs are orthogonal and thus are easily separated deterministically.With a 5-mm-long waveguide,our design can achieve a bandwidth of 5.56 THz(44.8 nm),which is 8.6 times larger than that of the bulk lithium niobate,and the central wavelength can be flexibly adjusted.Our design is a promising approach towards high-quality integrated photon sources and may have wide applications in photonic quantum technologies.

Continuous-wave mid-infrared intracavity singly resonant optical parametric oscillator based on periodically poled lithium niobate

This paper reports a continuous-wave （CW） mid-infrared intracavity singly resonant optical parametric oscillator based on periodically poled lithium niobate （PPLN） pumped by a diode-end-pumped CW Nd：YVO4 laser. Considering the thermal lens effects, it adopted an optical ballast lens and the near-concentric cavity for better operation. At the PPLN＇s grating period of 28.5 μm and the temperature of 140℃, the maximum idler output power of 155 mW at 3.86 μm has been achieved when the 808 nm pump power is 8.5 W, leading to an optical-to-optical conversion efficiency of 1.82%.

High efficiency continuous-wave tunable signal output of an intracavity singly resonant optical parametric oscillator based on periodically poled lithium niobate

In this paper we report on a continuous-wave （CW） intracavity singly resonant optical parametric oscillator （ICSRO） based on periodically poled LiNbO3 （PPLN） pumped by a diode-end-pumped CW Nd：YVO4 laser. Considering the thermal lens effects and diffraction loss, an optical ballast lens and a near-concentric cavity are adopted for better operation. Through varying the grating period and the temperature, the tunable signal output from 1406 nm to 1513 nm is obtained. At a PPLN grating period of 29 pm and a temperature of 413 K, a maximum signal output power of 820 mW at 1500 nm is achieved when the 808 nm pump power is 10.9 W, leading to an optical-to-optical conversion efficiency of 7.51%.

Four-channel CWDM transmitter chip based on thin-film lithium niobate platform

Multi-lane integrated transmitter chips are key components in future compact optical modules to realize high-speed optical interconnects.Thin-film lithium niobate(TFLN)photonics have emerged as a promising platform for achieving high-performance chip-scale optical systems.Combining a coarse wavelength-division multiplexing(CWDM)devices using fabrication-tolerant angled multimode interferometer structure and high-performance electro-optical modulators,we demonstrate monolithic on-chip four-channel CWDM transmitter on the TFLN platform for the first time.The four-channel CWDM transmitter enables high-speed transmissions of 100 Gb/s data rate per wavelength channel(i.e.,an aggregated date rate of 400 Gb/s).

Three-wavelength generation from cascaded wavelength conversion in monolithic periodically poled lithium niobate

Tunable coherent emission is generated in a single-pass, cascaded wavelength conversion process from mode-locked laser-pumped monolithic periodically poled lithium niobate（PPLN）. Three ranges of wavelength, including visible output from 628 nm to 639 nm, near-infrared output from 797 nm to 816 nm, and mid-infrared output from 3167 nm to 3459 nm,were obtained from the monolithic PPLN, which consists of a 10-mm section for 532-nm-pumped optical parametric generation（OPG） and a 7-mm section for 1064-nm-pumped sum frequency generation（SFG）. A pump-to-signal conversion efficiency of 23.4% for OPG at 50°C and a quantum efficiency of 26.2% for SFG at 200°C were obtained.

Photonic devices based on thin-film lithium niobate on insulator

Lithium niobate on insulator(LNOI)is rising as one of the most promising platforms for integrated photonics due to the high-index-contrast and excellent material properties of lithium niobate,such as wideband transparency from visible to mid-infrared,large electro-optic,piezoelectric,and second-order harmonic coefficients.The fast-developing micro-and nanostructuring techniques on LNOI have enabled various structure,devices,systems,and applications.In this contribution,we review the latest developments in this platform,including ultra-high speed electro-optic modulators,optical frequency combs,opto-electro-mechanical system on chip,second-harmonic generation in periodically poled LN waveguides,and efficient edge coupling for LNOI.

Effect of thickness variations of lithium niobate on insulator waveguide on the frequency spectrum of spontaneous parametric down-conversion

We study the effect of waveguide thickness variations on the frequency spectrum of spontaneous parametric downconversion in the periodically-poled lithium niobate on insulator(LNOI)waveguide.We analyze several variation models and our simulation results show that thickness variations in several nanometers can induce distinct effects on the central peak of the spectrum,such as narrowing,broadening,and splitting.We also prove that the effects of positive and negative variations can be canceled and thus lead to a variation-robust feature and an ultra-broad bandwidth.Our study may promote the development of on-chip photon sources in the LNOI platform,as well as opens up a way to engineer photon frequency state.

Growth and Holographic Storage Properties of Sc, Fe Co-Doped Lithium Niobate Crystals

The holographic storage properties of Fe （0.03% （mass fraction） Fe2O3）：LiNbO3 doped with Sc at different levels （0, 1%, 2%, 3%） were investigated. The Sc threshold concentration in Fe：LiNbO3 was implied to be about 3% （mole fraction） because O-H vibration absorption peak of Sc （3%）：Fe：LiNbO3 was at 3508 cm^-1, compared with 3484 cm^-1 of crystals with lower Sc doping level. Sc（3%）：Fe：LiNbO3 exhibited higher optical damage resistance ability. The threshold intensity （wavelength 488 nm） of Sc （3%）：Fe：LiNbO3 was 2.2 ×10^2 W ·cm^-2, two orders of masnitude higher than that of Fe：LiNbO3. Holographic storage properties of the crystals were determined in an extraordinary polarized laser of wavelength 632.8 nm by a two-wave coupling method. It was found that in terms of holographic storage properties, the optimal doping concentration of Sc was 2% （mole fraction） among this crystal series.

The two photorefractive centres in iron doped nearly stoichiometric lithium niobate crystals

This paper investigates the photorefractive properties of iron doped lithium niobate with different [Li]/[Nb] ratios The experimental results show two photorefractive centres for iron doped near-stoichiometric lithium niobate crystal Besides Fe^2＋ and Fe^3＋ ions, small polarons and bipolarons are considered as another photoactive centre.

Supercontinuum Generation in Lithium Niobate Ridge Waveguides Fabricated by Proton Exchange and Ion Beam Enhanced Etching

We report on the fabrication of the lO-mm-long lithium niobate ridge waveguide and its supercontinuum gen- eration at near-visible wavelengths （around 800hm）. The waveguides are fabricated by a combination of MeV copper ion implantation followed by wet etching in a proton exchanged lithium niobate planar waveguide. Using a mode-locked Ti：sapphire laser with a central wavelength of 800nm, the generated broadest supereontinuum through the ridge waveguides spans 302 nm （at -30 dB points）, from 693 to 995 nm. Temporal coherence proper- ties of the supercontinuum are experimentally studied by a Michelson interferometer and the coherence length of the broadest supercontinuum is measured to be 5.2 μm. Our results offer potential for a compact and integrated supercontinuum source for applications including bio-imaging, spectroscopy and optical communication.

Spatio-temporal isolator in lithium niobate on insulator

In this contribution,we simulate,design,and experimentally demonstrate an integrated optical isolator based on spatiotemporal modulation in the thin-film lithium niobate on an insulator waveguide platform.We used two cascaded travelling wave phase modulators for spatiotemporal modulation and a racetrack resonator as a wavelength filter to suppress the sidebands of the reverse propagating light.This enabled us to achieve an isolation of 27 dB.The demonstrated suppression of the reverse propagating light makes such isolators suitable for the integration with III-V laser diodes and Erbium doped gain sections in the thin-film lithium niobate on the insulator waveguide platform.

Ultra-broadband and low-loss edge coupler for highly efficient second harmonic generation in thin-film lithium niobate

Thin-film lithium niobate is a promising material platform for integrated nonlinear photonics,due to its high refractive index contrast with the excellent optical properties.However,the high refractive index contrast and correspondingly small mode field diameter limit the attainable coupling between the waveguide and fiber.In second harmonic generation processes,lack of efficient fiber-chip coupling schemes covering both the fundamental and second harmonic wavelengths has greatly limited the overall efficiency.We design and fabricate an ultra-broadband tri-layer edge coupler with a high coupling efficiency.The coupler allows efficient coupling of 1 dB∕facet at 1550 nm and 3 dB∕facet at 775 nm.This enables us to achieve an ultrahigh overall second harmonic generation normalized efficiency(fiber-to-fiber)of 1027%W^(−1)cm^(−2)(on-chip second harmonic efficiency∼3256%W^(−1)cm^(−2))in a 5-mm-long periodically-poled lithium niobate waveguide,which is two to three orders of magnitude higher than that in state-of-the-art devices.

Degenerate polarization entangled photon source based on a single Ti-diffusion lithium niobate waveguide in a polarization Sagnac interferometer

Combining a Ti-diffusion periodically poled lithium niobate(PPLN)waveguide with a Sagnac interferometer,two opposite directions type-II spontaneous parametric down conversions(SPDC)occur coherently and yield a high brightness,high stability polarization entanglement source.The source produces degenerate photon pairs at 1540.4 nm with a brightness of B=(1.36±0.03)×10^(6) pairs/(s·nm·m W).We perform quantum state tomography to reconstruct the density matrix of the output state and obtain a fidelity of F=0.983±0.001.The high brightness and phase stability of our waveguide source enable a wide range of quantum information experiments operating at a low pump power as well as hold the advantage in mass production which can promote the practical applications of quantum technologies.

Molecular dynamic simulations of surface morphology and pulsed laser deposition growth of lithium niobate thin films on silicon substrate

The molecular dynamic simulation of lithium niobate thin films deposited on silicon substrate is carried out by using the dissipative particle dynamics method. The simulation results show that the Si （111） surface is more suitable for the growth of smooth LiNbO3 thin films compared to the Si（100） surface, and the optimal deposition temperature is around 873 K, which is consistent with the atomic force microscope results. In addition, the calculation molecular number is increased to take the electron spins and other molecular details into account.

Two waveguide layers in lithium niobate crystal formed by swift heavy Kr ion irradiation

We report the formation of two waveguide layers in a lithium niobate crystal by irradiation with swift heavy Kr ions with high （GeV） energies and ultralow fluences. The micro-Raman spectra are measured at different depths in the irradiated layer and show that the high electronic energy loss can cause lattice damage along the ion trajectory, while the nuclear energy loss causes damage at the end of the ion track. Two waveguide layers are formed by confinement with two barriers associated with decreases in the refractive index that are caused by electronic and nuclear energy losses, respectively.

Generation of hyperentangled photon pairs based on lithium niobate waveguide

Generation of hyperentangled photon pairs is investigated based on the lithium niobate straight waveguide.We propose to use the nonlinear optical process of spontaneous parametric down-conversion(SPDC)and a well-designed lithium niobate waveguide structure to generate a hyperentangled(in the polarization dimension and the energy-time dimension)two-photon state.By performing numerical simulations of the waveguide structure and calculating the possible polarization states,joint spectral amplitudes(JSA),and joint temporal amplitudes(JTA)of the generated photon pair,we show that the generated photon pair is indeed hyperentangled in both the polarization dimension and the energy-time dimension.

Temperature-dependent second harmonic generation process based on an MgO-doped periodically poled lithium niobate waveguide

The temperature dependency of a 5-mo1% MgO-doped periodically poled lithium niobate waveguide was investi- gated in this paper. We started with the temperature-dependent refractive index equation for the waveguide. Secondly, the temperature dependency of the second harmonic generation effect was experimentally researched under different temperatures and pump powers. The quasi-phase matched wavelengths, efficiency bandwidths and peak efficiencies of the waveguide were measured. The experimental results agreed with theoretical simulations, which are indispensable in the following all-optical sampling studies based on the cascaded second harmonic generation/difference-frequency generation process in the current device.

Formation Enthalpy Calculation of Oxygen Vacancy Defect in Doped Lithium Niobate Crystals

The relationship between temperature and oxygen vacancy concentration is deduced in this paper. Based on the data of thermal weight-loss experiment, the formation enthalpies of congruent and several doped LN crystals have been calculated. It was found that the formation enthalpy of oxygen vacancies can be decreased evidently by doping valence-changeable ions. The experimental results were discussed and a new reduction process of the photorefractive LN crystal at a relatively low temperature was proposed, and the reduced crystals showed a good effect in practical use.

Deterministic N-photon state generation using lithium niobate on insulator device

The large-photon-number quantum state is a fundamental but nonresolved request for practical quantum information applications.We propose an N-photon state generation scheme that is feasible and scalable,using lithium niobate on insulator circuits.Such a scheme is based on the integration of a common building block called photon-number doubling unit(PDU)for deterministic single-photon parametric downconversion and upconversion.The PDU relies on a 107-optical-quality-factor resonator and mW-level on-chip power,which is within the current fabrication and experimental limits.N-photon state generation schemes,with cluster and Greenberger–Horne–Zeilinger state as examples,are shown for different quantum tasks.