• Chinese Optics Letters
  • Vol. 19, Issue 6, 060012 (2021)
Yang Li1, Zhijin Huang1, Wentao Qiu1, Jiangli Dong2, Heyuan Guan2、*, and Huihui Lu1、**
Author Affiliations
  • 1Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications, Jinan University, Guangzhou 510632, China
  • 2Key Laboratory of Optoelectronic Information and Sensing Technologies of Guangdong Higher Education Institutes, Jinan University, Guangzhou 510632, China
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    DOI: 10.3788/COL202119.060012 Cite this Article Set citation alerts
    Yang Li, Zhijin Huang, Wentao Qiu, Jiangli Dong, Heyuan Guan, Huihui Lu. Recent progress of second harmonic generation based on thin film lithium niobate [Invited][J]. Chinese Optics Letters, 2021, 19(6): 060012 Copy Citation Text show less

    Abstract

    Recently, nonlinear photonics has attracted considerable interest. Among the nonlinear effects, second harmonic generation (SHG) remains a hot research topic. The recent development of thin film lithium niobate (TFLN) technology has superior performances to the conventional counterparts. Herein, this review article reveals the recent progress of SHG based on TFLN and its integrated photonics. We mainly discuss and compare the different techniques of TFLN-based structures to boost the nonlinear performances assisted by localizing light in nanostructures and structured waveguides. Moreover, our conclusions and perspectives indicate that more efficient methods need to be further explored for higher SHG conversion efficiency on the TFLN platform.

    1. Introduction

    Nonlinear effects of optics have been explored since Maxwell’s time. However, much progress has been made in the field of nonlinear optics since the discovery of the laser, which made high-intensity optical fields easily feasible. The field started to grow with the first, to the best of our knowledge, experimental work of Franken et al. on optical second-harmonic generation (SHG) in 1961[1]. In SHG, input pump light can be converted to a signal with twice the frequency. The efficient second-order nonlinear process is a promising platform for many kinds of applications such as light sources[2,3], optical frequency conversion[4], optical focusing[5], supercontinuum generation[6,7], and biological applications like local fluorescent excitation[8]. For practical applications, it is of great significance to improve SHG conversion efficiency for the research on second-order nonlinear effects. Up to now, compact photonic micro- and nanostructures with enhanced optical nonlinearities at different wavelength ranges have been studied extensively. However, nonlinear optical performances still need to be improved[911].

    The lithium niobate (LiNbO3, LN) crystal is one of the most promising materials to address a considerable number of optical applications[1214], well-known for its excellent nonlinear optic, piezoelectric, electro-optic (EO), photoelastic, and photorefractive properties in the present days. Indeed, LN is naturally birefringent and has a broad transparency spectral window from the ultraviolet to the far infrared (350–5500 nm). Recently, due to the excellent optical properties of LN, the emerging LN technology has received great attention in the research of integrated and guided-wave optics. With the help of advanced micro–nano fabrication technologies, many high-performance LN integrated photonic devices have been realized[15]. Levy et al. reported on the exfoliation of the first high-quality (Q) LN film from a piece of bulk LN crystal by ion slicing in 1998[16]. Recently, thin film LN (TFLN) as a modern and emerging technology platform capable of tighter integration of nonlinear devices has been a topic of intense research and is uniquely well-suited for any functions requiring modulating light or shifting the frequency of light. TFLN can facilitate large-scale integration of photonic integrated circuits, as well as boost performance for optical devices compared to bulk LN crystals[1719]. Silicon is lack of high second-order susceptibilities (χ(2)) with more absorption losses. Moreover, the loss of TFLN-based devices is much lower than that of silicon-based optoelectronic devices[20]. So far, the major applications of TFLN in nonlinear optics are well-developed second-order nonlinear processes.

    As mentioned in Fig. 1, most nonlinear applications of TFLN are mainly divided into structured waveguides and nanostructures. Due to nonlinear effects that can be significantly enhanced inside nanophotonic waveguides with tight light confinement[2123], the most feasible approach to achieve near 100% nonlinear conversion efficiency is the waveguide. However, more recently, nanostructures made by TFLN supporting Fano resonances[24] and Mie resonances[25] are also widely used in phase-matching-free nonlinear optics fields such as compact SHG devices and integrated LN chips. In many cases, the nonlinear structures are optimized to realize and maximize their conversion efficiency.

    Summary of different approaches of SHG based on TFLN technology.

    Figure 1.Summary of different approaches of SHG based on TFLN technology.

    In this paper, we briefly summarize the recent developments and progresses of SHG based on TFLN technology. The review will focus on different techniques for enhancing SHG conversion efficiency based on TFLN. Lastly, this article also summarizes the latest advances in the functionality of TFLN nonlinear photonic devices and gives a short outlook on their future applications in the fast-growing field of multifunctional integrated photonics.

    2. Nonlinear Optical Structures Based on TFLN

    2.1. Second harmonic generation in structured waveguides

    SHG is the most straightforward nonlinear application of TFLN waveguides. In this section, we give a brief overview of the different waveguides in TFLN and discuss their developments. As mentioned before, TFLN can overcome the drawbacks associated with bulk LN devices. TFLN is oftentimes applied to the platform of LN-on-insulator (LNOI)[26], like the silicon-on-insulator (SOI), although silicon or LN is sometimes used as a substrate. Generally speaking, the insulator layer is comprised of a lower refractive index material to avoid optical mode leakage from the thin film into the substrate, such as silicon dioxide (SiO2). A high-Q LNOI substrate was recently commercialized by NANOLN Inc. in China, facilitating the area of photonics on the LNOI platform all over the world[27]. Traditional LN waveguides are fabricated by titanium indiffusion or by proton exchange[28], whereas waveguides in LNOI oftentimes are ridge waveguides fabricated by reactive ion etching (RIE)[29,30]. In this approach, the fabrication of TFLN-based optical elements requires advanced micro- and nanostructuring methods. SHG from waveguides has the advantages of combining the high optical power density inside the waveguides with the periodic modulation of nonlinear coefficients[31]. For efficient second-order nonlinear optical interactions, phase-matching conditions need to be fulfilled because the wave propagation is linked to the wavelength-dependent refractive index[32]. Various phase-matching techniques have been utilized in order to demonstrate thin film periodically poled LN (TF-PPLN) waveguides[33]. To date, quasi-phase matching (QPM)[34] as the most preferred approach to achieve phase matching can be achieved by inverting the spontaneous polarization of the optically ferroelectric crystals periodically along the waveguide to compensate the wave-vector mismatch Δk between the pump and SHG optical modes[35,36]. Furthermore, the QPM technique allows a free choice of different polarizations of the interacting light waves[37].

    Recently, much effort has improved conversion efficiencies by combining the QPM of periodically poled LN (PPLN) with the strong field confinement of the waveguide. In TFLN, periodic poling is most commonly achieved by the application of a strong electric field along the z axis of the crystal through electrodes. PPLN is made by using a static electric field to invert the orientation of the ferroelectric domains in LN thin film[38]. QPM SHG in bulk PPLN has been widely used among all the QPM wavelength converters with extremely high efficiencies in the visible and near infrared (NIR).

    Here, we will first refer to QPM nonlinear devices, because QPM interactions in waveguides with quadratic nonlinearities enable highly efficient nonlinear frequency conversion. For example, monolithic nanophotonic PPLN waveguides used for light propagating along the y axis were successfully prepared by standard lithography on a high-Q periodically poled X-cut Mg-doped LN film [Fig. 2(a)][39]. As indicated in Fig. 2(b), the periodically poled LNOI (PPLNOI) waveguide fabricated by the electrical field poling technique with active monitoring of the poling fidelity on the nanophotonic chip was characterized by SHG at 1470 nm, with the normalized conversion efficiency as high as 3061%W1cm2. Moreover, the reliability and repeatability of the LNOI platform can be improved by designing photonic structures with high tolerance to the fabrication errors[40]. The nonlinear frequency converter is schematically illustrated in Fig. 2(c) for a PPLNOI ridge waveguide with simultaneous transverse EO coupling and efficient conversion for SHG, which holds promise in realizing electrically controllable on-chip nonlinear devices[41]. A shallow-etched ridge waveguide has been fabricated in X-cut magnesium-oxide-doped TFLN, achieving the length-normalized conversion efficiency of 3757%W1cm2 to indicate promising applications for robust and efficient waveguides[42] [Fig. 2(d)].

    (a) Schematic and false-color SEM image of a periodically poled nanophotonic waveguide[39]; Copyright 2018, Optical Society of America. (b) SH confocal microscopy of the PPLN thin film fabricated by microelectrode poling and the cross section of the LNOI ridge waveguide[40]; Copyright 2020, AIP Publishing. (c) Schematic of the cascading EO coupling and SHG process in the PPLN ridge waveguide[41]; Copyright 2019, Optical Society of America. (d) Schematic illustration of the PPLN waveguide with poling electrodes[42]; Copyright 2019, Optical Society of America.

    Figure 2.(a) Schematic and false-color SEM image of a periodically poled nanophotonic waveguide[39]; Copyright 2018, Optical Society of America. (b) SH confocal microscopy of the PPLN thin film fabricated by microelectrode poling and the cross section of the LNOI ridge waveguide[40]; Copyright 2020, AIP Publishing. (c) Schematic of the cascading EO coupling and SHG process in the PPLN ridge waveguide[41]; Copyright 2019, Optical Society of America. (d) Schematic illustration of the PPLN waveguide with poling electrodes[42]; Copyright 2019, Optical Society of America.

    More and more researches on various structures designed with periodically poled TFLN waveguides can further enhance SHG conversion efficiency. These structures can achieve efficient SHG via different phase-matching mechanisms, such as type I phase matching by tuning temperature[43], modal phase matching by mode-shape modulation[44], modal dispersion phase matching by modal dispersion engineering[36], or grating-assisted QPM (GA-QPM)[45]. In addition to QPM in standard PPLN devices mentioned, other approaches for phase matching in TFLN waveguides can simplify their fabrication and device preparation because of the poling-free implementations.

    An example of such devices in a rotational symmetry Z-cut TFLN microring is shown in Fig. 3(a), which can be periodically poled by an external electric field between the bottom aluminium plate electrode and the top radial nickel electrodes. Due to the stronger mode confinement of the microring and the phase matching in the periodic domain structure, the QPM SHG conversion efficiency for the periodically poled microring can yield up to 250,000%W1[46]. Figure 3(b) displays two distinct schemes to achieve the phase-matching condition. Both fixed-width LN waveguides (modal phase matching) and periodically grooved LN (PGLN) waveguides (QPM) were fabricated on an X-cut LNOI platform, which were adopted to realize phase-matched SHG conversion efficiencies of 6.8%W1cm2 and 41%W1cm2, respectively[44]. To overcome careful dispersion engineering of traditional nonlinear phase matching, Wang et al. have demonstrated phase-matching-free SHG over many coherent lengths in TFLN waveguides and that gradient metasurface-based device can maintain a high efficiency over a broad range of pump wavelengths due to the strong nonlinear modal overlap[47] [see in Fig. 3(c)]. In terms of poling-free methods for SHG in the TFLN platform, employing a sinusoidal width perturbation of the rib-loaded waveguides with silicon nitride (SiN) can achieve GA-QPM based on TFLN [Fig. 3(d)]. The SHG conversion efficiency is 1%W1cm2 from 4.9-mm-long waveguides pumped at a 784 nm wavelength. An optimized width modulation pattern and a higher-refractive-index rib material can increase the conversion efficiency of such devices[45].

    (a) Demonstration of efficient SHG in PPLN microring resonators[46]; Copyright 2019, Optical Society of America. (b) Schematic of the periodically grooved structure of an LN waveguide and cross-section image of the X-cut LNOI waveguide[44]; Copyright 2017, Optical Society of America. (c) Schematic and working principle of the metasurface-assisted LN nanophotonic waveguide[47]; Copyright 2017, Springer Nature. (d) Schematic of a rib-loaded GA-QPM waveguide with a sinusoidal modulation of the width along with the optical mode profiles of the fundamental and SH TE modes at a grating width of 1095 nm[45]; Copyright 2017, AIP Publishing.

    Figure 3.(a) Demonstration of efficient SHG in PPLN microring resonators[46]; Copyright 2019, Optical Society of America. (b) Schematic of the periodically grooved structure of an LN waveguide and cross-section image of the X-cut LNOI waveguide[44]; Copyright 2017, Optical Society of America. (c) Schematic and working principle of the metasurface-assisted LN nanophotonic waveguide[47]; Copyright 2017, Springer Nature. (d) Schematic of a rib-loaded GA-QPM waveguide with a sinusoidal modulation of the width along with the optical mode profiles of the fundamental and SH TE modes at a grating width of 1095 nm[45]; Copyright 2017, AIP Publishing.

    Due to a well-designed grating metasurface in the LN slab waveguides to fulfill the phase-matching condition rather than using previous poling technology, SHG and nonlinear beam shaping have been experimentally observed, but the total efficiency has been decreased to 4.6×107[48]. Furthermore, the detailed comparisons between these TFLN photonics based on waveguides and their corresponding metrics are illustrated in Table 1.

    YearTFLN StructurePoled/Coupling Region Length L (mm)FF Power (λFF)Coupling Loss (dB/facet)Waveguide Propagation Loss (dB/cm)ηSH (%W1cm2)Institute
    2011Plasmonic waveguide[60]11 W (1550 nm)1.3%Nanjing University
    2015Nanoscale LN waveguides[61]0.9737 µW (1411 nm)616.9Friedrich Schiller Universität Jena
    2017PE channel waveguide[36]3.21 mW (1385 nm)2.548Shandong University
    2016Rib-loaded SiN-PPLN[22]4.80.5 mW (1530 nm)∼6.80.3±0.2160University of California
    2017Metasurface-assisted PM LN waveguide[47]0.019109 V/m/20 mW (1640 nm)1660Harvard University
    2017GA-QPM LN ridge waveguide[45]4.984 mW (1568 nm)6.510.8University of Central Florida
    2017Integrated TFLN waveguide[44]318.3 µW (1550 nm)4.83±0.241Harvard University
    2016Diced ridge PPLN waveguides[62]5.86.6 mW (1550 nm)0.5777.9Shandong University
    2018PPLN on silicon[63]2010 mW (1547 nm)0.21230University of Central Florida
    2018Nanostructured PPLN waveguide[39]4220 mW (1550 nm)∼102600Harvard University
    2018LN nanophotonic waveguide[64]8∼1 mW (1540 nm)50.5422.2University of Rochester
    2019PPLN microrings[46]115 µW (1617 nm)250,000%/WYale University
    2019PPLNOI ridge waveguide[41]1010 mW (1590 nm)0.04Shanghai Jiao Tong University
    2019Dry-etched[65]0.61 mW (1540 nm)634600University of Central Florida
    2019Dry-etched[21]42.95 mW (1550 nm)4.30.32200Stevens Institute of Technology
    2020Z-cut PPLNOI waveguide[66]1–(1550 nm)5.4±0.3<0.032400Stevens Institute of Technology
    2020Dry-etched PPLN[67]50.1 mW (1570 nm)70.542000University of California
    2020PPLNOI ridge waveguide[40]6397 µW (1470 nm)3061Nanjing University+Sun Yat-sen University
    2020Birefringent phase-matching LN waveguide[68]104500 W (1064 nm)0.580.87%Shandong University
    2020Shallow-etched TFLN waveguides[42]510 mW (1560 nm)7.713757University of California
    2020PPLN waveguide[69]660 fJ (2050 nm)<0.11000Stanford University
    2020LN slab waveguides by grating metasurfaces[48]0.0525 mW (1064 nm)4.6×107Nanjing University

    Table 1. Comparisons of SHG Conversion Efficiency of Different TF-PPLN Waveguides

    In conclusion, the limitation on poling period is taken into consideration for the optimization of the waveguide to obtain higher nonlinear conversion efficiency, because the required poling period for tightly confined modes in TFLN nanowaveguides will be very small for the required QPM. However, based on waveguide-width modulation or mode-shape modulation being studied, poling-free QPM in TFLN to achieve SHG without periodical poling is obtained[45]. There is no need to meet the strict phase-matching conditions for TFLN nanostructures in the nonlinear process, which will be discussed clearly in the next section.

    2.2. Second harmonic generation in resonant nanostructures

    Aside from the applications of PPLN waveguides, TFLN is also a promising candidate to achieve SHG from resonant nanostructures[49]. Up to now, more and more studies have been performed on TFLN nanostructures. The nanostructures different from waveguides have sizes smaller than the coherence length, and there is no need to consider their phase matching. Consequently, phase-matching techniques are not necessary. The recent experiment that applied LN nanocrystal powder as the random scattering medium can achieve the cavity-enhanced SHG process, which has broken phase-matching limitation due to random QPM (RQPM) [see in Fig. 4(a)], and important progress on the Anderson localization of the nonlinear signals has been indicated[50]. Furthermore, the light field is localized on micro- and nanostructures, which can be exploited in micro–nano photonics devices. It is very suitable for realizing many nonlinear optical frequency conversion effects. Nanostructures can locally confine electromagnetic fields due to their small size and resonant design. This confinement corresponds to resonances that occur at specific wavelengths that can be tuned by changing the size, shape, and material. On the other hand, electric and magnetic resonances and their interference in high-index dielectric nanostructures strongly influence the enhancement of the nonlinear optical interactions. As shown in Fig. 4(b), LN nanocubes with sizes from 200 to 300 nm presented by Timpu et al. are all-dielectric Mie-type nanoresonators to obtain strong SHG emission for the near ultraviolet (NUV) range[51].

    (a) Schematic of the LN powder to form the cavity behavior in the SH emission at a certain pump intensity[50]; Copyright 2019, American Physical Society. (b) SEM images of single LN nanocubes to obtain the maximal SHG[51]; Copyright 2019, American Chemical Society.

    Figure 4.(a) Schematic of the LN powder to form the cavity behavior in the SH emission at a certain pump intensity[50]; Copyright 2019, American Physical Society. (b) SEM images of single LN nanocubes to obtain the maximal SHG[51]; Copyright 2019, American Chemical Society.

    Because square-shaped nanoresonators can maximize the amount of the nonlinear material, metasurfaces based on densely packed arrangements of such nanoresonators can be used to realize enhanced optical nonlinearities. Figure 5(a) demonstrates the resonant enhancement of SHG via Mie-type resonances at a pump wavelength around 1550 nm experimentally, and the SHG signal can be emitted in the zeroth diffraction order normal to the metasurface[52]. Meanwhile, LN nanograting metasurfaces with controllable SHG properties have been experimentally demonstrated to enhance second harmonic (SH) signals via electric- and magnetic-natured Mie resonances [Fig. 5(b)]. Furthermore, the SHG efficiency spectra can be tuned by changing the geometric parameters[53].

    (a) Images of SHG in an LN metasurface and SHG power depending on average power of the fundamental harmonic (FH) beam[52]; Copyright 2020, American Chemical Society. (b) Schematic of LN nonlinear metasurfaces fabricated on an X-cut LN film residing on a fused quartz substrate. Left inset gives a typical SEM image of the cross section of the metasurface, and the right inset presents the measured second-order susceptibility of the LN film used in this study[53]; Copyright 2021, John Wiley and Sons.

    Figure 5.(a) Images of SHG in an LN metasurface and SHG power depending on average power of the fundamental harmonic (FH) beam[52]; Copyright 2020, American Chemical Society. (b) Schematic of LN nonlinear metasurfaces fabricated on an X-cut LN film residing on a fused quartz substrate. Left inset gives a typical SEM image of the cross section of the metasurface, and the right inset presents the measured second-order susceptibility of the LN film used in this study[53]; Copyright 2021, John Wiley and Sons.

    To realize strong local field confinement at subwavelength volumes, nanoscale resonators such as metallic nanostructures[54] exploiting plasmonic resonances are required[5557]. Moreover, resonant metallic nanostructures simply placed on top of a nonlinear material can provide an enhanced nonlinear response[58]. Due to the pronounced plasmonic field enhancement, plasmonic nanorings have the unique advantage for highly efficient SHG. Figure 6(a) shows metallic nanorings filled with crystalline LN as a nano-optical device to enhance SHG at NIR frequencies. At normal incidence, measurements and simulations confirm the SH enhancement of about 20[59]. In order to make meaningful comparisons between optical and plasmonic nonlinear effects, while pumped at a 1240 nm wavelength, the sample consisting of a thin gold film on LN [Fig. 6(b)] can obtain the non-phase-matched SHG response at a gold/air interface to calibrate the plasmonic signal. Phase matching needs to achieve efficient nonlinear conversion in plasmonic nanostructures owing to the nature of the nonlinearity, the plasmonic dispersion, and the lower dimensionality of the propagation[70].

    (a) SEM images showing the mask for ion-beam-enhanced etching (IBEE) (Cr/SiO2 pillars) and measured SH enhancement factor and linear reflection spectrum of the fabricated sample[59]; Copyright 2015, American Chemical Society. (b) Schematic of the experiment mounted using index matching oil in a typical Kretschmann geometry[70]; Copyright 2018, Optical Society of America.

    Figure 6.(a) SEM images showing the mask for ion-beam-enhanced etching (IBEE) (Cr/SiO2 pillars) and measured SH enhancement factor and linear reflection spectrum of the fabricated sample[59]; Copyright 2015, American Chemical Society. (b) Schematic of the experiment mounted using index matching oil in a typical Kretschmann geometry[70]; Copyright 2018, Optical Society of America.

    Therefore, free from phase-matching constraints, dielectric nanostructures have contributed significantly to the control of optical nonlinearity and enhancement of nonlinear generation efficiency by engineering subwavelength structures. In addition, it is of high interest to achieve on-chip LNOI microdisk resonators with high-Q factors[71,72], which may need phase-matching conditions to realize nonlinear effects[73]. Specially, by combining a 16 µm PPLN with microdisk cavities on a chip, the nonlinear optical performance was then investigated by optimizing the period and pattern of the poled structure and by improving the cavity Q factors (6.7×105). From this result, the SHG conversion efficiency can also be significantly improved by on-chip PPLN microcavities[74]. Then, as seen in Fig. 7(a), both SHG and spontaneous parametric down conversion with a bandwidth over 400 nm on chip can be demonstrated by a high-Q (1.2×105) X-cut LN microdisk resonator, which has a radius of 45 µm and a thickness of 300 nm[75]. High-Q TFLN microresonators provide an ideal platform for on-chip nonlinear optical applications[7476]. Due to the strong nonlinear effects in a photonic molecule (PM) structure composed of two strongly coupled LN microdisks with Q factors as high as 5.1×106 (at 1565.46 nm), which can enable phase matching, more nonlinear optical phenomena including cascaded four-wave mixing and stimulated Raman scattering were observed around the SH signal [Fig. 7(b)][77]. An X-cut LN microdisk resonator with a diameter of 30 µm, featuring Q factors as high as 9.61×106 (at 1547.8 nm), has been used to generate the SH and cascaded third-harmonic waves simultaneously. Using this device, the sign of the effective nonlinear coefficient inverts periodically as light propagates along the microdisk periphery, facilitating QPM without the necessity of domain engineering, as shown in Fig. 7(c). Hence, ultrahigh efficiencies over the broad wavelength range have been realized in nonlinear processes[78]. As such, the TFLN microresonators have been a promising method to promote the performance of current integrated nonlinear photonics to a large extent.

    (a) Scanning-electron micrograph of LN microresonators to achieve modal dispersion[75]; Copyright 2017, Optical Society of America. (b) SEM images of the LN microdisk PM[77]; Copyright 2020, IOP Publishing. (c) SEM image of the X-cut LN microdisk and spectra of the pump light, the second-harmonic wave, and the third-harmonic wave. SHG conversion efficiency as a function of the in-coupled power[78]; Copyright 2019, American Physical Society. (d) Schematic depiction of the proposed nanostructure for generating SH and nonlinear simulations[82]; Copyright 2020, De Gruyter.

    Figure 7.(a) Scanning-electron micrograph of LN microresonators to achieve modal dispersion[75]; Copyright 2017, Optical Society of America. (b) SEM images of the LN microdisk PM[77]; Copyright 2020, IOP Publishing. (c) SEM image of the X-cut LN microdisk and spectra of the pump light, the second-harmonic wave, and the third-harmonic wave. SHG conversion efficiency as a function of the in-coupled power[78]; Copyright 2019, American Physical Society. (d) Schematic depiction of the proposed nanostructure for generating SH and nonlinear simulations[82]; Copyright 2020, De Gruyter.

    More recently, research on non-radiating electromagnetic states such as the anapole mode[79,80] is under active study, in particular, at optical wavelengths. Such an interest has been boosted by many studies of light–matter interactions in resonant nanostructures. For example, LN nanodisks placed on aluminum substrates can be theoretically demonstrated to achieve efficient vacuum ultraviolet (VUV) radiation with an efficiency higher than 102%, assisted by anapole resonance at the pump wavelength[81]. The results obtained in this field very recently revealed that this platform allows creating field confinement in high-index contrast nanostructures. Based on this rapidly growing technology, the nanoparticles with moderate refractive indices can generate anapole modes with a highly enhanced near field when they are supported by metallic or near-zero-index substrates. By utilizing the higher index contrast between LN and hyperbolic metamaterial (HMM) with low losses, Li et al. have demonstrated record-high 104 normalized efficiency under pump intensity of 11GW/cm2 via anapole excitation, as shown in Fig. 7(d). The design of the mentioned configuration can be tuned by tailoring the geometric parameters of the nanostructure to achieve an enhanced and flexible SHG[82].

    In addition to the resonances mentioned above, Huang et al. reported that highly efficient TFLN periodic nanostructures assisted by Fano resonance with the Q of 2350 (at 1605 nm) in 2019[24] have also showed a large SH enhancement and yielded a higher SH conversion efficiency. Further studies on the EO effect in LN can achieve a tunable SHG with voltage excitations. Other research groups have also demonstrated various methods for nanostructures to excite a high field confinement of the different resonances to obtain higher nonlinear conversion efficiency[4959,7076,85,87,88].

    This section aims to provide a brief review of the key advances on SHG processes in TFLN micro- and nanostructures, concentrating on four important structures, namely nanoparticles, metasurfaces, plasmonics, and micro- and nanodisks. Different from the structured waveguides, these resonant structures have compact size and field confinement for nonlinear optical properties. The micro- and nanostructures based on TFLN technology introduced here, but not limited to these, will together pave the way to a wide range of functional devices and promising applications. Meanwhile, many ingenious methods for TFLN nanostructures with higher SHG conversion efficiency are also summarized in Table 2.

    YearStructureMechanismStructure Parameter (Radius R, Diameter D, Height H, Thickness T)Peak Pump Intensity/Power (λFF)Q Factor (λ)ηSHηdimSH (W1)/Unstructured LNInstitute
    2012–2013Embedded Ag-LN[83,84]Fabry–Perot resonanceCoaxial aperture (Rinner=65nm, Router=135nm, H = 120 nm)–1550 nm27timesFEMTO-ST, CNRS
    2014LN microdisk resonators[85]Cavity resonanceLN microdisk (D = 28 µm, T = ∼300 nm)1.8 mW (1546 nm)1.02×105 (1507 nm)0.109Harvard University
    2015High-Q LN microresonator[76]Femtosecond laser micromachiningLN microdisk (D = ∼82 µm, T = ∼670 nm)54.6 µW (1550 nm)2.45×106 (1550 nm)2.30×103Shanghai Institute of Optics and Fine Mechanics
    2015LN-filled gold nanorings[59]Plasmonic resonanceRing Rinner=80nm, Router=120nm, H = 100 nm)4GW/cm2 (820 nm)20timesFriedrich Schiller University Jena
    2017LN microdisk resonator[75]Broadband SPDCLN microdisk (R = 45 µm, T = 300 nm)115 µW (1549.32 nm)1.2×105 (1549.32 nm)3.6×103University of Rochester
    2018PPLN microcavity[74]Whispering gallery mode (WGM)PPLN microdisk (D = ∼80 µm, T = 700 nm)1.1 mW (1550 nm)6.7×1052.2×103Nankai University
    2018Gold deposited on TFLN[70]Plasmonic SHGGold film (T = ∼30 nm)60MW/cm2 (1240 nm)2×1013Macquarie University
    2018LN nanodisks on an Al substrate[81]Anapole resonancesLN nanodisk (D = 256 nm, H = 70 nm)5.31GW/cm2 (351.3 nm)1.1528×105Institute of Lasers, State Academy of Sciences
    2019On-chip monocrystalline TFLN microdisk resonator[78]QPMLN microdisk (D = ∼30 µm, T = 600 nm)0.25 mW (1547.8 nm)9.61×106 (1547.8 nm)9.9%/mWShanghai Institute of Optics and Fine Mechanics
    2019LNO nanocubes[51]Mie resonancesNanocube (200 nm)1.7GW/cm2 (720 nm)7.6×107ETH Zürich
    2019Periodic LN bar and LN disk[24]Fano resonancesBar and disk (D = 700 nm, T = 340 nm, L = 1100 nm)3.2GW/cm2 (1605 nm)2350 (1605 nm)3.165×104Jinan University
    2019Superfine LN powder[50]Cavity-enhanced SHG1.58GW/cm2 (793.5 nm)Shanghai Jiao Tong University
    2020BPPLN microcavities[49]Multiple reciprocal vectorsMinimum domain unit (width = 100 nm)0.02 mW (1550 nm)1.43×1055.1×101Nankai University
    2020LNOI wafer[86]Fabry–Perot resonanceLN film (H = 196.8 nm)4.05GW/cm2 (840 nm)1.6×105Nankai University
    2020Nanostructured LN[82]Anapole resonancesLN nanodisk (D = 432 nm, H = 104 nm)5.31GW/cm2 (565.4 nm)5.1371×1050.1711Jinan University
    2020LN metasurface[52]ED and MD Mie resonancesNanocube (period = 870 nm, length = 700 nm)4.3GW/cm2 (1550 nm)1061.14×103Friedrich Schiller University Jena
    2021LN nanograting metasurfaces[83]Mie resonanceMetasurface (period D = 600 nm, H = 235 nm)2.05GW/cm2 (820 nm)4.2×1062timesNankai University
    2021Integrated LN microresonators[87]Ultrahigh Q performanceLN microdisk (D = 1030 µm)5 µW (1550 nm)1.56×108 (1551.52nm)602%/mWShanghai Institute of Optics and Fine Mechanics

    Table 2. Performance Comparisons of Different Micro- and Nanostructures Based on TFLN

    To date, improvements in optical efficiency have been realized in nonlinear optics supported by technologies of TFLN nanostructures. The growing activities and the great potential of TFLN-based devices will lead to novel concepts and architectures of high-performance integrated optics with highly efficient nonlinear optical devices.

    3. Conclusions and Perspectives

    In summary, we have profoundly reviewed and discussed the recent progress in the intensely developing area of nonlinear waveguides and nanostructures made by TFLN to increase the SHG efficiency. First, the brief introduction of the rapid developments of SHG and TFLN material are demonstrated. What follows is the key section of this review, which, respectively, presents the comprehensive analyses of the waveguide and nanostructure to achieve high SHG efficiency. The SHG conversion efficiency of the structured waveguides and resonant nanostructures is approaching ηSH103%W1cm2 with lower waveguide propagation loss and ηdimSH101W1, respectively. Furthermore, these two kinds of structures have been widely applied in nonlinear optics and proven invaluable in the development of future nonlinear integrated photonics. Finally, the future perspectives and main challenges of TFLN nonlinear integrated photonics are discussed. It is important to mention that TFLN is a promising material to enhance light–matter interactions, whose integration helps improve the performances of LN photonic devices.

    Further, though much smaller poling periods are required for the PPLN approaches, many TFLN integrated waveguide devices are mature for industrial implementation. In fact, a thorough study needs to be done to make TFLN an attractive and competitive integrated nonlinear optical platform. The large-scale fabrication of high-Q TFLN structures and their integration with photonics devices still require suitable advanced production tools and relevant infrastructures. However, to utilize the remarkable optical properties of the TFLN material to the largest extent, various types of waveguides and nanostructures need to be carefully and properly designed to further optimize the nonlinear optical performances of TFLN devices, considering the underlying compatible technologies. Thus, it can be concluded that integrated TFLN-based nonlinear nanophotonics are still a rapidly developing platform, and more excellent optical properties need to be studied in this platform. This will pave the way for technological development and industrialization of high-performance TFLN integrated photonics in the future.

    Overall, there are various potential applications for booming TFLN-based nonlinear devices to overcome the challenges of TFLN from design, fabrication, and practical application in recent years. These can greatly inspire advancements in different kinds of fields, such as lasers, quantum communications, optical communications, imaging, optical memories, and diffractive optics.

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