Abstract
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[
The lithium niobate (, LN) crystal is one of the most promising materials to address a considerable number of optical applications[
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[
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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)[
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 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[
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 axis were successfully prepared by standard lithography on a high-Q periodically poled X-cut Mg-doped LN film [Fig. 2(a)][
Figure 2.(a) Schematic and false-color SEM image of a periodically poled nanophotonic waveguide[
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[
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 [
Figure 3.(a) Demonstration of efficient SHG in PPLN microring resonators[
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 [
Year | TFLN Structure | Poled/Coupling Region Length | FF Power ( | Coupling Loss (dB/facet) | Waveguide Propagation Loss (dB/cm) | Institute | |
---|---|---|---|---|---|---|---|
2011 | Plasmonic waveguide[ | 1 | 1 W (1550 nm) | – | – | 1.3% | Nanjing University |
2015 | Nanoscale LN waveguides[ | 0.9 | 737 µW (1411 nm) | – | 61 | 6.9 | Friedrich Schiller Universität Jena |
2017 | PE channel waveguide[ | 3.2 | 1 mW (1385 nm) | – | 2.5 | 48 | Shandong University |
2016 | Rib-loaded SiN-PPLN[ | 4.8 | 0.5 mW (1530 nm) | ∼6.8 | 160 | University of California | |
2017 | Metasurface-assisted PM LN waveguide[ | 0.019 | 109 V/m/20 mW (1640 nm) | – | – | 1660 | Harvard University |
2017 | GA-QPM LN ridge waveguide[ | 4.9 | 84 mW (1568 nm) | 6.5 | 1 | 0.8 | University of Central Florida |
2017 | Integrated TFLN waveguide[ | 3 | 18.3 µW (1550 nm) | 4.8 | 41 | Harvard University | |
2016 | Diced ridge PPLN waveguides[ | 5.8 | 6.6 mW (1550 nm) | – | 0.57 | 77.9 | Shandong University |
2018 | PPLN on silicon[ | 20 | 10 mW (1547 nm) | – | 0.2 | 1230 | University of Central Florida |
2018 | Nanostructured PPLN waveguide[ | 4 | 220 mW (1550 nm) | ∼10 | – | 2600 | Harvard University |
2018 | LN nanophotonic waveguide[ | 8 | ∼1 mW (1540 nm) | 5 | 0.54 | 22.2 | University of Rochester |
2019 | PPLN microrings[ | – | 115 µW (1617 nm) | – | – | 250,000%/W | Yale University |
2019 | PPLNOI ridge waveguide[ | 10 | 10 mW (1590 nm) | – | – | 0.04 | Shanghai Jiao Tong University |
2019 | Dry-etched[ | 0.6 | 1 mW (1540 nm) | 6 | 3 | 4600 | University of Central Florida |
2019 | Dry-etched[ | 4 | 2.95 mW (1550 nm) | 4.3 | 0.3 | 2200 | Stevens Institute of Technology |
2020 | 1 | –(1550 nm) | 2400 | Stevens Institute of Technology | |||
2020 | Dry-etched PPLN[ | 5 | 0.1 mW (1570 nm) | 2000 | University of California | ||
2020 | PPLNOI ridge waveguide[ | 6 | 397 µW (1470 nm) | – | – | 3061 | Nanjing University+Sun Yat-sen University |
2020 | Birefringent phase-matching LN waveguide[ | 10 | 4500 W (1064 nm) | – | 0.58 | 0.87% | Shandong University |
2020 | Shallow-etched TFLN waveguides[ | 5 | 10 mW (1560 nm) | 7.7 | 1 | 3757 | University of California |
2020 | PPLN waveguide[ | 6 | 60 fJ (2050 nm) | – | 1000 | Stanford University | |
2020 | LN slab waveguides by grating metasurfaces[ | 0.05 | 25 mW (1064 nm) | – | – | Nanjing 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[
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[
Figure 4.(a) Schematic of the LN powder to form the cavity behavior in the SH emission at a certain pump intensity[
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[
Figure 5.(a) Images of SHG in an LN metasurface and SHG power depending on average power of the fundamental harmonic (FH) beam[
To realize strong local field confinement at subwavelength volumes, nanoscale resonators such as metallic nanostructures[
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[
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[
Figure 7.(a) Scanning-electron micrograph of LN microresonators to achieve modal dispersion[
More recently, research on non-radiating electromagnetic states such as the anapole mode[
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[
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.
Year | Structure | Mechanism | Structure Parameter (Radius | Peak Pump Intensity/Power ( | Institute | |||
---|---|---|---|---|---|---|---|---|
2012–2013 | Embedded Ag-LN[ | Fabry–Perot resonance | Coaxial aperture ( | –1550 nm | – | – | FEMTO-ST, CNRS | |
2014 | LN microdisk resonators[ | Cavity resonance | LN microdisk ( | 1.8 mW (1546 nm) | – | 0.109 | Harvard University | |
2015 | High- | Femtosecond laser micromachining | LN microdisk ( | 54.6 µW (1550 nm) | – | Shanghai Institute of Optics and Fine Mechanics | ||
2015 | LN-filled gold nanorings[ | Plasmonic resonance | Ring | – | – | Friedrich Schiller University Jena | ||
2017 | LN microdisk resonator[ | Broadband SPDC | LN microdisk ( | 115 µW (1549.32 nm) | – | University of Rochester | ||
2018 | PPLN microcavity[ | Whispering gallery mode (WGM) | PPLN microdisk ( | 1.1 mW (1550 nm) | – | Nankai University | ||
2018 | Gold deposited on TFLN[ | Plasmonic SHG | Gold film ( | – | – | Macquarie University | ||
2018 | LN nanodisks on an Al substrate[ | Anapole resonances | LN nanodisk ( | – | – | Institute of Lasers, State Academy of Sciences | ||
2019 | On-chip monocrystalline TFLN microdisk resonator[ | QPM | LN microdisk ( | 0.25 mW (1547.8 nm) | – | 9.9%/mW | Shanghai Institute of Optics and Fine Mechanics | |
2019 | LNO nanocubes[ | Mie resonances | Nanocube (200 nm) | – | – | ETH Zürich | ||
2019 | Periodic LN bar and LN disk[ | Fano resonances | Bar and disk ( | 2350 (1605 nm) | – | Jinan University | ||
2019 | Superfine LN powder[ | Cavity-enhanced SHG | – | – | – | – | Shanghai Jiao Tong University | |
2020 | BPPLN microcavities[ | Multiple reciprocal vectors | Minimum domain unit (width = 100 nm) | 0.02 mW (1550 nm) | – | Nankai University | ||
2020 | LNOI wafer[ | Fabry–Perot resonance | LN film ( | – | – | Nankai University | ||
2020 | Nanostructured LN[ | Anapole resonances | LN nanodisk ( | – | 0.1711 | Jinan University | ||
2020 | LN metasurface[ | ED and MD Mie resonances | Nanocube (period = 870 nm, length = 700 nm) | – | Friedrich Schiller University Jena | |||
2021 | LN nanograting metasurfaces[ | Mie resonance | Metasurface (period | – | Nankai University | |||
2021 | Integrated LN microresonators[ | Ultrahigh | LN microdisk ( | 5 µW (1550 nm) | – | 602%/mW | Shanghai 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 with lower waveguide propagation loss and , 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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