3.1.1.

Li-out diffusion

The Li-out diffusion method was first demonstrated by Kaminow and Carruthers.⁹⁷ As LN can crystallize in a slightly nonstoichiometric form ${({\mathrm{Li}}_2\mathrm{O})}_y{({\mathrm{Nb}}_2{\mathrm{O}}_5)}_{1-y}$, its extraordinary index $n_e$ will increase linearly when the alloying variable $y$ decreases within a narrow range between 0.48 and 0.5. The change of $y$ can be realized by the Li-out diffusion of ${\mathrm{Li}}_2\mathrm{O}$ from the LN surface since the lithium ion has high mobility. Therefore, by applying a thermal treatment of an LN sample for several hours at high temperature, the refractive index will be changed in an LN target area as Li is diffused away.⁹⁶ This method has two main disadvantages. One is that the refractive-index change occurs only in $n_e$, and the other is that it is difficult to achieve selective diffusion.⁹⁸ Therefore, although Li-out diffusion is proposed in principle and has been demonstrated in experiment,^97,99,100 it is seldom used for photonics device fabrication. Alternatively, ion-in diffusion is more frequently adopted for LN device fabrication.¹⁹

3.1.2.

Metal ion-in diffusion

During the metal ion-in diffusion process, a thin layer of metal is first evaporated onto an LN crystal surface, and then the crystal is heated at a temperature in a nonreactive atmosphere for several hours to make the metal diffuse into the crystal.^{19,98,101–112} A schematic of a photonic waveguide fabrication process based on ion-in diffusion is shown in Fig. 2(a). Different metals have been used for ion-in diffusion. Some of these are summarized in Table 2. The most widely used metal is titanium (Ti).¹⁹ The metal ion-in diffusion method is an easy and economical method for fabricating photonics devices. For experimental demonstration, as shown in Table 2, propagation loss below $1\mathrm{dB}/\mathrm{cm}$ in photonics wires using such method has been realized.^98,113–115 Before the appearance of TFLN devices, metal ion-in diffusion was widely used in photonic device fabrication. However, the refractive index difference formed by this method is very small ($\text{normally}<0.02$). Another method called PE can result in a slightly larger refractive index contrast,^20,21 although also not very high overall.

Fig. 2

Process flow of planar LN device fabrication. Illustration of (a) metal ion-in diffusion and (b) PE methods for planar photonic device fabrication in bulk LN crystals (dimensions are not drawn to scale). PR, photoresist.

Table 2

Summaries of metal ion-in diffusion method. T, temperature; TE, transverse electric; TM, transverse magnetic; N.A., not available/applicable; Zn, Zinc.

3.1.3.

Proton exchange

In 1981, PE was first used to fabricate optical waveguides in LN.^20,21 Different from metal ion-in diffusion, hydrogen ions are diffused into an LN crystal and are then exchanged with lithium ions during the PE process. The lithium ions then diffuse out of the crystal and finally the LN crystal is partially transformed into a new chemical with composition ${\mathrm{H}}_x{\mathrm{Li}}_{1-x}{\mathrm{NbO}}_3$ within a certain surface depth. The ${\mathrm{H}}_x{\mathrm{Li}}_{1-x}{\mathrm{NbO}}_3$ compound has several different phase states depending on the component proportion ($x$), as validated by X-ray rocking curve analysis results and some other techniques.¹¹⁶ Several proton sources have been used for the PE process, such as benzoic, octanoic, adipic, glutaric, stearic acids, and their mixtures.^117–119 Among them, benzoic acid is the most widely used proton source, as it has a high boiling point and stability throughout its liquid phase.¹¹⁶ A typical process flow for photonic device fabrication using PE is shown in Fig. 2(b). Compared with the metal ion-in diffusion method, the PE process has a much higher photorefractive damage threshold for visible light confining and transmission,¹²⁰ as well as a higher resulting refractive index contrast.¹²¹ The PE process has been widely used to fabricate photonics devices in LN crystals with different orientations.^{116,121–126} Other than PE, a few other ions have also been used in such an exchange process with lithium to realize a refractive index change in LN.^127–130 For example, Shah et al. demonstrated a kind of LN waveguide by immersing X-cut LN crystals in silver nitrate at 360°C temperature for several hours, and lithium/silver ion exchange was observed.¹²⁷ Although with the more recent development of TFLN, direct dry etching has recently received more attention as it results in a much larger index contrast than either PE or ion-in diffusion, the PE method still has certain advantageous application scenarios, especially to assist dry/wet etching technologies to realize some innovative photonic devices.¹³¹

3.1.4.

Ion implantation

Ion implantation can also be used for fabricating waveguides in LN crystals.^132–137 Different from the above-mentioned methods, ion implantation results in a decrease of refractive index around the target area. Destefanis et al. implanted helium (He) ions into LN surfaces with about 1 to 2 MeV energies.¹³² They formed a low refractive index layer in the 2 to $4\mu \mathrm{m}$ range just below the LN surface. Such a region of lower refractive index can form sufficient refractive index contrast for light confinement. It is worth mentioning that such a low index layer is very easy to etch by wet etchant, compared with these areas without damage. Therefore, such a buried layer can also be etched away to obtain a much larger refractive index difference.¹³⁸ We will introduce this method again when we discuss wet etching. Other than the He ion, other ions have also been demonstrated for ion implantation of LN.^133,135 Although a very large index difference can be obtained based on such methods, many more crystal defects will also be produced during the physical bombardment. Therefore, ion implantation has not been widely used for the formation of photonics devices in LN crystals. But He ion implantation is widely used in CIS technology, which is used for fabrication of TFLN. More details about TFLN process technologies are introduced in the subsequent sections.

3.2.1.

Crystal ion slicing

In contrast, CIS technology can realize high-quality single crystal TFLN wafers and has widely been used since 1998.^5,142–146 Figure 3(a) shows a schematic of CIS technology. The processing starts from a bulk LN wafer, which is usually grown using the Czochralski method.¹⁴⁷ Then, the bulk LN is implanted with He ions to a specific layer thickness (which depends on the desired TFLN thickness) to form a sacrificial layer. Helium (He) implantation is performed using an ordinary ion implanter. The He ion is the most widely used ion due to its small atomic mass.¹⁴² As the ion implantation will cause LN crystal damage that results in a different subsequent etch rate or thermal properties compared with undamaged areas, it can be comparatively easy to separate the top thin film layer from a bulk LN using a simple etching method, such as hydrofluoric acid (HF) etching. Then, the thin film layer is bonded to a bottom insulator layer using mature wafer bonding methods.⁹ Usually, the bottom insulator is selected to be a material with a lower refractive index, such as silicon oxide. The large resulting refractive index contrast between the LN and the bottom insulator enables strong light confinement within the LN layer. It is worth mentioning that the bottom substrate is not only restricted to LN, glass or Si is also possible. Furthermore, a thin layer of metal can also be inserted between the substrate and insulator layer to form a bottom metal electrode.

Fig. 3

Process flows of (a) CIS and (b) lapping and polishing technologies. Dimensions are not drawn to scale.

3.2.2.

Lapping and polishing

In addition to CIS technology, the lapping and polishing method has also been developed for LN film fabrication. Figure 3(b) shows the process flow of the lapping and polishing method. The thick bulk LN wafer is first bonded onto a substrate with an insulator (of lower refractive index) inserted between them. Then, the top LN layer is thinned down to a few microns thickness by successive lapping and polishing.^29,30 Compared with the CIS method, the LN film thickness after lapping and polishing is a little bit thick (typically around a few microns) and thus results in a larger device size. However, there are some advantages of LN films made by this technology. First, lapping and polishing is a purely mechanical process and thus has a smaller influence on crystal quality compared with CIS technology. In addition, the larger waveguide core in thick LN films enables a better coupling efficiency and a much higher laser-damaged threshold compared with thinner LN films.^{29,30,148–154} Therefore, lapping and polishing is a good complement to CIS technology.

3.4.1.

Dry etching

For both bulk LN and TFLN, there are strong motivations toward direct etching to form 3D structures.¹⁷⁸ Dry etching is one of the main methods to realize such a goal. The plasma of a chemically active gas together with an inert ion plays the main roles during dry etching. As summarized in Table 4, many different kinds of dry etching methods have been demonstrated during the past decades. These demonstrations can be divided into three categories. One involves using the plasma of pure halogen ions,^{179,180,182,183,194,196} such as sulfur hexafluoride (${\mathrm{SF}}_6$), carbon tetrafluoride (${\mathrm{CF}}_4$), and boron trichloride (${\mathrm{BCl}}_3$). As halogen ions will chemically react with the lithium, the reactant produced in the process of dry etching will be a problem and later affect device performance. Figures 5(a) and 5(b) show the scanning electron microscopy (SEM) image and current changed along the etching depth in end point detection after ${\mathrm{SF}}_6$ etching, which clearly shows the byproduct layer.¹⁸² Nagata et al. tried to remove the reactant using an additional wet etching method.¹⁷⁹ The second category involves mixing the halogen ions with argon (Ar) in the plasma.^{178,182,185,186,188,189,191,193,195} Ar ion-based etching is a pure physical bombardment process. By engineering the gas flow ratio between the Ar and halogen element, the etch rate, profile, and surface conditions can be improved. In such a method, the halogen ions can still be a problem and affect the etching quality. Therefore, people have developed the third category of using pure Ar gas for etching.^{175,184,190,195} Compared with the first two methods, pure Ar-based etching has a lower etching rate, as the ion bombardment is a pure physical process. Its advantages are flat and clear surface condition, as evidenced by the SEM, AFM, and X-ray photoelectron spectroscopy (XPS) results shown in Figs. 5(c)–5(e).¹⁷⁵ By using Ar-based etching, nearly vertical side walls^52,175,197 and ultralow propagation loss¹⁹⁷ have been observed; this method can then find wide use for photonics device fabrication. During such dry etching processes, a hard mask is typically used in addition to photoresist to improve the etching selectivity, as summarized in Table 4.

Table 4

Summary of LN dry etching technologies. PMMA, polymethyl methacrylate; HSQ, hydrogen silsesquioxane; MMA, methyl methacrylate; N.A., not available/applicable; RIE, reactive ion etching; ICP, inductively coupled plasma.

Fig. 5

Dry etching results of LN. (a) SEM image of the LN cross section and (b) current changed along etching depth in end point detection after ${\mathrm{SF}}_6$ based etching. (a) and (b) Adapted from Ref. 182 © 2008 American Institute of Physics (AIP). (c) SEM, (d) AFM, and (e) XPS images of LN sample after Ar-based dry etching. (c)–(e) Adapted with permission from Ref. 175.

3.4.2.

Wet etching

LN can also be etched by the wet etching method. Compared with dry etching, wet etching can realize a more uniform surface and much higher etch rates.¹⁹⁸ In addition, wet etching is an economical and simple method compared with other etching technologies and is widely used in other material systems.¹⁰⁹ It has been demonstrated that after the PE process, for instance, LN shows a larger etch rate using the mixture of HF and nitric acid (${\mathrm{HNO}}_3$), compared with LN areas that are not subjected to the PE process.¹⁹⁹ Therefore, the combination of PE and a mixture of HF and ${\mathrm{HNO}}_3$ etchant is widely used for LN wet etching.^{109,199–204} Compared with dry etching, its etched sidewall is not too deep and there can be an underetching problem. Ting et al. demonstrated that the etching depth and aspect ratio can be improved using a diluted PE source with a lithium compound.¹⁹⁸ By optimizing different molar percentages of adipic acid and lithium compounds, a very high aspect ratio [defined as etched depth $D$ divided by horizontal distance of the slant $H$, as shown in Figs. 6(a)–6(f)] has been realized. Such an improvement can be clearly seen from the SEM images shown in Figs. 6(a)–6(f). The underetching problem can be alleviated by annealing (improve the adhesion of hard mask), as demonstrated by Hu et al.²⁰⁵ Some researchers also demonstrated that by adding some ethanol into the $\mathrm{HF}-{\mathrm{HNO}}_3$ mixture, the etched surface of LN can be much smoother.²⁰⁵ PE is not the only way to cause a subsequent LN etching rate difference. Wang et al. found that ${\mathrm{O}}^{+}$ and ${\mathrm{Si}}^{+}$ ion implanted regions can be etched more easily than those protected by a photoresist mask using a mixture of HF and ${\mathrm{HNO}}_3$ at room temperature.²⁰⁶ Si et al. demonstrated that He ion implantation caused crystal damage in LN, which consequently also showed a higher chemical etching rate,¹³⁸ and photonic crystal waveguides were successfully realized using such a method, as shown in Fig. 6(g). Using an ion beam enhanced etching method, a similar damaged layer can also be obtained.²⁰⁷ Copper (Cu) ion implantation with the assistance of HF solution has also been validated as a good way to achieve wet etching of LN crystals, and an etching rate of around $100\mathrm{nm}/\mathrm{s}$ has been observed.³² Up until now, most of the reported wet etching methods have been demonstrated in bulk LN crystals. But we believe wet etching is also suitable for TFLN devices, especially in some scenarios where cantilever structures are needed.

Fig. 6

Wet etching results of LN. SEM images of the LN etched cross section using undiluted (a) 0% and (b) 20% of adipic acid, (c) 0% and (d) 20% of adipic acid with 0.6% of lithium benzoate, (e) 20% and (f) 30% of adipic acid with 0.3% of lithium carbonate (concentrations in percent represent mole fractions). (a)–(f) Adapted from Ref. 198 © 2006 Wiley Periodicals, Inc. (g) SEM image of photonic crystals (PhCs) using ion implantation and wet etching. Adapted with permission from Ref. 138 © 2010 American Vacuum Society.

3.4.3.

Other patterning/etching technologies

In addition to the above-mentioned methods, some other methods have also been used for the patterning/etching of LN. Most of them have been based on the physical polishing/milling process, such as focused ion beam (FIB) milling,^208–211 dicing,^212,213 femtosecond laser micromachining,^214–218 and chemomechanical polish lithography (CMPL).^{214,215,217,219–223} FIB itself is a pure mechanical milling process, which either can be used along^208,209,211 or together with other dry etching methods for layout patterning.^208,210 For prototype validation, FIB is a good choice. But it will not be suitable for mass production of devices of any type due to economic and operability considerations. By contrast, dicing is an efficient method for quick fabrication of optical waveguides, as it can realize smooth sidewalls and a high aspect ratio.^212,213 Recently, femtosecond laser ablation combined with FIB^214–218 or CMPL^{214,215,217,219–221} has also been demonstrated for LN crystal patterning, which has received wide attention. Such a method can be divided into three steps. First, the LN crystal or metal mask is ablated with tightly focused femtosecond laser pulses, which is usually performed in water to reduce the possibility of debris and cracks.²¹⁵ Second, the LN crystal is polished by the FIB or CMPL process. Third, the underlying oxide is partially etched using a wet etching method. Very high quality factor ($Q$ factor) ($\sim {10}^8$) microresonators have been demonstrated based on such a method,²²⁴ which proves the very high quality of the etched sidewall. These mechanical milling methods together with the above introduced processing technologies pave the way for the fabrication of various LN photonics devices.

4.1.1.

Microresonator

In past years, various kinds of microresonators have been demonstrated both in bulk LN and TFLN platforms.^{43,181,192,193,197,214–218,221,222,224,227–240} In bulk periodically poled Z-cut LN crystals, a high $Q$ factor of $2\times {10}^7$ was measured by mechanically polishing the LN crystal,²²⁷ as shown in Fig. 7(a). The $Q$ factor of the TFLN-based microresonator is increased gradually. In 2014, Wang et al. demonstrated a kind of microdisk in TFLN using Ar-based electron-cyclotron resonance (ECR) reactive ion etching (RIE), and its measured $Q$ factor was around ${10}^5$ [Fig. 7(b)].²²⁸ This was just at the early stage of TFLN devices. Such a value can be further improved by optimizing design and processing technologies. For example, a microring with a $Q$ factor up to ${10}^7$ was demonstrated in 2017 using an optimized Ar-based dry etching process, which corresponds to a propagation loss as low as $2.7\mathrm{dB}/\mathrm{cm}$.¹⁹² An SEM image of this kind of etched microring and its corresponding measured transmission spectrum are shown in Figs. 7(c) and 7(d), respectively. These results validate the fact that Ar-based etching is a suitable method for TFLN patterning. Meanwhile, many other groups have also demonstrated high $\sim {10}^5$ $Q$ factor based on such a method.^{197,230,231,234,236,237} More recently, a record-high $Q$ factor up to ${10}^8$ (calibrated by considering transmission rates of modes) at 1550 nm wavelength was achieved using the femtosecond laser-assisted CMPL method²²⁴ [as shown in Figs. 7(e) and 7(h)], which indicates a propagation loss of around $0.28\mathrm{dB}/\mathrm{m}$. Such a result is realized using a pure mechanical polishing process, thus avoiding the possible ion-induced lattice damage. This ultralow loss device opens up many prospects toward broad application of LN photonics, especially for various nonlinear applications. Compared with bulk LN devices, low loss or high $Q$ factors are not the main advantages of TFLN-based microresonator, however, as ultrahigh $Q$ factors have also been demonstrated in bulk LN devices.²²⁷ More advantageous is the fact that the larger refractive index contrast in TFLN enables various microresonator forms that are ordinarily extremely difficult or impossible to realize in bulk LN. Here, our discussion on microresonators is limited to microrings or microdisks. Actually, some other types of microresonators have also been demonstrated in LN (mainly TFLN), such as photonics crystal (PhC)²⁴¹ and distributed Bragg reflector (DBR)-based Fabry–Perot (DBR-FP)^242,243 resonators. Recent progress of EO modulators based on PhC and Fabry–Perot microresonators is discussed in Sec. 4.2.1.

Fig. 7

LN-based microresonators. (a) Schematic experimental setup for characterizing a mechanical polishing bulk LN whispering-gallery resonator and its corresponding measured $Q$ factor. Adapted from Ref. 227 © 2011 AIP. (b) Resonance spectra of the fabricated microdisk using ECR RIE technology in TFLN. Inset shows the microscope image of tapered fiber coupling on top of the device. Zoom in views are the details of representative resonance dips. Adapted with permission from Ref. 228 © 2014 Optical Society of America (OSA). (c) SEM (top) and microscopic images (bottom) of microring and microracetrack ring with various lengths, and (d) its measured transmission spectrum. (c) and (d) Adapted with permission from Ref. 192 © 2017 OSA. (e) Microscope image of the waveguide coupled TFLN microring and (f) its measured transmission spectrum. The Q factors for (g) TE and (h) TM modes fitted by Lorentz-shape curves. (e)–(h) Adapted with permission from Ref. 224 © 2022 Chinese Optical Society (COS).

4.1.2.

Grating coupler

Due to the strong mode confinement in TFLN, many other passive blocks have subsequently been demonstrated.^{85,131,176,177,223,244–269} The fiber to chip interface is a basic function as it determines how much light can be coupled into a photonic device. There are two types of methods to couple the external light into a chip. One is vertical coupling of light onto a chip using a GC.^{176,177,245,247,248,252,254,259,265,266,268,269} The other is based on edge coupling,^{223,246,249,258} which couples light into a chip horizontally. According to the operation principle, GCs can also be divided into two categories, one is the one-dimensional (1D) GC and the other is the 2D GC, both of which have been demonstrated in the LN platform. For a 1D GC, the design strategy is to optimize the periodic structure to realize the phase matching condition for best coupling efficiency. A high coupling efficiency of $-1.42/-2.1\mathrm{dB}$ ($\sim 72\%/61.6\%$) has been realized in Z-cut TFLN with a bottom Au reflector, and the grating has been designed with a chirped structure to improve its coupling efficiency,²⁵⁹ as shown in Figs. 8(a)–8(c). The typical coupling efficiency of a 1D GC is between $-3$ and $-7\mathrm{dB}$.^{176,177,245,247,248,252,254,268,269} A 2D GC is more functional as it can realize the demultiplexing of orthogonal polarization multiplexed signals while coupling light into on-chip devices.²⁷⁰ Although there are many reports about 2D GC in other material systems, similar research is very limited in TFLN. Chen et al. demonstrated a kind of 2D GC in TFLN with measured coupling efficiencies of $-5.13\mathrm{dB}$ at 1561 nm for P-polarized light and $-7.6\mathrm{dB}$ at 1568 nm for S-polarized light,²⁶⁵ as shown in Figs. 8(d)–8(f). Their measured 1-dB bandwidths for both P- and S-polarized lights are around 30 nm. Such demonstrated results are far from comparable to those of its counterparts and hence need more efforts to improve. Both 1D and 2D GCs allow for wafer scale on-chip testing without the need of chip dicing. GCs are more difficult to realize in bulk LN compared with TFLN due to its smaller refractive index contrast.

Fig. 8

LN-based GCs. (a) Schematic structure, (b) simulated electric field distribution and (c) measured transmission spectrum of 1D chirped GC in TFLN. (a)–(c) Adapted with permission from Ref. 259 © 2020 OSA. (d) Schematic structure of a 2D GC in TFLN. Measured and simulated (e) transmission spectra and (f) polarization dependence loss of the TFLN 2D GC. (d)–(f) Adapted with permission from Ref. 265 © 2021 OSA.

4.1.3.

Edge coupler

Compared with the vertical GC, an edge coupler is less sensitive to polarization, has a larger operating bandwidth, and enables lower insertion loss. Its main drawbacks are that accurate facet polishing and sample dicing are needed. For edge coupling, the main optical loss mechanism is the mode mismatch between the fiber and on-chip waveguide. Therefore, design strategies are to tailor both the fiber and waveguide modes to reduce the mode mismatch and thus improve the coupling efficiency. By adiabatically tapering a standard single-mode fiber to match a specially designed LN waveguide, Yao et al. demonstrated a measured $-1.32/-1.88\mathrm{dB}$ coupling efficiency for transverse electric (TE)/magnetic (TM) modes.²²³ Using a monolithic bilayer mode size converter, He et al. also demonstrated a measured $1.7\text{-}\mathrm{dB}/\mathrm{facet}$ coupling loss,²⁴⁶ as shown in Figs. 9(a)–9(c). By combining a silicon oxynitride cladding waveguide with the bilayer LN taper, the coupling efficiency is further reduced to $0.54/0.59\mathrm{dB}$ per facet at 1550 nm for TE/TM light.²⁶⁶ A multiple layer mode size converter shows a possible solution for low loss edge coupling in TFLN. Compared with the SOI platform, the refractive index contrast in TFLN is smaller and thus results in weaker mode confinement and larger waveguide bending radii. However, a comparable low coupling loss can also be obtained in TFLN by reducing the mode size mismatch between the fiber and waveguide. We believe there is still room for performance enhancement of edge coupling in the LN platform, as lower coupling loss ($<0.5\mathrm{dB}/\text{facet}$) has already been demonstrated in ion-in diffused bulk LN devices.²⁷¹

Fig. 9

LN-based edge coupler. (a) Schematic structure of the bilayer edge coupler and its corresponding mode profiles at different positions. (b) Simulated and measured coupling efficiency versus different tip widths in the tapered slab region. (c) Additional insertion loss with respect to coupling misalignment (TE mode). (a)–(c) Adapted with permission from Ref. 246 © 2019 OSA.

4.1.4.

Other passive devices

In addition, mode-related devices,^{131,244,250,251,255,261,262,267} TO-based devices,^85–87 Bragg grating filters,^{253,256,257,264} optical true delay lines,²⁶³ and optical phased arrays²⁶⁰ have also been demonstrated in the LN platform. Most of them rely on the large refractive index contrast available in TFLN. Similar to its counterpart, which is SOI, these passive blocks can be combined together to form more powerful chip scale PICs. And researchers have already tried to do so. For example, a two-mode (de)multiplexer is realized by combining a passive Mach–Zehnder interferometer (MZI) and an EO tuning electrode.¹³¹ This will be a trend as PICs can solve power consumption and device size problems that are inherent in conventional bulk LN devices.

4.2.1.

Electro-optic modulator

Compared with other material systems, the most attractive feature of LN is its large EO coefficient, which can be used for fabricating high-performance modulators. Different from the plasma dispersion effect-based modulators in silicon photonics¹⁰ and the electro-absorption-based modulators in III–V platforms,¹¹ there is no carrier dynamic process in LN-based modulators where the speed is mainly limited by the microwave electrode. Therefore, Pockels effect-based linear LN modulators can achieve higher modulation speeds.^31,52 Therefore, LN-based modulators are widely used in current fiber-optical communication systems. For a long time, these modulators have been fabricated in bulk LN crystals using the technologies described in Sec. 2. For example, a Ti-diffused ring resonator in a bulk LN crystal can achieve around a $1.565\mathrm{pm}/\mathrm{V}$ tuning efficiency.⁴³ However, these bulky devices have a large device size and cannot meet the requirements of dense integration in current/future large capacity optical interconnect systems. The TFLN platform is well poised to solve these problems. In the same year, Guarino et al. demonstrated an EO tunable microring resonator in TFLN using CIS and wafer bonding technology.¹⁸¹ Its structure is shown in Fig. 10(a), where the Z-cut TFLN is directly etched and inserted between the top and bottom electrodes. As shown in Fig. 10(b), the measured tuning efficiency is about $0.105\mathrm{pm}/\mathrm{V}$ from the observed wavelength shift.¹⁸¹ Such a value is lower than results demonstrated in bulk LN, which is probably due to the lower electric field strength inside the waveguide and can perhaps be improved by changing the design. Subsequently, with the high-quality TFLN that is now commercially available, various kinds of modulators have been demonstrated.^{31,52,88,241–243,272–288} Some of the reported modulators in LN platforms are summarized in Table 5.

Fig. 10

TFLN EO tunable microring resonator. (a) Schematic structure (top), cross section (bottom left), and SEM images of the Z-cut TFLN microring modulator, and (b) its EO resonance shift curve. (a) and (b) Adapted with permission from Ref. 181 © 2007 Nature Publishing Group.

Table 5

Summary of LN-based EO modulators. HI, heterogeneous integration; DMT, discrete multitone; APE, annealed proton exchange; N.A., not available/applicable.

In terms of device configuration, there are some structures such as microrings,^272,292 MZIs,^{31,52,88,272,274,275,277,278,281–285,287} Michelson interferometers,^273,276,279 PhC cavities,²⁴¹ and DBR-FP modulators,^242,243 as shown in Table 5 and Fig. 11. Different configurations have different advantages. For example, microring-based modulators have compact sizes,^272,292 and Michelson interferometer modulators (MIMs) [Fig. 11(f)] can realize reduced half-wave voltage-length product ($V_{\pi }L$) due to doubled interaction between the light wave and electric field compared with MZI structures.^273,276,279 The improved tuning efficiency of PhC-based [Figs. 11(d) and 11(e)] and DBR-FP-based [Figs. 11(g)–11(i)] modulators is based on a similar principle.^241–243

Fig. 11

TFLN-based modulators. (a) Microscopic image of TFLN MZI modulator (inset is its schematic cross section). (b) Measured transmission spectrum of a 2-cm long device. (c) Measured high speed data transmission results of $100\mathrm{Gb}/\mathrm{s}$ NRZ, $140\mathrm{Gb}/\mathrm{s}$ 4-ASK, and $210\mathrm{Gb}/\mathrm{s}$ 8-ASK signals. (a)–(c) Adapted with permission from Ref. 31 © 2018 Springer Nature Limited (SNL). (d) SEM images (top: full SEM image; bottom: zoom-in image of the PhC details). (e) Schematic structure of the TFLN PhC modulator. (d) and (e) Adapted with permission from Ref. 241. (f) Schematic structure of the MIM, and insets are cross section mode profiles at different positions. Adapted from Ref. 276. (g) Schematic structure of the TFLN DBR modulator, and SEM images of the (h) DBR and (i) modulation region. (g)–(i) Adapted with permission from Ref. 242 © 2021 COS. (j) Schematic structure and (k) measured $S_{21}$ curves of the TFLN-based DP-IQ modulator. X and Y represent two orthogonal polarization states, I and Q represent in-phase and quadrature branches. (j) and (k) Adapted with permission from Ref. 294 © 2022 Optica.

In most reported results, the amplitude of the input light is modulated by an applied electrical signal (amplitude modulation). As shown in Figs. 11(a)–11(c), around $100\mathrm{Gb}/\mathrm{s}$ nonreturn to zero (NRZ) and up to $210\mathrm{Gb}/\mathrm{s}$ 8-ASK (8 level amplitude modulation) are measured using an optimized MZI TFLN modulator. For a 20-mm long arm length, its measured $S_{21}$ (defined as the forward transmission coefficient from port 1 as input to port 2 as output when port 2 is matched for a two-port network and was widely used to characterize the speed of optoelectronic devices) is above 40 GHz. The authors also demonstrated that its $S_{21}$ can be improved up to above 80 GHz by reducing the arm length.³¹ Some other modulation dimensions can also be added into the TFLN modulator.^88,294 In 2020, Xu et al. first introduced the phase modulation dimension into a TFLN modulator and realized transmission speeds up to $320\mathrm{Gb}/\mathrm{s}$ based on the 16 quadrature amplitude modulation (QAM) format.⁸⁸ More recently, researchers from the same group further demonstrated a TFLN-based dual-polarization in-phase (DP-IQ) modulator with record $1.96\mathrm{Tb}/\mathrm{s}$ data rate.²⁹⁴ Figures 11(j) and 11(k) show a schematic of the DP-IQ modulator with a 2.35-cm long arm and its corresponding measured $S_{21}$ curves. The researchers used double polarizations and a quadrature amplitude phase modulation format. For each single MZM, the measured $S_{21}$ is above 110 GHz under 1 V voltage. Such a high bandwidth enables 400-QAM and thus realized a record $1.96\mathrm{Tb}/\mathrm{s}$ total data capacity. By engineering the electrode transmission line^282,295 or introducing more multiplexing dimensions, such as mode division multiplexing,²⁴ the transmission speed/volume can be further improved.

In addition to speed, some other key metrics are also critical to evaluating modulator performance, such as $V_{\pi }L$, as well as insertion loss (IL). As shown in Table 5, most reported results have an average $V_{\pi }L$ between 1 and $3\mathrm{V}·\mathrm{cm}$.^{52,88,272,273,276–280,282–285,287} Using a dual-capacitor electrode layout, the $V_{\pi }L$ can be reduced to as low as $0.64\mathrm{V}·\mathrm{cm}$.²⁸¹ Such a factor is highly related to the device structure and can likely be improved by optimizing the design. The overall low $V_{\pi }L$ values of LN-based modulators are highly desirable for their application scenarios.

It can be observed from Table 5 that most of the reported LN-based modulators have low IL (excluding coupling loss),^{31,52,88,241–243,272,275,277,282,287} which depends on the device dimension and fabrication technology. A typical bulk LN-based modulator is usually fabricated using ion-in diffusion technology. For TFLN modulators, Ar plasma-based dry etching is widely used to pattern the LN waveguide structures, which can realize a low propagation loss (also depends on etching conditions) compared with bulk devices. By optimizing the processing technology, the TFLN waveguide with a more prevailing propagation loss of lower than $0.2\mathrm{dB}/\mathrm{cm}$ can be obtained.³¹ Thus, high-performance LN-based modulators with lower IL can be expected in the future.

Extinction ratio (ER) is another metric to characterize a modulator’s performance. In most published LN-related results, ER is defined by the valley value [such as the minimum value shown in Fig. 10(b)] of the transmission spectrum.¹⁸¹ A high ER gives a better modulation signal quality, such as more open eye diagrams during data transmission experiments. The typical ER in literature is between 10 and 30 dB,^282,285,292 which can be improved by optimizing either process technology or design. Take the MZI as an example, and usually the Y-branch is used as the power splitter. However, if the optical power difference between the two arms is too large due to the imperfect fabrication process technology, its ER will be degraded. By optimizing the fabrication process, or changing the Y-branch structure with a multimode interferometer (MMI, more tolerant to fabrication process error compared with Y-branch),²⁹⁶ a higher ER can be realized. In addition, using cascaded MZI²⁷⁴ or Bragg grating waveguide,²⁴³ ultrahigh ERs have been demonstrated, which are other examples of improving ER by optimizing design.

In summary, there are many different criteria to evaluate a modulator’s performance. Although there are many different kinds of LN-based modulators that have been demonstrated, none of them can realize all the best metrics at the same time. The tradeoffs exist, thus there are still optimizing spaces for the research community.

4.2.2.

Other electro-optic devices

The superior EO effect of LN is not only limited to the modulator application, as it can also be extended to various other kinds of application scenarios. One of the key benefits that may result from the development of TFLN is that a variety of different components can be integrated on the same chip to enable more functionality overall. The physics behind them is generally the same as EO modulators, which is by changing the material refractive index and thus the phase. Some typical EO-based applications demonstrated on TFLN platform have been summarized here.

First, the fast EO tuning features can be used in optical fiber communication or optical interconnects. For example, Fig. 12(a) shows EO tunable interleavers,²⁹⁷ which can be used as tunable filters or wavelength-selective switches. Their measured tuning sensitivity is $\sim 18$ [Fig. 12(b)] and $\sim 16\mathrm{pm}/\mathrm{V}$ [Fig. 12(c)] for TE and TM modes, respectively. Compared with TO-based tuning, EO tuning enables a much higher speed and thus has broader application perspectives.

Fig. 12

EO tunable interleaver in TFLN. (a) Schematic structure of TFLN waveguide interleaver, and its measured tunable transmission spectra for (b) TE and (c) TM polarized input light. (a)–(c) Adapted with permission from Ref. 297 © 2018 OSA.

Second, the EO effect of LN can also be used for controlling the optical frequency, which enables broad applications, such as advanced photonic computation and frequency-domain photonic quantum computers.^298,299 Figure 13(a) shows an example of a programmable photonic two-level system for controlling gigahertz microwave signals, and its working principle is shown in Fig. 13(b).²⁹⁸ Benefits from the low loss TFLN ring resonator and cointegrated microwave electrode, the authors have demonstrated $>30\mathrm{GHz}$ electrical bandwidth, around $0.5\mathrm{GHz}/\mathrm{V}$ modulation efficiency, and $\sim 2\mathrm{ns}$ photon lifetime.²⁹⁸ Figures 13(c)–13(e) show another example of an on-chip EO frequency shifter for frequency controlling using only a single-tone microwave signal.²⁹⁹ In that work, the authors have realized frequency shifts as high as 28 GHz with an approximately 90% on-chip conversion efficiency (CE) and $>0.99$ shift ratio (defined as the ratio of the output power at the shifted frequency and the output power inside the bus waveguide). Both works pave the way of efficiently and precisely manipulating light on gigahertz frequency, and open doors to many application scenarios.

Fig. 13

TFLN-based EO devices for optical frequency controlling. (a) False colored SEM image of an EO tunable coupled microring resonator. (b) The programmable photonic molecule consists of a pair of identical coupled rings (resonant frequency ${\omega }_1={\omega }_2$). Such a system has two distinct energy levels: symmetric (blue/blue shading) and antisymmetric (red/blue) optical modes are spatially out of phase by $\pi$. The microwave field interacts with the two-level system through the large EO effect of TFLN. (a) and (b) Adapted with permission from Ref. 298 © The Author(s), under exclusive license to SNL 2018. (c) SEM image of a reconfigurable electro-optic frequency shifter. (d) Upshift and (e) downshift under 12.5 GHz microwave frequency and at 1601.2 nm wavelength (${\omega }_1$) show measured 80% CE and >0.99 shift ratio (defined as the ratio of the output power at the shifted frequency and the output power inside the bus waveguide). Inset shows the directions of energy flow and the spectra in dB scale. (c)–(e) Adapted with permission from Ref. 299 © The Author(s), under exclusive license to SNL 2021.

Third, an LN-based microwave to optical transducer for quantum networks is another application scenario, especially under the conditions of rapidly developed quantum computation and long-haul quantum communication systems.^300–302 The typical microwave to optical conversion is based on electro-optomechanics (EOM) in a bulk optical cavity, which is difficult to operate at the quantum ground state as the mechanical cavity has a limited frequency.^302,303 An LN with the large Pockels effect enables GHz microwave to optical photons conversion and thus attracts significant attention. Holzgrafe et al. used the EO effect in coupled TFLN microrings, realizing an efficient microwave-to-optical transducer [Fig. 14(a)].³⁰⁰ Its measured on-chip transduction efficiency is up to $(2.7\pm 0.3)\times {10}^{-5}$ [Fig. 14(b)], which can be used to link up superconducting quantum devices with optical fibers. McKenna et al. demonstrated a similar microwave-to-optical transducer,³⁰¹ and the TFLN sits on a sapphire platform. Its device structure is shown in Fig. 14(c), which consists of triple resonators. According to the measured results shown in Fig. 14(d), such a device converts microwave photons to optical photons with an on-chip efficiency of around $6.6\times {10}^{-6}$. More recently, Xu et al. demonstrated an improved CE up to 1.02% in coupled ring resonators,³⁰² as shown in Figs. 14(e) and 14(f). Such an improvement is realized using an air-cladding structure to mitigate the prominent photorefractive (PR) effect, which is supposed to be the main limiting factor of CE.³⁰² Even though the highest CE in TFLN is still under expectation considering its high Pockels effects, we believe the gap between a typical EOM-based scheme (highest CE of 47%³⁰³) and a TFLN-based EO structure [currently of $(2.7\pm 0.3)\times {10}^{-5}$³⁰⁰ or $6.6\times {10}^{-6}$³⁰¹] can be further narrowed down by optimizing the design and fabrication technology. TFLN-based integrated transducers will play a significant role in future quantum networks.

Fig. 14

EO-based microwave to optical transducer in TFLN. (a) Microscopic image of a TFLN-based transducer, and (b) its corresponding measured maximum transduction efficiency with respect to optical pump powers. (a) and (b) Adapted with permission from Ref. 300 © 2020 OSA. (c) Microscopic image of a triply resonant LN on sapphire transducer (zoom in: device details), and (d) its measured photon count rate versus microwave drive frequency with respect to different input microwave powers. (c) and (d) Adapted with permission from Ref. 301 © 2020 OSA. (e) Schematic of an EO converter in TFLN based on two coupled microring resonators (red) and a cointegrated superconducting resonator (yellow). DC bias is applied for optical mode tuning. (f) False color SEM image of the EO converter detail. Inset is the electric field distribution. (e) and (f) Adapted with permission from Ref. 302.

The last example is a dynamic integrated Fourier-transform spectroscopy based heterogeneously integrated SiN on LN hybrid structure (detail about such integration technology can be found in Sec. 3.3), and its device details are shown in Fig. 15,¹⁷¹ where the EO properties of TFLN have been exploited for retrieving a complete spatial interferogram. This prototype device is capable of completely sampling the standing waves from signals over a 500-nm bandwidth. Using such a device, the authors have demonstrated a measured interferogram for a broadband optical signal from a super-luminescent light emitting diode, which has a 1550 nm center wavelength and a 50 nm 3 dB bandwidth. The above-discussed results are just a few examples based on the EO effect of LN for extended applications. We believe the applications are not limited to these and more advanced devices/PICs will be demonstrated in the future.

Fig. 15

EO waveguide spectrometer in TFLN. Microscopic image and device details of an EO TFLN waveguide spectrometer. Adapted with permission from Ref. 171 © The Author(s), under exclusive license to SNL 2019.

4.4.1.

Cavity optomechanics

Benefiting from commercially available high-quality TFLN and well-developed LN etching technologies, high $Q$ factor and small phonon mode size SAW resonators at gigahertz frequencies have been demonstrated by the engineering of photonic band structures,^{197,309–311} something that is inapplicable in the typical bulk LN platform. Figure 17(a) shows an SAW resonator based on chirping a quasi-1D PhC period, and its measured $Q$ factor is 6240 at a fundamental mode frequency of 3.07 GHz [Fig. 17(b)], which results in an $f·Q$ product of $2\times {10}^{13}\mathrm{Hz}$.³¹¹ In another optimized PhC structure, as shown in Figs. 17(c)–17(e), a mechanical mode frequency close to 2 GHz with a $Q$ factor of around 17,000 at 4 K was obtained, which corresponds to around $3.4\times {10}^{13}\mathrm{Hz}$.³¹⁰ These demonstrated high $f·Q$ product optomechanics resonators pave the way toward hybrid quantum systems, enabling the control of solid-state electronic spins, AO modulators, gigahertz frequency optical comb generation, and performing microwave to optical conversion.^17,311

Fig. 17

Cavity optomechanics devices in LN. (a) Schematic of a band structure engineered surface acoustic resonator on TFLN. Inset is the microscopic image of the fabricated device. (b) Measured $Q$ factor with respect to different resonator frequencies. (a) and (b) Adapted with permission from Ref. 311 © 2019 APS. Unit cell geometries of the (c) nanobeam optomechanical crystal and (d) 1D photonic shield. (e) SEM image of a 1D PhC cavity resonator for optomechanical mode generation. Left: full view of the device. Middle: top view of one device. Top right: top view of the 1D photonic shield region. Bottom right: SEM image of the nanobeam reflector coupling region. (c)–(e) Adapted with permission from Ref. 310 © 2019 OSA.

4.4.2.

Acousto-optic modulators

AO modulators have already been demonstrated in LN platforms.^{16,17,312,313} Different from high speed EO modulators, AO modulators can be enhanced by a mechanical quality factor with bandpass frequency selectivity and also have no critical requirement of placing interdigitated transducers (IDTs) close to the optical waveguide, since acoustic waves have a low propagation loss.³¹² Thus, AO modulators can be a good complement to existing EO modulators.

Surface elastic waves can be generated by mechanical coupling of a shear or compressional wave transducer.³¹⁴ However, for better interaction between acoustic and optical waves, the IDT structure is more frequently used.³¹⁴ Figure 18(a) shows a typical straight IDT structure, which consists of spatially periodic electrodes. Its key metrics involve the finger length ($W$), finger number ($n$), and finger period ($p$). Based on the angular spectrum of plane wave theory, the amplitude field of the IDT can be calculated.³¹⁵ To further improve the acoustic–photonic interaction, a concentric IDT is proposed, as shown in Fig. 18(b). In such a structure, the interdigitated electrodes are designed with circular arc shapes and hence can achieve higher intensity acoustic fields.^315,316

Fig. 18

Design of IDTs. Schematics of (a) straight and (b) concentric IDTs. (a) and (b) Adapted with permission from Ref. 315 © 2005 IEEE.

AO devices have been demonstrated in bulk LN platform. In 1992, Cheng et al. demonstrated an AO frequency shifter with a measured 121 MHz tunable bandwidth (center frequency is 0 MHz) in the visible band based on Y-cut bulk LN.³¹⁷ Passive waveguides are formed through Ti diffusion; thus, the device volume is typically in the cubic centimeter range. Kakio et al. demonstrated an AO modulator in a 128-deg rotated Y-cut LN crystal. Under a 17 V and 195 MHz driving voltage, about 84% diffractive efficiency was obtained.³¹⁸ Similarly, such a device is based on a Ti diffused waveguide and thus presents a large device size (with an IDT length of 2 or 3 mm). In addition, due to the large device size in bulk LN, the modulation frequency is typically limited to several hundreds of megahertz. The TFLN platform can compensate for these drawbacks and thus realize more compact and efficient AO devices.^{312,313,319,320}

Figures 19(a)–19(c) show both MZI and microring-based AO modulators in a TFLN platform, which uses IDTs for launching the SAW to reduce reflection losses.¹⁶ The photoelastic coefficient $p_{\mathrm{eff}}$ was extracted to be 0.053 from experimental data, which agrees well with the theoretical value. Based on the acoustic optic resonator/modulator, high-performance microwave to optical conversion has been demonstrated in TFLN.¹⁷ As shown in Figs. 19(d) and 19(e), in a suspended IDT-coupled TFLN MZI AO resonator, enhanced microwave to optical conversion at 2.24 and 3.33 GHz acoustic resonance modes is observed. From such an experimental result, the $V_{\pi }$ is estimated to be around 4.6 V, and $V_{\pi }L$ is around $0.046\mathrm{V}·\mathrm{cm}$ (acoustic resonator length is $100\mu \mathrm{m}$). It is known that the introduction of a resonant acoustic cavity will degrade the bandwidth of an AO modulator, and Hassanien et al. demonstrated a wideband (140 MHz) operation AO modulator without using any acoustic cavity.³¹² Its principle is shown in Fig. 19(f), which consists of a PhC waveguide for light confinement, and a split IDT designed to generate ${\mathrm{S}}_0$ mode Lamb waves. Such a device is power efficient, with phase shifts up to $0.0166\mathrm{rad}/\sqrt{\mathrm{mW}}$ over a $45\text{-}\mu \mathrm{m}$ modulation length, and its measured bandwidth is up to 140 MHz under a 1.9 GHz center frequency [Fig. 19(g)]. More recently, photonics BIC has also been demonstrated to realize high-performance AO modulation.^319,320 As discussed in Sec. 3.3, BIC structures can realize low loss light transmission without the need for etching of TFLN and thus enable various integrated devices/circuits. Figure 19(h) shows an example of GHz AO modulation based on a photonics BIC waveguide.³²⁰ Compared with a typical bulk LN device, the improved modulation frequency is a consequence of the reduced device size and enhanced acoustic-photonics interaction.

Fig. 19

LN-based AO modulators. Schematics of (a) MZI and (b) microring type AO modulators. (c) Cross section of the AO modulator. (a)–(c) Adapted with permission from Ref. 16 © 2019 Chinese Laser Press (CLP). (d) Microscopic image of a suspended AO MZI. (e) $S_{11}$ and $S_{21}$ spectra of microwave to optical conversion. The optical power detected by photodetector (PD) is 0.25 mW. (d) and (e) Adapted with permission from Ref. 17 © 2019 OSA. (f) Principal illustration of one AO modulator without resonator cavity. (g) Measured $S_{11}$ and $S_{21}$ spectra. (f) and (g) Adapted with permission from Ref. 312 © 2021 CLP. (h) Schematic of AO frequency shifter based on photonic BIC. Adapted with permission from Ref. 320 © 2021 American Chemical Society (ACS).

4.4.3.

Acoustic delay line

In addition, TFLN is also suitable for the fabrication of an acoustic delay line (ADL) that can be used for radio frequency (RF) acoustic signal processing.³²¹ By choosing the fundamental symmetrical (${\mathrm{S}}_0$) mode, low IL and large fractional bandwidth (FBW) at a high frequency can be realized,^321–323 which overcomes the drawback of fundamental shear horizontal ($\mathrm{S}{\mathrm{H}}_0$) mode³¹³ or first order antisymmetric (${\mathrm{A}}_1$) mode-based devices.³⁰⁸ Figures 20(a) and 20(b) show an optical image of a fabricated ADL in TFLN operating in the ${\mathrm{S}}_0$ mode, which realized 1 dB IL and 4.1% FBW at 300 MHz [Fig. 20(c)].³²¹

Fig. 20

LN-based ADL. (a) Microscopic image of the fabricated ADL. (b) Microscopic image of zoomed in view of the device. (c) Extracted IL and FBW of ADLs with respect to cell numbers. (a)–(c) Adapted with permission from Ref. 321 © 2018 IEEE.

4.5.1.

Rare earth doped bulk devices

In bulk LN crystals, rare earth doping has been explored for a long time. ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Er}}^{3+}$ are the two main ions used for doping in Ti-diffused or PE bulk LN devices.⁹¹ For a ${\mathrm{Nd}}^{3+}$ doped bulk LN device, the amplifier gain has been measured to be around 7.5 dB³²⁶ and with a lasing threshold near 1.5 mW,³²⁷ while in ${\mathrm{Er}}^{3+}$ doped and Ti-diffused bulk LN waveguides, a 13.8 dB gain³²⁸ at 1531 nm and 8 mW³²⁹ lasing threshold has been demonstrated. In these reported results, the device performance was not as good as expected and the main reason is likely poor optical mode confinement caused by a low refractive index contrast. TFLN provides an alternative for improved device performance in the future.

4.5.2.

Rare earth doped TFLN devices

Dutta et al.⁹⁴ demonstrated that ${\mathrm{Tm}}^{3+}$ doped TFLN exhibits identical optical lifetimes to those measured in bulk crystals, as shown in Figs. 21(a)–21(c). Such a result opens the door to applications in rare earth doped TFLN. Many types of integrated amplifiers/lasers based either on microresonators or straight waveguides have been demonstrated successively.^{33,92,93,95,238,330–334} Doping of rare earth ions can be either by implantation⁹⁵ or can take place during LN crystal growth.^{33,92–94,238,330–334} Figures 21(d)–21(f) show one example of an ${\mathrm{Er}}^{3+}$ implanted TFLN device.⁹⁵ The researchers have shown that ion implantation damage can be repaired by postannealing. As can be seen from the measured results shown in Fig. 21(e), a Q factor up to $9.2\times {10}^5$ can be measured after 550°C postannealing, and its mean value is $5\times {10}^5$. Other reports rely on high-quality TFLN with ${\mathrm{Er}}^{3+}$ doping achieved during crystal growth using CIS technology, which has been introduced in Sec. 2. Both high-performance amplifiers^33,92,333 and chip scale laser sources^{93,238,330–332,334} have been realized in such a platform. On-chip optical amplification with an 18-dB internal net gain was obtained using a 3.6-cm TFLN waveguide [Figs. 21(g)–21(i)], where the pump wavelength is 980 nm.³³ Benefiting from the microring with an ultrahigh $Q$ factor ($1.25\times {10}^6$ near 971.5 nm, $4.27\times {10}^5$ near 1531.8 nm) in TFLN, an on-chip laser with $\sim 20\mu \mathrm{W}$ threshold is realized [Figs. 21(j) and 21(k)], and differential CE was around $6.61\times {10}^{-5}\%$.³³⁰ In addition, these chip scale lasers also enable an emission wavelength tuning either by changing the pump power⁹³ or using the EO effect of LN.³³⁴ Wang et al. measured a $\sim -17.03\mathrm{pm}/\mathrm{mW}$ tuning efficiency with a pump power below 13 mW, and $10.58\mathrm{pm}/\mathrm{mW}$ with a pump power above 13 mW,⁹³ as shown in Fig. 21(l). Another example of a monolithically integrated electrical tunable microring laser is based on the EO effect of LN with estimated EO coefficient of around $0.33\mathrm{pm}/\mathrm{V}$.³³⁴ It is worth noting that there is also a heterogeneously integrated III-V laser recently demonstrated on TFLN based on transfer printing.³³⁵ Benefiting from a mature design in III-V semiconductors generally, its performance is better than that of rare earth doped devices. However, the reduced fabrication complexity and monolithic integration features make rare earth doped device a promising alternative.