The main applications of lithium niobate crystals

The primary applications of lithium niobate crystals are as follows:

1. Piezoelectric Applications

Lithium niobate crystals have a high Curie temperature, a small temperature coefficient of the piezoelectric effect, a high electromechanical coupling coefficient, low dielectric loss, stable physical and chemical properties, good machinability, and are easy to prepare into large-sized, high-quality crystals. This makes them an excellent piezoelectric crystal material. Compared with the commonly used piezoelectric crystal quartz, lithium niobate crystals have a higher velocity of sound, allowing for the production of high-frequency devices. Therefore, lithium niobate crystals can be used in resonators, transducers, delay lines, filters, etc., and are applied in civil fields such as mobile communication, satellite communication, digital signal processing, television, broadcasting, radar, remote sensing, and telemetry, as well as in military fields such as electronic countermeasures, fuses, and guidance systems. The most widely used application is the Surface Acoustic Wave Filter (SAWF), as shown in Figure 8. Since the 1970s, intermediate frequency surface acoustic wave filters made of lithium niobate crystals have been widely used in color televisions, cordless telephones, electronic remote controls, etc. By 2010, with the application of silicon tunable integrated chips, intermediate frequency surface acoustic wave filters in televisions have basically exited the market. In the field of mobile communication, from the 1980s, mobile communication has been continuously updated from 2G, 3G, 4G to 5G, and mobile terminals must be backward compatible, which has led to a dramatic increase in the demand for surface acoustic wave filters. Considering that two filters are needed for each frequency band, the number of surface acoustic wave filters required for each mobile phone can reach over a hundred, most of which are prepared using lithium niobate and lithium tantalate crystals, especially lithium niobate crystals, which have been widely used in temperature-compensated surface acoustic wave filters (TCSAW).

For piezoelectric applications, the composition of lithium niobate crystals has a significant impact on the velocity of sound, and it is necessary to strictly control its fluctuation range because the Curie temperature is very sensitive to the crystal composition. Therefore, the Curie temperature is often used to characterize the consistency of the crystal composition. In addition, the domaining of the crystal will directly affect the piezoelectric performance of the crystal. Therefore, the technical indicators required for lithium niobate crystals used in piezoelectric devices mainly include the Curie temperature, domaining, and internal scattering particles. Mechanical waves with longer wavelengths propagating in the crystal are not sensitive to lattice defects much smaller than the wavelength. Crystals that meet the requirements of piezoelectric applications are commonly referred to as "acoustic-grade lithium niobate crystals."

The cutting direction of acoustic-grade lithium niobate crystals is related to specific applications. Lithium niobate crystals cut along the Y-axis have a high electromechanical coupling coefficient, but the excitation of the bulk wave is too large, resulting in fewer applications. In contrast, crystals cut along the <101-4> direction have fewer bulk wave excitations and are more widely used, and this direction is also used for TCSAW; the <101-4> direction is the Y-axis rotated 127.86° counterclockwise around the X-axis, commonly known as 128°Y lithium niobate crystal. In addition, lithium niobate crystals cut along the 64°Y and 41°Y directions are more suitable for the preparation of high-frequency products and have been widely applied. Currently, the size of lithium niobate crystals used for piezoelectric applications has reached 6 inches.

Furthermore, in 1982, Lewis reported the impact of the pyroelectric effect of lithium niobate crystals on the preparation of surface acoustic wave devices, finding that the pyroelectric effect of lithium niobate crystals leads to the destruction of electrodes and crystals, which can be suppressed by using high-resistance metal to short-circuit the electrodes. In 1998, Standifer and others used a chemical reduction treatment method to increase the light absorption of lithium niobate crystals by 1,000 times, improving the exposure quality of narrower and finer lines during photolithography, and the electrical conductivity of the crystal also increased by more than 10^5, suppressing the damage to the interdigital electrodes by the pyroelectric effect during the thermal treatment process in the surface acoustic wave device process. The lithium niobate wafers prepared by this method are called "black lithium niobate" and have been widely applied in surface acoustic wave filters.

2. Optical Applications

In addition to the piezoelectric effect, lithium niobate crystals exhibit a rich variety of photoelectric effects, with the electro-optic effect and nonlinear optical effects standing out in terms of performance and wide application. Lithium niobate crystals can be prepared into high-quality optical waveguides through proton exchange or titanium diffusion, and can also be prepared into periodically poled crystals through polarization reversal. Therefore, they are widely used in devices such as electro-optic modulators (as shown in Figure 9), phase modulators, integrated optical switches, electro-optic Q-switches, electro-optic deflection, high-voltage sensors, wavefront detectors, optical parametric oscillators, and ferroelectric superlattices. Additionally, applications based on lithium niobate crystals such as birefringent wedge prisms, holographic optical devices, infrared pyroelectric detectors, and erbium-doped waveguide lasers have also been reported.

Unlike piezoelectric applications, these applications involving optical transmission have different requirements for lithium niobate crystals. First, the light waves that propagate through the lithium niobate crystals in optical applications have wavelengths ranging from hundreds of nanometers to several micrometers. It is not only necessary for the crystals to have excellent optical homogeneity, but also to strictly control crystal defects that can be comparable in scale to the wavelength of the light wave. Secondly, as optical applications typically require control over parameters such as phase and polarization when light waves propagate through the crystal, these parameters are directly related to the refractive index and its distribution within the crystal. Therefore, it is also necessary to eliminate internal and external stresses in the crystal as much as possible to avoid stress-induced birefringence caused by the photoelastic effect. Lithium niobate crystals that meet the requirements of optical applications are commonly referred to as "optical-grade lithium niobate crystals."

Optical-grade lithium niobate crystals are mainly grown along the Z-axis and X-axis. The Z-axis is the highest symmetry axis of the lithium niobate crystal. Growing crystals along this direction aligns the crystal's symmetry with that of the thermal field, which is conducive to the growth of high-quality crystals. Therefore, when devices require the crystal to be cut into blocks or irregular shapes, crystals grown along the Z-axis are often used, and ferroelectric superlattice devices are also prepared using Z-axis lithium niobate crystal slices. X-axis lithium niobate crystals are mainly used to prepare X-cut lithium niobate wafers to be compatible with subsequent processing techniques such as cutting, chamfering, grinding, polishing, and photolithography that have been developed from semiconductor processes, and are applied to most electro-optic modulators, phase modulators, birefringent wedge prisms, waveguide lasers, etc.

3 Dielectric Superlattices

In 1962, Armstrong et al. first introduced the concept of quasi-phase-matching (QPM), utilizing the reciprocal lattice vectors provided by superlattices to compensate for the phase mismatch in the optical parametric process. The direction of polarization in ferroelectric materials determines the sign of the nonlinear susceptibility χ^(2). By creating a periodic domain structure with opposite polarization directions within ferroelectric materials, quasi-phase matching technology can be realized. Crystals such as lithium niobate, lithium tantalate, and potassium titanyl phosphate are all capable of being prepared into periodically poled crystals, among which lithium niobate crystals are the earliest and most widely used materials for the preparation and application of this technology.

In 1969, Camlibel proposed that high electric fields above 30 kV/mm could be used to reverse the ferroelectric domains in ferroelectric crystals such as lithium niobate. However, such high electric fields are prone to break down the crystals, and the technical conditions at the time made it difficult to prepare fine electrode structures and precisely control the domain polarization reversal process. Subsequently, people attempted to construct a polydomain structure by alternating the polarization direction of lithium niobate crystals, but the number of wafers that could be achieved was limited . In 1980, Feng et al.  obtained a periodically poled domain structure by offsetting the crystal rotation center from the center of symmetry of the thermal field, using an eccentric growth method, and achieved frequency doubling output of 1.06 μm laser, verifying the quasi-phase-matching theory. However, this method faced significant difficulties in the fine control of the periodic structure. In 1993, Yamada et al. combined semiconductor photolithography with external electric field methods to successfully solve the periodic domain polarization reversal process, and the external electric field polarization method gradually became the mainstream preparation technology for periodically poled lithium niobate crystals. At present, periodically poled lithium niobate crystals have been commercialized, with thicknesses that can reach more than 5 mm.

The initial application of periodically poled lithium niobate crystals mainly considered the application in laser frequency conversion. As early as 1989, Ming et al.  proposed the concept of dielectric superlattices based on the ferroelectric domain structure of lithium niobate crystals, where the reciprocal lattice vector of the superlattice would participate in the excitation and propagation of light waves and acoustic waves. In 1990, Feng and Zhu et al. proposed the theory of multiple quasi-phase-matching. In 1995, Zhu et al. used room temperature polarization technology to prepare quasi-periodic dielectric superlattices, which were experimentally verified in 1997, achieving effective coupling of two optical parametric processes—frequency doubling and sum frequency—on a quasi-periodic superlattice for the first time, realizing efficient laser tripling. In 2001, Liu et al.  designed a scheme for three-primary-color lasers based on quasi-phase-matching. In 2004, Zhu et al. realized the design of optical superlattices for multi-wavelength laser output and their application in all-solid-state lasers. In 2014, Jin et al. designed an optical superlattice integrated photonic chip based on a reconfigurable lithium niobate waveguide optical path, as shown in Figure 10, which for the first time achieved efficient generation of entangled photons and high-speed electro-optic modulation on a chip. In 2018, Wei et al. and Xu et al.  prepared a 3D periodic domain structure based on lithium niobate crystals, and in 2019 , they achieved efficient nonlinear beam shaping using the 3D periodic domain structure.

The proposal and development of the dielectric superlattice theory have propelled the application of lithium niobate crystals and other ferroelectric crystals to a new level, with significant application prospects in areas such as all-solid-state lasers, optical frequency combs, laser pulse compression, beam shaping, and entangled light sources in quantum communication.