Nature Photonics reports a new type of nonlinear optical crystal - all band phase matched crystal

Short wave ultraviolet all solid-state coherent light sources have the characteristics of strong photon energy, practicality and precision, and high spectral resolution. They have significant application value in laser precision processing, information communication, cutting-edge science, and aerospace fields.

The core component of obtaining all solid-state shortwave ultraviolet lasers is nonlinear optical crystals. In the process of nonlinear optics, if the energy of the fundamental frequency light is continuously converted to the doubling frequency light, it is necessary to maintain the second polarization harmonic excited by the fundamental frequency light and the doubling frequency light at the same position and time in the crystal. However, due to the intrinsic dispersion of the crystal, the refractive indices of the fundamental frequency light and the doubling frequency light are different, resulting in different group velocities of the two beams in the crystal, Unable to achieve sustained growth of frequency doubling light, this is a phase mismatch.

Therefore, achieving the application of band phase matching in crystals is generally considered the most difficult technical challenge, determining the final laser output power and efficiency. At present, there are various technical solutions to choose from, such as birefringence phase matching technology for crystal anisotropy, random quasi phase matching technology for spontaneous domain structures within crystals, and artificial microstructure quasi phase matching technology. Among them, the birefringence phase matching technology using crystal anisotropy is the most widely used and effective way to compensate for phase mismatch. This technology utilizes the birefringence characteristics of anisotropic crystals to make a certain polarization of fundamental frequency light incident in a specific direction of the crystal, or change the temperature of the crystal to achieve angle or temperature phase matching, even if the refractive index of fundamental frequency light and doubling frequency light propagating in a specific direction of the crystal is the same, This scheme has high conversion efficiency, but existing crystals all have phase matching wavelength loss, which can be characterized by the difference between the UV cutoff edge of the crystal and the shortest photo matching wavelength（ λ Cutoff - λ PM).

Pan Shilie's research team from the Xinjiang Institute of Physical and Chemical Technology, Chinese Academy of Sciences, based on the previous research of cutting-edge progress in the field and the analysis of the current situation of birefringence phase matching of nonlinear optical crystals, put forward the assumption of an ideal state of nonlinear optical crystals in the invited review (Angel.Chem.Int.Ed.2020, 5920302-20317), that is, in nonlinear optical crystals based on birefringence phase matching, Can the ideal state of 'UV cut-off edge equal to the shortest matching wavelength' be achieved? If this assumption can be realized in the crystal, it will provide a new way and idea for the crystal to achieve birefringence phase matching throughout the entire transmission range.

Recently, the team has created a new type of nonlinear optical crystal, namely full band phase matching crystal. This type of crystal is based on the most widely used birefringence phase matching technology and can achieve phase matching for any wavelength within the transmission range of the crystal material. The physical mechanism of full band phase matched crystals was revealed. Starting from the microscopic expression of refractive index and optical conditions such as birefringence dispersion curve, refractive index dispersion curve, and phase matching, two independent evaluation parameters of full band phase matched crystals were given. This evaluation parameter was applied to some classic nonlinear optical crystal materials, and the feasibility and universality of using this parameter to evaluate the wavelength loss of crystal phase matching were discussed. Based on this, an example of nonlinear optical crystal (GFB) is obtained.

The author stated that the experiment investigated the direct frequency doubling output ability of crystals throughout the entire transmission range through multi-level frequency conversion schemes or optical parameter technology schemes. Based on phase matching devices, 193.2-266 nm UV/deep UV tunable laser output has been achieved, verifying the full band phase matching ability of the crystal, making it the first UV/deep UV frequency doubling crystal material to achieve full band birefringence phase matching, The crystal transmittance of this material at 193.2 nm is less than 0.02%, and it can still achieve frequency doubling laser output, verifying its full band phase matching characteristics. More importantly, the crystal has excellent linear and nonlinear optical properties, such as short ultraviolet cutoff edge (~193 nm), large effective frequency doubling coefficient (deff=1.42 pm/V), short phase matching wavelength (~194 nm), and high laser damage resistance threshold (>BBO @ 266/532 nm, 8 ns, 10 Hz), making it a promising nonlinear optical crystal material for 266 nm lasers.

The related research achievement "Achieving the full wavelength phase matching for effective nonlinear optical frequency conversion in C (NH2) 3BF4" was recently published in the journal Nature Photonics (2023, DOI: 10.1038/s41566-023-01228-7).

Figure: GFB crystal structure, microscopic performance analysis, and crystal photos

Source: Xinjiang Institute of Physical and Chemical Technology, Chinese Academy of Sciences