Design and Discovery of High-Performance Nonlinear Optical Crystals for Laser Applications
Open Access
- Author:
- He, Jingyang
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 23, 2023
- Committee Members:
- John Mauro, Program Head/Chair
Sahin Ozdemir, Outside Unit & Field Member
Zhiqiang Mao, Dissertation Co-Advisor
Joan Redwing, Major Field Member
Venkatraman Gopalan, Chair & Co-Dissertation Advisr - Keywords:
- nonlinear optics
second harmonic generation
nonlinear optical crystal
crystal synthesis - Abstract:
- Nonlinear optics (NLO) has had a remarkable impact on both fundamental science and technological applications thanks to their ability to convert one frequency to another of interest via combining or splitting photons. Through the NLO interactions, we can obtain a tunable spectrum, and the different lights generated are critical to a number of civil and military applications. For example, second harmonic generation (SHG) is a second-order NLO effect that combines two identical photons into one photon with twice the frequency. In addition, nonlinear optics is still the primary approach for generating entangled photons for quantum optics through the process of spontaneous parametric down-conversion (SPDC), which can be considered as a reverse process of SHG. However, the current challenge the NLO community faces is the lack of high-performance NLO single crystals, which is more severe in the infrared and ultraviolet regions. Developing new NLO crystals superior to the currently available materials is not an easy task because there are many competing demands on NLO crystals: high nonlinear coefficients, broad transparency range, large laser damage threshold (LDT), low optical absorption, phase-matchability of the NLO process for high-efficiency conversion, and the ability to grow large high-quality single crystals. The major obstacle is finding the balance between bandgap and nonlinearity: having a large bandgap generally leads to high LDT but small nonlinear coefficients. In this dissertation, several promising infrared and ultraviolet NLO crystals are explored and evaluated as future NLO candidates for practical applications. This dissertation in designing and discovering novel NLO materials aims to address these technological challenges. The central goal of this dissertation is to synthesize the promising NLO single crystals and perform optical characterization to show they exhibit high second-order NLO coefficients combined with large bandgaps for generating a broadband electromagnetic spectrum using nonlinear optics. This involves a number of different disciplines, including chemistry, physics, and materials science. Five promising NLO materials were identified and proven applicable for infrared laser systems. γ-NaAs1-xSbxSe2 (x=0 or 0.05) described in Chapter 3 exhibits the highest non-resonant SHG coefficient among all known materials. They consist of one-dimensional (1D) infinite [(Sb/As)Se2]- chains aligned along the a-axis. In general, low dimensional microscopic structure (1D chains in this case) within a 3D crystal lattice can give rise to relatively flat bands and high density of states (DOS), which is beneficial for having large birefringence (the difference in refractive indices in different orientations, required for phase matching), large refractive indices and large non-resonant SHG nonlinear susceptibility. Large γ-NaAs1-xSbxSe2 single crystals have been successfully synthesized, and giant SHG coefficients of d11 = 577 ± 60 pm V−1 and 648±74 pm V-1 were extracted for γ-NaAsSe2 and γ-NaAs0.95Sb0.05Se2 respectively, surpassing the commercial NLO crystal AgGaSe2 by over eighteen times. To understand the origin of the superior optical properties and develop the structure-property relationships, we further correlated the giant SHG coefficient with the crystallographic and electronic structure in collaboration with the first principles theory group from Northwestern University. In the case of γ-NaAsSe2, we found it shows relatively flat bands, providing abundant excitation routes and large dipole transition matrix elements, which enhances both resonant and non-resonant linear and nonlinear susceptibilities. The lone pair electrons on the Se and As atoms contribute to the optical anisotropy and are the origin of the higher linear and nonlinear optical response in the [100] direction. This provides not only an understanding of the origin of the excellent optical performance but also guidance for future NLO crystal design and discovery. Moreover, the superior properties make this family a promising system for high-efficient laser applications. Chapter 4 describes a chalcophosphate crystal SnP2S6, which is a promising infrared NLO crystal. Large single crystals with dimensions up to 5mm were synthesized using the chemical vapor transport method. It consists of a chiral and layered atomic arrangement. The low dimensional microscopic units (2D layers) also give rise to large birefringence and SHG response, similar to γ-NaAs1-xSbxSe2. Through optical SHG polarimetry, I extracted its complete NLO tensor and found a huge non-resonant SHG coefficient of d33=53 ± 6.4 pm V−1, much higher than AgGaSe2. In addition, it can be Type I and Type II phase-matched with high effective d of 20.4 pm V−1 and 15.2 pm V−1 respectively, promising for efficient frequency conversion. It also has a high LDT value, over three times better than the benchmark crystal ZnGeP2. These outstanding properties make SnP2S6 a promising bulk NLO crystal for high-power infrared lasers. A metal pnictide with the chalcopyrite structure MgSiP2 is described in Chapter 5, which also shows extraordinary optical properties, promising for NLO laser applications. A new synthesis method was discovered and modified for growing the polycrystalline sample. Large single crystals were also synthesized by the modified flux growth for optical studies. Different from the above-mentioned materials, it has a 3D diamond-like structure in which the bonds show strong covalency, leading to large dipole moments and thus high SHG response. It exhibits a high SHG coefficient of 89 pm V−1, about three times larger than AgGaSe2. The calculation predicts that its SHG coefficient can be directly Type I and Type II phase-matched. It also demonstrates a giant laser damage threshold value over six times better than commercial crystal ZnGeP2. Lastly, an ultraviolet NLO crystal EuBa3B9O18, is explored as described in Chapter 6. Single crystal synthesis was reported for the first time using the floating zone method. Different from previous literature, we find it has broken inversion symmetry, making it SHG active. The complete linear and nonlinear susceptibility tensors were investigated. Further, the SHG performance is correlated with the delocalized , which is highly polarizable when an electric field is applied. In summary, this dissertation discovers several excellent NLO crystals for infrared and ultraviolet laser applications and systematically investigates their optical properties necessary for commercialization. The chapters in this dissertation provide detailed information on synthesis and the optical characterization techniques used, such as ellipsometry, Fourier-transform infrared (FTIR) spectroscopy, and SHG polarimetry. Structure-property relationships were developed to understand the physical properties of these crystals and aim to shed light on the NLO crystal discovery and engineering.