♯SHAARP, an Open-source Package For Modeling Nonlinear Optical Processes For Integrated Photonics
Open Access
- Author:
- Zu, Rui
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 18, 2023
- Committee Members:
- Zhiwen Liu, Outside Unit Member
Venkatraman Gopalan, Chair & Dissertation Advisor
Zhiqiang Mao, Outside Field Member
Long-Qing Chen, Major Field Member
John Mauro, Program Head/Chair - Keywords:
- Nonlinear Optics
Electromagnetic waves
Single crystal
Thin films
Second harmonic generation
Ferroelectricity
Polarization
Optics
Modeling
Thin film - Abstract:
- Since the discovery of lasers in the 1960s, nonlinear optics has attracted significant interest due to its wide range of applications ranging from sensing, communication, wavelength conversion, surface chemistry, medical treatment, and biological and tissue imaging. Recently, quantum communications and quantum computing have employed nonlinear optics to generate entangled photons. With the growing interest in applications beyond the edge of the visible spectrum, nonlinear optical (NLO) materials that are suitable for deep ultraviolet (DUV) and terahertz (THz) applications are in critical need. To better model the optical response for a given system and to design superior materials for lasers and integrated photonic applications, this dissertation work focuses on two major directions: (1) Development of advanced nonlinear optical modeling of heterostructures for both experimental characterization and optical systems simulation, (2) Exploration of new wide bandgap materials with large nonlinear optical susceptibilities towards nonlinear photonic architectures. The first essential part of my dissertation is the advanced optical modeling and package development for linear and nonlinear optical responses in complex structures. The theoretical breakthroughs, thorough experimental verifications and the open-source package development with a user-friendly graphic user interface (GUI) integrate materials science, physics, optics, and various modeling and computational techniques. Second harmonic generation (SHG) is one of the nonlinear optical phenomena in which a material converts light from one frequency to twice the frequency, e.g., emitting blue light when an infrared laser shines on the material. Such frequency up-conversion (as well as the reverse process of down-conversion) processes are critical for generating a wide frequency spectrum using modern lasers. The reverse process of the SHG effect, namely the difference frequency conversion (DFG), is the principal means for creating entangled photons for quantum communications. SHG analysis involves more waves than linear optics due to the three-wave mixing phenomenon. Complexity in the SHG analysis builds up dramatically when numerous degrees of freedom are included. Many assumptions, such as high symmetry and lossless systems, have been adopted for nearly seven decades to simplify the problems. However, these assumptions can cause more than two orders of magnitude errors in measuring nonlinear coefficients. A comprehensive model that accurately describes the nonlinear waves picture is thus vital to understanding fundamental material properties and further accomplishing reliable optical system designs. In my thesis work, an open-source package, ♯SHAARP (Second Harmonic Analysis of Anisotropic Rotational Polarimetry), is developed to calculate analytical and numerical solutions of the SHG response in any given heterostructures. Eight key attributes are addressed and integrated into ♯SHAARP, i.e., arbitrary crystal symmetry, arbitrary crystal orientation, absorption, birefringence, dispersion, varying polarization states of light, multiple reflections within the sample, and any number of interfaces that light propagates through. The model was benchmarked with excellent agreement between ♯SHAARP and literature, covering all three optical classes, both transparent and absorbing systems. Eliminating simplifications mentioned above that have lasted for six decades, ♯SHAARP has achieved more than 90% accuracy in nonlinear optical analysis. The robust study and user-friendly GUI of ♯SHAARP are designed to promote comprehensive SHG analysis across broad research communities with different backgrounds and bridge the gap between experimentalists and theorists. Moreover, ♯SHAARP provides easy access to on-demand modality, benefiting nonlinear optical studies exploring new materials, understanding structural properties relations, and advanced optical system design. Building upon the ♯SHAARP, we have extended our research towards exploring new multifunctional NLO materials with sizable SHG effects and designing architectures for integrated photonics. The second part of my thesis consists of the enhanced piezoelectric and SHG response in ferroelectric Zn1-xMgxO thin films. Ferroelectrics possess switchable spontaneous polarization useful in nonvolatile random-access memory (RAM), where up (down) polarization corresponds to 1 (0). Piezoelectricity converts mechanical force to electrical signals or vice versa and has been widely used as sensors or transducers. With research efforts shifting towards low-dimensional applications for integrated systems, discovering new materials that can bring exceptional electrical and optical response into tens of nanometer device-compatible platform remains a grand challenge. (Ba,Sr)TiO3 is one of the few commercially available candidates. ZnO is a well-known wide band gap non-ferroelectric semiconductor, but recently Zn1-xMgxO thin films have been shown to possess large switchable ferroelectric polarization, nearly three times larger than BaTiO3. Using ♯SHAARP, we demonstrate a 30% tunability in the electronic band gap, a 500% increase in SHG coefficients, and near ~200% improvement in the piezoelectric coefficients over pure ZnO. Further investigation of the fundamental mechanisms reveals a design strategy for discovering new materials candidates with significant ferroelectric, piezoelectric, and SHG responses. Motivated by improving practical applications, we have demonstrated precisely controllable domains for SHG down to 500 nm and predicted orders of magnitude higher SHG efficiency than pristine ZnO, opening up exciting device possibilities for ultraviolet optical and piezoelectric devices. The novel approach not only aims to explore new material candidates but also focuses on enhancing multifunctional properties.