Theses Doctoral

Integrated and Phased-Matched Nonlinear Optics in 3R Phase Transition Metal Dichalcogenides

Xu, Xinyi

Nonlinear frequency conversion provides essential tools for generating new colors and quantum states of light. Conventional nonlinear crystals have the problem of relative lower nonlinear susceptibilities, which result in the large footprint of devices and low efficient. Transition metal dichalcogenides possess huge nonlinear susceptibilities; further, 3R-stacked transition metal dichalcogenide crystals possess aligned layers with broken inversion symmetry, representing ideal candidates to boost the nonlinear optical gain with minimal footprint.

Here we report the second-order nonlinear processes of 3R-MoS2 along the ordinary and extraordinary directions. Along the ordinary axis, by measuring the thickness-dependent second-harmonic generation, we present the first measurement of the second harmonic-generation coherence length of 3R-MoS2 and achieve record nonlinear optical enhancement from a van der Waals material, >104 stronger than a monolayer. It is found that 3R-MoS2 slabs exhibit similar conversion efficiencies of lithium niobate, but within 100-fold shorter propagation lengths. Furthermore, along the extraordinary axis, we achieve broadly tunable second-harmonic generation from 3R-MoS2 in a waveguide geometry, revealing the coherence length in such a structure. We characterize the full refractive-index spectrum and quantify its birefringence with near-field nanoimaging.

In order to bring 3R-MoS2 into the application field, we have developed two fabrication methods: low-cost femtosecond laser etching and cleanroom nanolithography-based processes. The femtosecond laser writing setup offers a rapid, residue-free, and in-situ method for patterning grating structures. On the other hand, the cleanroom process can provide structures with higher resolution. The cleanroom fabrication process is based on SF6 RIE and E-beam lithography, which can narrow down the minimum linewidth to ~120nm.

To achieve mode matching in waveguiding second-order nonlinear conversion, we utilized the mode dispersion relation calculated by an anisotropic model to find the overlapping of wavevectors among different photon energies. We proposed a molybdenum disulfide on silicon nitride structure (MOSS) to further unleash the potential of 3R-MoS2 in optical parametric conversion. Photonic structure optimization was performed using the Lumerical FDTD simulator, achieving a 90% coupling efficiency from SiN to 3R-MoS2 with a taper structure. With a taper length of 50μm, we successfully maintained a single mode of excitation wave in MoS2, which could provide a monotonoic mode source for nonlinear conversion.

Our work highlights the potential of 3R-stacked transition metal dichalcogenides for integrated photonics, providing critical parameters, developing high-resolution fabrication processes, and offering initial designs for highly efficient on-chip nonlinear optical devices including periodically poled structures, optical parametric oscillators and amplifiers, and quantum circuits.


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More About This Work

Academic Units
Mechanical Engineering
Thesis Advisors
Schuck, P. James
Hone, James C.
Ph.D., Columbia University
Published Here
February 28, 2024