Beyond 100 THz-spanning ultraviolet frequency combs in a non-centrosymmetric crystalline waveguide

Abstract

Ultraviolet frequency combs enable applications ranging from precision spectroscopy to atomic clocks by addressing electronic transitions of atoms and molecules. Access to ultraviolet light via integrated nonlinear optics is usually hampered by the strong material dispersion and large waveguide attention in ultraviolet regions. Here we demonstrate a simple route to chip-scale ultraviolet comb generators, simultaneously showing a gap-free frequency span of 128 terahertz and high conversion efficiency. This process relies on adiabatic quadratic frequency translation of a near-visible supercontinuum sourced by an ultrafast fiber laser. The simultaneous cubic and quadratic nonlinear processes are implemented in single-crystalline aluminum nitride thin films, where chirp-modulated taper waveguides are patterned to ensure a broad phase matching. The heterodyne characterization suggests that both the near-visible and ultraviolet supercontinuum combs maintain high coherence. Our approach is also adaptable to other non-centrosymmetric photonic platforms for ultrafast nonlinear optics with scalable bandwidth.

Nonlinear nanophotonic devices in the ultraviolet to visible wavelength range

“…Despite the successful demonstration of high Q resonators on AlN, the large chromatic dispersion and group velocity dispersion in the UV and Vis spectra remain to be the significant challenges towards realizing highly efficient frequency conversion through χ (2) or χ (3) process using AlN devices. Recently, X. Liu et al proposed a chirp-modulated taper AlN waveguide to achieve broad phase matching using a near-visible pumping wavelength [221]. By fully utilizing the χ (2) and χ (3) nonlinearities, over 100 THz of coherent frequency spanning (360-425 nm) was achieved on AlN, and the results are as shown in Figures 7a and b.…”

“…Sun et al demonstrated AlN ring resonators with intrinsic Q factors greater than 70,000 [219], while T. J. Lu et al reported AlN devices with intrinsic Q values exceeding 170,000 at 638 nm [220]. In the UV spectra region, X. Liu et al demonstrated AlN devices with intrinsic Q factors of 210,000 [221]. AlGaN/AlN alloys were also theoretically proposed [222], in which the low index contrast is promising for low loss waveguiding.…”

“…The AlN waveguides were pumped in TE 10 mode near the zero dispersion wavelength (ZDW) at 810 nm [225]. Due to the relatively large nonlinear parameter of AlN [221,225] compared with silica [224], the pulse energy of AlN waveguides was significantly reduced to less than 1 nJ while covering a much broader spectral region, while the typical operation pulse energy within silica is above 1 nJ [224]. This work opens door for integrated AlN nanophotonic devices and circuits, which will lead to potential applications in on-chip mode locking [226], parametric oscillator [227], and entangled photon generation [228].…”

“…(b) The broad spectrum obtained from near-the chirp-tapered AlN waveguide using a near-visible pumping wavelength. Reprinted from [221]. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC) http://creativecommons.org/licenses/bync/4.0/ (c) The coherent dispersive wave generation in the UV and visible spectra through soliton dynamics using silica ridge waveguides.…”

Supercontinuum Generation Assisted by Wave Trapping in Dispersion-Managed Integrated Silicon Waveguides

“…In 2018, an octave-spanning SC, covering 1.124-2.4 μm, was generated in SOI ridge waveguides [38]. In order to avoid the inherent nonlinear loss encountered in silicon in the near infrared and achieve broader SC spectra, the research moved toward other platforms, such as silicon nitride [39], chalcogenide [40], silicon-germanium [41], indium gallium phosphide [42], and aluminum nitride [43]. In the context of SC generation, Guo et al [44] have reported a mid-infrared comb source from 2.5 μm to 4 μm in silicon nitride (Si 3 N 4 ) waveguides and, recently, a (400-2400)-nm supercontinuum source has been demonstrated in integrated lithium niobate waveguides [45].…”