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In this work, we exploit adiabatic frequency conversion using chirp-modulated taper waveguides and construct a UV microcomb with broad and gap-free envelopes. On-chip implementation of adiabatic SF transfer processes are recently theoretically discussed in linear-tapered waveguides for enhanced efficiency and bandwidth 29. In contrast, adiabatic frequency conversion is known for broadband operation 25, and has primarily been employed for sum-frequency (SF) generation in aperiodically poled bulk nonlinear crystals 26, 27, 28. Yet, the operating bandwidth of such QPW structures is commonly limited (~10 THz) for either pulsed 23 or continuous-wave (cw) pump 24. By engineering the quasi-phase matching (QPM) in periodically poled lithium niobate waveguides 22 and microresonators 23, efficient UV harmonics below 400 nm have been accessed. Nonetheless, UV supercontinuum microcombs are still in its infancy, and have only been demonstrated in silica-based waveguides 21 owing to its small material dispersion and flexible waveguide fabrication for tailoring a zero-integrated dispersion in the UV region.Ĭascaded harmonic generation, on the other hand, provides an alternative route for spectral transfer to the UV wavelengths. Photonic chip-based supercontinua are equally important for serving as ultra-broadband comb sources in various photonic platforms 15, 16, 17, 18, 19, 20, where soliton-induced dispersive waves are employed for coherent spectral transfer into visible (VIS) and mid-infrared regions. Attempts to transfer such microcombs into UV regimes are usually hampered by the strong normal material dispersion and large waveguide attenuation therein.
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Nanophotonic architectures enable tight light confinement for enhanced optical nonlinearities as well as readily engineered dispersion, and have been exploited as a viable route towards four-wave mixing based microcombs with low power threshold and high scalability 14. While these schemes permit spectral transfer into deep-UV regimes, it requires bulk optics and intense pump energies, which are unfavorable for out-of-the-lab applications. Conventionally, UV frequency combs are achieved from mode-locked lasers by leveraging nonlinear frequency conversion including high-harmonic generation in atomic gases 10, cascaded harmonics in bulk nonlinear crystals 11, and supercontinua in microstructure optical fibers 12, 13. The broad UV spectra also enrich the trapping and cooling of atom and ion species for atomic clocks 5, 6, 7 and quantum memories 8, 9. A exemplary case is precision spectroscopic sensing for chemical detection 2, 3 and trace gas monitoring 4 by mapping their UV absorbance. Extending this technique into the ultraviolet (UV) region gives direct access to the electronic transitions of atoms and molecules, making it desirable for a myriad of applications. Optical frequency combs that transmit equally-spaced, mutually-coherent spectral lines have revolutionized the field of ultrafast science and metrology 1. Our approach is also adaptable to other non-centrosymmetric photonic platforms for ultrafast nonlinear optics with scalable bandwidth.
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The heterodyne characterization suggests that both the near-visible and ultraviolet supercontinuum combs maintain high coherence. 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. This process relies on adiabatic quadratic frequency translation of a near-visible supercontinuum sourced by an ultrafast fiber laser. 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.
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Access to ultraviolet light via integrated nonlinear optics is usually hampered by the strong material dispersion and large waveguide attention in ultraviolet regions. Ultraviolet frequency combs enable applications ranging from precision spectroscopy to atomic clocks by addressing electronic transitions of atoms and molecules.