High-harmonic generation (HHG) provides short-wavelength light that is useful for precision spectroscopy and probing ultrafast dynamics. We report efficient, phase-coherent harmonic generation up to the ninth order (333 nm) in chirped periodically poled lithium niobate waveguides driven by phase-stable$≤<#comment/>12nJ$, 100 fs pulses at 3 µm with 100 MHz repetition rate. A mid-infrared to ultraviolet-visible conversion efficiency as high as 10% is observed, among an overall 23% conversion of the fundamental to all harmonics. We verify the coherence of the harmonic frequency combs despite the complex highly nonlinear process. Accommodating the extreme spectral bandwidth, numerical simulations based on a single broadband envelope equation with only quadratic nonlinearity give estimates for the conversion efficiency within approximately 1 order of magnitude over a wide range of experimental parameters. From this comparison between theory and experiment, we identify a dimensionless parameter capturing the competition between three-wave mixing and group-velocity walk-off of the harmonics that governs the cascaded HHG physics. We also gain insights into spectral optimization via tuning the waveguide poling profile and pump pulse parameters. These results can inform cascaded HHG in a range of different platforms.

Authors:
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Award ID(s):
Publication Date:
NSF-PAR ID:
10273259
Journal Name:
Journal of the Optical Society of America B
Volume:
38
Issue:
8
Page Range or eLocation-ID:
Article No. 2252
ISSN:
0740-3224; JOBPDE
Publisher:
Optical Society of America
1. Vacuum-ultraviolet (VUV) light is critical for the study of molecules and materials, but the generation of femtosecond pulses in the VUV region at high repetition rates has proven difficult. Here we demonstrate the efficient generation of VUV light at megahertz repetition rates using highly cascaded four-wave mixing processes in a negative-curvature hollow-core fiber. Both even- and odd-order harmonics are generated up to the 15th harmonic (69 nm, 18.0 eV), with high energy resolution of$∼<#comment/>40meV$. In contrast to direct high harmonic generation, this highly cascaded harmonic generation process requires lower peak intensity and therefore can operate at higher repetition rates, driven by a robust$∼<#comment/>10W$fiber-laser system in a compact setup. Additionally, we present numerical simulations that explore the fundamental capabilities and spatiotemporal dynamics of highly cascaded harmonic generation. This VUV source can enhance the capabilities of spectroscopies of molecular and quantum materials, such as photoionization mass spectrometry and time-, angle-, and spin-resolved photoemission.
2. Quasi-phase-matched interactions in waveguides with quadratic nonlinearities enable highly efficient nonlinear frequency conversion. In this paper, we demonstrate the first generation of devices that combine the dispersion engineering available in nanophotonic waveguides with quasi-phase-matched nonlinear interactions available in periodically poled lithium niobate (PPLN). This combination enables quasi-static interactions of femtosecond pulses, reducing the pulse energy requirements by several orders of magnitude compared to conventional devices, from picojoules to femtojoules. We experimentally demonstrate two effects associated with second harmonic generation (SHG). First, we observe efficient quasi-phase-matched SHG with$<<#comment/>100fJ$of pulse energy. Second, in the limit of strong phase-mismatch, we observe spectral broadening of both harmonics with as little as 2 pJ of pulse energy. These results lay a foundation for a new class of nonlinear devices, in which coengineering of dispersion with quasi-phase-matching enables efficient nonlinear optics at the femtojoule level.
3. Thin-film lithium-niobate-on-insulator (LNOI) has emerged as a superior integrated-photonics platform for linear, nonlinear, and electro-optics. Here we combine quasi-phase-matching, dispersion engineering, and tight mode confinement to realize nonlinear parametric processes with both high efficiency and wide wavelength tunability. On a millimeter-long, Z-cut LNOI waveguide, we demonstrate efficient ($1900±<#comment/>500%<#comment/>W−<#comment/>1cm−<#comment/>2$) and highly tunable ($−<#comment/>1.71nm/K$) second-harmonic generation from 1530 to 1583 nm by type-0 quasi-phase-matching. Our technique is applicable to optical harmonic generation, quantum light sources, frequency conversion, and many other photonic information processes across visible to mid-IR spectral bands.
4. An optical parametric oscillator (OPO) is developed and characterized for the simultaneous generation of ultraviolet (UV) and near-UV nanosecond laser pulses for the single-shot Rayleigh scattering and planar laser-induced-fluorescence (PLIF) imaging of methylidyne (CH) and nitric oxide (NO) in turbulent flames. The OPO is pumped by a multichannel, 8-pulse Nd:YAG laser cluster that produces up to 225 mJ/pulse at 355 nm with pulse spacing of 100 µs. The pulsed OPO has a conversion efficiency of 9.6% to the signal wavelength of$∼<#comment/>430nm$when pumped by the multimode laser. Second harmonic conversion of the signal, with 3.8% efficiency, is used for the electronic excitation of the A-X (1,0) band of NO at$∼<#comment/>215nm$, while the residual signal at 430 nm is used for direct excitation of the A-X (0,0) band of the CH radical and elastic Rayleigh scattering. The section of the OPO signal wavelength for simultaneous CH and NO PLIF imaging is performed with consideration of the pulse energy, interference from the reactant and product species, and the fluorescence signal intensity. The excitation wavelengths of 430.7 nm and 215.35 nm are studied in a laminar, premixed–air flame. Single-shot CH and NO PLIF and Rayleigh scatter imaging is demonstrated in a turbulent$CH4−<#comment/>H2−<#comment/>NH3$diffusion flame using a high-speed intensified CMOS camera. Analysis of the complementary Rayleigh scattering and CH and NO PLIF enables identification and quantification of the high-temperature flame layers, the combustion product zones, and the fuel-jet core. Considerations for extension to simultaneous, 10-kHz-rate acquisition are discussed.
5. Optical nonlinearity plays a pivotal role in quantum information processing using photons, from heralded single-photon sources and coherent wavelength conversion to long-sought quantum repeaters. Despite the availability of strong dipole coupling to quantum emitters, achieving strong bulk optical nonlinearity is highly desirable. Here, we realize quantum nanophotonic integrated circuits in thin-film InGaP with, to our knowledge, a record-high ratio of$1.5%<#comment/>$between the single-photon nonlinear coupling rate ($g/2π<#comment/>=11.2MHz$) and cavity-photon loss rate. We demonstrate second-harmonic generation with an efficiency of$71200±<#comment/>10300%<#comment/>/W$in the InGaP photonic circuit and photon-pair generation via degenerate spontaneous parametric downconversion with an ultrahigh rate exceeding 27.5 MHz/µW—an order of magnitude improvement of the state of the art—and a large coincidence-to-accidental ratio up to$1.4×<#comment/>104$. Our work shows InGaP as a potentially transcending platform for quantum nonlinear optics and quantum information applications.