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1. A metasurface-based diamond frequency converter using plasmonic nanogap resonators

   https://doi.org/10.1515/nanoph-2020-0392  

   Shen, Qixin ; Shams-Ansari, Amirhassan ; Boyce, Andrew M. ; Wilson, Nathaniel C. ; Cai, Tao ; Loncar, Marko ; Mikkelsen, Maiken H. ( September 2020 , Nanophotonics)

   Abstract Diamond has attracted great interest as an appealing material for various applications ranging from classical to quantum optics. To date, Raman lasers, single photon sources, quantum sensing and quantum communication have been demonstrated with integrated diamond devices. However, studies of the nonlinear optical properties of diamond have been limited, especially at the nanoscale. Here, a metasurface consisting of plasmonic nanogap cavities is used to enhance both χ (2) and χ (3) nonlinear optical processes in a wedge-shaped diamond slab with a thickness down to 12 nm. Multiple nonlinear processes were enhanced simultaneously due to the relaxation of phase-matching conditions in subwavelength plasmonic structures by matching two excitation wavelengths with the fundamental and second-order modes of the nanogap cavities. Specifically, third-harmonic generation (THG) and second-harmonic generation (SHG) are both enhanced 1.6 × 10 7 -fold, while four-wave mixing is enhanced 3.0 × 10 5 -fold compared to diamond without the metasurface. Even though diamond lacks a bulk χ (2) due to centrosymmetry, the observed SHG arises from the surface χ (2) of the diamond slab and is enhanced by the metasurface elements. The efficient, deeply subwavelength diamond frequency converter demonstrated in this work suggests an approach for conversion of color center emission to telecom wavelengthsmore »directly in diamond.« less
2. Shallow-etched thin-film lithium niobate waveguides for highly-efficient second-harmonic generation

   https://doi.org/10.1364/OE.395545  

   Zhao, Jie ; Rüsing, Michael ; Javid, Usman A. ; Ling, Jingwei ; Li, Mingxiao ; Lin, Qiang ; Mookherjea, Shayan ( June 2020 , Optics Express)

   High-fidelity periodic poling over long lengths is required for robust, quasi-phase-matched second-harmonic generation using the fundamental, quasi-TE polarized waveguide modes in a thin-film lithium niobate (TFLN) waveguide. Here, a shallow-etched ridge waveguide is fabricated in x-cut magnesium oxide doped TFLN and is poled accurately over 5 mm. The high fidelity of the poling is demonstrated over long lengths using a non-destructive technique of confocal scanning second-harmonic microscopy. We report a second-harmonic conversion efficiency of up to 939 %.W⁻¹(length-normalized conversion efficiency 3757 %.W⁻¹.cm⁻²), measured at telecommunications wavelengths. The device demonstrates a narrow spectral linewidth (1 nm) and can be tuned precisely with a tuning characteristic of 0.1 nm/°C, over at least 40 °C without measurable loss of efficiency.
3. Spatial Beam Self-Cleaning in Second-Harmonic Generation

   https://doi.org/10.1038/s41598-020-64080-7  

   Krupa, K. ; Fona, R. ; Tonello, A. ; Labruyère, A. ; Shalaby, B. M. ; Wabnitz, S. ; Baronio, F. ; Aceves, A. B. ; Millot, G. ; Couderc, V. ( December 2020 , Scientific Reports)

   Abstract We experimentally demonstrate the spatial self-cleaning of a highly multimode optical beam, in the process of second-harmonic generation in a quadratic nonlinear potassium titanyl phosphate crystal. As the beam energy grows larger, the output beam from the crystal evolves from a highly speckled intensity pattern into a single, bell-shaped spot, sitting on a low energy background. We demonstrate that quadratic beam cleanup is accompanied by significant self-focusing of the fundamental beam, for both positive and negative signs of the linear phase mismatch close to the phase-matching condition.
4. Efficient second-harmonic generation in high Q-factor asymmetric lithium niobate metasurfaces

   https://doi.org/10.1364/OL.413764  

   Kang, Lei ; Bao, Huaguang ; Werner, Douglas H. ( January 2021 , Optics Letters)

   Lithium niobate (LN) has been widely used for second-harmonic generation (SHG) from bulk crystals. Recent studies have reported improved SHG efficiency in LN micro-ring resonators and hybrid waveguiding structures, as well as in LN nanostructures supporting anapole modes and plasmon-assisted dipole resonances. Here we numerically demonstrate that high$Q$-factor resonances associated with symmetry-protected bound states in the continuum can lead to highly efficient frequency doubling in LN metasurfaces. Simulations show that the radiative$Q$-factor and on-resonance field enhancement factor observed in the metasurface are closely dependent on the asymmetric parameter$\mathrm{\alpha <\#comment/>}$of the system. Furthermore, high$Q$-factor resonances boost the SH conversion process in the LN nanostructures. In particular, for a LN metasurface with a$Q$-factor of$\sim <\#comment/>8\times <\#comment/>{10}^4$, a 0.49% peak SH conversion efficiency is achieved at a pump intensity of$3.3{\mathrm{k}\mathrm{W}/\mathrm{c}\mathrm{m}}^2$. This suggests that such high$Q$-factor LN metasurfaces may be good candidates for practical blue–ultraviolet light sources. Our work provides insight into the possible implementation of metadevices based on nanoengineering of conventional nonlinear crystals.
5. Plasmonic nanoarcs: a versatile platform with tunable localized surface plasmon resonances in octave intervals

   https://doi.org/10.1364/OE.403728  

   Zhang, Kunyi ; Lawson, Andrew P. ; Ellis, Chase T. ; Davis, Matthew S. ; Murphy, Thomas E. ; Bechtel, Hans A. ; Tischler, Joseph G. ; Rabin, Oded ( October 2020 , Optics Express)

   The tunability of the longitudinal localized surface plasmon resonances (LSPRs) of metallic nanoarcs is demonstrated with key relationships identified between geometric parameters of the arcs and their resonances in the infrared. The wavelength of the LSPRs is tuned by the mid-arc length of the nanoarc. The ratio between the attenuation of the fundamental and second order LSPRs is governed by the nanoarc central angle. Beneficial for plasmonic enhancement of harmonic generation, these two resonances can be tuned independently to obtain octave intervals through the design of a non-uniform arc-width profile. Because the character of the fundamental LSPR mode in nanoarcs combines an electric and a magnetic dipole, plasmonic nanoarcs with tunable resonances can serve as versatile building blocks for chiroptical and nonlinear optical devices.