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Register a new userAnalytical Approximation of the Second-Harmonic Conversion Efficiency
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The second-harmonic generation process of a focused laser beam inside a nonlinear crystal is described by the Boyd-Kleinman theory. Calculating the actual conversion efficiency and upconverted power requires the solution of a double integral that is analytically intractable. We provide an expression that predicts the exact gain coefficient within an error margin of less than 2% over several orders of magnitude of the confocal parameter and as a function of the walk-off parameter. Our result allows for readily tuning the beam parameters to optimize the performance of the upconversion process and improve optical system designs.
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High quality factor optical microcavities have been employed in a variety of material systems to enhance nonlinear optical interactions. While single-crystalline aluminum nitride microresonators have recently emerged as a low loss platform for integrated nonlinear optics such as four wave mixing and Raman lasing, few studies have investigated this material for second-harmonic generation. In this Letter, we demonstrate an optimized fabrication of dually-resonant phase-matched ring resonators from epitaxial aluminum nitride thin films. An unprecendented second-harmonic generation efficiency of 17,000%/W is obtained in the low power regime and pump depletion is observed at a relatively low input power of 3.5 mW. This poses epitaxial aluminum nitride as the highest efficiency second-harmonic generator among current integrated platforms.
In this letter we experimentally demonstrate second harmonic conversion in the opaque region of a GaAs cavity with efficiencies of the order of 0.1% at 612nm, using 3ps pump pulses having peak intensities of order of 10MW/cm2. We show that the conversion efficiency of the inhomogeneous, phase-locked second harmonic component is a quadratic function of the cavity factor Q.
Second harmonic generation (SHG), as one of the most significant c{hi}(2) nonlinear optical processes, plays crucial roles in a broad variety of optical and photonic applications. Designing various delicate schemes to achieve highly efficient SHG has become a long standing and challenging topic in field of nonlinear optics. Despite numerous success on SHG based on birefringent phase matching and quasi-phase matching, so far, modal phase matching (MPM) for SHG in tightly light-confined structures has still in its infancy. Here, we propose a new scheme to realize highly-efficient SHG via MPM by using a nanophotonic LiNbO3 thin-film waveguide consists of two bonded layers with internally reversed polarizations. In such a dual-layer ridge waveguide based on lithium niobate on insulator, upon optical excitation at 1574.6 nm, we observe SHG at 787.3 nm with ultrahigh conversion efficiency of 5,540% /W/cm/cm experimentally. This work advances our understanding on modal-phase-matched SHG and other quadratic optical nonlinear process, offering additional strategies for development of high-performance nonlinear photonic devices in on-chip platforms.
We report the observations of unexpected layer-dependent, strong, and anisotropic second harmonic generations (SHGs) in atomically thin ReS2. Appreciable (negligible) SHGs are obtained from even (odd) numbers of ReS2 layers, which is opposite to the layer-dependence of SHGs in group VI transition metal dichalcogenides, such as MoS2 and WS2. The results are analyzed from ReS2s crystal structure, implying second harmonic polarizations generated from the interlayer coupling. Pumped by a telecomband laser, SHG from the bilayer ReS2 is almost one order of magnitude larger than that from the monolayer WS2. The estimated second-order nonlinear susceptibility of 900 pm/V is remarkably high among those reported in two-dimensional materials. The laser polarization dependence of ReS2s SHG is strongly anisotropic and indicates its distorted lattice structure with more unequal and non-zero second-order susceptibility elements.
Thin-film lithium niobate (TFLN) in the form of x- or z-cut lithium-niobate-on-insulator (LNOI) has recently popped up as a very promising and novel platform for developing integrated optoelectronic (nano)devices and exploring fundamental research. Here, we investigate the coherent interaction length $l_{c}$ of optical second-harmonic (SH) microscopy in such samples, that are purposely prepared into a wedge shape, in order to elegantly tune the geometrical confinement from bulk thicknesses down to $approx$ 50 nm. SH microscopy is a very powerful and non-invasive tool for the investigation of structural properties in the biological and solid-state sciences, especially also for visualizing and analyzing ferroelectric domains and domain walls. However, unlike bulk LN, SH microscopy in TFLN is largely affected by interfacial reflections and resonant enhancement that both rely on film thickness and substrate material. In this paper we show that the dominant SHG contribution measured in back-reflection, is the co-propagating phase-matched SH signal and textit{not} the counter-propagating SH portion as is the case for bulk LN samples. Moreover, $l_{c}$ dramatically depends also on the incident pump laser wavelength (sample dispersion) but even more on the numerical aperture of the focussing objective in use. These experimental findings on x- and z-cut TFLN are excellently backed up by our advanced numerical simulations.
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