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Optical characterization of SiC films grown on Si(111)
摘要： Thin SiC films, grown on Si by substitution of C into Si on Si substrates with and without a SiGe buffer layer, have been investigated with optical techniques. The formation of SiC domains leads to strong green and blue photoluminescence from stacking faults and surface oxides. Introduction of a 10-nm-thick SiGe buffer layer leads to improved crystallinity as evidenced by X-ray diffraction and optical second-harmonic generation (SHG). Nonlinear optical azimuthal rotational spectra demonstrate the presence of cubic SiC in the film. Furthermore, angle-of-incidence scans are consistent with simulations based on a film with cubic symmetry which demonstrates that the cubic phase dominates the SiC film. Growth on vicinal Si(111) leads to a SiC film with the same c1v symmetry as the substrate, demonstrating that the lattice planes of the SiC film follow those of the Si substrate. Spatially resolved SHG scans show structures that are related to the underlying structure of the Si interface resulting from the growth process.
关键词： second-harmonic generation,optical characterization,SiC films,photoluminescence,crystallinity
更新于2025-09-10 09:29:36
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Circular dichroism & isotropy - polarity Reversal of Ellipticity in molecular films of 1,1’-Bi-2-Naphtol
摘要： We have studied the circular dichroism (CD) in the ultra violet and visible region of the transparent, chiral molecule 1,1’-Bi-2-Naphtol (BINOL) in 1.5 μm thick films. The initial transparent film shows an additional negative cotton effect in the CD compared to solution. With time under room temperature the film undergoes a structural phase transition. This goes hand in hand with a cotton effect at the low energy absorption band which inverts with opposite propagation direction of light through the film which is revealed as a polarity reversal of ellipticity (PRE). After completion of the phase transition the film exhibits circular differential scattering throughout the visible range which also shows PRE. The structure change was studied with Raman, microscopy under cross polarization conditions and nonlinear second-harmonic generation circular dichroism (SHG-CD). The superposition of the optical activity of individual molecules and isotropy effects makes an interpretation challenging. Yet overcoming this challenge by finding a suitable model structural information can be derived from CD measurements.
关键词： Second-harmonic generation circular dichroism,Isotropy,Polarity reversal of ellipticity,Circular dichroism
更新于2025-09-10 09:29:36
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Time-resolved second harmonic generation with single-shot phase sensitivity
摘要： A time-resolved, phase-sensitive second harmonic generation (SHG) method to probe the excited state dynamics of interfacial species is presented. It is based on an interference measurement between the SHG from a sample and a local oscillator generated at a reference surface in which an entire interference pattern is recorded in a single shot by using a spatially varying phase unit comprised of a pair of wedges that sandwich the reference sample. In combination with 30 kHz modulation of the experiment, shot-to-shot pump-probe measurements are presented. The technique is characterised by measuring the time-resolved change in the amplitude and phase of the interference pattern due to the excited state dynamics of the dye malachite green at the air/water interface. The key attributes of the technique are its excellent phase stability and sensitivity, and relatively short data acquisition times.
关键词： air/water interface,excited state dynamics,time-resolved,second harmonic generation,malachite green,phase-sensitive,interference measurement
更新于2025-09-09 09:28:46
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[IEEE 2018 12th International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials) - Espoo, Finland (2018.8.27-2018.9.1)] 2018 12th International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials) - Nonlinear Quasi-Phase Matching with metasurfaces
摘要： Quasi-Phase Matching is an optical process needed to build up nonlinear signal from strong pump intensity and is generally obtained through periodical inversion of coherence domains of the crystal. We show that this effect can be arti?cially monitored through inserting metasurfaces along the optical path of the light in a waveguide. Sub-wavelength gratings of holes are etched in Gallium Nitride to locally change medium dispersion, allowing continuous increase of the second harmonic signal. This approach can be used to address any phase delay difference between pump and signal at will, not only π, opening up new perspectives for arbitrary phase matching conditions. It can also be implemented in highly nonlinear systems, in which it is generally hard to address phase-matching conditions, for instance waveguides with buried intersubband quantum wells or other materials such GaAs.
关键词： second harmonic generation,metasurfaces,Quasi-Phase Matching,Gallium Nitride,nonlinear optics
更新于2025-09-09 09:28:46
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Analysis of Second-Harmonic Generation of Low-Frequency Dilatational Lamb Waves in a Two-Layered Composite Plate
摘要： We analyze the effect of second-harmonic generation (SHG) of primary Lamb wave propagation in a two-layered composite plate, and then investigate the influence of interfacial properties on the said effect at low frequency. It is found that changes in the interfacial properties essentially affect the dispersion relation and then the maximum cumulative distance of the double-frequency Lamb wave generated. This will remarkably influence the efficiency of SHG. To overcome the complications arising from the inherent dispersion and multimode natures in analyzing the SHG effect of Lamb waves, the present work focuses on the analysis of the SHG effect of low-frequency dilatational Lamb wave propagation. Both the numerical analysis and finite element simulation indicate that the SHG effect of low-frequency dilatational Lamb wave propagation is found to be much more sensitive to changes in the interfacial properties than primary Lamb waves. The potential of using the SHG effect of low-frequency dilatational Lamb waves to characterize a minor change in the interfacial properties is analyzed.
关键词： composite plate,interfacial properties,low frequency,second-harmonic generation,Lamb waves
更新于2025-09-09 09:28:46
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Nonlinear optical susceptibility of atomically thin WX2 crystals
摘要： We have studied tungsten diselenide (WSe2) and tungsten disulfide (WS2) monolayer materials in second harmonic generation spectroscopy and microscopy experiments. Ultra-broadband continuum pulses served as the fundamental beam while its second harmonic spectrum in the visible and ultraviolet (UV) range was detected and analyzed with a better than 0.3 nm spectral resolution (< 2 meV). We provide dispersion data and absolute values for χ(2) for the materials within a photon energy range of 2.3–3.2 eV. Fine spectral features that were detected within the dispersion data for the optical nonlinearities indicate the impact of near bandgap exciton transitions. The fundamental bandgap of 2.35 eV and exciton binding energy of 0.38 eV were determined from the measurements for WS2 monolayers while the corresponding values in WSe2 monolayers were 2.22 eV and 0.71 eV. Ranges for the absolute values of the sheet nonlinearity for WS2 and WSe2 are shown to be 0.58–1.65 × 10?18 m2/V and 0.21–0.92 × 10?18 m2/V, correspondingly.
关键词： Nonlinear spectroscopy and microscopy,Transition metal dichalcogenides,Two-dimensional semiconductors,Second harmonic generation,Optical nonlinearity in semiconductors,Monolayer crystals,Optics at interfaces
更新于2025-09-09 09:28:46
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Anapole Resonances Facilitated by High Index Contrast between Substrate and Dielectric Nanodisk Enhance Vacuum Ultraviolet Generation
摘要： Anapole modes are known to be excited only in high-index nanoparticles. Here we show that the nanodisks made of dielectric materials with much lower refractive indices, such as LiNbO3, can generate anapole modes, provided that they are supported by metallic or near-zero-index substrate. The important parameter for exciting anapole mode is the index contrast between nanodisk and substrate, rather than the refractive index of the nanodisk. We numerically demonstrate highly efficient coherent generation of vacuum ultraviolet with the efficiency higher than 10-2 % based on the second-harmonic generation via the excitation of anapole modes in LiNbO3 nanodisks supported by aluminum substrate. Highly efficient vacuum ultraviolet nanosource proposed in this work can find important applications for spectroscopic and sensing purposes at the nanoscale.
关键词： Second-harmonic generation (SHG),Anapole mode,Refractive index,Nanodisks,Vacuum ultraviolet (VUV)
更新于2025-09-09 09:28:46
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Nonlinear emission from silver-coated 3D hollow nanopillars
摘要： High aspect ratio metal nanostructures have been the subject of a number of studies in the past, due to their pronounced resonances in the infrared that can be exploited to enhance vibrational spectroscopies. In this work, we investigate the nonlinear optical response of both individual and closely-packed assemblies of vertical hollow Ag nanopillars upon excitation with ultrafast laser pulses. In particular, the analysis of their nonlinear emission spectra evidences an intense two photon photoluminescence (TPPL) emission and a neat signature of second harmonic generation (SHG). Given the relatively low background, this pronounced nonlinear emission could be employed as a local probe for analytes trapped at the surface of the nanopillar or flowing through it. For this reason, these nanostructures may become appealing building blocks in multi-purpose devices for nonlinear photonics and sensing.
关键词： Two Photon Photoluminescence,Ag Nanopillars,Localized Surface Plasmons,3D nanofabrications,Second Harmonic Generation
更新于2025-09-09 09:28:46
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Spatially-Controllable Hot Spots for Plasmon-Enhanced Second-Harmonic Generation in AgNP-ZnO Nanocavity Arrays
摘要： Plasmon-enhanced second-harmonic generation (PESHG) based on hybrid metal-dielectric nanostructures have extraordinary importance for developing efficient nanoscale nonlinear sources, which pave the way for new applications in photonic circuitry, quantum optics, and biosensors. However, the relatively high loss of excitation energies and the low spatial overlapping between the locally enhanced electromagnetic field and nonlinear materials still limit the promotion of nonlinear conversion performances in such hybrid systems. Here, we design and fabricate an array of silver nanoparticle-ZnO (AgNP-ZnO) nanocavities to serve as an efficient PESHG platform. The geometry of AgNP-ZnO nanocavity arrays provides a way to flexibly modulate hot spots in three-dimensional space, and to achieve a good mutual overlap of hot spots and ZnO material layers for realizing efficient SH photon generation originating from ZnO nanocavities. Compared to bare ZnO nanocavity arrays, the resulting hybrid AgNP-ZnO design of nanocavities reaches the maximum PESHG enhancement by a factor of approximately 31. Validated by simulations, we can further interpret the relative contribution of fundamental and harmonic modes to Ag-NP dependent PESHG performances, and reveal that the enhancement stems from the co-cooperation effect of plasmon-resonant enhancements both for fundamental and harmonic frequencies. Our findings offer a previously unreported method for designing efficient PESHG systems and pave a way for further understanding of a surface plasmon-coupled second-order emission mechanism for the enhancement of hybrid systems.
关键词： second-harmonic generation,hybrid nanostructure,finite-difference time-domain,plasmonics
更新于2025-09-09 09:28:46
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Second Harmonic Generation Imaging Analysis of Collagen Arrangement in Human Cornea
摘要： PURPOSE. To describe the horizontal arrangement of human corneal collagen bundles by using second harmonic generation (SHG) imaging. METHODS. Human corneas were imaged with an inverted two photon excitation ?uorescence microscope. The excitation laser (Ti:Sapphire) was tuned to 850 nm. Backscatter signals of SHG were collected through a 425/30-nm bandpass emission ?lter. Multiple, consecutive, and overlapping image stacks (z-stacks) were acquired to generate three dimensional data sets. ImageJ software was used to analyze the arrangement pattern (irregularity) of collagen bundles at each image plane. RESULTS. Collagen bundles in the corneal lamellae demonstrated a complex layout merging and splitting within a single lamellar plane. The patterns were signi?cantly different in the super?cial and limbal cornea when compared with deep and central regions. Collagen bundles were smaller in the super?cial layer and larger in deep lamellae. CONCLUSIONS. By using SHG imaging, the horizontal arrangement of corneal collagen bundles was elucidated at different depths and focal regions of the human cornea.
关键词： collagen,corneal stroma,second harmonic generation,cornea
更新于2025-09-09 09:28:46