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Enhancement of the second harmonic signal of nonlinear crystals by self-assembled gold nanoparticles
摘要： In second harmonic generation (SHG), the energy of two incoming photons, e.g., from a femtosecond laser, can be combined in one outgoing photon of twice the energy, e.g., by means of a nonlinear crystal. The SHG efficiency, however, is limited. In this work, the harvested signal is maximized by composing a hybrid system consisting of a nonlinear crystal with a dense coverage of plasmonic nanostructures separated by narrow gaps. The method of self-assembled diblock-copolymer-based micellar lithography with subsequent electroless deposition is employed to cover the whole surface of a lithium niobate (LiNbO3) crystal. The interaction of plasmonic nanostructures with light leads to a strong electric near-field in the adjacent crystal. This near-field is harnessed to enhance the near-surface SHG signal from the nonlinear crystal. At the plasmon resonance of the gold nanoparticles, a pronounced enhancement of about 60-fold SHG is observed compared to the bare crystal within the confocal volume of a laser spot.
关键词： gold nanoparticles,nonlinear crystal,second harmonic generation,lithium niobate,plasmonic nanostructures,electroless deposition,self-assembled diblock-copolymer-based micellar lithography,LiNbO3,SHG
更新于2025-09-23 15:19:57
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Ultra-Compact Low-Noise Broad-Band Upconversion Detector at 6 μm
摘要： State-of-the-art mid-infrared (MIR: 2 – 15 μm) direct detectors (e.g. semiconductor based HgCdTe, PbS, PbSe, and microbolometer) suffer from high background noise when operating at room temperature. Low noise detection therefore requires multi-stage or cryogenic cooling (-195°C) and perfect shielding to avoid temperature fluctuations [1]. Such systems easily become sophisticated and bulky, non-suitable for widespread applications. As the MIR spectral range, especially around 6 μm, is very relevant for spectroscopic-imaging of bio-molecules (protein, lipids), tissues (cancer, tumour), or for sensing of environmental pollutants (CH4, NO, NO2, SO2), high resolution spectroscopic systems in the 6 μm range is much in need. The numbers of pixels in typical HgCdTe or microbolometer based array detectors are limited to few 100s, making them less than ideal for fast high resolution broad-band spectroscopy. This work aims to solve that problem, by using frequency upconversion detection (UCD) [2]. In the presence of a strong near-infrared (NIR) LASER pump (at 1.03 μm), the MIR signal (6 μm) is translated to the NIR wavelength range (below 1 μm) using parametric frequency conversion in a nonlinear crystal (AgGaS2), without losing the spectral information encoded in the MIR signal. After upconversion, a standard silicon-CCD based spectrometer (pixel number = 2064) is used to detect the upconverted signal. In this way, high resolution, sensitive spectroscopic measurements can be performed, without the need for sophisticated cooling. The proposed upconversion system is based on a diode (940 nm) pumped Yb:YAG based solid state LASER operating at 1.03 μm, where the nonlinear crystal AgGaS2 (bulk, 5×5×10 mm3) is placed inside the LASER cavity to access the high intracavity power (Fig. 1(a)). This arrangement essentially gives higher upconversion efficiency. A Globar (heat source at ~ 800°C) is used as MIR illumination in the 6 μm range, giving an upconverted signal in the 880 nm range (grey area plot in Fig. 1(b)). In comparison to the previous demonstrations [3], the primary novelties of this system are (i) long-wavelength pumping of the laser, meaning that the pump diode wavelength (940 nm) is longer than the upconverted wavelength (< 900 nm), simplifying the spectral filtering of the upconverted signal, (ii) the LASER (at 1.03 μm) cavity is only 4 cm long, which makes the footprint of the upconversion module < 10 cm2, and (iii) no moving parts are needed in the upconversion spectrometer system (Fig. 1(a)). Using this system, we have successfully upconverted the 6 – 6.8 μm range (800 nm wide) within a single acquisition (50 ms) using Type-II birefringent phase matching in an AgGaS2 crystal. The wide bandwidth of the upconversion detection is achieved by exploiting non-collinear interaction between the pump LASER and the MIR signal inside the crystal. As an experimental verification we measured the MIR absorption spectrum of a polystyrene film placed at the MIR input side (Fig. 1(b)). Further details and results will be presented. We believe this is a promising route towards a small footprint, low-noise, efficient upconversion detector for high resolution spectroscopic application, directly in the molecular fingerprint wavelength range.
关键词： upconversion detection,spectroscopic-imaging,mid-infrared,environmental pollutants,nonlinear crystal
更新于2025-09-12 10:27:22
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Theoretical Investigation of the Optimal Nonlinear Crystal Thickness for THz Generation from two-Color Laser Pulse Ionized Gas under Different Laser Pulse Parameters
摘要： The generation of terahertz (THz) radiation by two-color laser pulse induced gas plasma is an actively researched topic since its exploration. It is a tabletop, powerful spectroscopic tool in the far-infrared spectral regime. It is possible to generate THz radiation by remotely more than 100m from the laser system, the remote sensing of the THz radiation also possible. Furthermore, the THz spectroscopy is capable to identify chemicals in a nondestructive way. By the combination of the former mentioned features, the THz spectroscopy is a powerful tool to identify drugs, poisons and explosives with remotely in a nondestructive way. One of the drawbacks of the THz radiation generated from two-color laser pulse induced gas plasma is the low efficiency. In the last two decades many publications have been published about increasing the efficiency of this method. Spectrally shaped fundamental beam has been proved to be an effective way to control the generated THz field properties. Using a step phase plate is demonstrated to enhance the generated THz intensity with a factor of two. Combining the fundamental, the second harmonic and the third harmonic beams also increase the THz pulse generation efficiency. Increasing the fundamental laser pulse wavelength has proved to be improve the generation efficiency. In this publication, we examined the nonlinear crystal thickness effect on the THz generation from two-color laser pulse induced gas plasma. It was found the at a given fundamental pulse energy and pulse duration there is an optimal thickness for the nonlinear crystal, where the THz generation is the most effective as seen on Fig. 1. In case of a thinner crystal, than the optimal crystal thickness, the second harmonic generation effectivity decrease, which entails the THz generation effectivity also decrease. In case of thicker crystal, than the optimal crystal thickness, the material dispersion decreases both the fundamental and its second harmonic’s peak power, which entails the THz generation effectivity decrease. The optimal nonlinear crystal thickness was examined in the function of the fundamental laser pulse energy, pulse duration, central wavelength and the nonlinear crystal distance from the focal spot. It was found that all the former mentioned parameters have a great impact on the optimal nonlinear crystal thickness. In conclusion, choosing the proper crystal parameters considering the laser and the setup parameters are a crucial point of the THz generation from two-color laser pulse induced gas plasma.
关键词： optimal thickness,two-color laser pulse,gas plasma,nonlinear crystal thickness,THz generation
更新于2025-09-12 10:27:22