image: Two-photon induced fluorescence in polycrystalline ZnSe while varying incident laser intensity at 775 nm (a) intense front face blue emission; (b) side view, I = 25 GW cm-2; (c) I = 4 GW cm-2; (d) I = 1.5 GW cm-2; (e) 1 GW cm-2. Fluorescence appears deeper in sample as incident intensity decreases. view more
Credit: OEA
In a new publication from Opto-Electronic Advances; DOI 10.29026/oea.2022.210036, authors from the Laser Engineering Group in the School of Engineering at the University of Liverpool, Liverpool, United Kingdom, discuss two-photon absorption and stimulated emission in poly-crystalline zinc selenide with femtosecond laser excitation.
The material studied here is polycrystalline ZnSe – readily available, still very pure and much cheaper than single crystal material. The two-photon absorption coefficient (b) was also found to change with intensity and measured using a “Z-scan technique” where a thin sample of ZnSe is translated through a weakly focused laser beam while measuring the change in transmission. This variation in b also infers that further sequential photon absorption (or excited state absorption) takes place during laser exposure and termed reverse saturated absorption. At low peak intensities I < 5 GW cm-2, we measured b = 3.5 cm GW-1 at 775 nm, in agreement with other research – reducing significantly as intensitiy is increased.
The intense blue fluorescence observed encouraged us to consider whether at ultrahigh intensity, stimulated emission could be induced in polycrystalline ZnSe by two-photon absorption at 775 nm. This has previously been observed in single crystal ZnSe. As the fluorescence lifetime was measured to be te ~ 3.3 ns, a thin 0.5 mm thick sample was mounted in a short (10 cm) optical cavity providing feedback. Stimulated emission was indeed confirmed by significant line narrowing from a bandwidth Dl = 11 nm (cavity blocked) to Dl = 2.8 nm at peak wavelength lp = 475 nm while the upper state lifetime also decreased. This is the first reported observation of stimulated emission in polycrystalline material. These results suggest that with more optimum pumping conditions and crystal cooling, polycrystalline ZnSe might reach lasing threshold via two-photon pumping at l = 775 nm.
The Laser Engineering Group in the School of Engineering at the University of Liverpool is headed by Professor Geoff Dearden, an expert in Lasers and Photonics. Over a number of years, ultrafast laser-materials interactions (using femtosecond and picosecond pulses) have been studied in detail using, for example laser ablation for production of periodic complex surface micro-structures (< 1 mm pitch) using advanced optical techniques and laser beam engineering on metals, polymers and semi-conductors. Such structures have applications in controlling surface hydrophobicity, anti-bacterial response, security marking and precision micro-structuring of high value components for sectors such as aerospace. With femtosecond pulses, transparent polymers (PMMA) and dielectrics such as Sapphire have been internally micro-structured at high speed through parallel beam, multi-photon absorption. The resulting periodic refractive index engineering can create high quality, high efficiency Volume Bragg Gratings, useful in spectral analysis and creating high temperature sensors for extreme environments (aero-engines).
Article reference: Li QL, Perrie W, Li ZQ, Edwardson SP, Dearden G. Two-photon absorption and stimulated emission in poly-crystalline Zinc Selenide with femtosecond laser excitation. Opto-Electron Adv 5, 210036 (2022) . doi: 10.29026/oea.2022.210036
Keywords: ZnSe / femtosecond laser / nonlinear absorption / stimulated emission / fluorescence
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The Laser Engineering Group in the School of Engineering at the University of Liverpool was founded in 1988 by Professor Bill Steen and is currently headed by Professor Geoff Dearden. The current research interests include laser micromachining, ultra-short pulse laser-material interactions, laser beam engineering, Spatial Light Modulation, Additive Manufacturing, forming, Laser ignition of IC engines and optical trapping. The group’s main activity is therefore in advanced manufacturing with lasers which is a rapidly developing field due to the ongoing development and introduction of new laser technology and systems that provide advanced capabilities, addressing challenges that need to be met by high quality, high impact research. This bridges the gap between laboratory experiments and the manufacturing industry, allowing the laser to be applied in the sectors such as aerospace, automotive, biomaterials, art conservation and medicine.
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Opto-Electronic Advances (OEA) is a high-impact, open access, peer reviewed monthly SCI journal with an impact factor of 9.682 (Journals Citation Reports for IF 2020). Since its launch in March 2018, OEA has been indexed in SCI, EI, DOAJ, Scopus, CA and ICI databases over the time and expanded its Editorial Board to 36 members from 17 countries and regions (average h-index 49).
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