image: (a) Schematic graph of additional-phase-matching condition in arbitrary nonlinear optical crystals. The white and grey regions represent ordered crystal and disordered amorphous, respectively. The period length Λ equals the sum of ordered width La and disordered width Lb (Λ = La + Lb). Notably, La and Lb may be equivalent to coherent length Lc or integral multiple of Lc. deff/0 and n1/n2 represent the second-order nonlinear coefficient and refractive index of ordered and disordered regions, respectively. (b) Schematic estimation of the SH field amplitude of the APP quartz with different shifted phase (ΔφAPP) under the same crystal length. (c) Theoretical calculation of the APP (ΔφAPP) with the APP quartz samples of La=Lb= 2.1 μm, 1.4 μm, and 0.7 μm (d) 177.3 nm SHG output power in APP quartz (purple point) with La=Lb= 2.1 μm and Δφ=3π and in as-grown quartz (green point). view more
Credit: Mingchuan Shao, Fei Liang, Haohai Yu, Huaijin Zhang
Nonlinear optical frequency conversion is an important technique to extend the wavelength of lasers which has been widely used in modern technology. The efficiency of frequency conversion depends on the phase relationship among the interacting light waves. High conversion efficiency requires the satisfaction of phase matching. However, due to the dispersion property of nonlinear optical crystals, the phase mismatching always occurs, thus, phase matching condition should be specially designed. There are two widely used techniques for phase-matching: birefringence phase matching (BPM) and quasi-phase matching (QPM). Normally, BPM employs the natural birefringence properties of nonlinear optical crystals, and QPM is mainly focused on the periodically inversion of the ferroelectric domains. However, most of nonlinear optical crystals hold neither sufficient birefringence nor controllable ferroelectric domains. Therefore, it is in urgent demand to develop new route to meet phase-matching in arbitrary nonlinear crystals and in broad wavelength ranges.
In a new paper published in Light Science & Application, scientists from the State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, China, proposed a concept based on the basic principles of nonlinear frequency transformation, additional periodic phase (APP) from the disorder alignment, which can intercept the energy transmission channel of nonlinear light to fundamental light and compensate for mismatched phase. The APP concept means that after the light propagating at the coherence length Lc, the generated phase difference Δφ_PD was compensated by the additional phase difference Δφ_APP with Δφ_APP+Δφ_PD=2mπ (m is the integer). Based on the APP concept, a periodic ordered/disordered structure is introduced into crystal quartz by femtosecond laser writing technology to achieve an effective output from ultraviolet to deep-ultraviolet at the wavelength of 177.3nm. More interestingly, the APP phase matching may get rid of the limitations of birefringent and ferroelectric materials on nonlinear frequency conversion and should be applicable to all non-centrosymmetric nonlinear crystals for achieving effective output at any wavelength in the transmission range of the materials.
"To the best of our knowledge, the phase-matched deep-ultraviolet 177.3 nm generation was firstly achieved via quartz crystal with a high efficiency of 1.07‰." they added.
"This APP strategy may provide a versatile route for arbitrary nonlinear crystal in broadband wavelength. More importantly, this order/disorder alignment adds a variable physical parameter into optical system, thus leading to next-generation revolution in nonlinear or linear modulation and classical or quantum photonics." the scientists forecast.
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