image: a SHG intensity (I₂⍵) under phase mismatch and different APP phase-matching conditions with phase difference Δφ in a nonlinear crystal. The relative SHG intensities with light phase manipulation of Δφa :Δφb=π:π, 3π:π, and 3π:3π correspond to the red, blue and orange lines, respectively. b Distribution of the second-order nonlinear coefficient d with the phase difference Δφ. view more
Credit: by Mingchuan Shao, Fei Liang, Haohai Yu, Huaijin Zhang
Phase engineering of a wave function influences and determines the tendency and even results of physical processes, especially for efficient nonlinear optical frequency conversion. High conversion efficiency requires a strict phase-matching condition between the fundamental and harmonic electromagnetic, thus crystals without suitable birefringence and invertible ferroelectric domains were ruled out, which constrains the further development and application of nonlinear photonics in the broad spectral range, especially in the ultraviolet (UV) region. The novel strategy for the efficient tunable frequency conversion and universal nonlinear crystals are expected for dozens of years. Recently, scientists in China proposed a novel and universal phase-matching strategy associated with phase manipulation, named as additional periodic phase (APP) phase matching. Such phase-matching condition is attributed to the artificial manipulation of the optical phase in the periodic ordered/disordered alignments. Meanwhile, the light phase is related to its propagation length in the crystal, thereby giving rise to the possibility of the generation of tunable phase matching if the phase could be angularly engineered in angular APP phase matching, which has not been touched up to now and should be very important for the further applications of APP techniques.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Haohai Yu from the State Key Laboratory of Crystal Materials and Institute of Crystal Materials, Shandong University, China, and co-workers have theoretically investigated angular engineering of the additional periodic phase in APP phase matching and experimentally realized efficient tunable SHG (221-332 nm) covering almost the entire deep-UV spectral range with a tunable range of 111 nm with an APP quartz crystal. In addition, all possible phase-matching types are also originally simultaneously achieved with the participation of phase variation induced by the distribution of the grating period within a certain range. This novel and practicable work should be further development of APP phase matching and could inspire further studies in nonlinear optics, photonics and even physics.
For many nonlinear optical crystals, especially for the ultraviolet nonlinear crystals, angle tuning is only suitable for specific crystals at typical wavelength ranges. Compared with other common deep-UV nonlinear optical frequency conversion technologies, angular APP strategy shows a great advantage in wavelength tunability, regardless of BPM or QPM techniques. These scientists summarize the characteristics of Angular APP phase matching:
“The key of angular engineering strategy of APP is the effective optical path tunability, corresponding to the phase difference, by angle tuning to achieve phase difference of Δφ = 2Nπ with continuous phase compensation.”
“The angular engineering by tuning azimuth angle can be employed not only to select nonlinear coefficients, but also to control the polarization output in different phase matching types, which should be very difficult or unavailable in traditional phase-matching conditions due to the dispersion of refractive index, thus could be further applied in relevant scientific fields, such as laser-induced periodic surface structures, laser-scanning nonlinear optical techniques and even some quantum entanglement fields.” they added.
“Angular APP phase matching should provide a new route for nonlinear optics and thus will be key to opening the door for realization of tunable deep-UV lasers, which could have applications in modern photonics and physics.” the scientists forecast.
Journal
Light Science & Applications