Dark blue Cerenkov second harmonic generation in the two-layer-stacked hexagonal periodically poled MgO: LiNbO3s
By bonding two identical hexagon polarized lithium niobate crystals with a 30 degree intersection angle, the diffraction patterns at the second harmonic frequency for (left) a single PPMgOLN structure and (middle) a two-layer-stacked hexagonal PPMgOLN structure are recorded. (right) The magnified luminous spots on the inner and the outer ring from (middle).
The researchers, led by Prof. Wanhua Zheng, from Institute of Semiconductors, Chinese Academy of Sciences, use two identical hexagon polarized lithium niobate crystals to bond with a 30° intersection angle along Z axis to form a laminated structure. When a fundamental frequency laser with a pulse duration of picosecond or femtosecond perpendicularly on the bonded crystals, 12 diffraction points uniformly distributed on a ring are clearly observed due to the ÄŒerenkov frequency-doubled effect. As is known, there are only six diffraction points in the case of single layer hexagon polarized lithium niobate crystal. Thus, by bonding two identical hexagon polarized lithium niobate crystals with a 30 degree intersection angle, the diffraction points can be doubled. The third-order and even higher-order ÄŒerenkov effect are also observed experimentally. It is reported in Chinese Optics Letters Vol. 12, No. 3, 2014 (/col/abstract.cfm?uri=col-12-3-030501).
Given that a high-speed charged ion is propagating in transparent medium, if the ion speed is larger than the phase velocity of light in the medium, electromagnetic waves will be excited and transmit in the direction whose cosine is the ratio of phase velocity to ion speed. The intersection angle between the propagation direction of ion and electromagnetic waves is the ÄŒerenkov angle. This phenomenon is so called ÄŒerenkov effect. Similarly, in nonlinear optics, if the phase velocity of fundamental frequency light is larger than that of the second-harmonic light, a doubling-frequency light will be excited and propagates in the direction of Cerenkov angle.
A new phenomenon of Cerenkov frequency-doubled effect has been revealed in the past few years. Namely, when a beam of laser is transmitting in ferroelectric crystal such as lithium niobate, the nonlinear polarization excited at the ferroelectric domain wall will lead to the emission of second harmonics in particular direction that is called Cerenkov angle. Ever since then, this effect has been widely used in investigating the spatial distribution of the ferroelectric domain, the properties of the domain wall, and the nonlinear Talbot effect. The obtained achievements enrich the nonlinear optics significantly.
The authors report the dark blue nonlinear ÄŒerenkov radiation by second harmonic generation (SHG) in a two-layer-stacked hexagonal periodically-poled-MgO: LiNbO3s (PPMgOLNs). Based on the direct wafer bonding of two rotating PPMgOLNs, twelve bright spots as twice of those in a single PPMgOLN are observed at each second-harmonic ÄŒerenkov ring. This result suggests that the ferroelectric domain in the crystal not only can be used to generate short-wavelength laser, but also can be used to control the shape and the spatial distribution of the generated laser dot. Therefore, it can be widely used in the laser display, high-density storage, high-resolution printing, biological tissue imaging and so on.
Like the ÄŒerenkov diffraction pattern from two-layer-stacked structure, complex frequency-multipled patterns such as uniform diffraction rings would be obtained by stacking different polarized lithium niobate wafers. Moreover, if the relative displacement and rotation can made for the laminations, a dynamic or rotating diffraction pattern could be obtained, which can be used in developing special lasers, information coding, and dynamic laser display.