Significant progress has been made in the generation of single-harmonic nonlinear crystals
Since laser generation, the laser windows have been successfully extended to deep UV using various nonlinear optical effects (multiplier, sum frequency, difference frequency, etc.) in non-linear optical crystal materials, and visible infrared, terahertz, etc. Range, and to achieve a broadband coherent light source and ultra-fast pulsed laser. L01, a group led by researcher Li Zhiyuan from the Institute of Physics, Chinese Academy of Sciences / Beijing Condensed Matter Physics Laboratory, has been devoted to quasi-phase matching (QPM) for high-efficiency nonlinear transformation (Appl. Phys. Lett. 105, 151106 (2014)], as well as the simultaneous generation of wideband second and third harmonics by the use of lithium niobate superlattice nonlinear crystals [Light: Science & Applications 3, e189 (2014)]. However, achieving higher harmonics in a monolithic non-linear crystal is an insurmountable level due to the many nonlinear upconversions involved in the implementation of higher harmonics, whereas monoliths It is difficult for the crystal lattice to provide the off-lattice vector to compensate for the phase mismatch in these processes at the same time. In the world, in order to achieve high harmonic generation with high conversion efficiency, only a few non-linear crystal cascades can be used, and the phase matching condition of each crystal needs to be finely controlled so as to obtain the highest possible conversion efficiency . Since the advent of nonlinear optics for 50 years, no efficient high-order harmonic generation has been achieved in monolithic crystals. Recently, the research team used original scientific ideas and technical solutions to make breakthrough research on this important scientific issue. The group uses the chirp structure of non-linear photonic crystal with a wide-band chamfered vector distribution, the first realization of broadband supercontinuum generation, in a single crystal to achieve the second to eighth harmonic generation. The research group chose the width of the negative domain as a fixed value along the direction of light propagation and changed the period of the polarization by changing the width of the positive domain to realize periodically poled lithium niobate crystal with chirp structure ) And a 1.6 cm long experimental sample was prepared using high-voltage pulsed polarization. Through the Fourier transform of the position function of the domain distribution in the chirp structure, the chaff lattice distribution of the structure is obtained, as shown in FIG. 2 (a). Theoretical analysis shows that the chirp-like crystal has multiple wide-band undercut vector distributions, which can not only compensate the phase mismatch of each nonlinear process during the generation of high-order harmonics, but also make the incident mid-infrared fly Second pulse pump laser (fundamental frequency light) of the wavelength components can participate in the high harmonics generated nonlinear process, which make full use of the laser linewidth of the energy of each component, significantly improve the nonlinear interaction strength, Produce high-brightness high harmonics, as shown in Figure 2 (b) - (f) below. The group experimented with a mid-infrared femtosecond pulsed laser with an infrared femtosecond laser (pulse width 115 fs, average power 20 mW, bandwidth 3400-3800 nm, repetition rate 1 kHz, peak power 0.17 GW) into the chirp structure After the sample, a very bright white light spot was seen at the output and the output light was split by a grating to obtain diffracted light spots of order 0 and -1 (Fig. 3), which fully reflected that the light output from the chirped structure sample had Supercontinuum broadband visible light distribution. After careful analysis and calculation, the internal conversion efficiency of the crystal is about 18% (400-800 nm in the visible band), which is much higher than the conversion efficiency of high-order harmonics obtained by bombarding the atomic gas and the plasma with a strong laser. Among them, the conversion efficiencies of the various harmonics are: the fourth harmonic (850-950 nm) to 0.7%, the fifth harmonic (660-850 nm) to 4.5%, the sixth harmonic (560-660 nm) ~ 7.2%, the seventh harmonic (485-560 nm) ~ 5.1%, the eighth harmonic (350-485 nm) ~ 1.2%. The experimental results show that, after special design, the conversion efficiency of higher order harmonics can be much higher than the lower order harmonics. Relevant theoretical and experimental work was published in the Physical Review Letters, August 20 (Phy. Rev. Lett., 2015, 115, 083902) and applied for a national invention patent. The authors believe that there are many elements in the design and success of nonlinear superlattice samples with chirped structure. 1. The non-linear process makes use of the maximum nonlinear coefficient d33 of lithium niobate crystal; 2. The sample provides a series of down-vestigial vectors that basically meet the requirements of multi-order higher harmonics in the cascade process; Vector with enough bandwidth to cover the bandwidth of the pump femtosecond laser, so as to maximize the energy of all spectral components of the fundamental frequency light; 4. The pump light is a femtosecond pulsed laser with a high peak power level, Significantly enhance the nonlinear interaction intensity; 5 samples for one-dimensional nonlinear superlattice, the quasi-phase matching process are collinear. The collinear non-linear process effectively reduces the complexity of the optical path adjustment, and avoids the defects such as walk-away effect, increases the nonlinear effect length and further increases the conversion efficiency of higher harmonics. 6. All of the nonlinear processes Occurs inside the monolithic crystal, avoiding the problem of the coupling loss of the crystal interface caused by using a plurality of cascaded crystals. It is with so much merits that high-bandwidth, high-order harmonics are produced in a single nonlinear crystal, making it possible to achieve breakthroughs in the core strategic issues of nonlinear optics. Fig.1 Schematic diagram of chirped nonlinear photonic crystal structure, Source: Physical Review Letters Figure 2 High-order harmonics generated in chirped nonlinear photonic crystals. Source: Journal of Physical Review Letters Graphite Molds,High Purity Graphite,Medium Grain Graphite,High Strength Graphite Block CARBONS TECH&CREATION(HENAN) INDUSTRY CO.,LTD , https://www.kycarbon.com