Laser&Photonics Reviews New Type Quartz Crystal Space Harmonic Modulation for Efficient Vacuum UV Laser

Professor Zhang Huaijin and Yu Haohai from the Institute of Crystal Materials of Shandong University (the State Key Laboratory of Crystal Materials) proposed a spatial harmonic modulation strategy, which realizes the phase matching conditions that can be manipulated artificially in the new quartz crystal, and realizes the effective frequency doubling within the VUV range. The relevant research is titled "Spatial Frequency Manipulation of a Quartz Crystal for Phase Matched Second Harmonic Vacuum Ultraviolet Generation" and published in Laser&Photonics Reviews.

Second harmonic generation (SHG) is a nonlinear optical effect of electrons on strong light, which has been successfully applied in basic scientific research and modern optoelectronics.

However, due to strict and necessary phase matching conditions, efficient vacuum ultraviolet (VUV) SHG remains a challenge in solid-state lasers. This article adopts a spatial harmonic modulation strategy to achieve artificially manipulated phase matching conditions in ordinary quartz crystals, achieving effective frequency doubling within the VUV range. 31.6 mW of 177.3 nm (photon energy 7.0 eV) and 1.1 mW of 167.8 nm (photon energy 7.4 eV) vacuum ultraviolet laser output and conversion were obtained in quartz crystal devices. 

This design strategy solves the strict phase matching conditions of nonlinear optics, and provides a new approach for studying the nonlinear effects of light matter interaction, as well as for the development of high-resolution angle resolved photoelectron spectroscopy and Al+optical clocks in VUV photonics.

Keywords: artificial quartz crystal; Phase matching; Space harmonic modulation; Vacuum ultraviolet laser generation
Vacuum ultraviolet light with wavelengths below 200nm has advantages such as strong radiation energy and high resolution, and the radiation performance of VVV lasers is the fundamental light source for many applications in spectroscopy, lithography, micro/nano processing, and attosecond pulse generation. The frequency doubling treatment of solid-state ultraviolet lasers with wavelengths below 400 nm is a preferred technology for manufacturing compact and high light conversion efficiency VVV lasers, but it relies on nonlinear optical (NLO) crystals. 

However, the photon energy carried by VUV photons (>6.2 eV) exceeds the bandgap of ordinary nonlinear crystals, whether inorganic or organic. In addition, appropriate birefringence is also required for the phase matching conditions of the interaction between fundamental and frequency doubling light waves. 

Since the discovery of second harmonic generation (SHG) in quartz in 1961, only potassium fluoborate beryllite crystal (KBBF) laser crystals have been used to achieve VUV lasers with output power ranging from nanowatts to milliwatts, which has updated and upgraded some modern spectral systems such as angle resolved photoelectron spectroscopy (ARPES). 

However, due to the serious layered growth habit of KBBF crystals and the toxicity of beryllium oxide (BeO), it is urgently necessary to explore new deep ultraviolet NLO crystal materials. The study of ultraviolet nonlinear optics remains a huge challenge for international research.

Design of artificially manipulated periodic grating structure by introducing spatial harmonics
The phase matching condition, i.e. the equal phase velocity relationship of the interaction wave, determines the energy conversion efficiency of the input light to SHG. 

Based on the birefringence effect of non centrosymmetric materials, specific material orientations and polarization configurations can be selected to meet phase matching conditions and exhibit polarization dependent characteristics. In addition, in reversible ferroelectric domains, quasi phase matching (QPM) has been proposed and successfully implemented. Its nonlinear second-order nonlinear coefficient periodically reverses in the wavelength range from visible light to mid infrared or even terahertz, corresponding to the coherent length Lc of π phase shift at the interface. 

The research team proposed and demonstrated a phase matching condition, which involves adding an additional periodic phase (APP) from an ordered/disordered arrangement, where the disordered portion periodically increases the phase difference to satisfy a 2 π phase relationship without the need for reversible ferroelectric domains or appropriate birefringence.

The research team artificially manipulated the phase velocity of the interaction waves related to the spatial harmonics involved through a new APP phase strategy, and preliminarily designed a new type of quartz crystal, achieving efficient VUV lasers with power output of 177.3 nm and 167.8 nm at mW. This power value is equivalent to KBBF crystal and eliminates the toxicity of BeO. The research in this article should help to study the physical problems of artificially controlled spatial harmonic propagation speed, and the light source in this study should be suitable for the application of modern optoelectronic devices.

The preparation of spatial harmonic modulated quartz supercrystals was demonstrated by the research team through experiments using spatial harmonic modulated quartz supercrystals. In the production process of nonlinear quartz crystal devices, it is necessary to consider and optimize structural loss, crystal length, laser damage, and experimental conditions.

The second harmonic generation of 177.3 nm and 167.8 nm VUV lasers was studied by the research team using a VUV experimental setup to investigate the SHG effect of quartz supercrystals with different crystal lengths.

In summary, the research team theoretically studied the feasibility of using spatial harmonics generated by periodic grating structures to modulate phase velocity using spatial harmonics. Artificial quartz microcrystalline nonlinear photonic devices have been demonstrated through high-power SHG experiments within the VUV range. 

Space harmonic modulation provides an efficient method for exploring VUV laser sources. Currently, VUV with high energy resolution and high photon flux density is applied in modern instruments for stable, compact, and efficient coherent light sources. 

These modern systems include high-resolution ARPES, Al+optical clocks, light emission electron microscopes (PEEMs), Raman spectrometers, and light assisted scanning tunneling microscopes. In addition, the designed spatial harmonic processing strategy is also applicable to nonlinear optics in visible light, infrared, and even terahertz bands, and the design of linear and NLO processes will be more flexible.

Source: Sohu