Fifty years have passed since the landmark experiment by Peter Franken, generating second harmonic from a ruby laser in a quartz crystal, marked the birth of nonlinear optics. Nowadays, the latter still provides one of the most reliable means to extend the wavelength range of coherent light sources across the whole optical spectrum, from the ultraviolet (UV) to the far infrared, by means of frequency up- and down- conversion of available solid-state lasers.

The progress of nonlinear optics through the years has always been closely linked to the development of new nonlinear materials, and this is even truer for developments into the extreme regions of the optical spectrum, such as the deep-UV. UV radiation covers the wavelength range between 40 and 400 nm, i.e., the portion of the electromagnetic spectrum between X-rays and visible light. Coherent sources in the deep-UV (i.e., at wavelengths below 300 nm) are important for a broad range of applications, such as high-resolution lithography, micromachining, bioagent detection, and water purification, which rely on key features associated to the short wavelength of UV radiation, namely, the ability to achieve tight focusing and to provide highly energetic photons capable of, for example, breaking bonds in many materials and chemical compounds.

Unfortunately, not many sources are available to generate coherent high-power light in the deep-UV spectral range. Traditional excimer lasers have long been the only viable option. Nonlinear optics, in the form of frequency upconversion of well-established, solid-state lasers (such as Nd:YAG or Ti:sapphire) can in principle provide an appealing alternative to such bulky gas lasers, affording wavelength flexibility, compactness, and efficiency as well as high spatial and temporal coherence. Yet, for a long time, developments in this direction have been hindered by the substantial lack of suitable nonlinear materials for harmonic generation in the deep-UV. In particular, for many years scientists in the field have been speaking of a “200-nm wall,” referring to the difficulty of achieving wavelength conversion below 200 nm.

In this paper, scientists from the group that first synthesized one of the most widely used nonlinear UV crystal (?-barium borate), present the full crystallographic and optical characterization of a new promising material for nonlinear optics beyond the 200-nm wall—CBBF (CsBe2BO3F2). The crystal belongs to the same family of the recently discovered nonlinear optical borate crystal KBBF (KBe2BO3F2). The authors develop a high-temperature, flux-growth technique for CBBF, yielding high-quality crystals with thicknesses greater than 2 mm and overcoming the limitations observed in KBBF, where achieving such thickness has proved so far extremely challenging.

Thicker samples are important for high-power nonlinear applications. Moreover, they allow thorough material characterizations as the one presented in the paper, encompassing both crystallographic and optical properties. In agreement with theoretical predictions, the measurements performed by the authors on their flux-grown CBBF crystals highlight a cutoff wavelength on the UV side as low as 151 nm, nonlinear coefficients comparable with those of KBBF, as well as birefringence values high enough to envisage practical applications of this new borate-based material for frequency conversion to wavelengths below 200 nm.

All this points to prospective developments of nonlinear optics in CBBF well beyond the 200-nm wall, where many important applications, such as high-resolution semiconductor lithography, nanosurgery, material processing, and spectroscopic techniques await the implementation of efficient and compact all-solid-state coherent UV sources.

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