Nonlinear optics: Fibre sources in the deep ultraviolet
چکیده
Wavelength-tunable deep-ultraviolet (DUV; ~200–300 nm) and vacuum-ultraviolet (VUV; ~100–200 nm) lasers are needed in fields such as lithography1, medicine1 and spectroscopy2. However, many laser sources in this wavelength range, including excimerand ion-based DUV and VUV lasers, are complex and expensive to manufacture and operate because of their high power consumption, short lifetime, low efficiency and narrow accessible wavelength range1. Writing in a recent issue of Physical Review Letters, Nicolas Joly and co-workers now report3 a hollow-core argon-filled photonic crystal fibre (PCF) capable of wavelengthtunable near-infrared (NIR) to DUV frequency conversion at efficiencies of up to 8%. The output wavelength of the PCF can be tuned in the range of 200–320 nm by varying the input pulse energy or gas pressure, and does not require any external realignment of the system. One of the most compact, effective and simple ways of generating DUV and VUV light is to apply nonlinear optical frequency conversion processes such as harmonic generation1,4, Raman5 or four-wave mixing6 to the output of NIR all-solid-state lasers. The conversion efficiency of the process depends primarily on the input light intensity, the length of the optical medium and its nonlinear coefficient7. Typical efficiencies are ~10% for commercial third-harmonic generation-based systems and ~5% for systems based on fourthharmonic generation of an 800 nm ultrafast Ti:sapphire laser. Converting NIR to DUV/VUV wavelengths is particularly challenging because of the narrow transparency window of nonlinear materials in the UV range. For example, beta barium borate (BBO), a nonlinear conversion crystal widely used for generating UV wavelengths, has a UV transmission cut-off wavelength of ~189 nm (ref. 1). The process is further complicated by the large group velocity mismatch between interacting pulses and the requirement for multistage conversion (for example, two cascaded second-harmonic converters to change 800 nm light into 200 nm light)1. There are also other limitations, including the lack of large-size and high-damage-threshold nonlinear materials, and the need for critical optical alignment and complex optical set-ups such as delay lines and dispersion compensators5–7. It is therefore imperative to develop novel nonlinear optical conversion techniques that exhibit high efficiency, are cheap to fabricate and can be easily integrated into existing set-ups. One promising approach is optical-fibrebased frequency conversion (Fig. 1). This compact, alignment-free technique offers high efficiency and low threshold due to the tight mode confinement and long interaction length provided by the fibre. The guided mode-field diameter can be further reduced through fibre design optimization7,8 and, in principle, there is no limit to the interaction length, provided low-propagation-loss fibres are used. For example, a standard singlemode optical fibre (such as SMF-28) permits interaction lengths of several kilometres while maintaining low loss (~0.2 dB km–1 at ~1.5 μm)8. The key parameters that determine the propagation and interaction of light in a fibre are mode distribution, fibre dispersion, nonlinearity, propagation loss and length8. Attaining highly efficient nonlinear frequency conversion requires the interacting pulses to overlap in the space and time domains, which can be achieved by appropriate design of the guiding mode and fibre dispersion7,8. However, the optimization of conventional single-mode optical fibres is limited by their refractive index8,9. Such fibres are therefore normally used for frequency conversion in a narrow range around 1.5 μm, where they exhibit low loss and low dispersion8. Indeed, their use in the visible and UV range is severely restricted. NoNliNear optics