1. Introduction

Tunable ultrashort-pulsed laser emission in the ultraviolet (UV) region is highly sought after because of its numerous applications in many fields of science and technology [1, 2]. Ultrashort pulses are necessary for controlling ultrafast chemical processes [1], probing fast physical and chemical processes [3, 4], and investigating the relaxation of charge carriers in conductors [5],

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

to name a few. On the other hand, UV lasers have many applications in various fields such as surface structuring [6–8], micromachining [9], remote sensing [10], spectroscopy and imaging [11]. An important advantage of having ultrashort pulses in the UV wavelength region is its ability to modify material properties only within the laser focus where the peak power is high. This feature is especially critical in micromachining [12]. Ultrashort UV pulses also permit outstanding temporal resolutions for pump-probe experiments [13]. Available light sources that satisfy these requirements are limited, despite the many applications. Excimer lasers can emit UV wavelengths, but these are bulky and cumbersome to maintain [14, 15]. UV laser emission using frequency conversion in nonlinear crystals is well established but is complex, has limited spectral bandwidth, non-tunable, and has low conversion efficiency. In contrast, high-power, all-solid-state UV lasers are highly regarded for simplicity in operation and maintenance. Hence, there is a great deal of interest for developing all-solid-state UV lasers.

2. Review of cerium ion-doped fluoride crystals

Laser gain media based on wide bandgap fluoride hosts was first proposed as a result of spectroscopic studies on trivalent lanthanides such as neodymium (Nd), cerium (Ce), and thulium (Tm) doped in solid-state hosts [37]. The intense broad band UV fluorescence from 276 to 312 nm observed from Ce3+:LaF3 and the 288–322 nm fluorescence from Ce3+:LuF3 were attributed to dipole-allowed 5d to 4f (5d-4f) radiative transition of Ce ions in the LaF3 and LuF3 fluoride hosts [38]. Subsequently, the first laser emission from the 5d-4f transition was achieved in 1977 using Ce3+:YLiF4. It emitted at 325.5 nm when it was optically pumped at 249 nm [16]. However, the progress of Ce3+:YLiF4 was limited by poor performance characteristics brought about by an early onset of saturation and roll off in the above-threshold gain and power output as well as a drop in the output for pulse repetition rates above 0.5 Hz. Although lasing from Ce3+: YLiF4 is ground breaking, the existence of solarization or color center formation prevents this material from being of practical use, thereby hindering its further development. In 1980, operation of an optically pumped Ce3+:LaF3 laser was reported [17]. Limitations of this laser medium include low output power and high lasing threshold. Moreover, the lasing results have not been reproduced. Subsequent experiments on other Ce3+-doped fluorides have not been very successful due to the formation of transitory or permanent color centers. Such color centers were essentially due to absorption of the pump and/or the laser radiation from emitting 5d states leading to the promotion of an electron into the conduction band followed by trapping by impurities or defects [39–43]. Recent investigations, however, showed that by an appropriate choice of activator-matrix complexes and active medium-pump source combinations, efficient tunable lasers using d-f transitions can be created [19, 21, 22, 44]. In 1992, emission from Ce3+: LiLuF4 pumped by a KrF excimer laser was reported. This laser material has almost the same optical properties as Ce3+:YLiF4 but with smaller solarization effect [19]. As a result, slope efficiencies of more than 50% were obtained [45, 46]. Moreover, continuous tunability was achieved from 305 to 333 nm [46]. Subsequently, lasing from Ce3+:LiCaAlF6 (Ce:LiCAF) was reported. This was a milestone not only because Ce:LiCAF can be pumped by the fourth harmonic of a Nd:YAG laser, but also because remarkably, no solarizaion effect was observed

Ultrashort Pulse Generation in Ce:LiCAF Ultraviolet Laser http://dx.doi.org/10.5772/intechopen.73501 137

Figure 1. Ce3+-doped lasers for tunable UV radiation. Solid lines and dots indicate the confirmed tunable wavelength

region, dotted lines show potential tunable wavelength region.

Cerium ion (Ce3+)-doped wide band gap fluorides have been the most successful tunable solidstate laser media in the UV region. Direct UV emission has been reported from Ce3+-doped YLiF4, LaF3, LiLuF4, LiCaAlF6, and LiSrAlF6 crystals [16–23]. Among these known UV laser crystals, Ce3+-doped lithium calcium hexafluoroaluminate (Ce3+:LiCaAlF6 or Ce:LiCAF) is the most prominent and successful solid-state gain medium for amplifying short UV pulses as well as for generating ultrashort UV pulses because it is highly transparent, has low excited state absorption, not prone to color center formation and therefore tolerant to laser-induced damage. Most importantly, it is absorbing at around 266 nm, which makes the fourth harmonics of readily-available Nd:YAG lasers an ideal excitation source. It also has sufficiently large effective gain cross-section of 6.0 <sup>10</sup><sup>18</sup> cm<sup>2</sup> that is favorable for oscillators, and a high saturation fluence of 115 mJ/cm2 . Lastly, it has broad tunability from 280 to 325 nm and enough bandwidth to generate 3-fs pulses [2, 20–22, 24–30].

Direct generation of tunable UV short-pulses using solid-state gain media, such as Ce:LiCAF, is not as straight forward as using near IR tunable solid-state laser media, such as Ti:sapphire, due to the difficulty of obtaining continuous wave (CW) laser operation. As a result, Kerr lens mode-locking schemes that utilize spatial or temporal Kerr type nonlinearity are a real challenge in the UV region [31]. The lifetime of the upper energy level of Ce:LiCAF is about 25 ns, which is too short to directly generate tunable UV short-pulses. Therefore, high pump power densities are required to achieve CW and mode-locked operation [32]. Moreover, the cavity lengths of the pump and the Ce:LiCAF laser oscillator have to be matched for synchronous mode locking [33]. A typical oscillator in mode-locking schemes also uses a four-mirror z-fold cavity, which means that reducing losses in the laser cavity is also crucial. On the other hand, the transient cavity method is a simpler means of generating tunable UV ultrashort pulses because it only utilizes the usual two-mirror laser oscillator. Pulse shortening by resonator transients has been demonstrated in dye lasers where laser pulse durations that are an order of magnitude shorter than the pump pulse have been obtained [34–36]. This book chapter discusses numerical simulations that extend the resonator transient technique to solid-state gain media. Since formation of resonator transients that lead to picosecond pulses depend on the cavity length and the Q-factor of the laser oscillator cavity, numerical simulations are carried out to investigate the decay time of the photons within the laser cavity as well as the energy of the picosecond Nd:YAG pump laser that will support the formation of these resonator transients. The technique can be extended to other rare earth-doped fluoride laser materials.
