2. Review of cerium ion-doped fluoride crystals

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 main-

tenance. Hence, there is a great deal of interest for developing all-solid-state UV lasers.

saturation fluence of 115 mJ/cm2

136 Numerical Simulations in Engineering and Science

bandwidth to generate 3-fs pulses [2, 20–22, 24–30].

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

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.

. Lastly, it has broad tunability from 280 to 325 nm and enough

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

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.

in this crystal [21, 22]. Following the success of Ce:LiCAF, lasing from Ce3+:LiSrAlF6 was reported. It can also be pumped with the fourth harmonic of a Nd:YAG laser and it has similar laser properties as the Ce:LiCAF crystal [23, 47]. Figure 1 summarizes the tunable wavelength regions of the five currently known Ce-doped lasers.

fluorescence emission from Ce3+-doped fluoride crystals have smaller radiative lifetimes of a few tens of nanoseconds compared to IR emissions that have lifetimes within hundreds of microseconds. In addition, fluorescence from the 5d-4f transitions is characterized by broad bandwidths and large Stokes shifts. The broad gain bandwidth enables tunability and ultrashort laser pulse generation from Ce3+-activated laser crystals. The large energy gap between the excited state 5d configuration and the 4f ground state configuration laser levels results to low multi-phonon related nonradiative decay. Therefore, quantum efficiencies as high as 90%

Ultrashort Pulse Generation in Ce:LiCAF Ultraviolet Laser http://dx.doi.org/10.5772/intechopen.73501 139

4. Numerical simulation of the electronic properties of the LiCAF host

LiCAF is a colquiriite-type fluoride with a hexagonal crystal structure belonging to the P-31c space group (group number 163). It is optically a uniaxial crystal with two formula units per unit cell. Six fluorine (F) atoms surround a lithium (Li), calcium (Ca), or aluminum (Al) atom. Each Li, Ca, and Al cation occupies a deformed octahedral site as shown in Figure 3a. This structure is also described by an alternative stacking of metallic and fluorine atom layers parallel to the c-axis [49–51]. The fraction coordinates of the representative atoms in the unit

First-principles density functional theory (DFT) calculations are used to obtain the optimized volume, electronic band structure, total and partial density of states (DOS), and the band gap

Figure 3. (a) Colquiriite-type structure of LiCAF and (b) first Brillouin zone of the hexagonal unit cell of a LiCAF crystal.

Li 1/3 2/3 1/4 Ca 0 0 0 Al 2/3 1/3 1/4 F 0.3769 0.0312 0.1435

xyz

are expected from Ce3+-activated laser crystals [20, 48].

cell are shown in Table 1.

Table 1. Atomic positions of the LiCAF atoms.

Ce:LiCAF has the following advantages over the other known Ce3+-doped fluoride hosts: (1) the strong absorption band at 266 nm would allow direct optical pumping by the fourth harmonics of the Nd:YAG laser; (2) the wide fluorescence band offers a tuning range from the 280 to 320 nm for possible pulse compression; (3) the gain cross-section of Ce:LiCAF (6 10<sup>18</sup> cm2 ) is high, even higher compared to the Ti:Sapphire. This property is ideal for building a laser resonator; (4) the saturation fluence (~115 mJ/cm2 ) of Ce:LiCAF is higher than organic dyes. This is significant when using this crystal in power amplifiers; (5) the nanosecond lifetime of Ce:LiCAF maybe too short to for regenerative amplifications, but this is sufficiently long to allow multi-pass amplification.
