Oxide laser crystals doped with rare earth and transition metal ions

K. Petermann,     University of Hamburg, Germany

Abstract:

At the beginning of this chapter the most prominent transition metal-and rare earth-doped oxide lasers and their emission wavelengths are introduced. After a short section about the fabrication of the laser crystals, some aspects concerning the geometry of the active medium for high-power lasers are presented. Finally, the spectroscopy as well as the laser results of rare earth-doped sesquioxides are reported in more detail. The chapter ends with some expected trends for the future.

Key words

high-power solid-state lasers

transition metal-and rare earth-doped oxides

growth of laser crystals

spectroscopy of rare earth-doped sesquioxides

sesquioxide lasers

1.1 Introduction

Since the renaissance of solid-state lasers in the mid-1980s numerous types of continuous and pulsed lasers on the basis of transition metal (TM) and rare earth (RE) ions have been developed. Many new laser wavelengths have been realised, but a few wavelength gaps still exist, for example in the yellow and blue/near UV spectral range. Especially, UV lasers are challenging, because high-energy photons very often create colour centres in the active medium, resulting in high laser losses. Furthermore, compact and efficient pump sources are not yet available, except (In,Ga)N-diode lasers. So, further research is necessary to close these wavelength gaps.

Also, many new host materials like oxides, fluorides, and glasses have been investigated in the past. However, the most successful family of host lattices for high-power lasers are still the garnets. With nd-and Yb-doped YAG (Y3A15O12) multi-kilowatt lasers are available nowadays. But also the vanadates (YVO4 and GdVO4) are very important laser materials due to their high absorption and emission cross-sections. A new class of hosts are the sesquioxides (Sc2O3, Y2O3, and Lu2O3), which exhibit high thermal conductivity and can be doped with high concentrations of RE ions. Thus, these laser materials are predestined for high-power solid-state lasers, although the growth of single crystalline material is quite complicated.

This chapter on TM-and RE-doped oxides is structured as follows. In the next two sections the most important laser-active TM and RE ions are introduced as well as the commonly used host crystals and their growth techniques. In Section 1.4 a short introduction to thin-disk lasers is given, and in Section 1.5 the spectroscopic properties of RE-doped sesquioxides are presented as well as the results of the cw-laser experiments. With a brief section about pulsed laser systems and a short outlook into future developments, this chapter will close.

1.2 Laser-active ions

The basic properties of a solid-state laser are dominated by the interaction of the laser-active ion and the host lattice. This interaction is quite strong in the case of transition metal ions (TM ions) due to the non-shielded 3d-electrons, which couple easily with the phonons of the surrounding oxygen ligands resulting in broad 3d-3d absorption and emission bands. In contrast, the 4f-electrons of the rare earth ions (RE ions) are shielded by the electrons in the 5s-and 5p-orbitals and thus the 4f-4f transitions are narrow and only weakly influenced by the crystal field provided by the ligands. Consequently, the transition energies or laser wavelengths are rather independent of the host lattice. In Fig. 1.1 the basic difference between the energy level scheme of RE ions with weak electron-phonon coupling and the vibronic energy levels of the TM ions with strong electron-phonon coupling is demonstrated.

1.1 (a) Weak electron-phonon coupling of RE ions; (b) relatively strong electron-phonon coupling of TM ions described by theconfigurational coordinate model.

1.2.1 Transition metal ions

Beside Ti3+ the most important TM laser ion is chromium with its different valencies, i.e. Cr2+, Cr3+, and Cr4+. Cr3+:Al2O3 was the very first solid-state laser, developed by T. H. Maiman in 1960 (Maiman 1960), and Cr3+ doped into garnets like Gd3Sc2Ga3O12 (GSGG) was the first tunable TM ion laser in the deep red spectral range (Struve et al., 1983; Huber and Petermann 1985). The tuning range of all Cr3+ lasers is limited to about 100 nm, but the great advantage is that they can be pumped by diode lasers.

Due to the broader tuning range extending from about 680 nm to 1100 nm the Ti:Al2O3 laser has replaced the Cr3+ lasers and can be regarded as the most important tunable laser nowadays (Moulton 1985). Ti:sapphire lasers are typically pumped by frequency doubled Nd3+ lasers at 532 nm, but pumping by (Ga, In)N diode lasers will be possible in future, when diodes with sufficient cw-power are available. Presently the output power of green diode lasers is limited to about 50 mW (Avramescu et al. 2010). Tests with GaN diodes at 452 nm wavelength have already been performed, but the efficiency and stability were very low due to the creation of unknown parasitic losses (Roth et al. 2009).

For the near-to mid-infrared wavelength range between 1.9 µm and 3.4 µm Cr2+ is a suitable laser ion, if it is doped into various zinc-chalcogenides like ZnS or ZnSe (DeLoach et al. 1996). Presently, Cr2+:ZnSe is the most efficient TM laser system with more than 1 W output power and a slope efficiency of 73%, which is very close to the quantum limit of 77%. These excellent data are due to the tetrahedral coordination of the Cr2+ ion resulting in a huge emission cross-section of more than 10-18 cm2. Because of the wide tuning range of 1100 nm, Cr2+:ZnSe may be regarded as the 'Ti:sapphire laser of the infrared' (Sorokina 2004). However, a Cr2+:ZnO laser has not yet been developed because of the quite large ionic radius of the Cr2+ ion, which apparently prevents the diffusion of the ion into the host lattice. Here, other preparation techniques like layer growth by pulsed laser deposition (PLD) may be successful in future.

One interesting aspect of the Cr2+-doped chalcogenides is the fact that they could be pumped electrically, since they are semiconductors (Fedorov et al. 2007). Recently, the first electrically pumped Cr2+:ZnS waveguide laser was demonstrated (Vlasenko et al. 2009) and it can be expected that such laser structures will be realised in future also with ZnO as host lattice.

Another tunable laser ion in the near infrared (NIR) is tetrahedrally coordinated Cr4+. In the past it was doped into a large variety of oxides, that is garnets, silicates, and germanates (Kück 2001). The best laser performances have been obtained with Cr4+:Y3Al5O12 (YAG) and Cr4+:Mg2SiO4 (forsterite). Nearly 2 W cw output power at 42% slope efficiency and a tuning range from 1340 nm to 1570 nm for YAG (Sennaroglu et al. 1995) and 1.5 W output power at 34% slope efficiency for forsterite have been measured (Zhavoronkov et al. 1997). However, the efficiency as well as the tuning range is limited in many oxides by excited state absorption (ESA) within the Cr4+ ion (Kück 2001).

More exotic laser ions are Fe2+, Ni2+, Co2+, and Mn5+, which have been lasing mostly at low temperatures with quite low efficiencies and thus are more of academic value. Other ions like Mn3+ (octahedral coordination) or the 3d1 ions V4+, Cr5+, and Mn6+ (tetrahedral coordination) have been investigated, with the result that they exhibit an extremely low quantum efficiency due to non-radiative transitions or suffer from strong ESA at the emission wavelengths preventing laser action. A summary of all TM ions with their various valencies is given in Table 1.1. Roman lettering denotes octahedral coordination in the host lattice and italic lettering tetrahedral coordination. Ions which have shown laser action are distinguished by bold characters (Kück 2001).

Table 1.1

Overview of TM ions with different valencies and coordinations. Roman lettering denotes octahedral coordination, italic lettering denotes tetrahedral coordination, and bold characters indicate laser oscillation

Another interesting, new laser ion for the NIR spectral range is bismuth, which was investigated in various glasses (Bufetov and Dianov 2009). However, the structure of the Bi-centre has not yet been identified. Nevertheless, high power cw-, Q-switched, and mode-locked lasing has been achieved with Bi-doped fibre lasers (Bufetov et al. 2010) though not yet with crystalline laser materials.

1.2.2 Rare earth ions

The largest class of laser ions are the rare earth ions (RE). Hundreds of lasers have been realised with Nd3+ as active ion, most of them in oxide crystals. The Nd ion exhibits various groups of NIR laser transitions around 900 nm, 1060 nm, and 1300 nm wavelength. The strongest and mostly used laser transition...