Periodically poled lithium tantalate crystals for efficient SHG generation

There has been an increasing interest in compact coherent sources at 600 nm for a number of applications, e.g. orange-red components for laser projection displays [1], and excitation sources in photodynamic therapy [2]. The need for these sources become more apparent if one recalls that the amber-orange-red spectral region remains unreachable for conventional surface emitting diode lasers due to a lack of suitable direct bandgap materials providing sufficient carrier confinement in the quantum wells. One way forward is the frequency doubling of infrared light. Recently, a second-harmonic generation (SHG) at 610 nm with an output power of 2.7 W has been demonstrated from an optically-pumped semiconductor disk laser with a BBO crystal [3]. While nonlinear crystals such as BBO and LBO are highly resistant to optical damage inflicted by visible light, their effective non-linear coefficient, d
eff
is low (1–3 pm/V). Periodically poled lithium tantalate (LT) and lithium niobate crystals on the other hand offer d
eff
values above 10 pm/V. They are highly transparent from the UV to mid-IR spectral region. However, being photorefractive, these materials have traditionally been considered to be not suitable for intracavity generation of visible light. Recent advances in double-crucible Czochralski (DCZ) method made it possible to produce nearly stoichiometric LT materials (Mg:SLT) with increased resistance to optical damage [4].