Optical properties and passive self Q-switching of floating zone grown Cr–Nd co-doped YVO4 crystal

Abstract

Single crystals of Cr (0.7, 1.0, 1.4 and 2.0 at%) and Nd (0.7 at%) co-doped yttrium vanadate (YVO4) were grown by optical floating zone method. [100]-oriented elements were fabricated from the grown crystals for optical and lasing studies. The refractive index analysis revealed that the values of ordinary refractive indices (no) for 0.7, 1.0, 1.4 and 2.0 at% Cr doped samples were found to be almost constant except for a slight increase corresponding to 532 nm. But the extraordinary refractive indices (ne) was found to increase with Cr doping concentration and it is maximum at 1.4 at% for all the wavelengths. The thermal coefficient of ne and no increases slightly and becomes constant beynd 1.4 at% of Cr. The band gap of the crystal is polarization sensitive with the values in the range of 3.51–3.55 eV and 3.59–3.62 eV for electric field parallel and perpendicular to c axis, respectively. The band gap does not exhibit any significant change on Cr doping. The emission spectrum with excitation at 808 nm is characterized by three broad asymmetric bands centered at 896, 1064 and 1341 nm. The emission intensity for the peak around 1064 nm decreases with an increase in Cr due to a broad absorption around 1100 nm of Cr5+ ion and, therefore, this band is exploited for passive Q-switching in the crystal. Further, excitation at 400 nm of Cr band revealed that there is strong emission at 1064 nm, which may be explored for pumping the gain medium with solar radiation. Finally, a stable passive Q-switching at 1064 nm was demonstrated in [100] element fabricated from the gown crystal having 1.4 at% Cr. An output power of ∼108 mW having pulse-width ∼220 ns and repetition rate ∼239 kHz was achieved for a pumping of ∼14.6 W at 808 nm.