Diode-pumped, stimulated random emission using a powder of Nd:YVO4

Abstract

In random lasers, which refer to lasing in disordered media, strong multiple scattering plays a constructive role instead of being only a loss factor [1]. Random lasers have received considerable attention for several years due to its unique properties and its potential applications. Being demonstrated in a wide variety of media, including powders of rare-earth crystals, pulsed and continuous-wave lasing emission has been reported [2]. In this work, emission intensity and linewidth narrowing is experimentally analyzed in an optical pumped random media, observing a sharp threshold and pulsed emission on only one transition, namely, the 4 F3/2→4 I11/2. The powder was obtained by grinding a 1.4mol% Nd3+:YVO4 laser crystal using an agate mortar and a pestle. The mean diameter of the particles was 390 nm (determined by laser diffraction technique). A sample with flat surfaces and dimension of Ф5x1 mm3 was obtained by compressing ~33.7 mg of powder with a manual punch tablet. For the experiment, a QCW laser diode bar was used as the pump source, tuned to the maximum of the Nd3+ absorption for this sample (809 nm) with 100 µs pulse width and 3 Hz of repetition rate. The excitation beam was focused to a square shape with area of 5.33 mm2 . The samples’ backscattered luminescence, caused by the normal incidence beam from the diode bar, was separated from the pump excitation by a beam splitter, then collected and analyzed using a fast oscilloscope and a spectrometer. At low pump intensity (0.81 mW/mm2 ), several fluorescent emissions from 4G7/2→4 I9/2, 4G5/2→4 I9/2, 4 F3/2→4 I9/2, and 4 F3/2→4 I11/2 Nd3+ transitions were visible, as seen in Figure a. However, by increasing the pump intensity gradually, a threshold value is observed, accompanied by a sharp emission line at the 4 F3/2→4 I11/2 transition (1064.12 nm). Figure b shows the exponential growth for this transition, demonstrating that stimulated random emission for this sample has been obtained. Other fluorescent emissions suffered a spectral quenching. The spectral width of this transition decreased as a function of pump power, from 1.30 nm to 0.48 nm (Figure c). At higher duty cycles or longer pulses (400 µs pulse width and 30 Hz of repetition rate), a spectral broadening of the 4 F3/2→4 I11/2 transition with increased pump power is observed. This broadening is related to thermal effects that are more significant in a powder than in bulk crystal, on account of the poor thermal conductivity of the inhomogeneous structure. This temperature dependence is relatively strong for samples with a narrow gain spectrum, like the Nd3+:YVO4 and much smaller in systems with broad gain spectra, such as ZnO powder [3].