Broadband Laser Emission from Color Centers Inside MgO Microcrystals

We report room-temperature broadband laser emission in the near-ultraviolet to the blue-green spectral range using color centers in MgO microcrystals. The lasing MgO microcrystals are obtained through a solid phase reaction between SiO and Mg at $450°\mathrm{C}$ in an argon atmosphere and are mostly composed of an accumulation of microcubes enclosed by $\{100\}$ facets. The laser action was observed in the wavelength region from 350 to 600 nm without using cavity mirrors. In the present MgO microcrystals, some of the color centers will be stabilized at the interfaces and/or boundaries of the microcrystalline domains, probably explaining the stable laser action even at room temperature.

Figure 1

(a) XRD patterns of the powders deposited onto the top lid (upper) and the materials found in the bottom of the crucible (lower). (b) An EDX spectrum of the microcube regions of the deposited powders. The inset shows a SEM image of the deposited powders.

Figure 2

PL characteristics of the MgO powders deposited onto the top lid. (a) Typical PL and PLE spectra. The excitation wavelength for the PL spectrum is 256 nm, while the monitor wavelengths of the PLE spectra are 500 nm (upper line) and 400 nm (lower line). (b) The PL decay curves monitored at 400 and 500 nm. The inset shows the expanded plot of the PL decay curve at 400 nm in the time region from 0 to 100 ms.

Figure 3

PL spectra of the MgO powders deposited onto the lid before and after annealing in air. The annealing was performed for 1 h at the designated temperature. The excitation wavelength is 256 nm.

Figure 4

Lasing characteristics of the MgO powders. (a) Emission spectra when the pumping density is (from bottom to top) 7, 22, 45, 65, 88, 120, 160, 190, 230, $270\mathrm{mJ}/{\mathrm{cm}}^2$. The asterisk indicates the second harmonic (532 nm) of the Nd:YAG laser contaminated in the incident laser beam. The time-resolved emission spectrum (inset) is also shown to reduce the effect of contamination of the second harmonic light; the spectrum was obtained with a gate delay of 20 ns and a gate width of 5 ms at a pump density of $270\mathrm{mJ}/{\mathrm{cm}}^2$. (b) The emission intensity at the peak wavelength (384 nm, open circles) and the wavelength-integrated intensity of emission (closed circles) as a function of the pump density.

Figure 5

The decay of the luminescence of the MgO powders at the peak wavelength (384 nm). The measurements were performed at the pump density of $250\mathrm{mJ}/{\mathrm{cm}}^2$. A solid line is a pure exponential fit, yielding a decay time of 20 ns.