Photon Localization Laser: Low-Threshold Lasing in a Random Amplifying Layered Medium via Wave Localization

We demonstrate low-threshold lasing in random amplifying layered medium via photon localization. Lasing is facilitated by resonant excitation of localized modes at the pump laser wavelength, which are peaked deep within the sample with greatly enhanced intensity. Emission occurs into long-lived localized modes overlapping the localized gain region. This mechanism overcomes a fundamental barrier to reducing lasing thresholds in diffusive random lasers, in which multiple scattering restricts the excitation region to the proximity of the sample surface.

Figure 1

(a) Scheme of the experimental setup: ${\mathrm{L}}_{1,2}$, focusing and collection lenses; S, a stack of glass slides with gain medium inside; ${\mathrm{F}}_{1,2}$, color filters for blocking the pump and fluorescence light, respectively; $\lambda$, spectrometer; E, energy meter. (b)–(d) Emission spectra of the neat dye solution and of localization laser (see text), for the following experimental parameters: (b) single $25\mu \mathrm{m}$ dye layer (2 mg of dye in 1 ml of ethylene glycol) and $L=50$ glass slides; pump energy, $E_{\mathrm{p}\mathrm{u}\mathrm{m}\mathrm{p}}=110\mu \mathrm{J}$ and $130\mu \mathrm{J}/\mathrm{\text{pulse}}$ for the lower and middle curves, respectively. (c),(d) $L=100$ slides, every other air gap filled with the same dye solution; $E_{\mathrm{p}\mathrm{u}\mathrm{m}\mathrm{p}}=3.2\mu \mathrm{J}$ and $4.9\mu \mathrm{J}/\mathrm{\text{pulse}}$ for (c) and (d), respectively. Dashed curve in (c) corresponds to different gain medium (colored plastic sheet) in the middle of a stack excited by a chopped Ar-laser beam. The inset in (d) shows the total emission energy integrated over the spectral region 550–590 nm.

Figure 2

Average transmittance through a stack of glass slides without intervening dye solution. Experimental results (circles) are compared with the results of numerical calculations (diamonds), in which averaging was performed over 100 sample realizations of the glass stack with uniform distribution of the slide thickness between 80 and $120\mu \mathrm{m}$.

Figure 3

(a) High-resolution experimental spectrum of transmission through a stack of 50 glass slides. (b) Results of a scattering-matrix calculation of the transmittance spectrum for a stack of 50 slabs of random thickness illuminated by a plane wave, with the instrumental resolution taken into account. (c) Calculated spatial energy distribution of a plane wave incident upon the sample in (b) for the three wavelengths indicated. A stepwise change in energy density occurs at the air-glass interfaces.

Figure 4

Spatial variation of transmitted energy of the pump beam with fixed input power of $10\mu \mathrm{J}/\mathrm{\text{pulse}}$ (dashed curve), and of the total laser energy (solid curve) for a stack of 50 glass slides with a single layer of dye solution in the center.