Laser Particle Stimulated Emission Microscopy

We introduce an optical microscopy technique that utilizes micro- or nanolasers embedded in a sample as imaging probes. The narrow spectra and nonlinear power dependence of stimulated emission from the laser particles yield optical sectioning, subdiffraction resolution, and low out-of-focus background. A proof of concept is demonstrated using perovskite nanowires.

Figure 1

The principle of LASE microscopy. (a) Laser particles excited by a tightly focused optical pump beam. (b) Output energy from a laser particle as a function of pump intensity. The laser output increases steeply at pump energy above the threshold ($P_{\text{th}}$). (c) The PSF of laser emission (red line) in comparison to the traditional diffraction-limited PSF of fluorescence detection (green line). Both the resolution and signal-to-background contrast are superior in LASE microscopy.

Figure 2

Numerical simulation of LASE microscopy. (a) Axial PSF of wide-field fluorescence (FL) versus LASE microscopy at pump rates at the lasing threshold ($p=1$) and of 300% higher than lasing threshold ($p=3$). (b) Normalized axial resolution, ${\mathrm{\Delta }}_{\mathrm{LASE}}/{\mathrm{\Delta }}_0$, for $\beta ={10}^{-2}$ (green, solid) and ${10}^{-5}$ (red, dashed).

Figure 3

Schematic of the experimental setup. Spherical lenses ($L1$, $L2$, $L3$), half-wave plate ($\lambda /2$), cylindrical lens (CL), dichroic mirror (DM), mirror (M), objective lens (Obj) ($\mathrm{NA}=0.8$, water immersion), long pass filter (LF), and beam splitter (BS). SEM image of a typical lead iodide perovskite nanowire. Insets (from left to right): a typical fluorescence image of a perovskite nanowire below threshold, a stimulated emission image above threshold of the nanowire, and a pump beam profile recorded in the CCD camera.

Figure 4

LASE imaging of a nanowire laser. (a) A typical laser output spectrum (circles). Curves: the curve fit for fluorescence background (gray) and the stimulated-emission laser output spectrum (magenta) calculated by subtracting the fluorescence background from the measured spectrum. (b) The stimulated emission output power (squares) measured as a function of the pump pulse fluence level. Line: a curve fit based on Eq. (3). Inset: same plot in a log-log scale. (c) Representative pump-beam scan profiles of the nanowire, for fluorescence background at $p<1$ (green dashed line) and stimulated emission at $p=1$ (gold) and $p=1.8$ (cyan). (d) Measured FWHM of laser emission profiles (blue circles) and fluorescence profiles (green circles), and numerical simulation data (magenta, dotted line).