Fig. 1. Principle of light sheet scanning of biointegrated microlasers. (a) Zebrafish sample with a microlaser injected into the region of interest (ROI). (b) Illustration of the light sheet (LS) pumping (blue beam) of the microlaser. Also shown is the detection objective that collects the fluorescence of the microlaser or sample (green light) and the microlaser emission (purple light). (c) Optical sectioning is performed by moving the light sheet along the z-axis and acquiring N slices. Spatial data (top) as well as spectral data (bottom) can be acquired simultaneously in different spectral regions. The spectral data show the characteristic multimode emission of a WGM microlaser.

Fig. 2. Overview of the light sheet microscopy setup. The illumination path is shown in blue, the detection path for fluorescence and transmission imaging is shown in green, and the path for detecting the microlaser spectra is in purple. At the bottom of the figure viewpoints from different directions in the setup are shown with respect to the applied coordinate system. 1, the illumination arm camera (CAM1) view (y-z-plane); 2, the sample chamber from above (x-z-plane); and 3, the detection arm camera (CAM3) view (x-y-plane). The sample and microlaser are represented by the beige and pink circles inside the sample chamber, respectively.

Fig. 3. Scanning microlasers with different pump beam profiles. (a) Diagram of a microlaser (gray sphere) with different pump beam profiles (blue) as viewed by CAM1. A Gaussian beam (top) or light sheet (bottom) was scanned over the microlaser in steps of 2 μm. The excited WGMs (pink) are also shown. Note that for better visualization of the so-called “cross modes,” the position of the pump beam for the light sheet excitation case is not at zero but slightly shifted towards the center of the microlaser. Cross modes are excited at the point where the light sheet overlaps with the point of maximum light coupling efficiency. (b) Normalized maximum intensity profiles of the WGM lasing spectra under Gaussian beam (red circles) or light sheet (blue circles) pumping. The beam position is given with respect to the center of the microlaser and divided by the radius of the microlaser (rc=7.5  μm). Also shown is the theoretical coupling efficiency (black line) of a Gaussian beam into a circular cavity.

Fig. 4. Light sheet scanning of a microlaser embedded in agarose. (a) Images of a microlaser scanned through a static light sheet in z-direction as viewed by CAM1. The bright elongated area is the pump light sheet, which is labeled in the second image. Scanning is in the z-direction and from the left to the right of the image. The bright spots on the circumference of the laser originate from the lasing cross modes. The individual cross modes are highlighted by the black and gray arrows, and the thin dashed line indicates the circumference of the microlaser. Scale bar, 10 μm. (b) Intensity contour plot of WGM spectra of a microlaser as the microlaser is scanned in the z-direction. (c) A magnified view of two WGMs with fitted peak position (lines), where blue indicates a TE mode and red indicates a TM mode. (d) A plot of the refractive index shift Δnext (using the first spectrum as reference) as a function of light sheet position.

Fig. 5. Light sheet scanning of a microlaser inside zebrafish tail muscles. (a) Diagram of a microlaser inserted into the zebrafish tail muscles. (b) Fluorescence light sheet imaging of the zebrafish tail muscles. Scale bar, 50 μm. (c) Bright field image of a microlaser injected into the zebrafish tail. Scale bar, 100 μm. (d) Light sheet microscopy image of the injected microlaser under lasing conditions, viewed by CAM3. Scale bar, 10 μm. (e) Intensity contour plot of the WGM spectra of the injected microlaser as the sample is scanned through the static light sheet in the z-direction. (f) Calculated refractive index next at different z-positions. (g) Mode splitting of the TM mode highlighted in (e) for three different z-positions. The laser mode (black line) is fitted by two Voigt profiles (blue and red-shaded curves). The combined fitted profiles (pink dashed-dotted line) are also shown.