  Nanophotonic objects like plasmonic nanoparticles and colloidal quantum dots
can complement the functionality of molecular dyes in biomedical optics.
However, their operation is usually governed by spontaneous processes, which
results in broad spectral features and limited signal-to-noise ratio, thus
restricting opportunities for spectral multiplexing and sensing. Lasers provide
the ultimate spectral definition and background suppression, and their
integration with cells has recently been demonstrated. However, laser size and
threshold remain problematic. Here, we report on the design, high-throughput
fabrication and intracellular integration of semiconductor nanodisk lasers. By
exploiting the large optical gain and high refractive index of GaInP/AlGaInP
quantum wells, we obtain lasers with volumes 1000-fold smaller than the
eukaryotic nucleus ($V_{laser}$<0.1 $\mu$m$^3$), lasing thresholds 500-fold
below the pulse energies typically used in two-photon microscopy ($E_{th}
\approx $0.13 pJ), and excellent spectral stability (<50 pm wavelength shift).
Multiplexed labelling with these lasers allows cells-tracking through
micro-pores, thus providing a powerful tool to study cell migration and cancer
invasion.
