One Pulse Instead of a Thousand express
A laser hits the sample, and molecules inside start to vibrate. This vibration heats the material by tiny fractions of a degree. Previously, such heating was considered a nuisance, but now it's used to read chemical composition, like guessing who's coming down the hallway by the sound of footsteps.
The new optical microscope uses just one pair of laser pulses to trigger this heating. Conventional instruments fire thousands of pulses in a row, like knocking on a door many times, hoping someone will hear. Here, a single knock suffices. The laser is deliberately stretched in time so it hits more gently and doesn't damage living cells.
On living HeLa cells, researchers could spot patches of cholesterol in the cell membrane. Previously, these were labeled with glowing antibodies, but now they're visible directly, like grease spots on the surface of broth. The imaging speed is 10 frames per second, and the cell stays alive.
The heat left behind by a laser flash turns out to be a precise language of chemistry. One pulse, and the molecular map is ready.
Original abstract
as written by the authors arXiv:2603.01386
By measuring the thermal effects resulting from stimulated Raman excitations, stimulated Raman photothermal (SRP) microscope offers a new access to spatially and temporally resolved Raman signatures across various sample types. Unlike stimulated Raman scattering (SRS) microscopy, SRP is highly compatible with noisy ultrafast laser sources, allowing the use of high peak power, low repetition rate optical parametric amplifier (OPA) lasers to boost sensitivity and imaging speed. Here, we report a single pulse SRP (spSRP) microscope system in which SRP signals are induced by individual pairs of laser pulses generated by an OPA laser. Extensive pulse chirping is used to maximize SRS excitation rate and to minimize photodamage. The single-pixel limit of detection (LOD) of spSRP on dimethyl sulfoxide (DMSO) reaches 890 {\mu}M, which is a ~44-fold improvement over SRS microscope. Versatile applications to fungi, cells, and tissues are demonstrated. Live cell spSRP imaging was carried out at speed of 10 frames per second. Particularly, the spSRP system enables direct visualization of cholesterol-rich domains, co-localized with caveolin immunofluorescence, in the plasma membrane of HeLa cells.
A single laser pulse heats the sample by less than a thousandth of a degree, but that's enough to see individual molecules.
In the TV series "Fringe," characters use a device that "sees" the chemical composition of objects from a distance — the new microscope does something similar, but within a drop of solution.