Ground-State Depletion Microscopy: Detection Sensitivity of Single-Molecule Optical Absorption at Room Temperature

Abstract

Optical studies of single molecules in ambient environments, which have led to broad applications, are primarily based on fluorescence detection. Direct detection of optical absorption with single-molecule sensitivity at room temperature is difficult because absorption is not a background-free measurement and is often complicated by sample scattering. Here we report ground-state depletion microscopy for ultrasensitive detection of absorption contrast. We image 20 nm gold nanoparticles as an initial demonstration of this microscopy. We then demonstrate the detection of an absorption signal from a single chromophore molecule at room temperature. This is accomplished by using two tightly focused collinear continuous-wave laser beams at different wavelengths, both within a molecular absorption band, one of which is intensity modulated at a high frequency (>MHz). The transmission of the other beam is found to be modulated at the same frequency due to ground state depletion. The signal of single chromophore molecules scanned across the common laser foci can be detected with shot-noise limited sensitivity. This measurement represents the ultimate detection sensitivity of nonlinear optical spectroscopy at room temperature.

Exciton Dynamics in Suspended Monolayer and Few-Layer MoS2 2D Crystals

“…Such high frequency modulation is necessary for achieving shot noise limited detection. 39 Transient absorption traces were obtained by delaying the probe with respect to the pump with a mechanical translation stage (Thorlabs). The time-resolution at the sample was ca.…”

Two-dimensional fluorescence-detected coherent spectroscopy with absolute phasing by confocal imaging of a dynamic grating and 27-step phase-cycling

“…18,19 Note that despite the pioneering work by van Hulst and co-workers, [20][21][22] 2D-CS of isolated single molecules has not been demonstrated yet. Although detection of isolated (immobilized) single molecules by absorption was demonstrated both at low temperature 23 and quite recently at room temperature, [24][25][26] the sensitivity is greatly enhanced by fluorescence detection 27 which has been a standard tool for single molecule spectroscopy. 28 Phasematching, being a macroscopic phenomenon, does not apply in a small ensemble of molecules.…”

Ultrafast pump-probe force microscopy with nanoscale resolution

“…Optical detection of the pumpprobe effect is not background free: it relies on the ability to register a pump-induced intensity change in the scattered probe radiation. In the single molecule limit, despite recent successes, 13 the induced gain or loss in the probe detection channel may be well below the experimental shot noise. In this regard, detection strategies that circumvent the deleterious effects of the large optical background in nonlinear pump-probe measurements would be highly desirable.…”