Fig. 6: Optical spring shift in picocavities.

a Schematic of nanolens on NPoM (SPARK construct). b Generation of a picocavity when Au atom moves onto facet, enhancing field at a single BPT molecule. c Sawtooth modulation of 633 nm CW laser power from 60 to 300 μW at 50 Hz. d, e Fast spectral scans (0.5 ms integration time) of Stokes emission from the SPARK nanocavity (d), and after formation of a picocavity (e). f-h, Extracted fits to 1586 cm⁻¹ line in nanocavity (grey) and 1501 cm⁻¹ picocavity line (red). Peak area (f) is linear in laser power, while optical spring effect in the picocavity leads to a repeatable shift in position (g) and broadening (h) of the vibrational line. i, j Spectra of vibrational lines investigated in f-h averaged over 4 periods of laser modulation. Nanocavity line (i) shows constant width and position while picocavity line (j) shifts and broadens (colour gives laser power). k Optical spring shift \({\varDelta }_{{os}}^{1}\) dependence on laser intensity for several vibrational lines in nano- and picocavities. Each picocavity vibration experiences a different optical spring magnitude.