Frequency modulation spectroscopy with localized fluorescence

A frequency-modulation spectroscopy system comprising:

• an optical trap generator for forming an optical trap to localize molecular entities in a subregion of a cell, the subregion having a volume that is less than 1/10 of an internal volume of the cell;
• a frequency converter for frequency converting an optical source signal to provide a first probe beam having a first probe wavelength, the frequency conversion including frequency modulating the optical source signal using a radio-frequency signal, at least the first probe beam probing the molecular entities so that the molecular entities emit fluorescence having a first fluorescence wavelength different from the first probe wavelength;
• a spectral filter for spectrally filtering the fluorescence, the spectral filter being characterized by an effective angle of incidence on the spectral filter within which the spectral filter can effectively block the first probe wavelength and transmit the first fluorescence wavelength, the filter being less effective at blocking the first probe wavelength or transmitting the first fluorescence wavelength at angles larger than the effective angle of incidence, at least half of the molecular entities that emit the fluorescence being within a conical volume defined by the effective angle of incidence;
• a fluorescence detector for detecting the spectrally filtered fluorescence transmitted by the spectral filter to provide a fluorescence detection signal;
• and
• a demodulator for demodulating the fluorescence detection signal so as to provide a frequency-modulation spectroscopy output signal.