Fluorescence-detected Fourier transform electronic spectroscopy by phase-tagged photon counting

TL;DR

This paper introduces phase-tagged photon counting (PTPC), a novel fluorescence-detected Fourier transform spectroscopy technique that improves signal detection in low-flux conditions compared to traditional lock-in methods.

Contribution

The paper presents PTPC as a new approach for low-signal FT spectroscopy, demonstrating its advantages over lock-in detection in weak fluorescence measurements.

Findings

PTPC outperforms lock-in detection in low-flux scenarios.

Measurement uncertainty is mainly due to photon count statistics.

PTPC extends the range of measurable fluorescence signals.

Abstract

Fluorescence-detected Fourier transform (FT) spectroscopy is a technique in which the relative paths of an optical interferometer are controlled to excite a material sample, and the ensuing fluorescence is detected as a function of the interferometer path delay and relative phase. A common approach to enhance the signal-to-noise ratio in these experiments is to apply a continuous phase sweep to the relative optical path, and to detect the resulting modulated fluorescence using a phase-sensitive lock-in amplifier. In many important situations, the fluorescence signal is too weak to be measured using a lock-in amplifier, so that photon counting techniques are preferred. Here we introduce an approach to low-signal fluorescence-detected FT spectroscopy, in which individual photon counts are assigned to a modulated interferometer phase ('phase-tagged photon counting,' or PTPC), and the resulting data are processed to construct optical spectra. We studied the fluorescence signals of a molecular sample excited resonantly by a pulsed coherent laser over a range of photon flux and visibility levels. We compare the performance of PTPC to standard lock-in detection methods and establish the range of signal parameters over which meaningful measurements can be carried out. We find that PTPC generally outperforms the lock-in detection method, with the dominant source of measurement uncertainty being associated with the statistics of the finite number of samples of the photon detection rate.