Interferometric Signatures of Single Molecules

Abstract

We built an interferometer where one of the two slits of a classical Young's setup is replaced by a single molecule embedded in a solid matrix. This enabled direct measurement of the first order coherence of the 0-0 single-molecule emission, which at high excitation powers proves to be split in coherent and incoherent parts. We demonstrate an order of magnitude higher precision in axial localization of single molecules in comparison with that of confocal microscopy. These experiments open a possibility for single-molecule holography. Detection of single molecules with low luminescence quantum yields could be another application of this technique. DOI: 10.1103/PhysRevLett.87.183602 PACS numbers: 42.50.Ar, 32.50. +d, 42.25.Hz, 42.40.Kw Since the pioneering experiments of Young [1], interference remains one of the most spectacular phenomena in optics and a very informative approach. However, although characterization and manipulation of individual molecules has developed into a powerful tool [2][3][4][5][6][7], relevant optical methods on a single-molecule (SM) level are practically limited to the detection of Stokes shifted luminescence. Conventionally, a SM is excited by a laser but to protect a photodetector from the laser light, a cutoff filter is used which also blocks the unshifted SM emission. A general belief that SM absorption [8,9] provides a smaller signal-to-noise ratio (SNR) and should be based on complex modulation techniques to be competitive has created a psychological barrier for all methods where a cutoff filter does not block off the laser radiation. Our Letter shows that existing beliefs are limited and that SM interferometry can give an excellent SNR. This opens new options for SM applications and gives us a new tool for fundamental studies.The experimental setup is shown in Fig. 1 and is explained in the figure caption. A key element of the setup is the aperture A whose diameter is smaller than the diffraction limited spot size into which the lens L 2 focuses SM emission. Such an aperture blocks a part of the reference beam not overlapping the SM emission and removes phase inhomogeneities in the two beams caused by optical imperfections. The overlapping parts of the reference and signal beams result in interference. This becomes evident when the count rate of the photomultiplier (PM) is recorded as a function of the laser frequency which is scanned across a SM resonance. Three examples of interferograms are shown in Fig. 2. As demonstrated in Fig. 3, the interference patterns become smaller in the relative amplitude and broader in frequency at higher excitation laser powers.Only emission with a frequency equal to the frequency of the laser light contributes to the interferograms, whereas SM emission falling on the aperture has a complex spectrum which includes a resonance 0-0 line and a set of Stokes shifted vibronic lines. All lines are accompanied also by Stokes shifted phonon wings [10]. The intensity of the 0-0 emission under resonance laser excitation is given by a count rate R 0 of t...

Highly sensitive detection of nanoparticles with a self-referenced and self-heterodyned whispering-gallery Raman microlaser

“…In the past decade we have witnessed a boost in the number of label-free detection techniques with varying levels of sensitivities. Techniques relying on electrical conductance (5), light scattering and interferometry (6)(7)(8), surface and localized plasmon resonance (9, 10), nanomechanical resonators (11,12), and optical resonances (13)(14)(15)(16)(17) have been demonstrated.…”

Seeing small

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“…The emission coming directly from ZPL is discarded because it originates at the same frequency as the (typically overwhelming) strong background of the excitation laser light. Interference experiments [18,20], besides prospectives to learn about fundamental features of quantum optics, provide a new concrete approach to the study ZPLs, especially if the frequency-shifted emission is absent or of a too low intensity for a reliable detection.…”