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Room Temperature Extinction Imaging of Single Emitters: Fluorescence-Free Detection Down to the Single Molecule Level

Michele Celebrano

Published Jan 1, 2010• 0 citations
OpenAlex

Abstract

Since its first development and applications in the early 1990s, single molecule detection demonstrated to be a powerful tool for a wide range of studies, ranging from biophysics to quantum optics. However, this approach has been limited so far to species with high fluorescence quantum yield. Over the past few years, we have developed extinction detection and spectroscopy as an alternative to fluorescence for investigating single nano-objects such as metallic nanoparticles, viruses, dye molecules, and quantum dots [1-5]. The contrast mechanism at work in this coherent scheme can be understood as the interference between the incident beam and the light scattered by the nano-object. We showed that the sensitivity of this technique can be pushed to detect single semiconductor nanocrystals at room temperature and measured the absorption cross section of a single quantum dot before and after photobleaching as well as during the on and off photo-blinking times [4]. Here, I will concentrate on the recent improvements of such technique which allowed the direct detection and imaging of a single molecule by optical absorption at room temperature [6, 7]. Through a transmission microscopy setup in an index-matched configuration we were able to suppress the background scattering and shot-noise fluctuations below the part per million. Additionally, by means of three distinct approaches, the residual background fluctuations produced by the environment could be suppressed to about 1 × 10 -6 RMS. This allowed us to recover the extinction/absorption dip produced by a single molecule (Terrylene Diimide, TDI) on an impinging laser beam which can be evaluated to be about 3 × 10 -6 . Thanks to these background-subtraction methods, which allowed to extract the pure absorption signal, we could also exactly determine exactly the absorption cross section of TDI [7], which turned to be in good agreement with an evaluation made upon the ensemble cross-section. In this framework, we also demonstrated that spectroscopy at the single molecule level is within reach [7]. This simple contrast technique will allow interrogating a large class of nano-object and studying the photophysics of all the species which do not fluoresce or do so with very low quantum efficiency.

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Since its first development and applications in the early 1990s, single molecule detection demonstrated to be a powerful tool for a wide range of studies, ranging from biophysics to quantum optics. However, this approach has been limited so far to species with high fluorescence quantum yield. Over the past few years, we have developed extinction detection and spectroscopy as an alternative to fluorescence for investigating single nano-objects such as metallic nanoparticles, viruses, dye molecules, and quantum dots [1-5]. The contrast mechanism at work in this coherent scheme can be understood as the interference between the incident beam and the light scattered by the nano-object. We showed that the sensitivity of this technique can be pushed to detect single semiconductor nanocrystals at room temperature and measured the absorption cross section of a single quantum dot before and after photobleaching as well as during the on and off photo-blinking times [4]. Here, I will concentrate on the recent improvements of such technique which allowed the direct detection and imaging of a single molecule by optical absorption at room temperature [6, 7]. Through a transmission microscopy setup in an index-matched configuration we were able to suppress the background scattering and shot-noise fluctuations below the part per million. Additionally, by means of three distinct approaches, the residual background fluctuations produced by the environment could be suppressed to about 1 × 10 -6 RMS. This allowed us to recover the extinction/absorption dip produced by a single molecule (Terrylene Diimide, TDI) on an impinging laser beam which can be evaluated to be about 3 × 10 -6 . Thanks to these background-subtraction methods, which allowed to extract the pure absorption signal, we could also exactly determine exactly the absorption cross section of TDI [7], which turned to be in good agreement with an evaluation made upon the ensemble cross-section. In this framework, we also demonstrated that spectroscopy at the single molecule level is within reach [7]. This simple contrast technique will allow interrogating a large class of nano-object and studying the photophysics of all the species which do not fluoresce or do so with very low quantum efficiency.
The advent of single molecule optics has had a profound impact in fields ranging from biophysics to material science, photophysics, and quantum optics. However, all existing room-temperature single molecule methods have been based on fluorescence detection of highly efficient emitters. Here we demonstrate that standard, modulation-free measurements known from conventional absorption spectrometers can indeed detect single molecules. We report on quantitative measurements of the absorption cross section of single molecules under ambient condition even in their dark state, for example during photoblinking or strong quenching. Our work extends single-molecule microscopy and spectroscopy to a huge class of materials that absorb light but do not fluoresce efficiently.
Single semiconductor quantum dots, which are considered as two-level systems in the artificial atom model, are promising structures for the realization of integrated devices such as single photon sources for quantum information applications. However, contrary to genuine atoms, they are condensed matter systems which suffer from the coupling to their solid environment, leading to a degradation of the coherence of the emitted photons. One possibility to avoid this coupling, thereby improving the coherence of the photons until the one imposed by the radiative limit T2 = 2T1, is to perform strictly resonant excitation of a quantum dot at low temperature. Using an experimental setup that spatially decouples the excitation and detection paths, we showed that a fine control of the electrostatic environment related to the nanostructures residual doping remains a major challenge to answer the widely encountered problem of the inhibition of the resonance fluorescence of a quantum dot. Meanwhile, thanks to quantum optics experiments and high resolution optical spectroscopy, we demonstrated that the resonant Rayleigh scattering regime where quantum dots emit single photons with the laser coherence time, paves the way to the development of "ultra-coherent" sources of single photons with high degrees of indistinguishability. In this regime where the laser tailors the coherence of the photons and where the radiative limit becomes a secondary requirement, two photons emitted successively by the same quantum dot are indistinguishable on time scales never obtained before for a solid nano-emitter (up to ten nanoseconds, comparing to the carriers lifetime and decoherence time of about a hundred of picoseconds).
In this paper direct measurements of light extinction by single molecules are reported. We use both near-field aperture tips and confocal microscopy at cryogenic temperature to tightly confine the excitation light to an area comparable to the absorption cross section of a single molecule, which results in directly observable 10% extinction of the laser beam. It is concluded that the access to both the incoherent and coherent emission of single molecules opens up a broad range of studies on the light-matter interaction of single solid state emitters.
Abstract In recent years different microscopic and spectroscopic techniques were successfully employed to detect the fluorescence signals of single dye molecules in the condensed phase. In low temperature solids the fluorescence excitation line of a single dopant molecule is a high resolution optical probe of its local nanoenvironment. As inhomogeneous contributions and ensemble averaging are absent, many effects hidden when looking at large populations of molecules become observable. Two important research areas in single molecule spectroscopy deal with the investigation of quantum optical effects and the spectral jumping behaviour of single molecule excitation lines caused by fluctuations of two‐level systems inherent to disordered solids. Rather than delivering a complete presentation, we will focus on some selected examples which emphasize the unique features of spectroscopy with a single absorber.
The combination of single particle detection and ultrafast laser pulses is an instrumental method to track dynamics at the femtosecond time scale in single molecules, quantum dots and plasmonic nanoparticles. Optimal control of the extremely short-lived coherences of these individual systems has so far remained elusive, yet its successful implementation would enable arbitrary external manipulation of otherwise inaccessible nanoscale dynamics. In ensemble measurements, such control is often achieved by resorting to a closed-loop optimization strategy, where the spectral phase of a broadband laser field is iteratively optimized. This scheme needs long measurement times and strong signals to converge to the optimal solution. This requirement is in conflict with the nature of single emitters whose signals are weak and unstable. Here we demonstrate an effective closed-loop optimization strategy capable of addressing single quantum dots at room temperature, using as feedback observable the two-photon photoluminescence induced by a phase-controlled broadband femtosecond laser. Crucial to the optimization loop is the use of a deterministic and robust-against-noise search algorithm converging to the theoretically predicted solution in a reduced amount of steps, even when operating at the few-photon level. Full optimization of the single dot luminescence is obtained within ~100 trials, with a typical integration time of 100 ms per trial. These times are faster than the typical photobleaching times in single molecules at room temperature. Our results show the suitability of the novel approach to perform closed-loop optimizations on single molecules, thus extending the available experimental toolbox to the active control of nanoscale coherences. Femtosecond laser control allows to harness short-lived coherences and study molecular ultrafast dynamics. Translating such coherent-control schemes from the ensemble to single molecules is challenging due to signal fluctuation and photo-dissociation. Now, Nicolò Accanto and Pablo de Roque from ICFO - the Institute of Photonic Sciences in Barcelona, and colleagues, have reduced the time needed to perform closed-loop optimisations. By employing a new simple deterministic algorithm, and assuming simple optimisation landscapes, they accomplished a phase-only optimisation of the two-photon photoluminescence of a single quantum dot. Combining this rapid feedback process within the search algorithm with a sensitive microscopy technique, they were able to optimise on signals even at the few-photon level, enabling the coherent control of individual quantum-dots at room temperature in a matter of seconds - a speedup critical for capturing details of biomolecular interactions, for instance.
Simultaneous detection of single molecules by absorption and fluorescence is demonstrated using confocal microscopy at cryogenic temperature. Dynamical processes such as blinking and spectral jumping of single emitters are observed in both detection channels. The relative magnitude of fluorescence and absorption varies between molecules. In particular, we observe molecules that do not emit detectable Stokes-shifted fluorescence but show a strong absorption signal. The fact that coherent resonant scattering underlies the absorption process is demonstrated by a correlation between small linewidth and large absorption amplitude.