Coherent Interaction of Light and Single Molecules in a Dielectric Nanoguide

Many of the currently pursued experiments in quantum optics would greatly benefit from a strong interaction between light and matter. Here, we present a simple new scheme for the efficient coupling of single molecules and photons. A glass capillary with a diameter of 600 nm filled with an organic crystal tightly guides the excitation light and provides a maximum spontaneous emission coupling factor ($\beta$) of 18% for the dye molecules doped in the organic crystal. A combination of extinction, fluorescence excitation, and resonance fluorescence spectroscopy with microscopy provides high-resolution spatiospectral access to a very large number of single molecules in a linear geometry. We discuss strategies for exploring a range of quantum-optical phenomena, including polaritonic interactions in a mesoscopic ensemble of molecules mediated by a single mode of propagating photons.

Figure 1

(a) Schematics of a filled capillary butt coupled to a single-mode optical fiber. (b) A microscopic image of an empty capillary next to a grain of table salt. (c) A simplified Jablonski diagram of a DBT molecule. (d) Sketch of the experimental arrangement. A cw Ti:sapphire laser is coupled to a single-mode fiber that enters a helium bath cryostat, ending with the nanocapillary. An aspheric lens provides optical access to the sample from a side window. Synchronized cameras C1 and C2 and avalanche photodiodes APD1 and APD2 are used for detection. PD: photodiode; SM: single-mode fiber; DBS: dichromatic beam splitter; PM: fiber polarization management; FC: fiber coupler; FL: fiber leakage for intensity stabilization.

Figure 2

(a) Stokes-shifted fluorescence spectrum recorded via the angled end of the nanoguide. (b) Extinction spectrum recorded directly in transmission, i.e., without using lock-in detection. This spectrum was recorded some tens of minutes after (a). While the great majority of the resonances are reproduced, in a few cases one notices slight frequency jumps. (c) A zoom into an extinction dip of 6.5% recorded from a single molecule. The excitation wavelength for the data in this figure was around 771 nm. The right vertical axis shows the intensity of the excitation light in units of megacounts per second. (d) Second-order autocorrelation function of the photons emitted on the resonance of (c). Here, we detected the Stokes-shifted fluorescence from the side. The recorded value of $g^{(2)}(0)=0.01$ was limited by the APD dark counts.

Figure 3

(a) Nanoguide-emitter coupling efficiency $\beta$ (black solid curve, left axis) and the total decay rate (blue dashed curve, right axis) as a function of the emitter position inside a nanocapillary core of 600 nm. (b) $\beta$ as a function of the core radius (black solid curve, left axis). The blue dashed trace plots the effective mode area (right axis). The dot indicates the optimal core radius $r=300\mathrm{nm}$, which was used in the experiment. The nanocapillary core was assumed to have a refractive index of $n_1=1.59$ surrounded by a large cladding with $n_2=1.45$. The shaded region in (b) marks the regime where the waveguide becomes multimode.

Figure 4

(a) A resonance fluorescence spectrum of a single DBT molecule recorded from the side. (b) The Stokes-shifted fluorescence spectrum of the same molecule. (c) A section of the resonance fluorescence excitation spatiospectral map of the sample. (d) The same for the Stokes-shifted signal recorded simultaneously. The data in this figure were recorded at an excitation wavelength of around 757 nm. The linear color scales represent the signal strength in arbitrary units.

Figure 5

(a) Stokes-shifted excitation spectrum recorded in transmission. The inhomogeneous distribution of DBT lines in our system is considerably larger than that observed in sublimated naphthalene crystals. Furthermore, the current site in the region of 770 nm has not been reported previously. (b) Schematics of micro- or nanoelectrodes for controlling the resonance frequencies of the molecules along the nanoguide. (c) Arrangement for generating several indistinguishable photons which would be detected either from the side port or the end. Here, one would pump the medium via the $v=1$ vibrational level of the electronic excited state. (d) This graphic illustrates how a right-angle geometry detection allows resonant excitation and detection of photons on the 00ZPL.