Coherent Coupling of a Single Molecule to a Scanning Fabry-Perot Microcavity

Organic dye molecules have been used in a great number of scientific and technological applications, but their wider use in quantum optics has been hampered by transitions to short-lived vibrational levels, which limit their coherence properties. To remedy this, one can take advantage of optical resonators. Here, we present the first results on coherent molecule-resonator coupling, where a single polycyclic aromatic hydrocarbon molecule extinguishes 38% of the light entering a microcavity at liquid helium temperature. We also demonstrate fourfold improvement of single-molecule stimulated emission compared to free-space focusing and take first steps for coherent mechanical manipulation of the molecular transition. Our approach of coupling molecules to an open and tunable microcavity with a very low mode volume and moderately low quality factors of the order of $10^3$ paves the way for the realization of nonlinear and collective quantum optical effects.

Popular Summary

Organic dye molecules have found their way into many technological applications such as lasers, microscopy, and spectroscopy. These molecules might also be used in quantum information processing. It is widely believed that photons (particles of light) are ideal carriers of quantum information, while material entities (such as organic dye molecules) are advantageous for switching and storage. To optimize the transfer of information between the molecules (or any quantum system) and photons, they must interact efficiently. Using a device known as an optical resonator, which allows a beam of light to travel along a closed path, we have demonstrated an enhanced coupling efficiency between light and a single organic molecule.

We dope single dibenzoterrylene molecules into a thin organic crystal and place the crystal in a Fabry-Perot cavity made by nanomilling a mirror with a radius of curvature of $5\mu \mathrm{m}$. By tuning the cavity on and off the molecular resonance, we show that a single organic molecule could attenuate 38% of the light that enters the cavity. Furthermore, we exploit this efficient coupling to demonstrate controllable switching of a light beam from attenuation to amplification via stimulated emission. We also discuss the prospects of mechanical actuation of molecular emission via the microcavity.

Further improvements to our setup could lead to a 90% attenuation of light entering the cavity. Such dramatic improvements in coupling efficiency pave the way for using organic molecules as building blocks of quantum nanophotonic networks.

Figure 1

Schematics of the experimental arrangement. Slip-stick piezoelectric motors and transducers are used to control the position of the micromirror both in the lateral and axial directions. An aspheric lens with a numerical aperture $\mathrm{NA}=0.55$ is employed to couple an incoming laser beam to the microcavity. The dashed lines and elements signify a second aspheric lens and the laser beams used to access the sample from the other side in the absence of the micromirror. S, sample; PD, photodiode; BPD, balanced photodetector; APD, avalanche photodiode; BS, beam splitter; PBS, polarizing beam splitter; HWP, half-wave plate; QWP, quarter-wave plate; IF, interference filter; BD, beam dump; FM, flip mirror; CW, continuous-wave. See text for details.

Figure 2

(a) Microcavity design to scale, showing the electric field intensity distributions in a cavity mode. (b) The red curves show the measured (symbols) and calculated (solid line) transmission of the DBR at liquid helium temperature as a function of wavelength. The blue spectrum presents the emission of a molecule excited at 766 nm. The section beyond 785 nm is magnified by 5 times for clarity. (c) Relative position of AC crystal axes, laser, and detection polarizations as well as the orientation of the DBT dipole moment in the DBR plane. (d) The arrangement for cross-polarized measurements.

Figure 3

(a) The red and blue spectra show the detected Stokes-shifted fluorescence for $\lambda >785\mathrm{nm}$ when an incident laser is scanned across the inhomogeneous broadening of the molecules for the cases of cavity locked on resonance and micromirror retracted, respectively. The width of the red envelope gives a measure for the cavity FWHM of 250 GHz. The sharp resonances represent molecules coupled to the cavity. (b) A zoom of one sharp resonance as recorded in (a) with a Lorentzian fit. (c) $g^{(2)}$ measured via a coincidence Hanbury Brown–Twiss (HBT) study. (d) A lateral cross section of the cavity mode in the AC crystal mapped by measuring the fluorescence of a single molecule. The solid curve shows a Gaussian fit. (e) The red symbols show the decay time of the molecular excited state extracted from HBT measurements in a resonant cavity at low power. The blue symbols display the HBT decay time recorded in the absence of the curved mirror at different incident powers measured before the cryostat. Extrapolation of a linear fit gives the lifetime in the limit of zero excitation intensity. (f) Transmission of the resonant cavity-molecule system as a function of laser frequency detuning. The left-hand side shows the signal for the laser frequency far from the cavity resonance.

Figure 4

(a)–(d) Response of the resonant cavity-molecule system as a function of laser frequency detuning for four different cavity detunings by the amounts shown in each legend. See Supplemental Material for fitting procedure [24]. (e) The response of the system as a function of the excitation power. The dotted vertical line indicates saturation parameter $S=1$. In these measurements the finesse has dropped to about 100 due to condensation of residual gases (see Supplemental Material [24]).

Figure 5

(a)–(c) Coherent resonant response (transmission) of the cavity-molecule system as a function of the probe laser frequency. In each case, the pump rate $k_p$ is indicated in the legend. (d) Amplification as a function of pump power. (e) The green curve shows the emission of a molecule in response to a picosecond pulse in the absence of the probe laser. The black curve presents a fit, taking into account the instrumental response displayed by the dash-dotted black profile. The red and blue curves show the time evolution of the emission on the 00ZPL when the probe laser is on and away from the molecular resonance, respectively. The dash-dotted red and blue horizontal lines mark the background from the probe laser light for the on- and off-resonance cases, respectively. The shaded areas in the inset zoom correspond to the standard error of the occurrences at each delay time. (f) The net change due to stimulated emission obtained by subtracting the red and blue curves in (e) and normalizing to the intensity of the probe laser. The error bars represent the standard error of the mean at each delay time. The solid curve displays a fit according to analytical calculations. See Supplemental Material for details [24].

Figure 6

(a) 00ZPL emission of a molecule as a function of the cavity length modulation at 10 Hz. (b) Fourier transform of the signal in (a), revealing the second and third harmonics of it (left) together with the Fourier transform of the signal from a cavity modulation at 114 kHz (right). The inset shows the region of the cavity resonance that is scanned. The highest frequency achieved in this experiment is limited by the resolution of our data acquisition card.