Single Molecule as a Local Acoustic Detector for Mechanical Oscillators

A single molecule can serve as a nanometer-sized detector of acoustic strain. Such a nanomicrophone has the great advantage that it can be placed very close to acoustic signal sources and high sensitivities can be achieved. We demonstrate this scheme by monitoring the fluorescence intensity of a single dibenzoterrylene molecule in an anthracene crystal attached to an oscillating tuning fork. The characterization of the vibration amplitude and of the detection sensitivity is a first step towards detection and control of nanomechanical oscillators through optical detection and feedback.

Figure 1

(a) Sample mounting: an anthracene crystal doped with DBT was attached to the quartz crystal tuning fork. (b) The ZPL of a single DBT molecule is shifted upon deformation of its surrounding host crystal, as shown in (c) and (d). The real deformations are three-dimensional and much more complicated, as molecules can also rotate and be distorted.

Figure 2

Spectral trails of a single DBT molecule as functions of the driving frequency (a) from 19.820 to 19.850 kHz or (b) from 20.060 to 20.130 kHz. The driving voltage was 0.8 V. Lock-in signal of the reflection light intensity (c) from the tuning fork with sample attached and (d) from the bare tuning fork. The asymmetry of the main resonance around 20.091 kHz is due to the anharmonicity of the tuning fork’s oscillation. All the measurements were done in vacuum at 1.5 K.

Figure 3

(a) Fluorescence photon counts (with bin size of $10\mu \mathrm{s}$ for periods of 10 ms, 100 ms, and 1 s) from a single DBT molecule (fluorescence rate of $3300\text{counts}/\mathrm{s}$) and (b) fast Fourier transforms of such fluorescence intensity traces (1 s duration) when the tuning fork was driven at its resonant frequency (20.091 kHz), for various driving voltages (1 to 20 mV). The curves are shifted along $Y$ axis for clarity. The sharp peak at 20.091 kHz indicates the modulation due to coupling to the tuning fork. This signal becomes comparable to noise at a driving voltage of 1 mV.