Optomechanical detection of single bacterium mechanical modes

Abstract

Mechanical modes of biological particles such as proteins, viruses and bacteria involve coherent structural vibrations at frequencies in the THz and GHz domains, which carry very valuable information on their mechanical properties and structure that play a pivotal role in many relevant biological processes. Here we demonstrate the detection of single bacterium mechanical modes in ambient conditions, by using ultra-high frequency optomechanical resonators. We harness a particular regime in the physics of mechanical resonator sensing, which emerge when the sensor and the analyte support mechanical modes at similar frequencies (Figure 1). We develop a general theoretical framework to describe the different regimes that can be found when an analyte adsorbs on a mechanical resonant sensor. Our model recovers the classical inertial mass sensing regime as a limit case of a more general problem, showing excellent agreement with numerical simulations and experiments, which enables retrieving the eigenfrequencies and mechanical loss of the bacterium modes from the detected collective modes. We apply our method to study the effect of hydration on the properties of a single bacterium. This work opens the door for a new class of vibrational spectrometry based on the use of high frequency mechanical resonators with the unique capability to obtain information on single bioentities [1].