  In a canonical optomechanical system, mechanical vibrations are dynamically
encoded on an optical probe field which reciprocally exerts a backaction force.
Due to the weak single photon coupling strength achieved with macroscopic
oscillators, most of existing experiments were conducted with large photon
numbers to achieve sizeable effects, thereby causing a dilution of the original
optomechanical non-linearity. Here, we investigate the optomechanical
interaction of an ultrasensitive suspended nanowire inserted in a fiber-based
microcavity mode. This implementation allows to enter far into the hitherto
unexplored ultrastrong optomechanical coupling regime, where one single
intracavity photon can displace the oscillator by more than its zero point
fluctuations. To fully characterize our system, we implement nanowire-based
scanning probe measurements to map the vectorial optomechanical coupling
strength, but also to reveal the intracavity optomechanical force field
experienced by the nanowire. This work establishes that the single photon
cavity optomechanics regime is within experimental reach.
