Abstract
The interaction of a cavity with an external field is symmetric under time reversal. Thus, coupling to a resonator is most efficient when the incident light is the time reversed version of a free cavity decay, i.e. when it has a rising exponential shape matching the cavity lifetime. For light entering the cavity from only one side, the maximally achievable coupling efficiency is limited by the choice of the cavity mirrors' reflectivities. Such an empty-cavity experiment serves also as a model system for single-photon single-atom absorption dynamics. We present experiments coupling exponentially rising pulses to a cavity system which allows for high coupling efficiencies. The influence of the time constant of the rising exponential is investigated as well as the effect of a finite pulse duration. We demonstrate coupling 94% of the incident TEM00 mode into the resonator.
Highlights
The desired pulses are cut out of the continuous-wave laser beam by means of an acousto-optic modulator (AOM) working at a centre frequency of 400 MHz. It is driven by an arbitrary-waveform generator (AWG) providing the radiofrequency signal with a suitably shaped envelope according to the desired pulse shape
If plotting only the light in the TEM00 mode, hardly any rising of the photodiode signal can be observed until t = 0, indicating that a huge amount of the energy is stored inside the cavity and only emitted after the end of the incident pulse
Experiments with exponentially rising incident pulses demonstrate the power of time-reversal arguments in optics: by choosing a time constant matching the cavity lifetime, we achieved an energy coupling efficiency of EC,max = 94%
Summary
In order to tune the cavity length, the end mirror is mounted on a ring-shaped piezo actuator This cavity set-up corresponds to an atomic experiment where the incident light is focused from 97% of the full solid angle. For an asymmetric cavity the leakage through the end mirror is very weak, resulting in a signal to noise ratio smaller than one when measuring with sufficient detection bandwidth for the coupling experiment. PD2 in turn is used in 1 M coupling to determine the spatial mode matching of the incident beam to the cavity. For this purpose, the cavity length is scanned and transmission spectra of a continuous wave beam are taken. In the measurements presented here, the fraction η of the incident light found in the TEM00 mode lies between 94 and 96.5%
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