Normal-Mode Splitting in a Weakly Coupled Optomechanical System

Abstract

Normal-mode splitting is the most evident signature of strong coupling between two interacting subsystems. It occurs when two subsystems exchange energy between themselves faster than they dissipate it to the environment. Here we experimentally show that a weakly coupled optomechanical system at room temperature can manifest normal-mode splitting when the pump field fluctuations are anti-squashed by a phase-sensitive feedback loop operating close to its instability threshold. Under these conditions the optical cavity exhibits an effectively reduced decay rate, so that the system is effectively promoted to the strong coupling regime.Keywords: cavity optomechanics, active feedback, squashed states, strong coupling regime, normal mode spltting Normal-mode splitting is the hallmark of strongly coupled systems. In this regime two interacting systems exchange excitations faster than they are dissipated, and form collective normal modes the hybridized excitations of which are superpositions of the constituent systems' excitations [1,2]. This regime is necessary for the observation of coherent quantum dynamics of the interacting systems and is a central achievement in research aimed at the control and manipulation of quantum systems [3]. In cavity opto/electro-mechanics, where electromagnetic fields and mechanical resonators interact via radiation pressure, normal-mode splitting and strong coupling have already been obtained, using sufficiently strong power of the input driving electromagnetic field [4], or working at cryogenic temperatures with relatively large singlephoton coupling [5,6].In this letter we report on the oxymoron of observing normal-mode splitting in a weakly coupled system. Specifically, we have designed and implemented a feedback system [7,8] which permits the formation of hybridized normal modes also at room temperature and in a relatively modest device, in terms of single-photon optomechanical interaction strength (as compared to the devices used in Refs. [4][5][6]). Our system is basically weakly coupled at the driving power that we can use (limited by the onset of optomechanical bistability at stronger power), and the emergence of hybridized optomechanical modes is observed when the light amplitude at the cavity output is detected and used to modulate the amplitude of the input field driving the cavity itself. The feedback works in the anti-squashing regime, close to the feedback instability, where light fluctuations are enhanced over a narrow frequency range around the cavity resonance. In this regime the system behaves effectively as an equivalent optomechanical system with reduced cavity linewidth. This allows coherent energy oscillations between light and vibrational degrees of freedom when, for example, a coherent light pulse is injected into the cavity mode, similar to what has been discussed in Ref. [6].Light (anti-) squashing [9][10][11] refers to an in-loop (enhancement) reduction of light fluctuations within a (positive) negative feedback loop. Even if the sub-shot noise features of in-l...

An optomechanical heat engine with feedback-controlled in-loop light

“…In this case the engine described in [7,8] has low efficiency. Here we utilize feedback to effectively reduce the cavity linewidth and reach the regime of strong coupling [16] and significantly enhance the performance of the engine. Figure 5(a) shows that the lowest polariton B plays the main role in the dynamics of the system, and we can ignore the dynamics of the polariton A as the variation of its excitations is relatively small during each stroke of the Otto cycle.…”

“…It is therefore important to suggest strategies for the realization of a working optomechanical quantum engine. Here we have shown that the experimental realization of the polariton-based quantum heat engine proposed in [7][8][9] can be significantly eased by means of a feedback system [16][17][18][19] which allows to control the decay rate of the optical cavity. This engine exploits the lower polariton mode as working fluid and works between the hot phononic thermal reservoir and the cold photonic reservoir with which the polariton comes into contact as the cavity pump detuning is varied around the red mechanical sideband frequency.…”

“…This regime is in general not easily achievable and in some cases is inhibited by detrimental nonlinear processes, such as optical bistability or thermorefractive effects, which hamper the ability to carefully control the coupled dynamics of the systems. It has been shown [16] that feedback-controlled light [16][17][18][19] can be employed to significantly ease the onset of strong coupling in an optomechanical system. This suggests [20] that the feedback analysed in [19] can be used to enhance the efficiency of the quantum heat engine proposed in [7].…”

Mimicking a hybrid-optomechanical system using an intrinsic quadratic coupling in conventional optomechanical system

“…This can be deduced by applying Routh-Hurwitz criterion (30 ) using A given in Eqn. (6) yielding us the following conditions in terms of system parameters:…”

Exciting a Bound State in the Continuum through Multiphoton Scattering Plus Delayed Quantum Feedback

“…A perfectly sinusoidal photon wavefunction and stationary excitation of the emitters represents a clear signature of singlephoton trapping. This provides a solvable example of non-Markovian quantum dynamics in a nonlinear system, a scenario of interest in many areas of contemporary physics [27][28][29][30][31][32][33].…”