A robust single-beam optical trap for a gram-scale mechanical oscillator

Abstract

Precise optical control of microscopic particles has been mastered over the past three decades, with atoms, molecules and nano-particles now routinely trapped and cooled with extraordinary precision, enabling rapid progress in the study of quantum phenomena. Achieving the same level of control over macroscopic objects is expected to bring further advances in precision measurement, quantum information processing and fundamental tests of quantum mechanics. However, cavity optomechanical systems dominated by radiation pressure – so-called ‘optical springs’ – are inherently unstable due to the delayed dynamical response of the cavity. Here we demonstrate a fully stable, single-beam optical trap for a gram-scale mechanical oscillator. The interaction of radiation pressure with thermo-optic feedback generates damping that exceeds the mechanical loss by four orders of magnitude. The stability of the resultant spring is robust to changes in laser power and detuning, and allows purely passive self-locking of the cavity. Our results open up a new way of trapping and cooling macroscopic objects for optomechanical experiments.

Observation of nonlinear dynamics in an optical levitation system

“…We attribute this modification to the optical spring effect, which causes a reduction of the effective frequency in the red-detuned regime when the parametric amplification begins, and an increase in the blue-detuned regime when the cavity trails out of resonance [60]. We note that photothermal effects can also contribute to the optical modification of the natural mechanical frequency [35]. The pure optical spring is produced by the back-action of the intracavity optical field via the passive feedback loop L 1 between the cavity field and the acoustic mode.…”

“…These oscillations are linked to the excitation of the acoustic modes of the levitation mirror as they get parametrically amplified by the photothermal effect. Similarly to the radiation-pressure induced optical spring effect, the positive photothermal stiffness experienced during self-locking by the system is paralleled by a negative damping coefficient [34,35]. The amount by which the natural damping of the acoustic mode is modified by the photothermal interaction can be estimated by the eigenvalues of the Jacobian matrix [30,[55][56][57][58].…”

“…Reducing absorption beyond a certain level, however, may be technically challenging. Alternatively, one may attempt to switch the sign of photothermal interaction so that it collaborates with radiation pressure towards both static and dynamic stability [35,61]. In our system, the photothermal coefficient is β = 8.6 pm W −1 , which is positive.…”

“…The fluctuation of the cavity field is subsequently propagated to other degrees of freedom via optical back action. In particular, we show that the absorbed energy results in effective anti-damping and parametric amplification [33][34][35]. Finally, the mirror also interacts with the intracavity field via the radiation pressure force, inducing a displacement when the optical push is stronger than the gravitational force.…”

Observation of an optical spring with a beam splitter

“…An alternative method to stabilizing the optical spring is to modify the damping force by adding a second optical spring [10,11] or utilizing thermo-optic effects [12,13].…”