Absolute pressure and gas species identification with an optically levitated rotor

Abstract

We describe a novel variety of spinning-rotor vacuum gauge in which the rotor is a ∼4.7-µmdiameter silica microsphere, optically levitated. A rotating electrostatic field is used to apply torque to the permanent electric dipole moment of the silica microsphere and control its rotational degrees of freedom. When released from a driving field, the microsphere's angular velocity decays exponentially with a damping time inversely proportional to the residual gas pressure, and dependent on gas composition. The gauge is calibrated by measuring the rotor mass with electrostatic co-levitation, and assuming a spherical shape, confirmed separately, and uniform density. The gauge is crosschecked against a capacitance manometer by observing the torsional drag due to a number of different gas species. The techniques presented can be used to perform absolute vacuum measurements localized in space, owing to the small dimensions of the microsphere, as well as measurements in magnetic field environments. In addition, the dynamics of the microsphere, paired with a calibrated vacuum gauge, can be used to measure the effective molecular mass of a gas mixture without the need for ionization, and at pressures up to approximately 1 mbar.

Initiating revolutions for optical manipulation: the origins and applications of rotational dynamics of trapped particles

“…For example, Arita et al extracted the viscosity of picolitre volumes of air, CO 2 and Ar using optically levitated rotating vaterite microspheres [144]. These sensors can also be used as non-ionising residual gas analysers to measure the composition of a gaseous environment: the drag depends on the momentum exchange between the trapped particle and the gas molecules, and therefore the molecular mass of the gas [144,211].…”

“…Optically levitated spinning nano-and micro-particles can be used as miniaturised spinning-rotor vacuum gauges. Blakemore et al spin 5 μm-diameter silica microspheres using electrostatic forces, which can be switched off to remove the driving torque [211]. After the switch-off, the rotation rate decays exponentially with a time constant I=γ rot and is therefore inversely proportional to the pressure in the vicinity of the microsphere.…”

“…A faster method to extract pressure can be accomplished with optically levitated particles undergoing a driven rotation, where there is direct access to the drive signal. Such a scenario can be accomplished by fast-switching between linear and circular polarisation such that the particle's motion is frequency-locked to the drive [213], or by driving the rotation through interactions between the particle electric dipole moment and a rotating electrostatic field [211,214]. Access to the drive signal allows the measurement of the phase lag between drive and particle rotation.…”

“…While this linear relationship also holds for translational motion, at low pressures a particle's translational motion is only stable with the addition of feedback cooling [38], which mimics the damping provided by residual gas and therefore complicates the relationship between pressure and damping rate. To first order, the rotational motion is not affected by feedback cooling [211], making rotational motion a more convenient platform for pressure measurements with levitated particles in lower pressure regimes.…”

Optically levitated rotor at its thermal limit of frequency stability

“…Besides their promise to allow for the investigation of fundamental effects [30][31][32], freely rotating nanoparticles in optical traps have recently attracted considerable attention as the fastest rotating manmade objects [24,25,29] and have been identified as potential candidates for pressure [23,33], acceleration, and various torque-sensing schemes [29,[34][35][36][37]. On the one hand, phaselocked driven rotors have been considered for torque sensing [23,33], but the sensitivity of this scheme remains largely unexplored. On the other hand, the current state-of-the-art levitated torque-sensing technique detects changes in rotation frequency, such that its sensitivity depends on the stability of that frequency [29].…”