Cavity-enhanced Detection Research Articles

Quantum control of a nanoparticle optically levitated in cryogenic free space.

Tests of quantum mechanics on a macroscopic scale require extreme control over mechanical motion and its decoherence1-3. Quantum control of mechanical motion has been achieved by engineering the radiation-pressure coupling between a micromechanical oscillator and the electromagnetic field in a resonator4-7. Furthermore, measurement-based feedback control relying on cavity-enhanced detection schemes has been used to cool micromechanical oscillators to their quantum ground states8. In contrast to mechanically tethered systems, optically levitated nanoparticles are particularly promising candidates for matter-wave experiments with massive objects9,10, since their trapping potential is fully controllable. Here we optically levitate a femtogram (10-15grams) dielectric particle in cryogenic free space, which suppresses thermal effects sufficiently to make the measurement backaction the dominant decoherence mechanism. With an efficient quantum measurement, we exert quantum control over the dynamics of the particle. We cool its centre-of-mass motion by measurement-based feedback to an average occupancy of 0.65 motional quanta, corresponding to a state purity of 0.43. The absence of an optical resonator and its bandwidth limitations holds promise to transfer the full quantum control available for electromagnetic fields to a mechanical system. Together with the fact that the optical trapping potential is highly controllable, our experimental platform offers a route to investigating quantum mechanics at macroscopic scales11.

Cavity-enhanced detection of transient absorption signals

We present a simple, high-duty-cycle, cavity-enhanced optical absorption measurement technique based on delay-limited Pound-Drever-Hall (PDH) sideband locking. The chosen circuit naturally provides realtime readout of the amplitude quadrature of the PDH error signal, which can be mapped onto the cavity’s internal loss rate while using the phase quadrature to lock sideband frequency to the cavity mode. Our proofof-concept device comprises a 5-cm-long Fabry-Perot cavity with a 450 kHz bandwidth (finesse 6800, 350 ns power ringdown), and a feedback bandwidth of several MHz, limited primarily by the group delay of our electronics. This technique could readily be applied to other optical resonators such as fiber cavities, with potential applications in radiation dosimetry.

Optomechanical Dark State in a Dual-Species Bose—Einstein Condensate

We study cavity optomechanics of ultracold dual-species atomic mixtures with nonlinear collisions. Interspecies interactions provide a direct parametric coupling of fictitious mechanical elements which, through interfering with the intracavity optical field, leads to a switchable optically-dark state for either species. This demonstrates a matter-wave analog of recently observed mechanical wave mixing and quantum motional-state swapping, with applications in the construction of integrated phononic devices, and the cavity-enhanced detection of quantum degenerate atomic mixtures.

Microwave cavity-enhanced transduction for plug and play nanomechanics at room temperature.

Following recent insights into energy storage and loss mechanisms in nanoelectromechanical systems (NEMS), nanomechanical resonators with increasingly high quality factors are possible. Consequently, efficient, non-dissipative transduction schemes are required to avoid the dominating influence of coupling losses. Here we present an integrated NEMS transducer based on a microwave cavity dielectrically coupled to an array of doubly clamped pre-stressed silicon nitride beam resonators. This cavity-enhanced detection scheme allows resolving of the resonators' Brownian motion at room temperature while preserving their high mechanical quality factor of 290,000 at 6.6 MHz. Furthermore, our approach constitutes an 'opto'-mechanical system in which backaction effects of the microwave field are employed to alter the effective damping of the resonators. In particular, cavity-pumped self-oscillation yields a linewidth of only 5 Hz. Thereby, an adjustement-free, all-integrated and self-driven nanoelectromechanical resonator array interfaced by just two microwave connectors is realised, which is potentially useful for applications in sensing and signal processing.

Cavity-enhanced detection of magnetic order in lattice spin models

We develop a general scheme for detecting spin correlations inside a two-component lattice gas of bosonic atoms, stimulated by the recent theoretical and experimental advances on analogous systems for a single component quantum gas. Within a linearized theory for the transmission spectra of the cavity mode field, different magnetic phases of a two-component (spin $1∕2$) lattice bosons become clearly distinguishable. In the Mott-insulating (MI) state with unit filling for the two-component lattice bosons, three different phases: antiferromagnetic, ferromagnetic, and the $XY$ phases are found to be associated with drastically different cavity photon numbers. Our suggested study can be straightforwardly implemented with current cold atom experiments.

Theory of Resonant Cavity-Enhanced Detection Applied to Thermal Infrared Light

The performances of thermal infrared light detector based on a model system of resonant semiconductor microcavities are theoretically investigated. An original transfer matrix formalism of cavity enhanced absorption is presented which makes use of the small thickness of the absorbing layer compared to the light wavelengths. This formalism yields exact expressions which take standing wave effects into account in a built-in way. Approximations lead to tractable expressions which allow deriving asymptotic behaviors and general trends. The tradeoff between large cavity absorption enhancement and reduction of the detector bandwidth is particularly studied, leading to a gain-bandwidth product analysis. Approximated expressions for detectors based on resonant (i.e type I quantum dots) and nonresonant (bulk or type II quantum wells) optical transitions are also derived, which are physically meaningful and may be conveniently used for engineering purposes. It is found that the limitations due to the gain-bandwidth product conservation can be overcome. However, these cavity enhancement effects are only important for very small quantum efficiency for which the finesse of the microcavity is not seriously deteriorated.

Cavity-enhanced laser absorption spectroscopy using microresonator whispering-gallery modes

Tunable diode laser absorption spectroscopy using microresonator whispering-gallery modes (WGMs) is demonstrated. WGMs are excited around the circumference of a cylindrical cavity 125 mum in diameter using an adiabatically tapered fiber. The microresonator is very conveniently tuned by stretching, enabling the locking of an individual WGM to the laser. As the laser is scanned in frequency over an atmospheric trace-gas absorption line, changes in the fiber throughput are recorded. The experimental results of cavity-enhanced detection using such a microresonator are centimeter effective absorption pathlengths in a volume of only a few hundred microns cubed. The measured effective absorption pathlengths are in good agreement with theory.

Cavity Ring-Down and Cavity-Enhanced Detection Techniques for the Measurement of Aerosol Extinction

An instrument employing cavity ring-down (CRD) and cavity-enhanced detection (CED) for the local measurement of aerosol extinction is described and demonstrated. CRD measures the lifetime of photons in a high-quality optical cavity and thereby determines the sum of sample extinction between the cavity mirrors and that due to mirror losses. CRD systems can be calibrated with a single gas for the determination of extinction. A green laser emitting subnanosecond pulses is used as a light source, facilitating measurements free from optical interference in the cavity. The addition of a low-frequency chopper allows for the determination of extinction coefficients with simple linear fitting procedures and also facilitates CED measurements by providing laser power modulation for phase-sensitive detection. CED measures the average power transmitted by the optical cavity. After calibration with two gases, CED allows for the independent measurement of extinction with very high dynamic range and for an independent co...

Cavity-Enhanced Detection of Molecular Absorption under the Scheme of Wavelength Modulation Spectroscopy

We propose a new scheme of trace gas detection by the use of a Fabry–Perot cavity (FPC) and the wavelength modulation technique. Optical propagation length is enhanced using a 25 cm FPC with a finesse of ∼100. By applying the same modulation to both the laser and cavity frequency, we observe the first derivative signal of acetylene absorption at 1.5 µm. It is found that a sample-and-hold circuit serves well for restoring the signal level, which otherwise is degraded because of the pulsation of the detected laser intensity due to a feedback loop required for scanning the laser and cavity frequency simultaneously. Compared with the signal intensity observed with a 20 cm Brewster cell, this experiment yields the improvement of signal intensity by a factor of 81.2 with an acetylene pressure of 2.6 Pa.

Sensitive detection of laser-ablation plumes in an optical build-up cavity

Cavity-enhanced detection is used to monitor the dynamics of minute vapor plumes produced by focusing a pulsed laser beam onto a surface. The time evolution of the various processes, namely vaporization, acoustic wave generation and photothermal effect range from nsec to msec.

Cavity-enhanced detection of surface photovaporization

Cavity-enhanced detection is used to monitor minute vapor plumes produced by focusing a pulsed laser beam onto a surface placed inside a resonant optical cavity. The photovaporization signals from a variety of different materials are examined, with emphasis being placed on their amplitude and temporal structure.

Cavity-enhanced detection of surface ablation

A resonant optical cavity was used to amplify the signals from minute laser ablation plumes. Such photovaporization signals were investigated for more than 25 different materials. Three features studied in detail were amplitude, temporal shape of the signals, and the velocity of the ejected particles. The most interesting signals were obtained from metals and opaque films. The diameter of the focal spot of the pulsed dye laser was measured to be 20 μm using a Ronchi grating. The fixed focal spot was compared to the surface damage spots which, depending on the material, were quite different in size.