Measurement of quantum back action in the audio band at room temperature.

Quantum mechanics places a fundamental limit on the precision of continuous measurements. The Heisenberg uncertainty principle dictates that as the precision of a measurement of an observable (for example, position) increases, back action creates increased uncertainty in the conjugate variable (for example, momentum). In interferometric gravitational-wave detectors, higher laser powers reduce the position uncertainty created by shot noise (the photon-counting error caused by the quantum nature of the laser) but necessarily do so at the expense of back action in the form of quantum radiation pressure noise (QRPN)1. Once at design sensitivity, the gravitational-wave detectors Advanced LIGO2, VIRGO3 and KAGRA4 will be limited by QRPN at frequencies between 10 hertz and 100 hertz. There exist several proposals to improve the sensitivity of gravitational-wave detectors by mitigating QRPN5-10, but until now no platform has allowed for experimental tests of these ideas. Here we present a broadband measurement of QRPN at room temperature at frequencies relevant to gravitational-wave detectors. The noise spectrum obtained shows effects due to QRPN between about 2 kilohertz and 100 kilohertz, and the measured magnitude of QRPN agrees with our model. We now have a testbed for studying techniques with which to mitigate quantum back action, such as variational readout and squeezed light injection7, with the aim of improving the sensitivity of future gravitational-wave detectors.

Passive laser power stabilization via an optical spring.

Metrology experiments can be limited by the noise produced by the laser involved via small fluctuations in the laser's power or frequency. Typically, active power stabilization schemes consisting of an in-loop sensor and a feedback control loop are employed. Those schemes are fundamentally limited by shot noise coupling at the in-loop sensor. In this Letter, we propose to use the optical spring effect to passively stabilize the classical power fluctuations of a laser beam. In a proof of principle experiment, we show that the relative power noise of the laser is stabilized from approximately 2 × 10-5 Hz-1/2 to a minimum value of 1.6 × 10-7 Hz-1/2, corresponding to the power noise reduction by a factor of 125. The bandwidth at which stabilization occurs ranges from 400 Hz to 100 kHz. The work reported in this Letter further paves the way for high power laser stability techniques which could be implemented in optomechanical experiments and in gravitational wave detectors.

Laser power stabilization via radiation pressure.

This Letter reports the experimental realization of a novel, to the best of our knowledge, active power stabilization scheme in which laser power fluctuations are sensed via the radiation pressure driven motion they induce on a movable mirror. The mirror position and its fluctuations were determined by means of a weak auxiliary laser beam and a Michelson interferometer, which formed the in-loop sensor of the power stabilization feedback control system. This sensing technique exploits a nondemolition measurement, which can result in higher sensitivity for power fluctuations than direct, and hence destructive, detection. Here we used this new scheme in a proof-of-concept experiment to demonstrate power stabilization in the frequency range from 1 Hz to 10 kHz, limited at low frequencies by the thermal noise of the movable mirror at room temperature.

Observation of an optical spring with a beam splitter.

We present the experimental observation of an optical spring without the use of an optical cavity. The optical spring is produced by interference at a beam splitter and, in principle, does not have the damping force associated with optical springs created in detuned cavities. The experiment consists of a Michelson-Sagnac interferometer (with no recycling cavities) with a partially reflective GaAs microresonator as the beam splitter that produces the optical spring. Our experimental measurements at input powers of up to 360 mW show the shift of the optical spring frequency as a function of power and are in excellent agreement with theoretical predictions. In addition, we show that the optical spring is able to keep the interferometer stable and locked without the use of external feedback.

Stable Optical Trap from a Single Optical Field Utilizing Birefringence.

We report a stable double optical spring effect in an optical cavity pumped with a single optical field that arises as a result of birefringence. One end of the cavity is formed by a multilayer Al_{0.92}Ga_{0.08}As/GaAs stack supported by a microfabricated cantilever with a natural mode frequency of 274 Hz. The optical spring shifts the resonance to 21 kHz, corresponding to a suppression of low frequency vibrations by a factor of about 5 000. The stable nature of the optical trap allows the cavity to be operated without any external feedback and with only a single optical field incident.

Direct visualization of bactericidal action of cationic conjugated polyelectrolytes and oligomers.

The bactericidal mechanisms of poly(phenylene ethynylene) (PPE)-based cationic conjugated polyelectrolytes (CPE) and oligo-phenylene ethynylenes (OPE) were investigated using electron/optical microscopy and small-angle X-ray scattering (SAXS). The ultrastructural analysis shows that polymeric PPE-Th can significantly remodel the bacterial outer membrane and/or the peptidoglycan layer, followed by the possible collapse of the bacterial cytoplasm membrane. In contrast, oligomeric end-only OPE (EO-OPE) possesses potent bacteriolysis activity, which efficiently disintegrates the bacterial cytoplasm membrane and induces the release of bacterial cell content. Using single giant vesicles and SAXS, we demonstrated that the membrane perturbation mechanism of EO-OPE against model bacterial membranes results from a 3D membrane phase transition or perturbation.

Antibacterial activity of conjugated polyelectrolytes with variable chain lengths.

Cationic poly(phenylene ethynylene)- (PPE-) based conjugated polyelectrolytes (CPEs) with six different chain lengths ranging in degree of polymerization from ∼7 to ∼49 were synthesized from organic-soluble precursor polymers. The molecular weight of the precursor polymers was controlled by the amount of a monofunctional "end-capping" agent added to the polymerization reaction. Cationic CPEs were prepared by quaternization of amine groups to tetraalkylammonium groups. Their structure-property relationships were investigated by observing their photophysical properties and antibacterial activity. The polymers were found to exhibit a chain-length dependence in their photophysical properties. It has also been observed that the polymers exhibit effective antibacterial activity against both Gram-positive and Gram-negative bacteria under UV irradiation, whereas they show little antibacterial activity in the dark. An effect of chain length on the light-activated antibacterial activity was also found: The shortest polymer (n=7) exhibited the most effective antibacterial activity against both Gram-positive and Gram-negative bacteria.

Light-induced antibacterial activity of symmetrical and asymmetrical oligophenylene ethynylenes.

The light-induced antibacterial activity of symmetric and asymmetric oligophenylene ethynylenes (OPEs) was investigated against Gram-positive (Staphylococcus aureus and Staphylococcus epidermidis) and Gram-negative (Escherichia coli) bacteria. To understand the light-induced biocidal effect better, the transient absorption and triplet lifetime of OPEs were studied in methanol and water. A higher triplet lifetime was observed for OPE samples in water than in methanol. The magnitudes of the changes in optical density (ΔOD) of the S-OPE-n(H) series of symmetric oligomers are much higher than that of the asymmetric OPE-n series in water and are generally correlated with the singlet oxygen yield. It was found that the antibacterial activity against both Gram-positive and Gram-negative bacteria is size-, concentration-, and time-dependent. The light-induced antibacterial activity may result from the coordinated interactions of membrane disruption and interfacial or intracellular singlet oxygen generation, and the dominant factor is most likely the latter. The results obtained in this study will aid in the design of more efficient biocides in the future.

Photophysics and light-activated biocidal activity of visible-light-absorbing conjugated oligomers.

The photophysical properties of three cationic π-conjugated oligomers were correlated with their visible light activated biocidal activity vs S. aureus. The oligomers contain three arylene units (terthiophene, 4a; thiophene-benzotriazole-thiophene, 4b; thiophene-benzothiadiazole-thiophene, 4c) capped on each end by cationic -(CH2)3NMe3(+) groups. The oligomers absorb in the visible region due to their donor-acceptor-donor electronic structure. Oligomers 4a and 4b have high intersystem crossing and singlet oxygen sensitization efficiency, but 4c has a very low intersystem crossing efficiency and it does not sensitize singlet oxygen. The biocidal activity of the oligomers under visible light varies in the order 4a > 4b ≈ 4c.

Rapid evaluation of the antibacterial activity of arylene-ethynylene compounds.

A series of oligo(arylene-ethynylene) (1-3 repeat units) compounds functionalized with quaternary ammonium groups was screened for their antibacterial activity in the dark and with activation by long-wavelength (365 nm) UV irradiation. Several of these compounds have effective bactericidal activity (>99.9% killing) at concentrations between 0.01 and 10 µg/mL. Our approach uses flow cytometry to rapidly screen and evaluate the susceptibility of bacterial populations. The rapidity, high information content, and accuracy of this approach make it an extremely valuable method for the study of antibacterial compounds.

Light and dark-activated biocidal activity of conjugated polyelectrolytes.

This Spotlight on Applications provides an overview of a research program that has focused on the development and mechanistic study of cationic conjugated polyelectrolytes (CPEs) that function as light- and dark-active biocidal agents. Investigation has centered on poly-(phenylene ethynylene) (PPE) type conjugated polymers that are functionalized with cationic quaternary ammonium solubilizing groups. These polymers are found to interact strongly with Gram-positive and Gram-negative bacteria, and upon illumination with near-UV and visible light act to rapidly kill the bacteria. Mechanistic studies suggest that the cationic PPE-type polymers efficiently sensitize singlet oxygen ((1)O(2)), and this cytotoxic agent is responsible for initiating the sequence of events that lead to light-activated bacterial killing. Specific CPEs also exhibit dark-active antimicrobial activity, and this is believed to arise due to interactions between the cationic/lipophilic polymers and the negatively charged outer membrane characteristic of Gram-negative bacteria. Specific results are shown where a cationic CPE with a degree of polymerization of 49 exhibits pronounced light-activated killing of E. coli when present in the cell suspension at a concentration of 1 µg mL(-1).

Conjugated-polyelectrolyte-grafted cotton fibers act as "micro flypaper" for the removal and destruction of bacteria.

We demonstrate herein a method for chemically modifying cotton fibers and cotton-containing fabric with a light-activated, cationic phenylene-ethynylene (PPE-DABCO) conjugated polyelectrolyte biocide. When challenged with Pseudomonas aeruginosa and Bacillus atropheaus vegetative cells from liquid suspension, light-activated PPE-DABCO effects 1.2 and 8 log, respectively, losses in viability of the exposed bacteria. These results suggest that conjugated polyelectrolytes retain their activity when grafted to fabrics, showing promise for use in settings where antimicrobial textiles are needed.

Standard quantum limit for probing mechanical energy quantization.

We derive a standard quantum limit for probing mechanical energy quantization in a class of systems with mechanical modes parametrically coupled to external degrees of freedom. To resolve a single mechanical quantum, it requires a strong-coupling regime-the decay rate of external degrees of freedom is smaller than the parametric coupling rate. In the case for cavity-assisted optomechanical systems, e.g., the one proposed by Thompson et al. [Nature (London) 452, 72 (2008)], zero-point motion of the mechanical oscillator needs to be comparable to the linear dynamical range of the optical system which is characterized by the optical wavelength divided by the cavity finesse.

Light and dark biocidal activity of cationic poly(arylene ethynylene) conjugated polyelectrolytes.

In this paper we report a study of cationic poly(arylene ethynylene) conjugated polyelectrolytes. The objective of the study was to compare the behavior of a polymer where a thiophene has replaced a phenyl ring in poly(phenylene ethynylene) polycations (PPE) previously investigated. Properties of solution phase and physisorbed suspensions of the polymer on microspheres were investigated. The photophysical properties of the polymer are evaluated and used to understand the striking differences in biocidal activity compared to the PPE polymers previously examined. The principal findings are that the thiophene polymer has remarkable dark biocidal activity against Pseudomonas aeruginosa strain PAO1 but very little light-activated activity. The low light-activated biocidal activity of the thiophene polymer is attributed to a highly aggregated state of the polymer in aqueous solutions and on microspheres as a physisorbed coating. This results in low triplet yields and a very poor sensitization of singlet oxygen and other reactive oxygen intermediates. The highly effective dark biocidal activity of the thiophene-containing polymers is attributed to its high lipophilicity and the presence of accessible quaternary ammonium groups. The difference in behavior among the polymers compared provides insights into the mechanism of the dark process and indicates that aggregation of polymer can reduce light activated biocidal activity by suppressing singlet oxygen generation.

Conjugated polyelectrolyte capsules: light-activated antimicrobial micro "Roach Motels".

Microcapsules consisting of alternating layers of oppositely charged poly(phenylene ethynylene)-type conjugated polyelectrolytes (CPEs) were prepared via layer-by-layer deposition onto MnCO3 template particles followed by dissolution of the template particles using an ethylenediaminetetraacetate solution. The resulting microcapsules exhibit bright-green fluorescence emission characteristics of the CPEs. Strong antimicrobial activity was observed upon mixing of polyelectrolyte capsules with Cobetia marina or Pseudomonas aeruginosa followed by white-light irradiation. It was demonstrated that the materials act as highly effective light-activated micro "Roach Motels" with greater than 95% kill after exposure to approximately 1 h of white light.

Light-induced biocidal action of conjugated polyelectrolytes supported on colloids.

A series of water soluble, cationic conjugated polyelectrolytes (CPEs) with backbones based on a poly(phenylene ethynylene) repeat unit structure and tetraakylammonium side groups exhibit a profound light-induced biocidal effect. The present study examines the biocidal activity of the CPEs, correlating this activity with the photophysical properties of the polymers. The photophysical properties of the CPEs are studied in solution, and the results demonstrate that direct excitation produces a triplet excited-state in moderate yield, and the triplet is shown to be effective at sensitizing the production of singlet oxygen. Using the polymers in a format where they are physisorbed or covalently grafted to the surface of colloidal silica particles (5 and 30 microm diameter), we demonstrate that they exhibit light-activated biocidal activity, effectively killing Cobetia marina and Pseudomonas aeruginosa. The light-induced biocidal activity is also correlated with a requirement for oxygen suggesting that interfacial generation of singlet oxygen is the crucial step in the light-induced biocidal activity.

Optical dilution and feedback cooling of a gram-scale oscillator to 6.9 mK.

We report on the use of a radiation pressure induced restoring force, the optical spring effect, to optically dilute the mechanical damping of a 1 g suspended mirror, which is then cooled by active feedback (cold damping). Optical dilution relaxes the limit on cooling imposed by mechanical losses, allowing the oscillator mode to reach a minimum temperature of 6.9 mK, a factor of approximately 40 000 below the environmental temperature. A further advantage of the optical spring effect is that it can increase the number of oscillations before decoherence by several orders of magnitude. In the present experiment we infer an increase in the dynamical lifetime of the state by a factor of approximately 200.

An all-optical trap for a gram-scale mirror.

We report on a stable optical trap suitable for a macroscopic mirror, wherein the dynamics of the mirror are fully dominated by radiation pressure. The technique employs two frequency-offset laser fields to simultaneously create a stiff optical restoring force and a viscous optical damping force. We show how these forces may be used to optically trap a free mass without introducing thermal noise, and we demonstrate the technique experimentally with a 1 g mirror. The observed optical spring has an inferred Young's modulus of 1.2 TPa, 20% stiffer than diamond. The trap is intrinsically cold and reaches an effective temperature of 0.8 K, limited by technical noise in our apparatus.