Bradley Deutsch

Subkelvin Parametric Feedback Cooling of a Laser-Trapped Nanoparticle

We optically trap a single nanoparticle in high vacuum and cool its three spatial degrees of freedom by means of active parametric feedback. Using a single laser beam for both trapping and cooling we demonstrate a temperature compression ratio of four orders of magnitude. The absence of a clamping mechanism provides robust decoupling from the heat bath and eliminates the requirement of cryogenic precooling. The small size and mass of the nanoparticle yield high resonance frequencies and high quality factors along with low recoil heating, which are essential conditions for ground state cooling and for low decoherence. The trapping and cooling scheme presented here opens new routes for testing quantum mechanics with mesoscopic objects and for ultrasensitive metrology and sensing.

Nano-optofluidic Detection of Single Viruses and Nanoparticles

The reliable detection, sizing, and sorting of viruses and nanoparticles is important for biosensing, environmental monitoring, and quality control. Here we introduce an optical detection scheme for the real-time and label-free detection and recognition of single viruses and larger proteins. The method makes use of nanofluidic channels in combination with optical interferometry. Elastically scattered light from single viruses traversing a stationary laser focus is detected with a differential heterodyne interferometer and the resulting signal allows single viruses to be characterized individually. Heterodyne detection eliminates phase variations due to different particle trajectories, thus improving the recognition accuracy as compared to standard optical interferometry. We demonstrate the practicality of our approach by resolving nanoparticles of various sizes, and detecting and recognizing different species of human viruses from a mixture. The detection system can be readily integrated into larger nanofluidic architectures for practical applications.

Creation and detection of Skyrmions in a Bose-Einstein condensate.

We present the first experimental realization and characterization of two-dimensional Skyrmions and half-Skyrmions in a spin-2 Bose-Einstein condensate. The continuous rotation of the local spin of the Skyrmion through an angle of pi (and half-Skyrmion through an angle of pi/2) across the cloud is confirmed by the spatial distribution of the three spin states as parametrized by the bending angle of the l vector. The winding number w = (0,1,2) of the internal spin states comprising the Skyrmions is confirmed through matter-wave interference.

Focusing of surface phonon polaritons

Surface phonon polaritons (SPs) on crystal substrates have applications in microscopy, biosensing, and photonics. Here, we demonstrate focusing of SPs on a silicon carbide (SiC) crystal. A simple metal-film element is fabricated on the SiC sample in order to focus the surface waves. Pseudoheterodyne scanning near-field infrared microscopy is used to obtain amplitude and phase maps of the local fields verifying the enhanced amplitude in the focus. Simulations of this system are presented, based on a modified Huygens’ principle, which show good agreement with the experimental results.

Near-field amplitude and phase recovery using phase-shifting interferometry

Scattering-type scanning near-field optical microscopy has allowed for investigation of light-matter interaction of a large variety of samples with excellent spatial resolution. Light incident on a metallic probe experiences an amplitude and phase change on scattering, which is dependent on optical sample properties. We implement phase-shifting interferometry to extract amplitude and phase information from an interferometric near-field scattering system, and compare recorded optical images with theoretical predictions. The results demonstrate our ability to measure, with nanoscale resolution, amplitude and phase distributions of optical fields on sample surfaces. The here-introduced phase-shifting method is considerably simpler than heterodyne methods and less sensitive to errors than the two-step homodyne method.

Visualizing the Optical Interaction Tensor of a Gold Nanoparticle Pair

The control of optical fields on the nanometer scale is a central theme of plasmonics and nanophotonics. Methods for characterizing localized optical field distributions are necessary to validate theoretical predictions, to test nanofabrication procedures, and to provide feedback for design improvements. Typical methods of probing near fields (e.g., single molecule fluorescence and near-field microscopy) cannot probe both the complex-valued and vectorial nature of the field distributions. We demonstrate that a nanoparticle probe with isotropic polarizability in combination with polarization control of excitation and detection beams provides access to this information through the interaction tensor. For a sample consisting of a single nanoparticle we show that the recorded images correspond to maps of the local Greens function tensor elements that couple the probe and sample. The tensorial mapping of interacting nanoparticles is of interest for optical sensing, optical antennas, surface-enhanced Raman scattering, nonlinear optics, and molecular rulers.

Phase in Nanooptics

Quantitative phase measurements in imaging, microscopy, and nanooptics provide information not carried in amplitude measurements alone. In this issue of ACS Nano, Honigstein et al. present a new method in phase measurement. In this Perspective, we comment on this work and more broadly on the emerging role of phase and phase measurements in nanooptics.

Material-specific detection and classification of single nanoparticles

A material-specific dual-color common-path interferometric detection system for discriminating between nanoparticles in solution in real time is described. The detection technique is applicable to situations where both particle size and material are of interest.

Nanoparticle detection using dual-phase interferometry

The detection and identification of nanoparticles is of growing interest in atmospheric monitoring, medicine, and semiconductor manufacturing. While elastic light scattering with interferometric detection provides good sensitivity to single particles, active optical components prevent scalability of realistic sizes for deployment in the field or clinic. Here, we report on a simple phase-sensitive nanoparticle detection scheme with no active optical elements. Two measurements are taken simultaneously, allowing the amplitude and phase to be decoupled. We demonstrate the detection of 25 nm Au particles in liquid in Δt ∼ 1 ms with a signal-to-noise ratio of ∼ 37. Such performance makes it possible to detect nanoscale contaminants or larger proteins in real time without the need of artificial labeling.

Compositional prior information in computed infrared spectroscopic imaging

Compositional prior information is used to bridge a gap in the theory between optical coherence tomography (OCT), which provides high-resolution structural images by neglecting spectral variation, and imaging spectroscopy, which provides only spectral information without significant regard to structure. A constraint is proposed in which it is assumed that a sample is composed of N distinct materials with known spectra, allowing the structural and spectral composition of the sample to be determined with a number of measurements on the order of N. We present a forward model for a sample with heterogeneities along the optical axis and show through simulation that the N-species constraint allows unambiguous inversion of Fourier transform interferometric data within the spatial frequency passband of the optical system. We then explore the stability and limitations of this model and extend it to a general 3D heterogeneous sample.