Label-free detection of single nanoparticles with disordered nanoisland surface plasmon sensor.

We report sensing of single nanoparticles using disordered metallic nanoisland substrates supporting surface plasmon polaritons (SPPs). Speckle patterns arising from leakage radiation of elastically scattered SPPs provide a unique fingerprint of the scattering microstructure at the sensor surface. Experimental measurements of the speckle decorrelation are presented and shown to enable detection of sorption of individual gold nanoparticles and polystyrene beads. Our approach is verified through bright-field and fluorescence imaging of particles adhering to the nanoisland substrate.

Dielectric perturbations: anomalous resonance frequency shifts in optical resonators.

Small perturbations in the dielectric environment around resonant dielectric structures usually lead to a frequency shift of the resonator modes directly proportional to the polarizability of the perturbation. Here, we report experimental observations of strong frequency shifts that can oppose and even exceed the contribution of the perturbations' polarizability. We show in particular how the mode frequencies of a lithium niobate whispering-gallery-mode resonator are shifted by planar substrates-of refractive indices ranging from 1.50 to 4.22-contacting the resonator rim. Both blue- and redshifts are observed, as well as an increase in mode linewidth, when substrates are moved into the evanescent field of the whispering gallery mode. We compare the experimental results to a theoretical model by Foreman et al. [J. Opt. Soc. Am. B33, 2177 (2016)JOBPDE0740-322410.1364/JOSAB.33.002177] and provide an additional intuitive explanation based on the Goos-Hänchen shift for the optical domain, with applications to dielectric structures ranging from meta-surfaces to photonic crystal cavities.

Single pixel polarimetric imaging through scattering media.

Polarimetric imaging can provide valuable information about biological samples in a wide range of applications. Detrimental tissue scattering and depolarization however currently hamper in vivo polarization imaging. In this work, single pixel imaging is investigated as a means of reconstructing polarimetric images through scattering media. A theoretical imaging model is presented, and the recovery of the spatially resolved Mueller matrix of a test object behind a scattering phantom is demonstrated experimentally.

SNR enhancement in brillouin microspectroscopy using spectrum reconstruction.

Brillouin spectroscopy can suffer from low signal-to-noise ratios (SNRs). Such low SNRs can render common data analysis protocols unreliable, especially for SNRs below ∼10. In this work we exploit two denoising algorithms, namely maximum entropy reconstruction (MER) and wavelet analysis (WA), to improve the accuracy and precision in determination of Brillouin shifts and linewidth. Algorithm performance is quantified using Monte-Carlo simulations and benchmarked against the Cramér-Rao lower bound. Superior estimation results are demonstrated even at low SNRs (≥ 1). Denoising is furthermore applied to experimental Brillouin spectra of distilled water at room temperature, allowing the speed of sound in water to be extracted. Experimental and theoretical values were found to be consistent to within ±1% at unity SNR.

Erratum: Optimal Frames for Polarization State Reconstruction [Phys. Rev. Lett. 115, 263901 (2015)].

This corrects the article DOI: 10.1103/PhysRevLett.115.263901.

Field Correlations in Surface Plasmon Speckle.

In this work fluctuations in the electric field of surface plasmon polaritons undergoing random scattering on a rough metallic surface are considered. A rigorous closed form analytic expression is derived describing second order correlations in the resulting plasmon speckle pattern assuming statistically stationary and isotropic roughness. Partially coherent planar Schell-model source fields can also be described within the developed framework. Behaviour of the three-dimensional degree of cross polarisation and spectral degree of coherence is also discussed. Expressions derived take full account of dissipation in the metal with non-universal behaviour exhibited within the correlation length of the surface and source fields.

Precision and informational limits in inelastic optical spectroscopy.

Using Fisher information and the Cramér-Rao lower bound, we analyse fundamental precision limits in the determination of spectral parameters in inelastic optical scattering. General analytic formulae are derived which account for the instrument response functions of the dispersive element and relay optics found in practical Raman and Brillouin spectrometers. Limiting cases of dispersion and diffraction limited spectrometers, corresponding to measurement of Lorentzian and Voigt lineshapes respectively, are discussed in detail allowing optimal configurations to be identified. Effects of defocus, spherical aberration, detector pixelation and a finite detector size are also considered.

Whispering gallery mode single nanoparticle detection and sizing: the validity of the dipole approximation.

Interactions between whispering gallery modes (WGMs) and small nanoparticles are commonly modeled by treating the particle as a point dipole scatterer. This approach is assumed to be accurate as long as the nanoparticle radius, a, is small compared to the WGM wavelength λ. In this Letter, however, we show that the large field gradients associated with the evanescent decay of a WGM causes the dipole theory to significantly underestimate the interaction strength and, hence, the induced WGM resonance shift, even for particles as small as a∼λ/10. To mitigate this issue, we employ a renormalized Born approximation to more accurately determine nanoparticle-induced resonance shifts and, hence, enable improved particle sizing. The domain of validity of this approximation is investigated, and supporting experimental results are presented.

Integrated plasmonic metasurfaces for spectropolarimetry.

Plasmonic metasurfaces enable simultaneous control of the phase, momentum, amplitude and polarization of light and hence promise great utility in realization of compact photonic devices. In this paper, we demonstrate a novel chip-scale device suitable for simultaneous polarization and spectral measurements through use of six integrated plasmonic metasurfaces (IPMs), which diffract light with a given polarization state and spectral component into well-defined spatial domains. Full calibration and characterization of our device is presented, whereby good spectral resolution and polarization accuracy over a wavelength range of 500-700 nm is shown. Functionality of our device in a Müller matrix modality is demonstrated through determination of the polarization properties of a commercially available variable waveplate. Our proposed IPM is robust, compact and can be fabricated with a single photolithography step, promising many applications in polarization imaging, quantum communication and quantitative sensing.

Optical tracking of anomalous diffusion kinetics in polymer microspheres.

In this Letter we propose the use of whispering gallery mode resonance tracking as a label-free optical means to monitor diffusion kinetics in glassy polymer microspheres. Approximate solutions to the governing diffusion equations are derived for the case of slow relaxation and small Stefan number. Transduction of physical changes in the polymer, including formation of a rubbery layer, swelling, and dissolution, into detectable resonance shifts are described using a perturbative approach. Concrete examples of poly(methyl methacrylate) and polystyrene spheres in water are considered.

Whispering gallery mode sensors.

We present a comprehensive overview of sensor technology exploiting optical whispering gallery mode (WGM) resonances. After a short introduction we begin by detailing the fundamental principles and theory of WGMs in optical microcavities and the transduction mechanisms frequently employed for sensing purposes. Key recent theoretical contributions to the modeling and analysis of WGM systems are highlighted. Subsequently we review the state of the art of WGM sensors by outlining efforts made to date to improve current detection limits. Proposals in this vein are numerous and range, for example, from plasmonic enhancements and active cavities to hybrid optomechanical sensors, which are already working in the shot noise limited regime. In parallel to furthering WGM sensitivity, efforts to improve the time resolution are beginning to emerge. We therefore summarize the techniques being pursued in this vein. Ultimately WGM sensors aim for real-world applications, such as measurements of force and temperature, or alternatively gas and biosensing. Each such application is thus reviewed in turn, and important achievements are discussed. Finally, we adopt a more forward-looking perspective and discuss the outlook of WGM sensors within both a physical and biological context and consider how they may yet push the detection envelope further.

Optimal Frames for Polarization State Reconstruction.

Complete determination of the polarization state of light requires at least four distinct projective measurements of the associated Stokes vector. Stability of state reconstruction, however, hinges on the condition number κ of the corresponding instrument matrix. Optimization of redundant measurement frames with an arbitrary number of analysis states, m, is considered in this Letter in the sense of minimization of κ. The minimum achievable κ is analytically found and shown to be independent of m, except for m=5 where this minimum is unachievable. Distribution of the optimal analysis states over the Poincaré sphere is found to be described by spherical 2 designs, including the Platonic solids as special cases. Higher order polarization properties also play a key role in nonlinear, stochastic, and quantum processes. Optimal measurement schemes for nonlinear measurands of degree t are hence also considered and found to correspond to spherical 2t designs, thereby constituting a generalization of the concept of mutually unbiased bases.

Single-molecule nucleic acid interactions monitored on a label-free microcavity biosensor platform.

Biosensing relies on the detection of molecules and their specific interactions. It is therefore highly desirable to develop transducers exhibiting ultimate detection limits. Microcavities are an exemplary candidate technology for demonstrating such a capability in the optical domain and in a label-free fashion. Additional sensitivity gains, achievable by exploiting plasmon resonances, promise biosensing down to the single-molecule level. Here, we introduce a biosensing platform using optical microcavity-based sensors that exhibits single-molecule sensitivity and is selective to specific single binding events. Whispering gallery modes in glass microspheres are used to leverage plasmonic enhancements in gold nanorods for the specific detection of nucleic acid hybridization, down to single 8-mer oligonucleotides. Detection of single intercalating small molecules confirms the observation of single-molecule hybridization. Matched and mismatched strands are discriminated by their interaction kinetics. Our platform allows us to monitor specific molecular interactions transiently, hence mitigating the need for high binding affinity and avoiding permanent binding of target molecules to the receptors. Sensor lifetime is therefore increased, allowing interaction kinetics to be statistically analysed.

Experimental proof of concept of nanoparticle-assisted STED.

We imaged core-shell nanoparticles, consisting of a dye-doped silica core covered with a layer of gold, with a stimulated emission depletion, fluorescence lifetime imaging (STED-FLIM) microscope. Because of the field enhancement provided by the localized surface plasmon resonance of the gold shell, we demonstrate a reduction of the STED depletion power required to obtain resolution improvement by a factor of 4. This validates the concept of nanoparticle-assisted STED (NP-STED), where hybrid dye-plasmonic nanoparticles are used as labels for STED in order to decrease the depletion powers required for subwavelength imaging.

Optimizing detection limits in whispering gallery mode biosensing.

A theoretical analysis of detection limits in swept-frequency whispering gallery mode biosensing modalities is presented based on application of the Cramér-Rao lower bound. Measurement acuity factors are derived assuming the presence of uncoloured and 1/ f Gaussian technical noise. Frequency fluctuations, for example arising from laser jitter or thermorefractive noise, are also considered. Determination of acuity factors for arbitrary coloured noise by means of the asymptotic Fisher information matrix is highlighted. Quantification and comparison of detection sensitivity for both resonance shift and broadening sensing modalities are subsequently given. Optimal cavity and coupling geometries are furthermore identified, whereby it is found that slightly under-coupled cavities outperform critically and over coupled ones.

Revealing molecular structure and orientation with Stokes vector resolved second harmonic generation microscopy.

We report on measurements and characterization of polarization properties of Second Harmonic (SH) signals using a four-channel photon counting based Stokes polarimeter. In this way, the critical polarization parameters can be obtained concurrently without the need of repeated image acquisition. The critical polarization parameters, including the degree of polarization (DOP), the degree of linear polarization (DOLP), and the degree of circular polarization (DOCP), are extracted from the reconstructed Stokes vector based SH images in a pixel-by-pixel manner. The measurements are further extended by varying the polarization states of the incident light and recording the resulting Stokes parameters of the SH signal. In turn this allows the molecular structure and orientation of the samples to be determined. Use of Stokes polarimetry is critical in determination of the full polarization state of light, and enables discrimination of material properties not possible with conventional crossed-polarized detection schemes. The combination of SHG microscopy and Stokes polarimeter hence makes a powerful tool to investigate the structural order of targeted specimens.

Stokes vector based polarization resolved second harmonic microscopy of starch granules.

We report on the measurement and analysis of the polarization state of second harmonic signals generated by starch granules, using a four-channel photon counting based Stokes-polarimeter. Various polarization parameters, such as the degree of polarization (DOP), the degree of linear polarization (DOLP), the degree of circular polarization (DOCP), and anisotropy are extracted from the 2D second harmonic Stokes images of starch granules. The concentric shell structure of a starch granule forms a natural photonic crystal structure. By integration over all the solid angle, it will allow very similar SHG quantum efficiency regardless of the angle or the states of incident polarization. Given type I phase matching and the concentric shell structure of a starch granule, one can easily infer the polarization states of the input beam from the resulting SH micrograph.

Polarization-resolved second harmonic generation microscopy with a four-channel Stokes-polarimeter.

We developed a four-channel photon counting based Stokes-polarimeter for spatial characterization of polarization effects in second harmonic generation (SHG). We have implemented a calibration technique allowing quantitative measurement of polarization parameters, such as the degree of polarization (DOP), degree of linear polarization (DOLP), degree of circular polarization (DOCP), as well as anisotropy from the acquired Stokes parameters. The technique is used as contrast mechanism to characterize the polarization properties from two potassium dihydrogen phosphate (KDP) micro-crystals and collagen type-I in SHG microscopy.

Focusing of spatially inhomogeneous partially coherent, partially polarized electromagnetic fields.

We report a general framework capable of describing the focusing of electromagnetic waves with spatially varying coherence and polarization properties in optical systems of arbitrary numerical aperture and Fresnel number. We also investigate the reduction of the dimensionality of the requisite integrals by use of a coherent mode expansion. We find that coherent mode expansions treating each component of the electric field vector individually are unsuitable for describing focusing systems because of the inter-component mixing that can occur in high numerical aperture systems. In addition, we show that the assumption of harmonic angular dependence allows the azimuthal integration to be performed analytically, providing further simplification of the analysis. We also find that the effective degree of spectral coherence of an electromagnetic beam is unchanged upon focusing. Finally, as an illustration of the developed framework, we calculate the transverse and axial focal distributions for a partially coherent source formed by incoherent superposition of radially and azimuthally polarized Laguerre-Gauss modes.

A priori information and optimisation in polarimetry.

Polarimetric measurements are designed to obtain information pertaining to the system under study, however noise in the system limits the precision and hence information obtainable. Exploitation of a priori knowledge of the system allows for an improvement in the precision of experimental data. In this vein we present a framework for system design and optimisation based upon the Fisher information matrix, which allows easy incorporation of such a priori information. As such the proposed figure of merit is more complete than the commonly used condition number. Conditions of equivalence are considered, however a number of examples highlight the failings of the condition number under more general scenarios. Bounds on the achievable informational gains via multiple polarimeter arms are also given. Finally we present analytic results concerning error distribution in a Mueller matrix polar decomposition, allowing for a more accurate noise analysis in polarimetric experiments.