Precise Holographic Manipulation of Olfactory Circuits Reveals Coding Features Determining Perceptual Detection.

Sensory systems transform the external world into time-varying spike trains. What features of spiking activity are used to guide behavior? In the mouse olfactory bulb, inhalation of different odors leads to changes in the set of neurons activated, as well as when neurons are activated relative to each other (synchrony) and the onset of inhalation (latency). To explore the relevance of each mode of information transmission, we probed the sensitivity of mice to perturbations across each stimulus dimension (i.e., rate, synchrony, and latency) using holographic two-photon optogenetic stimulation of olfactory bulb neurons with cellular and single-action-potential resolution. We found that mice can detect single action potentials evoked synchronously across <20 olfactory bulb neurons. Further, we discovered that detection depends strongly on the synchrony of activation across neurons, but not the latency relative to inhalation.

Near- and far-field properties of plasmonic oligomers under radially and azimuthally polarized light excitation.

We present a comprehensive experimental and theoretical study on the near- and far-field properties of plasmonic oligomers using radially and azimuthally polarized excitation. These unconventional polarization states are perfectly matched to the high spatial symmetry of the oligomers and thus allow for the excitation of some of the highly symmetric eigenmodes of the structures, which cannot be excited by linearly polarized light. In particular, we study hexamer and heptamer structures and strikingly find very similar optical responses, as well as the absence of a Fano resonance. Furthermore, we investigate the near-field distributions of the oligomers using near-field scanning optical microscopy (NSOM). We observe significantly enhanced near-fields, which arise from efficient excitation of the highly symmetric eigenmodes by the radially and azimuthally polarized light fields. Our study opens up possibilities for tailored light-matter interaction, combining the design freedom of complex plasmonic structures with the remarkable properties of radially and azimuthally polarized light fields.

Subwavelength plasmonics for graded-index optics on a chip.

Planar plasmonic devices are becoming attractive for myriad applications, owing to their potential compatibility with standard microelectronics technology and the capability for densely integrating a large variety of plasmonic devices on a chip. Mitigating the challenges of using plasmonics in on-chip configurations requires precise control over the properties of plasmonic modes, in particular their shape and size. Here we achieve this goal by demonstrating a planar plasmonic graded-index lens focusing surface plasmons propagating along the device. The plasmonic mode is manipulated by carving subwavelength features into a dielectric layer positioned on top of a uniform metal film, allowing the local effective index of the plasmonic mode to be controlled using a single binary lithographic step. Focusing and divergence of surface plasmons is demonstrated experimentally. The demonstrated approach can be used for manipulating the propagation of surface plasmons, e.g., for beam steering, splitting, cloaking, mode matching, and beam shaping applications.

Generation of a periodic array of radially polarized Plasmonic focal spots.

This paper demonstrates experimentally the tight focusing of a 3X3 array of radially polarized diffraction orders, and the coupling of this array of spots to surface plasmon polaritons (SPPs), propagating on a uniform metal film, and effectively generating a periodic structure of plasmonic sources by the use of structured illumination pattern, rather than by structuring the plasmonic sample. Using near field measurements, we observed coherent interactions between these multiple plasmonic sources as they propagate towards each other. The demonstrated setup exploits the previously demonstrated advantages of radially polarized light in coupling to SPPs and in generating sharper plasmonic hot spots and expends its use towards mitigating parallel processing challenges. The experimental results are in good agreement with the theory, showing interference fringes having periodicity compatible with the plasmonic SPP wavelength. The demonstrated approach of generating array of hot spots on flat metallic films is expected to play a role in variety of applications, e.g. microscopy, lithography, sensing and optical memories.

Pin cushion plasmonic device for polarization beam splitting, focusing, and beam position estimation.

Great hopes rest on surface plasmon polaritons' (SPPs) potential to bring new functionalities and applications into various branches of optics. In this paper, we demonstrate a pin cushion structure capable of coupling light from free space into SPPs, split them based on the polarization content of the illuminating beam of light, and focus them into small spots. We also show that for a circularly or randomly polarized light, four focal spots will be generated at the center of each quarter circle comprising the pin cushion device. Furthermore, following the relation between the relative intensity of the obtained four focal spots and the relative position of the illuminating beam with respect to the structure, we propose and demonstrate the potential use of our structure as a miniaturized plasmonic version of the well-known four quadrant detector. Additional potential applications may vary from multichannel microscopy and multioptical traps to real time beam tracking systems.

Light transmission through a circular metallic grating under broadband radial and azimuthal polarization illumination.

We studied the characteristics of a circular metallic grating illuminated by broadband radial and azimuthal polarizations. We demonstrated that this scenario is the cylindrical analogue of a one-dimensional Cartesian grating illuminated by TM and TE polarizations. We measured the transmission spectra of this structure and observed strong polarization selectivity and, specifically, a resonance for radial polarization excitation, indicating a strong coupling to surface plasmons. The structure may be attractive for applications where pure radial polarization is needed, such as tight focusing, material processing, and particle trapping.

Near field phase mapping exploiting intrinsic oscillations of aperture NSOM probe.

An innovative, simple, compact and low cost approach for phase mapping based on the intrinsic modulation of an aperture Near Field Scanning Optical Microscope probe is analyzed and experimentally demonstrated. Several nanoscale silicon waveguides are phase-mapped using this approach, and the different modes of propagation are obtained via Fourier analysis. The obtained measured results are in good agreement with the effective indexes of the modes calculated by electromagnetic simulations. Owing to its simplicity and effectiveness, the demonstrated system is a potential candidate for integration with current near field systems for the characterization of nanophotonic components and devices.

Plasmonic resonance effects for tandem receiving-transmitting nanoantennas.

A nanoplasmonic "transceiver" was assembled to examine the efficiency of coupled plasmonic antennas and their resonance interactions. In particular, plasmonic focusing receiver antenna coupled to transmitting annular antenna having a short central plasmonic wire was measured. The receiver collected incoming radially polarized light and efficiently focused and coupled it to a rear side transmitter comprised of a short resonant plasmonic wire and annular aperture. Transmission spectra exhibited a substantial signature of the wire Fabry-Perot resonances. The wire antenna crossection was improved by nearly 3 orders of magnitude by the focusing antenna system.

Efficient coupling and field enhancement for the nano-scale: plasmonic needle.

Theoretical demonstration of efficient coupling and power concentration of radially-polarized light on a conical tip of plasmonic needle is presented. The metallic needle is grown at the center of radial plasmonic grating, engraved in a metal surface. The electromagnetic field distribution was evaluated by Finite Elements and Finite-Difference-Time-Domain methods. The results show that the field on the tip of the needle is significantly enhanced compared to the field impinging on the grating. The power enhancement exhibited a resonant behavior as a function of needle length and reached values of approximately 10(4). Test samples for few types of characterization schemes were fabricated.

Demonstration of an elliptical plasmonic lens illuminated with radially-like polarized field.

We demonstrate an elliptically symmetric plasmonic lens that is illuminated by a radially-like polarization field. This illumination function is TM polarized with regard to the plasmonic lens, ensuring optimum coupling of the incident light into surface plasmons polaritons. The structure is analyzed theoretically by using the Green function approach, and a finite difference time domain simulation. Both approaches provide similar results. Specifically we calculate and experimentally measure the field distribution on the surface and a few microns above it. The results show strong dependency of the electric field distribution on the eccentricity of the elliptic structure and the illumination wavelength. The interference of surface plasmons generates a structured pattern consisting of distinct peaks distributed inside the ellipse with locations that are wavelength dependent. This pattern can be used in several applications including structured illumination microscopy, particles beam trapping and sensing.

Generation and tight focusing of hybridly polarized vector beams.

We propose and experimentally demonstrate the generation of hybridly polarized beams by transmitting radially polarized light through a wave plate. We show that such beams span a closed circle on the surface of the Poincaré sphere whose center coincides with the center of the sphere. In addition we numerically investigate the field and energy density distribution across the focal plane of a high NA lens illuminated by such a hybrid beam. The results show an interesting polarization distribution with 3D orientation and space variant ellipticity. This kind of polarization distributions may be used for a variety of applications, e.g. particle orientation analysis, microscopy and in atomic systems.

Demonstration of spatially inhomogeneous vector beams with elliptical symmetry.

We experimentally demonstrate vector beams having an elliptical symmetry of polarization, breaking the cylindrical symmetry of vector beams (e.g., radially polarized beams). Applications of such beams vary from material processing, lithography, and optical memories to excitation of elliptically shaped nanoparticles and plasmonic structures.

Demonstration of nanofocusing by the use of plasmonic lens illuminated with radially polarized light.

We experimentally demonstrate the focusing of surface plasmon polaritons by a plasmonic lens illuminated with radially polarized light. The field distribution is characterized by near-field scanning optical microscope. A sharp focal spot corresponding to a zero-order Bessel function is observed. For comparison, the plasmonic lens is also measured with linearly polarized light illumination, resulting in two separated lobes. Finally, we verify that the focal spot maintains its width along the optical axis of the plasmonic lens. The results demonstrate the advantage of using radially polarized light for nanofocusing applications involving surface plasmon polaritons.

Radial polarization interferometer.

We propose and demonstrate a new interferometric approach in which a uniform phase difference between the arms of the interferometer manifests itself as spatially varying intensity distribution. The approach is based on interfering two orthogonal spatially varying vector fields, the radially and azimuthally polarized beams, and measuring the projection of the obtained field on an analyzer. This method provides additional spatial information that can be used to improve the smallest detectable phase change as compared with a conventional Michelson interferometer.

Generation of a radially polarized light beam using space-variant subwavelength gratings at 1064 nm.

The generation of radially polarized beams at a wavelength of 1064 nm by the use of a polarization transformer device consisting of space-variant subwavelength gratings (SGs) is demonstrated experimentally. The SG generates a pi phase retardation between the TE and TM polarizations, acting as a half-wave plate, reflecting the polarization vector with respect to the axes of the plate. The polarization transformer is characterized by polarization analysis and by far-field measurements. The characterization results show good agreement with theory. The device is suitable for operation with Nd:YAG lasers; thus it is attractive for biological, optical tweezers, and material processing applications.

Effect of radial polarization and apodization on spot size under tight focusing conditions.

We study the effect of polarization and aperture geometry on the focal spot size of a high numerical aperture (NA) aplanatic lens. We show that for a clear aperture geometry, illuminating the lens by linear or circular polarization is preferable over radial polarization for spot size reduction applications. For annular aperture and objective lenses of 0.85 NA and above we give the sizes of the inner annulus which constitute the transition points to a state where the radial polarization illumination gives smaller spot size. We analyze the evolution, the profile and the effect of transverse and longitudinal field components in the focal plane, and show that they play an opposite role on the spot size in the cases of circular and radial polarization illumination. We show that in the limit of a very thin annulus the radial polarization approaches the prediction of the scalar theory at high NA, whereas the linear and circular polarizations deviate from it. We verify that the longitudinal component generated by radially polarized illumination produces the narrowest spot size for wide range of geometries. Finally, we discuss the effects of tight focusing on a dielectric interface and provide some ideas for circumventing the effects of the interface and even utilize them for spot size reduction.

Tight focusing of spatially variant vector optical fields with elliptical symmetry of linear polarization.

We study the tight-focusing properties of spatially variant vector optical fields with elliptical symmetry of linear polarization. We found the eccentricity of the incident polarized light to be an important parameter providing an additional degree of freedom assisting in controlling the field properties at the focus and allowing matching of the field distribution at the focus to the specific application. Applications of these space-variant polarized beams vary from lithography and optical storage to particle beam trapping and material processing.