Metasurface-Enabled Generation of Circularly Polarized Single Photons

Single photons carrying spin angular momentum (SAM), i.e., circularly polarized single photons generated typically by subjecting a quantum emitter (QE) to a strong magnetic field at low temperatures, are at the core of chiral quantum optics enabling nonreciprocal single-photon configurations and deterministic spin-photon interfaces. Here, a conceptually new approach to the room-temperature generation of SAM-coded single photons (SSPs) is described, which entails QE nonradiative coupling to surface plasmons being transformed, by interacting with an optical metasurface, into a collimated stream of SSPs with the designed handedness. Design, fabrication, and characterization of SSP sources, consisting of dielectric circular nanoridges with azimuthally varying widths deterministically fabricated on a dielectric-protected silver film around a nanodiamond containing a nitrogen-vacancy center, are reported. With properly engineered phases of QE-originated fields scattered by nanoridges, the outcoupled photons are characterized by a well-defined SAM (with the chirality >0.8) and high directionality (collection efficiency up to 92%).     

Transformation design and nonlinear Hamiltonians

We study a class of nonlinear Hamiltonians, with applications in quantum optics. The interaction terms of these Hamiltonians are generated by taking a linear combination of powers of a simple ‘beam splitter’ Hamiltonian. The entanglement properties of the eigenstates are studied. Finally, we show how to use this class of Hamiltonians to perform special tasks such as conditional state swapping, which can be used to generate optical cat states and to sort photons.     

Mode-selective dynamic light scattering: theory versus experimental realization

We present a quantitative experimental comparison of fiber-based, single- and few-mode dynamic light scattering with the classical pinhole-detection optics. The recently presented theory of mode-selective dynamic light scattering [Appl. Opt. 32, 2860 (1993)] predicts a collection efficiency and a signal-tobaseline ratio superior to that of a classical pinhole setup. These predictions are confirmed by our experiments. Using single-mode optical fibers with different cutoff wavelengths and commercially available mechanical components, we have constructed a mode-selective detection optics in a simple and compact dynamic light-scattering spectrometer that permits an optimal compromise between signal intensity and dynamical resolution.     

Channel waveguides of insoluble conjugated polymers

We demonstrate a fast and simple technique of patterning insoluble, nonbleachable, conjugated polymer films into channel waveguides by striploading with photoresist. Two types of coupling gratings were tested for this new system. The coupling parameters, the linear losses and the beam confinement are discussed. This technique makes it possible to explore this class of materials in the field of integrated optics. A new data evaluation method for a more precise measurement of losses in slab waveguides is introduced.     

Plasmonic Luneburg and Eaton lenses

Plasmonics takes advantage of the properties of surface plasmon polaritons, which are localized or propagating quasiparticles in which photons are coupled to the quasi-free electrons in metals. In particular, plasmonic devices can confine light in regions with dimensions that are smaller than the wavelength of the photons in free space, and this makes it possible to match the different length scales associated with photonics and electronics in a single nanoscale device. Broad applications of plasmonics that have been demonstrated to date include biological sensing, sub-diffraction-limit imaging, focusing and lithography and nano-optical circuitry. Plasmonics-based optical elements such as waveguides, lenses, beamsplitters and reflectors have been implemented by structuring metal surfaces or placing dielectric structures on metals to manipulate the two-dimensional surface plasmon waves. However, the abrupt discontinuities in the material properties or geometries of these elements lead to increased scattering of surface plasmon polaritons, which significantly reduces the efficiency of these components. Transformation optics provides an alternative approach to controlling the propagation of light by spatially varying the optical properties of a material. Here, motivated by this approach, we use grey-scale lithography to adiabatically tailor the topology of a dielectric layer adjacent to a metal surface to demonstrate a plasmonic Luneburg lens that can focus surface plasmon polaritons. We also make a plasmonic Eaton lens that can bend surface plasmon polaritons. Because the optical properties are changed gradually rather than abruptly in these lenses, losses due to scattering can be significantly reduced in comparison with previously reported plasmonic elements.     

Comparative evaluation of two simple diffuse reflectance models for biological tissue applications

We present a comparative evaluation of two simple diffuse reflectance models for biological tissue applications. One model is based on a widely accepted and used in biomedical optics implementation of diffusion theory, and the other one is based on a semiempirical approach derived from basic physical principles. We test the models on tissue phantoms and on human skin, utilizing a standard six-around-one optical fiber probe for light delivery and collection. We show that both models are suitable for use with an optical fiber probe and illustrate the potential, applicability, and validity range of the models.     

Depth profiling by confocal Raman microspectroscopy: semi-empirical modeling of the Raman response

It has been well documented that the use of dry optics in depth profiling by confocal Raman microspectroscopy significantly distorts the laser focal volume, thus negatively affecting the spatial resolution of the measurements. In that case, the resulting in-depth confocal profile is an outcome of several contributions: the broadening of the laser spot due to instrumental factors and diffraction, the spreading of the illuminated region due to refraction of the laser beam at the sample surface, and the influence of the confocal aperture in the collection path of the laser beam. Everall and Batchelder et al. developed simple models that describe the effect of the last two factors, i.e., laser refraction and the diameter of the pinhole aperture, on the confocal profile. In this work, we compare these theoretical predictions with experimental data obtained on a series of well-defined planar interfaces, generated by contact between thin polyethylene (PE) films (35, 53, 75, and 105 microm thickness) and a much thicker poly(methyl methacrylate) (PMMA) piece. We included two refinements in the above-mentioned models: the broadening of the laser spot due to instrumental factors and diffraction and a correction for the overestimation in the decay rate of collection efficiency predicted by Batchelder et al. These refinements were included through a semiempirical approach, consisting of independently measuring the Raman step-response in the absence of refraction by using a silicon wafer and the actual intensity decay of a thick and transparent polymer film. With these improvements, the model reliably reproduces fine features of the confocal profiles for both PE films and PMMA substrates. The results of this work show that these simple models can not only be used to assist data interpretation, but can also be used to quantitatively predict in-depth confocal profiles in experiments carried out with dry optics.     

Absorptivity contrast in transillumination imaging of tissue abnormalities

We calculate the time-resolved flux of photons transmitted across an optically turbid slab containing a partially absorbing inclusion. An analytical expression is obtained for the flux at a detector positioned opposite a point source (at a distance equal to the thickness of the slab) when the center of the inclusion lies on the line connecting those points. The calculation employs a discrete-time lattice random-walk model of photon transport. The resulting expression is used to assess the affects of time resolution on the detectability of the inclusion.     

Singlet exciton fission-sensitized infrared quantum dot solar cells

We demonstrate an organic/inorganic hybrid photovoltaic device architecture that uses singlet exciton fission to permit the collection of two electrons per absorbed high-energy photon while simultaneously harvesting low-energy photons. In this solar cell, infrared photons are absorbed using lead sulfide (PbS) nanocrystals. Visible photons are absorbed in pentacene to create singlet excitons, which undergo rapid exciton fission to produce pairs of triplets. Crucially, we identify that these triplet excitons can be ionized at an organic/inorganic heterointerface. We report internal quantum efficiencies exceeding 50% and power conversion efficiencies approaching 1%. These findings suggest an alternative route to circumvent the Shockley-Queisser limit on the power conversion efficiency of single-junction solar cells.     

Evanescent surface-wave modes in a slab with application to miniaturised waveguide components

Characteristics of surface-wave modes in a slab with either negative permittivity or negative permeability are considered. It is shown that this kind of slab is a monomode structure which supports TM mode when the permittivity is negative and TE mode when the permeability is negative. It is shown that tightly bound surface-wave modes exist, also backward waves, in a slab with small thickness. These surface-wave modes will certainly have applications in trying to miniaturise waveguide components in microwave techniques and in optics. As an example, characteristics for a planar plasma waveguide are given     

Electron beam supercollimation in graphene superlattices

Although electrons and photons are intrinsically different, importing useful concepts in optics to electronics performing similar functions has been actively pursued over the last two decades. In particular, collimation of an electron beam is a long-standing goal. We show that ballistic propagation of an electron beam with virtual no spatial spreading or diffraction, without a waveguide or external magnetic field, can be achieved in graphene under an appropriate class of experimentally feasible one-dimensional external periodic potentials. The novel chiral quasi-one-dimensional metallic state that the charge carriers are in originates from a collapse of the intrinsic helical nature of the charge carriers in graphene owing to the superlattice potential. Beyond providing a new way to constructing chiral one-dimensional states in two dimensions, our findings should be useful in graphene-based electronic devices (e.g., for information processing) utilizing some of the highly developed concepts in optics.     

Penetration depth for diffusing-wave spectroscopy

The depth at which diffusing photons are assumed to be deposited in a random scattering medium has traditionally been treated as a phenomenological parameter comparable to the photon transport mean free path. We show how to average properly over an exponential distribution of depositions weighted additionally by the transmission probability, and compare our prediction for the autocorrelation of intensity fluctuations in the transmitted light with experimental data on an ideal system. The improved correlation function, where distinguishable from the prior form, provides slightly better agreement with data as long as the sample is thicker than approximately 10 transport mean free paths. However, in contrast with static transmission, proper averaging over a range of penetration depths does not extend the validity of diffusing-wave spectroscopy to significantly smaller slab thicknesses. The most significant errors in the theory must therefore arise from approximations other than the treatment of the source of diffusing photons.     

Generation of transducers for fluorescence-based microarrays with enhanced sensitivity and their application for gene expression profiling

The present paper describes a novel generation of microchips suitable for fluorescence-based assays, such as cDNA, oligonucleotide, or protein microarrays. The new transducers consist of a fully corrugated surface coated with a thin layer of Ta2O5 as a high refractive index material. Tuning of the incident excitation light beam to abnormal reflection geometry results in a confinement of the energy within the thin metal oxide layer. Consequently, strong evanescent fields are generated at the surface of these microchips and fluorophores located within the fields showed up to a 2 order of magnitude increase in fluorescence intensities relative to the epifluorescence signals. We have attributed this phenomenon as evanescent resonance (ER). Due to the surface architecture, propagation distances of the incident energy and fluorescence photons are in the micrometer range, thus preventing cross talk between adjacent regions. ER microchips offer a significant increase in fluorescence intensities in both "snapshot" fluorescence setups and commercial fluorescence scanners. The underlying principle of the novel chips is explained, and quantitative data on the fluorescence enhancement are provided. To demonstrate their potential, the novel chips are used to investigate the dependence of expression levels from metabolic genes in rat liver on drug treatment. In contrast to competitive hybridization, labeled samples were hybridized to individual ER microchips, and changes were observed by comparing with normalized data from different chips. Results obtained in gene expression profiling experiments with phenobarbital-treated rats are shown.     

Microscopic imaging through a turbid medium by use of annular objectives for angle gating

We report a new method for microscopic imaging of an object embedded in a turbid medium. The new method is based on the angle-gating mechanism achieved by the use of polarized annular objectives in the illumination and collection paths of a microscopic imaging system. A detailed experimental study is presented of the effects of the size of annular obstructions on image quality when turbid media, including polystyrene microspheres and milk suspensions, are imaged. Images of 22-mum polystyrene microspheres embedded in the turbid media show that misinterpretation can occur when circular objectives are used, because of the detection of mainly multiply scattered photons (i.e., diffusing photons). However, when annular objectives are employed, diffusing photons from a turbid medium can be efficiently suppressed; thus image contrast appears correctly, and image resolution is increased.     

Electron focusing and Fermi surface diameter

We perform a simple model calculation of the effects of electron focusing on the transport properties of a semiinfinite metallic slab in the presence of a magnetic field. We consider, in particular, the case in which current is fed into the slab through two superconducting strips, with the field applied parallel to the strips. A thin resistive layer is assumed to exist on the surface of the metal. We treat the cases of short and long mean free paths compared with the separation of the leads. For the latter case we derive an integral equation relating the normally flowing current to the potential at the surface. This equation is used to study the magnetoresistance for three models of the Fermi surface: a free-electron-like surface, a compensated free-electron, free-hole-like surface, and a compensated surface composed of three free-electron-like sheets and one free-hole-like sheet. This latter model is topologically equivalent to the Bi case. The results of the calculation agree qualitatively with the experimental results for fields of the order of and larger than the focusing field.Work supported in part by the National Science Foundation through Grant GH 34438.     

Limits to remote molecular detection via coherent anti-Stokes raman spectroscopy using a maximal coherence control technique

ABSTRACT

The demand for remote molecular detection has been rising in recent years. The technique of coherent anti-Stokes Raman spectroscopy (CARS) has become one of the most optimal methods due to its high efficiency, fast response time and ease of use. In this article, we estimate the number of detectable photons from a CARS signal by using a semiclassical nonlinear optics approach. Several key parameters and their effect on the signal are studied in the following discussion. We also provide a method to prepare the maximum coherence between vibrational states in an effective two level system.     

Analysis of powder X-ray diffraction resolution using collimating and focusing polycapillary optics

Focusing X-ray optics can be used to increase the intensity onto small samples, greatly reducing data collection time. Typically, the beam convergence is restricted to avoid loss of resolution, since the focused beams broaden the resulting powder diffraction rings. However, with smooth Gaussian peaks, the resolution defined by the uncertainty in peak location can be much less than the peak width. Polycapillary X-ray optics were used to collimate and focus X-rays onto standard inorganic powder diffraction samples. Comparisons were made of system resolution and diffracted beam intensity with and without focusing and collimating optics using a standard small spot rotating anode system in point source geometry. The area detector and optics also allowed for the use of a low power 60 W source, without increasing either the collection time or the peak center error compared to the rotating anode no optic case. Resolution and intensity were in good agreement with those obtained from a simple geometrical model developed for the optics, which allows for system design and optimization for the desired sample characteristics. Foils and powders were used to model thin film samples while allowing both reflection and transmission measurements to more effectively verify theoretical modeling of beam parameters.     

Design of a one-dimensional electromagnetic transparent wall

For a one-dimensional (1D) anisotropic slab, if its electromagnetic parameters satisfy certain conditions, an incident wave can be totally transmitted without any reflections. In this work, a classical method based on analytical results and a transformation optics method using an arbitrary piecewise continuous transformation function are applied to design a 1D electromagnetic transparent wall, whose presence does not disturb the field distribution in the ambient environment. Material parameters and the geometrical requirement of the layered structure using these two different methods are derived, and they agree well with each other. Full-wave simulations validate the transparency of the proposed wall. Because of the simple constitutive parameters and geometry, a transparent structure could be realized using anisotropic and homogeneous materials. The proposed structure has potential applications in radomes, anti-reflection films, and various sensor sectors.     

General analysis of slab lasers using geometrical optics

A thorough and general geometrical optics analysis of a slab-shaped laser gain medium is presented. The length and thickness ratio is critical if one is to achieve the maximum utilization of absorbed pump power by the laser light in such a medium; e.g., the fill factor inside the slab is to be maximized. We point out that the conditions for a fill factor equal to 1, laser light entering and exiting parallel to the length of the slab, and Brewster angle incidence on the entrance and exit faces cannot all be satisfied at the same time. Deformed slabs are also studied. Deformation along the width direction of the largest surfaces is shown to significantly reduce the fill factor that is possible.