The mechanisms of super-resolution imaging by contact microspherical or microcylindrical nanoscopy remain an enigmatic question since these lenses neither have an ability to amplify the near-fields like in the case of far-field superlens, nor they have a hyperbolic dispersion similar to hyperlenses. In this work, we present results along two lines. First, we performed numerical modeling of super-resolution properties of two-dimensional (2-D) circular lens in the limit of wavelength-scale diameters, λ ≤ D ≤ 2λ, and relatively high indices of refraction, n=2. Our preliminary results on imaging point dipoles indicate that the resolution is generally close to λ/4; however on resonance with whispering gallery modes it may be slightly higher. Second, experimentally, we used actin protein filaments for the resolution quantification in microspherical nanoscopy. The critical feature of our approach is based on using arrayed cladding layer with strong localized surface plasmon resonances. This layer is used for enhancing plasmonic near-field illumination of our objects. In combination with the magnification of virtual image, this technique resulted in the lateral resolution of actin protein filaments on the order of λ/7.
The transmission properties of side-coupled circular cavity systems are studied based on numerical two-dimensional finite-difference time domain modeling. The spatial asymmetry is introduced due to different separations between the circular resonators and side-coupled stripe waveguides. These structures can be viewed as 4-port routers where different ports are connected due resonant coupling between the guided modes in stripe-waveguides and whispering gallery modes in circle resonators. It is found that due to strongly asymmetric geometry, significant optical losses, and mode conversion processes, such structures display strongly asymmetric optical transmission properties for the waves propagating in forward and backward directions between the ports. In non-optimized single microcavity structures, it results in isolation ratios on the order of 10 dB for wavelengths resonant with WGMs. In structures formed by two closely spaced circular resonators, WGMs are strongly coupled leading to formation of bonding and antibonding photonic molecular modes. It is shown that at the wavelengths resonant with hybridized molecular modes the isolation ratios can be increased beyond 20 dB. At the same time, different wavelengths can be preferentially coupled to different ports resulting in wavelength demultiplexing functionality.
In recent years, optical super-resolution by microspheres and microfibers emerged as a new paradigm in nanoscale label-free and fluorescence imaging. However, the mechanisms of such imaging are still not completely understood and the resolution values are debated. In this work, the fundamental limits of super-resolution imaging by high-index barium-titanate microspheres and silica microfibers are studied using nanoplasmonic arrays made from Au and Al. A rigorous resolution analysis is developed based on the object’s convolution with the point-spread function that has width well below the conventional (~λ/2) diffraction limit, where λ is the illumination wavelength. A resolution of ~λ/6-λ/7 is demonstrated for imaging nanoplasmonic arrays by microspheres. Similar resolution was demonstrated for microfibers in the direction perpendicular to the fiber axis with hundreds of times larger field-of-view in comparison to microspheres. Using numerical solution of Maxwell’s equations, it is shown that extraordinary close point objects can be resolved in the far field, if they oscillate out of phase. Possible super-resolution using resonant excitation of whispering gallery modes is also studied.
Resonant light pressure effects provide new degrees of freedom for optical manipulation of microparticles. In particular, they can be used for optical sorting of photonic atoms with extraordinary uniform resonant properties. These atoms can be used as building blocks of structures and devices with engineered photonic dispersions. To study the spectral shape of the force peaks, we developed a method to precisely control the wavelength detuning between the tunable laser emission line and central position of the whispering gallery mode (WGM) peaks in tapered fiber-to-microsphere water-immersed couplers. Our method is achieved by integrating optical tweezers to individually manipulate microspheres and based on preliminary spectral characterization of WGM peak positions followed by setting a precise amount of laser wavelength detuning for optical propulsion experiments. We demonstrated dramatic enhancement of the optical forces exerted on 20 μm polystyrene spheres under resonant conditions. Spectral properties of the resonant force enhancement were studies with controlled laser line detuning. In addition, we observed the dynamics of radial trapping and longitudinal propelling process and analyzed their temporal properties. Our studies also demonstrated a stable radial trapping of microspheres near the surface of tapered fiber for high speed resonant optical propulsion along the fiber.
The unique properties of semiconductor nanowires pose promising applications in optoelectronics such as photo-detectors
and lasers. Owing to the increased surface/volume ratio, nanowire-based p-n junctions exhibit qualitatively
different properties from those of bulk cases. These include weaker electrostatic screening and stronger fringe field
effects. This work employs a general device simulator, PROPHET, to numerically investigate the unique electrical
properties of p-n junctions in single nanowires and nanowire arrays. The implications of such effects in nanowire-based
photo-detector design are also examined.
We explore the possibility of coating semiconductor nanowires with metal (Ag) to reduce the size of nanowire
lasers operating at photon energies around 0.8-2 eV. Our results show that the material gain of a typical III-V
semiconductor in nanowire may be sufficient to compensate Joule losses of such metal as Ag. The most promising
mode to achieve lasing is TM01 near its cutoff. To calculate the guiding properties of metal coated nanowires, we
developed a finite-difference discretization approach, the details of which we also present. This approach allowed
us to treat accurately the large index contrast of the nanowire/metal interface and to include nonperturbatively
the imaginary parts of dielectric constants of the semiconductor core and metal coating.
We propose to excite surface plasmon-polaritons using the moving spot of nonlinear polarization created by a laser pulse. Two perspective excitation schemes - with superluminal and subluminal spots - are considered and their efficiencies are compared. These techniques can be used for surface spectroscopy at terahertz frequencies.
We review the basic electromagnetic properties of semiconductor nanowires which are required to evaluate their performance as lasers. These properties include the dispersions for guided modes, mode spacing, reflectivities from the nanowire facets, directionality and
polarization of far fields, and confinement factors. We also discuss
features that distinguish nanowire lasers from the usual
The dynamics of electromagnetic fields in open waveguiding structures whose dielectric properties are changed in time due to rapid plasma creation is considered. The phenomenon of transient coupling between bulk and surface waves is found. The applicability of this phenomenon for input/output of electromagnetic energy is discussed for simple waveguiding structures with both semiconductor and gaseous plasmas. In particular, a component for transient input of an electromagnetic wave into a planar dielectric waveguide covered with a nonstationary semiconductor film is proposed. Using the example of standing waves we show that the transient dynamics is sensitive to the values of the electromagnetic fields at the moment of the plasma density growth. The knowledge of the transient dynamics in rapidly ionized material can be applied for the control of guided modes in open waveguiding structures of integrated optics and (sub)millimeter wave electronics, rapid manipulation by coupling between electromagnetic radiation and guided modes, and ultrafast transient spectroscopy of an electron-hole plasma in semiconductors.
We outline an engineering approach to modeling the optical properties of semiconductor quantum wells which are driven by a growth-direction polarized electric field at frequency in the THz range. The approach is based on solving the Schroedinger equation for the electron-hole envelope wavefunction with inclusion of the excitonic effects. Unlike the usual case of a dc applied field when the optical response is a time-independent function, the presence of the THz field requires introduction of a response function with periodicity given by the THz period. Our focus is on the linear, with respect to the optical power, regime while the THz field can be strong and thus must be accounted nonperturbatively.