Showing papers by "Qiwen Zhan published in 2013"

Vectorial optical field generator for the creation of arbitrarily complex fields

Abstract: Generation of vectorial optical fields with complex spatial distribution in the cross section is of great interest in areas where exotic optical fields are desired, including particle manipulation, optical nanofabrication, beam shaping and optical imaging. In this work, a vectorial optical field generator capable of creating arbitrarily complex beam cross section is designed, built and tested. Based on two reflective phase-only liquid crystal spatial light modulators, this generator is capable of controlling all the parameters of the spatial distributions of an optical field, including the phase, amplitude and polarization (ellipticity and orientation) on a pixel-by-pixel basis. Various optical fields containing phase, amplitude and/or polarization modulations are successfully generated and tested using Stokes parameter measurement to demonstrate the capability and versatility of this optical field generator.

Transverse mode switchable fiber laser through wavelength tuning

Abstract: We report a fiber laser design that is capable of producing switchable transverse modes through wavelength tuning. The transverse mode switching is realized by exploiting the particular transverse mode-wavelength association characteristics of the few-mode fiber Bragg grating. Different transverse mode outputs with high spatial mode quality can be obtained by adjusting the oscillating wavelength with a tunable filter within the fiber laser cavity. For each of the spatial mode outputs, the laser operates at the corresponding single wavelength with narrow linewidth. Through adding polarization controllers in the laser cavity, output modes with cylindrical vector polarization are also realized.

Demonstration of beam steering via dipole-coupled plasmonic spiral antenna

Abstract: Optical antennas have been utilized to tailor the emission properties of nanoscale emitters in terms of the intensity, directivity and polarization. In this letter, we further explore the capability of beam steering via the use a spiral plasmonic structure as a transmitting antenna. According to both numerical simulation and experimental observations, the beaming direction can be steered through introducing a displacement of the feeding point to the spiral antenna from the geometrical center. For a 3-turn Archimedes' spiral antenna, experimental results show that steering angles of 3° and 7° are obtainable when the excitation location is transversally shifted from the center by a displacement of 200 nm and 500 nm, respectively. Furthermore, the emitted photons carry spin angular momentum determined by the chirality of the spiral optical antenna. A steerable nanoscale spin photon source may find important applications in single molecule sensing, quantum optical information processing and integrated photonic circuits.

Detecting orbital angular momentum through division-of-amplitude interference with a circular plasmonic lens

Abstract: We demonstrate a novel detection scheme for the orbital angular momentum (OAM) of light using circular plasmonic lens. Owing to a division-of-amplitude interference phenomenon between the surface plasmon waves and directly transmitted light, specific intensity distributions are formed near the plasmonic lens surface under different OAM excitations. Due to different phase behaviors of the evanescent surface plasmon wave and the direct transmission, interference patterns rotate as the observation plane moves away from the lens surface. The rotation direction is a direct measure of the sign of OAM, while the amount of rotation is linked to the absolute value of the OAM. This OAM detection scheme is validated experimentally and numerically. Analytical expressions are derived to provide insights and explanations of this detection scheme. This work forms the basis for the realization of a compact and integrated OAM detection architect that may significantly benefit optical information processing with OAM states.

Detecting orbital angular momentum through division-of-amplitude interference with a circular plasmonic lens

Abstract: We demonstrate a novel detection scheme for the orbital angular momentum (OAM) of light using circular plasmonic lens. Owing to a division-of-amplitude interference phenomenon between the surface plasmon waves and directly transmitted light, specific intensity distributions are formed near the plasmonic lens surface under different OAM excitations. Due to different phase behaviors of the evanescent surface plasmon wave and the direct transmission, interference patterns rotate as the observation plane moves away from the lens surface. The rotation direction is a direct measure of the sign of OAM, while the amount of rotation is linked to the absolute value of the OAM. This OAM detection scheme is validated experimentally and numerically. Analytical expressions are derived to provide insights and explanations of this detection scheme. This work forms the basis for the realization of a compact and integrated OAM detection architect that may significantly benefit optical information processing with OAM states.

Compact flattop laser beam shaper using vectorial vortex

Abstract: In this paper, we demonstrate a compact flattop beam shaper to realize two-dimensional flattop focus through generating a second order full Poincare beam. Liquid crystal material is used in the device as the voltage-dependent birefringent material to provide appropriate phase retardation modulation. The beam shaper is fabricated and tested. Experimental results show that high quality flattop profiles can be obtained with steep edge roll-off. The tolerance of different input beam sizes of the beam shaper is also verified in the experimental demonstration.

Electric field sensor based on electro-optic polymer refilled silicon slot photonic crystal waveguide coupled with bowtie antenna

Abstract: We present the design of a compact and highly sensitive electric field sensor based on a bowtie antenna-coupled slot photonic crystal waveguide (PCW). An electro-optic (EO) polymer with a large EO coefficient, r 33 =100pm/V, is used to refill the PCW slot and air holes. Bowtie-shaped electrodes are used as both poling electrodes and as receiving antenna. The slow-light effect in the PCW is used to increase the effective in-device r 33 >1000pm/V. The slot PCW is designed for low-dispersion slow light propagation, maximum poling efficiency as well as optical mode confinement inside the EO polymer. The antenna is designed for operation at 10GHz.

Modified bow-tie antenna with strong broadband field enhancement for RF photonic applications

Abstract: We present a novel design of a subwavelength modified bow-tie antenna that is capable of generating strong broadband field enhancement in its extended feed gap. This modified bow-tie antenna is comprised of a conventional bow-tie antenna with capacitive extended bars attached to the apex points of the bow-tie. The feed gap between the two capacitive bars is separated with a deep subwavelength width for the generation of enhanced local electrical field. Three-dimensional finite element method model is utilized to systematically explore the properties of this antenna design. Through adjusting the bow-tie geometry and the substrate properties, the antenna structure is optimized with a central resonant frequency at 100 GHz. Highly enhanced electrical field is created between the extended bar under radio frequency (RF) illumination. With the optimized design, numer ical simulations show that a uniform field enhancement of more than 200 through the entire feed gap with a bandw idth of 40 GHz can be achiev ed. The strongly enhanced RF field within the gap can be applied to directly modulate guided optical wave propagating in a waveguide embedded in the substrate underneath the feed gap. This work builds up a bridge between devices in the RF and optical frequency regimes that may find many potential applications in RF photonic devices and systems. Keywords: Modified bow-tie antenna, broadband strong field enhancement, RF photonic applications

Potential energy profile of colloidal nanoparticles in optical confinement

Abstract: An optical bottle method is developed to determine the potential-energy profile of colloidal Rayleigh nanoparticles in an optical trap. The three-dimensional distribution of fluorescent particles in the trap is measured by laser scanning confocal fluorescence microscopy. At sufficiently low concentrations at which interactions between the particles are negligible, the single-particle trapping potential-energy profile is determined from the equilibrium number-density profile by use of the Boltzmann distribution. Fluorescence imaging as well as calculations based on a discrete dipole approximation show that effects due to scattering forces are negligible for polystyrene particles of size less than 10% of the wavelength of the trapping laser, thus justifying the assumption of conservative forces in the equilibrium potential-energy determinations. The new optical bottle method measures the entire two-dimensional trapping-potential profile for an individual nanoparticle without the restriction that only one particle be contained in the trap, thus obviating the need for high laser power.

Improving deep subwavelength imaging through terminal interface design of metallo-dielectric multilayered stacks

Abstract: Symmetric metallo-dielectric multilayered stacks (MDMS) are investigated to improve the spatial resolution of subwavelength imaging operated in canalization regime Simulation results revealed that subwavelength imaging capability is very sensitive to the thickness and material of the MDMS terminal layers Furthermore, the coupling and decoupling of the Bloch modes in MDMS, between the object and image space, strongly depend on the terminal layer parameters which can be tuned to achieve the optimal imaging improvement In contrast to metal-dielectric periodic MDMS, using MDMS with the developed symmetric surface termination, subwavelength imaging with optimal intensity throughput and improved field spatial resolution (∼204% ) can be obtained Moreover, optical singularity, in the form of Poynting vector saddle point, has been found in the free space after lens exit for the two kinds of symmetric MDMS that exhibit improved superresolution imaging performance with 100% energy flux visibility The improved subwavelength imaging capabilities, offered by this proposed termination design method, may find potential applications in the areas of biological imaging, sensing, and deep subwavelength lithography, and many others

Mapping two-dimension trapping potential of nanoparticles in an optical trap

Abstract: Combining confocal microscopy and optical tweezers, we map out the spatial distribution of the particle concentrations of quantum dots, fluorescent HIV pseudo virus particles and polystyrene nanospheres in an optical trap. By analyzing the Boltzmann distribution of local particle concentrations, we obtain the two-dimension single particle trapping potential profile at the center of the optical trap in the direction perpendicular to the beam propagation. We compare the trapping potential energies of pseudo HIV vesicles and same-sized polystyrene spheres. We also compare the trapping potential energy of polystyrene spheres of a focused Gaussian beam and two modes of cylindrical vector beams.

Dynamically steerable nanoscale photon source

Abstract: In metals or semiconductors, free electrons can move between atoms, giving rise to an electric current. However, highfrequency surface waves called plasmons can move by charge density oscillation of the free electrons without a net current. With a trend toward increasingly miniature devices and demand for higher information transmission speeds, interest in plasmonics is rising. Plasmonic antennas are nanoscale versions of conventional radio-frequency antennas. They work at optical frequencies where the interactions between the radiation and antennas are governed by plasmonics, and offer unprecedented capability to manipulate light-matter interaction on the nanoscale. Control of photon properties is key to a wide range of applications such as optical sensing and integrated photonic circuits, and a variety of optical antennas have previously been demonstrated to tailor the properties of photons from quantum emitters. Most of the previously reported optical antennas aimed to control only one or two properties. However, we recently showed that comprehensive control of emissive characteristics (in terms of intensity, angular momentum, and directivity, i.e., the ability to radiate into a desired direction) can be realized by coupling emitters to a spiral plasmonic antenna.1–3 In addition, many potential applications, such as chip-to-chip and board-toboard optical interconnects, require the ability to dynamically steer the photons radiated from the emitters. We have successfully demonstrated such a capability by conveniently adjusting the feed point location of a spiral plasmonic antenna.4 Figure 1 illustrates a simple model to understand the underlying physics of the proposed beam steering concept. For a device (in this study, a spiral plasmonic antenna) with an aperture size of D, the emission peak is normal to the surface when the emitter (optical feed) is located at the geometrical center. In this case, the optical path difference (OPD, l) between the two Figure 1. Schematic diagram for the steering angle calculation. The green and red dots stand for the nanoscale emitters with different locations. The color-filled ellipses represent the corresponding far-field emission patterns. OPD: Optical path difference.