Slow Group Velocity Research Articles

Printed Magnetoinductive-Wave (MIW) Delay Lines for Chipless RFID Applications

A novel fully passive and electromagnetic chipless radiofrequency identification (RFID) system is proposed. The system is based on printed tags implemented with magnetoinductive-wave (MIW) delay lines. Such lines are composed of a periodic array of coupled square split ring resonators (SSRRs) and propagate slow waves. The tag is codified by introducing reflectors (which provide the identification signature) between the elements of the array. When the tags are interrogated with a pulse in time domain, they produce replicas at the positions where the reflectors are placed. Thanks to the slow group velocity of the MIW delay line, the replicas of the original pulse are not overlapped in time domain and can be demodulated, thus providing the identification code of the tag. The design considerations to implement these chipless tags are studied in the present work. Moreover, a complete set of codified MIW lines for a two-bit system is designed, manufactured and measured. The reported experimental results validate the proposed approach.

Solar energy trapping with modulated silicon nanowire photonic crystals

We demonstrate the efficacy of nanostructured thin film silicon solar cells to trap and absorb approximately 75% of all sunlight incident (400 nm–1200 nm) with an equivalent bulk thickness of only 1 micron of silicon. This is achieved by sculpting the collection zone into a three-dimensional, simple-cubic-symmetry, photonic crystal consisting of modulated silicon nanowires embedded in SiO2 and sitting on a quartz substrate with no metallic mirrors. A specific modulation of the radius of nanowires provides antireflection, strong light trapping, and back-reflection mechanisms in targeted spectral regions. This modulation is linear at the top of the nano-rods leading to nanocones at the solar cell to air boundary. These silicon nanocones are very good absorbers at short wavelengths and act as broadband coupler to a light-trapping region below at longer wavelengths. In the light trapping region the modulation is periodic to form a simple cubic photonic crystal exhibiting a broad spectrum of strong parallel interface refraction resonances. Here, light incident from most angles is deflected into slow group velocity modes with energy flow nearly parallel to the interface, long dwell times, and strong light intensity enhancement (up to 150 times the incident intensity) in specific regions. Finally, a stronger and chirped modulation of the nanowire underneath provides back-reflection by means of a one-dimensional depth-dependent photonic stop-gap. The possibility of absorbing light at energies below the electronic band gap of silicon is illustrated using a graded index SixGe1−x alloy in the bottom section of each nanowire. Each nanowire is amenable to a radial P-N junction for proximal charge carrier separation and efficient collection of photo-generated current.

Circuit model for hybridization modes in metamaterials and its analogy to the quantum tight‐binding model

AbstractWe formulate a general method to analyze hybridization modes in metamaterials by employing electrical circuit models. By rewriting circuit equations for coupled inductor–capacitor circuit networks in the bra‐ket notation, we show that there exists a good analogy between circuit models and quantum tight‐binding models in solid‐state physics. Our picture is also applicable to transmission systems that do not have tightly bound eigenmodes.This analogy enables the synthesis of various electromagnetic metamaterials having dispersion relations similar to those for electrons in solids and consequently allows the systematic construction of metamaterials with desired characteristics. In this paper, we focus on the formation of flat bands in metamaterials with specific symmetries. The flat band, which corresponds to a slow group velocity of light or a heavy photon, is useful in designing functional metamaterials.

Pumping-power-dependent photoluminescence angular distribution from an opal photonic crystal composed of monodisperse Eu^3+/SiO_2 core/shell nanospheres

High quality opal photonic crystals (PhCs) were successfully fabricated by self-assembling of monodisperse Eu(3+)/SiO(2) core/shell nanospheres. Angular resolved photoluminescence (PL) spectra of a PhC sample were measured with different pumping powers, and its PL emission strongly depended on spectroscopic position of the photonic stop band and the optical pumping power. Suppression of the PL occurred in the directions where the emission lines aligned with the center of the photonic stop band. Suppression and enhancement of the PL were observed at low- and high-pumping powers, respectively, in the directions where the emission lines were located at the edges of the photonic stop band. When pumping power exceeded 6 µJ/pulse, a super-linear dependence was found between the pumping power and PL intensity. The dramatic enhancement of PL was attributed to the amplification of spontaneous emission resulted from the creation of large population inversion and the slow group velocity of the emitted light inside the PhC. The opal PhC provided highly angular-selective quasi-monochromatic PL output, which can be useful for a variety of optical applications.

Telecommunications-band heralded single photons from a silicon nanophotonic chip

We demonstrate room temperature heralded single photon generation in a CMOS-compatible silicon nanophotonic device. The strong modal confinement and slow group velocity provided by a coupled resonator optical waveguide produced a large four-wave-mixing nonlinearity coefficient γeff≈4100 W−1 m−1 at telecommunications wavelengths. Spontaneous four-wave-mixing using a degenerate pump beam at 1549.6 nm created photon pairs at 1529.5 nm and 1570.5 nm with a coincidence-to-accidental ratio exceeding 20. A photon correlation measurement of the signal (1529.5 nm) photons heralded by the detection of the idler (1570.5 nm) photons showed antibunching with g(2)(0)=0.19±0.03. The demonstration of a single photon source within a silicon platform holds promise for future integrated quantum photonic circuits.

The connection between electromagnetically induced transparency in the Zeeman configuration and slow light in hot rubidium vapor

We experimentally studied Zeeman electromagnetically induced transparency (EIT) resonances and slow light propagation in hot Rb vapor. Propagation of weak σ− polarized light pulses in the presence of stronger σ+ polarized background through Rb vapor was realized using a single laser beam and the Pockels cell. The dependences of slow light group velocity and fractional pulse delay on the overall laser beam intensity and temporal pulse length showed that lower optical power and longer light pulses lead to improved EIT and slow light features. The connection between EIT and slow light was also investigated showing that narrower and more contrasted EIT resonances are necessary for further decreasing a group velocity.

High-efficiency acousto-optical interaction in phoxonic nanobeam waveguide

We demonstrate the simultaneous existence of slow photonic and phononic modes in phoxonic nanobeam. The phoxonic nanobeam is formed by arranging air semi-cylinders along lateral sides of a suspended silicon waveguide. Because of the slow group velocities, the acousto-optical interactions are dramatically enhanced. The efficiencies of interaction are strongly related to the polarizations of both slow photonic and phononic modes. Our proposed structure is a potential high-efficiency acousto-optical modulator with ultra-small footprint size. The operating optical wavelength is about 1550 nm, while the acoustic frequency is about 6.8 GHz.

Cassini observation of Jovian anomalous continuum radiation

Jovian anomalous continuum is a narrowband electromagnetic radiation near 10 kHz that can escape from Jupiter's magnetosphere to interplanetary space. One possible source mechanism is the magnetosheath re‐radiation of the Jovian low frequency radio emissions such as the quasiperiodic (QP) radio emissions, broadband kilometric radiation (bKOM) and non‐thermal continuum. Jovian anomalous continuum was consistently observed by the Cassini Radio and Plasma Wave Science instrument from 2000 to 2004, right before the Saturn orbit insertion, which means the radiation can be detected as far as 8 AU away from Jupiter. An analysis of intensity versus radial distance shows that the Jovian anomalous continuum has a line source rather than a point source, consistent with the theory that the emission is radiated by the whole length of the magnetotail. The emissions are modulated at the system III period of Jupiter and are unpolarized. Since the lower cutoff frequency of the anomalous continuum is related to the plasma frequency in the magnetosheath of Jupiter, which is a function of solar wind density, the recurrent variations of the lower cutoff frequency can be used as a remote diagnostic of the solar wind condition at Jupiter. We propose that the frequency dispersion, a unique characteristic of the anomalous continuum, is likely a comprehensive effect of both the slow group velocity near the local plasma frequency and the refraction/scattering of the waves by density structures as they propagate in the magnetosheath.

ELECTROMAGNETICALLY INDUCED QUANTUM LATTICE SOLITON

An optical atomic discrete system through optical induction is proposed. A theoretical scheme to produce quantum discrete or lattice solitons (QLSs) in the system is presented with the use of the effects of enhanced self-phase modulation and cross-phase modulation through the giant kerr effect in the electromagnetically induced transparency. The power density and the photon flux can be tuned to a very low level by the coupling field and the soliton can propagate with very slow group velocity.

Photonic-chip-based tunable slow and fast light via stimulated Brillouin scattering

We report the first (to our knowledge) demonstration of photonic chip based tunable slow and fast light via stimulated Brillouin scattering. Slow, fast, and negative group velocities were observed in a 7 cm long chalcogenide (As(2)S(3)) rib waveguide with a group index change ranging from ~-44 to +130, which results in a maximum delay of ~23 ns at a relatively low gain of ~23 dB. Demonstration of large tunable delays in a chip scale device opens up applications such as frequency sensing and true-time delay for a phased array antenna, where integration and delays ~10 ns are highly desirable.

Enhanced spontaneous emission from quantum dots in short photonic crystal waveguides

We report a study of the quantum dot (QD) emission in short photonic crystal waveguides. We observe that the quantum dot photoluminescence intensity and decay rate are strongly enhanced when the emission energy is in resonance with Fabry-Perot (FP) cavity modes in the slow-light regime of the dispersion curve. The experimental results are in agreement with previous theoretical predictions and are further supported by three-dimensional finite element simulations. Our results show that the combination of slow group velocity and Fabry-Perot cavity resonance provide an avenue to efficiently channel photons from quantum dots into waveguides for integrated quantum photonic applications.

Nonlinear theory of mode conversion at plasma frequency

An electromagnetic (EM) wave at the mode conversion layer transfers its energy to the electrostatic (ES) wave whose amplitude can be greatly enhanced due to its slow group velocity near the cut off layer. The large amplitude ES wave produces the density compression, indispensable for the nonlinear coupling in the EM wave, leading to the localized vortex flow. A dc magnetic field, tantamount to the vorticity, is generated as a result of the conservation of canonical momentum.

Slow Light Properties of 2D Photonic Crystal Waveguide for Optical Storage in Optical Computers

Slow light properties of the photonic crystal line-defect waveguide are researched with the plane wave expansion method. The structure of the waveguide is adjusted with several methods mentioned above at the same time and the slow light properties get better. For the structure of dielectric rods, central frequency and the group velocity of the guided modes decrease with the increase of the radii of the defected rods as well as the dielectric constant. Effects on the slow light from the change of the defected rods’ position are also studied, through moving the rods up and down; we get the almost linear guide mode which has flat slow light curve and smaller group velocity. In a word, group velocity of the slow light is mainly affected by the radii and dielectric constant of the defected rods, and group velocity dispersion is decided by the change of the defected rods’ location.

Photonic-crystal-waveguide based Mach-Zehnder interferometer for terahertz switch and modulator

We propose a novel tunable photonic crystal (PC) waveguide Mach-Zehnder interferometer (MZI) based on nematic liquid crystals (LCs) 5CB and investigate its interference properties numerically by using the finite-difference time-domain method. We can change the refractive index of LC by rotating the directors of the LC molecules. The line defect modes of the PC waveguide with different liquid crystal refractive indices are analyzed by using the plane wave expansion method. Owing to the slow group velocity region of the line defect mode, when the index of 5CB changes from 1.53 to 1.63, the variation of the effective index of the line defect mode arrives at 0.168. This property helps to significantly control the phase of light propagation in a PC waveguide MZI. The novel interferometer can be used as either an optically controlled on-off switch or an amplitude modulator in optical circuits.

Slow surface plasmon polaritons with a large normalized delay bandwidth product in an ultracompact metal gap superlattice

We propose a metal gap superlattice that can decrease the group velocity of surface plasmon polaritons (SPPs) with a large normalized delay bandwidth product (NDBP) and small high-order dispersion. The length of the superlattice is only 8.60μm and the transmission is 0.4. The multiple reflections of photons in the nanocavity result in slow SPP group velocity. The numerical analysis exploiting the transfer matrix method (TMM) is confirmed by the finite-difference time-domain (FDTD) numerical simulations. In addition, we also analyze the reason for pulse deformation.

Lattice Solitons in Optical Lattice Controlled by Electromagnetically Induced Transparency

An optical four-level atomic discrete system in a solid through optical induction is proposed. A theoretical scheme to produce lattice solitons (LSs) in the system is presented by means of the effects of enhanced self-phase modulation and the giant kerr effect in the electromagnetically induced transparency. The power density and the photon flux can be tuned to a very low level by the controlling field and the soliton can propagate with very slow group velocity. By changing the sign of the detuning Δ 1 , both in-phase and π out-of-phase LSs can be produced in this system.

Bunching-induced optical nonlinearity and instability in cold atoms [Invited

We report a new nonlinear optical process that occurs in a cloud of cold atoms at low-light-levels when the incident optical fields simultaneously polarize, cool, and spatially-organize the atoms. We observe an extremely large effective fifth-order nonlinear susceptibility of χ(⁵) = 7.6 × 10⁻¹⁵ (m/V)⁴, which results in efficient Bragg scattering via six-wave mixing, slow group velocities (∼ c/10⁵), and enhanced atomic coherence times (> 100 μs). In addition, this process is particularly sensitive to the atomic temperatures, and provides a new tool for in-situ monitoring of the atomic momentum distribution in an optical lattice. For sufficiently large light-matter couplings, we observe an optical instability for intensities as low as ∼ 1 mW/cm² in which new, intense beams of light are generated and result in the formation of controllable transverse optical patterns.

Spontaneous emission dynamics in an omnidirectional waveguide made of photoniccrystals

The spontaneous emission dynamics of atoms embedded in an omnidirectional waveguide(ODWG), a novel optical waveguide, is studied on the basis of the complete reflectionof one-dimensional photonic crystals. With the dispersion curve of the singlewaveguide mode within the photonic band gap and various extents of backgrounddissipation, we characterize the photon–atom interaction in the ODWG. Thephoton emitter of the system is a two-level atom embedded in the low-indexmedium of the multilayer-film ODWG or the atom–ODWG system. Fractionalcalculus, an innovative mathematical method in optical systems, is applied to solvethe equation of motion for this atom–ODWG system. Two kinds of states withdifferent group velocities exhibit totally distinctive dynamical behavior. The highfrequency waveguide mode with a fast group velocity shows fast exponential decayin propagation while the band-edge mode with a slow group velocity displaysnon-Markovian dynamics with non-exponential oscillating time evolution. We thereforesuggest different functions of this atom–ODWG system for these two kinds ofstates. The richness of the physical content of the system is also revealed throughinvestigating the dynamical behavior of the band-edge mode. These results aid infurther application and fundamental understanding of the atom–ODWG system.

Absorbing boundary conditions for low group velocity electromagnetic waves in photonic crystals

We present an efficient method for the absorption of slow group velocity electromagnetic waves in photonic crystal waveguides (PCWs). We show that adiabatically matching the low group velocity waves to high group velocity waves of the PCW and extending the PCW structure into the perfectly matched layer (PML) region results in a 15 dB reduction of spurious reflections from the PML. We also discuss the applicability of this method to structures other than PCWs.

Observation of tunneling of slow and fast electromagnetic modes in coupled periodic waveguides

We report the experimental observation of tunneling of slow and fast electromagnetic modes in coupled periodic waveguides shifted longitudinally by half of modulation period. According to the symmetry analysis, such a coupler supports two electromagnetic modes with exactly matched slow or fast group velocities but different phase velocities for frequencies close to the edge of the photonic band. We confirm the predicted properties of the modes by directly extracting their dispersion and group velocities from the near-field measurements using specialized Bloch-wave spectral analysis method.