Showing papers on "Resonance published in 2005"

Resonant optical antennas.

Abstract: We have fabricated nanometer-scale gold dipole antennas designed to be resonant at optical frequencies. On resonance, strong field enhancement in the antenna feed gap leads to white-light supercontinuum generation. The antenna length at resonance is considerably shorter than one-half the wavelength of the incident light. This is in contradiction to classical antenna theory but in qualitative accordance with computer simulations that take into account the finite metallic conductivity at optical frequencies. Because optical antennas link propagating radiation and confined/enhanced optical fields, they should find applications in optical characterization, manipulation of nanostructures, and optical information processing.

Magnetic Metamaterials at Telecommunication and Visible Frequencies

Abstract: Arrays of gold split rings with a 50-nm minimum feature size and with an LC resonance at 200 THz frequency (1.5 microm wavelength) are fabricated. For normal-incidence conditions, they exhibit a pronounced fundamental magnetic mode, arising from a coupling via the electric component of the incident light. For oblique incidence, a coupling via the magnetic component is demonstrated as well. Moreover, we identify a novel higher-order magnetic resonance at around 370 THz (800 nm wavelength) that evolves out of the Mie resonance for oblique incidence. Comparison with theory delivers good agreement and also shows that the structures allow for a negative magnetic permeability.

Phase-locking in double-point-contact spin-transfer devices

Abstract: Spin-transfer in nanometre-scale magnetic devices results from the torque on a ferromagnet owing to its interaction with a spin-polarized current and the electrons' spin angular momentum. Experiments have detected either a reversal or high-frequency (GHz) steady-state precession of the magnetization in giant magnetoresistance spin valves and magnetic tunnel junctions with current densities of more than 10(7) A cm(-2). Spin-transfer devices may enable high-density, low-power magnetic random access memory or direct-current-driven nanometre-sized microwave oscillators. Here we show that the magnetization oscillations induced by spin-transfer in two 80-nm-diameter giant-magnetoresistance point contacts in close proximity to each other can phase-lock into a single resonance over a frequency range from approximately 24 GHz for contact spacings of less than about approximately 200 nm. The output power from these contact pairs with small spacing is approximately twice the total power from more widely spaced (approximately 400 nm and greater) contact pairs that undergo separate resonances, indicating that the closely spaced pairs are phase-locked with zero phase shift. Phase-locking may enable control of large arrays of coupled spin-transfer devices with increased power output for microwave oscillator applications.

Electrostatic (plasmon) resonances in nanoparticles

Abstract: A surface integral eigenvalue based technique for the direct calculation of resonance values of the permittivity of nanoparticles, and hence resonance frequencies, is discussed. General physical properties of electrostatic plasmon resonances are presented. Strong orthogonality properties of resonance modes, a twodimensional phenomenon of “twin” spectrum and explicit estimates of resonance frequencies in terms of geometrical characteristics of convex nanoparticles are reported. Second-order corrections for resonance values of the dielectric permittivity are derived. Tunability and optical controllability of plasmon resonances in semiconductor nanoparticles are discussed and, as a digression, a plausible plasmon resonance mechanism for nucleation and formation of ball lightning is outlined. An efficient numerical algorithm for the calculation of resonance frequencies is developed and illustrated by extensive computational results that are compared with theoretical results and available experimental data.

Playing with Plasmons: Tuning the Optical Resonant Properties of Metallic Nanoshells

Abstract: Nanoshells, concentric nanoparticles consisting of a dielectric core and a metallic shell, are simple spherical nanostructures with unique, geometrically tunable optical resonances As with all metallic nanostructures, their optical properties are controlled by the collective electronic resonance, or plasmon resonance, of the constituent metal, typically silver or gold In striking contrast to the resonant properties of solid metallic nanostructures, which exhibit only a weak tunability with size or aspect ratio, the optical resonance of a nanoshell is extraordinarily sensitive to the inner and outer dimensions of the metallic shell layer The underlying reason for this lies beyond classical electromagnetic theory, where plasmon-resonant nanoparticles follow a mesoscale analogue of molecular orbital theory, hybridizing in precisely the same manner as the individual atomic wave functions in simple molecules This plasmon hybridization picture provides an essential “design rule” for metallic nanostructures that can allow us to effectively predict their optical resonant properties Such a systematic control of the far-field optical resonances of metallic nanostructures is accomplished simultaneously with control of the field at the surface of the nanostructure The nanoshell geometry is ideal for tuning and optimizing the near-field response as a stand-alone surface-enhanced Raman spectroscopy (SERS) nanosensor substrate and as a surface-plasmon-resonant nanosensorTuning the plasmon resonance of nanoshells into the near-infrared region of the spectrum has enabled a variety of biomedical applications that exploit the strong optical contrast available with nanoshells in a spectral region where blood and tissue are optimally transparent

Fast, low-noise, mode-by-mode, cavity-enhanced absorption spectroscopy by diode-laser self-locking

Abstract: A new technique of cavity enhanced absorption spectroscopy is described. Molecular absorption spectra are obtained by recording the transmission maxima of the successive TEMoo resonances of a high-finesse optical cavity when a Distributed Feedback Diode Laser is tuned across them. A noisy cavity output is usually observed in such a measurement since the resonances are spectrally narrower than the laser. We show that a folded (V-shaped) cavity can be used to obtain selective optical feedback from the intracavity field which builds up at resonance. This induces laser linewidth reduction and frequency locking. The linewidth narrowing eliminates the noisy cavity output, and allows measuring the maximum mode transmissions accurately. The frequency locking permits the laser to scan stepwise through the successive cavity modes. Frequency tuning is thus tightly optimized for cavity mode injection. Our setup for this technique of Optical-Feedback Cavity-Enhanced Absorption Spectroscopy (OF-CEAS) includes a 50 cm folded cavity with finesse ∼20 000 (ringdown time ∼20 μs) and allows recording spectra of up to 200 cavity modes (2 cm−1) using 100 ms laser scans. We obtain a noise equivalent absorption coefficient of ∼5×10−10 cm−1 for 1 s averaging over scans, with a dynamic range of four orders of magnitude.

Magnetic vortex resonance in patterned ferromagnetic dots

Abstract: We report a high-resolution experimental detection of the resonant behavior of magnetic vortices confined in small disk-shaped ferromagnetic dots. The samples are magnetically soft Fe-Ni disks of diameter 1.1 and $2.2\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ and thickness 20 and 40 nm, patterned via electron-beam lithography onto microwave coplanar waveguides. The vortex excitation spectra were probed by a vector network analyzer operating in reflection mode, which records the derivative of the real and the imaginary impedance as a function of frequency. The spectra show well-defined resonance peaks in magnetic fields smaller than the characteristic vortex annihilation field. Resonances at 162 and 272 MHz were detected for disks of 2.2- and $1.1\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}\text{\ensuremath{-}}\mathrm{diameter}$ with thickness 40 nm, respectively. A resonance peak at 83 MHz was detected for 20-nm thick, $2\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}\text{\ensuremath{-}}\mathrm{diameter}$ disks. The resonance frequencies exhibit weak field dependence and scale as a function of the dot geometrical aspect ratio. The measured frequencies are well described by micromagnetic and analytical calculations that rely only on known properties of the dots (such as the dot diameter, thickness, saturation magnetization, and exchange stiffness constant) without any adjustable parameters. We find that the observed resonance originates from the translational motion of the magnetic vortex core.

Gigantic transmission band-edge resonance in periodic stacks of anisotropic layers.

Abstract: We consider Fabry-Perot cavity resonance in periodic stacks of anisotropic layers with misaligned in-plane anisotropy at the frequency close to a photonic band edge. We show that in-plane dielectric anisotropy can result in a dramatic increase in field intensity and group delay associated with the transmission resonance. The field enhancement turns out to be proportional to fourth degree of the number N of layers in the stack. By contrast, in common periodic stacks of isotropic layers, those effects are much weaker and proportional to N2 Thus, the anisotropy allows one to drastically reduce the size of the resonance cavity with similar performance. The key characteristic of the periodic arrays with gigantic transmission resonance is that the dispersion curve omega(k) at the photonic band edge has the degenerate form Deltaomega approximately (Deltak)4, rather than the regular form Deltaomega approximatley (Deltak)2. This can be realized in specially arranged stacks of misaligned anisotropic layers. The degenerate band-edge cavity resonance with similar outstanding properties can also be realized in a waveguide environment, as well as in a linear array of coupled multimode resonators, provided that certain symmetry conditions are in place.

Optimizing the process of nuclear magnetic resonance spectrum analysis and computer aided resonance assignment

Abstract: Nuclear magnetic resonance (NMR) spectroscopy is an important method which allows to determine the three-dimensional structure of proteins and other biological macromolecules. The process is based on the concept of sequence-specific resonance assignment, where cross-peaks between sequentially neighboring amino acids are observed in multidimensional NMR spectra and linked to fragments. The fragments can be uniquely mapped on the sequence, if they become sufficiently long, which allows the atoms of the residue types to be assigned to their corresponding chemical shifts. To assign a protein with 100 or more residues, several thousand signals have to be identified and analyzed. This task is very complex and usually takes several month of manual work. Up to now the available tools left most of the complex information management to the operator, who had to take care of plausibility and consistency by hirnself. One of the major achievements of this dissertation is a formal and sufficiently complete information model. This model is able to describe and capture all information coming up during the analysis and resonance assignment of NMR spectra and to ensure consistency. The new software package CARA is a comprehensive implementation of this information model. It follows a semi-automatic approach and causes a significant increase of process efficiency and a decrease of error probability. In contrast to previous solutions Iike XEASY, whose information management is primarily based on peaklists, CARA makes use of a central repository able to manage abstract and semantically interlinked information objects. The availability of this repository allows CARA to dynamically calculate the needed projections (e.g. the cross-peaks expected in a concrete spectrum) by means ot incremental inference algorithms. Further concepts have been developed to simulate the magnetization transter pathways ot NMR experiments, to integrate cross-peaks and to back-calculate and efficiently store NMR spectra.

Application of parametric resonance amplification in a single-crystal silicon micro-oscillator based mass sensor

Abstract: A mass sensing concept based on parametric resonance amplification is proposed and experimentally investigated using a non-interdigitated comb-finger driven micro-oscillator. Mass change can be detected by measuring frequency shift at the boundary of the first order parametric resonance ‘tongue’. Both platinum deposition using focused ion beam (FIB) and water vapor desorption and absorption are used to change the mass of a prototype sensor. Due to the sharp transition in amplitude caused by parametric resonance, the sensitivity is 1–2 order of magnitude higher than the same oscillator working at Simple Harmonic Resonance (SHR) mode in air. Picogram (10 −12 g) level mass change can be easily detected in the sensor with mass about 30 ng and resonance frequency less than 100 kHz. Damping effects and noise processes on sensor dynamics and sensing performance are also investigated and damping has no significant effect on sensor noise floor and sensitivity. Higher sensitivity is expected when the oscillator design is optimized and dimensions are scaled.

Magnetic vortex resonance in patterned ferromagnetic dots

Abstract: A microwave reflection method has been used to measure the spin excitations corresponding to the translational mode of magnetic vortices in samples containing either one or two vortices. Experimental findings are complemented by micromagnetic simulations. One-vortex systems are investigated in micron-sized circular and elliptical cylinders. For ellipses, the resonance frequency can effectively be tuned by applying static magnetic fields and the field dependence of the frequency is significant for fields applied along the short axes but negligible when applied along the long axes of the ellipses. This is contrary to the circular case, where virtually no field dependence was found. This can be understood by considering the shape of the vortex potential well. Further, it is found that the resonance frequency is independent on the direction of the excitation field for the one-vortex systems. Ellipses containing two interacting vortices are also investigated. It is shown that the relative vortex core polarizations dominate the vortex translational mode and cause, in the case of opposite polarizations, a dependence on the excitation field direction. For parallel core polarizations, no dependence on the excitation field direction is found. The dependence of the resonance frequencies on applied static fields along the long and short axes are also experimentally mapped out and compared with micromagnetic simulations, where the possible eigenmodes are determined. Another section of the thesis introduces the dawning of a device based on patterned magnetic elliptical elements for the manipulation and movement of magnetic particles on a surface. The controlled movement and separation of individual particles are successfully demonstrated. Contributions to micromagnetic standard problems and simulations on magnetization switching in nanoscale particles have also been performed. The standard problems highlight some important aspects of choosing the discretization cell sizes and the finite temperature simulations show that thermal fluctuations can alter the magnetization reversal paths.

Effects of spin-exchange collisions in a high-density alkali-metal vapor in low magnetic fields

Abstract: Spin-exchange collisions often play a dominant role in the broadening of Zeeman resonances in an alkali-metal vapor. Contrary to intuitive expectations, at high alkali-metal densities this broadening can be completely eliminated by operating in a low magnetic field, allowing construction of ultrasensitive atomic magnetometers. We describe a detailed study of the Zeeman resonance frequencies and linewidths as a function of the magnetic field, alkali-metal density, and the degree of spin polarization of the atoms. Due to the nonlinear nature of the density matrix equations describing the spin-exchange collisions both the gyromagnetic ratio and the linewidth change as a function of the polarization. The results of experimental measurements are in excellent agreement with analytical and numerical solutions of the density matrix equations.

Near-field optical imaging of plasmon modes in gold nanorods.

Abstract: We have investigated optical properties of single gold nanorods by using an apertured-type scanning near-field optical microscope. Near-field transmission spectrum of single gold nanorod shows several longitudinal surface plasmon resonances. Transmission images observed at these resonance wavelengths show oscillating pattern along the long axis of the nanorod. The number of oscillation increases with decrement of observing wavelength. These spatial characteristics were well reproduced by calculated local density-of-states maps and were attributed to spatial characteristics of plasmon modes inside the nanorods. Dispersion relation for plasmons in gold nanorods was obtained by plotting the resonance frequencies of the plasmon modes versus the wave vectors obtained from the transmission images.

Using Resonance Energy Transfer to Improve Exciton Harvesting in Organic-Inorganic Hybrid Photovoltaic Cells**

Abstract: 141.5, 141.2, 140.7, 140.6, 140.2, 140.0, 139.8, 139.7, 139.2, 138.9, 138.6, 135.2, 134.8, 132.5, 132.4, 132.0, 131.6, 130.7, 130.4, 129.4, 129.1, 128.8, 128.0, 127.3, 126.8, 126.1, 90.7 (CCluster), 90.6 (CCluster), 73.1 (CCluster), 73.0 (CCluster). H and C NMR spectra were recorded in deuterated solvents, such as CD2Cl2, on a Bruker DPX 250 and a Bruker DRX 500 spectrometer, using the proton or carbon signal of the solvent as an internal standard. The thermolysis reactions were carried out in sealed quartz tubes in a temperature controlled electromagnetic oven. SEM measurements were performed on a LEO 1530 field-emission scanning electron microscope. High-resolution TEM studies were conducted on a Philips Tecnai F30 analytical TEM at an operating voltage of 300 kV and on a TEM EM420 electron microscope at an operating voltage of 120 kV. The samples were dispersed in ethanol under ultrasonic irradiation and the suspension was dropped onto a TEM copper grid with a carbon film. TGA measurements were performed on a Mettler Toledo TS0801R0 device at a heating rate of 10 °C min between 0 and 900 °C under an inert atmosphere.

Controlling the resonance of a photonic crystal microcavity by a near-field probe.

Abstract: We demonstrate theoretically that the resonance frequencies of high-Q microcavities in twodimensional photonic crystal membranes can be tuned over a wide range by introducing a subwavelength dielectric tip into the cavity mode. Three-dimensional finite-difference time-domain simulations show that by varying the lateral and vertical positions of the tip, it is possible to tune the resonator frequency without lowering the quality factor. Excellent agreement with a perturbative theory is obtained, showing that the tuning range is limited by the ratio of the cavity mode volume to the effective polarizability of the nanoperturber.

On the resonances and polarizabilities of split ring resonators

Abstract: In this paper, the behavior at resonance of split ring resonators (SRRs) and other related topologies, such as the nonbianisotropic SRR and the broadside-coupled SRR, are studied. It is shown that these structures exhibit a fundamental resonant mode (the quasistatic resonance) and other higher-order modes which are related to dynamic processes. The excitation of these modes by means of a properly polarized time varying magnetic and/or electric fields is discussed on the basis of resonator symmetries. To verify the electromagnetic properties of these resonators, simulations based on resonance excitation by nonuniform and uniform external fields have been performed. Inspection of the currents at resonances, inferred from particle symmetries and full-wave electromagnetic simulations, allows us to predict the first-order dipolar moments induced at the different resonators and to develop a classification of the resonances based on this concept. The experimental data, obtained in SRR-loaded waveguides, are in agreement with the theory and point out the rich phenomenology associated with these planar resonant structures.

Photoconductivity Spectra of Single-Carbon Nanotubes: Implications on the Nature of Their Excited States

Abstract: We have measured the photoconductivity excitation spectra of individual semiconducting carbon nanotubes incorporated as the channel of field-effect transistors. In addition to the pronounced resonance that correlates with the second van Hove transition (E22) in semiconducting carbon nanotubes, a weaker sideband at about 200 meV higher energy is observed. Electronic structure calculations that include electron−phonon coupling indicate that the spectra originate from the simultaneous excitation of an exciton (main resonance) and a C−C bond stretching phonon (sideband). The spectral features are not compatible with an interband interpretation of the excitation involved.

Substrate effect on refractive index dependence of plasmon resonance for individual silver nanoparticles observed using darkfield microspectroscopy.

Abstract: We use optical darkfield micro-spectroscopy to characterize the plasmon resonance of individual silver nanoparticles in the presence of a substrate. The optical system permits multiple individual nanoparticles to be identified visually for simultaneous spectroscopic study. For silver particles bound to a silanated glass substrate, we observe changes in the Plasmon resonance due to induced variations in the local refractive index. The shifts in the plasmon resonance are investigated using a simple analytical theory in which the contributions from the substrate and environment are weighted with distance from the nanoparticle. The theory is compared with experimental results to determine a weighting factor which facilitates modeling of environmental refractive index changes using standard Mie code. Use of the optical system for characterizing nanoparticles attached to substrates for biosensing applications is discussed.

Tunable Microelectromechanical Filters that Exploit Parametric Resonance

Abstract: Background: This paper describes an analytical study of a bandpass filter that is based on the dynamic response of electrostatically-driven MEMS oscillators. Method of Approach: Unlike most mechanical and electrical filters that rely on direct linear resonance for filtering, the MEM filter presented in this work employs parametric resonance. Results: While the use of parametric resonance improves some filtering characteristics, the

Kondo effect of molecular complexes at surfaces: ligand control of the local spin coupling.

Abstract: The spin state of single magnetic atoms and molecules at surfaces is of fundamental interest and may play an important role in future atomic-scale technologies. We demonstrate the ability to tune the coupling between the spin of individual cobalt adatoms with their surroundings by controlled attachment of molecular ligands. The strength of the coupling is determined via the Kondo resonance by low-temperature scanning tunneling spectroscopy. Spatial Kondo resonance mapping is introduced as a novel imaging tool to localize spin centers in magnetic molecules with atomic precision.

Laser-based ultrasonic generation and detection of zero-group velocity Lamb waves in thin plates

Abstract: A novel laser-based ultrasonic technique for the inspection of thin plates and membranes is presented, in which a modulated continuous-wave laser source is used to excite narrow bandwidth Lamb waves. The dominant feature in the acoustic spectrum is a sharp resonance peak that occurs at the minimum frequency of the first-order symmetric Lamb mode, where the group velocity of the Lamb wave goes to zero while the phase velocity remains finite. Experimental results with the laser source and receiver on epicenter demonstrate that the zero-group velocity resonance generated with a low-power modulated excitation source can be detected using a Michelson interferometer coupled to a lock-in amplifier. This resonance peak is sensitive to the thickness and mechanical properties of plates and may be suitable, for example, for the measurement and mapping of nanoscale thickness variations.

Temperature and pressure dependence of resonance in multi-layer microcantilevers

Abstract: The resonance frequency of the fundamental and four higher order modes of a silicon dioxide microcantilever is measured. The effect on these modes of depositing a 400 nm gold coating is investigated theoretically and experimentally. We derive an analytical solution to the eigenmodes of a multi-layered cantilever and verify its validity by comparison to finite-element analysis as well as the experimentally obtained results. The temperature and pressure dependence of the resonance frequencies is investigated experimentally and found to be in good agreement with theoretical models. An experimentally obtained value for the temperature dependence of Young's modulus of elasticity for thermally grown SiO2 is presented.

Non-invasive MR thermography using the water proton chemical shift

Abstract: Among various proton magnetic resonance (MR) parameters, such as longitudinal relaxation time, transverse relaxation time, diffusion coefficient and chemical shift, the chemical shift of water protons is recognized as the most reliable indicator of temperature. The chemical shift is the only frequency-based parameter and is independent of the other parameters, which are measured based on the intensity of the MR signal. In this paper, the basic principle and the recent progress in imaging temperature by spectroscopic techniques using the water proton chemical shift are discussed. The advantages of spectroscopic imaging over phase mapping for measuring temperature are that the former can distinguish water resonance from other resonances, and that another resonance can be used as an internal reference to reduce the effects of external magnetic field instability, tissue susceptibility and inter-scan tissue movement or deformation. Methods utilizing various magnetic resonance spectroscopy (MRS) techniques, such as single voxel spectroscopy, conventional magnetic resonance spectroscopic imaging (MRSI), echo planar spectroscopic imaging (EPSI) and line scan echo planar spectroscopic imaging (LSEPSI) are discussed.

Ultracold Molecule Production via a Resonant Oscillating Magnetic Field

Abstract: A novel atom-molecule conversion technique has been investigated. Ultracold $^{85}\mathrm{Rb}$ atoms sitting in a dc magnetic field near the 155 G Feshbach resonance are associated by applying a small sinusoidal oscillation to the magnetic field. There is resonant atom to molecule conversion when the modulation frequency closely matches the molecular binding energy. We observe that the atom to molecule conversion efficiency depends strongly on the frequency, amplitude, and duration of the applied modulation and on the phase space density of the sample. This technique offers high conversion efficiencies without the necessity of crossing or closely approaching the Feshbach resonance and allows precise spectroscopic measurements. Efficiencies of 55% have been observed for pure Bose-Einstein condensates.

Damping of a Unitary Fermi Gas

Abstract: We measure the temperature dependence of the radial breathing mode in an optically trapped, unitary Fermi gas of 6Li, just above the center of a broad Feshbach resonance. The damping rate reveals a clear change in behavior which we interpret as arising from a superfluid transition. We suggest pair breaking as a mechanism for an increase in the damping rate which occurs at temperatures well above the transition. In contrast to the damping, the frequency varies smoothly and remains close to the unitary hydrodynamic value. At low temperature T, the damping depends on the atom number only through the reduced temperature, and extrapolates to 0 at T = 0.

Nanomechanical hydrogen sensing

Abstract: A nanomechanical beam resonator is used as a sensitive, specific hydrogen sensor. The beam is fabricated from AuPd alloy and tested by magnetomotive transduction at room temperature. The fundamental resonance frequency decreases significantly and reversibly at hydrogen pressures above 10−5Torr, whereas the frequency shifts observed for other gases are significantly smaller. The large frequency shift is likely due to the formation of interstitial hydrogen in the metal alloy lattice, which relieves the built-in tensile stress in the resonator beam. The uptake of hydrogen as measured by frequency shift is consistent with previous studies.

Surface plasmon resonance of capped Au nanoparticles

Abstract: In this Rapid Communication we show the relationship between surface plasmon resonance damping and the intensity of surface bonding for capped Au nanoparticles, (NPs). Up to now the influence of capping has been included as a phenomenological modification of the scattering constant. It is indicated here that the effective NP size is the parameter mainly affected by surface bonding. Experimental results in different Au-thiol NPs are shown to be in excellent agreement with the expression we propose for damping. Moreover, according to our model the resonance profile gives a deep insight of the interface bonding strength.