Cross-polarization Excitation Research Articles

THz Near-field spectroscopy of metamaterial resonators

Terahertz near-field spectroscopy is a powerful technique for the investigation of metamaterial resonators with subwavelength resolution. Here, we used an a-SNOM THz time domain spectroscopic system for the direct study of all the modes supported by individual D-split ring resonators, their E-field distributions and relative Q factors. A cross-polarized excitation and detection technique is also demonstrated, yielding a modes’ mapping by the resonators with unprecedented resolution and sharpness.

Rotation Sensor Based on the Cross-Polarized Excitation of Split Ring Resonators (SRRs)

This paper presents a novel rotation sensor based on the concept of cross-polarized excitation of split ring resonators (SRRs). The structure of the proposed sensor which is quite simple and as a result is low cost, involves a slotline loaded with a rotatable SRR. In this configuration, when the symmetry plane of the loading SRR is orthogonal to the orientation of the slotline, the reflection coefficients from the two ports of the slotline are in phase. However, if the symmetry of the structure is broken by the rotation of the loading SRR, a phase difference between the two reflection coefficients is observed. Therefore, a rotation angle up to 180° can be sensed by measuring the phase difference between the reflection coefficients. Furthermore, it is demonstrated that introducing a flag resonator with the same principle of operation can extend the dynamic range of the proposed sensor to full 360°. A prototype of the proposed sensor is designed and fabricated and its principle of operation and wide dynamic range are validated numerically and experimentally.

Design of frequency controlled 4G and 5G band dielectric sensors using cross-polarized metamaterial particle dedicated to communication industry

ABSTRACTA novel approach is presented to design the dual-band planar sensor for the communication system. The proposed sensor is based on the complementary split ring resonator (CSRR) topology employing the cross-polarized excitation, which facilitates its operation even at the higher frequency band (28 GHz). The cross-polarized excitation of a CSRR allows efficient coupling of both electric and magnetic fields from the microstrip line, thereby producing the frequency-controlled dual resonance response. The prime focus here is to use the proposed approach to design two sensors with one sensor being simultaneously operated in the X-band and 5G band, while another sensor is operated in the 4G LTE band. The cross-polarized arrangement helps to achieve the higher dielectric sensitivity (17.21%) and Q factor (90) as compared to that of the co-polarized CSRR-loaded microstrip line sensor (14%, 73), where split of rings is aligned in the direction orthogonal to the microstrip line.

Detection of single nano-defects in photonic crystals between crossed polarizers

We investigate, by simulations and experiments, the light scattering of small particles trapped in photonic crystal membranes supporting guided resonance modes. Our results show that, due to amplified Rayleigh small particle scattering, such membranes can be utilized to make a sensor that can detect single nano-particles. We have designed a biomolecule sensor that uses cross-polarized excitation and detection for increased sensitivity. Estimated using Rayleigh scattering theory and simulation results, the current fabricated sensor has a detection limit of 26 nm, corresponding to the size of a single virus. The sensor can potentially be made both cheap and compact, to facilitate use at point-of-care.

Ultrafast non-linear optical signal from a single quantum dot: exciton and biexcitoneffects

We present results on both the intensity and phase dynamics of the transient non-linearoptical response of a single quantum dot (SQD). The time evolution of the four wavemixing (FWM) signal on a subpicosecond timescale is dominated by biexcitoneffects. In particular, for the cross-polarized excitation case a biexciton boundstate is found. In this latter case, mean-field results are shown to give a poordescription of the non-linear optical signal at small times. By properly treatingexciton–exciton effects in an SQD, coherent oscillations in the FWM signal are clearlydemonstrated. These oscillations, with a period corresponding to the inverse of thebiexciton binding energy, are correlated with the phase dynamics of the system’spolarization, giving clear signatures of non-Markovian effects in the ultrafast regime.

Mobility-resolved 13C-NMR spectroscopy of primary plant cell walls

Primary plant cell walls contain highly hydrated biopolymer networks, whose major chemistry is known but whose relationship to architectural and mechanical properties is poorly understood. Nuclear magnetic resonance spectroscopy has been used to characterize segmental mobilities via relaxation and anisotropy effects in order to add a dynamic element to emerging models for cell wall architecture. For hydrated onion cell wall material, single pulse excitation revealed galactan (pectin side chains), provided that dipolar decoupling was used, and some of the pectin backbone in the additional presence of magic angle spinning. Cross-polarization excitation revealed the remaining pectin backbones, which exhibited greater mobility (contact time dependence, dipolar dephasing) than the cellulose component, whose noncrystalline and crystalline fractions showed no mobility discrimination. 1HT2 behavior could be quantitatively interpreted in terms of high resolution observabilities. Mobility-resolved spectroscopy of cell walls from tomato fruit, pea stem, and tobacco leaf showed similar general effects. Nuclear magnetic resonance study of the sequential chemical extraction of onion cell wall material suggests that galactans fill many of the network pores, that extractability of pectins is not dependent on segmental mobility, and that some pectic backbone (and not side chain) is strongly associated with cellulose. Analysis of the state of cellulose in four hydrated cell walls suggests a noncrystalline content of 60–80% and comparable amounts of Iα and Iβ polymorphs in the crystalline fraction. Comparison with micrographs for onion cell walls shows that noncrystalline cellulose does not equate to chains on fibril surfaces, and chemical shifts show that fully solvated cellulose is not a significant component in cell walls. © 1996 John Wiley & Sons, Inc.

27Al−H REDOR NMR and 27Al Spin-Echo Editing: A New Way To Characterize Brönsted and Lewis Acidity in Zeolites

A new approach to the NMR characterization of acid zeolites is described. The approach is based on (i) the quantitative comparison of the rotational echo double-resonance (REDOR) evolution curves obtained by one-pulse excitation and by cross-polarization excitation and (ii) the spin-echo editing technique. The procedure that is described allows a “reconstruction” of 27Al NMR spectra corresponding to either Lewis sites or nonframework (NF) Al. The approach is applied to the characterization of the dealuminated mordenite DHM600 with chemisorbed ammonia as a proton reservoir for polarization transfer. The second moment analysis suggests that the NF Lewis sites are constituted by a pair of Al atoms having chemisorbed NH3. The procedure provides a means for obtaining framework Si/Al ratios. The values obtained are more reliable than ones determined by the standard 29Si MAS NMR method, since a possible coupling between NF species and the framework is taken into account.

Mobility‐resolved 13C‐NMR spectroscopy of primary plant cell walls

Primary plant cell walls contain highly hydrated biopolymer networks, whose major chemistry is known but whose relationship to architectural and mechanical properties is poorly understood. Nuclear magnetic resonance spectroscopy has been used to characterize segmental mobilities via relaxation and anisotropy effects in order to add a dynamic element to emerging models for cell wall architecture. For hydrated onion cell wall material, single pulse excitation revealed galactan (pectin side chains), provided that dipolar decoupling was used, and some of the pectin backbone in the additional presence of magic angle spinning. Cross-polarization excitation revealed the remaining pectin backbones, which exhibited greater mobility (contact time dependence, dipolar dephasing) than the cellulose component, whose noncrystalline and crystalline fractions showed no mobility discrimination. 1HT2 behavior could be quantitatively interpreted in terms of high resolution observabilities. Mobility-resolved spectroscopy of cell walls from tomato fruit, pea stem, and tobacco leaf showed similar general effects. Nuclear magnetic resonance study of the sequential chemical extraction of onion cell wall material suggests that galactans fill many of the network pores, that extractability of pectins is not dependent on segmental mobility, and that some pectic backbone (and not side chain) is strongly associated with cellulose. Analysis of the state of cellulose in four hydrated cell walls suggests a noncrystalline content of 60–80% and comparable amounts of Iα and Iβ polymorphs in the crystalline fraction. Comparison with micrographs for onion cell walls shows that noncrystalline cellulose does not equate to chains on fibril surfaces, and chemical shifts show that fully solvated cellulose is not a significant component in cell walls. © 1996 John Wiley & Sons, Inc.

Determination of extent of reaction in dimethacrylate-based dental composites using solid-state 13C m.a.s. n.m.r. spectroscopy and comparison with FTi.r. spectroscopy

The instrumental conditions required for the determination of the extent of reaction in dimethacrylate-based dental composites using high-resolution solid-state 13C nuclear magnetic resonance (n.m.r.) techniques have been investigated. In the absence of paramagnetic fillers, clearly resolved carbonyl resonances for reacted and unreacted methacrylate groups were observed. The two peaks showed considerably different relaxation times using cross-polarization excitation, and accurate relative intensities could be only be obtained using direct excitation of the 13C nuclei with a pulse interval of at least 30s. Chemical shielding anisotropy spinning sidebands were observed for the reacted peak, but not for the unreacted. A spin-rate-dependent centreband intensity correction factor to account for this difference was evaluated using model triethyleneglycol dimethacrylate materials. The proportion of unreacted methacrylate groups determined using n.m.r. was essentially unaffected by spin rate, dipolar decoupling field strength and temperature, but was only about one-half that determined using infra-red spectroscopy. The origin of the discrepancy is as yet unclear.

1H- 27Al heteronuclear dipolar couplings and internuclear distances in alumina: SEDOR and REDOR 27Al study

Double-resonance spin-echo NMR techniques (SEDOR and REDOR) are used to study heteronuclear connectivities in two aluminas, gibbsite and super 5. In gibbsite (SEDOR), we describe a new procedure which allows one to estimate average internuclear distances when the ‘isolated pair approximation’ does not hold. In super5, a comparative examination of REDOR responses with one pulse and cross-polarization excitation results in obtaining structural information concerning the surroundings of four-, five- and six-coordinated Al sites and in discriminating between surface, intermediate and bulk species.

Magnetic-field-induced resonance in four-wave mixing in GaAs.

The frequency-domain four-wave-mixing response of excitons in GaAs in a magnetic field is theoretically and experimentally studied. For linearly cross-polarized excitation beams, the application of a magnetic field induces a resonance that leads to the observation of a narrow dip in the primary resonance. The primary resonance has a width given by the exciton spin-flip rate while the narrow dip has a width determined by the recombination rate. The results reflect the dynamics of two observables defined in terms of the appropriate Fourier components of the net population and the net population transfer between the two oppositely spin-oriented excitonic states, similar to the normal decay modes of the system. The relative amplitude of these observables is determined by the relative polarization of the incident fields and the strength of the applied magnetic field.