Magnetic Dipole Contributions Research Articles

Temperature dependence of the magnetic Casimir-Polder interaction

We analyze the magnetic dipole contribution to atom-surface dispersion forces. Unlike its electrical counterpart, it involves small transition frequencies that are comparable to thermal energy scales. A significant temperature dependence is found near surfaces with a nonzero dc conductivity, leading to a strong suppression of the dispersion force at $T>0$. We use thermal response theory for the surface material and discuss both normal metals and superconductors. The asymptotes of the free energy of interaction and of the entropy are calculated analytically over a large range of distances. Near a superconductor, the onset of dissipation at the phase transition strongly changes the interaction, including a discontinuous entropy. We discuss the similarities with the Casimir interaction between two surfaces and suggest that precision measurements of the atom-surface interaction may shed light upon open questions around the temperature dependence of dispersion forces between lossy media.

Surface and bulk contributions to the second-order nonlinear optical response of a gold film

We use two-beam second-harmonic generation to separate the surface (electric dipole origin) and bulk (magnetic dipole and electric quadrupole origin) contributions to the second-order nonlinear optical response of an isotropic gold film. The bulk response is unambiguously observed and explained by momentum damping of electrons in a free-electron model. Although bulk effects could be enhanced by inhomogeneous local fields in metal nanostructures and have been used to model second-harmonic generation from metamaterials [Y. Zeng et al., Phys. Rev. B 79, 235109 (2009)], we find that surface effects dominate the nonlinearity. Our quantitative results for the surface and bulk parameters set the baseline for future descriptions of the second-order response of nanostructured metals.

Upper limit on the magnetic dipole contribution to the5p−8ptransition in Rb by use of ultracold atom spectroscopy

We report on hyperfine-resolved spectroscopic measurements of the electric-dipole forbidden 5$p_{3/2} \to 8p_{1/2}$ transition in a sample of ultracold $^{87}$Rb atoms. The hyperfine selection rules enable the weak magnetic-dipole (M1) contribution to the transition strength to be distinguished from the much stronger electric-quadrupole (E2) contribution. An upper limit on the M1 transition strength is determined that is about 50 times smaller than an earlier experimental determination. We also calculate the expected value of the M1 matrix element and find that it is less than the upper limit extracted from the experiment.

Spin-Induced Optical Second Harmonic Generation in the Centrosymmetric Magnetic Semiconductors EuTe and EuSe

Spectroscopy of the centrosymmetric magnetic semiconductors EuTe and EuSe reveals spin-induced optical second harmonic generation (SHG) in the band gap vicinity at 2.1-2.4 eV. The magnetic field and temperature dependence demonstrates that the SHG arises from the bulk of the materials due to a novel type of nonlinear optical susceptibility caused by the magnetic dipole contribution combined with spontaneous or induced magnetization. This spin-induced susceptibility opens access to a wide class of centrosymmetric systems by harmonics generation spectroscopy.

Inhomogeneous and Spontaneous Formation of Chirality in the Langmuir Monolayer of Achiral Molecules at the Air/Water Interface Probed by In Situ Surface Second Harmonic Generation Linear Dichroism

The chirality of the Langmuir monolayer at different surface densities formed with achiral 5-octadecyloxy-2-(2-pyridylazo)phenol (PARC18) molecules at the air/water interface was measured with the in situ optical second harmonic generation linear dichroism (SHG-LD) technique. Using the s-polarization detection curve in SHG-LD, the degree of chiral excess (DCE) in the Langmuir monolayer can be determined as accurately as 2%. We found that the PARC18 Langmuir monolayer was chiral and the degree of chirality varied randomly between the two opposite chiral states in the same monolayer at different surface densities and at different locations, suggesting coexistence of the various enantiomers simultaneously in the PARC18 Langmuir monolayer. This inhomogeneity of the monolayer chirality was independent of the laser power and wavelength as well as the surface density and the surface pressure, and the chiral domains distributed randomly in the Langmuir monolayer. We also found that the chirality in the PARC18 Langmuir monolayer was spontaneously formed upon molecular aggregation, instead of being compression induced. There was no apparent magnetic dipole contribution observed throughout the different surface densities in the PARC18 Langmuir monolayer. These results indicate that the SHG-LD technique is not only an effective in situ probe of the interface chirality but also an effective tool to study the dynamic processes in the close-packed monolayer or in the self-assembled films.

Near-field induction heating of metallic nanoparticles due to infrared magnetic dipole contribution

We revisit the electromagnetic heat transfer between a metallic nanoparticle and a highly conductive metallic semi-infinite substrate, commonly studied using the electric dipole approximation. For infrared and microwave frequencies, we find that the magnetic polarizability of the particle is larger than the electric one. We also find that the local density of states in the near field is dominated by the magnetic contribution. As a consequence, the power absorbed by the particle in the near field is due to dissipation by fluctuating eddy currents. These results show that a number of near-field effects involving metallic particles should be affected by the fluctuating magnetic fields.

Strong Two-Photon Circular Dichroism in Helicenes: A Theoretical Investigation.

Using a recently derived origin-invariant quadratic response approach combined with time-dependent density functional theory, four representative helicenes are shown to present a very strong two-photon circular dichroism (TPCD) response, which makes them candidates for the first experimental observation of a TPCD effect. The large response is attributed to the unique combination of chirality and electron delocalization. Comparison with electronic circular dichroism and two-photon absorption (TPA) shows that the three effects exhibit complementary features for unravelling the molecular structures. In particular, for the four (M)-helicenes studied here, the first, i.e., low-energy, dominant Cotton band is always negative, whereas for TPCD it is positive. From an analysis of the frontier orbitals describing most of the one-electron excitation vectors, the largest TPCD response of tetramethoxy-bisquinone-dithia-[7]-helicene has been attributed to the charge-transfer character of the excited state, like for the parent TPA effect. Moreover, the TPCD intensities are found to be mostly governed by the electric and magnetic dipole contributions, while the electric quadrupole terms are, on a relative basis, less important.

Second-harmonic generation-circular dichroism in thin films of a chiral poly(3-alkyl)thiophene

Second-harmonic generation-circular dichroism (SHG-CD) effects in thin films of a chiral poly(3-alkylthiophene) have been investigated. Polarization patterns were recorded for several angles of incidence and analyzed using a recently developed optical model that takes all linear and non-linear optical achiral and chiral terms into account. From a complete analysis of the second-order susceptibility tensors, it is clear that SHG-CD in this particular system cannot be described by electric-dipole contributions only, but instead relies on the presence of strong magnetic-dipole contributions, similarly to linear optical CD effects.

LOCAL-FIELD AND MULTIPOLAR EFFECTS IN THE SECOND-HARMONIC RESPONSE OF ARRAYS OF METAL NANOPARTICLES

The distribution details of strongly-localized fields in metal nanostructures are important for strong interaction with the locally-varying non-linearity. Using gold nanodimers with nanogaps, we demonstrate that field localization alone is insufficient to drive a strong second-order response. Instead, polarization conversion of the local fundamental field combines with its asymmetric distribution along the dimer perimeter to yield strong responses. Strongly-localized fields may also favor higher-multipole (magnetic-dipole and electric-quadrupole) contributions to their nonlinear response. We show that the polarization-dependent interference of multipole sources in second-harmonic generation from gold nanoparticles allows the separation of their relative contributions.

Quadrupole contribution to the third-order optical activity spectroscopy

Time-resolved nonlinear optical activity measurement spectroscopy can be a useful tool for studying biomolecular and chemical reaction dynamics of chiral molecules. Only recently, the two-dimensional (2D) circularly polarized photon echo (CP-PE) spectroscopy of polypeptides and a photosynthetic light-harvesting complex were discussed, where the beam configuration was specifically controlled in such a way to eliminate the quadrupole contribution to the CP-PE signal. In this paper, we generalize the CP-PE spectroscopy by including the transition quadrupole contributions from peptide amide I vibrational transition and chlorophyll electronic transition. By using a density functional theory calculation method, the corresponding amide I vibrational and chlorophyll Q(y) electronic transition quadrupole tensor elements are determined. Amplitude of nonlinear optical transition pathway involving a quadrupole transition is found to be comparable to those of magnetic dipole terms for two different cases considered, i.e., dipeptides and photosynthetic antenna complex. However, due to the rotational averaging factors, the overall quadrupole contribution is an order of magnitude smaller than the magnetic dipole contribution. This suggests that the conventional 2D photon echo method and experimental scheme can be directly used to measure the 2D CP-PE signal from proteins and molecular complexes and that the 2D CP-PE signal is mainly dictated by the magnetic dipole contribution.

On electric-quadrupole and magnetic-dipole contributions to optical rectification in isotropic media near optical resonance

A molecular theory of resonant optical rectification via the second-order electric-dipole polarizability related to the magnetic-dipole and electric-quadrupole interaction of electromagnetic radiation with atoms and molecules of an isotropic medium is presented. The magnetic-dipole and electric-quadrupole contributions to dc electric-dipole polarization can be produced by arbitrarily polarized light which has to be at resonance with the molecule. This effect can appear in an isotropic medium composed of molecules with arbitrary symmetries.

Short-wavelength emission analysis in Dy:ZBLAN glasses

In this work we examine short-wavelength (blue, yellow and red) emission properties of dysprosium activated fluorozirconate ZBLAN glass. On the basis of the measured broad band absorption spectrum the intensity parameters Ωi were calculated using Judd–Ofelt formalism, yielding values of transition probabilities and radiative lifetimes. The basic spectroscopic characterization of Dy:ZBLAN was also performed, including visible emission and fluorescence decay measurements under pulsed, direct and two-photon up-conversion excitation. The theoretically predicted properties, specifically concerning the fluorescence lifetimes, were found to be in a rough agreement with experimentally determined values which was improved by isolation of magnetic dipole and impurity contributions to absorption spectrum.

Probing molecular chirality via excitonic nonlinear response**This article was originally submitted to the special issue on quantum electrodynamics published in August 2006.

The complete set of fifth-rank tensor components which describe the chirality-induced response of excitons to three femtosecond laser pulses is calculated to first order in the optical wavevector, by including the electric dipole and quadrupole and the magnetic dipole contributions to the induced current. The photon echo signal for the π − π* and n − π* electronic transitions of α helical polypeptides is calculated using the Green's function solution of the nonlinear exciton equations (NEE). Two-dimensional correlation plots of selected tensor components show considerably enhanced crosspeaks with high sensitivity to structural variations.

Scattering of surface plasmon polaritons by a nanoparticle with the inclusion of the magnetic dipole contribution

Using the method of the tensor Green’s function of the wave equation, the differential and total cross sections for the scattering of surface optical electromagnetic waves (surface plasmon polaritons) by a spherical nanoparticle into surface plasmon polaritons and light have been obtained with the inclusion of the magnetic dipole contribution. Using the example of noble metal systems, it has been shown that the magnetic dipole contribution may significantly affect the angular dependence of the differential cross sections and increase their anisotropy as the plasmon wavelength increases.

Ab InitioTreatment of Optical Second Harmonic Generation in NiO

In this Letter we develop a new systematic approach to study optical second harmonic generation in NiO, on both the (001) surface and the bulk. NiO is modeled as a doubly embedded cluster on which two highly correlated quantum chemistry methods are applied in order to obtain the wave functions of all the intragap d states and the low lying charge transfer states. The optical gap is calculated and the electric dipole, magnetic dipole, and electric quadrupole contributions to the second order susceptibility tensor are computed for the first time from first principles. Going beyond the electric dipole approximation gives new insight into the experimentally observed spectrum. A method is proposed for monitoring the spin dynamics of the NiO(001) surface.

Nonlinear optical properties of spincoated films of chiral polythiophenes

Spincoated thin films of a chiral poly(3-alkylthiophene) were investigated by second-harmonic generation. Based on a study of the polarization dependence of the second-harmonic signal, we were able to show that significant magnetic-dipole contributions are present in the nonlinearity of the films, in addition to the usual electric-dipole contributions.

The spin-vibration hyperfine interaction in the ν3 band of 189OsO 4 and 187OsO 4: a calculable example in high-resolution molecular spectroscopy

Osmium tetroxide offers the beautiful example of a central particle, the osmium nucleus, with several possible spin values (1/2 for 187Os and 3/2 for 189Os), interacting with the very symmetric and well-defined electromagnetic environment created by a tetrahedron of spinless oxygen nuclei. Among other nuclear hyperfine interactions, the magnetic field generated by the vibrations of these oxygen nuclei orientates this central spin. To study specifically this spin-vibration interaction, the magnetic dipole contributions to the hyperfine structures observed in the ν 3 band of 189OsO 4 and 187OsO 4 have been analyzed in detail. Besides the spin-rotation constants already deduced in a previous paper, we have obtained the spin-vibration constant, A, and a small change of the spin-rotation constant δc a in the upper vibrational level. We have verified the speculated relation A= ζ 3 c a at the 1% level ( ζ 3 is the Coriolis coupling constant, which multiplies the internal vibrational angular momentum to turn it into a physical angular momentum comparable to the pure rotation angular momentum). Starting from first principles, we have rederived the expressions of the magnetic dipole constants for any semi-rigid polyatomic molecule. When these are applied to OsO 4, they justify the previous approximate relationship between A and c a and give values for the correction terms which account for the 1% difference. To cite this article: C. Chardonnet et al., C. R. Physique 5 (2004).

Magnetic-dipole susceptibilities in electric-field induced second-harmonic generation

A new measurement technique has been developed to determine the magnitude of magnetic-dipole contributions to the second-order nonlinearity of oriented solutions of chiral molecules. The method is based on a modified electric-field induced second-harmonic generation technique.

Magnetic-dipole nonlinearities in chiral materials

We propose a new measurement procedure to study the role of magnetic-dipole contributions to the second-order nonlinearity in chiral materials. The procedure is based on the measurement of the ellipticity of the second-harmonic light generated in the chiral material. A better insight in the magnetic-dipole nonlinearities of materials could eventually lead to the observation of new phenomena in the field of photochemistry and photophysics.

Prediction of pure electric-dipole two-photon absorption circular dichroism in lanthanide compounds

Standard theories of chiral linear and nonlinear optical effects, such as circular intensity differentials and optical activity, require interference between electric-dipole and magnetic-dipole (or higher-order) contributions to the transition moments. Recent experimental and theoretical work on second-harmonic generation processes has shown that in the resonant case, pure electric-dipole contributions to such chiral effects are possible. In this paper, we consider the implications of pure electric-dipole contributions to circular dichroism in two-photon absorption transitions within the 4 f N configuration of lanthanide ions. We show that in some lanthanide compounds, when several intermediate states are near to resonance, the pure electric-dipole mechanism can give larger contributions than the standard electric-dipole magnetic-dipole mechanism.