Breakdown Of Inversion Symmetry Research Articles

Minimal model for charge transfer excitons at the dielectric interface

Theoretical description of the charge transfer (CT) exciton across the donor-acceptor interface without the use of a completely localized hole (or electron) is a challenge in the field of organic solar cells. We calculate the total wavefunction of the CT exciton by solving an effective two-particle Schrodinger equation for the inhomogeneous dielectric interface. We formulate the magnitude of the CT and construct a minimal model of the CT exciton under the breakdown of inversion symmetry. We demonstrate that both a light hole mass and a hole localization along the normal to the dielectric interface are crucial to yield the CT exciton.

Vibrational Sum Frequency Generation by the Quadrupolar Mechanism at the Nonpolar Benzene/Air Interface.

The interface selectivity of vibrational sum frequency generation (VSFG) spectroscopy is explained under the dipole approximation as resulting from the breakdown of inversion symmetry at the interface. From this viewpoint, VSFG is not expected to occur at the interface consisting of centrosymmetric molecules, because the inversion symmetry is preserved even at the interface. In reality, however, VSFG at the nonpolar benzene/air interface has been observed with traditional homodyne-detected VSFG. Here we report a heterodyne-detected VSFG study of the benzene/air interface. The result strongly indicates that VSFG at this interface cannot be explained within the framework of the dipole approximation. The selection rule and polarization dependence of the observed VSFG signal show that the quadrupole transition plays an essential role because of the field discontinuity at the interface. This finding implies the applicability of interface-selective VSFG to the nonpolar interfaces comprising centrosymmetric molecules, which opens a new possibility of VSFG spectroscopy.

Effect of mesoscopic breakdown of inversion symmetry on second‐order optical nonlinearity

The second-order nonlinear polarizability due to lack of inversion symmetry on a mesoscopic scale is investigated theoretically. We employ two-dimensional free electrons confined in a triangular cluster as the simplest system, and investigate the dependence of the second-harmonic polarizability on the system size. The second-harmonic polarizability has a nonzero value when the system size is small enough to yield a line spectrum. As the system size increases, the second-harmonic polarizability gradually cancels out to zero, which is also confirmed analytically. The limit in size to obtain nonzero second-harmonic polarizability is clarified.

Phenomena of g-u symmetry-breakdown in HD

Phenomena associated with the breakdown of inversion symmetry in the HD molecule are reviewed and discussed. A distinction is made between three kinds of physical effects observed in HD spectra. The existence of a small electric dipole moment in the ground state gives rise to vibrational and pure rotational transitions following selection rules of electric dipole transitions. Coupling between electronic states of g and u symmetry occurs, which is associated with the appearance of forbidden lines in the electronic spectrum. This effect occurs predominantly at near coincidences between levels of opposite inversion symmetry and a recently observed example of strongly interacting states (H¯ 1 + g and B¯ 1 + u ) is highlighted. Electronic coupling between states of g and u symmetry always takes place near dissociation threshold; as a result of the mass difference and the electronic isotope shift the behaviour at long range cannot be described in an adiabatic picture. A procedure is developed to construct long-range potentials near the n = 2 dissociation limit in which the breakdown of g-u symmetry is incorporated.

Electric-field-induced optical second-harmonic generation and nonlinear optical rectification in semiconducting carbon nanotubes

We have calculated the nonlinear susceptibility for the optical second-harmonic generation (SHG) from a bundle of aligned single-walled carbon nanotubes (SWCNs) of the `zig-zag' type subjected to a constant electric field E 0 , parallel to their axis. The breakdown of inversion symmetry caused by the electric field is accompanied by the occurrence of the parity-forbidden two-photon transitions between the valence- and conduction-band edges. As a result, the third-order nonlinear-susceptibility χ (3) spectrum for SHG clearly demonstrates two peaks strongly distinguished on intensity: one is the two-photon resonance exactly at the pump photon energy ℏ ω equal to one half of the band gap Δ g ; the other – small peak – sits at ℏ ω=Δ g , which corresponds to the one-photon interband transitions. It is found that the intensity of both peaks grows sharply with an increase of the SWCN radius. The susceptibility χ (3) relevant to the effect of nonlinear optical rectification in SWCNs has also been calculated. It is shown that near the fundamental absorption edge, the electric voltage appearing on the ends of SWCNs due to the optical rectification effect sharply changes its polarity. For a bundle of identical (25,0) SWCNs, 3 μm in length and placed in an electric field E 0∼10 4 V cm −1, the optical rectification voltage is found to be about 0.35 μV under excitation of the sample by a continuous laser with a radiation intensity of 30 mW cm −2, which may be of practical importance for mid-IR signal processing.