Vector optical vortices exhibit complex polarization patterns due to the interplay between spin and orbital angular momenta. Here we demonstrate, both analytically and with simulations, that certain polarization features of optical vortex beams maintain constant transverse spatial dimensions independently of beam divergence due to diffraction. These polarization features appear in the vicinity of the phase singularity and are associated with the presence of longitudinal electric fields. The predicted effect may prove important in metrology and high-resolution imaging applications.
There is an unsettled problem in choosing the correct expressions for the local momentum density and angular momentum density of electromagnetic fields (or indeed, of any non-scalar field). If one only examines plane waves, the problem is moot, as the known possible expressions all give the same result. The momentum and angular momentum density expressions are generally obtained from the energy-momentum tensor, in turn obtained from a Lagrangian. The electrodynamic expressions obtained by the canonical procedure are not the same as the symmetric Belinfante reworking. For the interaction of matter with structured light, for example, twisted photons, this is important; there are drastically different predictions for forces and angular momenta induced on small test objects. We show situations where the two predictions can be checked, with numerical estimates of the size of the effects.
In this paper we analyze the novel quantum properties of atomic photo-excitations generated by Orbital Angular Momentum (OAM) photons. It was previously predicted for Bessel beams (BB), that transitionsin atomic systems driven by twisted laser beams obey a modified set of position-dependent angular-momentum selection rules. It was also shown, that in the case of perfect alignment of the atom on the beam axis, total angular momentum of the photon must be fully transferred to the internal degrees of freedom of the target, which causes dramatic enhancement of higher-multipole processes. As a result, weak atomic transitions, which are hardly observable in interactions with simple states of light, become accessible with OAM photons. In the current paper, we extend the earlier developed theory to describe atomic photoexcitations by Bessel-Gauss (BG) and Laguerre-Gauss (LG) modes. It allowed us to infer the information about the phase structure of the beam, such as mode content and topological charge. The formalism exhibits a high level of sensitivity to the polarization content of the laser beam, which is on the order of 0.005% of the polarization admixture. The theoretical predictions are confirmed experimentally for photo-excitation of Ca40+ ions, trapped in a micro-structured, segmented Paul trap. The full set of quadrupole transitions from the S-state were generated by 729nm OAM laser beam shaped with a holographic phase-plate with a fork-dislocation. The experiment was conducted by Ferdinand Schmidt-Kaler's group at the University of Mainz.
We analyze the novel features of high-multipole excitation of quantum systems with twisted light, i.e, by the photons that carry extra orbital angular momentum (OAM) along the propagation direction. It was previously demonstrated experimentally [1] that OAM of the twisted light is passed to the internal degrees of freedom of an atom for electric quadrupole transitions. We extended these results to the analysis of individual S->D transition amplitudes by measuring Rabi frequencies for Ca40+ ions placed in a Paul trap. It is shown that relative strengths transitions into different Zeeman-split levels - most of them forbidden for plane-wave photons - depends on the target atom’s position with respect to the optical vortex center. The data compare favorably with the theoretical predictions. The selection rules also point to circular dichroism and birefringence of the twisted light interacting with non-chiral atomic matter.
Collaboration with Carl E. Carlson, Christian T. Schmiegelow, Ferdinand Schmidt-Kaler, and Maria Solyanik is gratefully acknowledged.
[1] C. T. Schmiegelow, J. Schulz, H. Kaufmann, T. Ruster, U. G. Poschinger, and F. Schmidt-Kaler, Nature communications, 7 (October 2016), DOI: 10.1038/ncomms12998.
We calculate transition amplitudes and cross sections for excitation of hydrogen like atoms by the twisted photon states, or photon states with large angular momentum projection on the direction of propagation. It is shown that when the transition rates are normalized to the local photon flux,the resulting cross sections for are singular near the optical vortex center [1], in close relation to the 'quantum core' concept introduced by Berry and Dennis. We also show that the photon state develops polarization singularity in the beam center due to circular dichroism of the photon absorption by atoms via higher multipoles.
[1]. A. Afanasev, C.E. Carlson, and A. Mukherjee, High-multipole excitations of hydrogen-like atoms by twisted photons near a phase singularity, Journal of Optics 18, 074013 (2016)
We calculate transition amplitudes and cross sections for excitation of hydrogen-like atoms by the twisted photon states, or photon states with more than one unit of angular momentum projection along the direction of propagation.
If the target atom is located at distances of the order of atomic size near the vortex center, the transitions rates into $l_f>1$ states become comparable with the rates for standard electric dipole transitions. It is shown that when the transition rates are normalized to the local photon flux, the resulting cross sections for $l_f>1$ are singular near the optical vortex center, i.e., high-multipole excitations take place in the region of zero field intensity near phase singularity. Relation to the "quantum core" concept introduced by Berry and Dennis is discussed.
We used AlGaSb/AlGaAs material system for a theoretical study of photovoltaic performance of the proposed GaAsbased
solar cell in which the type-II quantum dot (QDs) absorber is spatially separated from the depletion region. Due
to inelastic scattering of photoelectrons on QDs and proper doping of both QDs and their spacers, concentrated sunlight
is predicted to quench recombination through QDs. Our calculation shows that 500-sun concentration can increase the
Shockley-Queisser limit from 35% to 40% for GaAs single-junction solar cells.
We propose a new intermediate band GaAs solar cell comprising an AlxGa1-xAs absorber with built-in GaSb type-II quantum dots (QDs) [a gradual AlxGa1-xAs absorber with built-in AlyGa1-ySb QDs (0<x=y<0.40) as a variant] separated from the depletion region. We study the modification of the band alignment at type-II interface by two-photon absorption of concentrated sunlight. Our calculation shows that photogenerated carriers produce localized exciton-like electron-hole pairs spatially separated at QDs. Local field of such pairs may essentially modify potential barrier surrounding QDs, increase recombination lifetime of mobile carriers and additional photocurrent generated by two photon absorption. Concentration of about 300-sun pushes by 15% up the conversion efficiency as compared to the efficiency of the reference single junction GaAs solar cell without QDs.
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