Being motivated by the recent result on the emergence of superlattice properties of the helical nanoribbon in an electric field, we analyze its circular dichroism signal. We theoretically demonstrate that electric-field effect on the helical nanoribbon leads to appearance of new spectral lines in circular dichroism.
Here, we analytically study optical activity of chiral semiconductor gammadions whose chirality arises from the nonuniformity of their thickness. We show that such gammadions distinguish between the two circular polarizations upon the absorption of light, unlike two-dimensional semiconductor nanostructures with planar chirality. Chiral semiconductor gammadions of inverse conical shape are found to exhibit the highest dissymmetry of optical response among the nanostructures of the same size. The results of our theoretical study can be used in future applications of semiconductor gammadions in biomedicine and optoelectronics.
We present here a simple quantum-mechanical model that describes interband optical activity of cubical semiconductor nanocrystals with chiral shape irregularities. Using the developed model, we derive the analytical expression for the rotatory strengths of interband transitions and show that the circular dichroism spectra of the chiral-shape nanocrystal consists only of the electric dipole allowed transitions. Taking into account the splitting of the valence band, one can interpret experimental circular dichroism spectra using just a few fitting parameters. The results of our study may prove useful for various branches of nanophotonics, chiral chemistry, and biomedicine.
We develop a theory of time-resolved pump–probe optical spectroscopy for modelling interband absorption by an anisotropic semiconductor nanodumbbell. By considering three transition schemes where the pump and probe pulses are nearly resonant to a dipole-allowed interband transition of different elements of the nanodumbbell, and assuming that the populations of the exited states are coupled through the nonradiative relaxation processes, we analytically calculate the absorption efficiency of the probe as a function of its delay from the pump for relatively short pulses. The obtained functional dependency, being the sum of exponentials with exponents proportional to the energy relaxation rates of the excited electronic states, is useful for the analysis of experimental absorption spectra aiming at retrieving the relaxation parameters of the nanodumbbell’s electronic subsystem.
We develop a theory of time-resolved pump–probe optical spectroscopy of intraband absorption of a probe pulse inside
an anisotropic semiconductor nanorod. The absorption is preceded by the absorption of the pump pulse resonant to an
interband transition. It is assumed that the resonantly exited states of the nanorod are interrelated via the relaxation
induced by their interaction with a bath. We reveal the conditions for which the absorption of the probe’s pulse is
governed by a simple formula regardless of the pulse’s shape. This formula is useful for the analysis of the experimental
data containing information on the relaxation parameters of the nanorod’s electronic subsystem.
We propose a new type of optical spectroscopy of anisotropic semiconductor nanocrystals, which is based on the welldeveloped
stationary pump-probe technique, where the pump and probe fields are absorbed upon, respectively, interband
and intraband transitions of the nanocrystals’ electronic subsystem. We develop a general theory of intraband absorption
based on the density matrix formalism. This theory can be applied to study degenerate eigenstates of electrons in
semiconductor nanocrystals of different shapes and dimentions. We demonstrate that the angular dependence of
intraband absorption by nonspherical nanocrystals enables investigating their shape and orientation, as well as the
symmetry of quantum states excited by the probe field and selection rules of electronic transitions.
PbS quantum dots (QDs) with diameter of 2.9-7.4 nm were embedded into a porous matrix. The samples prepared by
developed low-cost effortless method demonstrate linear dependencies of optical density and luminescence intensity on
the QDs concentration and perfect homogeneity. Optical properties of quantum dots in the matrix were studied using
absorption and steady-state and time-resolved photoluminescence spectroscopy. Luminescence lifetimes were found to
be size-dependent and increase with decreasing of QDs size. The aging behavior of PbS QDs in a porous matrix was
explored for a variety of QDs sizes. The energy transfer process in quasi-monodispersed PbS QDs ensemble was
We develop a theory allowing one to calculate the energy spectra and wave functions of collective excitations in twoand
three-dimensional quantum-dot supercrystals. We derive analytical expressions for the energy spectra of twodimensional
supercrystals with different Bravias lattices, and use them to analyze the possibility of engineering the
supercrystals' band structure. We demonstrate that the variation of the supercrystal’s parameters (such as the symmetry
of the periodic lattice and the properties of the quantum dots or their environment) enables an unprecedented control over
its optical properties, thus paving a way towards the development of new nanophotonics materials.
We develop a theory of secondary emission from a single quantum dot, when the lowest-energy states of its
electron–hole pairs are involved in the photoluminescence process. For the sake of definiteness, our model allows
for two states contributing to the luminescence. We analyze the dependency of secondary emission intensity on
the energy gap between the states, while considering that the gap is determined by the quantum dot’s size. An
analytical expression for the time-dependent signal of thermalized luminescence is obtained using an analytical
solution to the kinetic Pauli equation. This expression yields the signal of stationary luminescence as the spectral
width of the excitation pulse tends to zero.
We study size dependence of kinetic and spectral properties of near-infrared luminescence from PbS quantum
dots in colloidal solution. Luminescence lifetimes are found to lie between 250 ns for the largest quantum
dots and 2:5 <i>μs </i>for the smallest ones, while the Stoke's shift is found to increase from 4-5 to 300 meV. These
results are explained by the presence of the long-living in-gap state, with the size-dependent energy. Analytical
modeling shows that the relaxation from this state is dominant in small quantum dots and negligible in large ones.
Biexponential luminescence decay with the size-dependent recombination rates is predicted for quantum dots
of all sizes.
We develop a low-temperature theory of the resonant Raman scattering from a semiconductor quantum dot, whose electronic subsystem is resonant with the confined longitudinal-optical (LO) phonon modes. Our theory employs a generalized model for the quantum dot's energy spectrum renormalization, which is induced by the polar electron-phonon interaction. The model takes into account the degeneration of electronic states and allows for arbitrary LO-phonon modes to be involved in the vibrational resonance. We solve the generalized master equation for the reduced density matrix, in order to derive an analytical expression for the differential cross section of the resonant Raman scattering from a single quantum dot.
Spectral distribution of emission was measured in a large angular range (8 deg to 180 deg) around a self-assembled photonic crystal synthesized from colloids of Rhodamine-B dye-doped polystyrene. Its comparison with the emission from the same dye-doped colloids in a liquid suspension provides a better understanding of the anisotropic propagation of light within the structure due to its pseudo-gap properties. The spontaneous emission is suppressed by 40% in the presence of the stop band over a large bandwidth (∼50%) of the first-order bandgap in the ΓL direction, due to the appropriate choice of the colloidal diameter. Spectral shifts in the spontaneous emission spectrum occur with the variation in the detection angle. The inevitable disorder in the self-assembled crystals and the resultant effect on emission was modeled by comparing the experimentally obtained reflection spectrum with the band structure calculated using the Korringa-Kohn-Rostoker method to exclude finite-size effects. Reflection and transmission are complementary because of the absence of strong absorptive effects. The extent of redistribution in the emission from a photonic crystalline environment with respect to a homogeneous emitter is significant in the spectral and spatial domains.
The design and realization of chip-scale plasmonic devices have been considerably facilitated by computational
electromagnetic simulations and sophisticated nanofabrication techniques. For rapid device optimization, numerical
simulations should be supplemented by simple analytical expressions capable of providing a reasonable
estimate of the initial design parameters. In this paper, we develop an analytic approach and derive approximate
expressions for the transmittance of metal-dielectric-metal (MDM) waveguides coupled to single, double, and
periodic stub structures. Our method relies on the well-known analogy between MDM waveguides and microwave
transmission lines, and enables us to use standard analytical tools in transmission-line theory. The advantage of
our analytic approach over the previous studies is in accounting for the plasmon damping due to Ohmic losses
and reflection-induced phase shift at the stub end. We found that the analyzed waveguide configurations can
exhibit the characteristics of nanoscale filters and reflectors. We validate our analytical model by comparing
its predictions with numerical simulations for several MDM waveguides with different stub configurations. The
proposed theoretical results are particularly useful to reduce lengthy simulation times and will prove valuable in
designing and optimizing MDM-waveguide-based photonic devices.