Vlastimil Křápek

Electronic structure of InAs quantum dots with GaAsSb strain reducing layer: Localization of holes and its effect on the optical properties

The electronic structure of InAs quantum dots covered with the GaAs1−ySby strain reducing layer has been studied using the k⋅p theory. We explain previous experimental observations of the red shift of the photoluminescence emission with increasing y and its blue shift with increasing excitation power. For y>0.19, type-II dots are formed with holes localized in the GaAsSb close to the dot base; two segments at the opposite sides of the dot, forming molecular-like states, result from the piezoelectric field. We also propose an experiment that could be used to identify the hole localization using a vertical electric field.

Volume dependence of excitonic fine structure splitting in geometrically similar quantum dots

We report on two-photon interferences from highly indistin guishable single photons emitted by a quantum dot. Stricly resonant excitation with picosecond laser pul ses allows coherent state preparation with a significantly increased coherence time ( T2 ∼ 1 ns) and reduced lifetime ( T1 ∼ 650 ps), as compared to a non-resonant excitation scheme. Building-up the Hong-Ou-Mandel dip wit hout post-selection of the interfering photons, visibilities greater than 70 % have been observed. Near-unity in dist guishable photons could be achieved for every dot if charge noise is controlled. Indeed, the remaining dec oh rence mechanism is likely due to the fluctuating electrostatic environment of the dots.

LDA+DMFT approach to ordering phenomena and the structural stability of correlated materials

Abstract Materials with correlated electrons often respond very strongly to external or internal influences, leading to instabilities and states of matter with broken symmetry. This behavior can be studied theoretically either by evaluating the linear response characteristics, or by simulating the ordered phases of the materials under investigation. We developed the necessary tools within the dynamical mean-field theory (DMFT) to search for electronic instabilities in materials close to spin-state crossovers and to analyze the properties of the corresponding ordered states. This investigation, motivated by the physics of LaCoO3, led to a discovery of condensation of spinful excitons in the two-orbital Hubbard model with a surprisingly rich phase diagram. The results are reviewed in the first part of the article. Electronic correlations can also be the driving force behind structural transformations of materials. To be able to investigate correlation-induced phase instabilities we developed and implemented a formalism for the computation of total energies and forces within a fully charge self-consistent combination of density functional theory and DMFT. Applications of this scheme to the study of structural instabilities of selected correlated electron materials such as Fe and FeSe are reviewed in the second part of the paper.

Polarization anisotropy of the emission from type-II quantum dots

We study the polarization response of the emission from type-II GaAsSb capped InAs quantum dots. The theoretical prediction based on the calculations of the overlap integrals of the single-particle states obtained in the (k) over right arrow . (p) over right arrow framework is given. This is verified experimentally by polarization resolved photoluminescence measurements on samples with the type-II confinement. We show that the polarization anisotropy might be utilized to find the vertical position of the hole wave function and its orientation with respect to crystallographic axes of the sample. A proposition for usage in the information technology as a room temperature photonic gate operating at the communication wavelengths as well as a simple model to estimate the energy of fine-structure splitting for type-II GaAsSb capped InAs QDs are given.

Spatially resolved electron energy loss spectroscopy of crescent-shaped plasmonic antennas

We present a study of the optical properties of gold crescent-shaped antennas by means of electron energy loss spectroscopy. These structures exhibit particularly large field enhancement near their sharp features, support two non-degenerate dipolar (i.e., optically active) localised surface plasmon resonances, and are widely tunable by a choice of their shape and dimensions. Depending on the volume and shape, we resolved up to four plasmon resonances in metallic structures under study in the energy range of 0.8 - 2.4 eV: two dipolar and quadrupolar mode and a multimodal assembly. The boundary-element-method calculations reproduced the observed spectra and helped to identify the character of the resonances. The two lowest modes are of particular importance owing to their dipolar nature. Remarkably, they are both concentrated near the tips of the crescent, spectrally well resolved and their energies can be tuned between 0.8 - 1.5 eV and 1.2 - 2.0 eV, respectively. As the lower spectral range covers the telecommunication wavelengths 1.30 and 1.55 μm, we envisage the possible use of such nanostructures in infrared communication technology.

Excitation intensity dependence of photoluminescence spectra of SiGe quantum dots grown on prepatterned Si substrates: evidence for biexcitonic transition

The pumping intensity (I) dependence of the photoluminescence (PL) spectra of perfectly laterally two-dimensionally ordered SiGe quantum dots on Si(001) substrates was studied. The PL results from recombinations of holes localized in the SiGe quantum dots and electrons localized due to the strain field in the surrounding Si matrix. The analysis of the spectra revealed several distinct bands, attributed to phonon-assisted recombination and no-phonon recombination of the excitonic ground state and of the excited excitonic states, which all exhibit a linear I dependence of the PL intensity. At approximately I larger than 3 W cm-2, additional bands with a nearly quadratic I dependence appear in the PL spectra, resulting from biexcitonic transitions. These emerging PL contributions shift the composite no-phonon PL band of the SiGe quantum dots to higher energies. The experimentally obtained energies of the no-phonon transitions are in good agreement with the exciton and biexciton energies calculated using the envelope function approximation and the configuration interaction method.

Excitonic fine structure splitting in type-II quantum dots

Excitonic fine structure splitting in quantum dots is closely related to the lateral shape of the wave functions. We have studied theoretically the fine structure splitting in InAs quantum dots with a type-II confinement imposed by a GaAsSb capping layer. We show that very small values of the fine structure splitting comparable with the natural linewidth of the excitonic transitions are achievable for realistic quantum dots despite the structural elongation and the piezoelectric field. For example, varying the capping layer thickness allows for a fine tuning of the splitting energy. The effect is explained by a strong sensitivity of the hole wave function to the quantum dot structure and a mutual compensation of the electron and hole anisotropies. The oscillator strength of the excitonic transitions in the studied quantum dots is comparable to those with a type-I confinement which makes the dots attractive for quantum communication technology as emitters of polarization-entangled photon pairs.

Boundary element method for 2D materials and thin films

2D materials emerge as a viable platform for the control of light at the nanoscale. In this context the need has arisen for a fast and reliable tool capable of capturing their strictly 2D nature in 3D light scattering simulations. So far, 2D materials and their patterned structures (ribbons, discs, etc.) have been mostly treated as very thin films of subnanometer thickness with an effective dielectric function derived from their 2D optical conductivity. In this study an extension to the existing framework of the boundary element method (BEM) with 2D materials treated as a conductive interface between two media is presented. The testing of our enhanced method on problems with known analytical solutions reveals that for certain types of tasks the new modification is faster than the original BEM algorithm. Furthermore, the representation of 2D materials as an interface allows us to simulate problems in which their optical properties depend on spatial coordinates. Such spatial dependence can occur naturally or can be tailored artificially to attain new functional properties.

Comparative study of plasmonic antennas fabricated by electron beam and focused ion beam lithography

We present a comparative study of plasmonic antennas fabricated by electron beam lithography and direct focused ion beam milling. We have investigated optical and structural properties and chemical composition of gold disc-shaped plasmonic antennas on a silicon nitride membrane fabricated by both methods to identify their advantages and disadvantages. Plasmonic antennas were characterized using transmission electron microscopy including electron energy loss spectroscopy and energy dispersive X-ray spectroscopy, and atomic force microscopy. We have found stronger plasmonic response with better field confinement in the antennas fabricated by electron beam lithography, which is attributed to their better structural quality, homogeneous thickness, and only moderate contamination mostly of organic nature. Plasmonic antennas fabricated by focused ion beam lithography feature weaker plasmonic response, lower structural quality with pronounced thickness fluctuations, and strong contamination, both organic and inorganic, including implanted ions from the focused beam. While both techniques are suitable for the fabrication of plasmonic antennas, electron beam lithography shall be prioritized over focused ion beam lithography due to better quality and performance of its products.

Near-field digital holography: a tool for plasmon phase imaging

The knowledge of the phase distribution of the near electromagnetic field has become very important for many applications. However, its experimental observation is still technologically a very demanding task. In this work, we propose a novel method for the measurement of the phase distribution of the near electric field based on the principles of phase-shifting digital holography. In contrast to previous methods the holographic interference occurs already in the near field and the phase distribution can be determined purely from the scanning near-field optical microscopy measurements without the need for additional far-field interferometric methods. This opens a way towards on-chip phase imaging. We demonstrate the capabilities of the proposed method by reconstruction of the phase difference between interfering surface plasmon waves and by imaging the phase of a single surface plasmon wave. We also demonstrate a selectivity of the method towards individual components of the field.