arXiv subjects

Vlastimil Křápek

Publications and source records attributed to Vlastimil Křápek.

At least 19 recordsLinked to original sources

arXiv2026

Reconfigurable plasmonic hot spots enabled by composite VO2-gold plasmonic antennas

We theoretically investigate the formation of electric and magnetic hot spots with reconfigurable plasmonic antennas. We consider three material systems offering different levels of reconfigurability: gold with the static response, vanadium dioxide which allows for ON/OFF switching, and composite gold-vanadium dioxide material platform which offers a possibility to switch between the electric and magnetic hot spot within a single antenna. Using bowtie and diabolo antennas as a case study, we evaluate optical response functions (scattering and absorption cross-sections, electric and magnetic field enhancement). We demonstrate that the composite material system brings, in addition to enhanced reconfigurability, also novel features of plasmonic antennas, such as strong optical absorption and a joint electric-magnetic hotspot.

physics.optics↗

arXiv2026

Tuning of Localized Surface Plasmons in Vanadium Dioxide Nanoparticles via Size and Insulator-Metal Transition

Vanadium dioxide has been identified as a promising phase-changing material for use in tunable plasmonic devices. In this study, we present a comprehensive modal analysis of single-phase and multi-phase vanadium dioxide nanoparticles. In-situ high-resolution electron energy loss spectroscopy was utilized to experimentally resolve the dipole plasmon peak, higher-order and breathing plasmonic modes, and bulk losses as a function of nanoparticle size. Furthermore, the focus is directed toward capturing the dynamic nanoscale optical response throughout the metal-insulator transition in a vanadium dioxide nanoparticle. This system possesses the ability to be gradually switched on and off in terms of the emergence of near-infrared plasmonic absorption. The switching is accompanied by a gradual spectral shift of the absorption peak, amounting to 0.18 eV for a 120 nm nanoparticle. It is envisioned that this phenomenon can be generalized to larger nanostructures with a higher aspect ratio, thereby introducing a wider tunability of the system, which is essential for functional nanodevices based on vanadium dioxide.

physics.optics↗

arXiv2025

Charge reservoir as a design concept for plasmonic antennas

Plasmonic antennas exploit localized surface plasmons to shape, confine, and enhance electromagnetic fields with subwavelength resolution. The field enhancement is contributed to by various effects, such as the inherent surface localization of plasmons or the plasmonic lightning-rod effect. Inspired by nanofocusing observed for propagating plasmons, we test the hypothesis that plasmonic antennas with a large cross-section represent a large charge reservoir, enabling large induced charge and field enhancement. Our study reveals that a large charge reservoir is accompanied by large radiative losses, which are the dominant factor, resulting in a low field enhancement.

physics.optics↗

arXiv2025

Ab initio study of angle-resolved electron reflection spectroscopy of few-layer graphene

We present ab initio theory for electron reflection spectroscopy of few-layer graphene for arbitrary angles of incidence. The inelastic effects are included in a consistent way using the optical potential retrieved from ab initio simulations of electron energy-loss spectra. We demonstrate a significant impact of inelastic effects even for single-layer graphene. Next, we address the ability of the electron reflection spectroscopy to determine specific parameters of graphene including not only the number of layers in the few-layer graphene but also the stacking type in the graphene multilayers, and to resolve moiré patterns in twisted graphene bilayers. We show that the predicted contrast, although significantly reduced by inelastic effects, is sufficient for the experimental detection of all considered parameters. Our findings are corroborated by a fair correspondence of our theoretical predictions with experimental data, both our own and recently published by other authors.

cond-mat.mes-hall↗

arXiv2024

Coexisting phases of individual VO$_2$ nanoparticles for multilevel nanoscale memory

Vanadium dioxide (VO$_2$) has received significant interest in the context of nanophotonic metamaterials and memories owing to its reversible insulator-metal transition associated with significant changes in its optical and electronic properties. While the VO$_2$ transition has been extensively studied for several decades, the hysteresis dynamics of individual single-crystal VO$_2$ nanoparticles (NPs) remains largely unexplored. Here, employing transmission electron microscopy techniques, we investigate phase transitions of single VO$_2$ NPs in real time. Our analysis reveals the statistical distribution of the transition temperature and steepness and how they differ during forward (heating) and backward (cooling) transitions. We assess the stability of coexisting phases in individual NPs and prove the persistent multilevel memory at near-room temperatures using only a few VO$_2$ NPs. Our findings shed new light on the underlying physical mechanisms governing the hysteresis of VO$_2$ and establish VO$_2$ NPs as a promising component of optoelectronic and memory devices with enhanced functionalities.

cond-mat.mes-hall↗

arXiv2024

Efficient nanoscale imaging of solid-state phase transitions by transmission electron microscopy demonstrated on vanadium dioxide nanoparticles

We present annular dark field scanning transmission electron microscopy (ADF-STEM) as an efficient, fast, and non-destructive nanoscale tool for monitoring solid-state phase transition. Using metal-insulator transition in vanadium dioxide nanoparticles as an example, we characterize lattice and electronic signatures of the phase transition using analytical transmission electron microscopy including diffraction and electron energy-loss spectroscopy. We demonstrate that ADF-STEM shows a clear contrast across the transition, interpreted with the help of convergent electron beam diffraction as stemming from the crystal-lattice modification accompanying the transition. In addition, ADF-STEM utilizes 3--6 orders of magnitude lower electron dose when compared to electron microscopy techniques able to reveal the phase transition with the same spatial resolution and universality. The benefits of ADF-STEM are emphasized by recording a full hysteresis loop for the metal-insulator transition of a single vanadium dioxide nanoparticle. Our study opens the prospect for fast, non-destructive, large-area and nanoscale characterization of solid-state phase transitions.

cond-mat.mtrl-sci↗

arXiv2024

Plasmonic lightning-rod effect

The plasmonic lightning-rod effect refers to the formation of a strong electric near field of localized surface plasmons at the sharp features of plasmonic antennas. While this effect is intuitively utilized in the design and optimization of plasmonic antennas, the relation between the magnitude of the electric field and the local curvature of the plasmonic antenna has not been yet rigorously established. Here, we provide such a study. We design sets of plasmonic antennas that allow to isolate the role of the local curvature from other effects influencing the field. The near electric field is inspected by electron energy loss spectroscopy and electrodynamic simulations. We demonstrate the existence of the plasmonic lightning-rod effect and establish its quantitative description, showing that its strength is comparable to the electrostatic lightning-rod effect. We also provide a simple phenomenological formula for the spatial dependence of the field. Finally, we introduce the effective radius of curvature related to the spatial distribution of induced charge in plasmonic antennas, significantly smaller than their geometrical radius.

physics.optics↗

arXiv2024

Analytical electron microscopy analysis of insulating and metallic phases in nanostructured vanadium dioxide

Vanadium dioxide (VO$_2$) is a strongly-correlated material that exhibits insulator-to-metal transition (IMT) near room temperature, which makes it a promising candidate for applications in nanophotonics or optoelectronics. However, creating VO$_2$ nanostructures with the desired functionality can be challenging due to microscopic inhomogeneities that can significantly impact the local optical and electronic properties. Thin lamellas, produced by focused ion beam milling from a homogeneous layer, provide a useful prototype for studying VO$_2$ at the truly microscopic level using a scanning transmission electron microscope (STEM). High-resolution imaging is used to identify structural inhomogeneities while electron energy-loss spectroscopy (EELS) supported by statistical analysis helps to detect V$_x$O$_y$ stoichiometries with a reduced oxidation number of vanadium at the areas of thickness below 70 nm. On the other hand, the thicker areas are dominated by vanadium dioxide, where the signatures of IMT are detected in both core-loss and low-loss EELS experiments with in-situ heating. The experimental results are interpreted with ab-initio and semi-classical calculations. This work shows that structural inhomogeneities such as pores and cracks present no harm to the desired optical properties of VO$_2$ samples.

cond-mat.mtrl-sci↗

arXiv2023

Plasmonic sensing using Babinet's principle

Developing methods to sense local variations in nearby materials, such as their refractive index and thickness, is important in different fields including chemistry and biomedical applications, among others. Localized surface plasmons (LSPs) excited in plasmonic nanostructures have demonstrated to be useful in this context due to the spectral location of their associated resonances being sensitive to changes near the plasmonic structures. In this manuscript, Babinet's principle is explored by exploiting LSP resonances excited in complementary metal-dielectric cylindrical plasmonic structures (plasmonic particle-dimers and aperture-dimers in our case). Both plasmonic structures are evaluated numerically and experimentally using Electron Energy Loss Spectroscopy (EELS), providing a full physical understanding of the complementary nature of the excited LSP resonances. The studied plasmonic structures are then exploited for dielectric sensing under two configurations: when a thin dielectric film is positioned atop the plasmonic structures and when the analyte surrounds/fills the plasmonic particles/apertures. The complementary sensing performance of both proposed structures is also evaluated, showing the approximate validity of the Babinet principle with sensitivities values of up to 700 nm/RIU for thin dielectric sensing.

physics.optics↗

arXiv2023

Surface quantum dots with pure, coherent, and blinking-free single photon emission

The surface of semiconductor nanostructures has a major impact on their electronic and optical properties. Disorder and defects in the surface layer typically cause degradation of charge carrier transport and radiative recombination dynamics. However, surface vicinity is inevitable for many scalable nano-optical applications. Epitaxially grown quantum dots are the best candidate for high-performance single photon emission and show great potential for quantum technologies. Yet, these emitters only reveal their excellent properties if they are deeply embedded in a semiconductor host. Until today, quantum dots close to surfaces yield weak, broad, and unstable emissions. Here, we show the complete restoration of optical properties from quantum dots grown directly on a semiconductor surface. The vanishing luminescence from the as-grown sample turns into bright, ultra-stable, coherent and blinking-free single photon emission after sulphur passivation. Under quasi-resonant excitation, single photons are generated with 98.8% purity, 77% indistinguishability, linewidths down to 4 $μ$eV and 99.69% persistency across 11 orders of magnitude in time. The emission is stable even after two years and when being subjected to nanomanufacturing processes. Some long-standing stumbling blocks for surface-dominated quantum dots are thereby removed, unveiling new possibilities for hybrid nano-devices and applications in quantum communication or sensing.

cond-mat.mes-hall↗

arXiv2023

Correlative Imaging of Individual CsPbBr3 Nanocrystals: Role of Isolated Grains in Photoluminescence of Perovskite Polycrystalline Thin Films

We report on the optical properties of CsPbBr3 polycrystalline thin film on a single grain level. A sample comprised of isolated nanocrystals (NCs) mimicking the properties of the polycrystalline thin film grains that can be individually probed by photoluminescence spectroscopy was prepared. These NCs were analyzed using correlative microscopy allowing the examination of structural, chemical, and optical properties from identical sites. Our results show that the stoichiometry of the CsPbBr3 NCs is uniform and independent of the NCs' morphology. The photoluminescence (PL) peak emission wavelength is slightly dependent on the dimensions of NCs, with the blue shift up to 9 nm for the smallest analyzed NCs. The magnitude of the blueshift is smaller than the emission linewidth, thus detectable only by high-resolution PL mapping. By comparing the emission wavelengths obtained from the experiment and a rigorous effective mass model we can fully attribute the observed variations to the size-dependent quantum confinement effect.

physics.chem-ph↗

arXiv2023

Spatio-spectral metrics in electron energy loss spectroscopy as a tool to resolve nearly degenerate plasmon modes in dimer plasmonic antennas

Electron energy loss spectroscopy (EELS) is often utilized to characterize localized surface plasmon modes supported by plasmonic antennas. However, the spectral resolution of this technique is only mediocre, and it can be rather difficult to resolve modes close in the energy, such as coupled modes of dimer antennas. Here we address this issue for a case study of the dimer plasmonic antenna composed of two gold discs. We analyze four nearly degenerate coupled plasmon modes of the dimer: longitudinal and transverse bonding and antibonding dipole modes. With a traditional approach, which takes into account the spectral response of the antennas recorded at specific points, the modes cannot be experimentally identified with EELS. Therefore, we employ the spectral and spatial sensitivity of EELS simultaneously. We propose several metrics that can be utilized to resolve the modes. First, we utilize electrodynamic simulations to verify that the metrics indeed represent the spectral positions of the plasmon modes. Next, we apply the metrics to experimental data, demonstrating their ability to resolve three of the above-mentioned modes (with transverse bonding and antibonding modes still unresolved), identify them unequivocally, and determine their energies. In this respect, the spatio-spectral metrics increase the information extracted from electron energy loss spectroscopy applied to plasmonic antennas.

physics.optics↗

arXiv2022

Plasmonic properties of individual gallium nanoparticles

Gallium is a plasmonic material offering ultraviolet to near-infrared tunability, facile and scalable preparation, and good stability of nanoparticles. In our contribution, we experimentally demonstrate the link between the shape and size of individual gallium nanoparticles and their optical properties. To this end, we utilize scanning transmission electron microscopy combined with electron energy loss spectroscopy. Lens-shaped gallium nanoparticles with a diameter between 10 nm and 200 nm were grown directly on a silicon nitride membrane using an in-house developed effusion cell operated at ultra-high vacuum conditions. We have experimentally proved that they support localized surface plasmon resonances and their dipole mode can be tuned through their size from ultraviolet to near-infrared spectral region. The measurements are supported by numerical simulations using realistic particle shapes and sizes. Our results open the way for future applications of gallium nanoparticles such as hyperspectral absorption of sunlight in energy harvesting or plasmon-enhanced luminescence of ultraviolet emitters.

physics.optics↗

arXiv2022

Fabrication and micro-Raman spectroscopy of arrays of copper phthalocyanine molecular-magnet microdisks

Phthalocyanines as organic semiconductors and molecular magnets provide plenty of industrial or high-tech applications from dyes and pigments up to gas sensors, molecular electronics, spintronics and quantum computing. Copper phthalocyanine (CuPc) belongs among the most used phthalocyanines, typically in the form of powder or films but self-grown nanowires are also known. Here we describe an opposite, i.e., top-down approach based on fabrication of ordered arrays of CuPc microstructures (microdisks) using electron beam lithography and other steps. Among critical points of this approach belongs a choice of a proper resist and a solvent. Fabricated CuPc microdisks have a diameter of 5 $μ$m and heights from 7 up to 70 nm. Micro-Raman spectroscopy of the films and microdisks reveals a crystalline $β$ phase associated with a paramagnetic form. Additional measurements with an increasing laser power show a significant shift ($Δω$ ~ 7.1 cm$^{-1}$ ) and broadening of a peak at 1532 rel$\cdot$cm$^{-1}$ corresponding to the phonon B1g mode. The observed smooth changes exclude a phase transition and confirm the thermally stable polymorph. Our versatile fabrication technique using the common lithographic resist brings new possibilities for the fabrication of various micro/nanostructures such as micromagnets, heterostructures or organic electronic devices.

physics.app-ph↗

arXiv2021

Strong coupling in a Au plasmonic antenna-SiO$_{2}$ layer system: a hybrid mode analysis

A detailed analysis of the optical response of a system accommodating several coupled modes is needed for the complete understanding of the strong coupling effect. In this paper, we report on the analysis of scattering cross section spectra of Au antennas on a SiO$_{2}$ layer on a Si substrate in the IR region. A classical model of coupled oscillators is used for determining the resonant energies, damping rates and coupling strengths of four phonon polariton modes in the SiO$_{2}$ layer coupled to a localized surface plasmon mode in a Au antenna. The calculated Hopfield mixing coefficients then show the contribution of the individual uncoupled modes to the hybrid modes of the coupled system.

cond-mat.mes-hall↗

arXiv2020

Structural and optical properties of monocrystalline and polycrystalline gold plasmonic nanorods

Plasmonic structures are often fabricated by lithographic patterning of a thin metallic film. Properties of the thin film are intimately related to the quality of the resulting structures. Here we compare two kinds of thin gold films on silicon nitride membrane: a conventional polycrystalline thin film deposited by magnetron sputtering, and monocrystalline gold microplates which were chemically synthesised directly on the membrane's surface for the first time. Both pristine metals were used to fabricate plasmonic nanorods using focused ion beam lithography. Structural and optical properties of the nanorods were characterized by analytical transmission electron microscopy. The dimensions of the nanorods in both substrates reproduced well the designed size of $240 \times 80\ \mathrm{nm^2}$ with the deviations up to 20 nm in both length and width. The shape reproducibility was considerably improved among monocrystalline nanorods fabricated from the same microplate. Interestingly, monocrystal nanorods featured inclined boundaries while the boundaries of the polycrystal nanorods were upright. Q factors and peak loss probabilities of the modes in both structures are within the experimental uncertainty identical. We conclude that the optical response of the plasmonic antennas is not deteriorated when the polycrystalline metal is used instead of the monocrystalline metal.

physics.app-ph↗

arXiv2020

Vacuum Rabi splitting of a dark plasmonic cavity mode revealed by fast electrons

Recent years have seen a growing interest in strong coupling between plasmons and excitons, as a way to generate new quantum optical testbeds and influence chemical dynamics and reactivity. Strong coupling to bright plasmonic modes has been achieved even with single quantum emitters. Dark plasmonic modes fare better in some applications due to longer lifetimes, but are difficult to probe as they are subradiant. Here, we apply electron energy loss (EEL) spectroscopy to demonstrate that a dark mode of an individual plasmonic bowtie can interact with a small number of quantum emitters, as evidenced by Rabi-split spectra. Coupling strengths of up to 85 meV place the bowtie-emitter devices at the onset of the strong coupling regime. Remarkably, the coupling occurs at the bowtie gap periphery, even while the electron beam probes their center. Our findings pave the way for using EEL spectroscopy to study exciton-plasmon interactions involving non-emissive photonic modes.

cond-mat.mes-hall↗

arXiv2019

Independent engineering of individual plasmon modes in plasmonic dimers with conductive and capacitive coupling

We revisit plasmonic modes in nanoparticle dimers with conductive or insulating junction resulting in conductive or capacitive coupling. In our study which combines electron energy loss spectroscopy, optical spectroscopy, and numerical simulations, we show coexistence of strongly and weakly hybridized modes. While the properties of the former ones strongly depend on the nature of the junction, the properties of the latter ones are nearly unaffected. This opens up a prospect for independent engineering of different plasmonic modes in a single plasmonic antenna. In addition, we show that Babinet's principle allows to engineer the near field of plasmonic modes independent of their energy. Finally, we demonstrate that combined electron energy loss imaging of a plasmonic antenna and its Babinet-complementary counterpart allows to reconstruct the distribution of both electric and magnetic near fields of localised plasmonic resonances supported by the antenna as well as charge and current antinodes of related charge oscillations.

physics.app-ph↗