Petr Dvořák

Control and near-field detection of surface plasmon interference patterns.

The tailoring of electromagnetic near-field properties is the central task in the field of nanophotonics. In addition to 2D optics for optical nanocircuits, confined and enhanced electric fields are utilized in detection and sensing, photovoltaics, spatially localized spectroscopy (nanoimaging), as well as in nanolithography and nanomanipulation. For practical purposes, it is necessary to develop easy-to-use methods for controlling the electromagnetic near-field distribution. By imaging optical near-fields using a scanning near-field optical microscope, we demonstrate that surface plasmon polaritons propagating from slits along the metal-dielectric interface form tunable interference patterns. We present a simple way how to control the resulting interference patterns both by variation of the angle between two slits and, for a fixed slit geometry, by a proper combination of laser beam polarization and inhomogeneous far-field illumination of the structure. Thus the modulation period of interference patterns has become adjustable and new variable patterns consisting of stripelike and dotlike motifs have been achieved, respectively.

Ultrasmooth metallic foils for growth of high quality graphene by chemical vapor deposition

Synthesis of graphene by chemical vapor deposition is a promising route for manufacturing large-scale high-quality graphene for electronic applications. The quality of the employed substrates plays a crucial role, since the surface roughness and defects alter the graphene growth and cause difficulties in the subsequent graphene transfer. Here, we report on ultrasmooth high-purity copper foils prepared by sputter deposition of Cu thin film on a SiO2/Si template, and the subsequent peeling off of the metallic layer from the template. The surface displays a low level of oxidation and contamination, and the roughness of the foil surface is generally defined by the template, and was below 0.6 nm even on a large scale. The roughness and grain size increase occurred during both the annealing of the foils, and catalytic growth of graphene from methane (≈1000 °C), but on the large scale still remained far below the roughness typical for commercial foils. The micro-Raman spectroscopy and transport measurements proved the high quality of graphene grown on such foils, and the room temperature mobility of the graphene grown on the template stripped foil was three times higher compared to that of one grown on the commercial copper foil. The presented high-quality copper foils are expected to provide large-area substrates for the production of graphene suitable for electronic applications.

Unmanned aircraft in nature conservation: an example from plant invasions

ABSTRACT To successfully fight plant invasions, new methods enabling fast and efficient monitoring are needed, and remote sensing can make their management more efficient and less expensive. However, the data resolution, cost, and availability can be limiting. Optimal solution depends on the species characteristics, where the spectral and spatial resolution can compensate each other to some extent, and phenology plays an important role. Available high spatial resolution satellite data are sufficient for recognition of species that are distinct and either large or form uniform patches at size comparable to the data pixel size. For other species, higher spatial resolution is needed, and unmanned aircraft (UAV) provide data of extremely high spatial resolution (cm) at low cost and high flexibility. We assess its potential to map invasive black locust (BL, Robinia pseudoaccacia), testing imagery of different origin (satellite, UAV), spectral (multispectral, red, green, and blue (RGB) + near-infrared (NIR)) and spatial resolution, and various technical approaches to choose the best strategy for the species monitoring balancing between precision of detection and economic feasibility. Using purposely designed low-cost UAV with tailless fixed wing design for two consumer cameras (RGB and modified NIR) ensures robustness and repeatable field performance while maintaining high aerodynamic efficiency, with resulting mapping capacity over 10 km2 per day. Several challenges exist in UAV application, such as lower spectral resolution, geometrical and radiometric distortions, and significant amount of data (necessity of automatic processing). In our study, we tested different options of UAV data processing and present comparison of resulting orthomosaic accuracies. For repeated measurements, it is extremely important to ensure spatial co-registration of pixels/objects from different phenological phases. Investment in GPS receiver in the UAV and GPS post-processing eliminated laborious collection of ground control points, while maintaining the co-registration of objects across multiple flights. In our study we provide evidence of benefit of the low cost unmanned system for species monitoring with high classification accuracies of target species from UAV orthomosaic outcompeting WorldView-2 satellite data, and describe methodology that can be used for practical management of invasions.

In-situ magnetic nano-patterning of Fe films grown on Cu(100)

Metastable paramagnetic face-centered cubic (fcc) Fe films grown on a Cu(100) single crystal at room temperature can be transformed to the ferromagnetic body-centered cubic (bcc) structure by ion irradiation. We have employed this technique to write small ferromagnetic patches by Ar+ irradiation through a gold coated SiN mask with regularly arranged 80-nm diameter holes, which was placed on top of the as-prepared fcc Fe films. Nanopatterning was performed on both 8-monolayer (ML) Fe films grown in ultrahigh vacuum as well as 22-ML films stabilized by dosing carbon monoxide during growth. The structural transformation of these nano-patterned films was investigated using scanning tunneling microscopy. In both 8 and 22-ML fcc Fe films, the bcc needles are found to protrude laterally out of the irradiated part of the sample, limiting the resolution of the technique to a few 10 nm. The magnetic transformation was confirmed by magnetic force microscopy.

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.

Imaging of near-field interference patterns by aperture-type SNOM – influence of illumination wavelength and polarization state

Scanning near-field optical microscopy (SNOM) in combination with interference structures is a powerful tool for imaging and analysis of surface plasmon polaritons (SPPs). However, the correct interpretation of SNOM images requires profound understanding of principles behind their formation. To study fundamental principles of SNOM imaging in detail, we performed spectroscopic measurements by an aperture-type SNOM setup equipped with a supercontinuum laser and a polarizer, which gave us all the degrees of freedom necessary for our investigation. The series of wavelength- and polarization-resolved measurements, together with results of numerical simulations, then allowed us to identify the role of individual near-field components in formation of SNOM images, and to show that the out-of-plane component generally dominates within a broad range of parameters explored in our study. Our results challenge the widespread notion that this component does not couple to the aperture-type SNOM probe and indicate that the issue of SNOM probe sensitivity towards the in-plane and out-of-plane near-field components - one of the most challenging tasks of near field interference SNOM measurements - is not yet fully resolved.

Patterning large area plasmonic nanostructures on nonconductive substrates using variable pressure electron beam lithography

Variable pressure electron beam lithography (VP-EBL) is a unique technique offering alternative cost-effective approach for patterning on nonconductive substrates that are often required for many applications in the field of plasmonics. Here, the authors present the use of the VP-EBL for accurate fabrication of nanoantennas with plasmonic resonances in visible range in order to achieve artificial sample coloring. Using confocal transmission spectroscopy, the authors show that optimized VP-EBL process enables fabrication of plasmonic nanoantennas with optical properties equivalent to those produced via traditional approach. Furthermore, the authors demonstrate high stability of the exposure process by fabricating a millimeter-sized color image composed of plasmonic nanoantennas.