Raul O. Freitas

X-ray phase measurements as a probe of small structural changes in doped nonlinear optical crystals

X-ray multiple diffraction experiments with synchrotron radiation were carried out on pure and doped nonlinear optical crystals: NH4H2PO4 and KH2PO4 doped with Ni and Mn, respectively. Variations in the intensity profiles were observed from pure to doped samples, and these variations correlated with shifts in the structure factor phases, also known as triplet phases. This result demonstrates the potential of X-ray phase measurements to study doping in this type of single crystal. Different methodologies for probing structural changes were developed. Dynamical diffraction simulations and curve fitting procedures were also necessary for accurate phase determination. Structural changes causing the observed phase shifts are discussed.

Growth and capping of InAs/GaAs quantum dots investigated by x-ray Bragg-surface diffraction

An x-ray diffraction method, based on the excitation of a surface diffracted wave, is described to investigate the capping process of InAs/GaAs (001) quantum dots (QDs). It is sensitive to the tiny misorientation of (111) planes at the surface of the buffer layer on samples with exposed QDs. After capping, the misorientation occurs in the cap-layer lattice faceting the QDs and its magnitude can be as large as 10° depending on the QDs growth rates, probably due to changes in the size and shape of the QDs. A slow strain release process taking place at room temperature has also been observed by monitoring the misorientation angle of the (111) planes.

Strain field of InAs QDs on GaAs (001) substrate surface: characterization by synchrotron X-ray Renninger scanning

Precise lattice parameter measurements in single crystals are achievable, in principle, by X-ray multiple diffraction (MD) experiments. Tiny sample misalignments can compromise systematic usage of MD in studies where accuracy is an important issue. In this work, theoretical treatment and experimental methods for correcting residual misalignment errors are presented and applied to probe the induced strain of buried InAs quantum dots on GaAs (001) substrates.

Infrared Fingerprints of Natural 2D Talc and Plasmon–Phonon Coupling in Graphene–Talc Heterostructures

Two-dimensional (2D) materials occupy noteworthy place in nanophotonics providing for subwavelength light confinement and optical phenomena dissimilar to those of their bulk counterparts. In the mid-infrared, graphene-based heterostructures and van der Waals crystals of hexagonal boron nitride (hBN) overwhelmingly concentrate the attention by exhibiting real-space nano-optics from plasmons, phonon-polaritons and hybrid plasmon phonon-polaritons quasiparticles. Here we present the mid-infrared nanophotonics of talc, a natural atomically flat layered material, and graphene-talc (G-talc) heterostructures using broadband synchrotron infrared nano-spectroscopy. We achieve wavelength tuning of the talc resonances, assigned to in- and out-of-plane vibrations by changing the thickness of the crystals, which serves as its infrared fingerprints. Moreover, we encounter coupling of the graphene plasmons polaritons with surface optical phonons of talc. As in the case of the G-hBN heterostructures, this coupling configures hybrid surface plasmon phonon-polariton modes causing 30 % increase in intensity for the out-of-plane mode, blue-shift for the in-plane mode and we have succeeded in altering the amplitude of such hybridization by varying the gate voltage. Therefore, our results promote talc and G-talc heterostructures as appealing materials for nanophotonics, like hBN and G-hBN, with potential applications for controllably manipulating infrared electromagnetic radiation at the subdiffraction scale.

Infrared nano-imaging and -spectroscopy of N graphene layers on SiO2 and hexagonal boron nitride substrates.

The optical response of exfoliated graphene on different surfaces (silicon dioxide (SiO2) and hexagonal boron nitride (hBN)) is investigated via scattering-type scanning near-field optical microscopy (s-SNOM) using broadband infrared synchrotron radiation. Basically, we use a commercial s-SNOM microscope integrated into the infrared synchrotron-based beamline to investigate with nanoscale resolution the optical response of different graphene layers on SiO2 or hBN substrates. Comparing atomic force microscopic topography and broadband mid-infrared images (lateral resolution of 30 nm), we confirm that optical response of both systems depends on the specific interactions between graphene and substrate as well as on the number of graphene layers. This dependence is explained by particular interactions of graphene and SiO2, wherein graphene plasmons couple to surface phonon-polaritons of SiO2. In the case of graphene and hBN, we observe coupling of the graphene plasmon to the hyperbolic phonon-polaritons of hBN.

Measuring Friedel pairs in nanomembranes of GaAs (001)

Friedel pairs are susceptible to symmetry breaking in crystals. Under resonant scattering conditions, non-centrosymmetric crystals can give rise to pairs of hkl and

Graphene/h-BN plasmon–phonon coupling and plasmon delocalization observed by infrared nano-spectroscopy

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Low-aberration beamline optics for synchrotron infrared nanospectroscopy

h¯k¯l¯ reflections with different diffracted intensities, which are quantified as an anomalous signal of the structure. In bulk crystals, the shift in the anomalous signal through an absorption edge can be measured with good accuracy regardless the crystalline quality of the sample, leading to experimental values in agreement with theoretical ones. With the advance of nanotechnology and synchrotron sources, it has been possible to produce free-standing nanomembranes of semiconductor crystals, opening the opportunity of checking the measurability of anomalous signal in nanoscale materials. In this study, we describe a successful procedure to measure the anomalous signal in nanomembranes of GaAs (001) 15-nm thick with synchrotron radiation. Different membrane processing methods and diffraction geometries were tested, and major sources of instrumental inaccuracy were identified. Relevances of this type of measurements in nanotechnology as well as in basic science are discussed.