Exposure to gallium arsenide nanoparticles in a research facility: a case study using molecular beam epitaxy.

We evaluated GaAs nanoparticle-concentrations in the air and on skin and surfaces in a research facility that produces thin films, and to monitored As in the urine of exposed worker. The survey was over a working week using a multi-level approach. Airborne personal monitoring was implemented using a miniature diffusion size classifier (DiSCMini) and IOM sampler. Environmental monitoring was conducted using the SKC Sioutas Cascade Impactor to evaluate dimensions and nature of particles collected. Surfaces contamination were assessed analyzing As and Ga in ghost wipes. Skin contamination was monitored using tape strips. As and Ga were analyzed in urines collected every day at the beginning and end of the shift. The greatest airborne exposure occurred during the cutting operations of the GaAs Sample (88883 np/cm3). The highest levels of contamination were found inside the hood (As max = 1418 ng/cm2) and on the laboratory floor (As max = 251 ng/cm2). The average concentration on the worker's skin at the end of the work shift (3.36 ng/cm2) was more than 14 times higher than before the start of the shift. In weekly urinary biomonitoring an average As concentration of 19.5 µg/L, which was above the Società Italiana Valori di Riferimento (SIVR) reference limit for the non-occupational population (2.0 - 15 µg/L), but below the ACGIH limit (30 µg/L). Overall, airborne monitoring, surface sampling, skin sampling, and biomonitoring of worker confirmed the exposure to As of workers. Systematic cleaning operations, hood implementation and correct PPE management are needed to improve worker protection.

Implication of the double-gating mode in a hybrid photon counting detector for measurements of transient heat conduction in GaAs/AlAs superlattice structures.

Understanding and control of thermal transport in solids at the nanoscale are crucial in engineering and enhance the properties of a new generation of optoelectronic, thermoelectric and photonic devices. In this regard, semiconductor superlattice structures provide a unique platform to study phenomena associated with phonon propagations in solids such as heat conduction. Transient X-ray diffraction can directly probe atomic motions and therefore is among the rare techniques sensitive to phonon dynamics in condensed matter. Here, optically induced transient heat conduction in GaAs/AlAs superlattice structures is studied using the EIGER2 detector. Benchmark experiments have been performed at the Austrian SAXS beamline at Elettra-Sincrotrone Trieste operated in the hybrid filling mode. This work demonstrates that drifts of experimental conditions, such as synchrotron beam fluctuations, become less essential when utilizing the EIGER2 double-gating mode which results in a faster acquisition of high-quality data and facilitates data analysis and data interpretation.

Two-dimensional quantum universality in the spin-1/2 triangular-lattice quantum antiferromagnet Na2BaCo(PO4)2.

An interplay of geometrical frustration and strong quantum fluctuations in a spin-1/2 triangular-lattice antiferromagnet (TAF) can lead to exotic quantum states. Here, we report the neutron-scattering, magnetization, specific heat, and magnetocaloric studies of the recently discovered spin-1/2 TAF Na2BaCo(PO4)2, which can be described by a spin-1/2 easy axis XXZ model. The zero-field neutron diffraction experiment reveals an incommensurate antiferromagnetic ground state with a significantly reduced ordered moment of about 0.54(2) µB/Co. Different magnetic phase diagrams with magnetic fields in the ab plane and along the easy c-axis were extracted based on the magnetic susceptibility, specific heat, and elastic neutron-scattering results. In addition, two-dimensional (2D) spin dispersion in the triangular plane was observed in the high-field polarized state, and microscopic exchange parameters of the spin Hamiltonian have been determined through the linear spin wave theory. Consistently, quantum critical behaviors with the universality class of d = 2 and νz = 1 were established in the vicinity of the saturation field, where a Bose-Einstein condensation (BEC) of diluted magnons occurs. The newly discovered quantum criticality and fractional magnetization phase in this ideal spin-1/2 TAF present exciting opportunities for exploring exotic quantum phenomena.

Metamaterial-enabled asymmetric negative refraction of GHz mechanical waves.

Wave refraction at an interface between different materials is a basic yet fundamental phenomenon, transversal to several scientific realms - electromagnetism, gas and fluid acoustics, solid mechanics, and possibly also matter waves. Under specific circumstances, mostly enabled by structuration below the wavelength scale, i.e., through the metamaterial approach, waves undergo negative refraction, eventually enabling superlensing and transformation optics. However, presently known negative refraction systems are symmetric, in that they cannot distinguish between positive and negative angles of incidence. Exploiting a metamaterial with an asymmetric unit cell, we demonstrate that the aforementioned symmetry can be broken, ultimately relying on the specific shape of the Bloch mode isofrequency curves. Our study specialized upon a mechanical metamaterial operating at GHz frequency, which is by itself a building block for advanced technologies such as chip-scale hybrid optomechanical and electromechanical devices. However, the phenomenon is based on general wave theory concepts, and it applies to any frequency and time scale for any kind of linear waves, provided that a suitable shaping of the isofrequency contours is implemented.

Synchrotron Radiation Study of Gain, Noise, and Collection Efficiency of GaAs SAM-APDs with Staircase Structure.

In hard X-ray applications that require high detection efficiency and short response times, such as synchrotron radiation-based Mössbauer absorption spectroscopy and time-resolved fluorescence or photon beam position monitoring, III-V-compound semiconductors, and dedicated alloys offer some advantages over the Si-based technologies traditionally used in solid-state photodetectors. Amongst them, gallium arsenide (GaAs) is one of the most valuable materials thanks to its unique characteristics. At the same time, implementing charge-multiplication mechanisms within the sensor may become of critical importance in cases where the photogenerated signal needs an intrinsic amplification before being acquired by the front-end electronics, such as in the case of a very weak photon flux or when single-photon detection is required. Some GaAs-based avalanche photodiodes (APDs) were grown by a molecular beam epitaxy to fulfill these needs; by means of band gap engineering, we realised devices with separate absorption and multiplication region(s) (SAM), the latter featuring a so-called staircase structure to reduce the multiplication noise. This work reports on the experimental characterisations of gain, noise, and charge collection efficiencies of three series of GaAs APDs featuring different thicknesses of the absorption regions. These devices have been developed to investigate the role of such thicknesses and the presence of traps or defects at the metal-semiconductor interfaces responsible for charge loss, in order to lay the groundwork for the future development of very thick GaAs devices (thicker than 100 µm) for hard X-rays. Several measurements were carried out on such devices with both lasers and synchrotron light sources, inducing photon absorption with X-ray microbeams at variable and controlled depths. In this way, we verified both the role of the thickness of the absorption region in the collection efficiency and the possibility of using the APDs without reaching the punch-through voltage, thus preventing the noise induced by charge multiplication in the absorption region. These devices, with thicknesses suitable for soft X-ray detection, have also shown good characteristics in terms of internal amplification and reduction of multiplication noise, in line with numerical simulations.

Fast amplitude modulation up to 1.5 GHz of mid-IR free-space beams at room-temperature.

Applications relying on mid-infrared radiation (λ ~ 3-30 µm) have progressed at a very rapid pace in recent years, stimulated by scientific and technological breakthroughs like mid-infrared cameras and quantum cascade lasers. On the other side, standalone and broadband devices allowing control of the beam amplitude and/or phase at ultra-fast rates (GHz or more) are still missing. Here we show a free-space amplitude modulator for mid-infrared radiation (λ ~ 10 µm) that can operate at room temperature up to at least 1.5 GHz (-3dB cutoff at ~750 MHz). The device relies on a semiconductor heterostructure enclosed in a judiciously designed metal-metal optical resonator. At zero bias, it operates in the strong light-matter coupling regime up to 300 K. By applying an appropriate bias, the device transitions towards the weak-coupling regime. The large change in reflectance is exploited to modulate the intensity of a mid-infrared continuous-wave laser up to 1.5 GHz.

Magnetic-Field-Dependent Equilibration of Fractional Quantum Hall Edge Modes.

Fractional conductance is measured by partitioning a ν=1 edge state using gate-tunable fractional quantum Hall (FQH) liquids of filling 1/3 or 2/3 for current injection and detection. We observe two sets of FQH plateaus 1/9, 2/9, 4/9 and 1/6, 1/3, 2/3 at low and high magnetic field ends of the ν=1 plateau, respectively. The findings are explained by magnetic field dependent equilibration of three FQH edge modes with conductance e^{2}/3h arising from edge reconstruction. The results reveal a remarkable enhancement of the equilibration lengths of the FQH edge modes with increasing magnetic field.

Departure from the Babinet principle in metasurfaces supported by subwavelength dielectric slabs.

Symmetry principles and theorems are of crucial importance in optics. Indeed, from one side, they allow obtaining direct insights into phenomena by eliminating unphysical interpretations; from the other side, they guide the designer of photonic components by narrowing down the parameter space of design variables. In this Letter, we illustrate a significant departure from the Babinet spectral complementarity in a very common and technologically relevant situation: that of a patterned conducting screen placed on a subwavelength dielectric slab. The symmetry property predicted by the Babinet theorem is correctly recovered for pairs of geometrically complementary-but less realistic in terms of applications-free-standing patterned screens. Our analysis merges experimental data with fully vectorial electromagnetic modeling and provides an alternative form of the Babinet theorem that highlights a connection with the concept of electromagnetic duality.

Picosecond pump-probe X-ray scattering at the Elettra SAXS beamline.

A new setup for picosecond pump-probe X-ray scattering at the Austrian SAXS beamline at Elettra-Sincrotrone Trieste is presented. A high-power/high-repetion-rate laser has been installed on-site, delivering UV/VIS/IR femtosecond-pulses in-sync with the storage ring. Data acquisition is achieved by gating a multi-panel detector, capable of discriminating the single X-ray pulse in the dark-gap of the Elettra hybrid filling mode. Specific aspects of laser- and detection-synchronization, on-line beam steering as well protocols for spatial and temporal overlap of laser and X-ray beam are also described. The capabilities of the setup are demonstrated by studying transient heat-transfer in an In/Al/GaAs superlattice structure and results are confirmed by theoretical calculations.

III-V on CaF2: a possible waveguiding platform for mid-IR photonic devices.

We developed a technique that enables replacement of a metallic waveguide cladding with a low-index (n≈1.4) material - CaF2 or BaF2. It is transparent from the mid-IR up to the visible range: elevated confinement is preserved while introducing an optical entryway through the substrate. Replacing the metallic backplane also allows double-side patterning of the active region. Using this approach, we demonstrate strong light-matter coupling between an intersubband transition (λ∼10 µm) and a dispersive resonator at 300 K and at 78 K. Finally, we evaluate this approach's potential as a platform for waveguiding in the mid-IR spectral range, with numerical simulations that reveal losses in the 1-10 cm-1 range.

First observation of the quantized exciton-polariton field and effect of interactions on a single polariton.

Polaritons are quasi-particles that originate from the coupling of light with matter and that demonstrate quantum phenomena at the many-particle mesoscopic level, such as Bose-Einstein condensation and superfluidity. A highly sought and long-time missing feature of polaritons is a genuine quantum manifestation of their dynamics at the single-particle level. Although they are conceptually perceived as entangled states and theoretical proposals abound for an explicit manifestation of their single-particle properties, so far their behavior has remained fully accounted for by classical and mean-field theories. We report the first experimental demonstration of a genuinely quantum state of the microcavity polariton field, by swapping a photon for a polariton in a two-photon entangled state generated by parametric downconversion. When bringing this single-polariton quantum state in contact with a polariton condensate, we observe a disentangling with the external photon. This manifestation of a polariton quantum state involving a single quantum unlocks new possibilities for quantum information processing with interacting bosons.

Site-Control of InAs/GaAs Quantum Dots with Indium-Assisted Deoxidation.

Site-controlled epitaxial growth of InAs quantum dots on GaAs substrates patterned with periodic nanohole arrays relies on the deterministic nucleation of dots into the holes. In the ideal situation, each hole should be occupied exactly by one single dot, with no nucleation onto planar areas. However, the single-dot occupancy per hole is often made difficult by the fact that lithographically-defined holes are generally much larger than the dots, thus providing several nucleation sites per hole. In addition, deposition of a thin GaAs buffer before the dots tends to further widen the holes in the [110] direction. We have explored a method of native surface oxide removal by using indium beams, which effectively prevents hole elongation along [110] and greatly helps single-dot occupancy per hole. Furthermore, as compared to Ga-assisted deoxidation, In-assisted deoxidation is efficient in completely removing surface contaminants, and any excess In can be easily re-desorbed thermally, thus leaving a clean, smooth GaAs surface. Low temperature photoluminescence showed that inhomogeneous broadening is substantially reduced for QDs grown on In-deoxidized patterns, with respect to planar self-assembled dots.

Integrated architecture for the electrical detection of plasmonic resonances based on high electron mobility photo-transistors.

We report the design of an integrated platform for on-chip electrical transduction of the surface plasmon resonance supported by a nanostructured metal grating. The latter is fabricated on the active area of a GaAs/AlGaAs photo-HEMT and simultaneously works as the electronic gate of the device. The gold plasmonic crystal has a V-groove profile and has been designed by numerical optical simulations. By showing that the numerical models accurately reproduce the phototransistors experimental response, we demonstrate that the proposed architecture is suitable for the development of a new class of compact and scalable SPR sensors.

Extremely sub-wavelength THz metal-dielectric wire microcavities.

We demonstrate minimal volume wire THz metal-dielectric micro-cavities, in which all but one dimension have been reduced to highly sub-wavelength values. The smallest cavity features an effective volume of 0.4 µm(3), which is ~5.10(-7) times the volume defined by the resonant vacuum wavelength (λ = 94 µm) to the cube. When combined with a doped multi-quantum well structure, such micro-cavities enter the ultra-strong light matter coupling regime, even if the total number of electrons participating to the coupling is only in the order of 10(4), thus much less than in previous studies.

Imaging fractional incompressible stripes in integer quantum Hall systems.

Transport experiments provide conflicting evidence on the possible existence of fractional order within integer quantum Hall systems. In fact, integer edge states sometimes behave as monolithic objects with no inner structure, while other experiments clearly highlight the role of fractional substructures. Recently developed low-temperature scanning probe techniques offer today an opportunity for a deeper-than-ever investigation of spatial features of such edge systems. Here we use scanning-gate microscopy and demonstrate that fractional features were unambiguously observed in every integer quantum Hall constriction studied. We present also an experimental estimate of the width of the fractional incompressible stripes corresponding to filling factors 1/3, 2/5, 3/5, and 2/3. Our results compare well with predictions of the edge-reconstruction theory.

Controlled coupling of spin-resolved quantum Hall edge states.

We introduce and experimentally demonstrate a new method that allows us to controllably couple copropagating spin-resolved edge states of a two-dimensional electron gas (2DEG) in the integer quantum Hall regime. The scheme exploits a spatially periodic in-plane magnetic field that is created by an array of Cobalt nanomagnets placed at the boundary of the 2DEG. A maximum charge or spin transfer of 28±1% is achieved at 250 mK.

Tuning nonlinear charge transport between integer and fractional quantum Hall states.

Controllable point junctions between different quantum Hall phases are a necessary building block for the development of mesoscopic circuits based on fractionally charged quasiparticles. We demonstrate how particle-hole duality can be exploited to realize such point-contact junctions. We show an implementation for the case of two quantum Hall liquids at filling factors nu=1 and nu*

Growth of vertical InAs nanowires on heterostructured substrates.

We investigate the Au-assisted growth of InAs nanowires on two different kinds of heterostructured substrates: GaAs/AlGaAs structures capped by a 50 nm thick InAs layer grown by molecular beam epitaxy and a 2 microm thick InAs buffer layer on Si(111) obtained by vapor phase epitaxy. Morphological and structural properties of substrates and nanowires are analyzed by atomic force and transmission electron microscopy. Our results indicate a promising direction for the integration of III-V nanostructures on Si-based electronics as well as for the development of novel micromechanical structures incorporating nanowires as their active elements.

Interedge strong-to-weak scattering evolution at a constriction in the fractional quantum Hall regime.

Gate-voltage control of interedge tunneling at a split-gate constriction in the fractional quantum Hall regime is reported. Quantitative agreement with the behavior predicted for out-of-equilibrium quasiparticle transport between chiral Luttinger liquids is shown at low temperatures at specific values of the backscattering strength. When the latter is lowered by changing the gate voltage, the zero-bias peak of the tunneling conductance evolves into a minimum, and a nonlinear quasiholelike characteristic emerges. Our analysis emphasizes the role of the local filling factor in the split-gate constriction region.

Microcavity polariton splitting of intersubband transitions.

The optical response of the intersubband excitation of multiple two-dimensional electron gases within a semiconductor microcavity has been studied through angle-dependent reflectance measurements. Using a resonator based on total internal reflection, a clear splitting of about 14 meV of the coupled intersubband cavity modes is observed from 10 K to room temperature, with resulting polaritonlike dispersion. The experimental findings are in good agreement with theoretical calculations performed in a transfer-matrix formalism.