Graphene strain engineering using micropatterned SiN membranes

Since its discovery in 2004 by Novoselov and Geim, graphene, a two-dimentional allotrope of carbon, has drawn the attention of the scientific community due to its surprising properties. The spatial arrangement of its carbon atoms in a honeycomb lattice creates in fact a unique electronic band structure, which causes conducting electrons to behave like massless Dirac fermions. Furthermore, the lattice configuration and the chemical composition of graphene leads to the formation of strong in-plane bonds, which grants this material an exceptional stability and mechanical strength. One of graphene's most interesting features is the possibility to tailor its electronic properties through the application of strain, and to generate a variety of phenomena such as -- to cite one of the most intriguing possibility -- the creation of pseudospin-dependent gauge magnetic fields, or pseudomagnetic field, when the lattice is subject to triaxial strain. Moderate deformations are expected to lead to magnetic fields of various Teslas and to significantly impact the quantum states of electrons in graphene. These perspectives, along with its two-dimensional nature, make graphene appealing for the realization of new devices both in the context of opto- and electro-mechanics. The control of strain in graphene, though, remains today a challenging objective. The aim of this thesis has been the creation and the investigation of different strain profiles on free-standing graphene monolayers. This target has been achieved by applying a differential pressure on silicon nitride (SiN) membranes patterned with pass-through holes of different sizes and shapes, onto which monolayer graphene has been deposited. A first part of the work has been focused on the preparation of the samples, which were obtained by micro-fabricating the SiN patterned membranes in a cleanroom, thanks to a combination of e-beam lithography, UV lithography, wet and dry etching, and by depositing on top of them monolayer graphene grown by chemical vapor deposition. Subsequently, the induced strain has been studied by micro-Raman spectroscopy, as a function of the hole geometry and of the applied differential pressure. The analysis of the Raman maps acquired over the suspended strained graphene indicates first of all a clear shift of both the G and 2D peaks: this is consistent with what expected in the presence of a hydrostatic deformation of graphene, due to the strain-induced modification of the energy of the phonons. The magnitude of the Raman peak shift can in fact be used as a direct measure of the local average strain. As a novel result, during my thesis I could also demonstrate the presence of an anisotropic component of the strain, in the case of devices obtained on elliptical holes in the SiN. Anysotropy was detected in terms of a non trivial evolution of the width of the doubly-degenerate peak of the G phononic mode and of its splitting in its two G+ and G- components. All the observed peak shifts were found to be in good agreement with the results reported in the literature for the value of the applied strain, which has been numerically estimated through simulations of the studied devices. The results here described indicate thus a new strategy for the creation of a broad range of custom strain profiles, which can be controlled by the applied pressure and by the geometry of the supporting SiN membrane

Probing the Purcell effect without radiative decay: Lessons in the frequency and time domains

The microscopic processes underlying electro-optic sampling of quantum-vacuum fluctuations are discussed, leading to the interpretation of these experiments in terms of an exchange of virtual photons. With this in mind it is shown how one can directly study the Purcell effect, i.e. the changes induced by cavities upon the quantum vacuum, in the frequency and time domains. This forges a link between electro-optic sampling of the quantum vacuum and geometry-induced vacuum effects

Anisotropic straining of graphene using micropatterned SiN membranes

We use micro-Raman spectroscopy to study strain profiles in graphene monolayers suspended over SiN membranes micropatterned with holes of non-circular geometry. We show that a uniform differential pressure load $\Delta P$ over elliptical regions of free-standing graphene yields measurable deviations from hydrostatic strain conventionally observed in radially-symmetric microbubbles. The top hydrostatic strain $\bar{\varepsilon}$ we observe is estimated to be $\approx0.7\%$ for $\Delta P = 1\,{\rm bar}$ in graphene clamped to elliptical SiN holes with axis $40$ and $20\,{\rm \mu m}$. In the same configuration, we report a $G_\pm$ splitting of $10\,{\rm cm^{-1}}$ which is in good agreement with the calculated anisotropy $\Delta\varepsilon \approx 0.6\%$ for our device geometry. Our results are consistent with the most recent reports on the Gr\"uneisen parameters. Perspectives for the achievement of arbitrary strain configurations by designing suitable SiN holes and boundary clamping conditions are discussed.Comment: 8 pages, 6 figure (including SI

Detection of quantum-vacuum field correlations outside the light cone

According to quantum field theory, empty space – the ground state with all real excitations removed –is not empty, but filled with quantum-vacuum fluctuations. Their presence can manifest itself through phenomena such as the Casimir force, spontaneous emission, or dispersion forces. These fluctuating fields possess correlations between space-time points outside the light cone, i.e. points causally disconnected according to special relativity. As a consequence, two initially uncorrelated quantum objects in empty space which are located in causally disconnected space-time regions, and therefore unable to exchange information, can become correlated. Here, we have experimentally demonstrated the existence of correlations of the vacuum fields for non-causally connected space-time points by using electro-optic sampling. This result is obtained by detecting vacuum-induced correlations between two 195 fs laser pulses separated by a time of flight of 470 fs. This work marks a first step in analyzing the space-time structure of vacuum correlations in quantum field theory.ISSN:2041-172

Terahertz waveform synthesis in integrated thin-film lithium niobate platform

Bridging the “terahertz gap“ relies upon synthesizing arbitrary waveforms in the terahertz domain enabling applications that require both narrow band sources for sensing and few-cycle drives for classical and quantum objects. However, realization of custom-tailored waveforms needed for these applications is currently hindered due to limited flexibility for optical rectification of femtosecond pulses in bulk crystals. Here, we experimentally demonstrate that thin-film lithium niobate circuits provide a versatile solution for such waveform synthesis by combining the merits of complex integrated architectures, low-loss distribution of pump pulses on-chip, and an efficient optical rectification. Our distributed pulse phase-matching scheme grants shaping the temporal, spectral, phase, amplitude, and farfield characteristics of the emitted terahertz field through designer on-chip components. This strictly circumvents prior limitations caused by the phase-delay mismatch in conventional systems and relaxes the requirement for cumbersome spectral pre-engineering of the pumping light. We propose a toolbox of basic blocks that produce broadband emission up to 680 GHz and far-field amplitudes of a few V m−1 with adaptable phase and coherence properties by using near-infrared pump pulse energies below 100 pJ.ISSN:2041-172

Electro-optic interface for ultrasensitive intracavity electric field measurements at microwave and terahertz frequencies

ISSN:2334-253

Enhancing adoption of fodder technologies: how can an innovation systems perspective help?

We report a thin film phase modulator employing organic nonlinear optical molecules, with an electro-optic bandwidth of 1.25 THz. The device acts as a polarization sensitive broadband Pockels medium for coherent electric field detection in a dual wavelength terahertz time-domain spectroscopy setup in the telecom band at 1550 nm. To increase the sensitivity, we combine a three-dimensional bow-tie antenna structure with strongly electro-optically active molecules JRD1 in poly­(methyl methacrylate) supporting polymer. The antenna provides subwavelength field confinement of the terahertz wave with its waveguide gap with lateral dimensions of 2.2 μm × 5 μm × 4 μm. In the gap, the electric field is up to 150× stronger than in a diffraction limited space-time volume, such that an interaction length of only 4 μm suffices for the detection of fields below 10 V/m. This device is promising in the growing field of quantum optics in the terahertz, single photon terahertz detection, nonlinear imaging, and on-chip telecommunication

"Delirium Day": A nationwide point prevalence study of delirium in older hospitalized patients using an easy standardized diagnostic tool

Background: To date, delirium prevalence in adult acute hospital populations has been estimated generally from pooled findings of single-center studies and/or among specific patient populations. Furthermore, the number of participants in these studies has not exceeded a few hundred. To overcome these limitations, we have determined, in a multicenter study, the prevalence of delirium over a single day among a large population of patients admitted to acute and rehabilitation hospital wards in Italy. Methods: This is a point prevalence study (called "Delirium Day") including 1867 older patients (aged 65 years or more) across 108 acute and 12 rehabilitation wards in Italian hospitals. Delirium was assessed on the same day in all patients using the 4AT, a validated and briefly administered tool which does not require training. We also collected data regarding motoric subtypes of delirium, functional and nutritional status, dementia, comorbidity, medications, feeding tubes, peripheral venous and urinary catheters, and physical restraints. Results: The mean sample age was 82.0 ± 7.5 years (58 % female). Overall, 429 patients (22.9 %) had delirium. Hypoactive was the commonest subtype (132/344 patients, 38.5 %), followed by mixed, hyperactive, and nonmotoric delirium. The prevalence was highest in Neurology (28.5 %) and Geriatrics (24.7 %), lowest in Rehabilitation (14.0 %), and intermediate in Orthopedic (20.6 %) and Internal Medicine wards (21.4 %). In a multivariable logistic regression, age (odds ratio [OR] 1.03, 95 % confidence interval [CI] 1.01-1.05), Activities of Daily Living dependence (OR 1.19, 95 % CI 1.12-1.27), dementia (OR 3.25, 95 % CI 2.41-4.38), malnutrition (OR 2.01, 95 % CI 1.29-3.14), and use of antipsychotics (OR 2.03, 95 % CI 1.45-2.82), feeding tubes (OR 2.51, 95 % CI 1.11-5.66), peripheral venous catheters (OR 1.41, 95 % CI 1.06-1.87), urinary catheters (OR 1.73, 95 % CI 1.30-2.29), and physical restraints (OR 1.84, 95 % CI 1.40-2.40) were associated with delirium. Admission to Neurology wards was also associated with delirium (OR 2.00, 95 % CI 1.29-3.14), while admission to other settings was not. Conclusions: Delirium occurred in more than one out of five patients in acute and rehabilitation hospital wards. Prevalence was highest in Neurology and lowest in Rehabilitation divisions. The "Delirium Day" project might become a useful method to assess delirium across hospital settings and a benchmarking platform for future surveys