Unveiling Low THz Dynamics of Liquid Crystals: Identification of Intermolecular Interaction among Intramolecular Modes.

Liquid crystals have found a wide area of application over the last few decades, proving to be excellent materials for tunable optics from visible to near-infrared frequencies. Currently, much effort is devoted to demonstrating their applicability at THz frequencies (1-10 THz), where tremendous advances of broadband and intense sources have been achieved. Yet, a detailed understanding of THz-triggered dynamics in liquid crystals is incomplete. Here, we perform broadband THz time domain spectroscopy on 4-cyano-4'-alkyl-biphenyl (nCB) and 5-phenylcyclohexanes (PCH5) across mesophases. Density functional theory calculations on isolated molecules capture the majority of the response. In particular, the pronounced modes around 4.5 and 5.5 THz mainly originate from bending modes of the cyano group. In contrast, the broad response below 3 THz, linked to modes of the alkyl chain, disagrees with the single molecule calculation. Here, we identify a clear intermolecular character of the response, supported by dimer and trimer calculations.

Non-equilibrium dynamics of spin-lattice coupling.

Quantifying the dynamics of normal modes and how they interact with other excitations is of central importance in condensed matter. Spin-lattice coupling is relevant to several sub-fields of condensed matter physics; examples include spintronics, high-Tc superconductivity, and topological materials. However, experimental approaches that can directly measure it are rare and incomplete. Here we use time-resolved X-ray diffraction to directly access the ultrafast motion of atoms and spins following the coherent excitation of an electromagnon in a multiferroic hexaferrite. One striking outcome is the different phase shifts relative to the driving field of the two different components. This phase shift provides insight into the excitation process of such a coupled mode. This direct observation of combined lattice and magnetization dynamics paves the way to access the mode-selective spin-lattice coupling strength, which remains a missing fundamental parameter for ultrafast control of magnetism and is relevant to a wide variety of materials.

Solid-state biased coherent detection of ultra-broadband terahertz pulses generated in a spintronic emitter for high repetition rate, low pulse energy lasers.

We report the coherent generation and detection of terahertz (THz) pulses featuring a spectral bandwidth in the range of 0.1-9 THz achieved via the use of a high repetition rate (250 kHz), low pulse energy (6.2 µJ) laser system. More specifically, we test and evaluate a solid-state biased coherent detection device in combination with a spintronic emitter. We demonstrate the use of this combination of techniques to measure the ultra-broadband THz frequency optical properties of bulk crystalline materials with time-domain spectroscopy.

Experimental station Bernina at SwissFEL: condensed matter physics on femtosecond time scales investigated by X-ray diffraction and spectroscopic methods.

The Bernina instrument at the SwissFEL Aramis hard X-ray free-electron laser is designed for studying ultrafast phenomena in condensed matter and material science. Ultrashort pulses from an optical laser system covering a large wavelength range can be used to generate specific non-equilibrium states, whose subsequent temporal evolution can be probed by selective X-ray scattering techniques in the range 2-12 keV. For that purpose, the X-ray beamline is equipped with optical elements which tailor the X-ray beam size and energy, as well as with pulse-to-pulse diagnostics that monitor the X-ray pulse intensity, position, as well as its spectral and temporal properties. The experiments can be performed using multiple interchangeable endstations differing in specialization, diffractometer and X-ray analyser configuration and load capacity for specialized sample environment. After testing the instrument in a series of pilot experiments in 2018, regular user operation begins in 2019.

Maxillofacial Injuries Due to Traffic Accidents.

PURPOSE: The purpose of this retrospective study was to analyze the epidemiology, patterns, and management of maxillofacial due to road traffic accidents over a 17-year period. METHODS: Between January 2001 and December 2017, 2924 patients with maxillofacial fractures were admitted to the Division of Maxillofacial Surgery, Turin, Italy.The following data were analyzed: age, gender, data of the trauma, alcohol and drug abuse, mechanism of injury, fracture site, facial injury severity scale, associated injuries, type of treatment, and length of hospital stay. RESULTS: Of the 605 patients included in the study, 419 were male and 186 were female (ratio, 2.2:1). The most common mechanism of injury was car accidents (62.6%).More than half of the patients had fractures of the middle third of the maxillofacial skeleton.Associated injuries were detected in 172 (45.5%) patients. In total 5.3% of patients did not undergo surgery. The average hospital stay was 7.3 days. CONCLUSIONS: This study shows an important reduction in maxillofacial fractures following road traffic accidents since the turn of the new millennium. At least in north-western Italy, road safety policies implemented in the last 30 years seem to have affected the behavior of motorists and motorcyclists.

Magnetic Switching in Granular FePt Layers Promoted by Near-Field Laser Enhancement.

Light-matter interaction at the nanoscale in magnetic materials is a topic of intense research in view of potential applications in next-generation high-density magnetic recording. Laser-assisted switching provides a pathway for overcoming the material constraints of high-anisotropy and high-packing density media, though much about the dynamics of the switching process remains unexplored. We use ultrafast small-angle X-ray scattering at an X-ray free-electron laser to probe the magnetic switching dynamics of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. We observe that the combination of laser excitation and applied static magnetic field, 1 order of magnitude smaller than the coercive field, can overcome the magnetic anisotropy barrier between "up" and "down" magnetization, enabling magnetization switching. This magnetic switching is found to be inhomogeneous throughout the material with some individual FePt nanoparticles neither switching nor demagnetizing. The origin of this behavior is identified as the near-field modification of the incident laser radiation around FePt nanoparticles. The fraction of not-switching nanoparticles is influenced by the heat flow between FePt and a heat-sink layer.

Time-Resolved X-Ray Powder Diffraction Study of Photoinduced Phase Transitions in Ti3 O5 Nanoparticles.

Nanoparticles of Ti3 O5 have been reported to show a permanent photoinduced phase transition at room temperature. This suggests that light-induced phase transformations of Ti3 O5 nanoparticles may be promising for technological applications. Here, we report a photoinduced semiconductor-to-metal phase transition from ß-Ti3 O5 to λ-Ti3 O5 nanoparticles at room temperature observed directly by time-resolved X-ray powder diffraction in a pump-probe setup. The results show a partial structural change, limited by differences between pumped and probed volumes, which persists a few microseconds after excitation. The first step in the relaxation back to the ground state can be described by a single exponential decay with time constant within microsecond timescales. Analysis of the change in lattice constants enables us to estimate an average temperature increase across the phase transition, consistent with a thermally driven process.

Long-term outcomes of implants placed after vertical alveolar ridge augmentation in partially edentulous patients: a 10-year prospective clinical study.

AIM: The aim of this study was to evaluate the long-term clinical results around non-submerged implants placed after vertical alveolar ridge augmentation. MATERIAL AND METHODS: The original population consisted of two groups of partially edentulous patients (Clinical Oral Implants Research, 15, 2004, 73; Clinical Oral Implants Research, 18, 2007, 286), receiving a total of 82 implants, after a vertical bone augmentation of at least 4 mm. Following cementation of the fixed dental prostheses, patients were asked to follow an individualized supportive periodontal therapy (SPT) program for an appropriate clinical and radiographic follow-up. RESULTS: At the 10-year examination, seven of the 41 patients were lost to follow-up. During SPT, additional antibiotic and/or surgical therapy was necessary in 18 implants, and four of these implants were removed for biological complications. The overall implant survival rate was 94.1%. The mean interproximal bone loss (BL) was 0.58 ± 0.57 mm. CONCLUSIONS: The results of this study confirmed that implants, placed after vertical augmentation and followed by an adequate SPT, offer predictable long-term results. Nevertheless, patients whose bone atrophy was consequence of a previous history of periodontitis presented a statistically significant greater BL.

Surgical Management of Myofibroma of the Gengiva.

The purpose of this paper is to report a rare patient of oral myofibroma in a 12-year old patient and to describe its clinical, histopathologic, and immunohistochemical features to establish the correct diagnosis and surgical management.Pathological and immunohistochemical examination is a mandatory method for establishing a definitive diagnosis of this lesion avoiding unnecessary treatment. Surgical excision and careful postoperative observation should be a treatment option.

An analysis of 711 victims of interpersonal violence to the face, Turin, Italy.

AIM: The aim of this work is to analyze the risk factors in interpersonal violence and to describe the epidemiology, patterns and management of maxillofacial fractures in high volume trauma center of the northern Italy. MATERIAL: This retrospective study recorded patients hospitalized with maxillofacial fractures, at the division of maxillofacial surgery, Città della Scienza e della Salute University Hospital, Torino, Italy, since January 2001. METHODS: Only patients who presented with "violence" in the database field for "cause of injury" and with "interpersonal violence" as a subtype of etiology were considered. Statistical analysis was determined using the χ(2) or Fisher's exact test. RESULTS: 2567 patients were admitted. 711 patients (27.7%) had undergone interpersonal violence that has increased from 20% to 35% in the study period. The male-to-female ratio was 11:1; the mean age was 32.7. 247 patients were foreigners (34.7%). 107 patients were enrolled in that study, referred alcohol or drugs abuse. IPV episodes account for 953 maxillofacial fractures localized in 55.3% of the cases in the midface, 43.1% in the lower third. Particularly the trauma involved the orbital region and the maxillo-orbito-zygomatic region. 4.4% of patients had combined trauma. CONCLUSION: IPV maxillofacial fractures in Europe such as in Italy are becoming one of the first cause of injuries. This study shows that young males and foreigners are involved in violence mostly during the weekend. These fractures occur due to fists frequently involving the maxillo-zygomatic-orbital complex.

Electric field generation of Skyrmion-like structures in a nematic liquid crystal.

Skyrmions are particle-like topological objects that are increasingly drawing attention in condensed matter physics, where they are connected to inversion symmetry breaking and chirality. Here we report the generation of stable Skyrmion-like structures in a thin nematic liquid crystal film on chemically patterned patchy surfaces. Using the interplay of material elasticity and surface boundary conditions, we use a strong electric field to quench the nematic liquid crystal from a fully aligned phase to vortex-like nematic liquid crystal structures, centered on patterned patches, which carry two different sorts of topological defects. Numerical calculations reveal that these are Skyrmion-like structures, seeded from the surface boojum topological defects and swirling towards the second confining surface. These observations, supported by numerical methods, demonstrate the possibility to generate, manipulate and study Skyrmion-like objects in nematic liquid crystals on patterned surfaces.

THz near-field enhancement by means of isolated dipolar antennas: the effect of finite sample size.

Generation of high intensity terahertz radiation in the low frequency region (f < 5 THz) is still a challenging task and only few experimental demonstrations exceeding 1 MV/cm have been reported so far. One viable option is the use of resonant metallic structures which act as amplifiers for the impinging radiation. Here with the aid of finite difference time domain simulations, we design and realize a set of isolated resonant elements which allow us to reach a 28-fold enhancement of freely propagating THz radiation at f ≈ 1 THz. These elements are deposited on a GaP sample allowing the direct measurement of the field enhancement using electro-optical sampling. Interestingly, we experimentally show strong modifications of the antennas resonance which is interpreted in terms of interference effects. These are particularly important in samples thinner than half the spatial pulse length.

Nanoscale Confinement of All-Optical Magnetic Switching in TbFeCo--Competition with Nanoscale Heterogeneity.

Single femtosecond optical laser pulses, of sufficient intensity, are demonstrated to reverse magnetization in a process known as all-optical switching. Gold two-wire antennas are placed on the all-optical switching film TbFeCo. These structures are resonant with the optical field, and they create a field enhancement in the near-field which confines the area where optical switching can occur. The magnetic switching that occurs around and below the antenna is imaged using resonant X-ray holography and magnetic circular dichroism. The results not only show the feasibility of controllable switching with antenna assistance but also demonstrate the highly inhomogeneous nature of the switching process, which is attributed to the process depending on the material's heterogeneity.

Ultrafast all-optical response of a nematic liquid crystal.

Liquid crystals are superior optical materials for large area displays, but it is considered that their collective and slow-millisecond response makes them useless for ultrafast optical applications. In contrast to that, we here demonstrate an ultrafast optical response of a nematic liquid crystal, which is induced by an intense femtosecond optical impulse. We show that the refractive index of the nematic liquid crystal pentyl-cyanobiphenyl can be modulated at a time scale as fast as 500 fs via a coherently excited optical Kerr effect. The change in the refractive index is in the order of 10-4 at a fluence of 4 mJ/cm2 and is strongly polarization dependent. This unprecedented result opens new ways towards ultrafast all-optical modulation in liquid crystal-based devices.

Gaining control through frustration: two-fold approach for Liquid Crystal three-dimensional command layers.

The alignment of Liquid Crystal (LC) molecules, essential for their applications in optical devices such as displays, is usually controlled by functionalizing their confining surfaces by either patterning or by specific surfactants that induce either parallel or perpendicular molecular arrangement. Inducing a bistable alignment, such as in the new zenithal bistable displays, offers new opportunities in terms of new functionalities and lower energy consumption but a full understanding of such bistable alignment appears still complicated. Here we present a simple phenomenological model that includes surface topography and chemistry. The predicted orientational transitions and bistable states are in excellent agreement with experiments, thus making this a proper tool to design multistable 3D command layers.

Self-assembled organic microfibers for nonlinear optics.

While highly desired in integrated optical circuits, multiresponsive and tunable nonlinear optical (NLO) active 1D (sub)wavelength scale superstructures from organic materials are rarely reported due to the strong tendency of organic molecules to self-assembly in centrosymmetric modes. Here a solution-processed assembly approach is reported to generate non-centrosymmetric single-crystalline organic microfibers with a cumulative dipole moment for anisotropic combined second- and third-order NLO.

Circular dichroism probed by two-photon fluorescence microscopy in enantiopure chiral polyfluorene thin films.

Two-photon fluorescence scanning confocal microscopy sensitive to circular dichroism with a diffraction-limited resolution well below 500 nm is demonstrated. With this method, the spatial variation of the circular dichroism of thermally annealed chiral polyfluorene thin films has been imaged. We observed circular dichroism associated with submicrometer-sized domains showing helicoidally twisted macromolecular organization. Domains with opposite chiroptical properties, corresponding to left- or right-handed molecular organization, coexist in the film. Our results are consistent with those obtained by one-photon imaging and illustrate the potential of two-photon imaging for use in studying helical macromolecular organization.

All-optical subdiffraction multilevel data encoding onto azo-polymeric thin films.

By exploiting photoinduced reorientation in azo-polymer thin films, we demonstrate all-optical polarization-encoded information storage with a scanning near-field optical microscope. In the writing routine, five-level bits are created by associating different bit values to different birefringence directions, induced in the polymer after illumination with linearly polarized light. The reading routine is then performed by implementing polarization-modulation techniques on the same near-field microscope in order to measure the encoded birefringence direction.