1,721,015 research outputs found
Thermal waves emitted by moving sources and the Doppler effect
No full textQuesto articolo studia teoricamente l'effetto Doppler per le onde termiche, trovando un'espressione dello spostamento di frequenza per i due casi di sorgente in movimento e osservatore in movimento. Vengono evidenziate differenze e analogie con l'effetto Doppler per onde elettromagnetiche e acustiche. Le onde d'urto termico vengono introdotte e simulate numericamente. Viene discussa anche un'applicazione per la valutazione e il test non distruttivo dei materialiThis paper theoretically studies the Doppler effect for thermal waves, finding an expression of the frequency shift for the two cases of moving source, and moving observer. Differences and analogies with the Doppler effect for electromagnetic and acoustic waves are highlighted. Thermal shock waves are introduced and simulated numerically. An application for nondestructive evaluation and testing of materials is also discussed
Doppler effect for thermal waves: theory and applications
No full textIn this paper we study theoretically the Doppler effect of thermal waves, deducing the frequency shift for the two cases of moving source, and moving observer
In-plane thermal diffusivity measurements of polyethersulfone woven textiles by infrared thermography
Full text linkLock-in thermography was applied to the measurement of the in-plane thermal diffusivity of three polyethersulfone (PES) textiles characterized by different weaving pattern as well as different mass density of interlacing fibers. The experimental results showed that the in-plane thermal diffusivity in each direction decreased with the increase of the fibers’ linear mass density, thus leading to an anisotropic behavior of the thermal diffusivity in the specimen where PES fibers with different density were interlaced. A new theoretical model for the study of the heat diffusion in textiles was specifically developed and, thereafter, employed for the analysis of the experimental results. As such, our textile model approach, shedding light on the role of different textile and fibers parameters on the resulting thermal diffusivity, paves the way for the development and design of textiles with tailored thermal behavior
Nanostructured materials for circular dichroism and chirality at the nanoscale: towards unconventional characterization [Invited]
Full text linkIn this work, we review the last attempts to use nanostructured materials for the enhancement of the chiro-optical effects at the nanoscale. Starting from the numerical design, we review different geometries that exhibit circular dichroic behavior in the far field; we then focus on the new branch of near-field chirality, where numerous nanostructures have been proposed for background-free chiral sensing. The next section reports on nanofabrication methods, with a special focus on self-assembling, cost- and time-efficient techniques. Finally, we review the chiro-optical experiments. Besides conventional extinction-based techniques, we are today able to reveal chiro-optical effects via photothermal behavior and photoluminescence, going down to single nanostructure chirality with sophisticated near-field techniques. We believe that the novel designs, state-of-the-art nanofabrication and modern characterization techniques have come to a stage to provide chiro-optical sensors and light components based on nanostructures
Extrinsic chirality tailors Stokes parameters in simple asymmetric metasurfaces
Full text linkMetasurfaces tailor electromagnetic confinement at the nanoscale and can be appropriately designed for polarization-dependent light-matter interactions. Adding the asymmetry degree to the desing allows for circular polarizations of opposite handedness to be differently absorbed or emitted, which is of interest in fields spanning from chiral sensing to flat optics. Here, we show that simple, low-cost asymmetric metasurfaces can control Stokes parameters in the transmitted far-field. With only 50 nm of asymmetric plasmonic shells on self-assembled polystyrene nanospheres, our metasurfaces allow for great spectral and incident angle tunability. We first investigated broadband extrinsic chirality in metasurfaces with asymmetric plasmonic semishells; we found high extinction circular dichroism (CD) in the near-infrared range. We then excited it with linear polarization and performed hyperspectral Stokes polarimetry on the transmitted field. We showed that the S3 parameter follows the behavior of CD in extinction, and that the output field position on the Poincaré sphere can be widely controlled by using the incidence angle and wavelength. Furthermore, simulations agreed well with the experiments and showed how the near-field chiro-optical response influences the extrinsic chiral behavior in absorption and the polarization state of the transmitted field
Quantitative evaluation of emission properties and thermal hysteresis in the mid-infrared for a single thin film of vanadium dioxide on a silicon substrate
Full text linkWe present a comparative study of the emission properties of a vanadium dioxide thin film (approximately 200 nm) deposited on a silicon wafer in different sub-spectral-ranges of the mid-infrared, with particular attention to the windows of transparency of the atmosphere to the infrared radiation (i.e., 3–5 μm, 8–12 μm). The infrared emission properties of the structure are closely related to the well-known phase transition of the first order, from semiconductor to metal, of the vanadium dioxide around the temperature of 68 °C. The characterization of the emissivity in the sub-regions of the mid-infrared was carried out both in the front configuration, that is on the VO2 film side, and in the rear configuration on the silicon wafer side, and showed a strong difference in the hysteresis thermal bandwidth, in particular between the short wave region and the long wave region. The bandwidth is equal to 12 °C for the front and 15 °C for the rear. The emissivity behaviors as a function of temperature during the semiconductor-metal transition in the mid-infrared subregions were analyzed and explained using the theories of the effective medium of Maxwell Garnett and Bruggeman, highlighting the greater functionality of one theory with respect to the other depending on the spectral detection band
Hybrid thermal Yagi-Uda nanoantennas for directional and narrow band long-wavelength IR radiation sources
Full text linkWe investigate the possibility of spatially and spectrally controlling the thermal infrared emission by exploitation of the Yagi-Uda antenna design. Hybrid antennas composed of both SiC and Au rods are considered and the contributions of emission from all the elements, at a given equilibrium temperature, are taken into account. We show that the detrimental effect due to thermal emission from the not ideal parasitic elements drastically affect the performances of conventional thermal Au antennas in the 12 μm wavelength range. Nevertheless, our results show that the hybrid approach allows the development of efficient narrow-band and high directivity sources. The possibility of exploiting the Yagi-Uda design both in transmission and in reception modes, may open the way to the realization of miniaturized, efficient, robust and cheap sensor devices for mass-market applications. 2020 Optical Society of America
Diffracted Beams from Metasurfaces: High Chiral Detectivity by Photothermal Deflection Technique
Full text linkMeasurements of difference in optical interactions between circularly polarized excitations of opposite handedness (circular dichroism) are highly important for both natural and artificial chiral structures. Here the photothermal deflection technique is proposed as a method to detect the optical chirality of a metasurface, analyzing the diffracted beams by the metasurface itself. Two metasurfaces are investigated, based on Au and Ag. The samples are fabricated by nanosphere lithography, with a tilted deposition of thin metal layer, which produces symmetry-breaking. The unit cell periodicity of these metasurfaces allows for multiple order diffraction in the 450–520 nm range, which encompasses the emission lines of an Ar laser. The metasurfaces are placed on a mirror and excited by an Ar pump beam at different orientations, laser wavelengths, and circular polarization degree; the probe beam scans the absorption of the diffraction orders back-reflected from the underlying mirror. In this way, the chiral investigation is simplified by placing the scanning of absorption-induced thermal effect into the metasurface plane, thus avoiding the transmission/reflection measurements of a specific order, which are done by angular placement of detector. Theoretical and numerical approaches are further developed to reconstruct both thermal and optical behavior of the chirality at the nanoscale
Chiro-optical characterization of self-assembled plasmonic nanostructures
No full textIn order to control the circular polarization of light at nanoscale, complex structures can be realized. Among different fabrication techniques, self-assembled approach can produce high reproducible, low cost, large area asymmetric metasurfaces that present different optical functionalities, and in particular can exhibit chiral optical response both in the near field and in the far field. Here we show the optical chiral properties of different self-assembled metasurfaces measured by different characterization techniques, both in linear and nonlinear regime
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