Recent discoveries have shown that, when two layers of van der Waals (vdW) materials are superimposed with a relative twist angle between them, the electronic properties of the coupled system can be ...dramatically altered. Here, we demonstrate that a similar concept can be extended to the optics realm, particularly to propagating phonon polaritons–hybrid light-matter interactions. To do this, we fabricate stacks composed of two twisted slabs of a vdW crystal (α-MoO3) supporting anisotropic phonon polaritons (PhPs), and image the propagation of the latter when launched by localized sources. Our images reveal that, under a critical angle, the PhPs isofrequency curve undergoes a topological transition, in which the propagation of PhPs is strongly guided (canalization regime) along predetermined directions without geometric spreading. These results demonstrate a new degree of freedom (twist angle) for controlling the propagation of polaritons at the nanoscale with potential for nanoimaging, (bio)-sensing, or heat management.
•Volatility forecasts are improved by stacking machine learning algorithms.•Merging Artificial Neural Networks with other models increases its forecasting power.•Regardless of the volatility model, ...high volatile regimes lead to higher error rates.•Risk measurements precision is increased by stacking machine learning models.
An appropriate calibration and forecasting of volatility and market risk are some of the main challenges faced by companies that have to manage the uncertainty inherent to their investments or funding operations such as banks, pension funds or insurance companies. This has become even more evident after the 2007–2008 Financial Crisis, when the forecasting models assessing the market risk and volatility failed. Since then, a significant number of theoretical developments and methodologies have appeared to improve the accuracy of the volatility forecasts and market risk assessments. Following this line of thinking, this paper introduces a model based on using a set of Machine Learning techniques, such as Gradient Descent Boosting, Random Forest, Support Vector Machine and Artificial Neural Network, where those algorithms are stacked to predict S&P500 volatility. The results suggest that our construction outperforms other habitual models on the ability to forecast the level of volatility, leading to a more accurate assessment of the market risk.
The appearance of single photon sources in atomically thin semiconductors holds great promises for the development of a flexible and ultracompact quantum technology in which elastic strain ...engineering can be used to tailor their emission properties. Here, we show a compact and hybrid two-dimensional semiconductor-piezoelectric device that allows for controlling the energy of single photons emitted by quantum emitters localized in wrinkled WSe2 monolayers. We demonstrate that strain fields exerted by the piezoelectric device can be used to tune the energy of localized excitons in WSe2 up to 18 meV in a reversible manner while leaving the single photon purity unaffected over a wide range. Interestingly, we find that the magnitude and, in particular, the sign of the energy shift as a function of stress is emitter dependent. With the help of finite element simulations we suggest a simple model that explains our experimental observations and, furthermore, discloses that the type of strain (tensile or compressive) experienced by the quantum emitters strongly depends on their localization across the wrinkles. Our findings are of strong relevance for the practical implementation of single photon devices based on two-dimensional materials as well as for understanding the effects of strain on their emission properties.
Van der Waals (vdW) materials host a variety of polaritons, which make them an emerging material platform for manipulating light at the nanoscale. Due to the layered structure of vdW materials, the ...polaritons can exhibit a hyperbolic dispersion and propagate as nanoscale‐confined volume modes in thin flakes. On the other hand, surface‐confined modes can be found at the flake edges. Surprisingly, the guiding of these modes in ribbons—representing typical linear waveguide structures—is widely unexplored. Here, a detailed study of hyperbolic phonon polaritons propagating in hexagonal boron nitride ribbons is reported. Employing infrared nanoimaging, a variety of modes are observed. Particularly, the fundamental volume waveguide mode that exhibits a cutoff width is identified, which, interestingly, can be lowered by reducing the waveguide thickness. Further, hybridization of the surface modes and their evolution with varying frequency and waveguide width are observed. Most importantly, it is demonstrated that the symmetrically hybridized surface mode does not exhibit a cutoff width, and thus enables linear waveguiding of the polaritons in arbitrarily narrow ribbons. The experimental data, supported by simulations, establish a solid basis for the understanding of hyperbolic polaritons in linear waveguides, which is of critical importance for their application in future photonic devices.
Infrared nanoimaging and theoretical simulations are applied to study phonon polariton waveguide modes in nanoscale hexagonal boron nitride ribbons. Fundamental volume and hybridized surface modes are identified. Most importantly, the symmetrically hybridized surface mode does not exhibit a cutoff width, and thus allows for linear waveguiding of infrared energy in the narrowest ribbons that can be fabricated.
We use Hubble Space Telescope (HST) NICMOS continuum and Paa observations to study the near-infrared and star formation properties of a representative sample of 30 local (d 635-75 Mpc) luminous ...infrared galaxies (LIRGs, infrared 8-1000 km luminosities of log L sub(IR) = 11-11.9 L sub( )). The data provide spatial resolutions of 25-50 pc and cover the central 63.3-7.1 kpc regions of these galaxies. About half of the LIRGs show compact (61-2 kpc) Paa emission with a high surface brightness in the form of nuclear emission, rings, and minispirals. The rest of the sample show Paa emission along the disk and the spiral arms extending over scales of 3-7 kpc and larger. About half of the sample contains H II regions with Ha luminosities significantly higher than those observed in normal galaxies. There is a linear empirical relationship between the mid-IR 24 km and hydrogen recombination (extinction-corrected Paa) luminosity for these LIRGs, and the H II regions in the central part of M51. This relation holds over more than four decades in luminosity, suggesting that the mid-IR emission is a good tracer of the star formation rate (SFR). Analogous to the widely used relation between the SFR and total IR luminosity of R. Kennicutt, we derive an empirical calibration of the SFR in terms of the monochromatic 24 km luminosity that can be used for luminous, dusty galaxies.
Terahertz (THz) fields are widely used for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated well below the classical ...diffraction limit through the excitation of graphene plasmons (GPs). These possibilities emerge from the strongly reduced GP wavelength, λ
, compared with the photon wavelength, λ
, which can be controlled by modulating the carrier density of graphene via electrical gating. Recently, GPs in a graphene/insulator/metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates. Although infrared GPs have been visualized by scattering-type scanning near-field optical microscopy (s-SNOM), the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far-only GPs with nearly free-space wavelengths have been observed. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically rather than optically. This on-chip detection simplifies GP imaging as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (λ
≈ λ
/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering.
We analyze a sample of 62600 Spitzer MIPS 24 km sources brighter than 680 kJy and located in the Chandra Deep Field-South to characterize the evolution of the comoving infrared (IR) energy density of ...the universe up to z 6 1. Using published ancillary optical data, we first obtain a nearly complete redshift determination for the 24 km objects associated with R 24 mag counterparts at z 1. These sources represent 655%-60% of the total MIPS 24 km population with f sub(24)k sub(m) 80 kJy, the rest of the sample likely lying at higher redshifts. We then determine an estimate of their total IR luminosities using various libraries of IR spectral energy distributions. We find that the 24 km population at 0.5 z 1 is dominated by "luminous infrared galaxies" (i.e., 10 super(11) L sub( ), L sub(IR) , 10 super(12) L sub( )), the counterparts of which appear to be also luminous at optical wavelengths and tend to be more massive than the majority of optically selected galaxies. A significant number of fainter sources (5 x 10 super(10) L sub( )L sub(IR) , 10 super(11) L sub( ))are also detected at similar distances. We finally derive 15 km and total IR luminosity functions (LFs) up to z 6 1. In agreement with the previous results from the Infrared Space Observatory (ISO) and SCUBA and as expected from the MIPS source number counts, we find very strong evolution of the contribution of the IR-selected population with look-back time. Pure evolution in density is firmly excluded by the data, but we find considerable degeneracy between strict evolution in luminosity and a combination of increases in both density and luminosity L IR a (1 + z) super(3.2) super(+) sub(-) super(0) sub(0) super(.) sub(.) super(7) sub(2) hIR a (1 + z) super(0.7) super(+) sub(-) super(0) sub(0) super(.) sub(.) super(6) sub(2). A significant steepening of the faint-end slope of the IR luminosity function is also unlikely, as it would overproduce the faint 24 km source number counts. Our results imply that the comoving IR energy density of the universe evolves as (1 + z) super(3.9c0.4) up to z 6 1 and that galaxies luminous in the infrared (i.e., L sub(IR) . 10 super(11) L sub( )) are responsible for 70% c 15% of this energy density at z 6 1. Taking into account the contribution of the UV luminosity evolving as (1 + z)6 super(2.5), we infer that these IR-luminous sources dominate the star-forming activity beyond z 6 0.7. The uncertainties affecting these conclusions are largely dominated by the errors in the k-corrections used to convert 24 km fluxes into luminosities.
Phonon polaritons-light coupled to lattice vibrations-in polar van der Waals crystals are promising candidates for controlling the flow of energy on the nanoscale due to their strong field ...confinement, anisotropic propagation and ultra-long lifetime in the picosecond range
. However, the lack of tunability of their narrow and material-specific spectral range-the Reststrahlen band-severely limits their technological implementation. Here, we demonstrate that intercalation of Na atoms in the van der Waals semiconductor α-V
O
enables a broad spectral shift of Reststrahlen bands, and that the phonon polaritons excited show ultra-low losses (lifetime of 4 ± 1 ps), similar to phonon polaritons in a non-intercalated crystal (lifetime of 6 ± 1 ps). We expect our intercalation method to be applicable to other van der Waals crystals, opening the door for the use of phonon polaritons in broad spectral bands in the mid-infrared domain.
In general insurance companies, a correct estimation of liabilities plays a key role due to its impact on management and investing decisions. Since the Financial Crisis of 2007–2008 and the ...strengthening of regulation, the focus is not only on the total reserve but also on its variability, which is an indicator of the risk assumed by the company. Thus, measures that relate profitability with risk are crucial in order to understand the financial position of insurance firms. Taking advantage of the increasing computational power, this paper introduces a stochastic reserving model whose aim is to improve the performance of the traditional Mack’s reserving model by applying an ensemble of Recurrent Neural Networks. The results demonstrate that blending traditional reserving models with deep and machine learning techniques leads to a more accurate assessment of general insurance liabilities.
•The proposed Mack-Net model leads to a more accurate reserve prediction.•Mack-Net model generates more appropriate risk measures than Mack’s model.•Mack-Net model does not increase the variance to generate an appropriate VaR.•Neural Networks estimate more accurate parameters than usual reserving models.
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