Spectral Extension Research Articles

Modeling Gamma-Ray SEDs and Angular Extensions of Extreme TeV Blazars from Intergalactic Proton-Initiated Cascades in Contemporary Astrophysical EGMF Models

The intrinsic spectra of some distant blazars known as “extreme TeV blazars” have shown a hint at an anomalous hardening in the TeV energy region. Several extragalactic propagation models have been proposed to explain this possible excess transparency of the Universe to gamma-rays starting from a model which assumes the existence of so-called axion-like particles (ALPs) and the new process of gamma-ALP oscillations. Alternative models suppose that some of the observable gamma-rays are produced in the intergalactic cascades. This work focuses on investigating the spectral and angular features of one of the cascade models, the Intergalactic Hadronic Cascade Model (IHCM) in the contemporary astrophysical models of Extragalactic Magnetic Field (EGMF). For IHCM, EGMF largely determines the deflection of primary cosmic rays and electrons of intergalactic cascades and, thus, is of vital importance. Contemporary Hackstein models are considered in this paper and compared to the model of Dolag. The models assumed are based on simulations of the local part of large-scale structure of the Universe and differ in the assumptions for the seed field. This work provides spectral energy distributions (SEDs) and angular extensions of two extreme TeV blazars, 1ES 0229+200 and 1ES 0414+009. It is demonstrated that observable SEDs inside a typical point spread function of imaging atmospheric Cherenkov telescopes (IACTs) for IHCM would exhibit a characteristic high-energy attenuation compared to the ones obtained in hadronic models that do not consider EGMF, which makes it possible to distinguish among these models. At the same time, the spectra for IHCM models would have longer high energy tails than some available spectra for the ALP models and the universal spectra for the Electromagnetic Cascade Model (ECM). The analysis of the IHCM observable angular extensions shows that the sources would likely be identified by most IACTs not as point sources but rather as extended ones. These spectra could later be compared with future observation data of such instruments as Cherenkov Telescope Array (CTA) and LHAASO.

Comparing Donor- and Acceptor-Originated Exciton Dynamics in Non-Fullerene Acceptor Blend Polymeric Systems.

Non-fullerene type acceptors (NFA) have gained attention owing to their spectral extension that enables efficient solar energy capturing. For instance, the solely NFA-mediated absorbing region contributes to the photovoltaic power conversion efficiency (PCE) as high as ~30%, in the case of the solar cells comprised of fluorinated materials, PBDB-T-2F and ITIC-4F. This implies that NFAs must be able to serve as electron donors, even though they are conventionally assigned as electron acceptors. Therefore, the pathways of NFA-originated excitons need to be explored by the spectrally resolved photovoltaic characters. Additionally, excitation wavelength dependent transient absorption spectroscopy (TAS) was performed to trace the nature of the NFA-originated excitons and polymeric donor-originated excitons separately. Unique origin-dependent decay behaviors of the blend system were found by successive comparing of those solutions and pristine films which showed a dramatic change upon film formation. With the obtained experimental results, including TAS, a possible model describing origin-dependent decay pathways was suggested in the framework of reaction kinetics. Finally, numerical simulations based on the suggested model were performed to verify the feasibility, achieving reasonable correlation with experimental observables. The results should provide deeper insights in to renewable energy strategies by using novel material classes that are compatible with flexible electronics.

Localized field induced room-temperature photoconduction far below bandgap

Long wavelength photodetectors, especially for the far-infrared or terahertz detection, have long been of great interest due to their myriad applications in biomedical diagnostic, spectroscopy, communication and sensing, but they still face challenges in room temperature performance. Here, we have established an efficient strategy for generating photocarriers in a sub-wavelength metal wrapped semiconductor or topological insulator layer with detectable wavelength depends only on the structure rather than the bandgap. The photocarriers, which are dislodged from metallic contacts, are ascribed to the trapping mechanism of a localized field-induced potential well (LFPW). When the structure is biased and illuminated, the unidirectional flow of non-equilibrium photocarriers will produce a large photo-condutive gain over several orders of magnitude. Owing to its electromagnetic nature, the photoconduction is still highly efficient even at photon energies far below the inter-band threshold at room temperature. The unique photoelectric mechanism provides high sensitivity, fast response and superb signal-to-noise ratio, facilitating the spectral extension beyond the standard limit of cryogenic cooling, undisputed benefits for addressing targeted application requirement in biomedical, security and fast imaging.

Enhanced Uplink Coverage for 5G NR: Frequency-Domain Spectral Shaping With Spectral Extension

This paper describes and investigates a novel concept of frequency-domain spectral shaping (FDSS) with spectral extension for the uplink (UL) coverage enhancement in 5G New Radio (NR), building on discrete Fourier transform spread orthogonal frequency-domain multiplexing (DFT-s-OFDM). The considered FDSS concept is shown to have large potential for reducing the peak-to-average-power ratio (PAPR) of the signal, which directly impacts the feasible maximum transmit power under practical nonlinear power amplifiers (PAs) while still meeting the radio frequency (RF) emission requirements imposed by the regulations. To this end, the FDSS scheme with spectral extension is formulated, defining filter windows that fit to the 5G NR spectral flatness requirements. The PAPR reduction capabilities and the corresponding maximum achievable transmit powers are evaluated for a variety of bandwidth allocations in the supported 5G NR frequency ranges 1 and 2 (FR1 and FR2) and compared to those of the currently supported waveforms in 5G NR, particularly π/2-BPSK with FDSS without spectral extension and QPSK without FDSS. Furthermore, an efficient receiver structure capable of reducing the noise enhancement in the equalization phase is proposed. Finally, by evaluating the link-level performance, together with the transmit power gain, the overall coverage enhancement gains of the method are analyzed and provided. The obtained results show that the spectrally-extended FDSS method is a very efficient solution to improve the 5G NR UL coverage clearly outperforming the state-of-the-art, while being also simple in terms of computational complexity such that the method is implementation feasible in practical 5G NR terminals.

Spectral extension and synchronization of microcombs in a single microresonator

Broadband optical frequency combs are extremely versatile tools for precision spectroscopy, ultrafast ranging, as channel generators for telecom networks, and for many other metrology applications. Here, we demonstrate that the optical spectrum of a soliton microcomb generated in a microresonator can be extended by bichromatic pumping: one laser with a wavelength in the anomalous dispersion regime of the microresonator generates a bright soliton microcomb while another laser in the normal dispersion regime both compensates the thermal effect of the microresonator and generates a repetition-rate-synchronized second frequency comb. Numerical simulations agree well with experimental results and reveal that a bright optical pulse from the second pump is passively formed in the normal dispersion regime and trapped by the primary soliton. In addition, we demonstrate that a dispersive wave can be generated and influenced by cross-phase-modulation-mediated repetition-rate synchronization of the two combs. The demonstrated technique provides an alternative way to generate broadband microcombs and enables the selective enhancement of optical power in specific parts of a comb spectrum.

Spatio-temporal control of dispersive waves trapping by solitons in graded-index multimode fibers

We investigate the impact of the spatial beam oscillations in a graded-index multimode fiber (MMF) upon the trapping and blue-shifting of dispersive pulses by a multimode soliton. We find that the dispersive pulses trapped by solitons can be controlled through adjusting the degree of compression of the spatial beam. This can provide an explanation for the enhanced blue spectral extension observed in the typical supercontinuum generation in graded-index MMFs. Besides, the spatial-temporal nonlinear effect may provide an additional degree of freedom for the spectrally tailored light generation and the ultrashort pulse generation with tunable wavelengths in the normal dispersion regime.

Upconversion nanoparticles extending the spectral sensitivity of silicon photodetectors to λ = 1.5 μm

The telecommunication wavelength of λ = 1.5 μm has been playing an important role in various fields. In particular, performing photodetection at this wavelength is challenging, demanding more performance stability and lower manufacturing cost. In this work,upconversion nanoparticle (UCNP)/Si hybrid photodetectors (hybrid PDs) are presented, made by integrating solution-processed Er3+-doped NaYF4 upconversion nanoparticles (UCNPs) onto a silicon photodetector. After optimization, we demonstrated that a layer of UCNPs can well lead to an effective spectral sensitivity extension without sacrificing the photodetection performance of the Si photodetector in the visible and near-infrared (near-IR) spectrum. Under λ = 1.5 μm illumination, the hybrid UCNPs/Si-PD exhibits a room-temperature detectivity of 6.15 × 1012 Jones and a response speed of 0.4 ms. These UCNPs/Si-PDs represent a promising hybrid strategy in the quest for low-cost and broadband photodetection that is sensitive in the spectrum from visible light down to the short-wave infrared.

Extremely nonlinear Raman interaction of an ultrashort nitrogen ion laser with an impulsively excited molecular wave packet

We report generation of cascaded rotational Raman scattering up to 58th orders in coherently excited CO_2 molecules. The high-order Raman scattering, which produces a quasiperiodic frequency comb with more than 600 sidebands, is obtained using an intense femtosecond laser to impulsively excite rotational coherence and the femtosecond-laser-induced N_2^+ lasing to generate cascaded Raman signals. The novel configuration allows this experiment to be performed with a single femtosecond laser beam at free-space standoff locations. It is revealed that the efficient spectral extension of Raman signals is attributed to the specific spectra-temporal structures of N_2^+ lasing, the ideal spatial overlap of femtosecond laser and N2+ lasing, and the guiding effect of molecular alignment. The Raman spectrum extending above 2000 cm^-1 naturally corresponds to a femtosecond pulse train due to the periodic revivals of molecular rotational wavepackets.

Broadband CrOCl Saturable Absorber with a Spectral Region Extension to 10.6 µm

AbstractNonlinear optical (NLO) materials with saturable absorption have attracted increasing attention because of their broad applications in the fields of optical communication, materials processing, biological detection, and surgery. However, their development is limited in the mid‐infrared (IR) range due to a lack of available materials with a wideband NLO response. Herein, the optical properties and applications of the transition metal oxychloride CrOCl, which has a broadband transparent region (0.8–18 µm) and a high laser damaged threshold, are studied. The saturable absorption of CrOCl is characterized at 1.06, 1.5, 2.0, 2.5, and 10.6 µm with nanosecond lasers by the Z‐scan method. With CrOCl as the saturable absorber (SA), passively Q‐switched solid‐state lasers are successfully realized at 1.06, 1.33, and 1.87 µm. This study is the first to report the use of a Cr3+ material as an optical SA. The high transmittance and wideband NLO response of CrOCl indicate that CrOCl can be an excellent NLO IR material with extensive application potential in photonics.

The 2- to 6- mid-infrared supercontinuum generation in cascaded ZBLAN and As2Se3 step-index fibers**Project supported by the National Natural Science Foundation of China (Grant Nos. 61435009, 61235008, and 61405254), the Fund from China Scholarship Council (Grant No. 201803170210), and Hunan Provincial Innovation Foundation for Postgraduate (Grant No. CX2018B008).

Fiber-based mid-infrared (MIR) supercontinuum (SC) sources benefit from their spectral brightness and spatial coherence that are needed for many applications, such as spectroscopy and metrology. In this paper, an SC spanning from to is demonstrated in cascaded ZrF4–BaF2–LaF3–AlF3–NaF (ZBLAN) and As2Se3 step-index fibers. The pump source is a ZBLAN fiber-based MIR SC laser with abundant high-peak-power soliton pulses between 3000 nm and 4200 nm. By concatenating the ZBLAN fiber and the As2Se3 fiber, efficient cascading red-shifts are obtained in the normal dispersion region of the As2Se3 fiber. The spectral behavior of cascaded SC generation shows that the long-wavelength proportion of MIR SC generated in the ZBLAN fiber plays a critical role for further spectral extension in the As2Se3 fiber.

Spatial Distribution of the Nonlinear Photoluminescence in Au Nanowires

When gold nanowires are excited with a tightly focused femtosecond laser a distributed nonlinear photoluminescence (N-PL) develops throughout the entire structure. A complete spaced-resolved analysis of the spectral signature of the nanowire nonlinear response is carried out to understand the origin of the distributed nonlinear response. We discuss various mechanisms to explain the experimental data and unambiguously demonstrate that the spatial and spectral extension of the N-PL in the nanowire are mainly dictated by the propagation of a surface plasmon excited at the pump wavelength. We also present experimental signature of near-field excitation of a broadband continuum of surface plasmons excited locally throughout the N-PL emission spectrum.

Electromagnetic coupling effect in the complex integrated channel of hypersonic vehicles and experimental verification

The communication link of hypersonic vehicles suffers in a complex electromagnetic channel environment with the multipath effect, large Doppler effect caused by the high speed of the hypersonic vehicles, and parasitic modulation caused by vortex turbulence of dynamic plasma sheath. The inherent coupling relationship between these effects exists even though they are with different physical generation mechanisms, which will seriously affect the reception effect of the communication signal from the perspective of the wireless channel. This study investigates the electromagnetic coupling relation between the hypersonic spatial channel and the dynamic plasma sheath channel. The coupling relation is derived in the frequency domain, and the integrated channel simulation model is established as well. Theoretical simulation results demonstrate that the harmonic spectral lines of the plasma sheath power spectrum are moved to the right with the line-of-sight Doppler frequency shift. The spectral spread of the hypersonic spatial channel is coupled to the channel power spectrum of the plasma sheath that forms multiple spectrum extension areas. The new spectral extensions caused by coupling effect will make the traditional narrowband matching receive mode face enormous challenges. The electromagnetic coupling effect in the integrated channel is also verified through channel sounding experiments on ground integrated channel emulation environment composed of plasma simulation device and wireless channel simulator. The deep understanding of this coupling effect is essential to wireless channel modeling and design of adaptive reception methods for hypersonic vehicles.

Watt-level, all-fiber, spectrally flat, and ZBLAN fiber-based MIR supercontinuum extending to 4.6 μm with a record power ratio beyond 3.8 μm

An all-fiber midinfrared supercontinuum with 20-dB spectral coverage from 1.9 to 4.6 μm is demonstrated with a record power ratio beyond 3.8 μm. The supercontinuum is generated in a piece of single-mode ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF) fiber pumped by a broadband, single-mode thulium-doped fiber amplifier. Under the optimized pulse repetition rate and ZBLAN fiber length, the output spectrum has a good flatness. The power ratio beyond 3.8 μm is measured to be over 30% when the average output power reaches 1.11 W. Based on the all-fiber configuration, we have provided a compact, reliable, and promising supercontinuum source for further spectral extension into the midinfrared region where a high long-wavelength ratio is preferred.

Study of infrared photodetectors with wavelength extension mechanism

The III-V semiconductor heterostructure-based photodetectors have been studied extensively for infrared detection, from near-infrared (NIR) to far-infrared (FIR) region. Due to the mature material system, GaAs/AlxGa1-xAs heterostructures are attractive options for development of infrared detectors. The conventional rule of photodetection, λt=hc/Δ, determines the wavelength threshold (λt) of spectral photoresponse, where Δ is the minimum energy gap of the material, or the interfacial energy gap of the heterostructure. In recent studies on the p-GaAs/AlxGa1-xAs heterostructure-based infrared photodetectors, spectral threshold limit due to Δ has been overcome owing to a detection mechanism arising from the hot-carrier effect driven extended wavelength photoresponse mechanism. It has been experimentally observed that a detector with a conventional spectral threshold of ∼3.1 μm shows an extended wavelength threshold of up to ∼68 μm. An important advantage of the wavelength extension mechanism is the lower dark current, which is determined by the designed Δ. Dark current fittings obtained from a 3D carrier drift model closely agree with experimentally measured dark current. A barrier energy offset (δEv) between AlxGa1-xAs barriers is found to be necessary for the spectral extension mechanism.

RF Up-Conversion and Waveform Generation Using a Frequency-Shifting Amplifying Fiber Loop, Application to Doppler Velocimetry

We investigate the radio-frequency (RF) up-conversion and waveform generation properties of an optical fiber loop including a frequency shifter and an amplifier. By seeding the loop with a single-frequency continuous-wave laser, one can develop a wide optically carried RF comb, whose spectral extension is governed by the loop net gain. In addition, by choosing the fiber loop length and the RF shifting frequency, arbitrary waveforms can be generated. We present an analytical interference model that includes the time delay, the frequency shift, and the gain. Experiments are conducted with 1.06-μm fiber-optic components. Using a 200-MHz acousto-optic frequency shifter, we find a 19-fold up-conversion up to 3.8 GHz with a typical in-loop gain of 3. Various waveforms including bright and dark pulses, square- or triangle shaped are achieved by properly adjusting the loop length and the frequency shift. A good agreement between experimental and theoretical results is obtained. The fully fibered microwave-photonic source is applied to a laboratory Doppler velocimetry demonstration. The gain in sensitivity obtained with the up-converted signal is readily observed.

Spectral properties of the Lau product $\mathcal{A}\times_{\theta}\mathcal{B}$ of Banach algebras

Let A and B be commutative Banach algebras. Then a multiplicative linear functional θ on B induces a multiplication on the Cartesian product space A×B given by (a,b)(c,d)=(ac+θ(d)a+θ(b)c,bd) for all (a,b),(c,d)∈A×B. We show that this Lau product is stable with respect to the spectral properties like the unique uniform norm property, the spectral extension property, the multiplicative Hahn–Banach property, and the unique semisimple norm property under certain conditions on θ.

Going deep into protein secondary structure with synchrotron radiation circular dichroism spectroscopy.

Circular dichroism (CD) spectroscopy is a fast, powerful, well-established, and widely used analytical technique in the biophysical and structural biology community to study protein secondary structure and to track changes in protein conformation in different environments. The use of the intense light of a synchrotron beam as the light source for collecting CD measurements has emerged as an enhanced method, known as synchrotron radiation circular dichroism (SRCD) spectroscopy, that has several advantages over the conventional CD method, including a significant spectral range extension for data collection, deeper access to the lower limit (cut-off) of conventional CD spectroscopy, an improved signal-to-noise ratio to increase accuracy in the measurements, and the possibility to collect measurements in highly absorbing solutions. In this review, we discuss different applications of the SRCD technique by researchers from Latin America. In this context, we specifically look at the use of this method for examining the secondary structure and conformational behavior of proteins belonging to the four main classes of the hierarchical protein domain classification CATH (Class, Architecture, Topology, Homology) database, focusing on the advantages and improvements associated with SRCD spectroscopy in terms of characterizing proteins composed of different structural elements.

Spectral properties and stability of perturbed Cartesian product

Let $${\mathcal {A}}$$ and $${\mathcal {B}}$$ be commutative Banach algebras, and let $$T:{\mathcal {B}} \rightarrow {\mathcal {A}}$$ be an algebra homomorphism with $${\Vert T\Vert }\le 1$$ . Then T induces a Banach algebra product $$\times _T$$ perturbing the coordinatewise product on the Cartesian product space $${\mathcal {A}} \times {\mathcal {B}}$$ . We show that the spectral properties like spectral extension property, unique uniform norm property, regularity, weak regularity as well as Ditkin’s condition are stable with respect to this product.

Spectral broadening of picosecond pulses forming dispersive shock waves in optical fibers.

We investigate analytically, numerically, and experimentally the spectral broadening of pulses that undergo the formation of dispersive shocks, addressing in particular pulses in the range of tens of ps generated via electro-optic modulation of a continuous-wave laser. We give an analytical estimate of the maximal spectral extension and show that super-Gaussian waveforms favor the generation of flat-topped spectra. We also show that the weak residual background of the modulator produces undesired spectral ripples. Spectral measurements confirm our estimates and agree well with numerical integration of the nonlinear Schrödinger equation.

A radiative transfer module for calculating photolysis rates and solar heating in climate models: Solar-J v7.5

Abstract. Solar-J is a comprehensive radiative transfer model for the solar spectrum that addresses the needs of both solar heating and photochemistry in Earth system models. Solar-J is a spectral extension of Cloud-J, a standard in many chemical models that calculates photolysis rates in the 0.18–0.8 µm region. The Cloud-J core consists of an eight-stream scattering, plane-parallel radiative transfer solver with corrections for sphericity. Cloud-J uses cloud quadrature to accurately average over correlated cloud layers. It uses the scattering phase function of aerosols and clouds expanded to eighth order and thus avoids isotropic-equivalent approximations prevalent in most solar heating codes. The spectral extension from 0.8 to 12 µm enables calculation of both scattered and absorbed sunlight and thus aerosol direct radiative effects and heating rates throughout the Earth's atmosphere.The Solar-J extension adopts the correlated-k gas absorption bins, primarily water vapor, from the shortwave Rapid Radiative Transfer Model for general circulation model (GCM) applications (RRTMG-SW). Solar-J successfully matches RRTMG-SW's tropospheric heating profile in a clear-sky, aerosol-free, tropical atmosphere. We compare both codes in cloudy atmospheres with a liquid-water stratus cloud and an ice-crystal cirrus cloud. For the stratus cloud, both models use the same physical properties, and we find a systematic low bias of about 3 % in planetary albedo across all solar zenith angles caused by RRTMG-SW's two-stream scattering. Discrepancies with the cirrus cloud using any of RRTMG-SW's three different parameterizations are as large as about 20–40 % depending on the solar zenith angles and occur throughout the atmosphere.Effectively, Solar-J has combined the best components of RRTMG-SW and Cloud-J to build a high-fidelity module for the scattering and absorption of sunlight in the Earth's atmosphere, for which the three major components – wavelength integration, scattering, and averaging over cloud fields – all have comparably small errors. More accurate solutions with Solar-J come with increased computational costs, about 5 times that of RRTMG-SW for a single atmosphere. There are options for reduced costs or computational acceleration that would bring costs down while maintaining improved fidelity and balanced errors.