Far-infrared Regime Research Articles

Teaming up Radio and Sub-mm/FIR Observations to Probe Dusty Star-Forming Galaxies

In this paper, we investigate the benefits of teaming up data from the radio to the far-infrared (FIR) regime for the characterization of dusty star-forming galaxies (DSFGs). These galaxies are thought to be the star-forming progenitors of local massive quiescent galaxies and to play a pivotal role in the reconstruction of the cosmic star formation rate density up to high redshift. Due to their dust-enshrouded nature, DSFGs are often invisible in the near-infrared/optical/UV bands. Therefore, they necessitate observations at longer wavelengths, primarily the FIR band, where dust emission occurs, and the radio band, which is not affected by dust absorption. Combining data from these two spectral windows makes it possible to characterize even the dustiest objects, enabling the retrieval of information about their age, dust temperature, and star-formation status, and facilitates the differentiation between various galaxy populations that evolve throughout cosmic history. Despite the detection of faint radio sources being a challenging task, this study demonstrates that an effective strategy to build statistically relevant samples of DSFGs would be reaching deep sensitivities in the radio band, even restricted to smaller areas, and then combining these radio observations with FIR/submm data. Additionally, this paper quantifies the improvement in the spectral energy distribution (SED) reconstruction of DSFGs by incorporating ALMA band measurements, in particular, in its upgraded status thanks to the anticipated Wideband Sensitivity Upgrade.

Quasi-bound state in the continuum based strong light confinement in graphene metasurfaces

The bound state in the continuum (BIC) is exploited to metasurfaces for the purpose of arising strong confinement of light and high quality (Q) factor resonances. In this paper, a BIC symmetry-protected metasurface consisting of rectangular graphene patches is synthesized and realistically modeled with intrinsic losses in a far-infrared regime. The quasi-BIC with three asymmetry approaches is applied to attain strong tailored confinement of the incident light, and the resulting resonances with tailored line widths and selective confinement of light are exhibited. Asymmetry parameters of length, rotation, and material feature of chemical potential regulate dual specifications of resonance linewidth and intensity. Efficiency analysis of the results illustrates that rotation asymmetry is the foremost among the three suggested methods, and it establishes the highest Q-factor of 945 with an achievable asymmetry parameter of 5.5%. The high Q and tunable resonances realized in the graphene quasi-BIC metasurfaces promise various practical applications in the infrared spectra, such as narrow band filters, sensors, optical switches, and light-matter interaction platforms.

Solving the spectral gap problem with a four-mirror bow-tie cavity in waveguide free-electron laser oscillators

The generation of intense coherent radiation pulses in the far-infrared and terahertz regimes is of considerable interest to the free-electron laser (FEL) radiation user community. At long wavelengths, the diffraction effect can be quite severe, therefore, an optical waveguide is required to confine the radiation field. However, it will also bring about some new phenomena, and the most noteworthy one is the spectral gap phenomenon: at some particular wavelengths, regardless of electron beam adjustments, the coupling efficiency and output power of waveguide FEL oscillators drop significantly. Such spectral gap has an adverse effect on experimental results since numerous experiments require continuous spectral scanning. In this paper, we propose to utilize a bow-tie cavity instead of conventional cavities to the waveguide FEL to solve the spectral gap problem. The simulation was carried out based on the parameters of FELiChEM, a newly built user facility in China. Numerical simulation code OPC combining with modified GENESIS is used to enable the modelling, for the first time, of a bow-tie cavity based FEL in the far-infrared wavelength regime. The simulation results indicate that this novel structure can effectively eliminate the spectral gaps and substantially enhance long-wavelength laser performance.

A platform for far-infrared spectroscopy of quantum materials at millikelvin temperatures.

Optical spectroscopy of quantum materials at ultralow temperatures is rarely explored, yet it may provide critical characterizations of quantum phases not possible using other approaches. We describe the development of a novel experimental platform that enables optical spectroscopic studies, together with standard electronic transport, of materials at millikelvin temperatures inside a dilution refrigerator. The instrument is capable of measuring both bulk crystals and micrometer-sized two-dimensional van der Waals materials and devices. We demonstrate its performance by implementing photocurrent-based Fourier transform infrared spectroscopy on a monolayer WTe2 device and a multilayer 1T-TaS2 crystal, with a spectral range available from the near-infrared to the terahertz regime and in magnetic fields up to 5T. In the far-infrared regime, we achieve spectroscopic measurements at a base temperature as low as ∼43 mK and a sample electron temperature of ∼450 mK. Possible experiments and potential future upgrades of this versatile instrumental platform are envisioned.

Atomically precise vacancy-assembled quantum antidots.

Patterning antidots, which are regions of potential hills that repel electrons, into well-defined antidot lattices creates fascinating artificial periodic structures, leading to anomalous transport properties and exotic quantum phenomena in two-dimensional systems. Although nanolithography has brought conventional antidots from the semiclassical regime to the quantum regime, achieving precise control over the size of each antidot and its spatial period at the atomic scale has remained challenging. However, attaining such control opens the door to a new paradigm, enabling the creation of quantum antidots with discrete quantum hole states, which, in turn, offer a fertile platform to explore novel quantum phenomena and hot electron dynamics in previously inaccessible regimes. Here we report an atomically precise bottom-up fabrication of a series of atomic-scale quantum antidots through a thermal-induced assembly of a chalcogenide single vacancy in PtTe2. Such quantum antidots consist of highly ordered single-vacancy lattices, spaced by a single Te atom, reaching the ultimate downscaling limit of antidot lattices. Increasing the number of single vacancies in quantum antidots strengthens the cumulative repulsive potential and consequently enhances the collective interference of multiple-pocket scattered quasiparticles inside quantum antidots, creating multilevel quantum hole states with a tunable gap from the telecom to far-infrared regime. Moreover, precisely engineered quantum hole states of quantum antidots are geometry protected and thus survive on oxygen substitutional doping. Therefore, single-vacancy-assembled quantum antidots exhibit unprecedented robustness and property tunability, positioning them as highly promising candidates for advancing quantum information and photocatalysis technologies.

Efficient Recursive Convolution Schemes for the Accurate FDTD Modeling of Graphene Interband Contribution via Complex Conjugate Pairs

The accurate time-domain numerical modeling of graphene surface conductivity due to the interband contribution is presented in this paper. Initially, the lower frequency limit for the interband conductivity inclusion is studied, highlighting that even in the far-infrared regime it should not be ignored. Then, a precise vector-fitting technique is utilized to decompose the analytical conductivity function into complex conjugate pairs. Every pair is connected to a complex surface current, while the frequency dispersion is evaluated via a proper recursive convolution scheme. Finally, a straightforward algebraic manipulation is conducted to eliminate the complex terms from the update equations of the electromagnetic components, concerning the finite-difference time-domain (FDTD) algorithm. The precision and efficiency of the proposed methodology are thoroughly validated by comparing the numerically extracted surface wave propagation characteristics to those obtained in terms of analytical expressions.

Design of Tunable Far-Infrared Plasmonic Absorber Based on Chalcogenide Phase Change Materials

We propose a wide-angle metamaterial absorber with more than 90% absorption in the far-infrared (F-IR) and terahertz (THz) regimes. Our metal-dielectric metamaterial absorber consists of a phase change layer (Ge2Sb2Te5), a dielectric spacer (MgF2), and a bottom refractory metal layer (TiN). We numerically designed the structure by finite-difference time-domain simulation method and demonstrated a perfect absorption in the spectral range from 10 μm to 50 μm (30 THz to 6 THz). Furthermore, it shows a broad peak with maximum absorption of 93% at the resonant wavelength of 22.5 μm when the phase change layer is in the amorphous (disorder) state. In contrast, the peak resonance is red-shifted to 29.5 μm when the Ge2Sb2Te5 switches to the crystalline (order) state, demonstrating a resonant band tunability of Δλ= 7μm. The proposed structure shown here is a simple planner structure, lithographic-free and easy to fabricate with spectral band tunability, which offers great potential for ultra-cooled detection, imaging, security scanning, gas leakage detection, and remote monitoring applications.

Investigation of low-confinement surface phonon polariton launching on SiC and SrTiO3 using scanning near-field optical microscopy

Surface phonon polaritons (SPhPs) are important building blocks of nanophotonics, as they enable strong light–matter interaction on the nanoscale, are well-suited for applications in the mid- to far-infrared regime, and can show low losses. SrTiO3 is an interesting material for SPhPs, because it allows for reversible, nonvolatile doping with free charge carriers via oxygen vacancies and for local switching with conductive AFM tips. As a result, SrTiO3 could enable programmable nanophotonics with tunable SPhPs and direct writing of metasurfaces. Surface polariton properties can be determined by mapping their real-space propagation using scattering-type scanning near-field optical microscopy (s-SNOM), which is sensitive to the high local electric fields with nanoscale lateral resolution. Low-confinement (LC) SPhPs with wavevectors close to that of free-space radiation, such as in SrTiO3 and the model polar dielectric SiC, can be difficult to investigate in s-SNOM due to interference effects with the incident illumination and fringe spacings exceeding the scan range or the size of the focus spot. Here, we present s-SNOM measurements of LC-SPhPs on SiC and SrTiO3 launched at gold stripes, retrieve physical quantities such as launching amplitude and phase, and show that they are influenced strongly by gold stripe geometry as well as illumination angle. Using two complementary measurements, we show a convenient way to determine the out-of-plane angle of the s-SNOM setup. Finally, we predict how control over the free charge carrier concentration in SrTiO3 could enable tunable LC-SPhPs, showing the potential of SrTiO3 for programmable nanophotonics.

Multi-wavelength metamaterial absorber for retrieving complex refractive index of thin-film materials at infrared regimes

Multi-wavelength absorption in uncomplicated system is pursued continuously due to its advantage on measuring thin-film material dispersion. Here, we experimentally demonstrate a single-periodic metamaterial with polarization-insensitive multi-wavelength absorption for the infrared thin-film material measurement. The meta-atom of metamaterial absorber is constructed by holed cylindrical gold disk and gold ground separated by a ZnS substrate. Simulation shows that the multi-wavelength absorption is related to the multiple resonant eigenmodes. The metamaterial absorber is proposed through photolithography process and multi-wavelength absorptions occur at 3.95 μm, 4.27 μm and 10.90 μm, respectively, in the experiment. We show that the multi-wavelength absorption is very sensitive to the dielectric property of the coating thin-film material, which indicates the proposed absorber can be used to retrieve the complex refractive index of coating thin film at various wavelengths. Our work provides a convenient way to measure refractive indices at both mid-infrared and far-infrared regimes.

The ALPINE-ALMA [CII] survey: The population of [CII]-undetected galaxies and their role in the L[CII]-SFR relation

The [CII] 158 μm emission line represents one of the most profitable tools for the investigation of the high-redshift galaxies in the early Universe so far. Being one of the brightest cooling lines in the rest-frame far-infrared regime of star-forming galaxies, it has been successfully exploited as a tracer of the star-formation rate (SFR) in local sources. The picture is more complex at higher redshifts, where its usability in this context is still under investigation. Recent results from the ALMA Large Program to INvestigate [CII] at Early times (ALPINE) survey suggest that there is no (or weak) evolution of the L[CII]-SFR relation up to z ∼ 6, but their reliability is hampered by the presence of a large population of [CII] nondetected galaxies. In this work, we characterize the population of [CII] nondetections in ALPINE. By stacking their ALMA spectra, we obtained a signal detected at ∼5.1σ, resulting in a [CII] luminosity of log(L[CII]/L⊙)∼7.8. When combining this value with those from the [CII] detections, we found a L[CII]-SFR relation with a slope b = 1.14 ± 0.11, which is in agreement within the uncertainties both with the linear relation found in the local Universe and with the previous findings from ALPINE at z ∼ 5. This suggests that the [CII] line can be considered a good tracer of star formation up to the distant Universe. Finally, we show that the galaxies of our sample that deviate from the observed L[CII]-SFR relation most could suffer from a less precise redshift estimation, perhaps artificially reducing their [CII] luminosity. In this respect, we claim that there is no evidence in favor of a deficit of [CII] content in high-z galaxies, in contrast with earlier studies.

In-Plane Anisotropic Plasmons in Van Der Waals Thin Films of WTe2

Anisotropic plasmonic surface supports elliptic, hyperbolic and even flattened polaritons, which is quite interesting for the diffractionless and highly collimated propagation of infrared light at the nanoscale. However, direct real-space near-field observation of anisotropic plasmons as well as frequency dependent topological transitions in natural materials have not been realized. In this paper, we theoretically investigate real-space anisotropic plasmons in WTe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin films by using a phenomenological cavity model, anisotropic near-field plasmonic images with specific interference patterns and isofrequency curves in momentum space have been demonstrated. Due to the frequency selective forbidden of plasmons along <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</i> axis, a topological transition from the elliptic to the hyperbolic regime is manifested. Moreover, the plasmons as well as topological transition present significant electrostatic-gating tunability. Our studies provide new insights into WTe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> based plasmonic components for the manipulation of plasmon propagation, which capable of tailoring anisotropic two-dimensional light confinement in the far-infrared regime and can be applied to investigate other anisotropic materials.

Accurate Time-Domain Modeling of Arbitrarily Shaped Graphene Layers Utilizing Unstructured Triangular Grids

The accurate modeling of curved graphene layers for time-domain electromagnetic simulations is discussed in the present work. Initially, the advanced properties of graphene are presented, focusing on the propagation of strongly confined surface plasmon polariton waves at the far-infrared regime. Then, the implementation of an unstructured triangular grid was examined, based on the Delaunay triangulation method. The electric-field components were placed at the edges of the triangles, while two different techniques were proposed for the sampling of the magnetic ones. Specifically, the first one suggests that the magnetic component is placed at the triangle’s circumcenter providing more accurate results, although instability may occur for nonacute triangles. On the other hand, the magnetic field was sampled at the triangle’s centroid, considering the second technique, ensuring the algorithm’s stability, but further approximations were required, leading to a slight accuracy reduction. Moreover, the updating equations in the time-domain were extracted via an appropriate approximation of Maxwell equations in their integral form. Finally, graphene was introduced in the computational domain as an equivalent surface current density, whose location matches the corresponding electric components. The validity of our methodology was successfully performed via the comparison of graphene surface wave propagation properties to their theoretical values, whereas the global error determination indicates the minimal triangle dimensions. Additionally, an instructive setup comprising a circular graphene scatterer was analyzed thoroughly, to reveal our technique’s advantages compared to the conventional staircase discretization.

Single-Step Fabrication of High-Performance Extraordinary Transmission Plasmonic Metasurfaces Employing Ultrafast Lasers.

Plasmonic metasurfaces based on the extraordinary optical transmission (EOT) effect can be designed to efficiently transmit specific spectral bands from the visible to the far-infrared regimes, offering numerous applications in important technological fields such as compact multispectral imaging, biological and chemical sensing, or color displays. However, due to their subwavelength nature, EOT metasurfaces are nowadays fabricated with nano- and micro-lithographic techniques, requiring many processing steps and carrying out in expensive cleanroom environments. In this work, we propose and experimentally demonstrate a novel, single-step process for the rapid fabrication of high-performance mid- and long-wave infrared EOT metasurfaces employing ultrafast direct laser writing. Microhole arrays composing extraordinary transmission metasurfaces were fabricated over an area of 4 mm2 in timescales of units of minutes, employing single pulse ablation of 40 nm thick Au films on dielectric substrates mounted on a high-precision motorized stage. We show how by carefully characterizing the influence of only three key experimental parameters on the processed micro-morphologies (namely, laser pulse energy, scan velocity, and beam shaping slit), we can have on-demand control of the optical characteristics of the extraordinary transmission effect in terms of transmission wavelength, quality factor, and polarization sensitivity of the resonances. To illustrate this concept, a set of EOT metasurfaces having different performances and operating in different spectral regimes has been successfully designed, fabricated, and tested. Comparison between transmittance measurements and numerical simulations has revealed that all the fabricated devices behave as expected, thus demonstrating the high performance, flexibility, and reliability of the proposed fabrication method. We believe that our findings provide the pillars for mass production of EOT metasurfaces with on-demand optical properties and create new research trends toward single-step laser fabrication of metasurfaces with alternative geometries and/or functionalities.

Analyzing broadband tunable metamaterial absorbers by using the symmetry model

In this paper, a broadband tunable absorber based on bulk Dirac semimetal (BDS) in the far-infrared regime is studied. By optimizing the Femi energy and geometric size, the structure can achieve absorption levels greater than 90% in the 8.11-13.94 THz range, with a total thickness of 5.1 µm. Further, the bandwidth of this proposed absorber can be dynamically controlled by changing the Femi energy of the BDS instead of geometry. Meanwhile, the polarization and oblique incident angles in the TE and TM electromagnetic waves are also investigated. Finally, a new symmetry model is adopted to analyze the absorption mechanism, which is the innovation of this paper. This research result may provide potential applications for all types of optical photovoltaic devices and tunable biological chemistry sensors.

Bringing high spatial resolution to the far-infrared

The far-infrared (FIR) regime is one of the wavelength ranges where no astronomical data with sub-arcsecond spatial resolution exist. None of the medium-term satellite projects like SPICA, Millimetron, or the Origins Space Telescope will resolve this malady. For many research areas, however, information at high spatial and spectral resolution in the FIR, taken from atomic fine-structure lines, from highly excited carbon monoxide (CO), light hydrides, and especially from water lines would open the door for transformative science. A main theme will be to trace the role of water in proto-planetary discs, to observationally advance our understanding of the planet formation process and, intimately related to that, the pathways to habitable planets and the emergence of life. Furthermore, key observations will zoom into the physics and chemistry of the star-formation process in our own Galaxy, as well as in external galaxies. The FIR provides unique tools to investigate in particular the energetics of heating, cooling, and shocks. The velocity-resolved data in these tracers will reveal the detailed dynamics engrained in these processes in a spatially resolved fashion, and will deliver the perfect synergy with ground-based molecular line data for the colder dense gas.

Constraining the magnetic field properties of Bok globule B335 using SOFIA/HAWC+

Thanks to their well-defined shape and mostly isolated locations, Bok globules are suitable objects for studying the physics of low-mass star formation. To study the magnetic field of the prototypical Bok globule B335, we obtained a spatially resolved polarization map with SOFIA/HAWC+ at a wavelength of 214 μm. For the first time, these observations reveal that polarization holes in Bok globules, that is, the decrease in polarization degree towards their dense centers, also occur in the far-infrared wavelength regime. The observed polarization pattern is uniform with a mean polarization angle of 48° ± 26° and a magnetic field strength of ~142 μG. Moreover, we use complementary polarimetic data for B335 obtained at near-infrared to millimeter wavelengths to analyze and constrain the magnetic field across different scales. By applying the 3D Monte-Carlo radiative transfer code POLARIS, we developed a model for the density and magnetic field structure as well as for the dust properties of this globule. We conclude that the column density towards the center of B335 is too low to cause the observed polarization hole in B335 via dichroic absorption. Furthermore, we conclude that the effect of self-scattering has no significant impact on the observed polarization. Adopting dust-grain alignment via the radiative torque mechanism, a combination of the interstellar radiation field and the central star as radiation sources is consistent with the decrease in polarization degree at the outer regions of B335 (≈104 au from the core). However, the model fails to explain the low polarization degree within the inner 5000 au.

Hydrostatic pressure and interlayer distance effects on non-linear optical proprieties in n-type double delta-doped GaAs quantum wells

Within the framework of the Thomas-Fermi theory, the electronic structure and the nonlinear optical properties in n-type GaAs double delta-doped quantum wells where determined. In particular, was studied the effects of the distance between the doping layers and the hydrostatic pressure on the absorption coefficient and the change in the refractive index. It was found that the interlayer distance has an important effect on potential geometry, while the pressure lowers the bottom of potential. It was found that the influence of the interlayer distance (hydrostatic pressure) generates a red-shift (blue-shift) on the absorption peak and the refractive index change node. The results indicate that the optoelectronic properties in small interlayer distances (25–50 Å) show a high sensitivity to the changes of hydrostatic pressure, whereas for distances larger than 100 Å the effects of hydrostatic pressure are unimportant for the 1-0 transition. On the far-infrared regime, the effects of these parameters can be applied to tuning the position and amplitude of the resonant optical signals.

Dynamic Absorption Enhancement and Equivalent Resonant Circuit Modeling of Tunable Graphene-Metal Hybrid Antenna.

Plasmonic antennas are attractive optical components of the optoelectronic devices, operating in the far-infrared regime for sensing and imaging applications. However, low optical absorption hinders its potential applications, and their performance is limited due to fixed resonance frequency. In this article, a novel gate tunable graphene-metal hybrid plasmonic antenna with stacking configuration is proposed and investigated to achieve tunable performance over a broad range of frequencies with enhanced absorption characteristics. The hybrid graphene-metal antenna geometry is built up with a hexagon radiator that is supported by the Al2O3 insulator layer and graphene reflector. This stacked structure is deposited in the high resistive Si wafer substrate, and the hexagon radiator itself is a sandwich structure, which is composed of gold hexagon structure and two multilayer graphene stacks. The proposed antenna characteristics i.e., tunability of frequency, the efficiency corresponding to characteristics modes, and the tuning of absorption spectra, are evaluated by full-wave numerical simulations. Besides, the unity absorption peak that was realized through the proposed geometry is sensitive to the incident angle of TM-polarized incidence waves, which can flexibly shift the maxima of the absorption peak from 30 THz to 34 THz. Finally, an equivalent resonant circuit model for the investigated antenna based on the simulations results is designed to validate the antenna performance. Parametric analysis of the proposed antenna is carried out through altering the geometric parameters and graphene parameters in the Computer Simulation Technology (CST) studio. This clearly shows that the proposed antenna has a resonance frequency at 33 THz when the graphene sheet Fermi energy is increased to 0.3 eV by applying electrostatic gate voltage. The good agreement of the simulation and equivalent circuit model results makes the graphene-metal antenna suitable for the realization of far-infrared sensing and imaging device containing graphene antenna with enhanced performance.

Reproducing the Universe: a comparison between the EAGLE simulations and the nearby DustPedia galaxy sample

ABSTRACTWe compare the spectral energy distributions (SEDs) and inferred physical properties for simulated and observed galaxies at low redshift. We exploit UV-submillimetre mock fluxes of ∼7000 z = 0 galaxies from the EAGLE suite of cosmological simulations, derived using the radiative transfer code skirt. We compare these to ∼800 observed galaxies in the UV-submillimetre range, from the DustPedia sample of nearby galaxies. To derive global properties, we apply the SED fitting code cigale consistently to both data sets, using the same set of ∼80 million models. The results of this comparison reveal overall agreement between the simulations and observations, both in the SEDs and in the derived physical properties, with a number of discrepancies. The optical and far-infrared regimes, and the scaling relations based upon the global emission, diffuse dust, and stellar mass, show high levels of agreement. However, the mid-infrared fluxes of the EAGLE galaxies are overestimated while the far-UV domain is not attenuated enough, compared to the observations. We attribute these discrepancies to a combination of galaxy population differences between the samples and limitations in the subgrid treatment of star-forming regions in the EAGLE-skirt post-processing recipe. Our findings show the importance of detailed radiative transfer calculations and consistent comparison, and provide suggestions for improved numerical models.

THz Near-Field Imaging of Extreme Subwavelength Metal Structures

Modern scattering-type scanning near-field optical microscopy (s-SNOM) has become an indispensable tool in material research. However, as the s-SNOM technique marches into the far-infrared (IR) and terahertz (THz) regimes, emerging experiments sometimes produce puzzling results. For example, anomalies in the near-field optical contrast have been widely reported. In this Letter, we systematically investigate a series of extreme subwavelength metallic nanostructures via s-SNOM near-field imaging in the GHz to THz frequency range. We find that the near-field material contrast is greatly impacted by the lateral size of the nanostructure, while the spatial resolution is practically independent of it. The contrast is also strongly affected by the connectivity of the metallic structures to a larger metallic ground plane. The observed effect can be largely explained by a quasi-electrostatic analysis. We also compare the THz s-SNOM results to those of the mid-IR regime, where the size-dependence becomes significant only for smaller structures. Our results reveal that the quantitative analysis of the near-field optical material contrasts in the long-wavelength regime requires a careful assessment of the size and configuration of metallic (optically conductive) structures.