Μm Band Research Articles

Assessments of the Above-Ocean Atmospheric CO2 Detection Capability of the GAS Instrument Onboard the Next-Generation FengYun-3H Satellite

The next-generation FengYun-3H satellite carrying a greenhouse gas absorption spectrometer (GAS) is planned for launch by 2024 with a strengthened ability to help researchers understand the global carbon cycle. However, assessments of the atmospheric CO2-detection capabilities of GAS are still incomplete, mainly in the following aspects: previous studies on the spectral range of GAS instruments often used the weak absorption band of CO2 molecules (1.61 μm); research on the measurement accuracies of different atmospheric environments above oceans is lacking; and most studies considered land surfaces as the bottom boundaries. Here, we simulated high spectral CO2 absorption spectra in both the strong and weak bands (2.06 and 1.61 μm) while considering the effects of different instrumental (spectral resolution and sampling rate) and environmental (wind speed, visibility, and rough sea surface) parameters. This is the first atmospheric CO2 absorption spectrum study to consider rough-sea-surface effects. The preliminary results show that the root mean squared error (RMSE) and mean absolute difference (MAD) values of the atmospheric CO2 transmittance spectra of GAS are 0.031 and 0.011, respectively, in the 1.61 μm band and 0.05 and 0.033 in the 2.06 μm band, revealing that GAS is competitive among similar CO2 instruments. This study provides a design reference for next-generation GAS instruments and contributes to spectral data CO2 processing in the above-sea atmosphere.

Highly stable and wavelength-switchable single-longitudinal-mode Tm3+-doped fiber laser with a specially-designed tri-coupler dual-ring filter

A single-longitudinal-mode (SLM) Tm3+-doped fiber laser (TDFL) with a narrow linewidth, less than 8 kHz, is proposed and demonstrated. A superimposed polarization-maintaining fiber Bragg grating (SI-PM-FBG), a specially-designed tri-coupler dual-ring (TCDR) cavity filter, and a section of unpumped Tm3+-doped fiber are combined to realize output lasing with SLM operation for each wavelength channel in 2 µm band. The design, fabrication, and characterization of the TCDR filter are demonstrated in detail. The proposed four-wavelength-switchable TDFL can be switched among fifteen lasing states, including four single-wavelength, six dual-wavelength, four triple-wavelength, and one quadruple-wavelength operations by adjusting the drop-in polarization controller. The experimental results show that the four SLM single-wavelength lasings are all with a relative intensity noise below − 113.06 dB/Hz at frequencies above 2 MHz, and the wavelength and output power fluctuation below 0.02 nm and 0.40 dB whihin 60 min, respectively. Four lasing wavelengths at 2048.28, 2048.71, 2049.05, and 2049.52 nm exhibit good wavelength and power stability and can maintain SLM operation.

10-Watt-Level 1.7 μm Random Fiber Laser with Super-High Spectral Purity in a Cost-Effective and Robust Structure

The 1.7 μm band eye-safe laser sources have recently received lots of attention thanks to the development of various applications. Although a variety of lasing configurations operating in this band have been demonstrated, one still needs to seek a good candidate for particular applications with a reasonable compromise between the relative performance targets (e.g., stability, output power, and spectral purity) and the construction cost. Here, we demonstrate a high-power 1694 nm random fiber laser (RFL) in a cost-effective structure pumped by a high-powered 1565 nm RFL. The maximum output power reached the 10 W level, and the output showed extremely low-intensity fluctuations for both the short-time and long-time regimes. Meanwhile, an excellent spectral purity as high as 26.9 dB was also realized. This work provides one of the most attractive approaches for constructing high-performance 1.7 μm band laser sources for practical applications.

Calibration of the Tully–Fisher relation in the WISE W1 (3.4 μm) and W2 (4.6 μm) bands

ABSTRACT In this paper, we present our calibrations of the Tully–Fisher relation in the mid-infrared W1 (3.4 $\mu$m) and W2 (4.6 $\mu$m) bands, using large samples of 877 galaxies and 876 galaxies from 31 clusters in the W1 and W2 bands, respectively. Using a new, iterative bivariate fitting procedure we performed two corrections, one for the cluster population incompleteness bias and a second morphological type correction. We initially performed calibrations using total absolute magnitudes and H i line widths WF50 derived from the H i global profiles as a measure of rotational velocity. We then used (i) isophotal magnitudes and (ii) average rotational velocities measured along the flat sections of the spatially resolved rotation curves of the galaxies, which were obtained from the empirical conversion between rotational velocity definitions. We determined that the calibrations using total magnitudes and H i line widths had smaller total scatter around the TF relations in the W1 and W2 bands than those using either isophotal magnitudes or spatially resolved rotational velocities. These calibrations are MTot,W1 = (1.77 ± 0.40) − (9.99 ± 0.16)log10(WF50) and MTot,W2 = (2.03 ± 0.40) − (10.12 ± 0.16)log10(WF50), with associated total scatters of σW1 = 0.66 and σW2 = 0.67. We compared these with previous calibrations in the near-infrared J, H, and K bands and the long-wavelength optical I band which used the same two corrections. The differences between calibrations can be explained by considering the different regions and components of spiral galaxies that are traced by the different wavelengths. The codes used for the calibrations of the Tully–Fisher relation are available from https://github.com/RiannaBell/WISE-TF-Calibration.

Surfactant effect of Bi on InAs quantum dot laser diode

In this study, InAs multiple-stacked quantum dot lasers were investigated in the 1.55 μm band using the Bi surfactant effect. The Bi surfactant effect increased the size of the quantum dot and changed the emission wavelength from 1522 to 1554 nm. Moreover, the surfactant effect enhanced the internal quantum efficiency of the fabricated broad-area laser from 33% to 54%.

Research on beam quality control technology of 2 μm antimonide semiconductor laser

Antimonide semiconductor laser is a new type of laser with unique advantages in the 2 μm band. However, employing FP cavities causes multiple transverse modes to degrade beam quality despite achieving higher power output. In this paper, an antimonide semiconductor laser operating in 2 μm band is realized by utilizing fiber coupling and combining. Fiber combining results in higher output power, while the uniform patterns in both near-field and far-field are obtained, and the beam quality is improved. The experimental results illustrate that the output power reaches 1.2 W after 7-channel beam combination, and the near-field distribution is approximately Gaussian, while the far-field distribution is a flat-top.

Visible transparent mid-infrared broadband absorbers based on gradient refractive indexes and multi-size cavity resonances.

We proposed a multi-layered nanorod structure with the same tilt angle and different diameters, which has high visible transmittance and strong 3-5 µm absorption based on the principles of the gradient of the refractive index and the multi-size cavity resonances. The indium tin oxide (ITO) was selected as the target material to fabricate the structure by using a glancing angle deposition method. The experimental results show that when the deposition angle θ is 80°, swing deposition is successively done with the rotation angle φ of ±8°, ± 5°, ± 3°, and 0° on the surface of the substrate, and the quartz crystal microbalance thicknesses of ITO nanorods are 220 nm for each deposition, the average transmittance is 80.5% in the range of 400-800 nm and the integrated absorption is 86% in the 3-5 µm band. Such a simple, low-cost, and easy-to-fabricate device has potential applications in window stealth materials and other related fields.

SFR estimations from z = 0 to z = 0.9

Context. The star formation rate (SFR) is a key ingredient for studying the formation and evolution of galaxies. Being able to obtain accurate estimations of the SFR, for a wide range of redshifts, is crucial for building and studying galaxy evolution paths over cosmic time. Aims. Based on a statistical sample of galaxies, the aim of this paper is to constrain a set of SFR calibrators that are able to work in a large redshift range, from z = 0 to z = 0.9. Those calibrators will help to homogenize SFR estimations of star-forming galaxies and to remove any possible biases from the study of galaxy evolution. Methods. Using the VIMOS Public Extragalactic Redshift Survey (VIPERS), we estimated a set of SFR based on photometric and spectroscopic data. We used, as estimators, photometric bands from ultraviolet (UV) to mid-infrared (mid-IR), and the spectral lines Hβ, [O II]λ3727, and [O III]λ5007. Assuming a reference SFR obtained from the spectral energy distribution reconstructed with Code Investigating GALaxy Emission (CIGALE), we estimated the reliability of each band as an SFR tracer. We used the GALEX-SDSS-WISE Legacy Catalog (GSWLC, z < 0.3) to trace the dependence of these SFR calibrators with redshift. Results. The far and near UV (FUV and NUV, respectively), u-band and 24 μm bands, as well as LTIR, are found to be good SFR tracers up to z ∼ 0.9 with a strong dependence on the attenuation prescription used for the bluest bands (scatter of SFR of 0.26, 0.14, 0.15, 0.23, and 0.24 dex for VIPERS, and 0.25, 0.24, 0.09, 0.12, and 0.12 dex for GSWLC). The 8 μm band provides only a rough estimate of the SFR as it depends on metallicity and polycyclic aromatic hydrocarbon properties (scatter of 0.23 dex for VIPERS). We estimated the scatter of rest-frame luminosity estimations from CIGALE to be 0.26, 0.14, 0.12, 0.15, and 0.20 dex for FUV, NUV, ugriz, Ks, and 8–24 μm-LTIR. At intermediate redshift, the Hβ line is a reliable SFR tracer (scatter of 0.19 dex) and the [O II]λ3727 line gives an equally good estimation when the metallicity from the R23 parameter is taken into account (0.17 for VIPERS and 0.20 dex for GSWLC). A calibration based on [O III] retrieves the SFR only when additional information such as the metallicity or the ionization parameter of galaxies are used (0.26 for VIPERS and 0.20 dex for GSWLC), diminishing its usability as a direct SFR tracer. Based on rest-frame luminosities estimated with CIGALE, we propose our own set of calibrations from FUV, NUV, u-band, 8, 24 μm, LTIR, Hβ, [O II], and [O III].

Tracing PAH Emission in λ-Orionis Using COBE/DIRBE Data

We use archival COBE/DIRBE data to construct a map of polycyclic aromatic hydrocarbon (PAH) emission in the λ-Orionis region. The presence of the 3.3 μm PAH feature within the DIRBE 3.5 μm band and the corresponding lack of significant PAH spectral features in the adjacent DIRBE bands (1.25, 2.2, and 4.9 μm) enable estimation of the PAH contribution to the 3.5 μm data. Having the shortest wavelength of known PAH features, the 3.3 μm feature probes the smallest PAHs, which are also the leading candidates for carriers of anomalous microwave emission (AME). We use this map to investigate the association between the AME and the emission from PAH molecules. We find that the spatial correlation in λ-Orionis is higher between AME and far-infrared dust emission (as represented by the DIRBE 240 μm map) than it is between our PAH map and AME. This finding, in agreement with previous studies using PAH features at longer wavelengths, is in tension with the hypothesis that AME is due to spinning PAHs. However, the expected correlation between mid-infrared and microwave emission could potentially be degraded by different sensitivities of each emission mechanism to local environmental conditions even if PAHs are the carriers of both.

Oil spills: Detection and concentration estimation in satellite imagery, a machine learning approach.

The method's development to detect oil-spills, and concentration monitoring of marine environments, are essential in emergency response. To develop a classification model, this work was based on the spectral response of surfaces using reflectance data, and machine learning (ML) techniques, with the objective of detecting oil in Landsat imagery. Additionally, different concentration oil data were used to obtain a concentration-estimation model. In the classification, K-Nearest Neighbor (KNN) obtained the best approximations in oil detection using Blue (0.453-0.520μm), NIR (0.790-0.891μm), SWIR1 (1.557-1.717μm), and SWIR2 (1.960-2.162μm) bands for 2010 spill images. In the concentration model, the mean absolute error (MAE) was 1.41 and 3.34, for training and validation data. When testing the concentration-estimation model in images where oil was detected, the concentration-estimation obtained was between 40 and 60%. This demonstrates the potential use of ML techniques and spectral response data to detect and estimate the concentration of oil-spills.

Evidence That Shocks Destroy Small PAH Molecules in Low-luminosity Active Galactic Nuclei

We combined mapping-mode mid-infrared Spitzer spectra with complementary infrared imaging to perform a spatially resolved study of polycyclic aromatic hydrocarbon (PAH) emission from the central regions of 66 nearby galaxies, roughly evenly divided into star-forming systems and low-luminosity active galactic nuclei (AGNs). In conjunction with similar measurements available for quasars, we aim to understand the physical properties of PAHs across a broad range of black hole accretion power, with the goal of identifying observational diagnostics that can be used to probe the effect of AGNs on the host galaxy. Whereas the PAH emission correlates tightly with far-ultraviolet luminosity in star-forming regions, the spatially resolved regions of AGNs tend to be PAH deficient. Moreover, AGN regions exhibit on average smaller PAH 6.2 μm/7.7 μm and larger PAH 11.3 μm/7.7 μm band ratios. Although the current data are highly restrictive, they suggest that these anomalous PAH band ratios cannot be explained by the effects of the AGN radiation field alone. Instead, they hint that small grains may be destroyed by the combined effects of radiative processes and shocks, which are plausibly linked to jets and outflows preferentially associated with highly sub-Eddington, radiatively inefficient AGNs. While quasars also present a PAH deficit and unusual PAH band ratios, their characteristics differ in detail compared to those observed in more weakly accreting AGNs, a possible indicator of fundamental differences in their modes of energy feedback.

Ultra-Short Polarization Rotator Based on Flat-Shaped Photonic Crystal Fiber Filled with Liquid Crystal.

In this study we demonstrate a high-performance polarization rotator (PR) based on flat-shaped photonic crystal fiber. The flat surfaces of the fiber are plated on gold films as electrodes, and the core of the structure is filled with liquid crystal. The polarization rotation characteristics of the flat-shaped fiber can be effectively adjusted by applying external voltage. The optical properties are analyzed using the finite element method (FEM). The results show that the magnitude of the modulation voltage is closely related to the thickness of the flat fiber. When the fiber thickness is 20 μm, only 100 V is required to achieve the highest PR performance. In the wavelength of the 1.55 μm band (~200 nm bandwidth), the conversion length of the PR is only 3.99 μm, the conversion efficiency is close to 100%, and the minimum crosstalk value is -26.2 dB. The presented PR, with its excellent performance, might enable promising applications in the communication system and the photonic integrated circuits.

D2-Filled Hollow-Core Fiber Gas Raman Laser at 2.15 μm

Fiber lasers around 2 µm band have attractive applications, such as coherent detecting, material processing, pump source for mid-IR lasers based on nonlinear frequency shift, etc. Fiber gas Raman lasers (FGRLs) based on the stimulated Raman scattering of the gas molecules filled in the hollow-core fibers (HCFs) have been proved an efficient method to enrich the wavelengths of fiber lasers. In this paper, we demonstrated a deuterium-filled fiber gas Raman laser working at 2147 nm. The pump laser is directly coupled into the HCF through the fusion splice between the HCF and the solid-core fiber. By adjusting the pressure, fiber length as well as the repetition frequency of the 1971 nm pump laser, a maximum average Raman power of ~2.57 W was obtained, with corresponding efficiency of ~40%. This work provides a simple and compact configuration for 2.1 µm fiber lasers, which is significant for their application.

Optimal Design and Analysis of 4.7 μm Hybrid Deep Dielectric High Efficiency Transmission Gratings

There is currently no transmission grating with good diffraction efficiency in the 4.7 μm band. Metal gratings at this wavelength are all reflective gratings which has a diffraction efficiency of lower than 90% and lower laser damage threshold. In this paper, we bring up a design of a multi-layer transmission grating with both high diffraction efficiency and wide working wavelength band. We have proved that the transmission grating made of composite materials has an average diffraction effectiveness of more than 96% throughout the whole spectral range of 200 nm. Meanwhile, the theoretically computed transmission grating has a highest first-order diffraction efficiency of more than 99.77% at 4746 nm. This multilayer dielectric film transmission grating’s optimized design may further boost spectral beam combining power, providing a practical technique for increasing SBC power and brightness.

A Near-Infrared pin Photodetector of Strain-Enhanced Ge Layer Epitaxially Grown on a Bonded Si-on-Quartz Wafer

In Si photonics, photodetectors (PDs) of a Ge epitaxial layer on Si have been widely used, which show a high photodetection efficiency, or a high responsivity in the O (1.260–1.360 µm) and C (1.530–1.565 µm) bands. However, an issue lies in a low responsivity in the longer wavelength range of L band (1.565–1.625 µm). In this regard, we have proposed a method using a Ge layer epitaxially grown on a Si-on-quartz (SOQ) wafer [1]. The tensile in-plane lattice strain, derived from the thermal expansion mismatch between the Ge layer and base substrate, is enhanced more than 0.3% in Ge on SOQ, despite ~0.2% on Si (or Si-on-insulator). The enhanced strain in Ge on SOQ extends the fundamental optical absorption edge to about 1.65 µm, leading to a potential increase in the L-band responsivity. In this study, a vertical pin PD is fabricated using a Ge layer on SOQ, showing an increased responsivity in the L band.A bonded SQQ wafer was used as the starting substrate, fabricated by Shin-Etsu Chemical Co. Ltd. The top Si (001) layer was 200 nm in thickness, and was heavily doped p-type by B implantation. An undoped Ge epitaxial layer (500 nm) was grown by chemical vapor deposition using GeH4 as the source gas, followed by a growth of Si cap layer (50 nm) using Si2H6. In the top region of Ge layer as well as in the Si cap layer, a heavily n-type doping was carried out by P implantation, forming a vertical pin junction. Finally, a Ti/TiN electrode was formed. Responsivity spectra were measured in the wavelength range of 1.46–1.64 µm at a normal incidence. A reference pin PD was also fabricated using a Ge layer on a standard bulk Si wafer. According to x-ray diffraction measurements, the Ge layer on SOQ showed a tensile strain of 0.33%, despite 0.14% for the reference Ge on Si.Figure 1(a) shows typical current–voltage curves for a pin diode of Ge on SOQ measured under dark and under an illumination of 1.55-µm light (1.1 mW). The diode showed a good rectifying property under dark, while under the illumination, the current in the reverse bias significantly increased, showing a reasonable PD operation. Figure 1(b) shows typical responsivity spectra at a reverse bias of 1 V for PDs of Ge on SOQ and reference Ge on Si. Circles and lines correspond to experimental data points and theoretical curves, respectively. The responsivity for Ge on SOQ was larger in the measured wavelength range of 1.46–1.64 µm, including the L band. The increased responsivity is ascribed to the enhanced tensile strain. However, the increase in the L band was found to be suppressed by a high reflectance because of the multiple reflections at the interfaces. An optimized Si cap thickness as well as an anti-reflection coating with SiO2 can increase the normal-incidence responsivity as high as 0.3 (0.1) A/W up to 1.60 (1.64) µm without an increase in the Ge thickness.[1] Noguchi et al., J. Appl. Phys. 128, 133107 (2020). Figure 1

Luminescence property in Er3+/Tm3+/Ag NPs doped tellurite glass applied for broadband amplifier

In this study, tellurite glasses co-doped with Er3+/Tm3+ were synthesized through the melt-quenching technique. Under 808 nm excitation, the spectral overlap of 1.53 µm band fluorescence generated by the transition of Er3+:4I13/2→4I15/2 and 1.47 µm band fluorescence generated by the transition of Tm3+:3H4→3F4 achieved the near-infrared (NIR) broadband flat fluorescence emission in the range of 1350–1650 nm, in which the most flat fluorescence emission was achieved when the doping amount of Er2O3 and Tm2O3 was 0.1 and 0.8 mol%, respectively, and the full width at half maximum (FWHM) reached 161 nm. Subsequently, the NIR fluorescence intensity was enhanced by introducing silver nanoparticles (Ag NPs) based on broadband flat fluorescence emission. The optimal conditions conducive to broadband fluorescence enhancement were explored by changing the heat-treated time that could affect the formation and growth of Ag NPs. When the amount of AgCl is 0.5 mol%, tellurite glass co-doped with Er3+/Tm3+ ions heat-treated at temperature 410 ℃ for 6 h has the best enhancement effect of 81 %. After transmission electron microscopy (TEM) measurement, the introduced Ag NPs’ diameter at this time is about 18 nm. Some important spectral parameters of Er3+ and Tm3+ ions, for instance the radiative transition probability together with radiative lifetime were calculated from the absorption spectra according to the Judd-Ofelt theory. Structural characteristics based on X-ray diffraction analysis and thermal stability based on differential scanning calorimeter analysis was presented. Based on the above research, the enhanced broadband and flat fluorescence in the 1350–1650 nm range is of great significance for the application of broadband optical amplifiers in modern WDM networks.

Red Spiral Galaxies at Cosmic Noon Unveiled in the First JWST Image

In the first image of the James Webb Space Telescope (JWST) of SMACS J0723.3-7327, one of the most outstanding features is the emergence of a large number of red spiral galaxies, because such red spiral galaxies are only a few percent in the number fraction among nearby spiral galaxies. While these apparently red galaxies were already detected with the Spitzer Space Telescope at ∼3–4 μm, the revolutionized view from the JWST’s unprecedented spatial resolution has unveiled their hidden spiral morphology for the first time. Within the red spiral galaxies, we focus on the two reddest galaxies that are very faint in the <0.9 μ m bands and show red colors in the 2–4 μm bands. Our study finds that the two extremely red spiral galaxies are likely to be in the cosmic noon (1 < z < 3). One of the extremely red spiral galaxies is more likely to be a passive galaxy having moderate dust reddening (i.e., ∼zero star formation rate with AV ~ 1 mag). The other is consistent with both passive and dusty starburst solutions (i.e., star formation rate > 100 M⊙ yr−1 with AV ∼ 3 mag). These “red spiral” galaxies would be an interesting, potentially new population of galaxies, as we start to see their detailed morphology using the JWST, for the first time.

Finding of a Population of Active Galactic Nuclei Showing a Significant Luminosity Decline in the Past ∼103–104 yr

Recent observations have revealed an interesting active galactic nucleus (AGN) subclass that shows strong activity at large scales (∼1 kpc) but weaker at small scales (<10 pc), suggesting a strong change in the mass accretion rate of the central engine in the past 103–104 yr. We systematically search for such declining or fading AGNs by cross-matching the Sloan Digital Sky Survey type 1 AGN catalog at z < 0.4, covering the [O iii] λ5007 emission line, which is a tracer for the narrow-line region emission, with the Wide-field Infrared Survey Explorer (WISE) mid-infrared (MIR) catalog covering the emissions from the dusty tori. Out of the 7653 sources, we found 57 AGNs whose bolometric luminosities estimated from the MIR band are at least one order of magnitude fainter than those estimated from the [O iii] λ5007 emission line. This luminosity-declining AGN candidate population shows four important properties: (1) the past AGN activity estimated from the [O iii] λ5007 line reaches approximately the Eddington limit; (2) more than 30% of the luminosity-declining AGN candidates show a large absolute variability of ΔW1 > 0.45 mag in the previous ∼10 yr at the WISE 3.4 μm band; (3) the median ratio of log([N ii] λ6584/Hα λ6563) = −0.52, suggesting a lower gas metallicity and/or higher ionization parameter compared to other AGN populations; and (4) the second-epoch spectra of the population indicate a spectral type change for 15% of the sources. This population provides insights on the possible connection between the luminosity decline that started ∼103–104 yr ago and the decline in the recent 10 yr.

Photometric behavior of Ryugu’s NIR spectral parameters

Context. JAXA’s Hayabusa2 mission rendezvoused the Ryugu asteroid for 1.5 years to clarify the carbonaceous asteroids’ record for Solar System origin and evolution. Aims. We studied the photometric behavior of the spectral parameters characterizing the near-infrared (NIR) spectra of Ryugu provided by the Hayabusa2/NIRS3 instrument, that is to say 1.9 µm reflectance, 2.7 and 2.8 µm band depths (ascribed to phyllosilicates), and NIR slope. Methods. For each parameter, we applied the following empirical approach: (1) retrieval of the equigonal albedo by applying the Akimov disk function (this step was only performed for the reflectance photometric correction); (2) retrieval of the median spectral parameter value at each phase angle bin; and (3) retrieval of the phase function by a linear fit. Results. Ryugu’s phase function shows a steepness similar to Ceres, according to the same taxonomy of the two asteroids. Band depths decrease with increasing phase angle: this trend is opposite to that observed on other asteroids explored by space missions and is ascribed to the very dark albedo. NIR and visible phase reddening are similar, contrary to other asteroids, where visible phase reddening is larger: this could be due to surface darkness or to particle smoothness. Albedo and band depths are globally uncorrelated, but locally anticorrelated. A correlation between darkening and reddening is observed.

Probing the surface environment of large T-type asteroids

Context. The thermal and radiative environments encountered by asteroids have shaped their surface features. Recent observations have focused on asteroids in the main belt and showed indications for ices and organics in the interiors of the asteroids that were likely significant on prebiotic Earth. They stand out in reflectance spectra as darker, redder colours than most colocated asteroids. Aims. We probe the surface environment of large (&gt;80km in diameter) T-type asteroids. This taxonomic type is relatively ill-constrained as an independent group. We discuss their place of origin based on our probing. Methods. We performed spectroscopic observations of two T-type asteroids, (96) Aegle and (570) Kythera, over the L band (2.8–4.0 µm) using the Subaru telescope. The spectra of other T-type asteroids are available in the literature, as are survey datasets. Based on this, we strove to find commonalities and global trends in this group. We also used the archival polarimetric data of the asteroids and meteorite spectra from laboratory experiments to constrain their surface texture and composition. Results. Our targets exhibit red L-band continuum slopes, (0.30±0.04) µm−1 for (96) Aegle and (0.31 ± 0.03) µm−1 for (570) Kythera, that are similar to those of (1) Ceres and 67P/Churyumov-Gerasimenko, and have an OH-absorption feature with band centres &lt;2.8 µm. For (96) Aegle, we find an indication of a shallow N–H band near 3.1 µm and a C–H band of organic materials over 3.4–3.6 µm, whereas no diagnostic bands of water ice and other volatiles exceeding the noise of the data were seen for both asteroids. The large T-type asteroids except for (596) Scheila display spectral shapes similar to those of to our targets. About 50% of large T-type asteroids contain an absorption band near 0.6−0.65 µm that is likely associated with hydrated minerals. For T-type asteroids (except for Jupiter Trojans) of all sizes, we found a weak correlation that the smaller the diameter and the closer to the Sun, the redder the visible (0.5−0.8 µm) slope. Conclusions. The 2.9 µm band depths of large T-type asteroids suggest that they might have experienced aqueous alteration comparable to Ch-type asteroids, but that it was more intense than for most of the main-belt asteroids. The polarimetric phase curve of the T-type asteroids is well described by a particular surface structure. The 0.5−4.0 µm reflectance spectra of large T-type asteroids appear most similar to those of CI chondrites with grain sizes of ~25−35 µm. Taken as a whole, we propose that large T-type asteroids may have been dislodged roughly around 10 au in the early Solar System.