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As a rediscovered ternary two-dimensional (2D) material, defect-rich Znln2S4 has great potential for energy-harvesting applications. However, the effect of defects on its physical properties and device performance remains elusive. Herein, we explored the influence of defects (S vacancies and In–Zn substitutions) in few-layer Znln2S4 on the charge transport and photoelectric performance. It is demonstrated that the defect-rich Znln2S4 device exhibits two-dimensional variable range hopping transport mechanism, with uniform charge transport along the channel and low contact resistance at the electrical contacts of Znln2S4/Au. Importantly, due to the contribution of the donor and acceptor energy levels inside the bandgap, the flake exhibits pronounced extrinsic absorption, leading to the competitive photodetector performance under sub-bandgap photo-excitation. Explicitly, the device exhibits a maximum responsivity of 4.08 × 104 A W−1, a photo-gain of >108 electrons per photon, and a specific detectivity of ∼1015 Jones under 532 nm laser excitation, with detection wavelength extending from 400 to 980 nm. Our findings underscore the significant potential of defect-engineering to enrich the functionalities of 2D semiconductors. [ABSTRACT FROM AUTHOR]

We report the wavelength dependencies of the two-photon absorption coefficients β as well as the nonlinear refractive index n2 of undoped β-Ga2O3 single crystal in the spectral range 350–515 nm (0.51 < Ephoto/Eg < 0.75). Femtosecond Z-scan measurements show that n2 decreases monotonically toward the shortwave side, while the maximum value of n2 is 5.0 × 10−19 m2/W near half of the bandgap, Eg. Interestingly, a second upward trend in β was observed when the incident photon energy is larger than 0.6Eg, indicating the impact of additional inter-band transitions with higher energy. A higher-energy band-to-band transition around 6.0 eV was determined by both femtosecond nondegenerate two-photon absorption spectroscopy and ab initio calculations. Our results could provide guidance for designing nonlinear β-Ga2O3 photonic devices in the UV-visible spectral range. [ABSTRACT FROM AUTHOR]

High-entropy alloys (HEAs), especially the FeCoNi-based HEAs with magnetic loss and dielectric loss, have attracted considerable attention in the field of electromagnetic-wave absorption (EMA). However, improper electromagnetic parameters of HEAs led to impedance mismatching, which considerably reduced its electromagnetic-wave absorbing efficiency. Dealloying in HEAs is an available approach to optimizing electromagnetic properties owing to its function of component tailoring and surface modification. Herein, FeCoNiCuAl HEAs were prepared by the high-energy balling method, and then, the as-milled powders were dealloyed in KOH solution with different concentrations. Undergoing the dealloying process, Al contents in FeCoNiCuAl HEAs decreased, while the surface area increased, which efficiently optimized impedance matching and promoted surface polarization loss. Profiting from these merits, samples A02 and A06 exhibit excellent EMA performance, with strong absorptions of −56.04 dB at 13.01 GHz and −56.12 dB at 6.88 GHz, corresponding to bandwidths of 3.29 GHz (1.69 mm) and 2.88 GHz (1.69 mm), respectively. [ABSTRACT FROM AUTHOR]

CsPbBr3 nanoparticles with averaged size of 14 and 2.1 nm (named as 14 and 2 nmNP, respectively) are synthesized capping with the ligand of dodecylbenzene sulfonic acid (DBSA). Using ultrafast transient absorption spectroscopy, free carriers in 14 nmNP characterize with the slow thermalization (∼0.76 ps) and long carrier cooling time (∼1 ns), which could be due to the phonon bottleneck effect. On the other hand, excitons are major photoexcitations in 2 nmNP, presenting with fast thermalization (<100 fs) and short cooling time (∼50 ps); this effective energy dissipation in 2 nmNPs can be ascribed to the involvement of phonons in DBSA ligands. In both 14 and 2 nmNP, we find that the photoexcitation accumulation leads to an increase in the bandgap that can be understood by the Burstein–Moss band filling effect. These results provide insights into the intrinsic photophysics of CsPbBr3 perovskite nanocrystals with different confinement effects, supplying direct implications for optoelectronic applications. [ABSTRACT FROM AUTHOR]

Magnetic damping rooted in various relaxation processes of spin angular momentum plays a crucial role in determining the energy consumption and the operating speed of emerging spintronic devices. Here, we reported a fourfold anisotropic magnetic damping extrinsically caused by spin current absorption in amorphous ferromagnetic/antiferromagnetic bilayers. The angular dependent broadband ferromagnetic resonance measurements were employed to investigate the effect of interfacial exchange coupling on magnetization dynamics of the CoFeB/IrMn3 bilayers. In addition to the conventional exchange bias, the amorphous CoFeB layer exhibits an induced fourfold magnetic anisotropy along the IrMn3⟨100⟩ axes because of the exchange coupling with the uncompensated IrMn3 moments along IrMn3⟨100⟩ caused by the 3Q antiferromagnetic structure. The magnetic damping of the CoFeB/IrMn3 bilayer also exhibits an obvious fourfold symmetry, which is ascribed to the anisotropic spin absorption caused by the uncompensated IrMn3 moments. The ratio of the fourfold anisotropic magnetic damping decreases dramatically with reducing the interfacial exchange coupling. When the interfacial exchange coupling is isolated by a Cu spacer layer, both the induced magnetic anisotropy and the magnetic damping exhibit a uniaxial symmetry. [ABSTRACT FROM AUTHOR]

We report on the nonlinear optical response of CdSe nanoplatelets and CdSe/CdS core/shell nanoplatelets in the heavy-hole resonant excitonic transition spectral regions with femtosecond pulses. The nonlinear refraction and saturable absorption coefficients were obtained using the Z-scan technique, and the results were explained theoretically. The nonlinear optical response under nonresonant excitation has been previously examined; comparing with the results presented in this work, at least a one order of magnitude increase in the nonlinear refraction coefficient is evident. [ABSTRACT FROM AUTHOR]

Tunable absorbers have attracted much attention due to their smart control of electromagnetic wave absorption. Despite much effort to enhance their performance, the absorption performance remains restricted by the conflict between absorption efficiency and tuning range. In this study, a wideband tunable absorber breaking the trade-off between the absorption efficiency and the tuning range is presented and investigated. First, an improved frequency-dependent resistance model is proposed and analyzed, in which the real part of input impedance varies with frequency. Subsequently, based on the frequency-dependent resistance circuit, a frequency tunable absorber (FTA) with a wide frequency tuning range is constructed by loading voltage-controlled varactors, which exhibits wide tuning bandwidth from 0.613 to 1.816 GHz with reflection coefficients below −10 dB. Additionally, by changing the location of varactor, a varactor-controlled amplitude tunable absorber (ATA) is further explored and designed. The prototypes of FTA and ATA are fabricated and measured, respectively, and good agreements between the simulated and measured results are observed. [ABSTRACT FROM AUTHOR]

We propose an amorphous silicon (a-Si) metasurface based on a quasi-bound state in the continuum (q-BIC) with a theoretical Q-factor up to 930, which consists of Π-like nanostructures with symmetrical defects. The influences of slot offset parameter and the size of added symmetric defect on the near- and far-field characteristics of the metasurface have been analyzed. Due to local near-field enhancement by the tailored high-Q quasi-BIC resonance, our designed metasurface exhibits significantly enhanced two-photon absorption (TPA) response in the experiment, i.e., the TPA coefficient β0 = 173.2 cm/MW (3–4 orders of magnitude larger than a-Si film) and the TPA saturation intensity Isat = 93.0 MW/cm2. Our work offers a promising alternative scheme to improve nonlinear optical effects at the nanoscale. [ABSTRACT FROM AUTHOR]

In electromagnetics, self-complementary antenna shapes possess a frequency-independent characteristic due to their particular shape. Based on this concept, this paper investigates the potential applications of self-complementary shapes for sound absorption to broaden the bandwidth and enhance the performances of resonators without altering the sizes and resonance frequencies. Self-complementary shapes, such as log-periodic planar tooth and log-spiral shapes, are used to demonstrate the effectiveness of our approach in enhancing the absorption bandwidth and performance of a resonator. Such shapes improve the absorption performance up to 58 % compared to a resonator with an equivalent circular area when the opening area is reduced, whereas they exhibit weak performance for wide opening areas. Numerical and experimental analyses are conducted to verify their performances and to investigate the effect of the materials inside the backing cavity, neck length, opening size, and geometry on absorption. By extending the use of frequency-independent antenna shapes as sound-absorbing structures, this approach overcomes the inherent limitations of resonant metamaterials to achieve an enhanced sound absorption for various applications. [ABSTRACT FROM AUTHOR]

A coherent perfect absorption-lasing (CPAL) point in non-Hermitian physics systems has infinite output amplitudes (or quality factor) theoretically and has been utilized for applications. However, practical implementation of CPAL systems faces extreme challenges because of the deviations of electronic components from ideal values, which makes it difficult to attain the CPAL state. Moreover, the amplitude-based output is easily interfered by noises or environments and is inferior as compared to frequency shift-based sensing. Through the investigation of the effects of component deviations on general CPAL systems, we propose an eigenstate tuning method specifically tailored for radio-frequency (RF) CPAL systems. The method enables the realization of CPAL states at any desired frequency with dynamic ranges of signal amplitudes exceeding 60 dB experimentally or to work at a frequency shift-based sensing. Then, a CPAL-based RF sensor system is developed for verification, showing an ultrahigh sensitivity of 1.9 dB/10 fF by measuring output coefficient variation or 9.4 MHz/pF by measuring frequency shift, significantly broadening the application scope of CPAL systems. [ABSTRACT FROM AUTHOR]

We report on photoluminescence emission in copper iodide bulk single crystals induced by two- and three-photon absorption around 1.525 eV. These non-linear optical processes are investigated utilizing density-dependent, steady-state, as well as time-resolved photoluminescence spectroscopy as a function of the excitation energy. Using an excitation energy that corresponds to half of the bandgap energy, the observed photoluminescence intensity dependence on the excitation power shows an almost parabolic behavior. By further reduction of the photon energy, a cubic contribution is observable, which increases with decreasing excitation energy. The experimentally observed behavior can be well described by taking into account two- and three-photon absorption. By a simultaneous analysis of the intensity behavior for all used excitation energies, we determined a ratio between the two- and three-photon absorption cross section on the order of σ 0 (3) / σ 0 (2) ≈ 10 − 28 cm 2 s. [ABSTRACT FROM AUTHOR]

An active electromagnetic (EM) absorption metasurface is proposed to achieve a switchable reflection band between two absorption bands. The metasurface consists of one lossy layer and one active layer separated by an air gap. The top lossy layer is composed of periodic metallic unit cells, designed to absorb the incident EM wave at the lower and higher absorption bands. PIN diodes are embedded on the bottom periodic array to control the reflection coefficient. When the PIN diodes are turned off, the incident EM wave is absorbed at lower and higher bands while reflected at the intermediate band; when the PIN diodes are turned on, the reflection band is switched off and a wide absorption band is achieved. Experimental results verify the numerical simulations. [ABSTRACT FROM AUTHOR]

Resonances are fundamentally important in the field of nano-photonics and optics. Thus, it is of great interest to know what are the limits to which they can be tuned. The bandwidth of the resonances in materials is an important feature, which is commonly characterized by using the Q-factor. We present tight bounds correlating the peak absorption with the Q-factor of two-phase quasi-static metamaterials and plasmonic resonators evaluated at a given peak frequency by introducing an alternative definition for the Q-factor in terms of the complex effective permittivity of the composite material. This composite may consist of well-separated clusters of plasmonic particles, and, thus, we obtain bounds on the response of a single cluster as governed by the polarizability. Optimal metamaterial microstructure designs achieving points on the bounds are presented. The most interesting optimal microstructure is a limiting case of doubly coated ellipsoids that attains points on the lower bound. We also obtain bounds on Q for three dimensional, isotropic, and fixed volume fraction two-phase quasi-static metamaterials and particle clusters with an isotropic polarizability. Some almost optimal isotropic microstructure geometries are identified. [ABSTRACT FROM AUTHOR]

Terahertz (THz) absorption spectroscopy has recently been developed as a diagnostic technique for measuring absolute ground-state atomic oxygen densities in plasmas. To demonstrate the validity of this approach, we present in this Letter a benchmark against a more established method. Atomic oxygen densities were measured with THz absorption spectroscopy and compared to those obtained from picosecond (ps) two-photon absorption laser induced fluorescence (TALIF) measurements on the same capacitively coupled radio frequency oxygen discharge. Similar changes in the atomic oxygen density were observed with both diagnostics when varying the applied power (20–100 W) and the gas pressure (0.7–1.3 mbar). Quantitatively, the results are in good agreement as well, especially when considering the total margin of error of the two diagnostics. For example, for a gas pressure of 1.3 mbar and an applied power of 30 W, atomic oxygen densities measured with THz absorption spectroscopy and TALIF were (7.0 ± 1.7) × 10 14 cm−3 and (5.3 ± 3.2) × 10 14 cm−3, respectively. This shows that THz absorption spectroscopy is an accurate technique that can be reliably used for real-world applications to determine atomic oxygen densities in plasmas. [ABSTRACT FROM AUTHOR]

The backprojection algorithm is a commonly used technique in non-line-of-sight imaging, but it is plagued by artifacts that significantly reduce the quality of the final reconstruction. To address this issue, we refer to the absorption of surrounding dust by celestial bodies and propose the absorption backprojection (ABP) algorithm by imitating the law of gravitation. This algorithm absorbs low confidence value voxels in hidden space into high confidence value voxels so that the majority of the artifacts surrounding the object are eliminated. The results of our study demonstrate that the ABP algorithm is highly effective in reducing the artifacts of backprojection in both simulation and laboratory experiments. Furthermore, it achieves better reconstruction results than other backprojection reconstruction algorithms, including filtered backprojection and error backprojection. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenide MoS2 is considered as a type of dielectric loss dominated electromagnetic wave absorbing material owing to the high specific surface area, layered structure, and lightweight. Introduction of interfaces will improve the electromagnetic wave absorbing performance. Here, the phase engineering of MoS2 is realized through W doping since of the microstrain effect, resulting in the appearance of 2H/1T MoS2 phase interface. Furthermore, MoS2/Ti3C2 MXene interface is realized through the construction of MoS2/Ti3C2 MXene heterostructure, leading to obvious improvements in electromagnetic wave absorbing. Because of the simultaneous introduction of 2H/1T MoS2 phase interface and the MoS2/Ti3C2 MXene interface, the microwave reflection loss can reach −45.2 dB with broad effective absorption bandwidth (<−10 dB) of 7.1 GHz (7.8–14.9 GHz) at the same thickness of 3 mm. The results shed light on enhancing electromagnetic wave absorbing performance by phase engineering as well as construction of two dimensional material/two dimensional material heterostructures, thereby introducing multiple interfaces. [ABSTRACT FROM AUTHOR]

The two-dimensional (2D) transition metal carbide Ti3C2Tx MXene is a potential candidate for efficient electromagnetic wave absorbers due to its excellent intrinsic conductivity and structural machinability. However, Ti3C2Tx MXene also has some problems (such as self-stacking and single loss mechanism) that limit its practical electromagnetic wave absorption. Based on the electromagnetic wave absorption mechanism, electromagnetic responsiveness of absorbers can be modulated by designing the composition and structure. Herein, a 1D/2D Co/Co9S8/Ti3C2Tx composite has been synthesized by assembling 2D Ti3C2Tx MXene with the designed 1D magnetic structure. The 1D Co/Co9S8 was designed as a core-sheath structure that avoids magnetic agglomeration, and the assembly with 2D Ti3C2Tx sheets alleviates the self-stacking problem of Ti3C2Tx MXene sheets. More importantly, the magnetic component enriches the electromagnetic wave dissipation mechanism, and the multiple heterojunction surfaces provide strong polarization loss capability for the Ti3C2Tx MXene-based absorber. Benefiting from the unique structure and dielectric-magnetic synergistic loss, the Co/Co9S8/Ti3C2Tx composite shows an effective absorption bandwidth of 5.36 GHz (10.08–15.44 GHz) at 2.1 mm and the optimal RLmin value of −52.02 dB at 1.8 mm. This work provides an innovative idea for the design of effective Ti3C2Tx MXene-based absorbers. [ABSTRACT FROM AUTHOR]

While a number of O-H and O-D vibrational lines have been observed for hydrogen and deuterium in β-Ga2O3, it has been commonly reported that there is no absorption with a component of the polarization E parallel to the [010], or b, axis. This experimental result has led to O-H defect structures that involve shifted configurations of a vacancy at the tetrahedrally coordinated Ga(1) site [VGa(1)] and have ruled out structures that involve a vacancy at the octahedrally coordinated Ga(2) site [VGa(2)], because these structures are predicted to show absorption for E//[010]. In this Letter, weak O-D lines at 2475 and 2493 cm−1 with a component of their polarization with E//[010] are reported for β-Ga2O3 that had been annealed in a D2 ambient. O-D defect structures involving an unshifted VGa(2) are proposed for these centers. An estimate is made that the concentration of VGa(2) in a Czochralski-grown sample is 2–3 orders of magnitude lower than that of VGa(1) from the intensities of the IR absorption lines. [ABSTRACT FROM AUTHOR]

An extremely low noise Separate Absorption and Multiplication Avalanche Photodiode (SAM-APD), consisting of a GaAs0.52Sb0.48 absorption region and an Al0.85Ga0.15As0.56Sb0.44 avalanche region, is reported. The device incorporated an appropriate doping profile to suppress tunneling current from the absorption region, achieving a large avalanche gain, ∼130 at room temperature. It exhibits extremely low excess noise factors of 1.52 and 2.48 at the gain of 10 and 20, respectively. At the gain of 20, our measured excess noise factor of 2.48 is more than three times lower than that in the commercial InGaAs/InP SAM-APD. These results are corroborated by a Simple Monte Carlo simulation. Our results demonstrate the potential of low excess noise performance from GaAs0.52Sb0.48/Al0.85Ga0.15As0.56Sb0.44 avalanche photodiodes. [ABSTRACT FROM AUTHOR]

We report a very long wave (14 μm) infrared quantum cascade detector based on a twin-well coupled absorption region operating at temperatures up to 130 K. By introducing two coupled absorption quantum wells that have the same width, the absorption strength and responsivity of the detector are increased relative to the single-well design. At 77 K, we observe a responsivity of 4.06 mA/W at zero bias, which is 4.27 times that of the single-well counterpart. The responsivity is further optimized for reverse bias operation, so that the obstruction of space charge field to electron transport is compensated. The photocurrent reaches a maximum value at 77 K for an applied bias of −1.3 V, and responsivity as high as 23.76 mA/W, which is 5.85 times that under zero bias, is obtained. [ABSTRACT FROM AUTHOR]

Oxygen in single crystalline antimony triselenide (Sb2Se3) is addressed by infrared (IR) absorption spectroscopy. Measurements conducted on Sb2Se3 samples doped—during growth, post-growth annealing in the O2 ambient, or by O ion implantation—with 16O reveal an IR absorption line at 527 cm−1 (10 K). Substitution of 16O by 18O "red"-shifts the signal down to 500 cm−1 based on which the line is assigned to a local vibrational mode of an isolated oxygen defect. Annealing of O-enriched samples in hydrogen atmosphere at temperatures above 380 °C results in the suppression of the 527-cm−1 line and concurrent appearance of the signals due to hydroxyl groups, suggesting formation of oxygen-hydrogen complexes. The configuration of the 527-cm−1 oxygen center is discussed. [ABSTRACT FROM AUTHOR]

Recent progress in minimizing sound absorbers is driven by their great scientific significance and engineering value; however, compact devices for low-frequency sound are still challenging. Here, we construct an ultra-thin metasurface by parallel connecting resonators with high dissipation loss to a non-resonant reactance-dominated boundary (RDB) with high radiation loss, which realizes extreme absorption asymmetry at the exceptional point of scattering eigenvalue. We develop a parallel transfer matrix method to design the system, and a deep-subwavelength absorber (the operating wavelength is 120 times of its thickness) with 99.2% and 0.5% absorption for sound incident from opposite ports is achieved. The extreme absorption asymmetry is ascribed to the distinct coupling between the RDB and resonant meta-atom in an unbalanced dissipating state with excellent robustness against geometrical reconfigurations originated from the broadband near-unity reflection characteristics of the RDB. In particular, the proposed strategy brings the design of a degree of freedom rather than typical multiple resonant modes, and an extensible prototype showing >90% (<1%) absorptance for left- (right-) incidence within a wavelength from 23.3 to 18 times its thickness is demonstrated. [ABSTRACT FROM AUTHOR]

In this work, scattering properties of a metal-dielectric structure are carried out in the optical range. In the proposed structure design, the absorption coefficient reaches more than 80% and has an almost constant character in the entire visible spectrum. Based on the obtained simulation results, such structures can be utilized in the field of solar energy storage applications. A review of works in the field of plasmonic solar cells is given, where the efficiency is usually increased due to the confinement of incident radiation in the active region of photoelectric conversion using localized modes, which are concentrated at the surface of conducting nanoparticles. That is, such batteries are operated in narrow wavelength intervals. However, in this article, the assumed solar cell efficiency enhancement can be achieved by incorporating the entire optical range due to the proposed metamaterial structure. [ABSTRACT FROM AUTHOR]

We report the spectral dependence of the three-photon absorption coefficient γ and third-order nonlinear refractive index n2 in pristine, Sn-doped and Fe-doped β-Ga2O3 crystals using 190 fs pulses in the range of 540–750 nm, i.e., photon energy between half and one-third of the bandgap Eg. The nonlinear coefficient n2 is found to be independent of Sn or Fe doping, and the maximum value of n2 is about 4 × 10−19 m2/W when the photon energy is near Eg/2. However, we observe that Fe doping can significantly enhance the γ value in the range of 540–600 nm (near Eg/2) while doping has negligible impact on γ for wavelength larger than 620 nm. Femtosecond pump-probe measurements show that Fe doping induced a fast (∼50 ps) carrier decay process after photo excitation, which could be attributed to the Fe induced deep trap states. We obtain three-photon figures of merit that allow operational intensities up to 100 GW/cm2. Our results indicate that β-Ga2O3 has great potential for all-optical switching applications in red and near IR regimes. [ABSTRACT FROM AUTHOR]

Bismuth triiodide (BiI3) is a two-dimensional (2D) layered semiconductor with a fairly narrow bandgap (∼2 eV) and strong excitonic absorption near the band edge. Being endowed with a large absorption coefficient in the visible range, BiI3 attracts a growing interest as a potential compound for thin-film optoelectronic devices. Although various growth techniques have been employed so far, thin films with a high enough crystallinity for the solid assignment of the exciton resonances have never been synthesized. Here, we demonstrate molecular beam epitaxy of high-quality BiI3 films by employing a thin buffer layer deposited at a low temperature (30 °C) to make high temperature (70 °C) growth possible. The films fabricated by this two-step method realize a single-crystalline structure and atomically flat surfaces. The absorption spectra of the films exhibit strong and sharp excitonic resonances near the band edge, indicating the high crystallinity of the films comparable to the bulk single crystals. The presently established fabrication process should be widely applicable to other 2D halide thin films and heterostructures, providing ideal platforms for observing unprecedented emergent phenomena. [ABSTRACT FROM AUTHOR]

The high-absorption ultra-thin metasurface absorber has important applications in EM radiation prevention and stealth design. In this paper, the proposed metasurface absorber composed of one-dimensional phase gradient supercell and uniform resonant supercell can reach perfect absorption at 10 GHz by reducing the scattering of the surface wave. Furthermore, the two-dimensional phase gradient metasurface absorber has better absorption of waves with different polarization angles and a wider range of incident angles. Our work provides a promising method to achieve high absorption, which is simpler and more robust than adjusting the effective EM parameters of metasurface absorber. [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal dichalcogenides (TMDs) are emerging as a promising complement for traditional semiconductor materials in ultrathin optoelectronic device fields. Developing a better understanding of high-energy C-exciton dynamics is essential for efficiently extracting hot carriers and building high-performance TMD-based light-harnessing devices; however, insight into the C-exciton dynamics remains scarce. To further understand the C-exciton dynamics, here, we have unraveled the interrelation between C-exciton and band edge A-exciton dynamics in monolayer WS2 by transient absorption spectroscopy. It is found that the band edge A-excitons could effectively generate high-energy C-excitons via the many-body process, and, in turn, the hot carriers relaxing from C-excitons to band edge states could compensate and slow the decay of the A-excitons. The comprehensive understanding of the interrelation between C-exciton and A-exciton dynamics in monolayer TMDs may trigger the potential applications for future TMD-based light-harvesting devices. [ABSTRACT FROM AUTHOR]

The single Helmholtz resonator obtains only one absorption peak in the broad frequency range, which limits its application in reducing the noise with multiple spectra. This paper reports an acoustic multi-layer Helmholtz resonance metamaterial, which can achieve multiple absorption peaks at given low-frequency targets. Meanwhile, through adjusting structural parameters of the multi-layer Helmholtz resonator, its impedance can be altered correspondingly to realize the absorption of noise with the multi groups of specific frequencies. In this paper, in order to achieve fine absorption performance with the specific frequencies of 100 and 400 Hz for a substation noise source, the sound absorption principle of a classical Helmholtz resonator with the embedded aperture is introduced theoretically, and then two series of multi-layer Helmholtz resonance structures with different parameters are designed. Thickness of the multi-layer structure is only 1/30th of the working wavelength, and two groups of resonance peaks are generated at 100 and 400 Hz, respectively. A finite element model of the multi-layer Helmholtz resonator is constructed to simulate its absorption performance. The samples are fabricated through the 3D light-curing printing, and their sound absorption performances are detected by the standing wave method. The simulation results are in good agreement with the experimental data, and two peaks with near-perfect absorptions are achieved at the target frequencies. The multi-layer Helmholtz resonator for achievement of three groups of absorption peaks is proposed later. This work provides an effective method to design a sound absorber with multiple absorption peaks, which can promote the application of acoustic metamaterials. [ABSTRACT FROM AUTHOR]

Efficient nanoprobes with unique optical properties are highly desirable for good-performance photoacoustic (PA) molecular imaging. The conventionally used PA nanoprobes basically take their ground-state absorption with contrast to the indelible wideband background absorption as the imaging mechanism, thus severely limiting the imaging contrast and sensitivity in practical applications. Herein, a nonlinearly enhanced PA microscopy mechanism with suppressed background interference and improved brightness has been proposed, by distinctively exploiting the picosecond-laser-pumped excited state absorption of the tin phthalocyanine (SnPc) nanoprobes that exhibit strong reverse saturable absorption below the laser damage threshold of tissues. Both theoretical simulation and experimental investigation have been performed to verify the nonlinearly enhanced optical and PA properties of the SnPc nanoprobes with comparison to conventional PA contrast agents. The enhanced PA imaging capability of the SnPc nanoprobes with improved sensitivity and contrast has been demonstrated by tissue-mimicking phantoms and in vivo mouse models. This work revolutionizes the traditional contrast mechanism of PA nanoprobes by introducing picosecond-laser-pumped nonlinear optical nanomaterials, which prefigures great potential for biosensing and bioimaging with improved contrast and sensitivity. [ABSTRACT FROM AUTHOR]

High-quality n-type continuously graded Ge-rich Si1−xGex parabolic quantum wells with different doping levels were grown by using ultrahigh-vacuum chemical vapor deposition on Si(001) substrates. A thorough structural characterization study highlights an ideal parabolic compositional profile. THz intersubband absorption has been investigated in modulation-doped samples and samples directly doped in the wells. The comparison of experimental absorption data and theoretical calculations allowed us to quantify the impact of electron correlation effects on the absorption resonances in the different doping conditions and for electron sheet densities in the (1 ÷ 6) × 10 11 cm−2 range. A single optical resonance is present in modulation doped samples. Its peak energy and line shape are independent of temperature-induced variations of the electron distribution in the subbands up to 300 K, in agreement with the generalized Kohn theorem. This achievement represents a relevant step forward for the development of CMOS compatible optoelectronic devices in the THz spectral range, where thermal charge fluctuations play a key role. [ABSTRACT FROM AUTHOR]

In this work, the authors propose a graphene-based tunable perfect metamaterial absorber at microwaves that can achieve perfect absorption at all angles. This is realized by stacking an unpatterned graphene sandwich structure and a dielectric layer on top of a mushroom-type high impedance surface whose resonant frequency is stable for all angles of incidence. A perfect-absorption-angle tunable absorber working at TM polarization is designed and realized, achieving perfect absorption covering a wide range of angles from normal to grazing incidence at the same frequency when the impedance of graphene is modulated by different bias voltages. The design concept of integrating different tunable materials and meta-structures opens up promising possibilities for wave manipulation with applications in smart and multifunctional electromagnetic devices. [ABSTRACT FROM AUTHOR]

As the hole mobility increases with free carrier absorption (FCA), heavily doped p-type silicon has emerged as a promising candidate for detecting near-infrared (NIR) light with photonic energy below the semiconductor bandgap, and the hot-carrier photodetector is capable of high responsivity approaching commercially available devices. To enhance performance-related FCA of the semiconductor, here we demonstrate an all-silicon metasurface absorber designed in nanohole arrays and present an in-depth analysis on the electromagnetic resonance mechanism in the NIR spectral of interest. Multipole decomposition under a Cartesian coordinate system reveals that combining with the dielectric loss introduced by the doping, magnetic dipole resonances contribute predominantly to the absorption enhancement. This simple and easy-fabricated architecture has great potential for silicon hot-carrier photodetectors. [ABSTRACT FROM AUTHOR]

We propose an efficient optically actuated THz modulator based on an ultrathin epsilon-near-zero (ENZ) slab photogenerated in an InAs semiconductor. We experimentally demonstrate a modulation depth of 90% at 1 THz obtained with a continuous laser at irradiation lower than 10 W cm − 2 . Beyond the strong attenuation of the THz transmission provided by the ENZ absorption effect, we also report a broadband modulation of the THz waves from 1 to 10 THz. In addition, our experimental results show that the cut-off frequency of 3 dB attains 2 MHz in the dynamic modulation regime. [ABSTRACT FROM AUTHOR]

We investigate coherent perfect absorption in a microcavity with C60 as an organic absorbing material. The experiment consists of a two-beam setup of counterpropagating laser beams incident on a single sample from opposite sides, resulting in two counterpropagating outputs consisting of the reflected and transmitted parts of the incident beams. The systematic behavior of the output intensities as a function of the relative phase of incident beams and the thickness of the absorbing cavity layer is investigated. We experimentally verify that the absorption resonance of the cavity is accompanied by a transition of the phase difference of the individual outputs, which is characterized by a passage from out-of-phase to in-phase behavior of the outputs for varying relative phases of incident beams. The analysis shows that the amplitudes and offsets of the outputs qualitatively follow the theoretical model. [ABSTRACT FROM AUTHOR]

Quantum-intensity-correlated twin beams of light can be used to measure absorption with precision beyond the classical shot-noise limit. The degree to which this can be achieved with a given estimator is defined by the quality of the twin-beam intensity correlations, which is quantified by the noise reduction factor. We derive an analytical model of twin-beam experiments, incorporating experimental parameters such as the relative detection efficiency of the beams, uncorrelated optical noise, and uncorrelated detector noise. We show that for twin beams without excessive noise, measured correlations can be improved by increasing the detection efficiency of each beam; notwithstanding, this may unbalance detection efficiency. However, for beams with excess intensity or other experimental noise, one should balance detection efficiency, even at the cost of reducing detection efficiency—we specifically define these noise conditions and verify our results with statistical simulation. This has application in design and optimization of absorption spectroscopy and imaging experiments. [ABSTRACT FROM AUTHOR]

Electric field-induced broadening and narrowing of an exciton absorption band have been observed to depend on the polarity of the applied electric field for methylammonium lead triiodide perovskite (MAPbI3) as a thin film sandwiched between a conducting film of fluorine-doped tin oxide (FTO) and an insulating film of poly(methyl methacrylate) (PMMA). The width of the absorption band increased or decreased when the direction of the applied field was toward FTO or PMMA, respectively, indicating that the spectral broadening and narrowing become switchable on altering the direction of the applied electric field. When titanium oxide (TiO2) in a compact layer was introduced between the MAPbI3 and FTO layers, the linear electric field effect that depended on the polarity was either not observed or decreased significantly although the quadratic field effects on the exciton absorption band were similarly observed with or without the TiO2 layer. [ABSTRACT FROM AUTHOR]

Quantum many-body theory is applied to discover the design principles of broadband absorbers based on III-nitride or transition metal dichalcogenide monolayer crystals. A combination of color-chirped exciton absorption and continuum-absorption-based coupling strategies is outlined and demonstrated to enable hyperspectral absorption with several eV bandwidth. The found structures are only a few micrometers thick and dominantly support direct generation of free carriers at optimal photovoltages. [ABSTRACT FROM AUTHOR]

The process of x-ray diffraction on the La3Ga5SiO14 (LGS) crystal modulated by surface acoustic waves (SAWs) near the K absorption edge of Ga (E = 10 367 eV) was studied. A redistribution of the diffracted x-ray intensity between the diffraction satellites occurs at the absorption edge due to the change in the x-ray penetration depth into the crystal and an effective change in the interaction of x-ray radiation with the near-surface crystal region modulated by SAW. The intensity distribution of the diffraction satellites starts to change smoothly immediately after the K absorption edge of Ga with a decrease in the x-ray penetration depth into the crystal. [ABSTRACT FROM AUTHOR]

Besides generating their own perfect absorption bands, the primary hybrid resonances of a hybrid membrane resonator (HMR) consisting of a decorated membrane resonator (DMR) backed by a cavity can also provide strong assistance for weak resonances to create secondary hybrid resonances and multiple-frequency perfect absorption by forming a low impedance region between the primary hybrid resonances. The weak resonances can be introduced by placing another DMR in front of a HMR or by using a DMR with an off-centered platelet in an HMR. In one such sample, five perfect absorption bands within the range of 200–1000 Hz are experimentally observed with minimum reflection values ranging from 1.2% to 7%. [ABSTRACT FROM AUTHOR]

SrFe12−2xNixRuxO19 become promising microwave absorbing materials at 0.1–18 GHz with Ni and Ru doping. Regular lattice dislocation stripes are observed in high-resolution transmission electron microscopy images of x = 0.5 and 0.7. The observed strong microwave absorption is mainly contributed by natural magnetic resonance, which is widened by local magnetic fields at dislocation stripes. At dislocation stripes, interface polarization also forms. Both the x = 0.5 and 0.7 samples are excellent single-layer microwave absorbers. In x = 0.5, the bandwidth of reflection loss (RL) < −10 dB is broader than 11.31 GHz of 6.69 GHz to >18 GHz. In x = 0.7, an ultra-low RL = −82 dB is observed with a fairly wide bandwidth (RL < −10 dB) of over 8.8 GHz in the range of 9.2 GHz to >18 GHz. As the thickness and the doping content vary, the absorption is able to cover the full range of 3.5–18 GHz. The excellent microwave absorption is based on the collaboration of good impedance matching and high attenuation factor. The quarter-wavelength criteria explain the relationship between the optimum thickness and the frequency. Compared to the advanced materials reported, both the absorption strength and the bandwidth of SrFe12−2xNixRuxO19 are very competitive. [ABSTRACT FROM AUTHOR]

Titanium trisulfide (TiS3) is regarded as a candidate material for optoelectronic devices and nano-transistors due to its photoresponse. However, its nonlinear optical response in a mode-locked laser is yet to be investigated. Here, the performance of TiS3 as a saturable absorber in a mode-locked laser is demonstrated. The generated mode-locked pulses achieve pulse duration as short as 147.72 fs at 1555 nm, which indicates that TiS3 as a potential functional material has applications in nanomaterial-related photonics. [ABSTRACT FROM AUTHOR]

The bandgap, electrical, and optical properties of PtSe2 depend dramatically on the vertical stacking and fabrication method. Here, we study the nonlinear absorption properties of the PtSe2 films composed of both semiconducting and semimetallic phases in a single film. These PtSe2 films exhibit remarkable thickness-dependent saturable absorption for femtosecond pulses at 400 nm and 800 nm. The saturation intensities decrease with the increase in the film thickness due to the accompanied increase in the semimetallic component and are much smaller than the reported values of PtSe2 synthesized by thermally assisted conversion. The saturable absorption characteristics are confirmed by time-resolved spectroscopies. The nonlinear refractive indexes of these PtSe2 films should be smaller than 1 × 10–12 cm2/W. Our results imply that the optical nonlinearities of PtSe2 could be flexibly tuned by the synthesis method and thickness. [ABSTRACT FROM AUTHOR]

We demonstrate terahertz intersubband absorptions in nitride step quantum wells (SQWs) grown by metal organic vapor phase epitaxy simultaneously on two different substrate orientations [Si(111) and Si(110)]. The structure of the SQWs consists of a 3 nm thick Al0.1Ga0.9N barrier, a 3 nm thick GaN well, and an Al0.05Ga0.95N step barrier with various thicknesses. This structure design has been optimized to approach a flatband potential in the wells to allow for an intersubband absorption in the terahertz frequency range and to maximize the optical dipole moment. Structural characterizations prove the high quality of the samples. Intersubband absorptions at frequencies of 5.6 THz (λ ≈ 54 μm), 7 THz (43 μm), and 8.9 THz (34 μm) are observed at 77 K on both substrate orientations. The observed absorption frequencies are in excellent agreement with calculations accounting for the depolarization shift induced by the electron concentration in the wells. [ABSTRACT FROM AUTHOR]

The twist-angle dependence of exciton absorption in a molybdenum disulfide (MoS2)/graphene heterostructure (MGH) is reported. Using photoluminescence (PL) spectroscopy, we found that, as the twist angle increased, the MGH demonstrated an enhancement of PL intensity and a peak position blueshift, indicating that the exciton radiative recombination was positively correlated with the twist angle. In addition, optical reflectance measurements were performed in order to investigate the exciton absorption in the MGH. It was found that the reflectance of MGH samples was twist-angle dependent at wavelengths of 620 and 670 nm, which corresponds to the PL peak positions of MoS2. This was attributed to the change in interlayer charge transfer for different twist angles. Our findings confirm the tunability of the electronic structure in MGHs via the interlayer twist, which enriches our understanding of interlayer coupling and is important for the future development of electronic and optoelectronic devices based on 2D material heterostructures. [ABSTRACT FROM AUTHOR]

We introduce a multicoiled acoustic metasurface providing quasiperfect absorption (reaching 99.99% in experiments) at an extremely low-frequency of 50 Hz, simultaneously featuring an ultrathin thickness down to λ/527 (1.3 cm). In contrast to the state of the art, this original conceived multicoiled metasurface offers additional degrees of freedom capable of tuning the acoustic impedance effectively without increasing the total thickness. We provide analytical derivation, numerical simulation, and experimental demonstrations for this unique absorber concept, and discuss its particular physical mechanism. Furthermore, based on the same conceptual approach, we propose a broadband low-frequency metasurface absorber by coupling unit cells exhibiting different properties. [ABSTRACT FROM AUTHOR]

A mechanism to electrically tune the frequency of terahertz quantum cascade lasers (QCLs) is developed that allows for tuning, while the QCL is operated close to its peak bias and temperature. Two optically coupled but electrically isolated cavities are used in which the bias of a control cavity tunes the resonant-mode of the coupled QCL cavity independent of the QCL's operating bias. Approximately 4 GHz electrical tuning is realized for a 3.6 THz distributed-feedback QCL operating in pulsed mode at 58 K in a Stirling cooler. The single-mode QCL emits near-constant peak-power in the range of 5 − 5.3 mW through the tuning range and radiates in a narrow single-lobed beam with a far-field divergence of ∼ 4 ° × 11 °. The superlattice structure of the QCL is designed to implement a low-voltage intersubband absorption transition that is detuned from that of its gain transition, the strength of which could be controlled sensitively with applied voltage utilizing resonant-tunneling injection of electrons in the absorption subband. The tuning is realized by the application of small bias voltages (∼ 6 − 7 V) and requires a narrow bias range (∼ 1 V, ∼ 40 A / cm 2 ) to traverse across the entire tuning range, and the method should be generally applicable to all intersubband lasers including mid-infrared QCLs. [ABSTRACT FROM AUTHOR]

A current assisted avalanche photodetector (CAAPD) is presented with a large detection window of 40 × 40 μm2, having a small 1-fF avalanche diode in its center. To quickly guide the photogenerated electrons to the center for avalanche multiplication, a drift field with associated majority hole current is applied across the neutral detection volume. This first type of CAAPD is fabricated in a conventional 350-nm CMOS process on a high resistive p− epi-layer. The low diode-junction capacitance can be of interest to integrated receivers. The CAAPD is characterized for its basic functionality, including the effects of lateral detection delay and gain. [ABSTRACT FROM AUTHOR]

We present a design of a low-frequency perfect acoustic absorption metaporous composite with frequency tunability and the insensitivity to the incident angle based on a critical coupling mechanism. The metaporous composite is constructed by embedding an Archimedean spiral structure in traditional porous materials. We verified the tunable balance of the dissipation and leakage of the metaporous composite by adjusting the slit orientation of the embedded spiral and the width and depth of the porous materials. Our simulations show that the composite's thickness is 1/20th of the corresponding wavelength at the perfect absorption (PA). Experiment and complex frequency plane analysis confirm the perfect acoustic absorption. The simulations show that the acoustic absorption of the system is highly efficient (0.95), up to a large oblique incidence, e.g., 60 °. We also achieve the dual-band PA by embedding two spirals or using the higher order mode of a single spiral. The proposed composite absorber provides promising potential for low-frequency noise reduction applications. [ABSTRACT FROM AUTHOR]

Sn-doped In2O3 or ITO is the most widely used transparent conducting oxide. We use first-principles calculations to investigate the limitations to its transparency due to free-carrier absorption mediated by phonons or charged defects. We find that the main contribution to the phonon-assisted indirect absorption is due to emission (as opposed to absorption) of phonons, which explains why the process is relatively insensitive to temperature. The wavelength dependence of this indirect absorption process can be described by a power law. Indirect absorption mediated by charged defects or impurities is also unavoidable since doping is required to obtain conductivity. At high carrier concentrations, screening by the free carriers becomes important. We find that charged-impurity-assisted absorption becomes larger than phonon-assisted absorption for impurity concentrations above 1020 cm–3. The differences in the photon-energy dependence of the two processes can be explained by band structure effects. [ABSTRACT FROM AUTHOR]