Thin Absorber Research Articles

Dramatically Enhanced Efficiency in Ultra-Fast Silicon MSM Photodiodes Via Light Trapping Structures

Due to relatively low responsivity at near infrared (NIR) wavelengths, surface-illuminated silicon (Si) photodiodes (PDs) are not attractive for ultra-fast data communication applications despite their CMOS-compatibility. Metal-semiconductor-metal (MSM) photodiodes are well-known for simplicity in fabrication compared to pin and pn junctions-based counterparts, but they usually work with lower efficiencies due to thin absorption layer that ensures high speed response. In this letter, we demonstrate a high efficiency and high-speed Si MSM PD with innovative photon-trapping surface structures. These wavelength-scale structures decrease the surface reflection and introduce laterally propagating waves parallel to the semiconductor surface. The responsivity of Si MSM PDs with the photon-trapping holes was measured to be ~0.59A/W, while the control devices without holes show ~0.08A/W. Such PDs with $50\mu \text{m}$ diameter and with 1 $\mu \text{m}$ finger spacing provide a pulse response of 60ps and 38ps full-width at half-maximum (FWHM) at 3V and 10V applied bias, respectively. Surface-illuminated PDs with these dimensions can offer easy optical coupling options for fiber optic links while offering complete CMOS compatibility.

Efficient and environmental-friendly perovskite solar cells via embedding plasmonic nanoparticles: an optical simulation study on realistic device architectures.

Solution-processed, lead halide-based perovskite solar cells have recently overcome important challenges, offering low-cost and high solar power conversion efficiencies. However, they still undergo unoptimized light collection due mainly to the thin (∼350 nm) polycrystalline absorber layers. Moreover, their high toxicity (due to the presence of lead in perovskite crystalline structures) makes it necessary that the thickness of the absorber layers to be further reduced. Here we address these issues via embedding spherical plasmonic nanoparticles of various sizes, composition, concentrations, and vertical positions, in realistic halide-based perovskite solar cells. We theoretically show that plasmon-enhanced near-field effects and scattering leads to a device photocurrent enhancement up to ∼7.3% when silver spheres are embedded inside the perovskite layer. An even further enhancement, up to ∼12%, is achieved with the combination of silver spheres in perovskite and aluminum spheres inside the hole transporting layer (PEDOT:PSS). The proper involvement of nanoparticles allows the employment of much thinner perovskite layers (up to 150 nm), reducing thus significantly the toxicity. Providing the requirements related to the design parameters of nanoparticles, our study establishes guidelines for a future development of highly-efficient, environmentally friendly and low-cost plasmonic perovskite solar cells.

Light induced degradation of the transport length of CH3NH3PbI3 studied by modulated surface photovoltage spectroscopy after Goodman

Abstract Transport length is one of the most important parameters of any absorber material in a solar cell. It affects the design and performance of solar cells given that it determines the photogeneration of charge carriers in the device. In this work, we studied the transport length of CH3NH3PbI3 perovskite layers deposited on sputtered TiO2 under blue light soaking using surface photovoltage (SPV) spectroscopy developed after Goodman. SPV developed after Goodman is a direct, sensitive and accurate method for determining the transport length of thin absorber layers. However, the method has been inadequately utilized. In the study, we used blue light for light soaking due to its short wavelength that can probe the surface properties of the perovskite layers in comparison to red light. We observed light induced degradation of transport length within 12 h of blue light soaking in TiO2/CH3NH3PbI3 layers. This is attributed to trap states at TiO2/CH3NH3PbI3 interface which create defect states that increases the recombination rate. In addition, the decrease in the transport length within 12 h of light soaking can be due to the charging and discharging effects at TiO2/CH3NH3PbI3 interface. With prolonged light soaking, we observe an increase in transport length, which is probably due to generation of compensating defects that neutralizes trap states at the TiO2/CH3NH3PbI3 interface. Above 12 h of light soaking, we observe dependence of transport length with surface photovoltage spectroscopy. This is probably related to change in quasi fermi level splitting in the space charge region which arises due to the in-homogeneity of the perovskite layers. We conclude that the strong variation of Transport length with SPV signals give an indication of the presence of defects and in-homogeneity of the perovskite films.

Direct Extraction of Complex Permittivity and Permeability of Materials on a Known-Substrate From Transmission/Reflection Measurements

In this letter, complex permittivity and permeability of single-layer materials on a known substrate are directly extracted from two measured S-parameters ( $S_{11}$ and $S_{21}$ ). The frequency-dependent formulas of the complex permittivity and permeability of the single-layer materials are derived for the first time. A thin absorber and a thin low-loss material on a known substrate are measured in the $X$ -band to experimentally validate the proposed technique. We also investigate the proposed technique with a thick low-loss material on a known substrate.

Nonlinear control of coherent absorption and its optical signal processing applications

All-optical data processing continues to attract significant interest as a way to overcome the electronic signal processing bottleneck of fiber telecommunication networks. Nonlinear optical devices such as limiters and saturable absorbers rely on intensity-dependent attenuation of light. However, making such devices using intensity-dependent multiphoton dissipation processes is an issue as these make complete absorption and transmission impossible. Here, we show that nonlinear phase retardation in an optical fiber can control the dissipation of coherent light waves interacting on a thin plasmonic absorber from total absorption to perfect transmission. The fiber’s instantaneous Kerr nonlinearity and the femtosecond coherent absorption time scale make this approach ultrafast. We report proof-of-principle demonstrations of all-optical intensity discrimination, power limiting, pulse restoration, pulse splitting, and signal transfer between carrier wavelengths within a fiber circuit. Our results indicate that nonlinear control of coherent absorption can imitate and outperform saturable and multiphoton absorption in terms of bandwidth and contrast.

Mechanism of stiffness reduction in a thin broadband noise absorber

Sound absorption by fibrous material is often considered broadband but poor in performance when the frequency is low. A new absorber is considered thin or otherwise when its thickness is compared with such a fibrous absorber for a given performance requirement. The performance of the latter is poor due to air stiffness in a compact design. This study first introduces a composite structure based on a parallel array of resonators with or without fibrous filling. The array is shown to introduce equivalent negative stiffness which is accompanied by a surge in system damping derived from resonator coupling. In principle, high damping is required for optimal absorption when the system as a whole is not operating at its resonance. However, the frequency dependency of such a composite system is too complex to be exactly optimal. The performance of the array structure is compared with a new fibrous absorber embedded with mass made from impervious membranes. The bandwidth and other characters of the two absorbers are compared when the incident sound has the typical environmental noise spectrum. Conclusions are shown to contradict with a few widespread misconceptions.

Ni-NPs doped PVA: An efficient saturable absorber for generation multiwavelength Q-switched fiber laser system near 1.5 μm

We report on the use of Nickel nanoparticles (Ni-NPs) as saturable absorber (SA) to generate Q-switched fiber laser pulses in C-band. The Ni-NPs were incorporated in polyvinyl alcohol (PVA) to produce thin film-based saturable absorber (SA) and integrated into an erbium-doped fiber laser (EDFL) ring cavity to generate passively Q-switching. The Ni-doped PVA SA reveals modulation depth of 15% and a saturation intensity of 200 MW/cm2. Lasing in CW region begins at 77 mW pump power, whereas stable self-starting Q-switching with a central wavelength of 1563.1 nm begins at 85 mW. By fine adjustment of the laser diode pump power up to 236.4 mW dual-, and triple- lasing lines have been observed at 1563.1, 1563.3 and 1563.4 with side mode suppression ratio (SMSR) of more than 49 dB. The proposed laser is useful for generating pulse laser with a minimum pulse width of 1.5 μs and a maximum repetition rate of 56.79 kHz with a pulse energy of about 4.33 nJ at the maximum pump power of 275 mW. The stability of the pulse is verified from the radio-frequency (RF) spectrum with a measured signal-to-noise ratio (SNR) of 44 dB. The ability of Ni-PVA as an effective SA may lead to further development of pulsed fiber laser in the field of photonics.

Excellent microwave absorbing performance of the sandwich structure absorber Fe@B2O3/MoS2/Fe@B2O3 in the Ku-band and X-band

In this paper, we present the design of a thin triple-layer sandwich-structure absorber (SSA) based on the composites of polyurethane (PU) with different mass filler fraction of the core–shell Fe@B2O3 nanoparticles and MoS2 microflowers for achieving a broadband absorption of EM waves. The absorbing performance of the proposed SSA is calculated using the EM parameters of the single-layer absorbers (SLA) prepared by the Fe@B2O3/PU and MoS2/PU composites. For designing the SSA, we optimized the thickness of each SLA first by studying the EM absorption characteristics of each layer. Here, we present the design of two SSAs: SSA–1 and SSA–2. Their calculated effective bandwidths reach 4.1 GHz (8.2–12.3 GHz) with a total thickness of 2.1 mm and 7.0 GHz (10.9–17.9 GHz) with a total thickness of 1.9 mm, respectively. Meanwhile, we fabricated the SSA-2 sample experimentally by hot-pressing the SLA. The reflection loss curve of the prepared sample was measured by the short-circuit method. The measured effective bandwidth (7.6 GHz) is in basically agreement with the calculated value (7.0 GHz), demonstrating the credibility of the proposed SSA design for its applications in the X-band or the Ku-band.

Crashworthiness analysis of foam filled star shape polygon of thin-walled structure

This paper aims to fabricate star polygon thin walled energy absorption structure that tends to relapse the intensity of set in decelerations during impact while escalating the amount of energy absorbed. The crashworthiness topology optimization is used for structural optimization of various foam-filled tubes. The relative advantages of 14 configurations are discussed, and the effects of filling five types of foam into the best configuration of the star-shaped tube over an empty one is investigated. Specific Energy Absorption, Peak Crush Force and response of weight of the members during frontal impact are the main dimensions parameters of the member's performance. Numerical simulation is carried out using the explicit dynamics of Ansys 17.1, and obtained results are compared with experimental results conferring crash behavior and energy absorption characteristics. Based on results, the suited configuration with required performance in crashworthiness is suggested, which shall be incorporated in automobiles for safety consideration of passengers during an impact. The results show an increment of 40% in Specific Energy Absorption suggesting a better choice of a particular type of foam over a hollow tube.

Magnetic-field-induced synthesis of one-dimensional core/shell Fe3O4/carbon nanorods composites as a highly efficient microwave absorber

In this paper, one-dimensional Fe3O4/C core/shell nanorods (NRs) were successfully prepared through magnetic-field-induced (MFI) assembly approach (0.3 T) and subsequent high-temperature carbonization using phenol formaldehyde resin as a carbon source. A serial of examinations were performed thoroughly, such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy. The results indicated that phenol formaldehyde resin (PFR) shell was transformed to carbon during the carbonization process and the thickness exhibited significant decrease. The Fe3O4/C core/shell nanoparticles (NPs) were prepared in the same way without magnetic-field-induced (0 T). The electromagnetic properties of Fe3O4/C core/shell composites were measured with coaxial reflection/transmission technique at 2.0–18.0 GHz. Electromagnetic performances indicated that the special structure played a crucial role in determining their dielectric behavior. Compared with the Fe3O4/C core/shell NPs, Fe3O4/C nanorods exhibited higher dielectric constant and higher reflection loss (RL) at a high frequency band with a thin absorber thickness. In detail, the optimal absorption peak value for Fe3O4/C core/shell NRs could reach a strong intensity of -44.1 dB at 1.6 mm, and the effective absorption bandwidth lower than −10 dB is 2.7 GHz (from 11.6 to 14.3 GHz). It is expected that our Fe3O4/C core/shell NRs synthesized via a magnetic field induced in this work could be used as a reference to design novel low-dimensional microwave absorption materials.

Design and Evaluation of Radiated Emission Metamaterial Absorbers

In this paper, mitigation of unwanted radiated electromagnetic emissions from a high-speed system enclosure using metamaterial-based absorbers is investigated. First, the performance of an earlier introduced frequency selective metamaterial absorber is thoroughly evaluated via full-wave simulations and compared with that of a commercial thin microwave absorber. An experimental in-lab test set-up is developed to validate simulation results confirming that a stronger mitigation of radiated emissions is delivered by the proposed absorber. The measured radiated emission patterns closely follow simulated results and confirm better than 5 dB reduction in emissions when the proposed absorber is included in the enclosure box. Simulations with simultaneous excitation of multiple radiation sources inside the box prove the effectiveness of the developed absorber in realistic operational conditions that could include array effects for unintentional radiation sources. A second absorber design is introduced in this paper with improved thermal conductivity while providing emission reduction better than 4.6 dB.

Critical analysis of periodic fractal frequency selective surfacescoupled with synthesised ferrite‐based dielectric substrates for optimal radarwave absorption

It is a very challenging task to develop a good radar wave absorber with wide bandwidth that corresponds to reflection loss (RL) ≤ − 10 dB and lower coating thickness using radar absorbing material alone. However, this goal can be achieved with the application of advanced electromagnetic (EM) structures such as fractal frequency selective surfaces (FFSSs). The prior knowledge of the suitability of an FFSS for a particular nanocomposite material based on its dielectric constant (ɛ′) is a very crucial factor. Therefore, a critical analysis has been carried out to choose the suitable FFSS geometries for a given set of nanocomposite materials having a ɛ′ ranging in between 1 and 20. The higher iterated Minkowski loop FFSS-impacted nanocomposite has been chosen for the experimental validation based on its good radar wave absorption capability. The Minkowski loop FFSS-impacted radar absorbing coating (RAC) has been fabricated, and its performance evaluation has been carried out. The measured RL of −17.9 dB at 10.8 GHz with a wide bandwidth of 2.9 GHz (RL≤−10 dB) has been noticed for 2.2 mm thick RAC. The findings provide an effective way to develop thin and broadband absorber for various practical EM applications.

Size-modulated electromagnetic properties and highly efficient microwave absorption of magnetic iron oxide ceramic opened-hollow microspheres

Hollow structured nanomaterials are considered as one of the most promising candidates of thin broadband absorbers due to the potential multiple scattering and resonance of electromagnetic waves. Here, magnetic iron oxide ceramics opened-hollow microspheres with tunable size were fabricated based on hydrothermal route and carbothermal reduction method. The electromagnetic parameters are obviously dependent on microsphere size due to size-dependent relaxation time of polarization and magnetization. The opened-hollow microspheres with diameter of 500 nm achieve the −10 dB absorption bandwidth of 6.1 GHz covering 7.5–13.6 GHz at only 2 mm thickness, which is the widest bandwidth in all reported nanostructures of magnetic iron oxide composites. Moreover, the broadband absorption covering the X band at such thin thickness is competitive in all absorbing materials. Such excellent microwave absorption benefits from the outstanding impedance matching caused by the multiple scattering of electromagnetic waves in large size open-hollow microspheres. This work provides a physical insight for obtaining high-performance microwave absorber.

An environment-friendly fabrication of nano-Co3O4 coating by aqueous electrophoretic deposition

Co3O4 coating is one of most studied transition metal oxide due to its wide application in many fields such as gas sensors, electrochromic thin films, supercapacitors, and solar selective absorber. Aqueous electrophoretic deposition (EPD), which is considered as a cost-effective and environment-friendly coating technology, was successfully employed in preparing nano-Co3O4 coating in this study. For the EPD process with the solid loading of 1 g L−1, 2.0 mM nitric acid was employed as suitable additive and 5 V was employed as deposition voltage. The prepared nano-Co3O4 coating exhibited good film characteristics, which was proved by scanning electron microscopy (SEM) technology. The result suggests that a rapid preparation of nano-Co3O4 coating in aqueous dispersion system can be easily accomplished. Moreover, the optimum concentrations of the added nitric acid would increase with the solid loading of Co3O4 nanoparticle, but the growth rate of the former was obviously lower than that of the latter.

Design of thin film solar cells based on a unified simple analytical model

Polycrystalline thin film solar cells made with absorber materials such as CdTe, CIGS, CZTS and metalorganic halides (perovskites) are currently important alternatives for the silicon solar cell technology, which still dominates the photovoltaic market. Then, it is important to have tools which can be used to design this kind of solar cells. For this purpose, we have developed a unified simple analytical model that can be applied to thin film solar cells. The model is based on the basic physics of hetero-junction devices, but it takes into consideration that the space charge region can extend along the major part of the cell length, particularly for very thin cells, causing important effects that typically are not observed in conventional junction devices. Photo-generated carriers are collected by electric field-drift instead of diffusion, and simultaneously strong recombination at this region may dominate the electrical I–V characteristic of the cell. Since the space-charge region width varies with the applied voltage, the illumination current density and the saturation dark current density are no longer independent of the voltage as is assumed for conventional solar cells. When the model is applied to CIS and CdTe solar cells as examples, it is found that it is possible to design very thin film solar cells (absorber less than 1 μm thick) with high efficiencies, whenever the recombination velocity at the back surface becomes small (102 cm/s), instead of the highrecombination velocities present at ohmic contacts (107 cm/s). This fact implies the cost reduction of thin film solar cells by reducing absorber material thickness, and therefore it poses a challenge to develop deposition methods for very thin CdTe and CIGS absorber materials without pinholes, so that improved efficiencies are obtained when the surface recombination velocity is made small at the back by having a p+ or an electron blocking region before the ohmic contact. This result also explains the high efficiencies achieved by very thin perovskite solar cells.

Core–shell structure BaFe12O19@PANI composites with thin matching thickness and effective microwave absorption properties

The BaFe12O19@PANI composites with different polyaniline (PANI) contents were successfully synthesized by an in situ polymerization method. The composition, phase and morphologies of these composites were characterized thoroughly. The result shows that the introduction of PANI can effectively the absorbing properties of barium ferrite. The prepared BaFe12O19@PANI composites had the best absorption performance when the amount of aniline addition was 400 μl with the reflection loss of − 65.354 dB, and the matching thickness is 1.47 mm. The introduction of PANI can decrease the weight of electromagnetic absorbing materials, and enhance the dielectric loss and the degree of polarization of the interface. The effective microwave absorption characteristics with thin thickness make the BaFe12O19@PANI composites have great potential application.

Understanding the nano-photonics absorption limit in both front-side and front/rear-side textured slabs

Surface texturing is one of the main techniques to enhance light absorption in solar cells. In thin film devices, periodic texturing can be used to excite the guided resonances supported by the structure. Therefore, total absorption is enhanced largely due to the excitation of these resonances. Although the maximum absorption enhancement limit in both bulk and photonic structures is known already, the weight of each resonance type in this limit is not yet clear. In this contribution, we extend the temporal couple-mode theory, deriving a closed formula to distinguish the contribution of Fabry-Perot and wave-guided modes within the absorption limit for 1-D grating structures. Secondly, using this analytical approach, we can clearly address cases of bulk and thin absorber thicknesses. Our results, supported by rigorous electromagnetic calculation, show that absorption enhancement in a 1-D grating structure can be much higher than the nano-photonic limit (2πn) reported by Yu et al. Thirdly, beyond the framework put forward by Yu et al., we extended our theory to describe the absorption enhancement in double side textured slabs. We have found that when the periods of top and bottom gratings are aliquant, absorption is enhanced in a wider frequency range. We provide rigorous numerical calculations to support our theoretical approach.

Passively Q-switched Ytterbium-doped fiber laser based on broadband multilayer Platinum Ditelluride (PtTe2) saturable absorber

Two-dimensional (2D) layered Platinum Ditelluride (PtTe2), a novel candidate of group 10 transition-metal dichalcogenides (TMDs), which provides enormous potential for pulsed laser applications due to its highly stable and strong nonlinear optical absorption (NOA) properties. PtTe2 saturable absorber (SA) is successfully fabricated with firstly demonstrated the passively Q-switched laser operation within a Yb-doped fiber laser cavity at 1066 nm. Few layered PtTe2 is produced by uncomplicated and cost-efficient ultrasonic liquid exfoliation and follow by incorporating into polyvinyl alcohol (PVA) polymer to form a PtTe2-PVA composite thin film saturable absorber. The highest achieved single pulse energy is 74.0 nJ corresponding to pulse duration, repetition rate and average output power of 5.2 μs, 33.5 kHz and 2.48 mW, respectively. This work has further exploited the immeasurable utilization potential of the air stable and broadband group 10 TMDs for ultrafast photonic applications.

Microwave absorption properties of easy-plane anisotropy Fe–Si powders with surface modification in the frequency range of 0.1–4 GHz

To satisfy impedance matching at low frequencies and obtain high microwave absorption performance, easy-plane anisotropy Fe–Si powders coated with SiO2 were produced by a ball milling technique and a chemical deposition method. Then, a scanning electron microscope, a transmission electron microscope, an X-ray photoelectron spectrometer, an X-ray diffractometer, a Mossbauer spectrometer, a vibrating sample magnetometer, and a vector network analyzer were used to characterize the samples. Compared to easy-plane anisotropy Fe–Si powders without surface modification, the dielectric constant of easy-plane anisotropy Fe–Si powders with SiO2 coating decreased dramatically and the microwave absorption performance was improved significantly. The reflection loss peak reached − 10 dB when the thickness was 1.4 mm and even reached − 34 dB when the thickness was 4.5 mm, exhibiting great potential for application in thin absorbers working in the frequency range of 0.1–4 GHz. In addition, the microwave absorption performance at oblique incidence was also investigated. A reflection loss peak of − 30 dB was obtained when the incident angle was 30°, and a reflection loss peak of more than − 10 dB can still be obtained even when the incident angle was 80°. The results show that the easy-plane anisotropy Fe–Si powders with SiO2 coating still possess high microwave absorption performance at oblique incidence.

A ultra-wideband thin microwave absorber using inkjet-printed frequency-selective surfaces combining carbon nanotubes and magnetic nanoparticles

A ultra-wideband microwave absorber (UWMA) combining frequency-selective surfaces (FSS) and dielectric layers is proposed. FSS patterns are printed by inkjet on dielectric layers using a resistive magnetic ink made of suspended carbon nanotubes decorated with $${\text {Fe}}_3{\text {O}}_4$$ nanoparticles. The UWMA exhibits a huge fractional bandwidth of 140%, corresponding to reflectivity lower than − 10 dB and absorption higher than 90%, observed from 7 GHz to 43 GHz, meaning a 36 GHz bandwidth, for a thickness of only 0.26 $$\lambda$$. This performance is achieved through a proper selection of FSS surface resistance, tuned by the number of printed layers. Excellent agreement is observed between designed and measured low reflectivity and high absorption.