Near-infrared Absorption Enhancement Research Articles

Comprehensive optical and thermal investigation of optimal near-infrared absorption enhancement of nano-patterned aluminum

Compelling electromagnetic (EM) properties, such as tight EM-confinement and fabrication-compatible architectures, propel the importance of metal-based nanophotonics in small-footprint devices, such as highly efficient optical absorbers. Here, we focus on boosting the absorption of optically thick bulk Aluminum (Al), the most abundant metal, solely by corrugating its surface periodically. Application-oriented, we utilize the simplest one-dimensional (1D) rectangular-grating pattern that can be remotely engraved on a large scale, e.g., using femtosecond pulsed lasers. We devise four designs for various utilizations optimized for perfect absorption around the application-abundant 1.06μm wavelength under transverse electric (TE), transverse magnetic (TM), and unpolarized (denoted TEM) irradiations. Lacking attractive plasmonic confinement properties, the s-polarization is considered electromagnetically “degenerate” to the p-polarization case, making it rarely explored in the literature of 1D-grating architectures. Nevertheless, we reveal that perfect s-polarized absorption is also attainable in such designs without adding extra layers. The designs’ performance, properties, limitations, and the propelling electromagnetic physics are theoretically and numerically discussed in-depth. A large-area sample of the TEM-optimized design was fabricated, facilitating uniquely extensive optical and radiative heating experimental investigation with realistic near-to-normal Gaussian-beam incidence. The absorption enhancement derived from wideband and single-frequency optical and radiative heating measurements agree well, also with the numerical simulations. Despite fabrication-derived thickness deviation, the design experimentally showed a 9.6-(TE), 6-(TM), and 7.8-(TEM) fold absorption enhancement over bulk Al. Considering the one-dimensional-patterning simplicity and its large-area fabrication compatibility, it is noteworthy and pioneering, particularly for the challenging s-polarization case. We also study a polarization-sensitive diffused reflection scattering pattern, evidently postulated to result from resonated surface roughness, highlighting the importance of radiative heating measurements. The elaborated original findings and methods are highly applicable alongside the prevalent material and spectrum.

Silicon based near-infrared absorption enhancement structure with gradient doping of nano metal particles

Silicon based near-infrared absorption enhancement structure with gradient doping of nano metal particles

Ultra-broadband near-infrared absorption enhancement of monolayer graphene by multiple-resonator approach

We theoretically study how to achieve an ultra-broadband near-infrared absorption enhancement of monolayer graphene through the multiple-resonator approach. The monolayer graphene (about 0.34 nm in thickness) is on nanostructured metal surfaces with multiple one-dimensional nanogrooves. The investigated nanostructure has an advantage of easy fabrication, because the monolayer graphene needs not to be sandwiched between different material layers. The metallic nanogrooves are able to support magnetic plasmon resonances whose resonance wavelengths are largely tuned, by changing the nanogroove depth from 70 to 210 nm or the nanogroove width from 20 to 40 nm. In a very wide near-infrared wavelength range from 850 nm to 1500 nm, the light absorption efficiency of graphene is enhanced to be more than 60 %, by carefully designing multiple magnetic plasmon resonances to be partly overlapped in optical spectra. Our work is of interest for some photoelectric devices, for example infrared photodetectors.

Deterministic Excitation of Polarization-Sensitive Extrinsic Anapole State in Si Nanodisk Clusters.

Nanoparticle clusters provide new degrees of freedom for light control due to their mutual interaction compared with an individual one. Here, the authors demonstrate theoretically and experimentally a type of optical anapole (a nonradiating state) termed as extrinsic anapole, with mode field spreading across Si nanodisk dimers unlike the intrinsic one that is confined within individual nanodisks. The extrinsic anapole is sensitive to the polarized excitation. When the electric vector E of excitation is perpendicular to the dimer axis, the coupled toroidal dipole (TD) mode is largely enhanced and broadened to be spectrally overlapped with the electric dipole (ED) mode. The destructive interference of these two modes results in the generation of the extrinsic anapole. However, it vanishes when E is parallel to the dimer axis. Such polarization dependence can be relieved with the participation of the third nanodisk. Scattering spectra of Si nanodisk trimers stay almost unchanged under different polarized excitations, although the near-field distributions are quite different. Finally, enhanced white-light emission is observed in Si nanodisk clusters, which can be attributed to the near-infrared absorption enhancement induced by extrinsic anapole states. The findings manifest that high-index all-dielectric nanodisk clusters are promising for light manipulation based on mode interference.

Photon confinement in a silicon cavity of an image sensor by plasmonic diffraction for near-infrared absorption enhancement.

Silicon-based image sensors are attractive for applications in the near-infrared (NIR) range owing to their low-cost and high availability. However, novel approaches are required to enhance their light absorption, hindered by the silicon band gap. In this study, we proposed a light trapping strategy in a silicon absorption layer by plasmonic diffraction and reflection within a pixel to improve the sensitivity at a specific NIR wavelength for complementary metal-oxide semiconductor image sensors. The plasmonic grating diffracted light under the quasi-resonant condition of the surface plasmon polaritons. We simulated the silicon absorption efficiency for plasmonic diffraction combined with metal-filled trenches and a pre-metal dielectric (PMD) layer. Backward propagation light in silicon by a total internal reflection at the bottom decoupled with plasmonic grating. A single SiO2 protrusion was added at the silicon bottom to prevent decoupling by scattering the light in the silicon and trapping it within the pixel. In addition, the light transmitted to the PMD layer is reflected by the wiring layer used as a mirror. The photon confinement in silicon by these constructions improved the absorption by approximately 8.2 times at an NIR wavelength of 940 nm with 3-µm-thick. It is useful for NIR imaging system with active laser illumination.

Design of ultrathin hole-transport-layer-free perovskite solar cell with near-infrared absorption enhancement using Ag NPs

Hole transfer layer-free perovskite solar cells (HTL-free PSCs) have been proposed to reduce manufacturing costs and improve stability. However, the weak optical absorption of the perovskite layer in the infrared region and ultrathin PSCs design have received less attention. In this study, ultrathin HTL-free PSC based on silver nanoparticles (NPs) at the perovskite layer/back electrode interface is proposed to increase the light trapping. The thickness of the perovskite layer is only 150 nm. The NPs radius is designed based on Mie theory to lead the highest scattering. By employing a numerical analysis, the effect of the periodicity of NPs on the performance of the PSCs is studied to obtain improved structure. The improved structure showed an absorption enhancement of 48% compared with the device without NPs within 300 nm–1400 nm wavelength. Light scattering and near field enhancement supplied by the Ag NPs are responsible for the broadband absorption increase. Photovoltaic characterizations of the improved proposed solar cell are obtained with coupled optical–electrical numerical analysis. The results showed 25.09 mA/cm2 and 14.96%, respectively, for short-circuit current density and efficiency that the efficiency improved by 56% compared to PSCs without NPs. It is believed that this investigation can provide approaches for the design and production of simple, efficient, and economical PSCs.

Dual-channel tunable near-infrared absorption enhancement with graphene induced by coupled modes of topological interface states

The dual-channel nearly perfect absorption is realized by the coupled modes of topological interface states (TIS) in the near-infrared range. An all-dielectric layered heterostructure composed of photonic crystals (PhC)/graphene/PhC/graphene/PhC on GaAs substrate is proposed to excite the TIS at the interface of adjacent PhC with opposite topological properties. Based on finite element method (FEM) and transfer matrix method (TMM), the dual-channel absorption can be modulated by the periodic number of middle PhC, Fermi level of graphene, and angle of incident light (TE and TM polarizations). Especially, by fine-tuning the Fermi level of graphene around 0.4 eV, the absorption of both channels can be switched rapidly and synchronously. This design is hopefully integrated into silicon-based chips to control light.

Near-infrared absorption enhancement for perovskite solar cells via the rear grating design

Optical management and design is one of the most effective methods to unlock the full potential of photocurrent density and power conversion efficiency for perovskite solar cells (PSCs). However, the common-used optical designs are still limited by the near-infrared response. Here, we proposed an effective optical structure in PSCs by texturing the rear-side perovskite layer with a rectangular grating to promote the optical response in the near-infrared region. Through the comprehensive numerical simulations, the new design under the optimized structural configuration shows an improved photocurrent density, i.e., from 22.75 (flat structure) to 23.72 mA/cm2. We confirm that the optical absorption enhancement occurs at the near-infrared region by the wavelength-dependent absorption and reflection spectra. Besides, the light-trapping mechanism was clarified by the comprehensive analysis of the electric field distributions. Moreover, PSCs having the rear-grating design demonstrate the superior optical performance compared to that of flat design in the varied incident angles. Our simulation results displayed in this study provide an easy scheme to promote the optical absorption in near-infrared region.

Electrically modulating and switching infrared absorption of monolayer graphene in metamaterials

Electrically modulating and switching the light absorption properties of monolayer graphene in near-infrared region has potentials in optoelectronic devices (e.g., photodetectors) and optical communication systems (e.g., modulators). In this work, we demonstrate numerically a broadband near-infrared absorption enhancement of monolayer graphene, due to the magnetic dipole resonance in metamaterials. The broadband light absorption in monolayer graphene can be largely modulated to realize an electrically switchable effect, via bias voltage for the interband transition of graphene to be near the magnetic dipole resonance. The absorption modulation depth is able to vary quickly from almost zero to nearly 100% in a very narrow wavelength range around the interband transition.

Holey graphene with enhanced near-infrared absorption: Experimental and DFT study

In this report, we use optical absorption spectroscopy and density functional theory simulations to investigate the optical behavior of a graphitic material with nanoscale holes. The material, produced by heating of graphite oxide in concentrated sulfuric acid followed by annealing at 1000 °C, demonstrated enhanced near-infrared absorption as compared to the pristine graphitic material. The computational study of graphene models containing holes of different sizes and different edge terminations revealed the major interband transitions defining the peaks in the absorption spectra. Our results suggest that the enhancement of near-infrared absorption of the material is caused by electron excitations involving hole edge states. The optical spectrum is strongly dependent on the distance between the holes and almost independent of both hole sizes and the functionalization family.

Photonic band engineering in absorbing media for spectrally selective optoelectronic films.

Spectrally selective materials are of great interest for optoelectronic devices in which wavelength-selectivity of the photoactive material is necessary for applications such as multi-junction solar cells, narrow-band photodetectors, transparent photovoltaics, and tailored emission sources. Achieving controlled transparency or opacity within multiple wavelength bands in the absorption, reflection, and transmission spectra are difficult to achieve in traditional semiconductors that typically absorb at all energies above their electronic band gap and is generally realized by the use of external bandpass filters. Here, we propose an alternate method for achieving spectral selectivity in optoelectronic thin films: the use of photonic band engineering within the absorbing region of a semiconductor in which resonant photonic bands are strongly coupled to the external reflectivity and transmission spectra. As a first step, we use optical simulations to systematically study the effect of material absorption on the properties of the photonic bands in a photonic crystal slab structure. We find that adding a weak loss to the materials model does not appreciably change the frequencies of the photonic bands but does reduce the quality factor of the associated photonic modes. Critically, the radiating photonic bands induce strong Fano resonance features in the transmission and reflection spectra, even in the presence of material absorption, due to coupling between the bands and external electromagnetic plane waves. These resonances can be tuned by adjusting the photonic crystal structural properties to induce spectral selectivity in the absorbing region of semiconductors. Lastly, we demonstrate this tuning method experimentally by fabricating a proof-of-principle photonic structure consisting of a self-assembled polystyrene bead monolayer infiltrated with PbS CQDs that displays both near-infrared absorption enhancement and visible transparency enhancement over a homogeneous control film, qualitatively matching predictions and showing promise for optoelectronic applications.

Broadband near-infrared absorption enhancement in Si substrate via random distributed Ag nanoparticles

A broadband near-infrared (NIR) absorption from 1200 to 2500 nm in Si substrate is investigated by Ag film deposition and subsequent thermal annealing. Under thermal annealing, Ag films can crack and shrink into Ag nanoparticles (NPs). Annealing temperature greatly affects the size, distribution status and morphology of Ag NPs, resulting in different NIR absorption. The influence of the size and distribution of Ag NPs on NIR absorption originated from localized surface plasmon resonance (LSPR) is theoretically simulated by a finite difference time domain method. The band width of LSPR belongs to different type Ag NPs are discussed.

Band engineering of amorphous silicon ruthenium thin film and its near-infrared absorption enhancement combined with nano-holes pattern on back surface of silicon substrate

Silicon is widely used in semiconductor industry but has poor performance in near-infrared photoelectronic devices because of its bandgap limit. In this study, a narrow bandgap silicon rich semiconductor is achieved by introducing ruthenium (Ru) into amorphous silicon (a-Si) to form amorphous silicon ruthenium (a-Si1-xRux) thin films through co-sputtering. The increase of Ru concentration leads to an enhancement of light absorption and a narrower bandgap. Meanwhile, a specific light trapping technique is employed to realize high absorption of a-Si1-xRux thin film in a finite thickness to avoid unnecessary carrier recombination. A double-layer absorber comprising of a-Si1-xRux thin film and silicon random nano-holes layer is formed on the back surface of silicon substrates, and significantly improves near-infrared absorption while the leaky light intensity is less than 5%. This novel absorber, combining narrow bandgap thin film with light trapping structure, may have a potential application in near-infrared photoelectronic devices.

Near-infrared absorption enhancement in microstructured silicon by Ag film deposition

We investigate the influence of spike-like microstructures formed on the silicon surface via fs-laser scanning with different-size on its near-infrared absorption from 1200 to 2500 nm. Although the infrared absorption of the small size microstructures is obviously lower than the large size, it can be further improved to 90 % by a large amount of random and irregular Ag nano particles from a subsequent deposition of Ag thin film. The origins of absorption enhancement are discussed and theoretically analyzed in detail.

Near-Infrared Absorption Enhancement Mechanism Investigations of Deep-Trench Silicon Microstructures Covered with Gold Films

We design a deep-trench microstructure covered with thin gold films to enhance near-infrared absorption of silicon material. This deep-trench microstructure exhibits a much higher absorption compared with plane nanoantenna arrays. We investigate its absorption enhancement in detail and find that the trench-shaped plasmonic waveguide greatly contributes an absorption enhancement by concentrating light effectively. Further, we clarify the influence of both trench depth and gold films covering on different positions of deep trench on the absorption. Finally, we use surface plasmon polaritons offered by plasmonic waveguide to explain well the significant enhancement of near-infrared absorption.

Enhancement of near-infrared absorption in graphene with metal gratings

Graphene has been demonstrated as a good candidate for ultrafast optoelectronic devices. However, graphene is essentially transparent in the visible and near infrared with an absorptivity of 2.3%, which has largely limited its application in photon detection. This Letter demonstrates that the absorptance in a monatomic graphene layer can be greatly enhanced to nearly 70%, thanks to the localized strong electric field resulting from magnetic resonances in deep metal gratings. Furthermore, the resonance frequency is essentially not affected by the additional graphene layer. The method presented here may benefit the design of next-generation graphene-based optical and optoelectronic devices.

Enhancement of near-infrared absorption with high fill factor in lead phthalocyanine-based organic solar cells

Enhancing the short circuit current (JSC) by extending the absorption to the near-infrared (NIR) spectrum, which has 50% of the total solar photon flux, is a remaining task in small molecular solar cells. Here, high efficiency NIR absorbing solar cells based on lead phthalocyanine (PbPc) are reported using copper iodide (CuI) as a templating layer to control the crystal structure of PbPc. Devices with CuI inserted between the ITO and PbPc layers exhibit a two times enhancement of the JSC compared to the case in the absence of the CuI layer. This is explained by the increase of crystallinity in the molecules grown on the CuI templating layer, which is investigated via an X-ray diffraction study. Moreover, fill factor is also enhanced to 0.63 from 0.57 due to low series resistance although the additional CuI layer is inserted between the ITO and the PbPc layer. As a result, the corrected power conversion efficiency of 2.5% was obtained, which is the highest one reported up to now among the PbPc based solar cells.

Red and near-infrared absorption enhancement for low bandgap polymer solar cells by combining the optical microcavity and optical spacers

Metal-mirror microcavity (MMC) structure is incorporated into the low bandgap polymer solar cells (LBPSCs) by sandwiching the active layer between the semitransparent Ag electrode and top Ag electrode. Optical simulations demonstrate that significant absorption enhancement can be achieved in the red and near infrared (near-IR) wavelength range due to the large optical electric field confined by the MMC structure. By inserting optical spacers to control the spectral and spatial distribution of the electric field in the LBPSC devices, the MMC structure can improve the total absorbed photons (TAPs) at the red and near-IR spectral range (620–900 nm) by 42.6% and yield an improvement of 14.0% in TAPs across the whole spectrum (400–900 nm) for the device with a 100 nm-thick active layer. For devices with thinner active layer, the improving rate is significantly raised to over 100%. Additionally, it is revealed that the effects of the optical spacers on the electric field distribution vary with their positions in the MMC structure. Finally, the MMC-based devices are optimized by tailoring the electric field distribution in the devices.

Strain-induced enhancement of near-infrared absorption in Ge epitaxial layers grown on Si substrate

Epitaxially grown Ge layers on Si substrate are shown to reveal an enhanced absorption of near-infrared light, which is effective for the photodiode application in Si-based photonics. Ge layers as thick as 1μm were grown on Si substrate by ultrahigh-vacuum chemical-vapor deposition with a low-temperature buffer layer technique. X-ray-diffraction measurements showed that the Ge layer possesses a tensile strain as large as 0.2%, which is generated during the cooling from the high growth temperature due to the thermal-expansion mismatch between Ge and Si. Photoreflectance measurements showed that the tensile strain reduces the direct band-gap energy to 0.77 eV (c.f. 0.80 eV for unstrained Ge), as expected from the theory. Reflecting the band-gap narrowing, photodiodes fabricated using the Ge layer revealed an enhanced absorption of near-infrared light with the photon energy below 0.80 eV, i.e., with the wavelength above 1.55μm. This property is effective to apply the photodiodes to the L band (1.56–1.62μm) in the optical communications as well as the C band (1.53–1.56μm). It is shown that the experimental absorption spectrum agrees with the theoretical one taking into account the splitting of light-hole and heavy-hole valence bands accompanied by the band-gap narrowing. Based on the calculation, the performance of the photodiode using the tensile-strained Ge is discussed.