Infrared hot-carrier photodetection based on planar perfect absorber

Grating-assisted hot-electron photodetectors for S- and C-band telecommunication

Although outstanding detectivities, InGaAs photodetectors for optic fiber communication are often costly due to the need for cooling. Therefore, cryogen-free and cost-effective alternatives working in telecommunication bands are highly desired. Here, we present a design of hot-electron photodetectors (HE PDs) with attributes of room-temperature operation and strong optical absorption over S and C bands (from 1460 to 1565 nm). The designed HE PD consists of a metal-semiconductor-metal hot-electron stack integrated with a front grating. Optical simulations reveal that mode hybridizations between Fabry-Pérot resonance and grating-induced surface plasmon excitation lead to high absorption efficiencies (≥0.9) covering S and C bands. Probability-based electrical calculations clarify that device responsivity is mainly determined by working wavelength on the premise of broadband strong absorption. Moreover, through comparison studies between the grating-assisted HE PD and purely planar microcavity system that serves as a reference, we highlight the design superiorities in average absorption and average responsivity with optimized values of 0.97 and 0.73 mA W-1, respectively. The upgraded peformances of the designed device are promising for efficient photoelectric conversion in optic fiber communication systems.

Bias voltage-tuned hot-electron optical sensing with planar Au-MoS2-Au junction

Harvesting photoexcited hot electrons in metals promises a number of benefits in optical sensing. In practice, hot-electron optical sensors with tunable performance in electrical sensitivity are still absent. Herein, we propose a design to realize tunable hot-electron optical sensing. The proposed device consists of a one-dimensional grating deposited on a planar Au-MoS₂-Au junction that is used for efficient hot-electron harvesting. Photoelectric simulations show that when grating-assisted plasmonic resonance is excited, bias voltage between two Au layers can be used to manipulate the magnitude and polarity of responsivity at the working wavelength. Therefore, the change in responsivity that originates from the change in refractive index of analyte in which the device is immersed can also be tuned by applied voltage. It is found that when bias voltage is 1 V, the electrical sensitivity doubled compared with that when applied voltage is absent. We believe the bias voltage-tuned strategy that is applied to planar hot-electron harvesting junctions facilitates the development of optical sensing.

Five-layer planar hot-electron photodetectors at telecommunication wavelength of 1550 nm

Cost-effective and high-responsivity photodetectors at a telecommunication wavelength of 1550 nm are highly desired in optical communication systems. Differing from conventional semiconductor-based photodetectors, several planar hot-electron photodetectors (HE PDs) that operate at 1550 nm have been reported. However, these devices were often comprised of many planar layers and exhibited relatively low responsivities. Herein, we propose a design of high-performance planar HE PDs consisting of five layers. Utilizing Fabry-Pérot (FP) resonance, the nearly perfect absorption of the proposed device can be achieved at the targeted wavelength of 1550 nm. Simulation results show that FP resonance orders are crucial for the optical absorption efficiencies, and then electrical responses. Analytical electrical calculations reveal that, benefiting from the strong absorption (>0.6) in the ultrathin Au layer with a thickness of 5 nm and the low Schottky barrier (0.5 eV) of Au-MoS₂ contact, predicted responsivity of proposed HE PD at zero-order FP resonance is up to ∼10 mA/W. Our design provides a new approach to realize low-cost and efficient photodetection for optical communication technology.

Reconfigurable and spectrally switchable perfect absorber based on a phase-change material

In this paper, we propose a lithography-free spectrally tunable prefect absorber based on an asymmetric Fabry-Perot cavity using Ge₂Sb₂Te₅ (GST), a phase-change material, as the cavity layer. The proposed device shows a maximum absorption of 99.7% at 1550 nm, at a particular angle of incidence and polarization when the phase of GST is in the amorphous state. The absorption spectrum is spectrally switched to longer wavelength when the phase of GST is transformed from amorphous to crystalline. The tuning range is about 866 nm, and the maximum absorption is maintained above 99% in the whole tuning range. The crystallinity ratio of GST is varied by applying voltage pulses of different amplitudes and durations. The electrothermal cosimulations show that the phase change is obtained in the whole GST layer. Furthermore, by reamorphization of GST, the absorption spectrum can be switched back, enabling a reconfigurable perfect absorber. This work shows a viable path toward achieving a tunable perfect absorber covering a 1550 nm communication wavelength window as well as an emerging optical communication window around 2 µm wavelength.

Ultra-narrowband resonant light absorber for high-performance thermal-optical modulators

Herein, a tunable thermal-optical ultra-narrowband grating absorber is realized. Four ultra-sharp absorption peaks in the infrared region are achieved with the absorption efficiency of 19.89%, 98.41%, 99.14%, and 99.99% at 1144.34 nm, 1190.92 nm, 1268.58 nm, and 1358.70 nm, respectively. Benefiting from an extremely narrow bandwidth (0.27 nm), a maximum Q-factor over 4400 is obtained for the absorber. Moreover, the spectral response can be artificially tuned by controlling the temperature via the strong thermo-optic effect of silicon resonator. The high absorption contrast ratio of 23 dB is demonstrated by only increasing the temperature by 10 °C, showing an order of magnitude better than that of the previously demonstrated performance in the infrared image contrast manipulation. Also, the absorption intensity can be precisely regulated via tuning the polarization state of incident light. Strong tunability extending to temperature and polarization states makes this metasurface promising for applications in a high-performance switch, notch filter, modulator, etc.

Planar hot-electron photodetector utilizing high refractive index MoS2 in Fabry-Pérot perfect absorber

Hot electron photodetection (HEPD) excited by surface plasmon can circumvent bandgap limitations, opening pathways for additional energy harvesting. However, the costly and time-consuming lithography has long been a barrier for large-area and mass production of HEPD. In this paper, we proposed a planar and electron beam lithography-free hot electron photodetector based on the Fabry-Pérot (F-P) resonance composed of Au/MoS2/Au cavity. The hot electron photodetector has a nanoscale thickness, high spectral tunability, and multicolour photoresponse in the near-infrared region due to the increased round-trip phase shift by using high refractive index MoS2. We predict that the photoresponsivity can achieve up to 23.6 mA W-1 when double cavities are integrated with the F-P cavity. The proposed hot electron photodetector that has a nanoscale thickness and planar stacking is a perfect candidate for large-area and mass production of HEPD.

Photodetection by Hot Electrons or Hot Holes: A Comparable Study on Physics and Performances

Hot-carrier photodetectors are drawing significant attention; nevertheless, current researches focus mostly on the hot-electron devices, which normally show low quantum efficiencies. In contrast, hot-hole photodetectors usually have lower barriers and can provide a wide spectral range of photodetection and an improved photoconversion efficiency. Here, we report a comparable study of the hot-electron and hot-hole photodetectors from both underlying physics and optoelectronic performance perspectives. Taking the typical Au/Si Schottky contact as an example, we find obvious differences in the energy band diagram and the sequent hot-carrier generation/transport/emission processes, leading to very distinguished photodetection performances. Compared with hot electrons, hot holes show higher density below the Fermi level, the longer mean free path arising under the lower electron-electron and electron-phonon scatterings, a lower barrier height, and a lighter effective mass in Si, all of which lead to larger number of high-energy hot holes, larger transport probability, higher emission efficiency, and higher photoresponsivity. However, the low barrier height can cause poor performances of hot-hole device in dark current density and detectivity. The study elucidates the intrinsic physical differences and compares the key performance parameters of the hot-hole and hot-electron photodetections, with the objective of providing complete information for designing hot-carrier devices.

Planar dual-cavity hot-electron photodetectors

Hot-electron photodetectors (HE PDs) are attracting increasing interests. However, the nanostructured HE PDs are fabricated via complicated and costly techniques, while the planar counterparts can hardly achieve outstanding photon absorption and hot-electron collection simultaneously. To address the incompatibility in optical and electrical domains, herein, we propose an HE PD based on planar dual cavities (i.e., DC-HE PD) one each for photon absorption and triple Schottky junctions for carrier collection. Optoelectronic simulation demonstrates that the resonant wavelength and the absorption efficiency of the device can be manipulated conveniently by tailoring the planar thickness. Compared with the single-cavity system, the absorption efficiency of the DC-HE PD with the multi-junction configuration doubled (∼100%) and the responsivity tripled (∼2 mA W-1). The high-performance optoelectronic responses are shown to be sustained over a wide range of incident angles. The detailed physical property, namely, the coupled-cavity nature and the detailed analysis of the hot electron dynamics are presented.

Perfect meta-absorber by using pod-like nanostructures with ultra-broadband, omnidirectional, and polarization-independent characteristics

The on-chip perfect meta-absorber (PMA) is an important optical and thermal energy component in photovoltaics, thermal emitters, and energy harvesting applications. However, most reported PMAs rely on the complicated lithography techniques, which imposed a serious cost barrier on the development of practical applications, especially in the visible to near-infrared (NIR) wavelength range and at very large scales. Importantly, it is hard to realize PMA in the UV wavelength range by using current lithography techniques. In this article, we develop an ultra-broadband PMA by using natural lithography (NL) technique. The morphology of proposed PMA is randomly distributed pod-like nanostructures composed of a nanocomposite (Au/SiO₂) covered a gold layer. It can be formed easily on Si substrate to function as an ultra-broadband, omnidirectional, and polarization-independent PMA by controlling the conditions of sputtering deposition and thermal annealing treatment. We experimentally realized an on-chip ultra-broadband PMA with almost 100% absorption spanned from UV-visible to NIR wavelength ranges. This cost-effective and high-efficiency approach would release the manufacturing barrier for previously reported PMAs and therefore open an avenue to the development of effectively energy harvesting, energy recycling, and heat liberation applications.

Photovoltaic absorber with different grating profiles in the near-infrared region

We theoretically introduce a Si-based photovoltaic absorber with different grating profiles, which demonstrates a desirable enhancement of light absorption in the near-infrared region by increasing the degree of the grating's profile function. The mechanisms of light absorption enhancement originate from the synergetic effect of optical waveguide modes, light scattering, and Fabry-Perot resonances. Moreover, numerical results indicate that the convex grating structure is more conducive to the excitation of optical waveguide modes compared with the concave grating structure. The research findings can be utilized to guide the design of thin-film solar cells based on grating structures.

Ultra-broadband, wide angle absorber utilizing metal insulator multilayers stack with a multi-thickness metal surface texture

In this paper, we propose a facile route to fabricate a metal insulator multilayer stack to obtain ultra-broadband, wide angle behavior from the structure. The absorber, which covers near infrared (NIR) and visible (Vis) ranges, consists of a metal-insulator-metal-insulator (MIMI) multilayer where the middle metal layer has a variant thickness. It is found that this non-uniform thickness of the metal provides us with an absorption that is much broader compared to planar architecture. In the non-uniform case, each thickness is responsible for a specific wavelength range where the overall absorption is the superposition of these resonant responses and consequently a broad, perfect light absorption is attained. We first numerically examine the impact of different geometries on the overall light absorption property of the multilayer design. Afterward, we fabricate the designs and characterize them to experimentally verify our numerical findings. Characterizations show a good agreement with numerical results where the optimum absorption bandwidth for planar design is found to be 620 nm (380 nm-1000 nm) and it is significantly boosted to an amount of 1060 nm (350 nm-1410 nm) for multi-thickness case.

Au nanoparticle-decorated silicon pyramids for plasmon-enhanced hot electron near-infrared photodetection

The heterojunction between metal and silicon (Si) is an attractive route to extend the response of Si-based photodiodes into the near-infrared (NIR) region, so-called Schottky barrier diodes. Photons absorbed into a metallic nanostructure excite the surface plasmon resonances (SPRs), which can be damped non-radiatively through the creation of hot electrons. Unfortunately, the quantum efficiency of hot electron detectors remains low due to low optical absorption and poor electron injection efficiency. In this study, we propose an efficient and low-cost plasmonic hot electron NIR photodetector based on a Au nanoparticle (Au NP)-decorated Si pyramid Schottky junction. The large-area and lithography-free photodetector is realized by using an anisotropic chemical wet etching and rapid thermal annealing (RTA) of a thin Au film. We experimentally demonstrate that these hot electron detectors have broad photoresponsivity spectra in the NIR region of 1200-1475 nm, with a low dark current on the order of 10-5 A cm-2. The observed responsivities enable these devices to be competitive with other reported Si-based NIR hot electron photodetectors using perfectly periodic nanostructures. The improved performance is attributed to the pyramid surface which can enhance light trapping and the localized electric field, and the nano-sized Au NPs which are beneficial for the tunneling of hot electrons. The simple and large-area preparation processes make them suitable for large-scale thermophotovoltaic cell and low-cost NIR detection applications.

Planar microcavity-integrated hot-electron photodetector

Hot-electron photodetectors are attracting increasing interest due to their capability in below-bandgap photodetection without employing classic semiconductor junctions. Despite the high absorption in metallic nanostructures via plasmonic resonance, the fabrication of such devices is challenging and costly due to the use of high-dimensional sub-wavelength nanostructures. In this study, we propose a planar microcavity-integrated hot-electron photodetector (MC-HE PD), in which the TCO/semiconductor/metal (TCO: transparent conductive oxide) structure is sandwiched between two asymmetrically distributed Bragg reflectors (DBRs) and a lossless buffer layer. Finite-element simulations demonstrate that the resonant wavelength and the absorption efficiency of the device can be manipulated conveniently by tailoring the buffer layer thickness and the number of top DBR pairs. By benefitting from the largely increased electric field at the resonance frequency, the absorption in the metal can reach 92%, which is a 21-fold enhancement compared to the reference without a microcavity. Analytical probability-based electrical calculations further show that the unbiased responsivity can be up to 239 nA mW(-1), which is more than an order of magnitude larger than that of the reference. Furthermore, the MC-HE PD not only exhibits a superior photoelectron conversion ability compared to the approach with corrugated metal, but also achieves the ability to tune the near infrared multiband by employing a thicker buffer layer.