Angle-independent hot carrier generation and collection using transparent conducting oxides

Planar hot-electron photodetection with polarity-switchable photocurrents controlled by the working wavelength

Hot-electron photodetection is attracting increasing interests. Based on internal photoemission mechanism, hot-electron photodetectors (HE PDs) convert incident photon energy into measurable photocurrent. To obtain polarity-switchable photocurrent, one often applies electric bias to reverse the hot-electron flow. However, the employment of bias reduces the device flexibility and increasing the bias voltage degrades the detectivity of the device. Herein, we design a planar HE PD with the polarity-switchable photocurrent controlled by the working wavelength. Optical simulations show that the device exhibits two absorption peaks due to the resonances of two Tamm plasmons (TPs). Electrical calculations predict two corresponding TP-assisted responsivity peaks, but with opposite photocurrent polarities, which are determined by the hot-electron flows with opposite directions. We find that the hot-electron flows are closely related with the population differences of TP-induced hot electrons in two electrodes. We further demonstrate that the photocurrent polarity of the HE PD can be switched by altering working wavelength from one TP wavelength to the other. We believe that this approach paves a route to achieve flexible hot-electron photodetection for extensive applications.

A broadband and polarization-independent metasurface perfect absorber for hot-electron photoconversion

We report an ultra-broadband metasurface perfect absorber from the UV to NIR region based on TiN nanostructures. A polarization-independent experimental average absorption of 0.900 (0.921 in simulation) at the wavelength band from 300 nm to 1500 nm is realized with only an 82 nm-thick TiN layer with TiO₂ and MgF₂ on top, which is efficiently fabricated by utilizing double-beam UV interference lithography followed by sputter coating deposition. A TiN-TiO₂ hot-electron photoelectric conversion system is also simulated. An IPCE of 4% is realized at the wavelength of 710 nm and the average IPCE is 2.86% in the wavelength range of 400 nm to 1500 nm. The demonstrated device suggests an efficient way of designing and fabricating broadband perfect absorbers, which has great application potential in efficient hot-electron optoelectronic and photocatalytic systems.

Enhancing Hot Electron Injection in Plasmonic Photodetectors through Roughened Interfaces

Ongoing efforts have been made to improve the photoresponsivity of plasmonic photodetectors. In this work, the photodetectors based on transparent conductive oxide (TCO)/Semiconductor/Metal configuration especially with a roughened interface were investigated numerically, and the effect of the roughness on the injection efficiency of hot electrons was analyzed. The simulated results indicate that a roughened structure alleviates effectively the momentum mismatch of hot electrons at the metal/semiconductor interface due to asymmetry factor, and greatly improves the injection efficiency as well as photoresponsivity. At the incidence wavelength of 1550 nm, the photoresponsivity increased by about 8 times. Meanwhile, the influence on the resonant wavelength shift is negligible where the roughness is nano-scale. Our work provides a valuable guidance for the theoretical and experimental research of plasmonic photodetectors.

Highly switchable absorption in a metal hydride device using a near-zero-index substrate

Optical switchability is an important functionality for photonic devices, which allows them to accommodate a wide range of applications. One way to achieve this switchability is to utilize the reversible and tunable optical changes of metal hydrides. When exposed to H₂ gas, certain metals go through dramatic changes in optical properties as hydrogen atoms expand the lattice spacing. In this paper, we propose a switchable absorption device consisting of a Pd-capped Mg thin film deposited onto a near-zero-index substrate. By utilizing Mg's extreme optical changes upon hydrogenation and combining it with the high optical contrast of the near-zero-index substrate, we can create a device that is fully switchable from a highly reflective state to a broadband absorbing state. When modeling the substrate as a Drude material with a plasma wavelength of 600 nm, we calculate an absorption change of > 70% from 650-1230 nm, with a peak total absorption of 78% at 905 nm. We experimentally demonstrate this effect using 25 nm of Mg with a 3 nm Pd capping layer deposited onto an ITO-coated glass substrate. This device achieves an absorption change of 76% at 1335 nm illumination, with a maximum absorption of 93% in the hydride state, utilizing ITO's near-zero-index region in the near-infrared. By tuning the near-zero-index region of the substrate, this effect can be extended from the visible through the infrared.

Broadband Tamm Plasmons in Chirped Photonic Crystals for Light-Induced Water Splitting

An electrode of a light-induced cell for water splitting based on a broadband Tamm plasmon polariton localized at the interface between a thin TiN layer and a chirped photonic crystal has been developed. To facilitate the injection of hot electrons from the metal layer by decreasing the Schottky barrier, a thin n-Si film is embedded between the metal layer and multilayer mirror. The chipping of a multilayer mirror provides a large band gap and, as a result, leads to an increase in the integral absorption from 52 to 60 percent in the wavelength range from 700 to 1400 nm. It was shown that the photoresponsivity of the device is 32.1 mA/W, and solar to hydrogen efficiency is 3.95%.

Broadband Tamm plasmon-enhanced planar hot-electron photodetector

Here, we propose a planar hot-electron photodetector based on broadband Tamm plasmon resonance using a TiN layer, n-type doped Si layer, and seven pairs of DBRs. Simulation results show high absorption (94.2%) with a full width at half maximum of 239.3 nm, which is 2.9 times that of the Au/DBR configuration. We predict that the photoresponsivity can reach 26.1 mA W^-1 at 1140 nm. Since the planar nanofilms for TP resonance are facile to fabricate, this work promotes hot-electron applications in broadband photodetection and other broadband light-harvesting applications.

Control of hot-carrier relaxation time in Au-Ag thin films through alloying

The plasmon resonance of a structure is primarily dictated by its optical properties and geometry, which can be modified to enable hot-carrier photodetectors with superior performance. Recently, metal alloys have played a prominent role in tuning the resonance of plasmonic structures through chemical composition engineering. However, it has been unclear how alloying modifies the time dynamics of the generated hot-carriers. In this work, we elucidate the role of chemical composition on the relaxation time of hot-carriers for the archetypal Au_xAg_1-x thin film system. Through time-resolved optical spectroscopy measurements in the visible wavelength range, we measure composition-dependent relaxation times that vary up to 8× for constant pump fluency. Surprisingly, we find that the addition of 2% of Ag into Au films can increase the hot-carrier lifetime by approximately 35% under fixed fluence, as a result of a decrease in optical loss. Further, the relaxation time is found to be inversely proportional to the imaginary part of the permittivity. Our results indicate that alloying is a promising approach to effectively control hot-carrier relaxation time in metals.

Quantum Leap from Gold and Silver to Aluminum Nanoplasmonics for Enhanced Biomedical Applications

Nanotechnology has been used in many biosensing and medical applications, in the form of noble metal (gold and silver) nanoparticles and nanostructured substrates. However, the translational clinical and industrial applications still need improvements of the efficiency, selectivity, cost, toxicity, reproducibility, and morphological control at the nanoscale level. In this review, we highlight the recent progress that has been made in the replacement of expensive gold and silver metals with the less expensive aluminum. In addition to low cost, other advantages of the aluminum plasmonic nanostructures include a broad spectral range from deep UV to near IR, providing additional signal enhancement and treatment mechanisms. New synergistic treatments of bacterial infections, cancer, and coronaviruses are envisioned. Coupling with gain media and quantum optical effects improve the performance of the aluminum nanostructures beyond gold and silver.

Plasmonic hot electrons for sensing, photodetection, and solar energy applications: A perspective

In plasmonic metals, surface plasmon resonance decays and generates hot electrons and hot holes through non-radiative Landau damping. These hot carriers are highly energetic, which can be modulated by the plasmonic material, size, shape, and surrounding dielectric medium. A plasmonic metal nanostructure, which can absorb incident light in an extended spectral range and transfer the absorbed light energy to adjacent molecules or semiconductors, functions as a "plasmonic photosensitizer." This article deals with the generation, emission, transfer, and energetics of plasmonic hot carriers. It also describes the mechanisms of hot electron transfer from the plasmonic metal to the surface adsorbates or to the adjacent semiconductors. In addition, this article highlights the applications of plasmonic hot electrons in photodetectors, photocatalysts, photoelectrochemical cells, photovoltaics, biosensors, and chemical sensors. It discusses the applications and the design principles of plasmonic materials and devices.

Hot electron photodetection with spectral selectivity in the C-band using a silicon channel-separated gold grating structure

Photodetection based on hot electrons is attracting interest due to its capability of enabling photodetection at sub-bandgap energies of semiconductor materials. Si-based photodetectors incorporating hot electrons have emerged as one of the most widely studied devices used for near infrared (NIR) photodetection. However, most reported Si-based NIR photodetectors have broad bandwidths with responsivities that change slowly with the target wavelength, limiting their practicality as spectrally selective photodetectors. This paper reports a Si channel-separated Au grating structure that exhibits the spectrally selective photodetection in the C-band (1530–1565 nm). The measured responsivity of the structure drops from 64.5 nA mW−1 at 1530 nm to 19.0 nA mW−1 at 1565 nm, representing a variation of 70.5% over the C-band. The narrowband, ease of tuning the resonant wavelength, and spectral selectivity of the device not only help bridge the gap between the optical and electrical systems for photodetection but are also beneficial in other potential applications, such as sensing, imaging, and communications systems.

Planar, narrowband, and tunable photodetection in the near-infrared with Au/TiO2 nanodiodes based on Tamm plasmons

There is increasing interest in hot-electron photodetection due to the extended photoresponse well below the semiconductor band edge. However, the photoresponsivity is extremely low and the metallic nanostructures used to excite surface plasmons (SPs) for improved quantum yield are too complex for practical applications. Here, we show that by exciting Tamm plasmons (TPs), a planar device consisting of a thin metal film of 30 nm on a distributed Bragg reflector (DBR) can absorb ∼93% of the incident light, resulting in a high hot-electron generation that is over 34-fold enhanced compared to that of the reference without the DBR. Besides, the electric field increases with the light penetration depth in the metal, leading to hot-electron generation that is strongly concentrated near the Schottky interface. As a result, the photoresponsivity can be over 30 (6) times larger than that of the reference (conventional grating system). Moreover, the planar device exhibits an easily tunable working wavelength from the visible to the near-infrared, sustained performance under oblique incidences, and a multiband photodetection functionality. The proposed strategy avoids the complicated fabrication of the metallic nanostructures, facilitating the compact, large-area, and low-cost photodetection, biosensing, and photocatalysis applications.

Alloying: A Platform for Metallic Materials with On-Demand Optical Response

Metallic materials with engineered optical properties have the potential to enhance the performance of energy harvesting and storage devices operating at the macro- and nanoscale, such as solar cells, photocatalysts, water splitting, and hydrogen storage systems. For both thin films and subwavelength nanostructures, upon illumination, the coherent oscillation of charge carriers at the interface with a dielectric material gives rise to resonances named surface plasmon polariton (SPP) and localized surface plasmon resonance (LSPR), respectively. These resonances result in unique light absorption, scattering, and transmission responses over the electromagnetic spectrum, which, in turn, can be exploited to tailor the behavior of active metallic components in optoelectronic devices containing Ag, Au, Cu, Al, Mg, among other metals. The wavelength in which the resonances occur primarily depends on the metal itself (i.e., the dielectric function or permittivity), the dielectric medium surrounding the metals, and the size, geometry, and periodicity of the metallic nanostructures. Nevertheless, the aforementioned parameters allow a limited modulation of both SPP and LSPR over a narrow window of frequencies. To overcome this constraint, we have proposed and realized the alloying of metals via physical deposition methods as a paradigm to almost arbitrarily tuning their optical behavior in the UV-NIR, which leads to permittivity values currently not available. Our approach offers an additional knob, chemical composition, to engineer light-matter interactions in metallic materials. This Account highlights recent progress in using alloying as a pathway to control the optical behavior of metallic thin films and nanostructures for energy harvesting and storage applications, including (photo)catalysis, photovoltaics, superabsorbers, hydrogen storage, among other systems. We choose to primarily focus on the optical properties of the metallic mixtures and in their near- to far-field responses in the UV-NIR range of the spectrum as they represent key parameters for materials' selection for the devices above. By alloying, it is possible to obtain metallic materials with LSPR not available for pure metals, which can enable the further control of the electromagnetic spectrum. First, we discuss how the permittivity of binary mixtures of coinage metals (Au, Ag, and Cu) can be tailored based on the chemical composition of their pure counterparts. Second, we present how novel metallic materials can be designed through band structure engineering through density functional theory (DFT), a paradigm that could benefit from artificial intelligence methods. Concerning alloyed thin films, we discuss the promise of earth-abundant metals and provide an example of the superior performance of AlCu in superabsorbers. In the realm of nanostructures, we focus the discussion on physical deposition methods, where we provide a detailed analysis of how chemical composition can affect the far- and near-field responses of metallic building blocks. Finally, we provide a brief outlook of promising next steps in the field.

Hot-electron photodetector with wavelength selectivity in near-infrared via Tamm plasmon

Tamm plasmonic (TP) structures, consisting of a metallic film and a distributed Bragg reflector (DBR), can exhibit pronounced light confinement allowing for enhanced absorption in the metallic film at the wavelength of the TP resonance. This wavelength dependent absorption can be converted into an electrical signal through the internal photoemission of energetic hot-electrons from the metallic film. Here, by replacing the metallic film at the top of a TP structure with a hot-electron device in a metal-semiconductor-ITO (M-S-ITO) configuration, for the first time, we experimentally demonstrate a wavelength-selective photoresponse around the telecommunication wavelength of 1550 nm. The M-S-ITO junction is deliberately designed to have a low energy barrier and asymmetrical hot-electron generation, in order to guarantee a measurable net photocurrent even for sub-bandgap incident light with a photon energy of 0.8 eV (1550 nm). Due to the excitation of TPs between the metallic film in the M-S-ITO structure and the underlying DBR, the fabricated TP coupled hot-electron photodetector exhibits a sharp reflectance dip with a bandwidth of 43 nm at a wavelength of 1581 nm. The photoresponse matches the absorptance spectrum, with a maximum value of 8.26 nA mW-1 at the absorptance peak wavelength that decreases by more than 80% when the illumination wavelength is varied by only 52 nm (from 1581 to 1529 nm), thus realizing a high modulation wavelength-selective photodetector. This study demonstrates a high-performance, lithography-free, and wavelength-selective hot-electron near-infrared photodetector using an M-S-ITO-DBR planar structure.

Progress in the Utilization Efficiency Improvement of Hot Carriers in Plasmon-Mediated Heterostructure Photocatalysis

The effect of plasmon-induced hot carriers (HCs) enables the possibility of applying semiconductors with wide band gaps to visible light catalysis, which becomes an emerging research field in environmental protections. Continued efforts have been made for an efficient heterostructure photocatalytic process with controllable behaviors of HCs. Recently, it has been discovered that the improvement of the utilization of HCs by band engineering is a promising strategy for an enhanced catalytic process, and relevant works have emerged for such a purpose. In this review, we give an overview of the recent progress relating to optimized methods for designing efficient photocatalysts by considering the intrinsic essence of HCs. First, the basic mechanism of the heterostructure photocatalytic process is discussed, including the formation of the Schokkty barrier and the process of photocatalysis. Then, the latest studies for improving the utilization efficiency of HCs in two aspects, the generation and extraction of HCs, are introduced. Based on this, the applications of such heterostructure photocatalysts, such as water/air treatments and organic transformations, are briefly illustrated. Finally, we conclude by discussing the remaining bottlenecks and future directions in this field.

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.

Polarization-insensitive hot-electron infrared photodetection by double Schottky junction and multilayer grating

Infrared photodetection based on hot electrons is drawing increasing interest due to the capabilities of below-bandgap detection, high tunability of working wavelength, compact size, and room-temperature operation. However, conventional hot-electron photodetectors are mostly based on surface plasmons with a strong polarization preference. In this Letter, we propose a multilayer grating double-junction hot-electron photodetector by introducing an ultrathin Au layer sandwiched between two Au-Si-Au cavities. The multilayer grating system allows the excitation of the guided-mode resonance that shows a weak reliance on the incident polarization and, therefore, realizes the polarization-insensitive optical absorption up to 98%. The special multilayer design facilitates hot-electron generation in the ultrathin Au layers with high carrier transport efficiency, as well as enabling the formation of a double Schottky junction, which doubles the carrier emission probability. The optical and electrical benefits ensure a polarization-independent photoresponsivity ∼1  mA/W at the wavelength of 1470 nm.

Ultraviolet Wavelength Identification Using Energy Distribution of Hot Electrons

Light wavelength identification is essential for many optical and optoelectronic applications. Here, we report a novel wavelength identification photodetector based on the energy distribution of hot electrons at the metal/insulator interface. The information of the light wavelength can be stored in the energy distribution of the hot electrons, which can then be readout in the form of the current-voltage characteristics. On the basis of this principle, the high-reliability wavelength identification of the monochromatic light has been realized with a simple Al/SiO₂/Si structure. The device has an excellent stability with dark current below 1 × 10^-7 A/m². Moreover, the wavelength of the monochromatic light in the deep ultraviolet range can be identified. This new principle will pave a new solution to design high-performance single-chip wavelength identification photodetectors and integrated miniaturized wavelength identification systems.

Planar Hot-Electron Photodetection with Tamm Plasmons

There is an increasing interest in harvesting photoejected hot-electrons for sensitive photodetectors, which have highly tunable detection wavelengths controlled by structural engineering rather than the classic doped semiconductors. However, the widely employed metallic nanostructures that excite surface plasmons (SPs) to enhance the photoemission of hot-electrons are usually complex with a high fabrication challenge. Here, we present a purely planar hot-electron photodetector based on Tamm plasmons (TPs) by introducing a distributed Bragg reflector integrated with hot-electron collection layers in metal/semiconductor/metal configuration. Results show that the light incidence can be strongly confined in the localized region between the top metal and the adjacent dielectric layer due to the excitation of TP resonance so that more than 87% of the light incidence can be absorbed by the top metal layer. This enables a strong and unidirectional photocurrent and a photoresponsivity that can even be higher than that of the conventional nanostructured system. Moreover, the planar TP system shows a narrow-band resonance with high tunability, good resistance against the change of the incident angle, and the possibility for extended functionalities. The proposed TP-based planar configuration significantly simplifies the conventional SP-based systems and opens the pathway for high-performance, low-cost, hot-electron photodetection.

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.

Infrared hot-carrier photodetection based on planar perfect absorber

Hot-carrier based photodetectors are independent on the semiconductor bandgap, thus paving a new paradigm of photovoltaic conversion. Herein, we propose a non-nanostructured and multilayered metal/insulator/transparent conductive oxide/silica/reflector system, and explore in detail the optical response and the electrical transport in the device via the finite-element electromagnetic simulation and the probability-based analytical carrier-transport calculation. Results show that the planar system can function as a planar perfect absorber at the targeted wavelength under the inbuilt cavity resonance with a very high tunability by tailoring the cavity length and the metal thickness. Moreover, a strong asymmetrical absorption is formed in the two electrode layers, yielding strong unidirectional photocurrents and output power densities. This Letter suggests a more simple and feasible way to realize hot-carrier infrared photodetectors.

Ultra-thin broadband nanostructured insulator-metal-insulator-metal plasmonic light absorber

An ultra-thin nanostructured plasmonic light absorber with an insulator-metal-insulator-metal (IMIM) architecture is designed and numerically studied. The IMIM structure is capable to absorb up to about 82.5% of visible light in a broad wavelength range of 300-750 nm. The absorption by the bottom metal is only 6% of that of the top metal. The results show that the IMIM architecture has weak dependence of the angle of the incident light. Interestingly, by varying the top insulator material the optical absorption spectrum can be shifted more than 180 nm as compared to the conventional air-metal-insulator-metal structure. The IMIM structure can be applied for different plasmonic devices with improved performance.