Fabry-Perot Cavity-Enhanced Optical Absorption in Ultrasensitive Tunable Photodiodes Based on Hybrid 2D Materials

Validated enhancement and temperature modulated absorbance of a WS2 monolayer based on a planar structure

Transition-metal dichalcogenides (TMDCs), as emerging optoelectronic materials, necessitate the establishment of an experimentally viable system to study their interaction with light. In this study, we propose and analyze a WS2/PMMA/Ag planar Fabry-Perot (F-P) cavity, enabling the direct experimental measurement of WS2 absorbance. By optimizing the structure, the absorbance of A exciton of WS2 up to 0.546 can be experimentally achieved, which matches well with the theoretical calculations. Through temperature and thermal expansion strain induced by temperature, the absorbance of the A exciton can be tuned in situ. Furthermore, temperature-dependent photocurrent measurements confirmed the consistent absorbance of the A exciton under varying temperatures. This WS2/PMMA/Ag planar structure provides a straightforward and practical platform for investigating light interaction in TMDCs, laying a solid foundation for future developments of TMDC-based optoelectronic devices.

Piezo-photovoltaic Effect in Monolayer 2H-MoS2

Noncentrosymmetric bulk materials effectively convert light energy into electricity by making use of the bulk photovoltaic effect (BPVE). However, whether such an effect persists when reducing the thickness of materials down to atomic-scale remains to be revealed. Here, we show the piezo-photovoltaic effect in atomically thin two-dimensional materials, where the strain-induced polarization can generate photovoltaic outputs in the noncentrosymmetric mono- and few-layer 2H-MoS2 crystals. The photocurrent is enhanced by orders of magnitude when the MoS2 crystals experience an in-plane strain of about 0.2%, with photopower-dependent responsivity up to 0.1 A/W that rivals other state-of-the-art BPVE materials. In addition, studies on the spatial distributions of photocurrents on MoS2 with a controlled number of layers also allow us to disentangle various factors that couple the piezoelectricity and photovoltaics. Therefore, our results also provide insights into the mechanisms of the piezo-photovoltaic effect in two-dimensional materials with thicknesses at the atomic-scale limit.

2D-materials-integrated optoelectromechanics: recent progress and future perspectives

The discovery of two-dimensional (2D) materials has gained worldwide attention owing to their extraordinary optical, electrical, and mechanical properties. Due to their atomic layer thicknesses, the emerging 2D materials have great advantages of enhanced interaction strength, broad operating bandwidth, and ultralow power consumption for optoelectromechanical coupling. The van der Waals (vdW) epitaxy or multidimensional integration of 2D material family provides a promising platform for on-chip advanced nano-optoelectromechanical systems (NOEMS). Here, we provide a comprehensive review on the nanomechanical properties of 2D materials and the recent advances of 2D-materials-integrated nano-electromechanical systems and nano-optomechanical systems. By utilizing active nanophotonics and optoelectronics as the interface, 2D active NOEMS and their coupling effects are particularly highlighted at the 2D atomic scale. Finally, we share our viewpoints on the future perspectives and key challenges of scalable 2D-materials-integrated active NOEMS for on-chip miniaturized, lightweight, and multifunctional integration applications.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS₂, WS₂, MoSe₂, and WSe₂, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.

Photoluminescence enhancement in monolayer MoS2 and self-assembled 3D photonic crystal heterostructures

Self-assembled photonic crystals (PCs) have promising applications in enhancing and directional manipulation of the photoemission due to their photonic bandgaps. Here, we employed self-assembled 3D polystyrene PCs to enhance the photoluminescence (PL) of monolayer molybdenum disulfide (MoS₂). Through tuning the photonic bandgap of the polystyrene crystals to overlap with the direct emission band of monolayer MoS₂, the MoS₂/3D-PC heterostructure showed a maximum 12-fold PL enhancement, and Rabi splitting was also observed in the reflection spectrum. The heterostructure is expected to be useful in nanophotonic emitting devices.

Polarization-Dependent Purcell Enhancement on a Two-Dimensional h-BN/WS2 Light Emitter with a Dielectric Plasmonic Nanocavity

Integrating two-dimensional (2D) transition-metal dichalcogenides (TMDCs) into dielectric plasmonic nanostructures enables the miniaturization of on-chip nanophotonic devices. Here we report on a high-quality light emitter based on the newly designed 2D h-BN/WS₂ heterostructure integrated with an array of TiO₂ nanostripes. Different from a traditional strongly coupled system such as the TMDCs/metallic plasmonic nanostructure, we first employ dielectric nanocavities and achieve a Purcell enhancement on the nanoscale at room temperature. Furthermore, we demonstrate that the light emission strength can be effectively controlled by tuning the polarization configuration. Such a polarization dependence meanwhile could be proof of the resonant energy transfer theory of dipole-dipole coupling between TMDCs and a dielectric nanostructure. This work gains experimental and simulated insights into modified spontaneous emission with dielectric nanoplasmonic platforms, presenting a promising route toward practical applications of 2D semiconducting photonic emitters on a silica-based chip.

Interfacing transition metal dichalcogenides with chromium germanium telluride quantum dots for controllable light-matter interactions

In this work, we unravel a facile solution-based method to prepare chromium germanium telluride, Cr₂Ge₂Te₆ (CGT) quantum dots (QDs), which present strong light-matter interactions with monolayer transition metal dichalcogenides (TMDs) in their CGT/TMD vertical heterostructures. The heterostructures' optoelectronic properties were controlled by simply varying the QDs thickness. We observed contrasting emissions from monolayer TMDs in the various CGT QDs-TMDs (of WS₂, WSe₂ and MoS₂) heterostructures depending on the density of QDs in the heterostructures. Low-density CGT QDs-based heterostructures demonstrated a reduced light emission intensity compared to the isolated monolayers, but with an increased trion ratio due to the electron doping effect of CGT QDs. In contrast, high-density CGT QDs-based heterostructures showed an increased light emission intensity and a broadened, red-shifted emission peak in comparison to the bare TMDs, attributed to the enhanced optical absorption in the heterostructures arising from the assembled CGT QDs. Finally, proof-of-concept field-effect transistor (FET) and photodetector devices based on the created CGT QDs-WS₂ heterostructures were designed, which showed an enhanced optoelectronic performance.

Highly Polarized Single Photons from Strain-Induced Quasi-1D Localized Excitons in WSe2

Single photon emission from localized excitons in two-dimensional (2D) materials has been extensively investigated because of its relevance for quantum information applications. Prerequisites are the availability of photons with high purity polarization and controllable polarization orientation that can be integrated with optical cavities. Here, deformation strain along edges of prepatterned square-shaped substrate protrusions is exploited to induce quasi-one-dimensional (1D) localized excitons in WSe₂ monolayers as an elegant way to get photons that fulfill these requirements. At zero magnetic field, the emission is linearly polarized with 95% purity because exciton states are valley hybridized with equal shares of both valleys and predominant emission from excitons with a dipole moment along the elongated direction. In a strong field, one valley is favored and the linear polarization is converted to high-purity circular polarization. This deterministic control over polarization purity and orientation is a valuable asset in the context of integrated quantum photonics.

Enhanced Spontaneous Emission of Monolayer MoS2 on Epitaxially Grown Titanium Nitride Epsilon-Near-Zero Thin Films

Room-temperature photoluminescence enhancement of molybdenum disulfide (MoS₂) monolayers on epitaxial titanium nitride (TiN) thin films grown by molecular-beam-epitaxy as well as magnetron-sputtered TiN films is observed by a confocal laser scanning microscope with excitation wavelengths covering the transition of TiN's macroscopic optical properties from dielectric to plasmonic. The photoluminescence enhancement increases as TiN becomes more metallic, and strong enhancement is obtained at the excitation wavelengths equal to or longer than the epsilon-near-zero (ENZ) wavelength of TiN films. A good agreement is observed between measured and calculated enhancements. The enhancement is attributed to the increased excitation field in MoS₂ at TiN's ENZ wavelength and interference effects for thick spacers that separate the MoS₂ flakes from TiN films in the metallic regime. This study enriches the fundamental understanding of emission properties on ENZ substrates that could be important for the development of advanced nanoscale lasers/light sources, optical/biosensors, and nano-optoelectronic devices.

Upconversion Photovoltaic Effect of WS2/2D Perovskite Heterostructures by Two-Photon Absorption

Photovoltaic devices work by converting sunlight energy into electric energy. The efficiency of current photovoltaic devices, however, is significantly limited by the transmission loss of photons with energies below the bandgap of channel semiconductors, which can be circumvented by photon energy upconversion. Energy upconversion has been widely employed to improve the efficiency of traditional solar cells. However, the employment of energy upconversion in two-dimensional (2D) heterostructure photovoltaic devices has not been investigated yet. Here, we report the upconversion photovoltaic effect of WS₂ monolayer/(C₆H₅C₂H₄NH₃)₂PbI₄ (PEPI) 2D perovskite heterostructures by below-bandgap two-photon absorption via a virtual intermediate state. An open circuit voltage of 0.37 V and short circuit current of 7.4 pA are obtained with a photoresponsivity of 771 pA/W and current on/off ratio of 130:1. This work demonstrates that upconversion by two-photon absorption may potentially be a strategy for boosting the efficiency of 2D material-based photovoltaic devices by virtue of the absorption of photons below the bandgap energy of channel semiconductors.

Photoluminescence upconversion of 2D materials and applications

Photoluminescence (PL) upconversion is a phenomenon involving light-matter interactions, where the energy of emitted photons is higher than that of the incident photons. PL upconversion is an intriguing process in two-dimensional materials and specifically designed 2D heterostructures, which have potential upconversion applications in optoelectronic devices, bioimaging, and semiconductor cooling. In this review, we focus on the recent advances in photoluminescence upconversion in two-dimensional materials and their heterostructures. We discuss the upconversion mechanisms, applications, and future outlook of upconversion in two-dimensional materials.

Spectrally Selective Mid-Wave Infrared Detection Using Fabry-Pérot Cavity Enhanced Black Phosphorus 2D Photodiodes

Thin two-dimensional (2D) material absorbers have the potential to reduce volume-dependent thermal noise in infrared detectors. However, any reduction in noise must be balanced against lower absorption from the thin layer, which necessitates advanced optical architectures. Such architectures can be particularly effective for applications that require detection only within a specific narrow wavelength range. This study presents a Fabry-Pérot cavity enhanced bP/MoS₂ midwave infrared (MWIR) photodiode. This simple structure enables tunable narrow-band (down to 0.42 μm full width at half-maximum) photodetection in the 2-4 μm range by adjusting the thickness of the Fabry-Pérot cavity resonator. This is achieved while maintaining room-temperature performance metrics comparable to previously reported 2D MWIR detectors. Zero bias specific detectivity and responsivity values of up to 1.7 × 10⁹ cm Hz^1/2 W^-1 and 0.11 A W^-1 at λ = 3.0 μm are measured with a response time of less than 3 ns. These results introduce a promising family of 2D detectors with applications in MWIR spectroscopy.

Optoelectronic Properties of a van der Waals WS2 Monolayer/2D Perovskite Vertical Heterostructure

Two-dimensional (2D) Ruddlesden-Popper perovskites have been demonstrated to possess great potential for optical and optoelectronic devices. Because they exhibit better ambient stability than three-dimensional (3D) perovskites, they have been considered as potential substitutes for 3D perovskites as light absorbing layers to improve the photoresponsivity of monolayer transition metal dichalcogenide (TMDC)-based photodetectors. Investigation of the optoelectronic properties of TMDC monolayer/2D perovskite vertical heterostructures is however at an early stage. Here, we address the photovoltaic effect and the photodetection performance in tungsten disulfide (WS₂) monolayer/2D perovskite (C₆H₅C₂H₄NH₃)₂PbI₄ (PEPI) vertical heterostructures. A vertical device geometry with separate graphene contacts to both heterointerface constituents acted as a photovoltaic device and self-driven photodetector. The photovoltaic device exhibited an open circuit voltage of -0.57 V and a short circuit current of 41.6 nA. A photoresponsivity of 0.13 mA/W at the WS₂/PEPI heterointerface was achieved, which was signified by a factor of 5 compared to that from the individual WS₂ region. The current on/off ratio of the self-driven photodetector was approximately 1500. The photoresponsivity and external quantum efficiency of the self-driven photodetector were estimated to be 24.2 μA/W and 5.7 × 10^-5, respectively. This work corroborates that 2D perovskites are promising light absorbing layers in optoelectronic devices with a TMDC-based heterointerface.

Strain Effect Enhanced Ultrasensitive MoS2 Nanoscroll Avalanche Photodetector

Two-dimensional (2D) materials and their derived quasi one-dimensional structure provide incredible possibilities for the field of photoelectric detection due to their intrinsic optical and electrical properties. However, the photogenerated carriers in atomically thin media are poor due to the low optical absorption, which greatly limits their performance. Here, in the MoS₂ nanoscroll photodetector, we meticulously investigated the avalanche multiplication effect. The results show that by employing the nanoscroll structure, the required threshold electrical field for triggering avalanche multiplication is significantly lower than that of MoS₂ flake due to the modulation of the energy band and intervalley scattering through the strain effect. Consequently, avalanche multiplication could efficiently enhance the photoresponsivity to >10⁴ A/W. Furthermore, enhanced avalanche multiplication could be generalized to other TMDCs through theoretical prediction. The results not only are significant for the understanding of the intrinsic nature of 2D materials but also reveal meaningful advances in high-performance and low-power consumption photodetection.

Near-Unity Light Absorption in a Monolayer WS2 Van der Waals Heterostructure Cavity

Excitons in monolayer transition-metal-dichalcogenides (TMDs) dominate their optical response and exhibit strong light-matter interactions with lifetime-limited emission. While various approaches have been applied to enhance light-exciton interactions in TMDs, the achieved strength have been far below unity, and a complete picture of its underlying physical mechanisms and fundamental limits has not been provided. Here, we introduce a TMD-based van der Waals heterostructure cavity that provides near-unity excitonic absorption, and emission of excitonic complexes that are observed at ultralow excitation powers. Our results are in full agreement with a quantum theoretical framework introduced to describe the light-exciton-cavity interaction. We find that the subtle interplay between the radiative, nonradiative and dephasing decay rates plays a crucial role, and unveil a universal absorption law for excitons in 2D systems. This enhanced light-exciton interaction provides a platform for studying excitonic phase-transitions and quantum nonlinearities and enables new possibilities for 2D semiconductor-based optoelectronic devices.

Localized Nanoresonator Mode in Plasmonic Microcavities

Submicron-thick hexagonal boron nitride crystals embedded in noble metals form planar Fabry-Perot half-microcavities. Depositing Au nanoparticles on top of these microcavities forms previously unidentified angle- and polarization-sensitive nanoresonator modes that are tightly laterally confined by the nanoparticle. Comparing dark-field scattering with reflection spectroscopies shows plasmonic and Fabry-Perot-like enhancements magnify subtle interference contributions, which lead to unexpected redshifts in the dark-field spectra, explained by the presence of these new modes.

Tunable narrow terahertz absorption of one-dimensional photonic crystals embedded with Dirac semimetal-dielectric defect layers

The absorption characteristics of one-dimensional photonic crystals embedded with Dirac semimetal-dielectric defect layers are studied using the transfer matrix method. Numerical results show that our proposed structure can realize near-perfect narrow absorption for its strong field localization effects. The absorption frequency is tunable by adjusting the Fermi energy of the Dirac semimetal, temperature, permittivity of the dielectric, and the structural parameters. Moreover, double or multiple absorption channels can be achieved by changing the structure. Furthermore, the absorption performance is wide-angle and insensitive to polarization of the incident wave. Such properties exhibit potential value in designing selective absorbers and thermal detectors.

High-Energy Gain Upconversion in Monolayer Tungsten Disulfide Photodetectors

Photodetectors usually operate in the wavelength range with photon energy above the bandgap of channel semiconductors so that incident photons can excite electrons from valence band to conduction band to generate photocurrent. Here, however, we show that monolayer WS₂ photodetectors can detect photons with energy even lying 219 meV below the bandgap of WS₂ at room temperature. With the increase of excitation wavelength from 620 to 680 nm, photoresponsivity varies from 551 to 59 mA/W. This anomalous phenomenon is ascribed to energy upconversion, which is a combination effect of one-photon excitation and multiphonon absorption through an intermediate state created most likely by sulfur divacancy with oxygen adsorption. These findings will arouse research interests on other upconversion optoelectronic devices, photovoltaic devices, for example, of monolayer transition metal dichalcogenides (TMDCs).

High-Performance, Room Temperature, Ultra-Broadband Photodetectors Based on Air-Stable PdSe2

Photodetection over a broad spectral range is crucial for optoelectronic applications such as sensing, imaging, and communication. Herein, a high-performance ultra-broadband photodetector based on PdSe₂ with unique pentagonal atomic structure is reported. The photodetector responds from visible to mid-infrared range (up to ≈4.05 µm), and operates stably in ambient and at room temperature. It promises improved applications compared to conventional mid-infrared photodetectors. The highest responsivity and external quantum efficiency achieved are 708 A W^-1 and 82 700%, respectively, at the wavelength of 1064 nm. Efficient optical absorption beyond 8 µm is observed, indicating that the photodetection range can extend to longer than 4.05 µm. Owing to the low crystalline symmetry of layered PdSe₂ , anisotropic properties of the photodetectors are observed. This emerging material shows potential for future infrared optoelectronics and novel devices in which anisotropic properties are desirable.

Photoluminescence Upconversion by Defects in Hexagonal Boron Nitride

Hexagonal boron nitride (h-BN) was recently reported to display single photon emission from ultraviolet to near-infrared range due to the existence of defects. Single photon emission has potential applications in quantum information processing and optoelectronics. These findings trigger increasing research interests in h-BN defects, such as revealing the nature of the defects. Here, we report another intriguing defect property in h-BN, namely photoluminescence (PL) upconversion (anti-Stokes process). The energy gain by the PL upconversion is about 162 meV. The anomalous PL upconversion is attributed to optical phonon absorption in the one-photon excitation process, based on excitation power, excitation wavelength, and temperature-dependence investigations. Possible constitutions of the defects are discussed from the results of scanning transmission electron microscopy (STEM) studies and theoretical calculations. These findings show that defects in h-BN exhibit strong defect-phonon coupling. The results from STEM and theoretical calculations are beneficial for understanding the constitution of the h-BN defects.