Focus on nanophotonics and nano-optics

We studied the structural, electronic, and optical characters of SiS₂, a new type of group IV–VI two-dimensional semiconductor, in this article. We focused on monolayer SiS₂ and its characteristic changes when different strains are applied on it. Results reveal that the monolayer SiS₂ is dynamically stable when no strain is applied. In terms of electronic properties, it remains a semiconductor under applied strain within the range from −10% to 10%. Besides, its indirect band-gap is altered regularly after applying a strain, whereas different strains lead to various changing trends. As for its optical properties, it exhibits remarkable transparency for infrared and most visible light. Its main absorption and reflection regions lie in the blue and ultraviolet areas. The applied uniaxial strain causes its different optical properties along the armchair direction and zigzag direction. Moreover, the tensile strain could tune its optical properties more effectively than the compressive strain. When different strains are applied, the major changes are in blue and ultraviolet regions, but only minor changes can be found in infrared and visible regions. So its optical properties reveal good stability in infrared and visible regions. Therefore, SiS₂ has a promising prospect in nano-electronic and nano-photoelectric devices.

All-inorganic cesium lead bromine (CsPbBr₃) perovskites quantum dots (QDs) are one of the most photoelectric materials due to their high absorption coefficient, pronounced quantum-size effect, tunable optical property. Here, a self-powered PD based on all-inorganic CsPbBr₃ perovskites QDs is fabricated and demonstrated. The light-induced pyroelectric effect is utilized to modulate the optoelectronic processes without the external power supply. The working mechanism of the PD is carefully investigated upon 532 nm laser illumination and the minimum recognizable response time of the self-powered PD is 1.5 μs, which are faster than those of most previously reported wurtzite nanostructure PDs. Meanwhile, the frequency and temperature independence of the self-powered PD are experimented and summarized. The self-powered PD with high performance is expected to have extensive applications in solar cell, energy harvesting, resistive random access memory.

The physical laws of diffraction limit the spatial resolution of optical systems. In contrary to most superresolution microscopy approaches used today, in our novel idea we are aiming to overcome this limit by developing a spatially resolved illumination source based on semiconductor nanoscale light emitting diode (nanoLED) arrays with individual pixel control. We present and discuss the results of optical simulations performed for such nanoLED emitter arrays and analyze the theoretical limits of this approach. As possible designs we study arrays of GaN nanofins and nanorods (obtained by etching nanofin arrays), with InGaN/GaN multi quantum wells embedded as active regions. We find that a suitable choice of the array dimensions leads to a reasonably directed light output and concentration of the optical power in the near field around an activated pixel. As a consequence, the spatial resolution for this type of microscopy should only be limited by the pixel pitch, and no longer by the optical diffraction. Realization of optimized nanoLED arrays has a potential to open new field of chip based superresolution microscopy, making super-high spatial resolution ubiquitously available.

Structural phase transition induced by temperature or voltage in phase change materials has been used for many tunable photonic applications. Exploiting reversible and sub-ns fast switching in antimony trisulfide (Sb₂S₃) from amorphous (Amp) to crystalline (Cry), we introduced a reflection modulator based on metal–dielectric–metal structure. The proposed design exhibits tunable, perfect, and multi-band absorption from visible to the near-infrared region. The reflection response of the system shows >99% absorption of light at normal incidence. The maximum achievable modulation efficiency with a narrow line width is ∼98%. Interestingly, the designed cavity supports critical resonance in an ultrathin (∼λ/15) Sb₂S₃ film with perfect, broadband, and tunable absorption. Finally, we proposed a novel hybrid cavity design formed of Cry and Amp Sb₂S₃ thin films side-by-side to realize an optical modulator via relative motion between the incident light beam and cavity. The proposed lithographic free structure can be also used for filtering, optical switching, ultrathin photo-detection, solar energy harvesting, and other energy applications.

Infrared (IR) and terahertz plasmons in two-dimensional (2D) materials are commonly excited by metallic or dielectric grating couplers with deep-submicron features fabricated by e-beam lithography. Mass reproduction of such gratings at macroscopic scales is a labor-consuming and expensive technology. Here, we show that localized plasmons in graphene can be generated on macroscopic scales with couplers based on randomly oriented particle-like nanorods (NRs) in close proximity to graphene layer. We monitor the excitation of graphene plasmons indirectly by tracking the changes in reflection/absorption spectra of methylene blue (MB) or polymethyl methacrylate(PMMA) molecules deposited on the structure. Hybridization of spectrally broad graphene plasmon and narrow molecular oscillators results in enhanced oscillator strengths and Fano scattering related lines asymmetry in reflection spectra.

Some advances have been achieved in developing heterojunctions consisting of indium-gallium-zinc oxide (a-IGZO) films and two dimensional (2D) van der Waals materials for optoelectronic applications in recent years, however, the improvement of IGZO channel itself via constructing such heterojunctions is rarely reported. Here, we report the huge improvement in photoresponse performances for the IGZO phototransistor devices by introducing boron nitride (BN)/black phosphorus (BP) interface engineering. By creating an appropriate band bending and an efficient photo-generated carrier transfer path between IGZO and BP, the recombination of the photo-generated carriers in the IGZO channel is significantly suppressed. As a result, the corresponding photoresponsivity at a wavelength of 447 nm can be promoted from 0.05 A W⁻¹ to 0.3 A W⁻¹. A corresponding maximum external quantum efficiency of 83.4% was obtained for the BN/BP decorated IGZO phototransistor. The results imply that such interface engineering via 2D materials can be used as a general route to high performance oxide-semiconductor based optoelectronic devices.

Symmetric droplet-etched quantum dots (QDs) are the leading candidate for generating high-performance polarization-entangled photon pairs. One of the challenges is how to precisely engineer the properties of QDs by controlling the morphology of etched nanoholes. In this paper, we systematically investigate the influence of the underlying material, showing the morphological evolution of the nanohole structure as well as symmetric GaAs QDs with an average fine-structure splitting (FSS) of (5.9 ± 1.2) μeV. Moreover, we develop a theoretical model that quantitatively reproduces the experimental data and provides insights into the mechanisms governing the relationship between the anisotropy of nanoholes in the $\left[ {1\bar 10} \right]$ crystallographic direction and the growth parameters. Our theoretical analysis also indicates how to improve the symmetry of nanoholes to meet the requirements for implementing QDs in entangled photon sources.

Amorphous hydrogenated silicon carbonitride (a-SiCN:H) thin films were grown using electron cyclotron resonance chemical vapour deposition using a mixture of methane, nitrogen, and silane as precursors. The origin of the variation of macroscopic properties such as hardness (H), elastic modulus (E), photoluminescence (PL), and the optical band gap was investigated through their correlation with the microscopic features of a-SiCN:H thin films as a function of the process parameters, including the deposition temperature and methane gas flow rate. From a microstructural perspective, the thin films were investigated using x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, elastic recoil detection, transmission electron microscopy, and x-ray diffraction. It is verified that an increase of the substrate temperature resulted in the substitution of hydrogen atoms mainly by carbon atoms, causing the density of the silicon carbide-related structures to increase in the amorphous structure of the a-SiCN:H thin films. Hardness and elastic modulus were found to increase with the deposition temperature and decreased with an increase of the methane gas flow during the deposition, resulting in higher carbon content in the films. The observed changes are ascribed to the reduced density of the weak hydrogen terminated bonds and the variation of the relative bond density of Si–C to Si–N bonds. In addition, the thin films were depth profiled using a slow positron beam to investigate the role of vacancies. The observed increase of the positronium formation with increasing deposition temperature was found to correlate with the variation of PL, where an enhancement of the visible emission originating from carbon-related defects was observed. A set of optimized process parameters to fabricate a-SiCN:H thin films with improved visible emission and hardness properties is suggested.

The recently emerged concept of all-dielectric nanophotonics based on optical Mie resonances in high-index dielectric nanoparticles has proven to be a promising pathway to boost light–matter interactions at the nanoscale. In this work, we discuss the opportunities enabled by the interaction of dielectric nanoresonators with 2D transition metal dichalcogenides (2D TMDCs), leading to weak and strong coupling regimes. We perform a comprehensive analysis of bright exciton photoluminescence (PL) enhancement from various 2D TMDCs, including WS₂, MoS₂, WSe₂, and MoSe₂ via their coupling to Mie resonances of a silicon nanoparticle. For each case, we find the system parameters corresponding to maximal PL enhancement taking into account excitation rate, Purcell factor and radiation efficiency. We demonstrate numerically that all-dielectric Si nanoantennas can significantly enhance the PL intensity from 2D TMDC by a factor of hundred through precise optimization of the geometrical and material parameters. Our results may be useful for high-efficiency 2D TMDC-based optoelectronic, nanophotonic, and quantum optical devices.

Nonradiating sources are nontrivial charge–current distributions that do not generate fields outside the source domain. The pursuit of their possible existence has fascinated several generations of physicists and triggered developments in various branches of science ranging from medical imaging to dark matter. Recently, one of the most fundamental types of nonradiating sources, named anapole states, has been realized in nanophotonics regime and soon spurred considerable research efforts and widespread interest. A series of astounding advances have been achieved within a very short period of time, uncovering the great potential of anapole states in many aspects such as lasing, sensing, metamaterials, and nonlinear optics. In this review, we provide a detailed account of anapole states in nanophotonics research, encompassing their basic concepts, historical origins, and new physical effects. We discuss the recent research frontiers in understanding and employing optical anapoles and provide an outlook for this vibrant field of research.

Bottom-up fabricated single-junction III–V nanowire array solar cells have shown efficiency up to 15.3%, which is approximately half of the conventional Shockley–Queisser detailed balance efficiency limit of 33.6%. Here, based on numerical and analytical opto-electronics modeling and analysis, we give guidelines for (i) geometry that gives strong absorption as well as (ii) the design of efficient p–n junction and electrical contacts in the nanowires to reach 20% and 25% efficiency. We exemplify the impact of eight different optical and electrical loss mechanisms in a 15% and a 25% design. We also provide an analytical equation for estimating the efficiency drop due to resistive losses in the top contact layer for varying cell size.

In last few decades, micro- and nano-fabrication techniques based on photolithography and electron beam lithography have advanced greatly, mainly in the field of semiconductor fabrication. Such techniques are generally transferrable to the fabrication of plasmonic structures and metamaterials. However, plasmonic devices often require a transparent insulating substrate to be operational at visible or near-infrared wavelengths. Here we report a resist-on-metal bilayer lift-off technique enabling the fabrication of plasmonic structures on insulating substrates. The metal layer under the resist eliminates major difficulties in lithography, such as charging during electron beam exposure and uncontrolled diffuse optical scattering during photolithography. In addition, the resist-on-metal bilayer can be migrated to different substrates with minimal process alteration, because the material properties of the substrate, such as secondary electron emission or optical reflectance, become irrelevant due to the shielding provided by the metal layer. As demonstrations, we fabricate large-scale plasmonic waveguides and Bragg gratings, adiabatically-modulated plasmonic waveguide couplers, and plasmonic nanoantenna arrays using the resist-on-metal bilayer lift-off process. The process can also be used to define structures formed of other materials such as dielectrics.

Photodetection at short- and mid-wavelength infrared (SWIR and MWIR) enables various sensing systems used in heat seeking, night vision, and spectroscopy. As a result, uncooled photodetection at these wavelengths is in high demand. However, these SWIR and MWIR photodetectors often suffer from high dark current, causing them to require bulky cooling accessories for operation. In this study, we argue for the feasibility of improving the room-temperature detectivity by significantly suppressing dark current. To realize this, we propose using (1) a nanowire-based platform to reduce the photoabsorber volume, which in turn reduces trap state population and hence generation-recombination current, and (2) p–n heterojunctions to prevent minority carrier diffusion from the large bandgap substrate into the nanowire absorber. We prove these concepts by demonstrating a comprehensive three-dimensional photoresponse model to explore the level of detectivity offered by vertical InAs(Sb) nanowire photodetector arrays with self-assembled plasmonic gratings. The resultant electrical simulations show that the dark current can be reduced by three to four orders at room temperature, leading to a peak detectivity greater than 3.5 × 10¹⁰ cm Hz^1/2 W⁻¹ within the wavelength regime of 2.0–3.4 μm, making it comparable to the best commercial and research InAs p–i–n homojunction photodiodes. In addition, we show that the plasmonic resonance peaks can be easily tuned by simply varying the exposed nanowire height. Finally, we investigate the impact of nanowire material properties, such as carrier mobility and carrier lifetime, on the nanowire photodetector detectivity. This work provides a roadmap for the electrical design of nanowire optoelectronic devices and stimulates further experimental validation for uncooled photodetectors at SWIR and MWIR.

The performance of nanowire-based devices is predominantly affected by nonradiative recombination on their surfaces, or sidewalls, due to large surface-to-volume ratios. A common approach to quantitatively characterize surface recombination is to implement time-resolved photoluminescence to correlate surface recombination velocity with measured minority carrier lifetime by a conventional analytical equation. However, after using numerical simulations based on a three-dimensional (3D) transient model, we assert that the correlation between minority carrier lifetime and surface recombination velocity is dependent on a more complex combination of factors, including nanowire geometry, energy-band alignment, and spatial carrier diffusion in 3D. To demonstrate this assertion, we use three cases—GaAs nanowires, InGaAs nanowires, and InGaAs inserts embedded in GaAs nanowires—and numerically calculate the carrier lifetimes by varying the surface recombination velocities. Using this information, we then investigate the intrinsic carrier dynamics within those 3D structures. We argue that the conventional analytical approach to determining surface recombination in nanowires is of limited applicability, and that a comprehensive computation in 3D can provide more accurate analysis. Our study provides a solid theoretical foundation to further understand surface characteristics and carrier dynamics for 3D nanostructured materials.

We report the structural, optical and electrical properties of GaAs quantum dots (QDs) embedded along GaP nanowires. The GaP nanowires contained p–i–n junctions with 15 consecutively grown GaAs QDs within the intrinsic region. The nanowires were grown by molecular beam epitaxy using the self-assisted vapor–liquid–solid process. The crystal structure of the NWs alternated between twinned ZB and WZ as the composition along the NW alternated between the GaP barriers and the GaAs QDs, respectively, leading to a polytypic structure with a periodic modulation of the NW sidewall facets. Photodetector devices containing QDs showed absorption beyond the bandgap of GaP in comparison to nanowires without QDs. Voltage-dependent measurements suggested a field emission process of carriers from the QDs.