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Vol. 4 ， Issue 1 ， 2015
- Abstract：In this work, the fundamental mechanism of ultrabright fluorescence from surface-modified colloidal silicon quantum dots is investigated in depth using ultrafast spectroscopy. The underlying energy band structure corresponding to such highly efficient direct bandgap-like emissions in our surface-modified silicon quantum dots is unraveled by analyzing the transient optical spectrum, which demonstrates the significant effect of surface molecular engineering. It is observed that special surface modification, which creates novel surface states, is responsible for the different emission wavelengths and the significant improvement in the photoluminescence quantum yields. Following this essential understanding, surface-modified silicon quantum dots with deep blue to orange emission are successfully prepared without changing their sizes.Keywords：quantum confinement;silicon quantum dots;surface molecular engineering;ultrafast spectroscopy;wave function modification
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Published：2024-09-26 - Abstract：Organic light-emitting diodes (OLEDs) have gained considerable attention because of their use of inherently flexible materials and their compatibility with facile roll-to-roll and printing processes. In addition to high efficiency, flexibility and transparency, reliable color tunability of solid state light sources is a desirable feature in the lighting and display industry. Here, we demonstrate a device concept for highly efficient organic light-emitting devices whose emission color can be easily adjusted from deep-blue through cold-white and warm-white to saturated yellow. Our approach exploits the different polarities of the positive and negative half-cycles of an alternating current (AC) driving signal to independently address a fluorescent blue emission unit and a phosphorescent yellow emission unit which are vertically stacked on top of each other. The electrode design is optimized for simple fabrication and driving and allows for two-terminal operation by a single source. The presented concept for color-tunable OLEDs is compatible with application requirements and versatile in terms of emitter combinations.Keywords：alternating current;color mixing;color tuning;vertical stacking;white organic light-emitting devices
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Published：2024-09-26 - Abstract：Multicolor organic electrochromic materials are important for the generation of full-color devices. However, achieving multiple colors using a single-molecule material has proved challenging. In this study, a multicolor electrochromic prototype device is generated by integrating medium engineering/in situ 'electro base'/laminated electrode technologies with the simple flying fish-shaped methyl ketone TM1. This multicolor electrochromic (green, blue and magenta) device is durable and has a high coloration efficiency (350 cm2 C−1), a fast switching time (50 ms) and superior reversibility. This study is a successful attempt to integrate solvatochromism and basochromism in an electronic display. This integration not only introduces a new avenue for color tuning, in addition to the structural design of the colorant, but will also inspire further developments in the tuning of many other properties by this medium engineering approach, such as conductance and the redox property, and thereby accelerate versatile applications in data recording, ultrathin flexible displays, and optical communication and sensing.Keywords：electrochromic materials;laminated electrodes;microenvironment;multicolor devices;solvatochromic materials
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Published：2024-09-26 - Abstract：Versatile devices, especially tunable ones, for terahertz imaging, sensing and high-speed communication, are in high demand. Liquid crystal based components are perfect candidates in the optical range; however, they encounter significant challenges in the terahertz band, particularly the lack of highly transparent electrodes and the drawbacks induced by a thick cell. Here, a strategy to overcome all these challenges is proposed: Few-layer porous graphene is employed as an electrode with a transmittance of more than 98%. A subwavelength metal wire grid is utilized as an integrated high-efficiency electrode and polarizer. The homogeneous alignment of a high-birefringence liquid crystal is implemented on both frail electrodes via a non-contact photo-alignment technique. A tunable terahertz waveplate is thus obtained. Its polarization evolution is directly demonstrated. Furthermore, quarter-wave plates that are electrically controllable over the entire testing range are achieved by stacking two cells. The proposed solution may pave a simple and bright road toward the development of various liquid crystal terahertz apparatuses.Keywords：liquid crystal;porous graphene;terahertz;waveplate
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Published：2024-09-26 - Abstract：Carrier multiplication is demonstrated in a solid-state dispersion of germanium nanocrystals in a silicon–dioxide matrix. This is performed by comparing ultrafast photo-induced absorption transients at different pump photon energies below and above the threshold energy for this process. The average germanium nanocrystal size is approximately 5–6 nm, as inferred from photoluminescence and Raman spectra. A carrier multiplication efficiency of approximately 190% is measured for photo-excitation at 2.8 times the optical bandgap of germanium nanocrystals, deduced from their photoluminescence spectra.Keywords：carrier multiplication;germanium nanocrystals;transient absorption
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Published：2024-09-26 - Abstract：Energy-efficient electro-optic modulators are at the heart of short-reach optical interconnects, and silicon photonics is considered the leading technology for realizing such devices. However, the performance of all-silicon devices is limited by intrinsic material properties. In particular, the absence of linear electro-optic effects in silicon renders the integration of energy-efficient photonic–electronic interfaces challenging. Silicon–organic hybrid (SOH) integration can overcome these limitations by combining nanophotonic silicon waveguides with organic cladding materials, thereby offering the prospect of designing optical properties by molecular engineering. In this paper, we demonstrate an SOH Mach–Zehnder modulator with unprecedented efficiency: the 1-mm-long device consumes only 0.7 fJ bit−1 to generate a 12.5 Gbit s−1 data stream with a bit-error ratio below the threshold for hard-decision forward-error correction. This power consumption represents the lowest value demonstrated for a non-resonant Mach–Zehnder modulator in any material system. It is enabled by a novel class of organic electro-optic materials that are designed for high chromophore density and enhanced molecular orientation. The device features an electro-optic coefficient of r33≈180 pm V−1 and can be operated at data rates of up to 40 Gbit s−1.Keywords：electro-optic materials;electro-optic modulation;nonlinear organic materials;silicon-organic hybrid
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Published：2024-09-26 - Abstract：High-sensitivity photodetection is at the heart of many optoelectronic applications, including spectroscopy, imaging, surveillance, remote sensing and medical diagnostics. Achieving the highest possible sensitivity for a given photodetector technology requires the development of ultra-small-footprint detectors, as the noise sources scale with the area of the detector. This must be accomplished while sacrificing neither the optically active area of the detector nor its responsivity. Currently, such designs are based on diffraction-limited approaches using optical lenses. Here, we employ a plasmonic flat-lens bull's eye structure (BES) to concentrate and focus light into a nanoscale colloidal quantum dot (CQD) photodetector. The plasmonic lenses function as nanofocusing resonant structures that simultaneously offer color selectivity and enhanced sensitivity. Herein, we demonstrate the first CQD photodetector with a nanoscale footprint, the optically active area of which is determined by the BES; this detector represents an exciting opportunity for high-sensitivity sensing.Keywords：PbS quantum dots;photodetectors;plasmonic bull's eye structures;plasmonic lens
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Published：2024-09-26 - Abstract：As a potential candidate for quantum computation and metrology, the nitrogen vacancy (NV) center in diamond presents both challenges and opportunities resulting from charge-state conversion. By utilizing different lasers for the photon-induced charge-state conversion, we achieved subdiffraction charge-state manipulation. The charge-state depletion (CSD) microscopy resolution was improved to 4.1 nm by optimizing the laser pulse sequences. Subsequently, the electron spin-state dynamics of adjacent NV centers were selectively detected via the CSD. The experimental results demonstrated that the CSD can improve the spatial resolution of the measurement of NV centers for nanoscale sensing and quantum information.Keywords：charge state;NV center;photon ionization;super-resolution microscopy
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Published：2024-09-26 - Abstract：The high-precision integration of three-dimensional (3D) microoptical components into microfluidics in a customizable manner is crucial for optical sensing, fluorescence analysis, and cell detection in optofluidic applications; however, it remains challenging for current microfabrication technologies. This paper reports the in-channel integration of flexible two-dimensional (2D) and 3D polymer microoptical devices into glass microfluidics by developing a novel technique: flat scaffold-supported hybrid femtosecond laser microfabrication (FSS-HFLM). The scaffold with an optimal thickness of 1–5 µm is fabricated on the lower internal surface of a microfluidic channel to improve the integration of high-precision microoptical devices on the scaffold by eliminating any undulated internal channel surface caused by wet etching. As a proof of demonstration, two types of typical microoptical devices, namely, 2D Fresnel zone plates (FZPs) and 3D refractive microlens arrays (MLAs), are integrated. These devices exhibit multicolor focal spots, elongated (> three times) focal length and imaging of the characters 'RIKEN' in a liquid channel. The resulting optofluidic chips are further used for coupling-free white-light cell counting with a success rate as high as 93%. An optofluidic system with two MLAs and a W-filter is also designed and fabricated for more advanced cell filtering/counting applications.Keywords：cell counting;3D optofluidic chips;hybrid femtosecond laser microfabrication;microlens arrays;two-photon polymerization
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Published：2024-09-26 - Abstract：To attain high efficiencies in hybrid white organic light-emitting diodes (WOLEDs), mutual quenching of the fluorophors and phosphors should be minimized. Efforts have been devoted to reducing the triplet quenching of phosphors; however, the quenching of fluorophors by the external heavy-atom effect (EHA) introduced by the phosphors is often ignored. Here, we observed that conventional fluorophors and fluorophors with thermally activated delayed fluorescence (TADF) behave differently in the presence of EHA perturbers. The efficiencies of the conventional fluorophors suffer greatly from the EHA, whereas the TADF fluorophors exhibit negligible changes, which makes TADF materials ideal fluorophors for hybrid devices. WOLEDs using a blue TADF fluorophor and an orange phosphor achieve a maximum forward viewing external quantum efficiency of 19.6% and a maximum forward viewing power efficiency of 50.2 lm W−1, among the best values for hybrid WOLEDs. This report is the first time that the EHA effect has been considered in hybrid WOLEDs and that a general strategy toward highly efficient hybrid WOLEDs with simple structures is proposed.Keywords：external heavy-atom effect;hybrid white OLEDs;thermally activated delayed fluorescence
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Published：2024-09-26 - Abstract：Recent theoretical and experimental studies have indicated the existence of a new stable phase of carbon with mixed sp2 and sp3 hybridized bonds—diaphite. Such a two-layered structure with sp2/sp3 bonds may be observed after the photostimulation of highly oriented pyrolytic graphene with femtosecond laser pulses. This hidden multistability of graphene may be used to create a semiconducting phase immersed in the semimetallic continuum, resulting in bandgap opening. We demonstrate that bandgap opening and light emission from graphene is possible using continuous-wave laser beams with wavelengths from the visible (405 nm) to the near-infrared range (975 nm). We demonstrate that without the application of cooling, the effective temperature of the emitting sample remains lower than 900 K, which is far below the value predicted by the theory of black-body radiation. Moreover, light emission from a graphene sample may be observed at temperatures as low as 10 K.Keywords：ceramics;graphene;photoluminescence;white light
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Published：2024-09-26 - Abstract：The rare earth Er3+ and Yb3+ codoped system is the most attractive for showcasing energy transfer upconversion. This system can generate green and red emissions from Er3+ under infrared excitation of the sensitizer Yb3+. It is well known that the red-emitting state can be populated from the upper green-emitting state. The contribution of multiphonon relaxation to this population is generally considered important at low excitation densities. Here, we demonstrate for the first time the importance of a previously proposed but neglected mechanism described as a cross relaxation energy transfer from Er3+ to Yb3+, followed by an energy back transfer within the same Er3+–Yb3+ pair. A luminescence spectroscopy study of cubic Y2O3: Er3+, Yb3+ indicates that this mechanism can be more efficient than multiphonon relaxation, and it can even make a major contribution to the red upconversion. The study also revealed that the energy transfers involved in this mechanism take place only in the nearest Er3+–Yb3+ pairs, and thus, it is fast and efficient at low excitation densities. Our results enable a better understanding of upconversion processes and properties in the Er3+–Yb3+ system.Keywords：energy transfer;erbium–ytterbium system;upconversion luminescence
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Published：2024-09-26 - Abstract：We show that highly doped graphene ribbons can support surface plasmons at near-infrared frequencies when their width is in the nanometer range, leading to important nonlocal and finite quantum-size corrections, such as sizable blueshifts. The magnitude of these effects is assessed by comparing classical and quantum-mechanical models to describe graphene plasmons. More precisely, we examine individual and interacting 6–8 nm wide zigzag and armchair ribbons doped to 0.4–1.5 eV Fermi energies. We find a strong influence of nonlocal effects on the orientation of graphene edges, with plasmons in zigzag ribbons undergoing strong quenching when their energy is below the Fermi level. Nonlocality is also affecting the hybridization between ribbon plasmons in dimers and arrays for separations below a few nanometers. Remarkably, the removal of a single row of atomic bonds in a ribbon produces a strong plasmon frequency shift, whereas the removal of bonds along an array of rows separated by several nanometers in an extended sheet causes a dramatic increase in the absorption. Besides the fundamental interest of these results, our work supports the use of narrow ribbons to achieve electro-optical modulation in the near infrared.Keywords：graphene;graphene plasmons;graphene ribbons;metamaterials;plasmonics
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Published：2024-09-26 - Abstract：Male Lawes's Parotia, a bird of paradise, use the highly directional reflection of the structurally colored, brilliant-silvery occipital feathers in their courtship display. As in other birds, the structural coloration is produced by ordered melanin pigmentation. The barbules of the Parotia's occipital feathers, with thickness ~3 µm, contain 6–7 layers of densely packed melanin rodlets (diameter ~0.25 µm, length ~2 µm). The effectively ~0.2 µm thick melanin layers separated by ~0.2 µm thick keratin layers create a multilayer interference reflector. Reflectance measurements yielded peak wavelengths in the near-infrared at ~1.3 µm, i.e., far outside the visible wavelength range. With the Jamin-Lebedeff interference microscopy method recently developed for pigmented media, we here determined the refractive index of the intact barbules. We thus derived the wavelength dependence of the refractive index of the barbules' melanin to be 1.7–1.8 in the visible wavelength range. Implementing the anatomical and refractive index data in an optical multilayer model, we calculated the barbules' reflectance, transmittance and absorptance spectra, thereby confirming measured spectra.Keywords：bird of paradise;interference reflector;iridescence;Jamin-Lebedeff microscopy;multilayer
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Published：2024-09-26
Original Articles
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