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Vol. 4 ， Issue 4 ， 2015
- Abstract：In this review, we introduce the current state of the art of the growth technology of pure, lightly doped, and heavily doped (solid solution) nonlinear gallium selenide (GaSe) crystals that are able to generate broadband emission from the near infrared (IR) (0.8 μm) through the mid- and far-IR (terahertz (THz)) ranges and further into the millimeter wave (5.64 mm) range. For the first time, we show that appropriate doping is an efficient method controlling a range of the physical properties of GaSe crystals that are responsible for frequency conversion efficiency and exploitation parameters. After appropriate doping, uniform crystals grown by a modified technology with heat field rotation possess up to 3 times lower absorption coefficient in the main transparency window and THz range. Moreover, doping provides the following benefits: raises by up to 5 times the optical damage threshold; almost eliminates two-photon absorption; allows for dispersion control in the THz range independent of the mid-IR dispersion; and enables crystal processing in arbitrary directions due to the strengthened lattice. Finally, doped GaSe demonstrated better usefulness for processing compared with GaSe grown by the conventional technology and up to 15 times higher frequency conversion efficiency.Keywords：frequency conversion;GaSe;solid solution crystal;THz
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Published：2024-09-25 - Abstract：Serving as the electrical to optical converter, the on-chip silicon light source is an indispensable component of silicon photonic technologies and has long been pursued. Here, we briefly review the history and recent progress of a few promising contenders for on-chip light sources in terms of operating wavelength, pump condition, power consumption, and fabrication process. Additionally, the performance of each contender is also assessed with respect to thermal stability, which is a crucial parameter to consider in complex optoelectronic integrated circuits (OEICs) and optical interconnections. Currently, Ⅲ-Ⅴ-based silicon (Si) lasers formed via bonding techniques demonstrate the best performance and display the best opportunity for commercial usage in the near future. However, in the long term, direct hetero-epitaxial growth of Ⅲ–Ⅴ materials on Si seems more promising for low-cost, high-yield fabrication. The demonstration of high-performance quantum dot (QD) lasers monolithically grown on Si strongly forecasts its feasibility and enormous potential for on-chip lasers. The superior temperature-insensitive characteristics of the QD laser promote this design in large-scale high-density OEICs. The Germanium (Ge)-on-Si laser is also competitive for large-scale monolithic integration in the future. Compared with a Ⅲ-Ⅴ-based Si laser, the biggest potential advantage of a Ge-on-Si laser lies in its material and processing compatibility with Si technology. Additionally, the versatility of Ge facilitates photon emission, modulation, and detection simultaneously with a simple process complexity and low cost.Keywords：on-chip light sources;optical interconnections;optoelectronic integrated circuit;silicon photonics
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Published：2024-09-25
Reviews
- Abstract：We report the first demonstration of a unidirectional, isolator-free 2-μm thulium-doped fiber (TDF) laser, relying on the properties of the theta cavity (ring resonator with S-shaped feedback). The core pumped theta cavity TDF laser provides sub-Watt output power with a slope efficiency of 25%, a 2 dB flat tuning range of 1900–2050 nm, and a linewidth of 0.2 nm, and achieves the extinction ratio of 18–25 dB (depending on the feedback value) between the favored and suppressed lasing directions. It is shown that these characteristics are competitive with, if not superior to, those of conventional ring cavities. The simulation results of the linear and Kerr-nonlinear theta cavities are also presented, explaining certain unexpected features of the laser behavior and establishing the importance of the doped fiber nonlinearity on the spectral shaping of the emitted signal.Keywords：fiber laser;nonlinear amplifying loop mirror;short-wave infrared coherent sources;thulium-doped fiber
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Published：2024-09-25 - Abstract：Unlike inorganic quantum dots, fluorescent graphene quantum dots (GQDs) display excitation-dependent multiple color emission. In this study, we report N-doped GQDs (N-GQDs) with tailored single color emission by tuning π-conjugation degree, which is comparable to the inorganic quantum dot. Starting from citric acid and diethylenetriamine, as prepared N-GQDs display blue, green, and yellow light emission by changing the reaction solvent from water, dimethylformamide (DMF), and solvent free. The X-ray photoelectron spectroscopy, ultraviolet-visible spectra results clearly show the N-GQDs with blue emission (N-GQDs-B) have relatively short effective conjugation length and more carboxyl group because H2O is a polar protic solvent, which tends to donate proton to the reagent to depress the H2O elimination reaction. On the other hand, the polar aprotic solvent (DMF) cannot donate hydrogen, the elimination of H2O is promoted and more nitrogen units enter GQD framework. With the increase of effective π-conjugation length and N content, the emission band of N-GQDS red-shifts to green and yellow. We also demonstrate that N-GQDs could be a potential great biomarker for fluorescent bioimaging.Keywords：excitation wavelength independent;graphene quantum dots;nitrogen doped;pure color emission
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Published：2024-09-25 - Abstract：Graphene-plasmonic hybrid platforms have attracted an enormous amount of interest in surface-enhanced Raman scattering (SERS); however, the mechanism of employing graphene is still ambiguous, so clarification about the complex interaction among molecules, graphene, and plasmon processes is urgently needed. We report that the number of graphene layers controlled the plasmon-driven, surface-catalyzed reaction that converts para-aminothiophenol (PATP)-to-p,p′-dimercaptoazobenzene (DMAB) on chemically inert, graphene-coated, silver bowtie nanoantenna arrays. The catalytic reaction was monitored by SERS, which revealed that the catalytic reaction occurred on the chemical inertness monolayer graphene (1G)-coated silver nanostructures. The introduction of 1G enhances the plasmon-driven surface-catalyzed reaction of the conversion of PATP-to-p,p′-DMAB. The chemical reaction is suppressed by bilayer graphene. In the process of the catalytic reaction, the electron transfer from the PATP molecule to 1G-coated silver nanostructures. Subsequently, the transferred electrons on the graphene recombine with the hot-hole produced by the localized surface plasmon resonance of silver nanostructures. Then, a couple of PATP molecules lost electrons are catalyzed into the p,p′-DMAB molecule on the graphene surface. The experimental results were further supported by the finite-difference time-domain method and quantum chemical calculations.Keywords：graphene;graphene-mediated SERS;plasmon-driven reaction;SERS
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Published：2024-09-25 - Abstract：Optical selection rules fundamentally determine the optical transitions between energy states in a variety of physical systems, from hydrogen atoms to bulk crystals such as gallium arsenide. These rules are important for optoelectronic applications such as lasers, energy-dispersive X-ray spectroscopy, and quantum computation. Recently, single-layer transition metal dichalcogenides have been found to exhibit valleys in momentum space with nontrivial Berry curvature and excitons with large binding energy. However, there has been little study of how the unique valley degree of freedom combined with the strong excitonic effect influences the nonlinear optical excitation. Here, we report the discovery of nonlinear optical selection rules in monolayer WS2, an important candidate for visible 2D optoelectronics because of its high quantum yield and large direct bandgap. We experimentally demonstrated this principle for second-harmonic generation and two-photon luminescence (TPL). Moreover, the circularly polarized TPL and the study of its dynamics evince a sub-ps interexciton relaxation (2p → 1s). The discovery of this new optical selection rule in a valleytronic 2D system not only considerably enhances knowledge in this area but also establishes a foundation for the control of optical transitions that will be crucial for valley optoelectronic device applications such as 2D valley-polarized THz sources with 2p–1s transitions, optical switches, and coherent control for quantum computing.Keywords：nonlinear optical selection rule;2D materials;2p–1s exciton relaxation;valley exciton
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Published：2024-09-25 - Abstract：Conventional photoluminescence (PL) yields at most one emitted photon for each absorption event. Downconversion (or quantum cutting) materials can yield more than one photon by virtue of energy transfer processes between luminescent centers. In this work, we introduce Gd2O2S:Tm3+ as a multi-photon quantum cutter. It can convert near-infrared, visible, or ultraviolet photons into two, three, or four infrared photons of ~1800 nm, respectively. The cross-relaxation steps between Tm3+ ions that lead to quantum cutting are identified from (time-resolved) PL as a function of the Tm3+ concentration in the crystal. A model is presented that reproduces the way in which the Tm3+ concentration affects both the relative intensities of the various emission lines and the excited state dynamics and providing insight in the quantum cutting efficiency. Finally, we discuss the potential application of Gd2O2S:Tm3+ for spectral conversion to improve the efficiency of next-generation photovoltaics.Keywords：downconversion;infrared emission;quantum cutting;solar cells;spectral conversions
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Published：2024-09-25 - Abstract：In this paper, an irregular displacement-based lensless wide-field microscopy imaging platform is presented by combining digital in-line holography and computational pixel super-resolution using multi-frame processing. The samples are illuminated by a nearly coherent illumination system, where the hologram shadows are projected into a complementary metal-oxide semiconductor-based imaging sensor. To increase the resolution, a multi-frame pixel resolution approach is employed to produce a single holographic image from multiple frame observations of the scene, with small planar displacements. Displacements are resolved by a hybrid approach: (ⅰ) alignment of the LR images by a fast feature-based registration method, and (ⅱ) fine adjustment of the sub-pixel information using a continuous optimization approach designed to find the global optimum solution. Numerical method for phase-retrieval is applied to decode the signal and reconstruct the morphological details of the analyzed sample. The presented approach was evaluated with various biological samples including sperm and platelets, whose dimensions are in the order of a few microns. The obtained results demonstrate a spatial resolution of 1.55 μm on a field-of-view of ≈30 mm2.Keywords：digital in-line holography;image registration;lensless imaging;point-of-care platform;pixel super-resolution
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Published：2024-09-25 - Abstract：Graphene-doped polymer nanofibers are fabricated by taper drawing of solvated polyvinyl alcohol doped with liquid-phase exfoliated graphene flakes. Nanofibers drawn this way typically have diameters measured in hundreds of nanometers and lengths in tens of millimeters; they show excellent uniformity and surface smoothness for optical waveguiding. Owing to their tightly confined waveguiding behavior, light–matter interaction in these subwavelength-diameter nanofibers is significantly enhanced. Using approximately 1350-nm-wavelength femto-second pulses, we demonstrate saturable absorption behavior in these nanofibers with a saturation threshold down to 0.25 pJ pulse−1 (peak power ~1.3 W). Additionally, using 1064-nm-wavelength nanosecond pulses as switching light, we show all-optical modulation of a 1550-nm-wavelength signal light guided along a single nanofiber with a switching peak power of ~3.2 W.Keywords：graphene;nanofiber;optical modulation;saturable absorption
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Published：2024-09-25 - Abstract：We propose theoretically and demonstrate experimentally the generation of light pulses whose polarization varies temporally to cover selected areas of the Poincaré sphere with both tunable swirling speed and total duration (1 ps and 10 ps, respectively, in our implementation). The effect relies on the Rabi oscillations of two polariton polarized fields excited by two counter-polarized and delayed pulses. The superposition of the oscillating fields result in the precession of the Stokes vector of the emitted light while polariton lifetime imbalance results in its drift from a circle of controllable radius on the Poincaré sphere to a single point at long times. The positioning of the initial circle and final point allows to engineer the type of polarization spanning, including a full sweeping of the Poincaré sphere. The universality and simplicity of the scheme should allow for the deployment of time-varying full-Poincaré polarization fields in a variety of platforms, timescales, and regimes.Keywords：full Poincaré beams;polaritons;polarization;pulse shaping;Rabi oscillations
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Published：2024-09-25 - Abstract：In photodegenerative diseases such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD), progressive loss of vision occurs as a result of degeneration of the periphery of the retina and the macula, respectively. Current optogenetic stimulation-based approaches to vision restoration offer the advantages of cellular specificity, high resolution, and minimal invasiveness over electrode arrays; however, the clinical translation of optogenetic activation suffers from the lack of a method for the delivery of opsins into spatially targeted regions of a retina that has degenerated. Non-targeted opsin delivery through viral or non-viral methods to non-photodegenerated retinal areas will perturb these already functioning retinal regions. Furthermore, viral methods are subject to limitations on the delivery of large plasmids, such as fusion constructs of multiple spectrally separated opsins (e.g., channelrhodopsin-2 (ChR2), chimeric opsin variants (C1V1), ReaChR), which can provide higher photo-excitability than can a single narrow-band opsin under ambient light conditions. Here, we report the ultrafast near-infrared laser-based spatially targeted transfection of single and multiple opsins and present a comparison with the opsin expression distribution achieved using another non-viral, but non-targeted, transfection method, lipofection. Functional evaluation of cells transfected with multiple opsins using the laser method revealed a significantly higher white-light-induced photocurrent than in cells expressing a single opsin (ChR2). The laser-assisted targeted delivery of multiple opsin-encoding genes to the peripheral retina/macula is ideal for sensitizing retinal areas that have degenerated, thus paving the way toward the restoration of lost vision in RP/AMD patients.Keywords：gene delivery;optogenetics;optoporation;vision restoration
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Published：2024-09-25 - Abstract：The importance of integrated quantum photonics in the telecom band is based on the possibility of interfacing with the optical network infrastructure that was developed for classical communications. In this framework, femtosecond laser-written integrated photonic circuits, which have already been assessed for use in quantum information experiments in the 800-nm wavelength range, have great potential. In fact, these circuits, being written in glass, can be perfectly mode-matched at telecom wavelength to the in/out coupling fibers, which is a key requirement for a low-loss processing node in future quantum optical networks. In addition, for several applications, quantum photonic devices must be dynamically reconfigurable. Here, we experimentally demonstrate the high performance of femtosecond laser-written photonic circuits for use in quantum experiments in the telecom band, and we demonstrate the use of thermal shifters, which were also fabricated using the same femtosecond laser, to accurately tune such circuits. State-of-the-art manipulation of single- and two-photon states is demonstrated, with fringe visibilities greater than 95%. The results of this work open the way to the realization of reconfigurable quantum photonic circuits based on this technological platform.Keywords：femtosecond laser micromachining;integrated quantum photonics;thermal shifters;tunable optical circuits
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Published：2024-09-25 - Abstract：This article reports a new phosphorescent material, CaZnOS:Cu, that exhibits two types of mechano-optical conversion: mechanical quenching and mechanoluminescence. An intense mechanical quenching of phosphorescence corresponding to mechanical stimuli can be achieved in CaZnOS:Cu within a short decay time period. Over time, it gradually changes to mechanoluminescence when a mechanical load is applied. We propose that the mechanical quenching and mechanoluminescence arise from the different roles of shallow and deep traps in CaZnOS:Cu. CaZnOS:Cu has promising applications in monitoring mechanical stress in industrial plants, structures, and living bodies.Keywords：mechanical quenching;mechanoluminescence;phosphorescence;traps
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Published：2024-09-25 - Abstract：The lack of capability to quantify oxygen metabolism noninvasively impedes both fundamental investigation and clinical diagnosis of a wide spectrum of diseases including all the major blinding diseases such as age-related macular degeneration, diabetic retinopathy, and glaucoma. Using visible light optical coherence tomography (vis-OCT), we demonstrated accurate and robust measurement of retinal oxygen metabolic rate (rMRO2) noninvasively in rat eyes. We continuously monitored the regulatory response of oxygen consumption to a progressive hypoxic challenge. We found that both oxygen delivery, and rMRO2 increased from the highly regulated retinal circulation (RC) under hypoxia, by 0.28 ± 0.08 µL min−1 (p <0.001), and 0.20 ± 0.04 µL min−1 (p <0.001) per 100 mmHg systemic pO2 reduction, respectively. The increased oxygen extraction compensated for the deficient oxygen supply from the poorly regulated choroidal circulation. Results from an oxygen diffusion model based on previous oxygen electrode measurements corroborated our in vivo observations. We believe that vis-OCT has the potential to reveal the fundamental role of oxygen metabolism in various retinal diseases.Keywords：oxygen metabolism;retinal circulation;visible light optical coherence tomography
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Published：2024-09-25 - Abstract：Strain-engineered silicon nanocrystals (SiNCs) have recently been shown to possess direct bandgap. Here, we report the observation of a rich structure in the single-nanocrystal photoluminescence spectra of strain-engineered direct-bandgap SiNCs in the temperature range of 9–300 K. The relationship between individual types of spectra is discussed, and the numerical modeling of spectral diffusion of the experimentally acquired spectra reveals a common origin for most types. The intrinsic spectral shape is shown to be a structure that contains three peaks, approximately 150 meV apart, each of which possesses a Si phonon substructure. Narrow spectral lines, reaching ≤1 meV at 20 K, are detected. The observed temperature dependence of the spectral structure can be assigned to the radiative recombination of positively charged trions, in contrast to several previous reports linking a very similar shape to phonons in the surface capping layers. Our result serves as strong additional support for the direct-bandgap nature of the investigated SiNCs.Keywords：direct bandgap;low-temperature single-nanocrystal spectroscopy;silicon nanocrystals;trions
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Published：2024-09-25 - Abstract：We discover a new kind of surface wave on biaxial hyperbolic metamaterial, which, in the k-space, connects the two diabolical points (or conical singularities) of the equifrequency surface. Interestingly, the propagation of such surface wave is found to be sensitive to the refractive index of the surrounding dielectric medium, showing a convex, concave or flat phase front when the refractive index is varied. Furthermore, the surface wave shows an elliptically polarized state, in which helicity is dependent on the propagation direction. This feature can be utilized for the spin-controllable excitation of surface waves, opening a gateway towards integrated photonic circuits with reconfigurable functionalities.Keywords：metamaterial;spin orbital coupling;surface wave
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Published：2024-09-25 - Abstract：As one of the recent advances of optics and photonics, plasmonics has enabled unprecedented optical designs. Having a vectorial configuration of surface plasmon field, metallic nanostructures offer efficient solutions in polarization control with a very limited sample thickness. Many compact polarization devices have been realized using such metallic nanostructures. However, in most of these devices, the functions were usually simple and limited to a few polarization states. Here, we demonstrated a plasmonic polarization generator that can reconfigure an input polarization to all types of polarization states simultaneously. The plasmonic polarization generator is based on the interference of the in-plane (longitudinal) field of the surface plasmons that gives rise to versatile near-field polarization states on a metal surface, which have seldom been considered in previous studies. With a well-designed nanohole array, the in-plane field of SPPs with proper polarization states and phases can be selectively scattered out to the desired light beams. A manifestation of eight focusing beams with well-routed polarizations was experimentally demonstrated. Our design offers a new route to achieve the full control of optical polarizations and possibly advance the development in photonic information processing.Keywords：near-field interference;phase modulation;plasmonics;polarization generator
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Published：2024-09-25 - Abstract：Photonic quantum technologies hold promise to repeat the success of integrated nanophotonic circuits in non-classical applications. Using linear optical elements, quantum optical computations can be performed with integrated optical circuits and can therefore overcome the existing limitations in terms of scalability. In addition to passive optical devices for realizing photonic quantum gates, active elements, such as single-photon sources and single-photon detectors, are essential ingredients for future optical quantum circuits. Material systems that allow for the monolithic integration of all components are particularly attractive, including Ⅲ-Ⅴ semiconductors, silicon and diamond. Here, we demonstrate nanophotonic integrated circuits made from high-quality polycrystalline diamond thin films in combination with on-chip single-photon detectors. By using superconducting nanowires that are coupled evanescently to traveling waves, we achieve high detection efficiencies of up to 66% as well as low dark count rates and a timing resolution of 190 ps. Our devices are fully scalable and hold promise for functional diamond photonic quantum devices.Keywords：diamond nanophotonics;integrated optics;superconducting single-photon detectors
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Published：2024-09-25 - Abstract：An in-line, all-optical fiber modulator based on a stereo graphene–microfiber structure (GMF) utilizing the lab-on-rod technique was demonstrated in this study. Owing to its unique spring-like geometry, an ultra-long GMF interaction can be achieved, and a modulation depth of ~7.5 dB (~2.5 dB) and a modulation efficiency of ~0.2 dB mW−1 (~0.07 dB mW−1) were demonstrated for two polarization states. The modulation depth and modulation efficiency are more than one order of magnitude larger than those of other graphene–microfiber hybrid all-optical modulators, although at the cost of a higher insertion loss. By further optimizing the transferring and cleaning process, the upper limit of the modulation depth is mainly determined by the loss from the intrinsic absorption, which depends on the light–graphene interaction. Then, the modulator can quickly switch between the on-state and the off-state with a theoretically maximized modulation depth of tens of decibels. This modulator is compatible with the current fiber-optic communication systems and may be applied in the near future to meet the impending need for ultrafast optical signal processing.Keywords：graphene;microfiber;optical modulation;polarization
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Published：2024-09-25
Original Articles
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