Frontiers of Optoelectronics

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Status and prospects for phosphor-based white LED packaging
Zongyuan LIU, Sheng LIU, Kai WANG, Xiaobing LUO
Front Optoelec Chin    2009, 2 (2): 119-140.   https://doi.org/10.1007/s12200-009-0011-2
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The status and prospects for high-power, phosphor-based white light-emitting diode (LED) packaging have been presented. A system view for packaging design is proposed to address packaging issues. Four aspects of packaging are reviewed: optical control, thermal management, reliability and cost. Phosphor materials play the most important role in light extraction and color control. The conformal coating method improves the spatial color distribution (SCD) of LEDs. High refractive index (RI) encapsulants with high transmittance and modified surface morphology can enhance light extraction. Multi-phosphor-based packaging can realize the control of correlated color temperature (CCT) with high color rendering index (CRI). Effective thermal management can dissipate heat rapidly and reduce thermal stress caused by the mismatch of the coefficient of thermal expansion (CTE). Chip-on-board (CoB) technology with a multi-layer ceramic substrate is the most promising method for high-power LED packaging. Low junction temperature will improve the reliability and provide longer life. Advanced processes, precise fabrication and careful operation are essential for high reliability LEDs. Cost is one of the biggest obstacles for the penetration of white LEDs into the market for general illumination products. Mass production in terms of CoB, system in packaging (SiP), 3D packaging and wafer level packaging (WLP) can reduce the cost significantly, especially when chip cost is lowered by using a large wafer size.

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Brillouin distributed time-domain sensing in optical fibers: state of the art and perspectives
Front. Optoelectron.    2010, 3 (1): 13-21.   https://doi.org/10.1007/s12200-009-0086-9
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Optical fiber sensors based on stimulated Brillouin scattering have now clearly demonstrated their excellent capability for long-range distributed strain and temperature measurements. The fiber is used as sensing element, and a value for temperature and/or strain can be obtained from any point along the fiber. After explaining the principle and presenting the standard implementation, the latest developments in this class of sensors will be introduced, such as the possibility to measure with a spatial resolution of 10 cm and below while preserving the full accuracy on the determination of temperature and strain.
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Characterization of basic physical properties of Sb2Se3 and its relevance for photovoltaics
Chao CHEN,David C. BOBELA,Ye YANG,Shuaicheng LU,Kai ZENG,Cong GE,Bo YANG,Liang GAO,Yang ZHAO,Matthew C. BEARD,Jiang TANG
Front. Optoelectron.    2017, 10 (1): 18-30.   https://doi.org/10.1007/s12200-017-0702-z
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Antimony selenide (Sb2Se3) is a promising absorber material for thin film photovoltaics because of its attractive material, optical and electrical properties. In recent years, the power conversion efficiency (PCE) of Sb2Se3 thin film solar cells has gradually enhanced to 5.6%. In this article, we systematically studied the basic physical properties of Sb2Se3 such as dielectric constant, anisotropic mobility, carrier lifetime, diffusion length, defect depth, defect density and optical band tail states. We believe such a comprehensive characterization of the basic physical properties of Sb2Se3 lays a solid foundation for further optimization of solar device performance.

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Hydrazine processed Cu2SnS3 thin film and their application for photovoltaic devices
Jun HAN, Ying ZHOU, Yang Tian, Ziheng HUANG, Xiaohua WANG, Jie ZHONG, Zhe XIA, Bo YANG, Haisheng SONG, Jiang TANG
Front Optoelec    2014, 7 (1): 37-45.   https://doi.org/10.1007/s12200-014-0389-3
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Copper tin sulfide (Cu2SnS3) was a potential earth abundant absorber material for photovoltaic device application. In this contribution, triclinic Cu2SnS3 film with phase pure composition and large grain size was fabricated from a hydrazine solution process using Cu, Sn and S as the precursors. Absorption measurement revealed this Cu2SnS3 film had a direct optical band gap of 0.88 eV, and Hall effect measurement indicated the film was p-type with hole mobility of 0.86 cm2/Vs. Finally Mo/Cu2SnS3/CdS/ZnO/AZO/Au was produced and the best device efficiency achieved was 0.78%. Also, this device showed improved device performance during ambient storage. This study laid some foundation for the further improvement of Cu2SnS3 solar cell.

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Review of fiber Bragg grating sensor technology
Jinjie CHEN, Bo LIU, Hao ZHANG
Front Optoelec Chin    2011, 4 (2): 204-212.   https://doi.org/10.1007/s12200-011-0130-4
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The current status of the fiber Bragg grating (FBG) sensor technology was reviewed. Owing to their salient advantages, including immunity to electromagnetic interference, lightweight, compact size, high sensitivity, large operation bandwidth, and ideal multiplexing capability, FBG sensors have attracted considerable interest in the past three decades. Among these sensing physical quantities, temperature and strain are the most widely investigated ones. In this paper, the sensing principle of FBG sensors was briefly introduced first. Then, we reviewed the status of research and applications of FBG sensors. As very important for industrial applications, multiplexing and networking of FBG sensors had been introduced briefly. Moreover, as a key technology, the wavelength interrogation methods were also reviewed carefully. Finally, we analyzed the problems encountered in engineering applications and gave a general review on the development of interrogation methods of FBG sensor.

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Recent progress on tandem structured dye-sensitized solar cells
Dehua XIONG, Wei CHEN
Front Optoelec    2012, 5 (4): 371-389.   https://doi.org/10.1007/s12200-012-0283-9
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Tandem structured dye-sensitized solar cells (DSSCs) can take full advantage of sunlight, effectively broadening the absorption spectrum of the cell, resulting in a higher open circuit voltage or short circuit current than that of the conventional DSSC with single light absorber. The theoretical maximum efficiency is therefore suggested to be over the Schottky-Queisser limit of 33%. Accordingly, tandem design of DSSC is thought to be a promising way to break the performance bottleneck of DSSC. Besides, the tandem designs also broaden the application diversity of DSSC technology, which will accelerate its scale-up industrial application. In this paper, we have reviewed the recent progress on photo-electrochemical applications associated with kinds of tandem designs of DSSCs, in general, which are divided into three kinds: “n-type DSSC+n-type DSSC,” “n-type DSSC+p-type DSSC” and “n-type DSSC+other solar conversion devices.” The working principles, advantages and challenges of these tandem structured DSSCs have been discussed. Some possible solutions for further studies have been also pointed out together.

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Recent progresses on optical arbitrary waveform generation
Ming LI,José AZA?A,Ninghua ZHU,Jianping YAO
Front. Optoelectron.    2014, 7 (3): 359-375.   https://doi.org/10.1007/s12200-014-0470-y
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This paper reviews recent progresses on optical arbitrary waveform generation (AWG) techniques, which could be used to break the speed and bandwidth bottlenecks of electronics technologies for waveform generation. The main enabling techniques for optically generating optical and microwave waveforms are introduced and reviewed in this paper, such as wavelength-to-time mapping techniques, space-to-time mapping techniques, temporal pulse shaping (TPS) system, optoelectronics oscillator (OEO), programmable optical filters, optical differentiator and integrator and versatile electro-optic modulation implementations. The main advantages and challenges of these optical AWG techniques are also discussed.

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Parameters that control and influence the organo-metal halide perovskite crystallization and morphology
Bat-El COHEN,Lioz ETGAR
Front. Optoelectron.    2016, 9 (1): 44-52.   https://doi.org/10.1007/s12200-016-0630-3
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This review discusses various parameters that influence and control the organo-metal halide perovskite crystallization process. The effect of the perovskite morphology on the photovoltaic performance is a critical factor. Moreover, it has a dramatic effect on the stability of the perovskite, which has significant importance for later use of the organo-metal perovskite in assorted applications. In this review, we brought together several research investigations that describe the main parameters that significantly influence perovskite crystallization, for example, the annealing process, the precursor solvent, anti-solvent treatment, and additives to the iteite solutions.

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Photonic crystal fibers, devices, and applications
Wei JIN, Jian JU, Hoi Lut HO, Yeuk Lai HOO, Ailing ZHANG
Front Optoelec    2013, 6 (1): 3-24.   https://doi.org/10.1007/s12200-012-0301-y
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This paper reviews different types of air-silica photonic crystal fibers (PCFs), discusses their novel properties, and reports recent advances in PCF components and sensors as well as techniques for splicing PCFs to standard telecomm fibers.

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Brief introduction to optical microfibers and nanofibers
Limin TONG,
Front. Optoelectron.    2010, 3 (1): 54-60.   https://doi.org/10.1007/s12200-009-0073-1
Abstract   PDF (192KB)
When its diameter goes close to or below the wavelength of the guided light, an optical microfiber/nanofiber (MNF) exhibits favorable properties such as tight optical confinement, strong near-field interaction, and excellent mechanical strength, which offers plenty of choices for combining a variety of functionalized materials ranging from semiconductors and metals to laser dyes; opens up plenty of opportunities for developing microphotonic or nanophotonic devices; and inspires new opportunities for near-field optics, nonlinear optics, and quantum optics.
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Homeostatic photobiomodulation
Timon Chengyi LIU , Ruochun LIU , Ling ZHU , Jianqin YUAN , Min HU , Songhao LIU ,
Front. Optoelectron.    2009, 2 (1): 1-8.   https://doi.org/10.1007/s12200-009-0010-3
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Photobiomodulation (PBM) is a modulation of laser irradiation or monochromatic light (LI) on biosystems, which stimulates or inhibits biological functions but does not result in irreducible damage. LI might be of low intensity LI (LIL) (about 10 mW/cm2), or moderate intensity LI (MIL) (102―103 mW/cm2). PBM of LIL or MIL (LPBM or MPBM) is studied from the homeostatic viewpoint in this paper. Homeostasis is redefined as the function-specific homeostasis (FSH), a negative-feedback response of a biosystem which maintains the function-specific conditions inside it. PBM is classified into two kinds, the FSH-specific PBM (fPBM) and developmental PBM (dPBM). For fPBM, there is no PBM of LI on the function in FSH, but there is PBM of LI on the function far from FSH. dPBM can disrupt FSH. It can be found that LPBM is an fPBM, and whether MPBM is fPBM or dPBM depends on MIL dose and cell sensitivity. Low level LI therapy is just clinical applications of fPBM, so that it is a cellular rehabilitation.
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Asymmetric resonant cavities and their applications in optics and photonics: a review
Yun-Feng XIAO, Yan LI, Qihuang GONG, Chang-Ling ZOU, Chun-Hua DONG, Zheng-Fu HAN,
Front. Optoelectron.    2010, 3 (2): 109-124.   https://doi.org/10.1007/s12200-010-0003-2
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Asymmetric resonant cavities (ARCs) with smoothly deformed boundaries are currently under intensive study because they possess distinct properties that conventional symmetric cavities cannot provide. On one hand, it has been demonstrated that ARCs allow for highly directional emission instead of the in-plane isotropic light output in symmetric whispering-gallery cavities, such as microdisks, microspheres, and microtoroids. On the other hand, ARCs behave like open billiard system and thus offer an excellent platform to test classical and quantum chaos. This article reviews the recent progresses and prospects for the experimental realization of ARCs, with applications toward highly directional microlasing, strong-coupling light-matter interaction, and highly sensitive biosensing.
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2-channel all optical demultiplexer based on photonic crystal ring resonator
Hadi GHORBANPOUR, Somaye MAKOUEI
Front Optoelec    2013, 6 (2): 224-227.   https://doi.org/10.1007/s12200-013-0322-1
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In this paper, we proposed a 2-channel demultiplexer based on photonic crystal ring resonator (PCRR). For performing wavelength selection, we used two ring resonators, two different wavelengths were obtained by using two resonant rings with different values for the radius of dielectric rods. All the simulations and calculations have been done using Rsoft Photonic CAD software, which employs finite difference time domain (FDTD) method. The output channels were respectively at 1590.8 and 1593.8 nm, correspondingly had the quality factors of 7954 and 3984, the crosstalk values of -22 and -11 dB separately. The total footprint of our proposed structure is 681.36 μm2. Results suggest that 2-channels in the proposed structure are characterized with high transmission efficiency and low band width, resulting in a very sharp output spectrum and high quality factor values.

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Direct band gap luminescence from Ge on Si pin diodes
E. KASPER, M. OEHME, J. WERNER, T. AGUIROV, M. KITTLER
Front Optoelec    2012, 5 (3): 256-260.   https://doi.org/10.1007/s12200-012-0235-4
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Germanium (Ge) pin photodiodes show clear direct band gap emission at room temperature, as grown on bulk silicon in both photoluminescence (PL) and electroluminescence (EL). PL stems from the top contact layer with highly doped Ge because of strong absorption of visible laser light excitation (532 nm). EL stems from the recombination of injected carriers in the undoped intrinsic layer. The difference in peak positions for PL (0.73 eV) and EL (0.80 eV) is explained by band gap narrowing from high doping in n+-top layer. A superlinear increase of EL with current density is explained by a rising ratio of direct/indirect electron densities when quasi Fermi energy level rises into the conduction band. An analytical model for the direct/indirect electron density ratio is given using simplifying assumptions.

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Low phase noise hybrid silicon mode-locked lasers
Sudharsanan SRINIVASAN,Michael DAVENPORT,Martijn J. R. HECK,John HUTCHINSON,Erik NORBERG,Gregory FISH,John BOWERS
Front. Optoelectron.    2014, 7 (3): 265-276.   https://doi.org/10.1007/s12200-014-0420-8
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In this paper, we review recent results on hybrid silicon mode-locked lasers with a focus on low phase noise optical pulse generation. Taking a high level design approach to lowering phase noise, we show the need for long on-chip optical delay lines for mode-locked lasers to reach and overcome material limits. Key results include demonstration of the longest (cavity length 9 cm) integrated on-chip mode locked laser, 14 dB reduction of Lorentzian noise on a 20 GHz radio-frequency (RF) signal, and greater than 55 dB optical supermode noise suppression using harmonically mode locked long cavity laser, 10 GHz passively mode locked laser with 15 kHz linewidth using on-chip all optical feedback stabilization.

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FBG-based smart bed system for healthcare applications
Jianzhong HAO, Maniyeri JAYACHANDRAN, Poh Leong KNG, Siang Fook FOO, Phyo Wai AUNG AUNG, Zhaohui CAI,
Front. Optoelectron.    2010, 3 (1): 78-83.   https://doi.org/10.1007/s12200-009-0066-0
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This paper presents a smart fiber Bragg grating (FBG) sensor system with an unobtrusive and easy-to-use FBG sensor bed, which automatically monitors the behavior of bedridden patients and their vital signs based on indicative spatio-temporal signature for adaptive intervention triggering and activity planning. We present the subtle design, fabrication, calibration, implementation and deployment issues of the FBG pressure sensors to be used in hospitals or nursing homes to prevent bedsore generation, patient falling out of the bed, and life-threatening situations such as patient’s heart rate weakening, breathing pattern change, etc. Through trials conducted in the laboratory for respiratory rate monitoring with a sample group of 10 subjects, the system showed maximum error of±€1 breaths per minute as compared to manual counting.
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Thermal conductivity of doped YAG and GGG laser crystal
WANG Baosong, JIANG Haihe, JIA Xiande, ZHANG Qingli, SUN Dunlu, YIN Shaotang
Front. Optoelectron.    2008, 1 (1-2): 138-141.   https://doi.org/10.1007/s12200-008-0049-6
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High-performance, stable and low-cost mesoscopic perovskite (CH3NH3PbI3) solar cells based on poly(3-hexylthiophene)-modified carbon nanotube cathodes
Xiaoli ZHENG,Haining CHEN,Zhanhua WEI,Yinglong YANG,He LIN,Shihe YANG
Front. Optoelectron.    2016, 9 (1): 71-80.   https://doi.org/10.1007/s12200-016-0566-7
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This work explores the use of poly(3-hexylthiophene) (P3HT) modified carbon nanotubes (CNTs@P3HT) for the cathodes of hole transporter free, mesoscopic perovskite (CH3NH3PbI3) solar cells (PSCs), simultaneously achieving high-performance, high stability and low-cost PSCs. Here the thin P3HT modifier acts as an electron blocker to inhibit electron transfer into CNTs and a hydrophobic polymer binder to tightly cross-link the CNTs together to compact the carbon electrode film and greatly stabilize the solar cell. On the other hand, the presence of CNTs greatly improve the conductivity of P3HT. By optimizing the concentration of the P3HT modifier (2 mg/mL), we have improved the power conversion efficiencies (PCEs) of CNTs@P3HT based PSCs up to 13.43% with an average efficiency of 12.54%, which is much higher than the pure CNTs based PSCs (best PCE 10.59%) and the sandwich-type P3HT/CNTs based PSCs (best PCE 9.50%). In addition, the hysteresis of the CNTs@P3HT based PSCs is remarkably reduced due to the intimate interface between the perovskite and CNTs@P3HT electrodes. Degradation of the CNTs@ P3HT based PSCs is also strongly retarded as compared to cells employing the pure CNTs electrode when exposed to the ambient condition of 20%-40% humidity.

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Donor design and modification strategies of metal-free sensitizers for highly-efficient n-type dye-sensitized solar cells
Xiaoyu ZHANG,Michael Grätzel,Jianli HUA
Front. Optoelectron.    2016, 9 (1): 3-37.   https://doi.org/10.1007/s12200-016-0563-x
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Dye-sensitized solar cells (DSSCs) cannot be developed without the research on sensitizers. As the key of light harvesting and electron generation, thousands of sensitizers have been designed for the application in DSSC devices. Among them, organic sensitizers have drawn a lot of attention because of the flexible molecular design, easy synthesis and good photovoltaic performance. Recently, new record photovoltaic conversion efficiencies of 11.5% for DSSCs with iodide electrolyte and 14.3% for DSSCs with cobalt electrolyte and co-sensitization have been achieved with organic sensitizers. Here we focus on the donor design and modification of organic sensitizers. Several useful strategies and corresponding typical examples are presented.

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Terahertz pulse imaging in archaeology
J. Bianca JACKSON,Julien LABAUNE,Rozenn BAILLEUL-LESUER,Laura D'ALESSANDRO,Alison WHYTE,John W. BOWEN,Michel MENU,Gerard MOUROU
Front. Optoelectron.    2015, 8 (1): 81-92.   https://doi.org/10.1007/s12200-014-0446-y
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The work presented in this paper was performed at the Oriental Institute at the University of Chicago, on objects from their permanent collection: an ancient Egyptian bird mummy and three ancient Sumerian corroded copper-alloy objects. We used a portable, fiber-coupled terahertz (THz) time-domain spectroscopic imaging system, which allowed us to measure specimens in both transmission and reflection geometry, and present time- and frequency-based image modes. The results confirm earlier evidence that THz imaging can provide complementary information to that obtainable from X-ray computed tomography (XRCT) scans of mummies, giving better visualisation of low density regions. In addition, we demonstrated that THz imaging can distinguish mineralized layers in metal artifacts.

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One-dimensional nanostructures for electronic and optoelectronic devices
Guozhen SHEN, Di CHEN,
Front. Optoelectron.    2010, 3 (2): 125-138.   https://doi.org/10.1007/s12200-010-0001-4
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One-dimensional (1-D) nanostructures have been the focus of current researches due to their unique physical properties and potential applications in nanoscale electronics and optoelectronics. They address and overcome the physical and economic limits of current microelectronic industry and will lead to reduced power consumption and largely increased device speed in next generation electronics and optoelectronics. This paper reviews the recent development on the device applications of 1-D nanostructures in electronics and optoelectronics. First, typical 1-D nanostructure forms, including nanorods, nanowires, nanotubes, nanobelts, and hetero-nanowires, synthesized from different methods are briefly discussed. Then, some nanoscale electronic and optoelectronic devices built on 1-D nanostructures are presented, including field-effect transistors (FETs), p-n diodes, ultraviolet (UV) detectors, light-emitting diodes (LEDs), nanolasers, integrated nanodevices, single nanowire solar cells, chemical sensors, biosensors, and nanogenerators. We then finalize the paper with some perspectives and outlook towards the fast-growing topics.
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Prospects and challenges of silicon/germanium on-chip optoelectronics
Erich KASPER,
Front. Optoelectron.    2010, 3 (2): 143-152.   https://doi.org/10.1007/s12200-010-0007-y
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On-chip optoelectronics allows the integration of optoelectronic functions with microelectronics. Recent advances in silicon substrate fabrication (silicon-on-insulator (SOI)) and in heterostructure engineering (SiGe/Si) push this field to compact (chipsize) waveguide systems with high-speed response (50-GHz subsystems realized, potential with above 100 GHz). In this paper, the application and requirements, the future solutions, the components and the physical effects are discussed.
A very high refractive index contrast of the waveguide Si-core/SiO2-cladding is responsible for the submicron line widths and strong bendings realized in chipsize waveguide lines and passive devices. The SiGe/Si heterostructure shifts the accessible wavelength into infrared up to telecommunication wavelengths 1.30–1.55 µm. Germanium, although also an indirect semiconductor as silicon, offers direct optical transitions which are only 140 meV above the dominant indirect one. This is the basic property for realizing high-speed devices for future above 10 GHz on-chip clocks and, eventually, a laser source monolithically integrated on the Si substrate.
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Key technologies and system proposals of TWDM-PON
Zhengxuan LI, Lilin YI, Weisheng HU
Front Optoelec    2013, 6 (1): 46-56.   https://doi.org/10.1007/s12200-012-0305-7
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In this paper, key technologies, system proposals and future directions of next generation passive optical networks stage 2 (NG-PON2) are reviewed. We first discuss the potential solutions for NG-PON2 standardization. Then we focus on time and wavelength division multiplexed PON (TWDM-PON), which is the primary solution selected by Full Service Access Network (FSAN). The key technologies in TWDM-PON configuration are analyzed, including how to improve the bandwidth capacity and power budget of the system, and choose upstream tunable transceiver, etc. Several system proposals are illustrated as candidates for NG-PON2 configuration.

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Substrate effect on morphology and photoluminescence from ZnO monopods and bipods
Pijus Kanti SAMANTA, Partha Roy CHAUDHURI
Front Optoelec Chin    2011, 4 (2): 130-136.   https://doi.org/10.1007/s12200-011-0168-3
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A simple wet chemical bath deposition has been successfully deployed to fabricate zinc oxide (ZnO) nanostructures. For substrate free growth, the nanostructure is spindle like monopods. But when the nanostructures grow on the glass and quartz substrates, they are bipods (two monopods joined together base to base). Variation in the size of the spindles of the monopods and bipods and the particle size was observed due to the strain exists in the thin film due to lattice mismatch at the interface of the thin film and the substrates. The X-ray diffraction (XRD) and selected area diffraction results confirmed the hexagonal unit cell structures of the monopods and bipods. Also the growth rates of various planes are different and the growth is anisotropic. The substrate free grown monopods show visible photoluminescence (PL) at 421 nm. But the emission gets shifted by 3 and 6 nm for ZnO thin film deposited on quartz and glass substrates respectively due to interfacial strain. In case of ZnO on quartz substrate a strong ultra-violet (UV) peak was observed at 386 nm due to band edge transition. These emissions are also accompanied by few weaker emission peaks due to various defect related transition.

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Laser-based micro/nanofabrication in one, two and three dimensions
Wei XIONG,Yunshen ZHOU,Wenjia HOU,Lijia JIANG,Masoud MAHJOURI-SAMANI,Jongbok PARK,Xiangnan HE,Yang GAO,Lisha FAN,Tommaso BALDACCHINI,Jean-Francois SILVAIN,Yongfeng LU
Front. Optoelectron.    2015, 8 (4): 351-378.   https://doi.org/10.1007/s12200-015-0481-3
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Advanced micro/nanofabrication of functional materials and structures with various dimensions represents a key research topic in modern nanoscience and technology and becomes critically important for numerous emerging technologies such as nanoelectronics, nanophotonics and micro/nanoelectromechanical systems. This review systematically explores the non-conventional material processing approaches in fabricating nanomaterials and micro/nanostructures of various dimensions which are challenging to be fabricated via conventional approaches. Research efforts are focused on laser-based techniques for the growth and fabrication of one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) nanomaterials and micro/nanostructures. The following research topics are covered, including: 1) laser-assisted chemical vapor deposition (CVD) for highly efficient growth and integration of 1D nanomaterial of carbon nanotubes (CNTs), 2) laser direct writing (LDW) of graphene ribbons under ambient conditions, and 3) LDW of 3D micro/nanostructures via additive and subtractive processes. Comparing with the conventional fabrication methods, the laser-based methods exhibit several unique advantages in the micro/nanofabrication of advanced functional materials and structures. For the 1D CNT growth, the laser-assisted CVD process can realize both rapid material synthesis and tight control of growth location and orientation of CNTs due to the highly intense energy delivery and laser-induced optical near-field effects. For the 2D graphene synthesis and patterning, room-temperature and open-air fabrication of large-scale graphene patterns on dielectric surface has been successfully realized by a LDW process. For the 3D micro/nanofabrication, the combination of additive two-photon polymerization (TPP) and subtractive multi-photon ablation (MPA) processes enables the fabrication of arbitrary complex 3D micro/nanostructures which are challenging for conventional fabrication methods. Considering the numerous unique advantages of laser-based techniques, the laser-based micro/nanofabrication is expected to play a more and more important role in the fabrication of advanced functional micro/nano-devices.

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Investigation of ultra-broadband terahertz time-domain spectroscopy with terahertz wave gas photonics
Xiaofei LU,Xi-Cheng ZHANG
Front. Optoelectron.    2014, 7 (2): 121-155.   https://doi.org/10.1007/s12200-013-0371-5
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Recently, air plasma, produced by focusing an intense laser beam to ionize atoms or molecules, has been demonstrated to be a promising source of broadband terahertz waves. However, simultaneous broadband and coherent detection of such broadband terahertz waves is still challenging. Electro-optical sampling and photoconductive antennas are the typical approaches for terahertz wave detection. The bandwidth of these detection methods is limited by the phonon resonance or carrier’s lifetime. Unlike solid-state detectors, gaseous sensors have several unique features, such as no phonon resonance, less dispersion, no Fabry-Perot effect, and a continuous renewable nature. The aim of this article is to review the development of a broadband terahertz time-domain spectrometer, which has both a gaseous emitter and sensor mainly based on author’s recent investigation. This spectrometer features high efficiency, perceptive sensitivity, broad bandwidth, adequate signal-to-noise ratio, sufficient dynamic range, and controllable polarization.

The detection of terahertz waves with ambient air has been realized through a third order nonlinear optical process: detecting the second harmonic photon that is produced by mixing one terahertz photon with two fundamental photons. In this review, a systematic investigation of the mechanism of broadband terahertz wave detection was presented first. The dependence of the detection efficiency on probe pulse energy, bias field strength, gas pressure and third order nonlinear susceptibility of gases were experimentally demonstrated with selected gases. Detailed discussions of phase matching and Gouy phase shift were presented by considering the focused condition of Gaussian beams. Furthermore, the bandwidth dependence on probe pulse duration was also demonstrated. Over 240 times enhancement of dynamic range had been accomplished with n-hexane vapor compared to conventional air sensor. Moreover, with sub-20 fs laser pulses delivered from a hollow fiber pulse compressor, an ultra-broad spectrum covering from 0.3 to 70 THz was also showed.

In addition, a balanced detection scheme using a polarization dependent geometry was developed by author to improve signal-to-noise ratio and dynamic range of conventional terahertz air-biased-coherent-detection (ABCD) systems. Utilizing the tensor property of third order nonlinear susceptibility, second harmonic pulses with two orthogonal polarizations was detected by two separated photomultiplier tubes (PMTs). The differential signal from these two PMTs offers a realistic method to reduce correlated laser fluctuation, which circumvents signal-to-noise ratio and dynamic range of conventional terahertz ABCD systems. A factor of two improvement of signal-to-noise ratio was experimentally demonstrated.

This paper also introduces a unique approach to directly produce a broadband elliptically polarized terahertz wave from laser-induced plasma with a pair of double helix electrodes. The theoretical and experimental results demonstrated that velocity mismatch between excitation laser pulses and generated terahertz waves plays a key role in the properties of the elliptically polarized terahertz waves and confirmed that the far-field terahertz emission pattern is associated with a coherent process. The results give insight into the important influence of propagation effects on terahertz wave polarization control and complete the mechanism of terahertz wave generation from laser-induced plasma.

This review provides a critical understanding of broadband terahertz time-domain spectroscopy (THz-TDS) and introduces further guidance for scientific applications of terahertz wave gas photonics.

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Challenges in the ambient Raman spectroscopy characterization of methylammonium lead triiodide perovskite thin films
Yuanyuan ZHOU,Hector F. GARCES,Nitin P. PADTURE
Front. Optoelectron.    2016, 9 (1): 81-86.   https://doi.org/10.1007/s12200-016-0573-8
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The importance of methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) organic-inorganic hybrid perovskites has shot up dramatically since their use in highly efficient thin-film perovskite solar cells (PSCs). However, the basic structural characterization of these fascinating materials remains sparse. In particular, Raman spectroscopy, which is a powerful vibrational spectroscopy characterization tool and complements other characterization methods, of MAPbI3 under ambient conditions is plagued with difficulties. Here, a systematic ambient Raman spectroscopy characterization study of MAPbI3 thin films is conducted under different conditions (excitation laser wavelength, integration time, filter characteristic). The results from this study help elucidate the possible sources of artifacts in the Raman spectra, and raise the awareness of the challenges in the ambient Raman spectroscopy of MAPbI3 perovskites. Approaches to overcome these challenges are suggested.

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Microwave photonics connected with microresonator frequency combs
Xiaoxiao XUE,Andrew M. WEINER
Front. Optoelectron.    2016, 9 (2): 238-248.   https://doi.org/10.1007/s12200-016-0621-4
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Microresonator frequency combs (microcombs) are very promising as ultra-compact broadband sources for microwave photonic applications. Conversely, microwave photonic techniques are also employed intensely in the study of microcombs to reveal and control the comb formation dynamics. In this paper, we reviewed the microwave photonic techniques and applications that are connected with microcombs. The future research directions of microcomb-based microwave photonics were also discussed.

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Recent advances in holographic data storage
Hao RUAN
Front. Optoelectron.    2014, 7 (4): 450-466.   https://doi.org/10.1007/s12200-014-0458-7
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Nowadays, big-data centers still rely on hard drives. However, there is strong evidence that these surface-storage technologies are approaching fundamental limits that may be difficult to overcome, as ever-smaller bits become less thermally stable and harder to access. An intriguing approach for next generation data-storage is to use light to store information throughout the three-dimensional (3D) volume of a material. Holographic data storage (HDS) is poised to change the way we write and retrieve data forever. After many years of developing appropriate recording media and optical read–write architectures, this promising technology is now moving industriously to the market. In this paper, a review of the major achievements of HDS in the past ten years is presented and the key technique details are discussed. The author concludes that HDS technology is an attractive candidate for big data centers in the future. On the other hand, there are many challenges ahead for HDS technology to overcome in the years to come.

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Optical fiber microwires and nanowires manufactured by modified flame brushing technique: properties and applications
G. BRAMBILLA, Y. JUNG, F. RENNA,
Front. Optoelectron.    2010, 3 (1): 61-66.   https://doi.org/10.1007/s12200-009-0081-1
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The modified “flame brushing” technique has been used to manufacture microwires and nanowires from both silica and compound glasses. In this paper, the properties of the wires manufactured by this technique are presented. Applications fabricated from microwires are also discussed.
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