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A randomly connected single-walled carbon nanotube (CNT) network film is suggested as an optically homogenous thin film to implement a tunable diffractive optical element with a subwavelength thickness. A Fresnel zone plate (FZP) as a thin-film lens is successfully realized by mask-free direct laser writing onto the CNT network film with a thickness of 450 nm. The fabricated FZP exhibits an intense three-dimensional focus having lateral and axial focal sizes of 0.95λ and 7.10λ, respectively, at the wavelength of 1550 nm. Furthermore, we show that the intensities at focal points of the first and second diffraction orders can be significantly modulated by 72% and 40% through ion-gel gating between +1.8 V and −1.8 V. These results may offer the potential for electro-optic tunability in multifocal diffraction flat optics and the like.

We report a 47.1% efficiency improvement resulting from the combination of device-architecture modification and inclusion of poly(3-hexylthiophene-2,5-diyl) nanowires (P3HT NWs) in bulk heterojunction (BHJ) active layers (ALs). Modelling of ellipsometry spectra shows substantial changes in inhomogeneity and optical constants of BHJ ALs with P3HT-NW inclusion. Furthermore, finite-difference time-domain simulation results based on actual device structures with inhomogeneous AL models indicate that enhanced light harvesting is a main contributing factor to efficiency improvement. On the contrary, P3HT-NW inclusion has no significant effect on charge carrier collection, other than suppressing occurrence of cul-de-sac in hole-transport pathways and unfavourable indene-C60 bisadduct domains near top anodes.

Biological polymers offer new opportunities for flexible electronics, which are necessary for applications in soft and biological systems, because of their bio- and eco-friendly material traits. However, integrating an arbitrarily patterned electrode, the backbone of electronic devices, is still challenging when transparency of the electrode is required. Here, we report the fabrication of patterned silver nanowire (AgNW) networks buried at the surface of a silk fibroin film, which are suitable for bioelectronic applications that require biocompatibility, flexibility, high conductivity, and optical transparency. The AgNW-buried silk film exhibits excellent low sheet resistance of ∼15 Ω sq.−1 and high optical transparency of over 80%, along with smoothness of the surface. A light-emitting diode (LED) chip is successfully integrated on the patterned electrodes and can be stably turned on and off. Furthermore, we can produce a transparent resistor and a radio-frequency (RF) antenna on the silk film, and use them together as a food sensor that responds to the decreased conductivity caused by rancidity of spoiled food.

We investigate the polarizability of terahertz (THz) waves emitted from undoped In0.2Ga0.8As nanowires (NWs). THz emission amplitude shows strong enhancement in vertically aligned NWs compared to less-aligned NWs. In particular, polarized THz waves are clearly demonstrated in aligned NWs via a drastic variation of amplitudes as a function of the axis angle in polarization-sensitive photoconductive antenna. In addition, phase reversal between aligned and less-aligned NWs substantiates the geometrical dependence of electronic diffusion in generating the transient THz electric fields.

With poly(3-hexylthiophene) (P3HT) nanowire (NW) inclusion in active layers (ALs), organic solar cells (OSCs) based on P3HT donor and indene-C60 bisadduct (ICBA) acceptor showed power conversion efficiency (PCE) improvements for both bulk heterojunction (BHJ)- and bilayer (BL)-structure AL devices. The PCE increase was approximately 14 % for both types of P3HT:ICBA OSCs. However, improvements in short-circuit current density (Jsc ) were about 4.4 and 6.4 % for BHJ- and BL-type AL devices, respectively. A systematic study showed that the addition of P3HT NWs did not result in enhanced internal quantum efficiencies for either type of device. However, the difference in light-harvesting efficiency was important in accounting for Jsc variations. Interestingly, there was no correlation between Jsc and PCE variations, whereas the open-circuit voltage (Voc ) and fill factor (FF) showed correlations with the PCE. The variation in FF is discussed in terms of Voc and equivalent-circuit parameters based on a nonideal diode model.

We have investigated THz radiation characteristics along different directions, either reflective or along lateral by using InAs-based heterostructures. Firstly, we demonstrate the phase shift with InAs layer thickness, revealing the change of dominant THz wave generation mechanism along both directions. Along the lateral direction, the time-domain signals in thin InAs epilayers showed an abrupt phase and amplitude change at certain time delays which suggest the interference between two rays at the photoconductive switch. This behavior was further substantiated by the multiple cavity modes in Fourier-transformed spectra and by the amplitude variation with excitation spot displacement.

We report the fabrication of efficient indium-tin-oxide-free organic solar cells based on poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). All layers of the devices from the lowermost silver nanowire cathode to the uppermost conducting polymer anode are deposited from solution and processed at plastic-compatible temperatures

We demonstrate the fabrication of plasmonic and metamaterials devices, operating in the terahertz frequency range, by using highly conductive, single-walled carbon nanotube (SWNT) network films. We fabricated various patterns on the SWNT films using photolithography or laser machining techniques, whose resonance behaviors are determined by geometric parameters such as the periodicity of the array patterns or the shape of the individual elements. The excellent mechanical properties of SWNT films enabled us to fabricate free-standing and highly flexible devices. More importantly, using postprocessings such as chemical treatments and nanoparticle coatings, we were able to engineer the dielectric constants of the SWNT films, such as enhancing or degrading the conductive properties. As a result of the postprocessings, the resonance peak of the plasmonic devices was suppressed or retrieved, which is not achievable in conventional metal films. In particular, we were able to control the metamaterials resonances, ...

We manipulate the phase and amplitude of terahertz waves emitted from graphitic materials by utilizing doping types, work function offsets, and circular transports.

Universal use of single-walled carbon-nanotube based saturable absorber devices for mode-locking of bulk solid-state lasers between 0.8 and 2 μm is discussed. The advantages in comparison to semiconductor saturable absorbers are emphasised. We briefly describe the manufacturing process and the essential optical properties, and review experimental results obtained with various types of femtosecond and picosecond solid-state lasers in the steady-state regime. We also demonstrate that a single hybrid saturable absorber used in transmission can be used to mode-lock four different types of lasers operating between 1 and 2 μm.

Organic bulk-heterojunction solar cells using thin-film single-walled carbon-nanotube (SWCNT) anodes deposited on glass are reported. Two types of SWCNT films are investigated: spin-coated films from dichloroethane (DCE), and spray-coated films from deionized water using sodium dodecyl sulphate (SDS) or sodium dodecyl benzene sulphonate (SDBS) as the surfactant. All of the films are found to be mechanically robust, with no tendency to delaminate from the underlying substrate during handling. Acid treatment with HNO 3 yields high conductivities >1000 S cm ―1 for all of the films, with values of up to 7694 ± 800 S cm ―1 being obtained when using SDS as the surfactant. Sheet resistances of around 100 Ω sq ―1 are obtained at reasonable transmission, for example, 128 ± 2 Ω sq ―1 at 90% for DCE, 57 ± 3 Ω sq ―1 at 65% for H 2 O:SDS, and 68 ± 5 Ω sq ―1 at 70% for H 2 O:SDBS. Solar cells are fabricated by successively coating the SWCNT films with poly(3,4-ethylenedioxythiophene) : p o l y(st y rene sulphonate) (PEDOT:PSS), a blend of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxy-carbonyl)-propyl-1-phenyl-(6,6)C 61 (PCBM), and LiF/Al. The resultant devices have respective power conversions of 2.3, 2.2 and 1.2% for DCE, H 2 O:SDS and H 2 O:SDBS, with the first two being at a virtual parity with reference devices using ITO-coated glass as the anode (2.3%).

Single-walled carbon-nanotube absorbers are experimentally demonstrated for laser mode-locking. A saturable absorber device is used to mode-lock three different bulk solid-state lasers in a 500 nm-wide wavelength interval. The devices exhibit a low saturation fluence of

Nonlinear optical characteristics of single-walled carbon nanotubes (SWCNTs) dispersed in dichlorobenzene and imbedded in polymer were investigated at 800 nm using the time-resolved optical Kerr gate technique. For systematic study of the influence of SWCNT bundling on optical nonlinearities, SWCNT solutions with different concentrations and a series of SWCNT/polymer composites deposited on glass substrates with different concentrations and thicknesses were prepared. The nonlinear response was comparable to the pulse duration of the laser used (∼90 fs) both in SWCNT solutions and SWCNT/polymer composites. Over three orders of magnitude enhancement was observed in the third-order nonlinear susceptibility of SWCNT/polymer composite film compared with that of SWCNT solution. An appreciable reduction of microscopic and macroscopic nonlinearities was observed with increasing SWCNT concentrations due to stronger bundling of SWCNTs.

We devised directionally controllable THz emission sources based on lateral composition modulation (LCM) structures. LCM structures were composed of In-rich Ga0.47In0.53P and Ga-rich Ga0.51In0.49P layers whose period was in quantum scale of ~`5 nm. The inherent type II band alignment in these structures leads to electron-hole (e-h) separation and plays a key role in generating later- ally polarized dipole ensembles, thus concomitantly emitting enhanced transmissive THz waves as compared to bulk sample. On the other hand, in lateral geometry, changes in THz fields between LCM and bulk structures turned out to negligible since the vertical electronic diffusion was allowed in both samples.

Carbon nanostructures such as carbon nanoparticle (CNP), carbon nanotube (CNT), and carbon nanofiber (CNF) were synthesized via hot-filament chemical vapor deposition using various catalysts. CNPs were grown with sputter coated permalloy catalysts on Cr or W buffer layers, whereas unaligned CNTs were synthesized on Ti or Ta buffer layers. Aligned CNTs were synthesized using H 2 -plasma-treated Fe and Fe-impregnated silica or alumina as catalysts. However, CNFs were synthesized using electroplated Ni catalysts. In general, the quality of the graphene layers of the CNFs was inferior to those of CNTs and CNPs. However, all these carbon nanostructures showed good field emission properties.

The effect of inserting an SU-8 dielectric interlayer into inverted bulk heterojunction (BHJ) organic solar cells (OSCs) was studied. Insertion of an ultrathin layer of SU-8 between the zinc oxide (ZnO) electron transport layer and the photoactive layer resulted in a smoother interface and a 14% enhancement in power conversion efficiency. The properties of devices with and without an SU-8 interlayer were investigated using transient photovoltage (TPV) and double injection (DoI) techniques, and it was found that devices with SU-8 show longer carrier lifetimes and greater mobility–lifetime (μ–τ) products than those without. Devices with SU-8 were also found to have improved stability. The results indicate that the insertion of an SU-8 interlayer reduces the recombination rate for photogenerated carriers without affecting the charge transport properties, improving overall performance and stability.

The terahertz (THz) radiation from transient dipoles, formed by distinct diffusion coefficients between oppositely charged carriers as often observed in low band gap semiconductors, propagates with an anisotropic amplitude distribution perpendicular to the dipole axis along the diffusive motion. By directionally adjusting the electronic diffusion, we conceptualize groove-patterned THz emitters based on (100) InAs thin films and demonstrate the unidirectional radiation. Line-of-sight emission along the surface-normal direction is greatly enhanced in a distributed asymmetric trapezoid with its period similar to the electronic diffusion length of InAs. This directional enhancement is in clear contrast to the constant emission amplitude along the lateral direction, regardless of pattern scale, which manifests the role of groove patterns as microscale reflectors in laterally corrugating the carrier density. In contrast to the rather limited nonlinearity in (100) plane, the azimuthal angle dependence of the THz field amplitude in corrugated samples shows a combined effect of diffusive transport and second-order nonlinearity, whose compositional contributions varies in different structures. (C) 2013 Optical Society of America

The polarity change of THz electromagnetic waves radiated from single-crystalline graphite and polycrystalline graphite films has been studied to identify the main generation mechanism in conventional reflective THz time-domain spectroscopy scheme. The excitation wavelength variation around the K-valley produces no significant changes in THz field strength. We further found that THz waves become fully dispersed without polarity change in lateral detection geometry.

The operation characteristics of nominal bilayer (BL) organic solar cells (OSCs), the active layers (ALs) of which consisted of sequentially casted bottom P3HT donor and top ICBA acceptor layers, resembled those of OSCs with bulk heterojunction (BHJ) ALs. Optical analysis and device simulations showed that such resemblance can be attributed to a similarity in the micromorphology of ALs; as-deposited BL-type ALs transformed spontaneously into BHJ-type ALs. The inclusion of P3HT nanowires (NWs) in the donor layers resulted in different AL micromorphology and consequently a larger power conversion efficiency. Separate assessment of the exciton generation and charge–carrier transport and/or extraction showed that the contribution of P3HT NWs was more prominent in optical effects. Transformation of sequentially casted P3HT and ICBA layers into a bulk heterojunction layer accounts for good agreement between measured and simulated J–V curves.

We manipulate the transient terahertz (THz) waves emitted from metal-graphite interfaces, where potential barriers were formed because of work function differences. To flip the phase of the THz waves, two distinct groups of metals were evaporated on n-type doped highly oriented pyrolytic graphite (HOPG): group A, which consisted of Pt, Au, and Ag with work functions larger than that of HOPG and group B, which consisted of Al and Ti with work functions smaller than that of HOPG. The phase of the transient THz lineshapes from group A was opposite to that of group B under infrared laser excitation, which is indicative of opposite band bending and concomitant interfacial doping for ambipolar transport at the metal-graphite junctions. The amplitude of the THz waves could also be manipulated by the work function differences and further quantified based on modified minority carrier mobilities at the depletion regions.

We report efficient organic bulk heterojunction solar cells, utilizing spray-patterned films of single-wall carbon nanotubes for the transparent electrode. High power conversion efficiencies of up to 3.6% were obtained using a blend of poly(3-hexylthiophene) and phenyl-C61 butyric acid methyl ester as the active layer, comparable to conventional devices utilizing indium tin oxide as the transparent electrode.

CNT-coated transparent substrates and their applications for field emission devices were reported. The advantage of this approach is that dispersed CNTs are strongly adhered on the transparent glass or polyethylene naphthalate (PEN) plastic while not ruining the transparency of the host platform. Such CNTs coated substrates have revealed effective and uniform field electron emission. In particular, turn-on and threshold applied fields were about 3.8 and 8.4 V/μm, respectively.

Femtosecond mode-locking of Yb:KYW at 1.04 μm, Cr:forsterite at 1.25 μm and a Cr:YAG at 1.50 μm lasers using one and the same transmission-type single-walled carbon nanotube saturable absorber is demonstrated.

Controlling bundling and curl of carbon nanotubes permits ultrafast optical switching, which was demonstrated in a >500 nm near-infrated band by femtosecond mode-locking of three bulk lasers using one and the same saturable absorber.

Stable and self-starting mode-locking of a Tm:KLu(WO4)2 laser using a single-walled carbon nanotubes based saturable absorber is demonstrated generating ~10-ps pulses at 1.94 µm with powers up to 220 mW at 126 MHz.

Transmission- and reflection-type single-walled carbon nanotube saturable absorbers (SWCNT-SAs) were designed and fabricated for passive mode-locking of bulk lasers in the 1 microm spectral range. Mode-locked laser operation based on a diffusion-bonded Yb:KYW/KYW crystal was demonstrated, and pulses as short as 83 fs and 140 fs were achieved applying reflection-type and transmission-type SWCNT-SA, respectively. The nonlinear parameters of the absorbers were measured to be in close vicinity to those of a semiconductor saturable absorber mirror for the same wavelength range. Mode-locking performance with SWCNT-SAs and the SESAM was compared utilizing the same cavity, with the SESAM resulting in only slightly shorter pulses of 66 fs duration. The nearly identical performance indicates that well-optimized SWCNT-SAs can substitute SESAMs even in the 1 microm region.

We performed time-domain terahertz (THz) spectroscopy on single-walled carbon nanotube films. We found that a filtration method provides an efficient way of fabricating thick nanotube films which is capable of effective shielding of THz waves. Shielding effectiveness of more than 40 has been achieved from our method. In addition, all of the films we fabricated demonstrate a good metallic behavior with a dispersion of optical constant relevant to simple Drude model.

Novel transmission- and reflection-type ultrafast saturable absorbers were developed based on single-walled carbon nanotubes (SWCNTs). Their nonlinear optical characteristics, crucial for mode-locking, such as modulation depth, saturation fluence and recovery time were studied. Additionally, we demonstrated passive mode-locking of different bulk solid-state lasers operating above 1 μm using these SWCNT-SAs and achieved stable femtosecond pulses without q-switching instabilities.

Stable and self-starting mode-locking of a Tm:KLu(WO(4))(2) crystal laser is demonstrated using a transmission-type single-walled carbon nanotube (SWCNT) based saturable absorber (SA). These experiments in the 2 microm regime utilize the E11 transition of the SWCNTs for nonlinear saturable absorption. The recovery time of the SWCNT-SA is measured by pump-probe measurements as approximately 1.2 ps. The mode-locked laser delivers approximately 10 ps pulses near 1.95 microm with a maximum output power of up to 240 mW at 126 MHz repetition rate.

A Cr:forsterite laser passively mode-locked with a transmission-type single-walled carbon nanotube saturable absorber delivers pulses as short as 120 fs near 1.25 μm with a maximum average power of 202 mW at 79.1 MHz.

Mode-locking of the disordered cubic sesquioxide crystal Yb:LuScO 3 using a reflection-type single-walled carbon nanotube saturable absorber is demonstrated under diode pumping and compared with passive mode-locking by a commercial SESAM.

We performed time-domain terahertz spectroscopy on thin layers of single-walled carbon nanotubes (SWCNTs) coated on flexible films. The SWCNT layers demonstrate good shielding of electromagnetic waves in the terahertz range while maintaining good transparency for visible light. The shielding efficiency can be engineered through the thickness control and/or chemical treatment of SWCNT layers. The frequency-dependent dielectric constants of SWCNT layers are in good agreement with the Drude free-electron model.

Ultrafast single-walled carbon nanotube saturable absorbers were fabricated and optimized for mode-locking solid-state lasers near 1 mum. Their typical modulation depth and recovery time were measured to be Gt 0.3% and ~ 750 fs, respectively.

Single-walled carbon nanotube saturable absorbers were designed for passive mode-locking near 1 mum. Using Yb:KYW and Yb:KLuW, nearly transform-limited sub-150 fs pulses were generated at 1037 nm and 1048 nm, respectively.

Mode locking of an Yb-doped bulk laser in the 1 microm spectral range using a single-walled carbon nanotube saturable absorber (SWCNT-SA) is demonstrated for the first time, to our knowledge. Passive mode locking of an Yb:KLuW laser resulted in nearly transform-limited pulses as short as 115 fs at 1048 nm. In addition, the nonlinear response of the SWCNT-SA was measured, yielding a modulation depth of 0.25% and a relaxation time of 750 fs.

Single-walled carbon nanotube saturable absorbers were designed and characterized (modulation depth

Solid single-walled carbon nanotube (SWCNT)/silica composites were prepared by a sol-gel method. Their third-order susceptibilities were estimated by using z-scan technique at 532 nm and 1064 nm, respectively. The results reveal negative nonlinear refraction and saturable absorption, which magnitudes depend on kinds of used SWCNTs prepared by different techniques.

We present low-power, femtosecond laser machining on vertically aligned array of carbon nanotube (CNT) films synthesized by two different growth techniques. Arrays of holes can be efficiently fabricated for a possible application as CNT based metamaterials, showing completely different features for the two samples.

The authors report the fabrication of free-standing carbon nanotube (CNT) array films supported by PDMS (poly-dimethylsiloxane) matrix, which opens up the possibility to transform deliberate CNT array architecture into flexible electronic and photonic components. The low viscosity and good wetting characteristics of uncured PDMS solution are important for thorough infiltration, and robustness and elasticity of cured PDMS are important for realization of deformable free-standing films detached from substrates (Duffy, 1999 and Lee, 2003). The characteristics of field-electron emission from PDMS-supported CNT arrays are also reported.

HiPco and Arc-made single-walled carbon nanotubes (SWCNTs) were incorporated into silica matrices via a conventional sol-gel method to form SWCNTs/silica composites. Optical absorption and FT-Raman spectroscopy revealed that purification process using acid treatment resulted in preferential removal of metallic SWCNTs and severe damage and/or side-wall functionalization of HiPco SWCNTs. Z-scan measurements of SWCNTs/silica composites showed self-defocusing and bleached absorption phenomena which indicated that both the real and imaginary parts of X3, the third-order nonlinear optical susceptibility, of these composites had negative sign.

The authors developed the protocol to fabricate transparent single wall carbon nanotube (SWCNT) films on polyethylene terephthalate (PET) film via spin coating of SWCNT-dispersion solution in dichloroethane (DCE). As it turned out, preparation of good SWCNT-dispersion solution was essential for making good transparent films with low sheet resistance. The posttreatment with nitric acid was also performed to reduce the sheet resistance of as-prepared films. Our acid-treated SWCNT films showed visible-range transmittance of about 80% at sheet resistance of about 85 Ω/sq.

Carbon nanotube (CNT) films were grown via hot-filament chemical-vapor deposition on silicon substrate covered with electroplated nickel catalyst. This resulted in less crystalline and defect rich carbon nanofibers (CNF). Scanning electron microscopy and FT-Raman spectra of the CNF films showed that a high density of CNFs was formed and that it was constituted of well-ordered graphene layer. High current density of 170 mA/cm/sup 2/ at 7.3 V//spl mu/m was obtained from field emission. Emission-stability test carried out at the current density of 20 mA/cm/sup 2/ showed stable emission currents with fluctuation of only 3.4% over 2 hours.

We prepared iron-impregnated silica films, which turned out to be very efficient catalysts for carbon nanotube synthesis, by a sol-gel process starting from the mixture of tetraethyl orthosilicate (TEOS), aqueous solution of iron nitrate (Fe(NO3)3⋅9H2O), and ethanol. The Fe/silicate-composite catalyst films consisted of nanoparticles, the composition and diameter of which depended on the iron-nitrate molarity in aqueous solution. Within the range of 0.5–1.5 M, the nanocomposite particles maintained good catalytic capacity, and very long and dense bundles of multiwall carbon nanotubes were synthesized regardless of the iron-nitrate molarity. The carbon nanotube density was so high that the nanotube bundles appeared made of aligned nanotubes in spite of the curliness of the individual nanotubes. However, carbon nanotubes were scarcely synthesized below 0.4 M of iron-nitrate molarity, which we attributed to the unfavorable composition and distribution of Fe-species in nanocomposite particles.

We investigate characteristics of THz waves radiated from differently doped graphite samples excited by femtosecond laser pulses. Between n-type single-crystalline graphite and p-type polycrystalline graphite films, we observe the phase reversal of THz waves regardless of excitation energy variations around K-valley. In addition, variations in other parameters such as excitation fluence and azimuthal angle produce no changes in the phase of THz waves, which correlate well with the opposite dipole polarization between differently doped samples rather than unidirectional diffusive transport.

We have performed terahertz (THz)-time domain spectroscopy in various geometries, for characterizing the directivity of THz waves emitted from groove-patterned InAs structures. First, we have distinguished the THz emission mechanisms as a function of epilayer thickness. The carrier drift was predominant in thin sample group (10-70 nm) which the electronic diffusion motion was overriding the oppositely aligned drifting dipoles in thick sample group (370-900 nm) as revealed via amplitude and phase variations. By combined use of the electron-beam lithography and the inductively coupled plasma etching in 1 {\mu}m-thick InAs epilayers, we have further fabricated either asymmetric V-groove patterns or symmetric parabolic patterns. The THz amplitude was enhanced, particularly along line-of-sight transmissive direction when the groove patterns act as microscale reflective mirrors periodically separated by a scale of diffusion length., Comment: 5 pages, 4 figures