Advanced 1D heterostructures based on nanotube templates and molecules

Recent advancements in materials science have shed light on the potential of exploring hierarchical assemblies of molecules on surfaces, driven by both fundamental and applicative challenges. This field encompasses diverse areas including molecular storage, drug delivery, catalysis, and nanoscale chemical reactions. In this context, the utilization of nanotube templates (NTs) has emerged as promising platforms for achieving advanced one-dimensional (1D) molecular assemblies. NTs offer cylindrical, crystalline structures with high aspect ratios, capable of hosting molecules both externally and internally (Mol@NT). Furthermore, NTs possess a wide array of available diameters, providing tunability for tailored assembly. This review underscores recent breakthroughs in the field of Mol@NT. The first part focuses on the diverse panorama of structural properties in Mol@NT synthesized in the last decade. The advances in understanding encapsulation, adsorption, and ordering mechanisms are detailed. In a second part, the review highlights the physical interactions and photophysics properties of Mol@NT obtained by the confinement of molecules and nanotubes in the van der Waals distance regime. The last part of the review describes potential applicative fields of these 1D heterostructures, providing specific examples in photovoltaics, luminescent materials, and bio-imaging. A conclusion gathers current challenges and perspectives of the field to foster discussion in related communities.

Transient breathing dynamics during extinction of dissipative solitons in mode-locked fiber lasers

The utilization of the dispersive Fourier transformation approach has enabled comprehensive observation of the birth process of dissipative solitons in fiber lasers. However, there is still a dearth of deep understanding regarding the extinction process of dissipative solitons. In this study, we have utilized a combination of experimental and numerical techniques to thoroughly examine the breathing dynamics of dissipative solitons during the extinction process in an Er-doped mode-locked fiber laser. The results demonstrate that the transient breathing dynamics have a substantial impact on the extinction stage of both steady-state and breathing-state dissipative solitons. The duration of transient breathing exhibits a high degree of sensitivity to variations in pump power. Numerical simulations are utilized to produce analogous breathing dynamics within the framework of a model that integrates equations characterizing the population inversion in a mode-locked laser. These results corroborate the role of Q-switching instability in the onset of breathing oscillations. Furthermore, these findings offer new possibilities for the advancement of various operational frameworks for ultrafast lasers.

Tapered Optical Fiber Sensor Coated with Single-Walled Carbon Nanotubes for Dye Sensing Application

The present study aimed to improve the optical sensing performance of tapered optical fiber sensors toward aqueous Rhodamine B solution of different concentrations by applying single-walled carbon nanotubes (SWCNTs). The functional coating was formed on the surface of the tapered optical fiber sensor using an aerosol layer-by-layer deposition method. Before deposition, the SWCNTs were processed with multistage liquid-phase treatment in order to form a stable dispersion. The effect of SWCNT treatment was investigated through Raman spectroscopy. The deposition of 220 layers caused a reduction of up to 60% of the initial optical power of radiation propagating through the optical fiber core. The optical fiber sensor coated with SWCNTs demonstrated significantly higher sensitivity compared to a non-coated sensor in the range of 2-32 mg/L of Rhodamine B concentration in an aqueous solution. The experimental results demonstrated that the sensitivity was increased 10 times from 32 (mg/L)^-1, for the non-coated sensor, up to 317 (mg/L)^-1 after SWCNT coating deposition. Moreover, the SWCNT-coated sensor demonstrated high repeatability that allowed for the evaluation of the concentration regardless of the previously analyzed dye concentration.

Extremely large static and dynamic nonlinear optical response of small superalkali clusters NM3M' (M, M'=Li, Na, K)

Exploring novel nonlinear optical (NLO) materials with excess electron properties is essential for advancing the use of excess electron compounds in optics. The studied superalkali clusters NM₃M' (M, M' = Li, Na, K) are thermodynamically stable and their binding energies range from -27.10 to -53.84 kcal mol^-1. The observed significant values for VIPs suggest their electronic stabilities. Being excess electron candidate these clusters show significant β_o value (3.9 × 10⁷ au), which nicely correlates the hyperpolarizability reported by a two-level model (β_tl). Furthermore, these clusters exhibit a remarkable static second hyperpolarizability (γ_o) value of 1.1 × 10¹⁰ au for the NK₄ superalkali cluster. The hyper Rayleigh scattering (β_HRS) is also computed where the highest value of 2.9 × 10⁷ is recorded for NNa₃K superalkali. The obtained values of β_vec values (projection of hyperpolarizability on dipole moment vector) also signify the excellent nonlinearity of clusters. Besides, the calculated electro-optica pockel's effect β(-ω; ω,0) and second harmonic generation β(-2ω; ω, ω) values are much pronounced at larger dispersion frequency ω = 1064 nm. Moreover, the frequency-dependent second hyperpolarizability γ(ω) with dc-Kerr effect γ(-ω; ω,0,0) and electric field induced second harmonic generation γ(-2ω; ω,ω,0) show larger values at ω = 1064 nm. Thus the highest value of the dc-Kerr constant increases up to 1.0 × 10¹¹ au which also signifies the larger nonlinear refractive index of the studied cluster. We hope this work could open up new possibilities using superalkali clusters as NLO materials for optoelectronics, laser, second harmonic generation and as frequency doubler.

Enhanced saturable absorption in the laser-treated free-standing carbon nanotube films

We demonstrate an increase of optical transmittance and saturable absorption of laser-treated free-standing single-walled carbon nanotube (SWNT) films. The combined acid and low-power non-destructive laser treatment ensures an enhancement of linear transmittance across the visible range and double-digit increase of the saturable absorption of femtosecond laser radiation at 795 nm. The saturable absorption coefficient and the ratio of saturable to non-saturable losses increase by 26% and 35%, correspondingly, while the saturation intensity decreases by 20% because of the treatment. Our analysis indicates that with the performed treatment one can significantly improve the nonlinear optical properties of free-standing SWNT-based ultrafast saturable absorbers.

Ionic Liquid Gated Carbon Nanotube Saturable Absorber for Switchable Pulse Generation

Materials with electrically tunable optical properties offer a wide range of opportunities for photonic applications. The optical properties of the single-walled carbon nanotubes (SWCNTs) can be significantly altered in the near-infrared region by means of electrochemical doping. The states' filling, which is responsible for the optical absorption suppression under doping, also alters the nonlinear optical response of the material. Here, for the first time we report that the electrochemical doping can tailor the nonlinear optical absorption of SWCNT films and demonstrate its application to control pulsed fiber laser generation. With a pump-probe technique, we show that under an applied voltage below 2 V the photobleaching of the material can be gradually reduced and even turned to photoinduced absorption. Furthermore, we integrated a carbon nanotube electrochemical cell on a side-polished fiber to tune the absorption saturation and implemented it into the fully polarization-maintaining fiber laser. We show that the pulse generation regime can be reversibly switched between femtosecond mode-locking and microsecond Q-switching using different gate voltages. This approach paves the road toward carbon nanotube optical devices with tunable nonlinearity.

Recent Progress in Two-Dimensional Nanomaterials for Laser Protection

The Nobel Prize in Physics 2018, “For groundbreaking inventions in the field of laser physics”, went to Arthur Ashkin and Gérard Mourou and Donna Strickland. Their inventions have revolutionized laser physics and greatly promoted the development of laser instruments, which have penetrated into many aspects of people’s daily lives. However, for the purpose of protecting human eyes or optical instruments from being damaged by both pulsed and continuous wave laser radiation, the research on laser protective materials is of particular significance. Due to the intriguing and outstanding physical, chemical, and structural properties, two-dimensional (2D) nanomaterials have been extensively studied as optical limiting (OL) materials owing to their broadband nonlinear optical (NLO) response and fast carrier relaxation dynamics that are important for reducing the laser intensity. This review systematically describes the OL mechanisms and the recent progress in 2D nanomaterials for laser protection.

Coherent optical modulation of graphene based on coherent population oscillation

The optical modulation of graphene circumvents the "electrical bottleneck" in electrical field tuning of the Fermi level and motivates diverse graphene-based controllable photonic devices with extraordinary performances. Unfortunately, pervious optical modulation schemes are incoherent, and the Fermi-Dirac distribution formed from a strong pump laser prevents the absorption of a weak probe laser due to the Pauli blocking, making the modulation inconvenient and low in efficiency. Here we demonstrate the coherent optical modulation of graphene based on coherent population oscillation, where ground state population oscillates with a beat frequency equal to the pump and probe frequency difference. To distinguish it from the coexisting incoherent modulation in graphene, a phase-sensitive pump-probe system is constructed with a fiber-based Mach-Zehnder interferometer. Clear resonance within the burning hole of a pump laser is observed in the interference spectrum of a coherent probe laser. The discovery of highly coherent ground state population oscillation in graphene offers new possibilities for manipulating and controlling the phase response of graphene-based photonics with high efficiency.

Poly(arylene ether)s with aromatic azo-coupled cobalt phthalocyanines in the side chain: synthesis, characterization and nonlinear optical and optical limiting properties

A novel poly(arylene ether) with azo-coupled cobalt phthalocyanine in the side chain (PAE-azo-CoPc) was prepared by 1,2-benzodinitrile, anhydrous cobaltous chloride and a novel azobenzene-containing poly(aryl ether) (PAE-azo-DCN) via a nucleophilic substitution polycondensation based on a novel bisfluoro monomer, 4-[(3,4-cyanophenyl)diazenyl]phenyl-2,6-difluorobenzoate. The obtained polymers were characterized and evaluated by FT-IR, ¹H NMR, DSC and TGA. PAE-azo-CoPc exhibited higher glass transition temperature and better thermal stability than PAE-azo-DCN because of the introduction of cobalt phthalocyanine groups. The results of Z-scan measurements demonstrated that PAE-azo-CoPc showed reversible saturable absorption and positive refraction, and PAE-azo-DCN showed saturable absorption and negative refraction. By calculation, PAE-azo-CoPc exhibited larger third-order nonlinear optical susceptibilities than that of PAE-azo-DCN. The results of optical limiting measurements demonstrated that the PAE-azo-CoPc exhibited an excellent optical limiting response.

A novel poly(arylene ether) with azo-coupled cobalt phthalocyanine in the side chain was prepared by 1,2-benzodinitrile, anhydrous cobaltous chloride and a novel azobenzene-containing poly(aryl ether).

Graphene and Carbon-Nanotube Nanohybrids Covalently Functionalized by Porphyrins and Phthalocyanines for Optoelectronic Properties

In recent years, there has been a rapid growth in studies of the optoelectronic properties of graphene, carbon nanotubes (CNTs), and their derivatives. The chemical functionalization of graphene and CNTs is a key requirement for the development of this field, but it remains a significant challenge. The focus here is on recent advances in constructing nanohybrids of graphene or CNTs covalently linked to porphyrins or phthalocyanines, as well as their application in nonlinear optics. Following a summary of the syntheses of nanohybrids constructed from graphene or CNTs and porphyrins or phthalocyanines, explicit intraconjugate electronic interactions between photoexcited porphyrins/phthalocyanines and graphene/CNTs are introduced classified by energy transfer, electron transfer, and charge transfer, and their optoelectronic applications are also highlighted. The major current challenges for the development of covalently linked nanohybrids of porphyrins or phthalocyanines and carbon nanostructures are also presented.

1.6 μm dissipative soliton fiber laser mode-locked by cesium lead halide perovskite quantum dots

We demonstrate a stable, picosecond fiber laser mode-locked by cesium lead halide perovskite quantum dots (CsPbBr₃-QDs). The saturable absorber is produced by depositing the CsPbBr3-QDs nanocrystals onto the endface of a fiber ferrule through light pressure. A balanced two-detector measurement shows that it has a modulation depth of 2.5% and a saturation power of 17.29 MW/cm². After incorporating the fabricated device into an Er³⁺-doped fiber ring cavity with a net normal dispersion of 0.238 ps², we obtain stable dissipative soliton with a pulse duration of 14.4 ps and a center wavelength at 1600 nm together with an edge-to-dege bandwidth of 4.5 nm. The linear chirped phase can be compensated by 25 m single mode fiber, resulting into a compressed pulse duration of 1.046 ps. This experimental works proves that such CsPbBr3-QDs materials are effective choice for ultrafast laser operating with devious mode-locking states.

Ultra-broadband nonlinear optical response of two-dimensional h-BN nanosheets and their hybrid gel glasses

Hydrophilic hydroxylated hexagonal boron nitride nanosheets (BNNSs), also called 'white graphene', exhibit high water solubility, compatibility, dispersity with physical and/or chemical effects on the gel matrix, and unexpected but outstanding near-infrared adsorption and nonlinear optical limiting properties. These properties mean that hydroxylated BNNSs are suitable for practical applications in solidification and devices. Hydroxylated BNNSs are doped into an organically modified silicate matrix through a facile sol-gel process. The resulting BNNS-organically modified silicate glass hybrids demonstrate various solid structures (glass, coating, and film), good uniformity, thermostability, toughness, and high linear transmittance in the visible and near-infrared regions. Hybrid glasses showed large optical limiting effects and high doping concentrations without the addition of a compatibiliser, and their limiting thresholds and concentration exceed those of the corresponding suspensions by factors of 1.3-2 and 20-130, respectively. The optical limiting performance of BNNSs is almost equal to or may exceed those of graphene oxide and graphene in solutions and doped glasses. The combined mechanisms of nonlinear absorption, refraction, and scattering are deduced. A solid organically modified silicate hybrid transparent system doped with BNNSs is a promising candidate material for optical limiters and nonlinear optical devices. This system could be an efficient solid-state optical limiter in the visible and long-wavelength near-infrared regions (532-2000 nm).

Facile one-step fabrication of upconversion fluorescence carbon quantum dots anchored on graphene with enhanced nonlinear optical responses

A novel nanocomposite hybrid, carbon quantum dots (CQD)/graphene oxide (GO), which combines the favorable optical properties of both its components, is synthesized by a facile one-step electrochemical method. Transmission electron microscopy, Raman spectroscopy, UV-vis spectroscopy, and fluorescence studies show that the CQDs uniformly attach on the GO surface, which enables highly efficient energy transfer between CQDs and GO. The nonlinear optical and optical limiting (OL) performances are investigated by the open-aperture Z-scan technique in the nanosecond regime using a laser with a wavelength of 532 nm. The as-prepared CQD/GO composite offers a significantly improved OL performance compared with GO because of the charge/energy transfer process between the CQDs and GO. The main contributors to the enhanced OL effect in the CQD/GO hybrid are a combination of nonlinear scattering and increased nonlinear absorption resulting from efficient charge/energy transfer at the CQD/GO interface.

A novel nanocomposite hybrid, carbon quantum dots (CQD)/graphene oxide (GO), which combines the favorable optical properties of both its components, is synthesized by a facile one-step electrochemical method.

Photon-Pair Generation with a 100 nm Thick Carbon Nanotube Film

Nonlinear optics based on bulk materials is the current technique of choice for quantum-state generation and information processing. Scaling of nonlinear optical quantum devices is of significant interest to enable quantum devices with high performance. However, it is challenging to scale the nonlinear optical devices down to the nanoscale dimension due to relatively small nonlinear optical response of traditional bulk materials. Here, correlated photon pairs are generated in the nanometer scale using a nonlinear optical device for the first time. The approach uses spontaneous four-wave mixing in a carbon nanotube film with extremely large Kerr-nonlinearity (≈100 000 times larger than that of the widely used silica), which is achieved through careful control of the tube diameter during the carbon nanotube growth. Photon pairs with a coincidence to accidental ratio of 18 at the telecom wavelength of 1.5 µm are generated at room temperature in a ≈100 nm thick carbon nanotube film device, i.e., 1000 times thinner than the smallest existing devices. These results are promising for future integrated nonlinear quantum devices (e.g., quantum emission and processing devices).

MoS2 /Carbon Nanotube Core-Shell Nanocomposites for Enhanced Nonlinear Optical Performance

Nanocomposites of layered MoS₂ and multi-walled carbon nanotubes (CNTs) with core-shell structure were prepared by a simple solvothermal method. The formation of MoS₂ nanosheets on the surface of coaxial CNTs has been confirmed by scanning electron microscopy, transmission electron microscopy, absorption spectrum, Raman spectroscopy, and X-ray photoelectron spectroscopy. Enhanced third-order nonlinear optical performances were observed for both femtosecond and nanosecond laser pulses over a broad wavelength range from the visible to the near infrared, compared to those of MoS₂ and CNTs alone. The enhancement can be ascribed to the strong coupling effect and the photoinduced charge transfer between MoS₂ and CNTs. This work affords an efficient way to fabricate novel CNTs based nanocomposites for enhanced nonlinear light-matter interaction. The versatile nonlinear properties imply a huge potential of the nanocomposites in the development of nanophotonic devices, such as mode-lockers, optical limiters, or optical switches.

Polyfluorenylacetylene for near-infrared laser protection: polymer synthesis, optical limiting mechanism and relationship between molecular structure and properties

The incorporation of styryl/stilbene–fluorene into polyacetylenes not only endowed the polymers with novel near-infrared optical limiting properties based on a two-photon absorption mechanism but good solubility and high thermal stability.

CH3NH3PbI3 perovskite:poly(N-vinylcarbazole) blends for broadband optical limiting

The superior optical limiting performance of the CH₃NH₃PbI₃ perovskite: poly(N-vinylcarbazole) (weight ratio: 1 : 1) blends was experimentally studied in both DMF and in the PMMA matrix, respectively, by open aperture Z-scan method.

Role of a singlet diradical character in carbon nanomaterials: a novel hot spot for efficient nonlinear optical materials

Carbon atoms have the potential to produce a variety of fascinating all-carbon structures with amazing electronic and mechanical properties. Over the last few decades, several efforts have been made using experimental and computational techniques to functionalize graphene, carbon nanotubes and fullerenes for potential use in modern hi-tech electronic, medicinal, optical and nonlinear optical (NLO) applications. Since photons replaced electrons as a carrier of information, the field of NLO material design has drawn immense interest in contemporary materials science. There have been several reports of bridging the gap between the exciting fields of carbon nanomaterials and NLO materials by functionalizing carbon nanomaterials for potential NLO applications. In contrast to previous reports of the design of third-order NLO materials using conventional closed-shell materials, a novel strategy using open-shell diradical molecular systems has recently been proposed. Quantum chemically, diradical character is explained in terms of the instability of the chemical bonds in open-shell molecular systems. Interestingly, several carbon nanomaterials, which naturally possess open-shell singlet configurations, have recently gained momentum in the design of these classes of open-shell NLO materials with excellent NLO properties, stability and diversity. The present review establishes a systematic sequence of different studies (spanning over two decades of intense research efforts), starting from the simplest theoretical two-site diradical model, continuing to its experimental and theoretical realization in actual chemical systems, and finally applying the abovementioned model/rule to novel carbon nanomaterials to tune their NLO properties, particularly their second hyperpolarizability (γ). In the present report, we highlight several recent efforts to functionalize carbon nanomaterials by exploiting their open-shell diradical character to achieve efficient third-order NLO properties. Several issues and opportunities are discussed, including the inherited disadvantages of both experimental (the high reactivity and short life of diradical compounds) and quantum (need for multi-reference methodology) techniques when dealing with carbon nanomaterials. A comparative analysis of several quantum chemical investigations, along with contemporary experimental results, will be performed to emphasize the core issues and opportunities related to carbon nanomaterials with singlet open-shell diradical character. Thus, the present review will highlight carbon nanomaterials with diradical/biradical character for their prospective applications in the NLO field.

Controllable Laser Reduction of Graphene Oxide Films for Photoelectronic Applications

This article presents a new simple method of creating light-absorbing carbon material for optical devices such as bolometers. A simple method of laser microstructuring of graphene oxide is used in order to create such material. The absorption values of more than 98% in the visible and more than 90% in the infrared range are achieved. Moreover thermal properties of the films, such as temperature dependence and the thermal response of the samples, are studied. The change in resistance with temperature is 13 Ohm K^-1, temperature coefficient of resistance (TCR) is 0.3% K^-1, and the sensitivity is 0.17 V W^-1 at 300 K. Thermal conductivity is rather high at ∼104 W m^-1 K^-1 at 300 K. The designed bolometer operates at room temperature using incandescent lamp as a light source. This technique suggests a new inexpensive way to create a selective absorption coating and/or active layer for optical devices. Developed GO and rGO films have a large surface area and high conductivity. These properties make carbon coatings a perfect candidate for creating a new type of optoelectronic devices (gas sensors, detectors of biological objects, etc.).

First-order hyperpolarizabilities of chiral, polymer-wrapped single-walled carbon nanotubes

Manipulation of polymer electronic structure provides a new means to modulate the first-order hyperpolarizabilities (β_HRS values) of chiral, individualized polymer-wrapped single-walled carbon nanotube superstructures at a telecommunication-relevant wavelength (1280 nm).

Porphyrin–poly(arylene ether sulfone) covalently functionalized multi-walled carbon nanotubes: synthesis and enhanced broadband nonlinear optical properties

A porphyrin polymer functionalized MWNT hybrid was synthesized with enhanced NLO performance due to the synergistic effect of different mechanism and photo-induced ET/PET process.

Size-dependent nonlinear optical properties of atomically thin transition metal dichalcogenide nanosheets

Size-dependent nonlinear optical properties of modification-free transition metal dichalcogenide (TMD) nanosheets are reported, including MoS2 , WS2 , and NbSe2 . Firstly, a gradient centrifugation method is demonstrated to separate the TMD nanosheets into different sizes. The successful size separation allows the study of size-dependent nonlinear optical properties of nanoscale TMD materials for the first time. Z-scan measurements indicate that the dispersion of MoS2 and WS2 nanosheets that are 50-60 nm thick leads to reverse saturable absorption (RSA), which is in contrast to the saturable absorption (SA) seen in the thicker samples. Moreover, the NbSe2 nanosheets show no size-dependent effects because of their metallic nature. The mechanism behind the size-dependent nonlinear optical properties of the semiconductive TMD nanosheets is revealed by transient transmission spectra measurements.

Multiwalled carbon nanotubes noncovalently functionalized by electro-active amphiphilic copolymer micelles for selective dopamine detection

We have synthesized an electro-active amphiphilic copolymer with carbazole side chains via free radical polymerization using 7-(4-vinylbenzyloxy)-4-methyl coumarin and 9-(4-vinylbenzyl)-9H-carbazole as the monomers.

Nonlinear absorption of SWNT film and its effects to the operation state of pulsed fiber laser

We study a single-wall carbon nanotube (SWNT) Polyvinyl alcohol (PVA) composite as a saturable absorber (SA) for pulse generation in Yb-doped fiber lasers. The saturable absorption and optical limiting (OL) characteristics of the SWNT device are investigated. By combing these two nonlinear effects, we find out for the first time, to the best of our knowledge, that mode-locking can be obtained in the dissipative soliton regime at low pumping followed by Q-switching at high pumping, which is quite different from conventional pulse dynamic evolutions. The Q-switched state operating at higher pump powers is due to the OL effect. The inverted operating fiber laser can be applied in various potential applications such as versatile material processing, optical communication and radar system etc.

Tuning the size and configuration of nanocarbon microcapsules: aqueous method using optical tweezers

To date, optical manipulation techniques for aqueous dispersions have been developed that deposit and/or transport nanoparticles not only for fundamental studies of colloidal dynamics, but also for either creating photonic devices or allowing accurate control of liquids on micron scales. Here, we report that optical tweezers (OT) system is able to direct three-dimensional assembly of graphene, graphite, and carbon nanotubes (CNT) into microcapsules of hollow spheres. The OT technique facilitates both to visualize the elasticity of a CNT microcapsule and to arrange a triplet of identical graphene microcapsules in aqueous media. Furthermore, the similarity of swelling courses has been found over a range of experimental parameters such as nanocarbon species, the power of the incident light, and the suspension density. Thanks to the universality in evolutions of rescaled capsule size, we can precisely control the size of various nanocarbon microcapsules by adjusting the duration time of laser emission.

Enhanced optical limiting in polystyrene-ZnO nanotop composite films

In this work we investigate the optical limiting property of polystyrene-zinc-oxide (ZnO) nanotop composite films, using an open aperture Z-scan technique. The nanocomposites are prepared for different loading concentrations of ZnO and are fabricated using spin and dip coating techniques. On exposing the films to a pulsed nanosecond laser at 532 nm, the nonlinear absorption (NLA) coefficient is found to be greater for spin-coated films compared to dip-coated films. The measured NLA coefficient is found to be enhanced with an increase in loading concentration of ZnO in the monomer for both spin- and dip-coated films.

Facile synthesis of carbon doped TiO2 nanowires without an external carbon source and their opto-electronic properties

The present study demonstrates a simple protocol for the preparation of one dimensional (1D) oxidized titanium carbide nanowires and their opto-electronic properties. The oxidized titanium carbide nanowires (Ox-TiC-NW) are prepared from TiC nanowires (TiC-NW) that are in turn synthesized from micron sized TiC particles using the solvothermal technique. The Ox-TiC-NW is characterized by X-ray diffraction, UV-Vis spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Raman spectroscopy. Thermal oxidation of TiC-NW yields carbon doped TiO2-NW (C-TiO2-NW), a simple methodology to obtain 1D C-TiO2-NW. Temperature dependent Raman spectra reveal characteristic bands for TiO2-NW. Electrical characterization of individual C-TiO2-NW is performed by fabricating a device structure using the focused ion beam deposition technique. The opto-electronic properties of individual C-TiO2-NW demonstrate visible light activity and the parameters obtained from photoconductivity measurements reveal very good sensitivity. This methodology opens up the possibility of using C-TiO2-NW in electronic and opto-electronic device applications.

Ultrafast saturable absorption of two-dimensional MoS2 nanosheets

Employing high-yield production of layered materials by liquid-phase exfoliation, molybdenum disulfide (MoS2) dispersions with large populations of single and few layers were prepared. Electron microscopy verified the high quality of the two-dimensional MoS2 nanostructures. Atomic force microscopy analysis revealed that ~39% of the MoS2 flakes had thicknesses of less than 5 nm. Linewidth and frequency difference of the E(1)2g and A1g Raman modes confirmed the effective reduction of flake thicknesses from the bulk MoS2 to the dispersions. Ultrafast nonlinear optical (NLO) properties were investigated using an open-aperture Z-scan technique. All experiments were performed using 100 fs pulses at 800 nm from a mode-locked Ti:sapphire laser. The MoS2 nanosheets exhibited significant saturable absorption (SA) for the femtosecond pulses, resulting in the third-order NLO susceptibility Imχ((3)) ~ 10(-15) esu, figure of merit ~10(-15) esu cm, and free-carrier absorption cross section ~10(-17) cm(2). Induced free carrier density and the relaxation time were estimated to be ~10(16) cm(-3) and ~30 fs, respectively. At the same excitation condition, the MoS2 dispersions show better SA response than the graphene dispersions.

Optical limiting effect and ultrafast saturable absorption in a solid PMMA composite containing porphyrin-covalently functionalized multi-walled carbon nanotubes

A versatile solid Poly-methyl-methacrylate (PMMA) composite containing porphyrin-covalently functionalized multi-walled carbon nanotubes (MWNTs-TPP) was prepared through free radical polymerization without additional dispersion stabilizer. Using nanosecond, femtosecond pulse Z-scan and degenerate femtosecond pump-probe techniques, we studied the optical limiting effect, ultrafast saturable absorption and transient differential transmission of the composite. Results show that the solid composite exhibits weaker optical limiting effects than that of the suspension at 532 nm under nanosecond pulse, due to the absence of nonlinear scattering mechanism. The composite also shows ultrafast saturable absorption with a relaxation time about 190 fs at 800 nm under femtosecond pulse due to band-filling effect, comparably to the suspension. The versatile solid composite can be the candidate for uses in applications of ultrafast optical switching and mode-locking element or optical limiter for nanosecond pulse.

Controllable broadband nonlinear optical response of graphene dispersions by tuning vacuum pressure

Nonlinear scattering, originating from laser induced solvent micro-bubbles and/or micro-plasmas, is regarded as the principal mechanism for nonlinear optical (NLO) response of graphene dispersions at ns timescale. In this work, we report the significant enhancement of NLO response of graphene dispersions by decreasing the atmospheric pressure, which has strong influence on the formation and growth of micro-bubbles and/or micro-plasmas. A modified open-aperture Z-scan apparatus in combination with a vacuum system was used to study the effect of vacuum pressure on the NLO property of graphene dispersions prepared by liquid-phase exfoliation technique. We show that the atmospheric pressure can be utilized to control and tune the nonlinear responses of the graphene dispersions for ns laser pulses at both 532 nm and 1064 nm. The lower the vacuum pressure was, the larger the NLO response was. In contrast, the NLO property of fullerene was found to be independent of the pressure change, due to its nature of nonlinear absorption. This work affords a simple method to distinguish the nonlinear scattering and absorption mechanisms for NLO nanomaterials.

Organic-inorganic hybrid functional carbon dot gel glasses

Silane pre-functionalized carbon dots, arbitrarily doped (0-100% scale) carbon dot nanohybrid gel glasses and macrostructures with high luminescence (quantum yields = 47% and 88%, respectively) and broadband optical limiting properties (532 and 1064 nm) are reported. These glasses are optically, thermally, and mechanically stable, as well as highly transmissive (ca. 90%) in the 400-1350 nm region.