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1.
Size‐dependent nonlinear optical properties of modification‐free transition metal dichalcogenide (TMD) nanosheets are reported, including MoS 2, WS 2, and NbSe 2. 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 MoS 2 and WS 2 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 NbSe 2 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. 相似文献
2.
Recent observations of facet‐dependent electrical conductivity and photocatalytic activity of various semiconductor crystals are presented. Then, the discovery of facet‐dependent surface plasmon resonance absorption of metal–Cu 2O core–shell nanocrystals with tunable sizes and shapes is discussed. The Cu 2O shells also exhibit a facet‐specific optical absorption feature. The facet‐dependent electrical conductivity, photocatalytic activity, and optical properties are related phenomena, resulting from the presence of an ultrathin surface layer with different band structures and thus varying degrees of band bending for the {100}, {110}, and {111} faces of Cu 2O to absorb light of somewhat different wavelengths. Recently, it is shown that the light absorption and photoluminescence properties of pure Cu 2O cubes, octahedra, and rhombic dodecahedra also display size and facet effects because of their tunable band gaps. A modified band diagram of Cu 2O can be constructed to incorporate these optical effects. Literature also provides examples of facet‐dependent optical behaviors of semiconductor nanostructures, indicating that optical properties of nanoscale semiconductor materials are intrinsically facet‐dependent. Some applications of semiconductor optical size and facet effects are considered. 相似文献
3.
The effects of composition, film thickness, substrate temperature, and annealing of amorphous thin films of Se 75Ge 25−x
As
x
(5⩽ x⩽20) on their optical properties have been investigated. X-ray diffraction revealed the formation of amorphous films. The
absorbance and transmission of vacuum-evaporated thin films were used to determine the band gap and refractive index. Optical
absorption measurements showed that the fundamental absorption edge is a function of glass composition and the optical absorption
is due to indirect transition. The energy gap increases linearly with increasing arsenic content. The optical band gap, E
opt, was found to be almost thickness independent. The shapes of the absorption edge of annealed samples displayed roughly the
same characteristic as those of the unannealed films, but were shifted towards shorter wavelengths; as a result, E
opt increased and E
e, the width of the band tails, decreases. The increase in E
opt is believed to be associated with void removal and microstructural re-arrangement during annealing. The influence of substrate
temperature on the optical parameters is discussed. 相似文献
4.
Linear and nonlinear optical properties of (Pb,La)(Zr,Ti)O 3 (PLZT) ferroelectric thin films were presented in this paper. The PLZT ferroelectric thin films have been in situ grown on
quartz substrates by radio-frequency (RF) magnetron sputtering at 650 °C. Their crystalline structure and surface morphologies
were examined by X-ray diffraction and atomic force microscopy, respectively. It can be found that the PLZT thin films exhibit
well-crystallized perovskite structure and good surface morphology. The fundamental optical constants (the band gap energy,
linear refractive index, and linear absorption coefficient) were obtained through the optical transmittance measurements.
A Z-scan technique was used to investigate the optical nonlinearity of the PLZT thin films on quartz substrates. The films display
the strong third-order nonlinear optical effect. A large and negative nonlinear refractive index n
2 is determined to be 1.21 × 10 −6 esu for the PLZT thin films. All results show that the PLZT ferroelectric thin films have potential applications in optical
limiting, switching, and modulated-type optical devices. 相似文献
5.
Graphene is a promising candidate material for high‐speed and ultra‐broadband photodetectors. However, graphene‐based photodetectors suffer from low photoreponsivity and Ilight/Idark ratios due to their negligible‐gap nature and small optical absorption. Here, a new type of graphene/InAs nanowire (NW) vertically stacked heterojunction infrared photodetector is reported, with a large photoresponsivity of 0.5 AW ?1 and Ilight/Idark ratio of 5 × 10 2, while the photoresponsivity and Ilight/Idark ratio of graphene infrared photodetectors are 0.1 mAW ?1 and 1, respectively. The Fermi level ( EF ) of graphene can be widely tuned by the gate voltage owing to its 2D nature. As a result, the back‐gated bias can modulate the Schottky barrier (SB) height at the interface between graphene and InAs NWs. Simulations further demonstrate the rectification behavior of graphene/InAs NW heterojunctions and the tunable SB controls charge transport across the vertically stacked heterostructure. The results address key challenges for graphene‐based infrared detectors, and are promising for the development of graphene electronic and optoelectronic applications. 相似文献
6.
Pulsed lasers operating in the mid‐infrared (3–25 µm) are increasingly becoming the light source of choice for a wide range of industrial and scientific applications such as spectroscopy, biomedical research, sensing, imaging, and communication. Up to now, one of the factors limiting the mid‐infrared pulsed lasers is the lack of optical switch with a capability of pulse generation, especially for those with wideband response. Here, a semiconductor material of bismuth oxyselenide (Bi 2O 2Se) with a facile processibility, constituting an ultrabroadband saturable absorber for the mid‐infrared (actually from the near‐infrared to mid‐infrared: 0.8–5.0 µm) is exhibited. Significantly, it is found that the optical response is associated with a strong nonlinear character, showing picosecond response time and response amplitude up to ≈330.1% at 5.0 µm. Combined with facile processibility and low cost, these solution‐processed Bi 2O 2Se materials may offer a scalable and printable mid‐infrared optical switch to open up the long‐sought parameter space which is crucial for the exploitation of compact and high‐performance mid‐infrared pulsed laser sources. 相似文献
7.
The effect of film thickness on the optical and electrical properties of Cu-30 wt % GeO 2-70 wt % thin cermet films prepared by electron-beam deposition at about 10 –3 Pa and at a substrate temperature of 300 K is reported. The ultraviolet, visible and direct current (d.c.) conductivity results are analysed with the aim of determining the optical band gap, E
opt, the width of the band tails, E
e, and the d.c. thermal activation energy, E
a. It was found that the optical energy gap increases with increasing thickness and that the absorption was due to indirect transitions in k-space. The general feature of the absorption edge remains similar for both unannealed and annealed films, but annealing has the effect of decreasing E
opt. The d.c. conductivity results show that E
a decreases with increasing thickness. From a knowledge of E
opt and E
a, a probable model of the electronic band structure in Cu-GeO 2 thin films has been suggested. 相似文献
8.
CdO doped (doping concentration 0, 1, 3 and 16 wt%) ZnO nanostructured thin films are grown on quartz substrate by pulsed laser deposition and the films are annealed at temperature 500 °C. The structural, morphological and optical properties of the annealed films are systematically studied using grazing incidence X-ray diffraction (GIXRD), energy dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), atomic force microscopy (AFM), Micro-Raman spectra, UV–vis spectroscopy, photoluminescence spectra and open aperture z-scan. 1 wt% CdO doped ZnO films are annealed at different temperatures viz., 300, 400, 500, 600, 700 and 800 °C and the structural and optical properties of these films are also investigated. The XRD patterns suggest a hexagonal wurtzite structure for the films. The crystallite size, lattice constants, stress and lattice strain in the films are calculated. The presence of high-frequency E2 mode and the longitudinal optical A1 (LO) modes in the Raman spectra confirms the hexagonal wurtzite structure for the films. The presence of CdO in the doped films is confirmed from the EDX spectrum. SEM and AFM micrographs show that the films are uniform and the crystallites are in the nano-dimension. AFM picture suggests a porous network structure for 3% CdO doped film. The porosity and refractive indices of the films are calculated from the transmittance and reflectance spectra. Optical band gap energy is found to decrease in the CdO doped films as the CdO doping concentration increases. The PL spectra show emissions corresponding to the near band edge (NBE) ultra violet emission and deep level emission in the visible region. The 16CdZnO film shows an intense deep green PL emission. Non-linear optical measurements using the z-scan technique indicate that the saturable absorption (SA) behavior exhibited by undoped ZnO under green light excitation (532 nm) can be changed to reverse saturable absorption (RSA) with CdO doping. From numerical simulations the saturation intensity ( Is) and the effective two-photon absorption coefficient ( β) are calculated for the undoped and CdO doped ZnO films. 相似文献
9.
Two-photon absorption induced optical limiting action was demonstrated in cytosinium benzoate (CB) under nanosecond laser (532 nm, 9 ns, and 10 Hz) excitation. Intensity dependent open aperture Z-scan experiment exposed the presence of reverse saturable absorption ascribed due to sequential two-photon absorption. Initially CB single crystals were grown at room temperature by slow evaporation solution technique. Single crystal XRD shows that CB belongs to monoclinic crystal system with P21/c space group. Fourier Transform Infrared spectrum was recorded to identify the presence of functional groups. Thermal studies shows that the crystal is stable upto 168 °C. Vickers microhardness studies confirm that the grown crystal was belongs to soft material category. Etching study shows linear rectangular etch patterns (5 s) and well defined stacking planes (10 s) for water etchant. Optical studies demonstrate that CB crystal possess lower cut-off (287 nm) and moderate linear transmittance in visible region. The optical energy band gap of CB crystal was estimated from photoluminescence studies as 3.1 eV. CB with higher two-photon absorption coefficient (1.26?×?10–10 m/W) and lower onset limiting threshold (1.92?×?1012 W/m2) can be a potential candidate for developing laser safety devices under nanosecond green laser excitation regime. 相似文献
10.
Linear and nonlinear optical properties of a phthalocyanine (Pc)-based nanohybrid material PCIGS [Cu 2( tBu 4PcGa)( tBu 4PcIn)S 2TPP 2] are described. The overall aggregation of phthalocyanines in poly(methylmethacrylate) (PMMA) films was evident, which is indicated by the broadening of linear spectra in the Q-band region and the shift of wavelength. Upon excitation with nanosecond laser pulse at 355 nm, the transient absorption band appeared at about 500 nm is attributed to the triplet–triplet absorption of the Pcs. For PCIGS and its starting materials tBu 4PcGaCl and tBu 4PcInCl, all Z-scans exhibit a decrease in transmittance about the focus typical of an induced positive nonlinear absorption of incident light. The absorption mechanism is due to population of excited states through a multi-step nonlinear absorption. When these Pc compounds were embedded into a commercially available polymer PMMA, all the Pc/PMMA composites display much larger nonlinear absorption coefficient and lower saturable fluence for optical limiting when compared to the same Pc molecules in solution. However, in contrast to tBu 4PcGaCl and tBu 4PcInCl, PCIGS displayed decreased optical limiting response, possibly due to competing electron accepting processes in the In and Ga metals, and the highly ordered structure of the PCIGS complex itself. 相似文献
11.
In the present study, it has been reported on the effect of Al doping on linear and nonlinear optical properties of ZnO thin films synthesized by spray pyrolysis method. The structural properties of ZnO thin films with different Al doping levels (0–4 wt%) were analyzed using X-ray diffraction (XRD). The results obtained from XRD analysis indicated that the grain size decreased as the Al doping value increased. The UV–Vis diffused refraction spectroscopy was used for calculation of band gap. The optical band gap of Al-doped ZnO (AZO) thin films is increased from 3.26 to 3.31 eV with increasing the Al content from 0 to 4 wt%. The measurements of nonlinear optical properties of AZO thin films have been performed using a nanosecond Nd:YAG pulse laser at 532 nm by the Z-scan technique. The undoped ZnO thin film exhibits reverse saturation absorption (RSA) whereas the AZO thin films exhibit saturation absorption (SA) that shows RSA to SA process with adding Al to ZnO structure under laser irradiation. On the other hand, all the films showed a self-defocusing phenomenon because the photons of laser stay on below the absorption edge of the ZnO and AZO films. The third-order nonlinear optical susceptibility, χ (3), of AZO thin films, was varied from of the order of 10 ?5–10 ?4 esu. The results suggest that AZO thin films may be promising candidates for nonlinear optical applications. 相似文献
12.
Optical absorption and transmittance spectra of 60B 2O 3-(40- x)PbO- xMCl 2 and 50B 2O 3-(50- x) PbO-xMCl 2 (M = Pb, Cd) (10 ≤ x ≤ 20) glasses of varying composition were recorded in the UV-visible region. Various optical parameters such as optical energy
gap ( E
opt), Urbach energy ( E
e), refractive index ( n
0), optical dielectric constant (ε∞), and ratio of carrier concentration to the effective mass ( N/m*;) were determined. The variation of optical energy gap with increase in the concentration of PbCl 2 or CdCl 2 is discussed. 相似文献
13.
A polymer based on fluorene, thiophene, and benzothiadazole as the donor–spacer–acceptor triad is covalently coupled to reduced graphene oxide (rGO) sheets via diazonium coupling with phenyl bromide, followed by Suzuki coupling. These polymer–graphene hybrids show good solubility in organic solvents, such as chloroform, tetrahydrofuran (THF), toluene, dichlorobenzene, and N, N‐dimethylformamide (DMF), and exhibit an excellent optical‐limiting effect with a 532‐nm laser beam. The optical‐limiting threshold energy values (0.93 J cm ?2 for G–polymer 1 and 1.12 J cm ?2 for G–polymer 2) of these G–polymer hybrids are better than that of carbon nanotubes (3.6 J cm ?2). 相似文献
14.
MXenes comprise a new class of 2D transition metal carbides, nitrides, and carbonitrides that exhibit unique light–matter interactions. Recently, 2D Ti 3CNT x (T x represents functional groups such as ? OH and ? F) was found to exhibit nonlinear saturable absorption (SA) or increased transmittance at higher light fluences, which is useful for mode locking in fiber‐based femtosecond lasers. However, the fundamental origin and thickness dependence of SA behavior in MXenes remain to be understood. 2D Ti 3C 2T x thin films of different thicknesses are fabricated using an interfacial film formation technique to systematically study their nonlinear optical properties. Using the open aperture Z‐scan method, it is found that the SA behavior in Ti 3C 2T x MXene arises from plasmon‐induced increase in the ground state absorption at photon energies above the threshold for free carrier oscillations. The saturation fluence and modulation depth of Ti 3C 2T x MXene is observed to be dependent on the film thickness. Unlike other 2D materials, Ti 3C 2T x is found to show higher threshold for light‐induced damage with up to 50% increase in nonlinear transmittance. Lastly, building on the SA behavior of Ti 3C 2T x MXenes, a Ti 3C 2T x MXene‐based photonic diode that breaks time‐reversal symmetry to achieve nonreciprocal transmission of nanosecond laser pulses is demonstrated. 相似文献
15.
Molybdenum disulfide (MoS 2) and tungsten disulfide (WS 2), two representative transition metal dichalcogenide materials, have captured tremendous interest for their unique electronic, optical, and chemical properties. Compared with MoS 2 and WS 2, molybdenum ditelluride (MoTe 2) and tungsten ditelluride (WTe 2) possess similar lattice structures while having smaller bandgaps (less than 1 eV), which is particularly interesting for applications in the near‐infrared wavelength regime. Here, few‐layer MoTe 2/WTe 2 nanosheets are fabricated by a liquid exfoliation method using sodium deoxycholate bile salt as surfactant, and the nonlinear optical properties of the nanosheets are investigated. The results demonstrate that MoTe 2/WTe 2 nanosheets exhibit nonlinear saturable absorption property at 1.55 μm. Soliton mode‐locking operations are realized separately in erbium‐doped fiber lasers utilizing two types of MoTe 2/WTe 2‐based saturable absorbers, one of which is prepared by depositing the nanosheets on side polished fibers, while the other is fabricated by mixing the nanosheets with polyvinyl alcohol and then evaporating them on substrates. Numerous applications may benefit from the nonlinear saturable absorption features of MoTe 2/WTe 2 nanosheets, such as visible/near‐infrared pulsed laser, materials processing, optical sensors, and modulators. 相似文献
16.
The schematic of the energy band gap figure of the graded optical band gap ( Egopt) in p-i-n layer in na-Si:H solar cells was given in the paper. The intrinsic hydrogenated nanoamorphous silicon(na-Si:H) thin films with the graded band gap as a function of depth through the films were prepared by varying the processing power, gas pressure, gas composition, and etc., We have carried out a investigation of the relationships between the Egopt with the crystallization ratio (Xc) and the Egopt with the nanocrystalline grain size ( D) in na-Si:H thin films grown by PECVD on glass substrates through XRD, Raman scattering, transmission. The Egopt increase with the decreases of the crystallization ratio (Xc) and the nanocrystalline grain size ( D). The hydrogen dilution ratio is found to increase basically both the crystallization ratio (Xc) and the nanocrystalline grain size ( D). Two relationships in na-Si:H are discussed by the etching effect of atomic hydrogen in the framework of the growth mechanism and the quantum size effect (QSE). 相似文献
17.
A microcrystalline mixture of cadmium carbonate (CdCO 3) and cadmium sulfide (CdS) were grown in the thin film format onto glass substrates by means of chemical bath. The temperature
of the bath ( Td) was selected in the interval 23–80 ∘C. At low temperatures, CdCO 3 is the compound predominant in the layers. At high temperatures CdS is the compound deposited on the substrate. At intermediate
Td-values a mixture of both materials are present, i.e., the gradual transition from an insulator (CdCO 3) to a semiconductor (CdS) growth occurs when Td increases. Physical properties of films were studied by means of X-ray diffraction and optical absorption. The forbidden
energy band gap of direct electronic transitions ( Eg) was calculated by applying the α 2
∝ ( hν − Eg) relation to the optical absorption spectra. 相似文献
18.
Periodically hydrogenated graphene is predicted to form new kinds of crystalline 2D materials such as graphane, graphone, and 2D C xH y, which exhibit unique electronic properties. Controlled synthesis of periodically hydrogenated graphene is needed for fundamental research and possible electronic applications. Only small patches of such materials have been grown so far, while the experimental fabrication of large‐scale, periodically hydrogenated graphene has remained challenging. In the present work, large‐scale, periodically hydrogenated graphene is fabricated on Ru(0001). The as‐fabricated hydrogenated graphene is highly ordered, with a √3 × √3/R30° period relative to the pristine graphene. As the ratio of hydrogen and carbon is 1:3, the periodically hydrogenated graphene is named “one‐third‐hydrogenated graphene” (OTHG). The area of OTHG is up to 16 mm 2. Density functional theory calculations demonstrate that the OTHG has two deformed Dirac cones along one high‐symmetry direction and a finite energy gap along the other directions at the Fermi energy, indicating strong anisotropic electrical properties. An efficient method is thus provided to produce large‐scale crystalline functionalized graphene with specially desired properties. 相似文献
19.
The spectral properties of undoped and Al doped ZnO nano thin films prepared using double dip method otherwise called SILAR
method (Successive Immersion Layer Adsorption Reaction) are reported. The thin films were having polycrystalline hexagonal
structure. The optical properties of these films are studied and reported. The optical constants like the band gap ( E
g
), refractive indices ( n, k), dielectric constant (ε), optical conductivity (σ), were estimated using an approximation algorithm developed from established
procedures using transmittance spectrum of the thin films. The average excitation energy ( E
0), oscillator strength ( E
d
), effective mass ( m*), plasma frequency ( ω
p
), static dielectric constant (ε ∞) and carrier concentration ( N) are also estimated and reported. The highly transparent thin films showed nanowires protruding from stacked nanorods on
SEM inspection that signifies the suitability of these thin films for gas sensors. 相似文献
20.
Van der Waals hybrids of graphene and transition metal dichalcogenides exhibit an extremely large response to optical excitation, yet counting of photons with single‐photon resolution is not achieved. Here, a dual‐gated bilayer graphene (BLG) and molybdenum disulphide (MoS 2) hybrid are demonstrated, where opening a band gap in the BLG allows extremely low channel (receiver) noise and large optical gain (≈10 10) simultaneously. The resulting device is capable of unambiguous determination of the Poissonian emission statistics of an optical source with single‐photon resolution at an operating temperature of 80 K, dark count rate 0.07 Hz, and linear dynamic range of ≈40 dB. Single‐shot number‐resolved single‐photon detection with van der Waals heterostructures may impact multiple technologies, including the linear optical quantum computation. 相似文献
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