垂直电场下扭转双层石墨烯光学吸收性质的理论研究

层间扭转角度是对石墨烯物理性质宽波段可调谐的一个新参量.本文采用2°<θ<15°扭转角度下的连续近似模型,获得了不同扭转角度双层石墨烯分别在有、无电场下的能带结构,通过电子-光子相互作用跃迁速率,计算模拟了范霍夫奇点附近电子带内跃迁和带间跃迁所引起的光学吸收谱.结果表明,在无外加电场时,带间跃迁吸收峰的位置随着扭转角度的增大而发生从红外到可见光波段的蓝移,且吸收系数增大,带内跃迁的光学吸收系数相对于带间跃迁高出2个数量级;而存在外加电场时,两个范霍夫奇点在波矢空间的位置发生偏移,带间跃迁吸收峰发生分裂,且两个分裂的吸收峰位置随着电场强度的不断增大而反向行进.上述研究结果对石墨烯材料在光电器件方面的应用有一定指导作用.     

Optical spectroscopy of graphene: From the far infrared to the ultraviolet

The unique electronic structure of graphene leads to several distinctive optical properties. In this brief review, we outline the current understanding of two general aspects of optical response of graphene: optical absorption and light emission. We show that optical absorption in graphene is dominated by intraband transitions at low photon energies (in the far-infrared spectral range) and by interband transitions at higher energies (from mid-infrared to ultraviolet). We discuss how the intraband and interband transitions in graphene can be modified through electrostatic gating. We describe plasmonic resonances arising from the free-carrier (intraband) response and excitonic effects that are manifested in the interband absorption. Light emission, the reverse process of absorption, is weak in graphene due to the absence of a band gap. We show that photoluminescence from hot electrons can, however, become observable either through femtosecond laser excitation or strong electrostatic gating.     

石英玻璃中导带电子的光吸收

分别用二阶和三阶微扰理论计算了 １９３ｎｍ 、３５５ｎｍ 激光照射下石英玻璃中导带电子的单光子吸收速率和双光子吸收速率。结果表明,电子空穴散射参与的单光子吸收和声学声子参与的双光子吸收都是材料中导带电子吸收激光能量的重要过程。     

Influence of direct transitions between spin branches on free-carrier absorption of ferromagnetic semiconductors

The direct spin-flip intraband absorption coefficient due to transitions of conduction electrons between the spin branches in degenerate ferromagnetic semiconductors is calculated. It is shown that the absorption coefficient consists essentially of a sharp peak at the band splitting frequency which shifts towards low frequencies as the temperature increases from T = 0 K.     

Electromodulation of the interband and intraband absorption of Ge/Si self-assembled islands

We have investigated the interband and the intraband absorption properties of Ge/Si self-assembled islands. The investigated structure consists of a p–i–n junction containing Ge/Si self-assembled islands embedded in a Si_0.98Ge_0.02 waveguiding layer. The variation of transmission associated with carrier injection under forward bias is monitored both in the near-infrared and in the mid-infrared spectral ranges. We show that the carrier injection leads to an absorption resonant at 185 meV which is polarized along the growth axis of the islands. This transition corresponds to an intraband optical transition from the island ground states to the two-dimensional wetting layer states. This assignment is supported by a two-dimensional band structure calculation performed in a 14 band k·p formalism. Meanwhile, the carrier injection leads to a bleaching of the interband absorption. We show that this electroabsorption spectroscopy is a useful tool for the study of self-assembled islands that is complementary of standard photoluminescence, electroluminescence or absorption spectroscopies.     

石英玻璃中导带电子的光吸收

 分别用二阶和三阶微扰理论计算了193nm、355nm激光照射下石英玻璃中导带电子的单光子吸收速率和双光子吸收速率。结果表明,电子空穴散射参与的单光子吸收和声学声子参与的双光子吸收都是材料中导带电子吸收激光能量的重要过程。     

Momentum-resolved lifetimes of image-potential States on cu(100)

The dependence of the inelastic lifetime of electrons in the image-potential states of Cu(100) on their momentum parallel to the surface has been studied experimentally by means of time- and angle-resolved two-photon photoemission and theoretically by evaluating the electron self-energy within the GW approximation. The pronounced decrease of the n = 1 lifetime from 40 fs at normal emission (k(parallel) = 0) to 20 fs for k(parallel) = 0.33 A(-1) cannot be accounted for by interband decay processes to bulk states. We show that intraband transitions within the image-state band give a contribution to this decrease comparable in magnitude with the interband channel.     

Two color laser ablation: Enhanced yield, improved machining

Laser ablation of semiconductors with nano- and picosecond lasers can be significantly improved, both in terms of yield and surface quality, by simultaneous irradiation of the sample with the fundamental beam (λ = 1064 nm) and a small amount of its second harmonic (SH) produced in a thin nonlinear crystal. While the total energy fluence is conserved, the small fraction of the second harmonic serves to excite electrons into the conduction band to get the ablation process started. For femtosecond laser pulses, the effect becomes insignificant, since sufficient conduction band population is provided by multiphoton absorption.     

Observation of simultaneous reverse saturation absorption and saturation absorption in silver nanoparticles incorporated into europium oxide thin film

In this work, nano silver clusters incorporated into europium oxide thin films at a level of 3.8% and 12.5% have been prepared by a vacuum evaporation method on glass and silicon substrates. Samples were investigated by X-ray fluorescence, X-ray diffraction, and linear and nonlinear optical absorption methods. The X-ray diffraction reveals that the Eu oxide of these samples remains amorphous after pre-annealing at 400 °C. The linear optical absorption of the samples shows surface plasmon resonance (SPR) phenomena, which varies with the Ag content of the samples. The optical nonlinear absorption properties of the prepared films were investigated using an open Z-scan technique with cw laser at wavelengths 476 nm and 514 nm. A changeover from reverse saturation absorption (RSA) to saturation absorption (SA) was observed. RSA is attributed to interband transition via two photon absorption. SA is attributed to plasmon bleach.     

Ultrafast electron dynamics in amorphous and crystalline D2O layers on Ru(0 0 1)

Femtosecond dynamics of excess electrons photo-injected into amorphous and crystalline D₂O layers on Ru(0 0 1) have been investigated by time-resolved two-photon photoelectron spectroscopy. In the crystalline case, excited electrons are transferred into delocalized states considered as image potential states in the conduction band of ice and relax back to the metal on an ultrafast time scale. The life time of the n = 1 image potential state is <5 fs. In the amorphous case, spectral features arise from delocalized and localized electronic states. Relaxation of delocalized electrons back to the metal is as fast as in the crystalline case. The binding energy of localized electrons, however, is found to increase as a function of time delay by 1 eV/ps, which is attributed to the formation of solvated electrons. Such energetic stabilization starting at the bottom of the conduction band is clearly absent in crystalline layers. This pronounced correlation of electronic structure and electron dynamics with molecular structure is associated with the presence of localized states near the bottom of the conduction band in amorphous ice. Such localized states are absent for perfect periodic crystalline structures but prevail in amorphous systems where they serve as precursor sites for electron solvation.     

水溶性CdTe:Mn量子点的双光子吸收

采用激发波长800 nm、脉宽50 fs、重复频率1 kHz的Ti:sapphire放大飞秒激光器作为激发光源,利用开孔Z扫描技术研究了不同粒径的CdTe:Mn量子点的非线性吸收性质。理论计算结果表明,同一生长时间CdTe:Mn量子点的双光子吸收系数是CdTe量子点的1.1倍,其双光子吸收系数随量子点尺寸的减小而增大,这是由于CdTe:Mn量子点非线性吸收属于反饱和吸收,掺杂了Mn元素,减小了表面缺陷浓度,表明掺杂量子点具有很好的双光子吸收现象。     

Multiphoton generation of electron-hole pairs involving free carriers in an indirect-gap crystal

The paper reports on a study of nonlinear absorption of high-intensity laser radiation in the case where the photon energy is less than one half of the indirect-gap width in a crystal. The deficiency in the energy needed for two-photon excitation of the electron-hole pair is made up by the kinetic energy of free electrons, which was acquired in intraband nonlinear absorption of light. The probabilities of Auger-type indirect two-photon interband transitions involving free electrons have been calculated by perturbation theory. It is shown that, for a free-carrier concentration in the conduction band n _c ? 10¹⁵ cm^?3, and the radiation intensity range of interest for the experiment, j ～ 3–10 GW/cm², the calculated probabilities of such transitions exceed by several orders of magnitude those of “conventional” direct and indirect (involving phonons) multiphoton transitions which can take place in the system considered.     

Modeling non-Lorentzian two-photon absorption line shape in dipolar chromophores

We present a new approach to modeling of homogeneous line shape in two-photon absorption (2PA) spectra of chromophores with large permanent dipole moment difference between the ground- and excited electronic states using numerical solution of stochastic two-level density matrix equation of motion. Good agreement with experimental 2PA line shapes is obtained for S₁←S₀ transition of Styryl 9 M, which allows us to estimate that the permanent dipole moment difference varies in this chromophore within the S₁←S₀ band in the range, Δμ=12-25 D.     

Size-dependent nonlinear absorption and refraction of Ag nanoparticles excited by femtosecond lasers

Silver (Ag) nanoparticles with different average sizes are prepared, and the nonlinear absorption and refraction of these nanoparticles are investigated with femtosecond laser pulses at 800 nm. The smallest Ag nanoparticles show insignificant nonlinear absorption, whereas the larger ones show saturable absorption. By considering the previously reported positive nonlinear absorption of 9 nm Ag nanoparticles, the nonlinear absorptions of Ag nanoparticles are found to be size-dependent. All these nonlinear absorptions can be compatibly explained from the viewpoints of electronic transitions, energy bands and electronic structures in the conduction band of Ag nanoparticles. The nonlinear refraction is attributed to the effect of hot electrons arising from the intraband transition in the s–p conduction band of Ag nanoparticles.     

Two types of fundamental luminescence of ionization-passive electrons and holes in optical dielectrics—Intraband-electron and interband-hole luminescence (theoretical calculation and comparison with experiment)

A short high-power pulse of ionizing radiation creates a high concentration of nonequilibrium electrons and holes in a dielectric. They quickly lose their energy, generating a multiplicity of secondary quasiparticles: electron—hole pairs, excitons, plasmons, phonons of all types, and others. When the kinetic energy of an electron becomes less that some value E_Δ≈(1.3-2)E_g it loses the ability to perform collisional ionization and electron excitations of the dielectric medium. Such an electron is said to be ionization-passive. It relaxes to the bottom of the lower conduction band by emitting phonons. Similarly a hole becomes ionization-passive when it “floats up” above some level E_H and loses the ability for Auger ionization of the dielectric medium. It continues to float upward to the ceiling of the upper valance band only by emitting phonons. The concentrations of ionization-passive electrons and holes are larger by several orders of magnitude than those of the active electrons and holes and consequently make of a far larger contribution to many kinetic processes such as luminescence. Intraband and interband quantum transitions make the greatest contribution to the fundamental (independent of impurities and intrinsic defects) electromagnetic radiation of ionization-passive electrons and holes. Consequently the brightest types of purely fundamental luminescence of strongly nonequilibrium electrons and holes are intraband and interband luminescence. These forms of luminescence, discovered relatively recently, carry valuable information on the high-energy states of the electrons in the conduction band and of the holes in the valence band of a dielectric. Experimental investigations of these types of luminescence were made, mainly on alkali halide crystals which were excited by nanoseconal pulses of high-current-density electrons and by two-photon absorption of the ultraviolet harmonics of pulsed laser radiation beams of nanosecond and picosecond duration. The present article gives the results of theoretical calculations of the spectra and other characteristics of intraband electron and interband hole luminescence which are compared with the experimental data. Institute of High-Current Electronics, Sibrian Branch of the Russian Academy of Sciences, Polytechnic University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 13–41, November, 1997.     

Light-induced ultrafast phase transitions in VO2 thin film

Vanadium dioxide shows a passive and reversible change from a monoclinic insulator phase to a metallic tetragonal rutile structure when the sample temperature is close to and over 68 °C. As a kind of functional material, VO₂ thin films deposited on fused quartz substrates were successfully prepared by the pulsed laser deposition (PLD) technique. With laser illumination at 400 nm on the obtained films, the phase transition (PT) occurred. The observed light-induced PT was as fast as the laser pulse duration of 100 fs. Using a femtosecond laser system, the relaxation processes in VO₂ were studied by optical pump-probe spectroscopy. Upon a laser excitation an instantaneous response in the transient reflectivity and transmission was observed followed by a relatively longer relaxation process. The alteration is dependent on pump power. The change in reflectance reached a maximum value at a pump pulse energy between 7 and 14 mJ/cm². The observed PT is associated with the optical interband transition in VO₂ thin film. It suggests that with a pump laser illuminating on the film, excitation from the d_θ,? - state of valence band to the unoccupied excited mixed d_θ,?-π* - state of the conduction band in the insulator phase occurs, followed by a resonant transition to an unoccupied excited mixed d_θ,?-π* - state of the metallic phase band.     

Measurement of nonlinear absorption coefficients in GaAs, InP and Si by an optical pump THz probe technique

An optical pump terahertz (THz) probe method for measuring carrier mobility and multiphoton absorption coefficients in semiconductors is demonstrated. A THz probe pulse is used to detect the transient photoconductivity generated by an optical pump pulse. The change in transmission coefficient at THz frequencies due to a pump pulse with photon energy greater than the band gap energy is used to determine the sum of electron and hole mobilities. The weak nonlinear absorption of a pump pulse with photon energy less than the band gap energy produces an approximately uniform free carrier distribution. The THz transmission coefficient vs. pump fluence, and the mobility, are used in a bulk photoconductivity model to determine the multiphoton absorption coefficients. For GaAs, InP and Si we find two photon absorption coefficients at 1305 nm of 42.5 ± 11, 70 ± 18 and 3.3 ± 0.9 cm/GW, respectively. For GaAs and InP we determine three photon absorption coefficients at 2144 nm of 0.19 ± 0.07 and 0.22 ± 0.08 cm³/GW².     

Etude de l'absorption optique du vanadium en couches minces interpretation par la theorie des bandes d'energie

Optical absorption of vanadium thin films has been determinated from 0.32 to 5.5 eV from reflectance and transmittance data. The films have been deposited in ultra high vacuum and the measurements have been realised in situ. Between 1 to 5.5 eV, we note a large absorption band which is independant of the photons angle of incidence and polarisation; with an important maximum and a shoulder respectively at 3.1 and 2 eV. We explain this absorption band by direct interband transitions deduced from theorical bands calculated by Yasui et al. From 0.32 to 1 eV, the observed absorption is principally due to intraband transitions.     

Investigation of nonlinear optical parameters of zinc based amorphous chalcogenide films

Third order nonlinear optical properties of amorphous Zn_x–S_y–Se_100−x−y chalcogenide films have been investigated using single beam transmission z-scan technique at 1064 nm of Nd:YAG laser. Measurement of optical properties of amorphous Zn_x–S_y–Se_100−x−y chalcogenide films prepared by thermal evaporation technique has been made. X-ray diffraction patterns of chalcogenide films confirm the amorphous nature. Optical band gap (E_g) has been estimated using Tauc's plot method from transmission spectra that is found to decrease with increase in content due to valence band broadening and band tailing the system. Nonlinear refractive index (n₂), nonlinear absorption coefficient (β) and third order nonlinear susceptibility (χ³) of chalcogenide films have been estimated. Self-focusing effect has been observed in closed aperture and reverse saturable absorption in open aperture scheme. Limiting threshold and dynamic range have been calculated from optical limiting studies. The increase in nonlinearity with increase in Zn content has been observed that is understood to be due to decrease in band gap on Zn doping. High nonlinearity makes these films a potential candidate for waveguides, fibers and two photon absorption in optical limiters.     

Z-scan measurement for the nonlinear absorption and the nonlinear refraction of poly1,4-diazophenylene-bridged-tris(8-hydroxy-quinoline) aluminum (PDPAlq3)

The nonlinear optical properties of the poly1,4-diazophenylene-bridged-tris(8-hydroxy-quinoline) aluminum (PDPAlq₃) solution were studied using single beam Z-scan technique with a continuous-wave Diode laser radiation at 657.2 nm with 10 Hz repetition rate. The results show that the solution of PDPAlq₃ exhibits large nonlinear refractive index (n₂ = −1.7642 × 10⁻¹² m²/W) and nonlinear absorption coefficient (β = 1.12 × 10⁻⁶ m/W). The negative sign of the nonlinear refractive index n₂ indicates that the material exhibits self-defocusing optical nonlinearity. The evaluation of the figure of merit (W = 1.8) shows that the solution of PDPAlq₃ is sufficient for application in all-optical switching technology. These results show that the solution of PDPAlq₃ have potential application in nonlinear optics.