Five-wave-mixing spectroscopy of ultrafast electron dynamics at a si(001) surface

The optically induced electron dynamics at a Si(001) surface is studied using a five-wave-mixing setup which measures the diffracted second-harmonic intensity induced by three ultrashort (13 fs) laser pulses. Depending on the time ordering of the pulses, this technique is capable of monitoring the temporal evolution of photoexcited one- or two-photon coherences, or populations. For a particular pulse sequence, the experiments show a delayed rise and a decay of the diffracted signal intensity on time scales of 50 and 250 fs, respectively. This response can be described by optical Bloch equations by including rapid scattering of the photoexcited carriers in the D(down) band of Si(001).     

Near-surface modification of optical properties of fused silica by low-temperature hydrogenous atmospheric pressure plasma

In this Letter, we report on the near-surface modification of fused silica by applying a hydrogenous atmospheric pressure plasma jet at ambient temperature. A significant decrease in UV-transmission due to this plasma treatment was observed. By the use of secondary ion mass spectroscopy, the composition of the plasma-modified glass surface was investigated. It was found that the plasma treatment led to a reduction of a 100 nm thick SiO2 layer to SiOx of gradual depth-dependent composition. For this plasma-induced layer, depth-resolved characteristic optical parameters, such as index of refraction and dispersion, were determined. Further, a significant plasma-induced increase of the concentration of hydrogen in the bulk material was measured. The decrease in transmission is explained by the plasma-induced near-surface formation of SiOx on the one hand and the diffusion of hydrogen into the bulk material on the other hand.     

All-optical switching in silicon photonic crystal waveguides by use of the plasma dispersion effect

Silicon photonic crystals offer new ways of controlling the propagation of light as well as new tools for the realization of high-density optical integration on monolithic substrates. However, silicon does not possess the strong nonlinearities that are commonly used in the dynamic control of optical devices. Such dynamic control is nevertheless essential if silicon is to provide the higher levels of functionality that are required for optical integration. We demonstrate that the combination of the refractive index change caused by the presence of photoexcited carriers, or so-called plasma dispersion, and photonic crystal properties such as photonic bandgaps, constitutes a powerful tool for active control of light in silicon integrated devices. We show close to 100% modulation depth near the photonic crystal band edge.     

Plasma dynamics and determination of ablation parameters using the near-target magnified imaging during pulsed laser ablation

A comparative study of the ultrafast dynamics on ZnO thin films deposited on flat Si substrates and on Si micro-cones following ultrashort laser excitation has been carried out using time-resolved optical spectroscopy. By monitoring the transient band gap renormalization induced by nonlinearly excited carriers it is found that fast electron scattering and trapping occurs more efficiently in the micro-cones as compared to the flat films. This enhanced trapping efficiency is attributed to the defects and imperfections that are introduced by the increased surface roughness due to the conical shape.     

Electron-acceptor luminescence from a non-Maxwellian distribution of photoexcited electrons in GaAs

Distribution functions are calculated for photoexcited electrons in GaAs, under conditions of continuous, monochromatic excitation. The lattice temperature is taken to be 1.2 K and the excitation intensity such that the density of photoexcited carriers is insufficient for the distribution to be affected by intercarrier scattering. A Boltzmann equation approach is used to take account of the effects of, injection of electrons into the conduction band, at an energy below the threshold for longitudinal optical phonon emission, scattering by acoustic phonons, via the deformation potential and piezoelectric interactions, and recombination. The equation is solved numerically using an iterative technique and the distribution functions are found to differ significantly from a Maxwellian form. Emission spectra due to conduction band to neutral acceptor transitions are derived from the computed distribution functions and are compared with recent experimental results.     

Formation of metastable above-barrier hole states in ZnSe/BeTe type II heterostructures under high-density optical excitation

A considerable slowing down of the luminescence kinetics of the direct optical transitions has been discovered in ZnSe/BeTe type II heterostructures under high-density optical pumping by femtosecond laser pulses. The effect is attributed to the potential barrier that appears due to the strong band bending at a high density of spatially separated photoexcited carriers and forms a metastable above-barrier hole state in the ZnSe layer. This yields a longer energy relaxation time of the holes migrating to the adjacent BeTe layer. The experimental results agree well with the numerical calculations.     

Generation mechanism for coherent LO phonons in surface-space-charge fields of III-V-compounds

We report on details of coherent LO phonon generation in surface-space-charge regions of III-V-compounds by optical injection of free carriers with laser pulses of 50 fs duration at 2 eV. Both the dynamics of the transient surface field, as well as the coherent lattice vibration, are measured via electro-optic sampling techniques under different experimental conditions. The driving force for the coherent phonon vibration is the sudden depolarization of the crystal lattice due to ultrafast screening of the intrinsic electrical surface field by photoexcited free carriers.     

Direct observation of a transition of a surface plasmon resonance from a photonic crystal effect

Transition of surface-plasmon resonance from out-of-plane photonic crystal effect is observed in a semiconductor array of subwavelength holes by optical pump-terahertz probe measurements. The dielectric properties of the photoexcited array are essentially altered by the intense optical excitation due to photogenerated free carriers. As a result, the array becomes metallic and favors the coupling and propagation of surface plasmons. The photoinduced resonant extremes agree well with the Fano model.     

Direct photoexcitation of appreciable size of domains without lattice motion in neutral-ionic transition systems

In a recent ultrafast pump-probe experiment on a typical neutral-(N-)ionic (I) transition material, tetrathiafulvalene-p-chloranil [S. Iwai, Phys. Rev. Lett. 96, 057403 (2006)10.1103/PhysRevLett.96.057403], it was claimed that an I-phase domain in the N-phase background was very quickly formed even before the lattice started to move. This study proves this idea in terms of an extended Hubbard model that has site-potential alternation and has been frequently used so far. Our focus is on the photoexcited states of the N phase and the optical conductivity spectrum. As a result of accurate spectrum calculation based on a dynamical density-matrix renormalization group technique, we clearly observe photoexcited ionic domains accompanied by no lattice dimerization. Their contribution almost dominates the spectrum near the N-I phase boundary, and we relate this property to the long-tailed spectrum observed in experiments.     

Strain-free polarization superlattice in silicon carbide: a theoretical investigation

A strain-free superlattice of inversion domains along the hexagonal axis of SiC is investigated by theoretical calculations. The induced polarization causes a zigzag shape in the band edges, leading to spatial separation of photoexcited carriers and to an effective band gap narrowing tunable over a wide range by the geometry and on a smaller scale by the intensity of the excitation. Calculations on the SiC surface indicate that preparation of such a superlattice might be possible in atomic layer epitaxy with properly chosen sources and temperatures.     

Circular Photogalvanic Effect in the Weyl Semimetal TaAs

Weyl semimetal(WSM) is expected to be an ideal spintronic material owing to its spin currents carried by the bulk and surface states with spin-momentum locking. The generation of a sizable photocurrent is predicted in non-centrosymmetric WSM arising from the broken inversion symmetry and the linear energy dispersion that is unique to Weyl systems. In our recent measurements, the circular photogalvanic effect(CPGE) is discovered in the TaAs WSM. The CPGE voltage is proportional to the helicity of the incident light, reversing direction if the radiation helicity changes handedness, a periodical oscillation therefore appears following the alteration of light polarization. We herein attribute the CPGE to the asymmetric optical excitation of the Weyl cone, which could result in an asymmetric distribution of photoexcited carriers in momentum space according to an optical spin-related selection rule.     

Terahertz electromagnetic waves emitted from semiconductor investigated using terahertz time domain spectroscopy

Ultrafast electromagnetic waves radiated from semiconductor material under high electric fields and photoexcited by femtosecond laser pulses have been recorded by using terahertz time domain spectroscopy (THz-TDS).The waveforms of these electromagnetic waves reflect the dynamics of the photoexcited carriers in the semiconductor material,thus,THz-TDS provides a unique opportunity to observe directly the temporal and spatial evolutions of non-equilibrium transport of carriers within sub-picosecond time scale....     

Simplified calculations of band-gap renormalization in quantum-wells

Non-linear optical properties of photoexcited semiconductor quantum-wells are of interest because of their opto-electronic device application possibilities. Many-body interactions of the optically created electrons and holes lead to the band-gap renormalization which in turn determines the absorption spectra of such systems. We employ a simplified approach to calculate the band-gap renormalization in quantum-well systems by considering the interaction of a single electron-hole pair with the collective excitations (plasmons). This method neglects the exchange-correlation effects but fully accounts for the Coulomb-hole term in the single-particle self-energy. We demonstrate that the density, temperature, and well-width dependence of the band-gap renormalization for GaAs quantum-wells within our model is in good agreement with the experimental results.     

Ratio problem in single carbon nanotube fluorescence spectroscopy

The electronic band gaps measured in fluorescence spectroscopy on individual single wall carbon nanotubes isolated within micelles show significant deviations from the predictions of one electron band theory. We resolve this problem by developing a theory of the electron-hole interaction in the photoexcited states. The one-dimensional character and tubular structure introduce a novel relaxation pathway for carriers photoexcited above the fundamental band edge. Analytic expression for the energies and line shapes of higher subband excitons are derived, and a comparison with experiment is used to extract the value of the screened electron-hole interaction.     

Dielectric function dynamics during femtosecond laser excitation of bulk ZnO

Using a broad band dual-angle pump-probe reflectometry technique, we obtained the ultrafast dielectric function dynamics of bulk ZnO under femtosecond laser excitation. We determined that multiphoton absorption of the 800-nm femtosecond laser excitation creates a large population of excited carriers with excess energy. Screening of the Coulomb interaction by the excited free carriers causes damping of the exciton resonance and renormalization of the band gap causing broadband (2.3–3.5 eV) changes in the dielectric function of ZnO. From the dielectric function, many transient material properties, such as the index of refraction of ZnO under excitation, can be determined to optimize ZnO-based devices.     

Ultrafast optical switching in three-dimensional photonic crystals

We present the first experimental investigation of ultrafast optical switching in a three-dimensional photonic crystal made of a Si-opal composite. Ultrafast (30 fs) changes in reflectivity around the photonic stop band up to 1% were measured for moderate pump power (70 microJ/cm(2)). Short-lived photoexcited carriers in silicon induce changes in the dielectric constant of Si and diminish the constructive interference inside the photonic crystal. The results are analyzed within a model based on a two-band mixing formalism.     

碳-锌共掺杂锐钛矿相TiO2 电子结构与光学性质的第一性原理研究

近年来的理论和实验研究表明,通过不同离子共掺杂TiO₂是减小其禁带宽度的一种有效方法.本文采用基于第一性原理的平面波超软赝势方法研究了C和Zn共掺杂TiO₂的能带结构、态密度和光学性质.计算结果表明C-Zn共掺杂导致导带相对Fermi能级发生了明显的下降,同时在TiO₂的导带下方与价带上方形成了新的杂质能级,使TiO₂的禁带宽度变小, TiO₂的光学吸收带边产生红移. 杂质能级可以降低光激发产生的电子-空穴对的复合概率, 提高TiO₂的光催化效率. 此外, 掺杂后TiO₂在可见光区的吸收系数有明显增加, 能量损失也明显减小.     

Influence of the absorbed optical quanta energy on GHz ultrasound generation in GaAs

Experimental results of the investigation of the optoacoustic processes taetang place in GaAs semiconductor at ultrashort time scales are reported. Femtosecond laser has been used both for the generation (through the deformation potential mechanism by the interband absorption of laser radiation) and detection of GHz ultrasound waves. First experimental observation of an abrupt change in the phase of the photoexcited GHz ultrasound, when with increase of the energy of optical quanta direct generation of the electron-hole pairs in the side-valleys of GaAs becomes allowed by the energy and momentum conservation laws, is reported. We relate this observation, at least partially, to abrupt change in the ultrafast dynamics of photogenerated electron-hole plasma, in particular to deceleration of plasma diffusion when heavy carriers in the high energy side valley are photogenerated instead of light carriers in the lowest energy valley.     

Stimulated recombination from the defect level in doped lead telluride

The relaxation processes of the photoexcited carriers from the defect level in the band gap to the valence band states were investigated in Na and Tl doped p-type PbTe single crystals at T = 77 K. The observed photosignal oscillations were proved to be induced by stimulated recombination of photoexcited carriers from the defect level E_d ≈ 50 meV above the top of the valence band. Non-equilibrium carrier inversion population was produced by impulses of a TEA CO₂-laser. The observed stimulated recombination may presumably be used for designing IR semiconductor lasers operating in the wavelength range of λ ∼ 25 μm at T = 77 K.