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.     

Determination of photoexcited carrier concentration and mobility in GaAs doping superlattices by hall effect measurements

The photo-Hall effect in a new type of periodicp-n doping multilayer structures (superlattices) of GaAs grown by molecular beam epitaxy has been investigated. In these space charge systems electrons and holes are separated in real space. As a consequence, large deviations from thermal equilibrium become quasi-stable. Carrier generation by optical absorption occurs in these doping superlattices even at photon energies far below the gap of the homogeneous semiconductor material. The photoexcitation results in a strong enhancement of the conductivityparallel to the layers and in a substantial photovoltaic response. An increase in carrierconcentration as well as an increase in carriermobility both contribute to the observed enhancement of the conductivity under excitation. The absolute values of changes in free-carrier concentration are very large due to the manyfold active layers of the structure. The measured free-carrier mobilities depend on the population of the multilayer system. A reduction in mobility as compared to bulk material is found to be more pronounced in weakly populated systems. This finding is caused by the larger weight of the boundary regions of the total active layers where the free-carrier density is lower than the density of ionized impurities resulting in an enhanced impurity scattering.     

硒化镉载流子的超快动力学研究

硒化镉是一种可用于X射线全光分幅相机和全光条纹相机的重要探测材料。用基于相位物体的泵浦探测方式,研究了硒化镉在1030nm波长,飞秒脉冲下的载流子超快动力学和非线性光学特性。得到了双光子吸收系数、载流子吸收截面、载流子复合时间等参数。实验表明,硒化镉载流子的动力学和非线性特性是由束缚电子和载流子共同决定的。束缚电子的克尔效应和双光子激发都是瞬态的,而载流子复合持续了较长时间。这些参数和载流子图像的的获得,为X射线超快探测器件的设计和改进提供了参考。     

双光子激发ZnSe自由载流子超快动力学研究

采用Z扫描和抽运-探测实验技术, 在波长为532 nm、脉冲宽度为41 fs的条件下测得ZnSe晶体的双光子吸收系数, 并获得了不同激发光强下的自由载流子吸收截面、电子-空穴带间复合时间和电子-声子耦合时间. 研究发现, 随着激发光强的增大, 自由载流子吸收截面减小, 复合时间变短. 当激发光强增大导致载流子浓度大于10¹⁸ cm^-3时, 抽运-探测信号出现明显改变, 原因归结为强光场激发导致样品在短时间内带隙变窄和电子-空穴等离子体的形成.     

Measurements of dynamics of nondegenerate optical nonlinearity in ZnS with pulses from optical parameter generation

We expand the degenerate PO pump-probe technique to nondegenerate field and use it to investigate optical nonlinear dynamics in ZnS single crystal. Excited by 532-nm laser pulses with 21-ps duration, the temporal response of nondegenerate nonlinear absorption and nondegenerate nonlinear refraction are probed by laser pulses from optical parameter generation (OPG) at 600 and 680 nm with pulse width of 10 ps. Based on the theory of free-carrier optical nonlinearity, we study the pure free-carrier refraction in ZnS. By numerically fitting based on the nondegenerate pump-probe theory, the nondegenerate two-photon absorption coefficient, the free-carrier lifetime, the free-carrier absorptive cross section and refractive coefficient at both probe wavelengths are determined respectively. The dispersion of the free-carrier refractive coefficients is discussed.     

Free-carrier absorption in quantum well structures for acoustic phonon scattering

Free-carrier absorption has been studied for quantum well structures fabricated from III-V semiconducting materials where the acoustic phonon scattering is important. The energy band of carriers is assumed to be nonparabolic. We discuss the effect of acoustic phonon scattering on the free-carrier absorption for both deformation-potential coupling and piezoelectric coupling. It is found that the free-carrier absorption coefficient depends upon the polarization of the electromagnetic radiation relative to the layer plane or quantum well, the photon frequency, and the temperature. When the deformation-potential coupling is dominant, the free-carrier absorption coefficient increases with increasing temperature for photons polarized in the layer plane or perpendicular to the layer plane. However, when the piezoelectric coupling is dominant, the free-carrier absorption coefficient increases with increasing temperature for photons polarized in the layer plane, but for photons polarized perpendicularly to the layer plane, the free-carrier absorption coefficient decreases with increasing temperature. Moreover, at high temperatures such as T = 300 K, the free-carrier absorption coefficient oscillates with the film thickness in a small quantum well region and then decreases monotonically with increasing the film thickness. This is different from the result for three-dimensional semiconducting solids.     

GaAs在光通讯波段的超快非线性光学研究

采用Z-扫描和泵浦-探测技术,在光通讯波段对砷化镓(GaAs)单晶进行了非线性动力学以及非线性光学的实验研究.飞秒泵浦-探测实验结果表明,三阶非线性光学效应源于砷化镓单晶对飞秒激光的瞬态双光子吸收,而五阶非线性光学效应源于砷化镓单晶双光子吸收诱导的自由载流子吸收效应.通过Z扫描实验,得到了关于GaAs单晶所有的非线性光学参数,包括双光子吸收系数、三阶非线性折射系数、双光子吸收诱导的自由载流子吸收截面以及双光子吸收诱导的自由载流子折射截面.结果表明,砷化镓单晶在制造光限幅器件和光电探测器方面具有良好的发展前景.     

THz Generation by Optical Rectification and Competition with Other Nonlinear Processes

We present a study of the competition between tera-hertz （THz） generation by optical rectification in （110） ZnTe crystals, two-photon absorption, second harmonic generation and free-carrier absorption. The two-photon nonlinear absorption coefficient, second harmonic generation efficiency and free-carrier absorption coefficient in the THz range are measured independently. The incident pump field is shown to be depleted by two-photon absorption and the THz radiation is shown to be reduced, upon focusing, by free-carrier absorption. The reduction of the generated THz radiation upon tight focusing is explained, provided that one also takes into account diffraction effects from the sub-wavelength THz source.     

Impact of two-photon absorption on self-phase modulation in silicon waveguides

We study the effects of two-photon absorption on the self-phase modulation (SPM) process in silicon waveguides while including both free-carrier absorption and free-carrier dispersion. An analytical solution is provided in the case in which the density of generated carriers is relatively low; it is useful for estimating spectral bandwidth of pulses at low repetition rates. The free-carrier effects are studied numerically with emphasis on their role on the nonlinear phase shift and spectral broadening. We also consider how the repetition rate of a pulse train affects the SPM process.     

A Novel Approach for the Evaluation of Band Gap Energy in Semiconductors

On the basis of absorption nature of semiconductors, we present a novel and simple method to determine the band gap energies of semiconductors directly from their absorption spectra at any temperatures, without any fitting processes and restrictions of sample thickness. The key point of the approach is the different dependence of the absorption coefficient derivative on the photon energy at different absorption regions in semiconductors. We first demonstrate and verify the approach by detailed temperature-dependent absorption measurements, combined with photoluminescence measurements and empirical band gap equations for the direct band gap of uniform InAs films, and then extend successfully to the indirect band gap of elemental Ge and to the ternary HgCdTe alloys with composition gradient. Furthermore, we have also shown that our approach can not only evaluate the average band gap energy for ternary semiconductor alloys, but also estimate their composition uniformity to monitor the material quality.     

Origin of the resonant optical Kerr nonlinearity in Cd(S,Se)-doped glasses and related topics

After having reported preliminary results related to saturation, we first theoretically consider the various mechanisms contributing to the resonant optical Kerr effect in Cd(S, Se)-doped glasses. We obtain the expression for the expected effective susceptibility in different possible cases. This nonlinearity is studied experimentally using optical-phase conjugation in the low-intensity regime. We show that, by time resolving the nonlinear response of such glasses having experienced various degrees of photodarkening, we can clearly assess the origin of the resonant optical Kerr effect in these materials. Usually, a combination of a fast free-carrier contribution due to particles without traps and of a slow trapped-carrier one due to particles with traps is observed. For the free-carrier contribution, induced absorption is observed to be almost as important as absorption saturation. We also report frequency-dependent measurements and discuss the change in absorption spectrum and the increase of the nonradiative decay rate that accompany darkening.     

Interband impact ionization and nonlinear absorption of terahertz radiation in semiconductor heterostructures

We have theoretically investigated nonlinear free-carrier absorption of terahertz (THz) radiation in InAs/AlSb heterojunctions. By considering multiple photon process and conduction-valence interband impact ionization (II), we have determined the field and frequency dependent absorption rate. It is shown that (i). electron-disorder scatterings are important at low to intermediate field, and (ii). most importantly, the high field absorption is dominated by II processes. Our theory can satisfactorily explain a long-standing experimental result on the nonlinear absorption in the THz regime.     

Spin relaxation by transient monopolar and bipolar optical orientation

We have used two-color time-resolved spectroscopy to measure the relaxation of electron spin polarizations in a bulk semiconductor. The circularly polarized pump beam induces a polarization either by direct excitation from the valence band, or by free-carrier (Drude) absorption when tuned to an energy below the band gap. We find that the spin relaxation time, measured with picosecond time resolution by resonant induced Faraday rotation in both cases, increases in the presence of photogenerated holes. In the case of the material chosen, n-InSb, the increase was from 14 to 38 ps.     

Demonstration of high Raman gain in a submicrometer-size silicon-on-insulator waveguide

We show high Raman gain in a silicon submicrometer-size planar waveguide. Using high-confinement structures and picosecond pump pulses, we show 3.1-dB net internal gain with 2.8-W peak pump power in a 7-mm-long waveguide. We also analyze experimentally and theoretically the effect of free-carrier absorption on the Raman gain.     

Free-carrier absorption and dielectric losses in crystals with dipole-impurities

In the one-band isotropic effective-mass approximation and the first Born approximation of the scattering the high-frequency electrical conductivity, dielectric losses and free-carrier absorption in crystals are calculated when the scattering mechanism is connected with the screened electrical dipoles. As in the case of low-frequency conductivity [1] it is shown that in some cases the scattering from dipole impurity centres may be essential and even more effective for the free-carrier absorption and dielectric losses than the other scattering mechanisms.     

Free carrier modulation of a sub-bandgap CW laser beam: A Si optoelectronic chopper

In this work we introduced two beam photocarrier cross-modulation for creation of an optically driven photonic laser beam modulator using a semiconductor wafer as the active medium. Unlike other laser beam modulators, the process of modulation of an unmodulated sub-bandgap laser beam was made possible by generating a spatially- and free-carrier density-wave-dependent infrared absorption coefficient in the bulk of the semiconductor, following absorption of a collinear super-bandgap modulated laser beam. The experimental results showed that the modulation efficiency strongly depends on the transport parameters of the semiconductor material and on the power of the super-bandgap laser beam.     

Spectral broadening enhancement in silicon waveguides through pulse shaping

Spectral broadening in silicon waveguides is usually inhibited at telecom wavelengths due to some adverse effects related to semiconductor dynamics, namely, two-photon and free-carrier absorption (FCA). In this Letter, our numerical simulations show that it is possible to achieve a significant enhancement in spectral broadening when we properly preshape the input pulse to reduce the impact of FCA on spectral broadening. Our analysis suggests that the use of input pulses with the correct skewness and power level is crucial for this achievement.     

Non-linear photo-induced desorption of GaP

Laser-induced desorption of P from GaP at various photon energies near the absorption edge has been measured. The desorption yield is found to start increasing above a certain threshold laser fluence, of which the dependence on the photon energy exhibits a sharp dip near the indirect band gap besides a gradually decreasing component from the indirect band gap to the direct band gap energy. The sharp dip is ascribed to desorption induced by dense excitation of the surface states.     

The refractive index enhancement at Eg in narrow-gap semiconductors: comparison between the interband absorption edge and the oscillator model

A comparison is made between the refractive index enhancement near the M₀-type absorption edge in direct narrow-gap semiconductors and the dispersive structure of a Lorentz oscillator. Phenomenologically, both absorptive structures can be described by three parameters, and the analogy between both models concerning n(E) is discussed. Examples for the predicted dependencies are given. With increasing energy gap the n(E) enhancement becomes related to discrete excitonic bands rather than to free-to-free transitions, thereby undergoing a spectral shift from energies above gap to energies below gap.     

Propagation of ultrafast pulses in the anomalous-dispersion regime of silicon-on-insulator strip nanowaveguides

The dynamic evolution of ultrafast high-intensity pulses with a 100 fs half-width at 1/e intensity point based on the silicon-on-insulator (SOI) strip nanowaveguides is considered and investigated numerically under the condition of anomalous group-velocity dispersion (GVD) regime. For ultrafast high-intensity pulses propagating in millimeter-long SOI nanowaveguides, the interplay between the dispersion and nonlinear effects such as the two-photon absorption, free-carrier absorption, free-carrier dispersion, and self-phase modulation has to be taken into account, which results in the significant optical wave breaking phenomenon that occurs near the pulse leading edge for an unchirped Gaussian pulse in the anomalous GVD regime. However, when the input Gaussian pulse with linear up-chirp is introduced, the position of the optical wave breaking shifts from the leading pulse edge to its trailing edge along the several millimeters-long SOI nanowaveguides.