Interband Relaxation Time Research Articles

Optical gain of AlGaN/GaN multiquantum well laser diode influenced by hydrostatic pressure

A numerical model allows analysis of optical gain according to the electronic properties of AGaN/GaN multiple quantum well laser diode (MQWLD) under hydrostatic pressure. Finite difference techniques are used to acquire energy eigenvalues and their corresponding eigenfunctions of AGaN/GaN MQWLD. The hole eigenstates are calculated via a 6 × 6 k . p method under applied hydrostatic pressure. It was found that the depth of the quantum wells, bandgaps, band offset, electron, and hole density increased with the increase of hydrostatic pressure. The electron and hole wave functions will have less overlap, the amplitude of the optical gain increases, and heavy hole and light hole interband relaxation time, and their excitons binding energy decreases with increasing pressure. A change in pressure of up to 10 GPa caused the optical gain peaks of light and heavy holes to shift to a high photon energy of up to 120 meV and enhanced the optical gain up to 1.7 × 105 m − 1.

Alpha-phase indium selenide saturable absorber for a femtosecond all-solid-state laser.

We successfully fabricated the high-quality few-layered α-In2Se3 and investigated its carrier dynamics and nonlinear optical absorption properties by pump-probe and open-aperture Z-scan technologies. Intra- and inter-band relaxation times were determined to be 7.2ps and 270ps, respectively, and the effective nonlinear absorption coefficient was measured as βeff=-3.9×103 cm/GW (1064nm). Based on α-In2Se3 saturable absorber, a continuous-wave mode-locking operation was realized. Pulse duration, repetition rate, and maximum output power were measured to be 352fs, 42.4MHz, and 0.56W, corresponding to pulse energy of 13.2nJ and peak power of 37.5kW. To the best of our knowledge, this is the first demonstration of non-transition metal chalcogenides applied in ultrafast all-solid-state bulk lasers. This work well verified that α-In2Se3 should be a promising optical modulator candidate for all-solid-state lasers.

Interband and intraband relaxation dynamics in InSb based quantum wells

We utilize pump/probe spectroscopy to determine the interband and intraband relaxation dynamics in InSb based quantum wells. Using non-degenerate pump/probe techniques, we observed several time scales for relaxation. One time scale τ3 ranging from 2 ps to 5 ps is due to the intraband relaxation dynamics. Here, both the emission of LO phonons (within the Γ valley) and carrier scattering between the X, L, and Γ valleys contribute to the relaxation. An observed longer relaxation time, τ2 ≈ 20 ps, is attributed to electron–hole recombination across the gap (the interband relaxation time). Finally, using a mid-infrared (MIR) degenerate pump/probe scheme, we observed a very fast relaxation time of ∼1 ps, which is due to the saturation of the band-to-band absorption. Our results are important for developing concepts for InSb devices operating in the THz or MIR optical ranges with the endless need for faster response.

Observation of large nonlinear responses in a graphene-Bi2Te3 heterostructure at a telecommunication wavelength

We report the large nonlinear response and ultrafast carrier relaxation dynamics of a graphene-Bi2Te3 heterostructure produced by two-step chemical vapour deposition. The nonlinear refractive index reaches n2 = 0.2 × 10−7 cm2/W at the telecommunication wavelength of 1550 nm, which is almost seven orders of magnitude larger than that of the bulk Si material. Additionally, a pump-probe experiment is performed to investigate the ultrafast dynamic process (intraband relaxation time τ1 = 270 ± 20 fs; interband relaxation time τ2 = 3.6 ± 0.2 ps) of the graphene-Bi2Te3 heterostructure. Then, based on the donor-acceptor structure model, we propose a theoretical model to explain the dynamic relaxation process. Our results show that the graphene-Bi2Te3 heterostructure is a promising saturable absorber for ultrafast pulse laser applications at telecommunication wavelengths.

Parametric wave phase conjugation of nonlinear ultrasound waves

Real time acoustic wave phase conjugation (WPC), based on parametric self-consistent physical mechanisms, was realized up to the present time only for the monochromatic waves [A. P. Brysev et al., Phys.-Usp. 41, 793 (1998)]. Here the possibility of WPC of nonmonochromatic ultrasound waves is considered. For simultaneous WPC of the entire series of spectral components generated by nonlinear propagation of the incident wave we propose the use of phonon-plasmon interaction in piezosemiconductors. WPC of nonlinear acoustic waves can be accomplished by modulation of the electron density provided by a sequence of short laser pulses pumping the sample. If the periodicity of the optical pulses is half the period of the fundamental component of the acoustic wave, such wide-band, excitation leads to self-synchronized parametric conjugation of each spectral component in the incident wave. The conjugation efficiency depends sharply on relations between acoustical frequency content, laser pulse duration, and interband relaxation time. It is shown that under certain conditions the time profile of the conjugate wave may be efficiently controlled by varying the duration of the laser pulses. The time profile of the conjugate wave is investigated for some physical conditions of practical interest.

Study of characteristics of single-frequency GaAs/AlGaAs semiconductor lasers

The characteristics of single-frequency lasers are investigated experimentally and theoretically. It is shown that the model of spectral hole burning with a varying interband relaxation time adequately describes the spectral and modulation characteristics of the laser (taking into account the transport of carriers). The time of carrier capture in a quantum well is 4 ps and the time of their escape is 80 ps.

Photoluminescence in delta-doped InGaAs/GaAs single quantum wells

We have studied the time integrated (cw) and time resolved photoluminescence (PL) spectra of Si δ-doped In0.2Ga0.8As/GaAs quantum wells (QWs), in which the δ doping layer was either at the center of the quantum well or outside the well, in the barrier region. We found that both the cw and the time resolved PL depended significantly on the position of the doping sheet. When the doping was at the center of the quantum well the luminescence spectrum displayed the characteristic features of the Fermi edge singularity, while in the case of barrier-doped QW, the PL spectra showed well-defined emission lines originating from transitions between subbands in the conduction and valence bands. From low-temperature time resolved PL experiments, we determined the effective hole capture times, the interband relaxation times (for holes), and the radiative decay times for both types of δ doping. We found that the interband relaxation time in the center-doped QWs is nearly two orders of magnitude shorter (τ=3 ps) than in samples doped in the barrier (τ=200 ps).

Dynamics of photoexcited holes in n-doped InGaAs/GaAs single quantum well

We have measured the temporal evolution of the photoluminescence (PL) of a Si δ-doped In0.2Ga0.8As/GaAs quantum wells using the PL up-conversion technique. The luminescence spectrum of this sample displayed the characteristic features of the Fermi edge singularity. The temporal evolution of the luminescence is described in terms of the dynamics of the hole population. From the experiments, we have determined the effective hole capture time (15 ps), the interband relaxation time (3 ps), and the radiative decay time (>1 ns) at T=8 K. We have found that the radiative decay time decreases dramatically with increasing temperature (τr=45 ps at T=125 K) which, we believe, is the result of the smearing of the Fermi edge and the delocalization of the holes that are responsible for the luminescence.

Third-order nonlinearities and coherent transient grating effects of narrow-gap semiconductors in the midinfrared

Picosecond excitation-probe measurements using a far-infrared free-electron laser (CLIO) have revealed large nonlinearities for indium antimonide at 4.7 μm. A theoretical model is described to determine the cw third-order nonlinear susceptibility and the interband relaxation time of the semiconductor which were found to be −8.6×10−11 m2 V−2 and 0.3 ns, respectively. Furthermore, the observation of the associated coherent transient grating effects allows us to obtain a coherence time of the laser system (2.5 ps) and the χ(3) of the transient grating which was found to be −7.2×10−13 m2 V−2. The measurements were performed at room temperature on undoped bulk InSb.

Evolution of the optical functions of thin-film aluminum: A real-time spectroscopic ellipsometry study.

We report a comprehensive study of the optical functions of thin-film aluminum from initial nucleation through continuous film growth. This study relies on ellipsometric spectra from 1.3 to 4.0 eV collected in real time with a multichannel instrument during both thermal evaporation and magnetron sputtering of Al onto ${\mathrm{SiO}}_{2}$ substrates at room temperature. The spectra for all films in the particle growth regime can be modeled with a Maxwell-Garnett-type effective medium theory, modified to include dipole interactions between spheroidal particles arranged on a square grid. The dielectric functions of the Al particles themselves, as well as those of continuous films, are interpreted assuming variable relaxation times for both Drude and interband electronic contributions. The relaxation times are determined by a common value of the mean free path, reduced from its bulk film value owing to electron scattering at defects, particle surfaces, and grain boundaries. For all films in the particle regime of growth, i.e., for thicknesses \ensuremath{\sim}50 \AA{}, the deduced relaxation times are independent of thickness (and hence particle size), and are more than an order of magnitude lower than bulk film values. This suggests a defective structure in which electron scattering at internal particle defects limits the relaxation time and determines the particle optical functions. For Al prepared by high-rate evaporation, a transition is observed at a thickness of 55--60 \AA{}, just after continuous film formation. At the transition, the interband electron relaxation time increases abruptly, signaling the development of higher-quality crystalline grains that extend throughout the film thickness. Only after this transition is the (200) parallel-band absorption feature visible in the Al dielectric function at 1.5 eV. For thicknesses >60 \AA{}, the interband relaxation time increases with thickness, providing evidence that grain-boundary scattering is the dominant mechanism controlling the optical properties in the bulk film stage.

Electron States and Conductivity of Size Quantized Systems with Rough Boundaries

AbstractA new approach to the study of electron properties of the systems with rough boundaries is proposed. The boundary problem is reduced to the analysis of Schrödinger equation with effective Hamiltonian and boundary conditions for flat surfaces by using a curvilinear coordinate system. A criterion for existence of localized states is obtained and evaluation of the lifetime of resonance states arising near the bulges is presented. Asymptotic behaviour of the state density tails from the subbands of the size quantization, intraband and interband relaxation time, and conductivity of the film are estimated. The influence of the scattering by boundaries on the localization effects in two‐dimensional systems is investigated.

A model for interband electroreflectance in polyvalent nearly-free-electron metals, with application to Pb

The interband contribution to the electroreflectance spectrum of Pb has been calculated with a model based on the optical properties of nearly-free-electron polyvalent metals. For such metals parallel band interband transitions dominate. The modulation arises from the uncertainty in wave vector parallel to the applied electric field, which then is simulated by a decrease in interband relaxation time. The model, although lacking microscopic justification, fits the electroreflectance spectra of Pb obtained at two angles of incidence for each of two polarizations using essentially only two fitting parameters, the depth of the modulated region and the (average) field-induced change in the interband relaxation time in this region. The model should be equally applicable to Mg, Al, In, Ga, and Tl.

Low energy interband transitions in aluminium

Low energy interband transitions in aluminium

Thermomodulation Spectra of Al, Au, and Cu

Thermotransmission and thermoreflection measurements were made on semitransparent films of Al, Au, and Cu at about 370 and 120 K in the range 0.5-5 eV. The data yield the thermomodulation spectrum $\ensuremath{\Delta}{\ensuremath{\epsilon}}_{2}$ of the imaginary part of the dielectric constant directly, without Kramers-Kronig analysis. A comparison of the interband region of $\ensuremath{\Delta}{\ensuremath{\epsilon}}_{2}$ for Cu with the piezo-modulation spectrum of a single crystal shows that broadening of the Fermi distribution and volume strain caused by thermal expansion are the principal causes of the $\ensuremath{\Delta}{\ensuremath{\epsilon}}_{2}$ spectrum. The $\ensuremath{\Delta}{\ensuremath{\epsilon}}_{2}$ spectrum for Al is particularly simple and can be discussed using closed-form expressions for the optical conductivity. It appears that the temperature dependence of the interband relaxation time for transitions across gaps produced by $|{V}_{111}|$ is smaller than that for transitions across gaps caused by $|{V}_{200}|$, which in turn is smaller than that for the infrared intraband transitions.