Resonant harmonic generation and dynamic screening in a double quantum well

Dynamics of Correlated Double-Ionization of Two-Electron Quantum Dots in Laser Fields

Using an ab initio, time-dependent calculational method, we study the non-linear dynamics of a two-electron quantum dot in the presence of ultrashort Thz laser pulses. The analysis of the contribution of the various partial waves to two-electron joint radial and energy distribution patterns revealed strongly correlated electron ejection channels. In the double-ionization process, regardless of the photon energy, the two-electron wave packets are born and remain concentrated until the pulse's peak; at later times, and depending on the photon energy of the field, distinctly different patterns emerge. Our calculations also showed the gradual transition of the radial and energy patterns from a single-peak to a doubly peaked structure, associated with the direct and the sequential double-ionization mechanisms, respectively.

Radial and Angular Electron Ejection Patterns of Two-Electron Quantum Dots in THz Fields

In this work, we develop and apply an ab-initio method to calculate the joint radial- and- angular electron distributions following the interaction of two-electron spherical quantum dots (QD) with intense terahertz pulses of subpicosecond duration. By applying the method to two QDs of different size, we could investigate two particular ionization mechanisms: the direct and the sequential two-photon double ionization. According to our results, the two ionization mechanisms show some similarity in the angular distribution patterns, whereas the corresponding radial distributions are distinctly different, associated with their joint kinetic energy spectrum. We also discuss the time-evolution of the ionization process in the context of the different nature of the interaction of the QD with the external radiation and the electron–electron correlation interactions.

Control of the nonlinear response of bulk GaAs induced by long-wavelength infrared pulses

The nonlinear optical response of GaAs is studied using extremely nonresonant 10 μm laser pulses with peak intensities greater than 2 GW/cm ². We observe over an order of magnitude enhancement in the four-wave mixing efficiency by decreasing the CO ₂ laser beat-wave frequency. This enhancement is attributed to currents of photoexcited unbound carriers modulated at the beat frequency, confirmed by measurements of nonlinear absorption at this long wavelength as well as a fully microscopic analysis of the excitation dynamics. Modeling of such nonperturbative semiconductor-laser interactions predicts that further decreasing the beat frequency can increase the nonlinear response and allow for its control over two orders of magnitude.

Coherent power amplification of third-order harmonic femtosecond pulses at thin-film up-conversion nanoparticles

Third harmonic generation (THG) is a nonlinear optical process attractive in high-resolution interfacial studies, sub-wavelength light manipulation, and bio-molecular detection due to its capability of converting low-energy quanta into a quantum of a higher energy. One of the limitations in utilizing THG is its low power conversion efficiency; thus, various THG enhancement methods have been researched by involving plasmonic coupling effects or utilizing electric band gap resonances at quantum dots or two-dimensional materials. Meanwhile, lanthanide ion-doped up-conversion nanoparticles (UCNPs) can be excited by a multi-photon process similar to THG, but its interaction or resonance with THG has not been studied to date. In this Communication, we demonstrate the first coherent amplification of third-order harmonic femtosecond pulses at multi-layered UCNP thin-film with an amplification factor of 7.8. This amplification is made by the resonance interaction of incident femtosecond laser field, generated third-order harmonics, and the electric band gaps of UCNPs. The power contribution of the third-order harmonic and the up-conversion luminescence (UCL) is strongly dependent on the sample geometry due to the reabsorption effect. For in-depth understanding of the emission characteristics, spectral-domain, time-domain, radio-frequency (RF) domain, and polarization-dependence analysis were addressed. This coherent amplification of third harmonic (TH) at UCNP thin-films enables us to attain higher power, shorter wavelength, and ultra-short femtosecond pulses generated from a simple thin-film structure near to the target samples, which will pave a way to an ultrafast short-wavelength laser platform for material characterization, sub-wavelength photonics, and biomolecular detection.

Ultrashort electromagnetic pulse control of intersubband quantum well transitions

: We study the creation of high-efficiency controlled population transfer in intersubband transitions of semiconductor quantum wells. We give emphasis to the case of interaction of the semiconductor quantum well with electromagnetic pulses with a duration of few cycles and even a single cycle. We numerically solve the effective nonlinear Bloch equations for a specific double GaAs/AlGaAs quantum well structure, taking into account the ultrashort nature of the applied field, and show that high-efficiency population inversion is possible for specific pulse areas. The dependence of the efficiency of population transfer on the electron sheet density and the carrier envelope phase of the pulse is also explored. For electromagnetic pulses with a duration of several cycles, we find that the change in the electron sheet density leads to a very different response of the population in the two subbands to pulse area. However, for pulses with a duration equal to or shorter than 3 cycles, we show that efficient population transfer between the two subbands is possible, independent of the value of electron sheet density, if the pulse area is Π.

Rabi flopping in a two-level system with a time-dependent energy renormalization: intersubband transitions in quantum wells

We obtain pulse-driven Rabi oscillations guided by a generalization of the rotating-wave approximation to include, in the optical-Bloch equations, two-level systems with a time-varying transition energy. We achieve this by using chirped pulses with the central frequency given by the time-varying transition energy. Using this approach, we predict Rabi oscillations in intersubband transitions in a two-subband n-type modulation-doped quantum well by taking into account the time-dependent intersubband energy-gap renormalization due to depolarization-shift effects. We obtain Rabi oscillations for jpi (j=0,1,2, ) pulses in the presence of dephasing.

Brewster's angle attenuator for terahertz pulses

A variable attenuator for terahertz (THz) pulses is developed on the basis of the change in reflectivity of lithium niobate wafers with incident angle in a Brewster configuration. We can vary the THz field transmission from 22% to 54%, a change of a factor of 2.5, while preserving the shape of the THz pulse spectrum. Changes in the THz spectrum are shown to be much smaller when the Brewster attenuator is used than when either the near-infrared pump power or the bias voltage on a THz photoconductive antenna is varied. The Brewster attenuator should prove especially useful for varying THz field strength in nonlinear optical studies that use broadband THz pulses.

Terahertz nonlinear optics with strained p-type quantum wells

Valence-subband nonparabolicity is shown theoretically to lead to nonlinearities associated with terahertz third-harmonic generation in strained p -type quantum wells. For strained InAs quantum wells it is found that the corresponding value of chi((3)) can be as large as ~10(-12)(m/V)(2). The predicted values of chi((3)) are in the range of those associated with intersubband transitions in the mid-infrared region of the spectrum.