Nonlinear Optical Spectra Research Articles

Electromagnetically Induced Transparency in Quantum Wells

We show how to compute nonlinear optical absorption spectra of an asymmetric double quantum well in the region of intersubband electronic transitions. The method uses the microscopic calculation of the dephasing due to electron-electron and electron-phonon scattering rates and the macroscopic real density matrix approach to compute the electromagnetic fields and susceptibilities. For a proper choice of the QW widths and of the driving fields we obtain electromagnetically induced transparency. Results are given for GaAs/GaAlAs QWs and experiments are proposed.

Optical second harmonic spectroscopy of semiconductor surfaces: advances in microscopic understanding

Even-order non-linear optical spectroscopy has emerged as an unusually sensitive technique for non-invasive analysis of surfaces and buried interfaces of centrosymmetric materials. The forefront challenges are: to develop reliable microscopic computational methods for calculating and interpreting measured surface non-linear spectra; to relate non-linear surface spectra quantitatively to linear optical surface probes such as reflectance-difference spectroscopy (RDS); and to develop convenient methods for acquiring non-linear optical spectra over bandwidths (several electron-volts) that encompass multiple electronic surface resonances. We review recent advances in both calculation and measurement of non-linear spectra, with emphasis on reconstructed and adsorbate-covered silicon surfaces in epitaxial growth environments. Copyright © 2001 John Wiley & Sons, Ltd.

Linear and nonlinear optical properties of semiconductor nanorings with magnetic field and disorder - Influence on excitons and biexcitons

Linear and nonlinear optical absorption spectra are studied theoretically for semiconductor nanorings penetrated by a magnetic field. Due to the Aharanov-Bohm effect the spectral position as well as the oscillator strength of the exciton change periodically as function of the magnetic flux enclosed by the ring. In the nonlinear differential absorption spectra it is found that the magnetic field strongly modifies Coulomb many-body correlations. In particular, the magnetic-field-induced increase of the exciton binding energy is accompanied by a decrease of the biexciton binding energy. The persistence of these effects in the presence of energetic disorder is analyzed.

Polarizable bond model for optical spectra of Si(100) reconstructed surfaces

We calculate the linear and nonlinear optical spectra of clean $\mathrm{Si}(100)2\ifmmode\times\else\texttimes\fi{}1$ reconstructed surfaces based on the polarizable bond model. The crystal is treated as an array of pointlike polarizable dipoles in which a dipole replaces each Si-Si bond. The model incorporates the reconstruction of the surface through the local field effect. As a function of the dimer buckling of the $2\ifmmode\times\else\texttimes\fi{}1$ reconstruction, we calculate the reflectance anisotropy (linear response), and the second-harmonic generation (nonlinear response). We find that a surface with a dimer buckling of $0.6 \AA{}$ qualitatively reproduces the linear and nonlinear experimental spectra. A number of physical processes, such as charge transfer in the dimer, are explored within this model and compared with experimental results.

Anharmonic Oscillator Approach to the Exciton‐Exciton Annihilation Dynamics in Molecular Aggregates

AbstractThe dynamics of Frenkel excitons in aggregates of coupled electronic three‐level molecules in the presence of strong external fields is investigated. Particular emphasis is paid to the microscopic inclusion of exciton‐exciton annihilation. It appears as a combination of exciton fusion and internal conversion processes which are taken into account within perturbation theory. The influence of an environment on the exciton dynamics is treated by means of stochastic fluctuations of the electronic transition energies. A closed set of equations of motion is derived and it is shown how nonlinear optical spectra can be calculated nonperturbatively.

Model-independent maximum-entropy method for the analysis of sum-frequency vibrational spectroscopy

We have developed and applied a maximum-entropy phase-retrieval procedure to analyze sum-frequency vibrational spectra from a CCl4/octadecyl tricholosilane/silica interface and a hydrogen-terminated diamond C(111) surface. Some a priori knowledge of a nonlinear optical spectrum was employed for determining the phase of nonlinear optical susceptibility, and therefore the requirement for experimental phase measurement can be avoided. The results agree well with those from the Lorentzian line-shape model and justify the applicability of the a priori constraints employed in our phase-retrieval procedure.

Phonon-mediated coherent transients in pseudoisocyanine J aggregates

We report results of femtosecond pump and probe absorption spectroscopy under the virtual excitation in pseudoisocyanine J aggregates. When the pump photon energy is tuned to the 2.000 eV which is 173 meV lower than the J band peak, the resonant inverse Raman type nonlinear optical spectrum is observed. With the increase and decrease of the pump photon energy the blue-shifted transient spectra are dominant. The blue-shifted spectra are ascribed to the nonresonant two-level contribution of virtual excitons, because the excitonic transition is much larger than the vibrational transition. The major character of the observed spectra agrees with the model calculation based on the third-order nonlinear optical process of the resonant inverse Raman effect and nonresonant two-level contribution.

Piezoelectric field-enhanced second-order nonlinear optical susceptibilities in wurtzite GaN/AlGaN quantum wells

Second-order nonlinear optical processes including second-harmonic generation, optical rectification, and difference-frequency generation associated with intersubband transitions in wurtzite GaN/AlGaN quantum well (QW) are investigated theoretically. Taking into account the strain-induced piezoelectric (PZ) effects, we solve the electronic structure of the QW from coupled effective-mass Schrödinger equation and Poisson equation including the exchange-correlation effect under the local-density approximation. We show that the large PZ field in the QW breaks the symmetry of the confinement potential profile and leads to large second-order susceptibilities. We also show that the interband optical pump-induced electron-hole plasma results in an enhancement in the maximum value of the nonlinear coefficients and a redshift of the peak position in the nonlinear optical spectrum. By use of the difference-frequency generation, THz radiation can be generated from a GaN/Al0.75Ga0.25N with a pump laser of 1.55 μm.

Linear and nonlinear optical properties of group-III nitrides

Linear and nonlinear [second harmonic generation (SHG)] optical functions of cubic and hexagonal BN, AlN, GaN, and InN have been studied by using the first-principles full-potential linearized augmented plane-wave method. Equilibrium lattice constants are determined from the total-energy minimization method. The calculated spectra of the second-order optical susceptibility show pronounced structures related to the two-photon resonances. In materials with heavy metals there are remarkable contributions from the single-photon transitions. Line shapes of the linear and particularly the nonlinear optical spectra of GaN and InN crystals are very sensitive to the interactions between the conduction bands and metallic d states. Studies of the nonlinear optical susceptibilities in both wurtzite and cubic crystals show high sensitivity of the SHG spectra to the changes of atomic structure.

From Band Structures to Linear and Nonlinear Optical Spectra in Semiconductors

The theory of the relation between optical properties and electronic band structures is reviewed. In the first part, we describe various approaches to go beyond the long-wavelength limit perturbation theoretical approach in the independent particle approximation for linear optical response functions. In the latter, electron-electron interaction effects are only included in as far as they are implicit in the underlying band structure (typically within the local density functional approximation). We discuss the inclusion of quasiparticle corrections to the band structures, local-field and electron-hole interaction effects. A case study of GaN is used to illustrate the discrepancies between theory and experiment that arise from neglect of these effects. On the other hand, the paper also illustrates that in the process of extracting the band structure information from optical spectra, the independent particle model still plays a central role. In the case of nonlinear optical response, even the independent particle model was only recently fully developed. Recent progress in this area and their implementation within the context of first-principles band structure methods is presented next. Some examples are used to illustrate the potentially richer information contained in NLO spectra in relation to the underlying band structures. Secondly, progress in understanding the trends in NLO coefficients in some classes of materials is illustrated with a study of chalcopyrites.

Computation of Optical Properties for Quantized Electronic Systems

ABSTRACTA density-matrix description has been developed to treat relaxation (decoherence) phenomena during resonant and non-resonant radiative transitions of quantized electronic systems, including many-electron atoms and quantum-confinement systems (e. g., semiconductor microstructures). Radiative and collisional relaxation phenomena have been treated using Liouville-space proj ecti on-operator techniques. Both time-independent (resolvent-operator) and time-dependent (equation-of-motion) formulations have been developed. The self-energy operators that occur in these formulations can provide the fundamental basis for a self-consistent determination of the non-equilibrium and coherent electronic-state kinetics together with the homogeneous spectral-line shapes. This density-matrix description can be adapted for the computer simulation of electromagnetic processes. From first-principles electronic-structure calculations or from semi-empirical approaches, the parameters describing the elementary collisional and radiative interactions can be evaluated and organized into the basic data set for the application of the density-matrix description. The final product is a theoretical prediction for the linear or non-linear optical absorption or emission spectrum corresponding to a given set of values for the appropriate physical variables, such as temperatures, densities, and electric or magnetic field strengths.

1PB060 光合成細菌の光捕獲アンテナ系における励起子の非線形光学スペクトル

1PB060 光合成細菌の光捕獲アンテナ系における励起子の非線形光学スペクトル

Few-Particle Effects in Nonlinear Optical Spectra of Semiconductor Quantum Dots

ABSTRACTWe present a density-matrix approach for the description of nonequilibrium carrier dynamics in optically excited semiconductor quantum dots, that explicitly accounts for exciton-exciton as well as exciton-carrier interactions. Within this framework, we analyze few-particle effects in the optical spectra and provide a consistent description of additional peaks appearing at high photoexcitation density. We discuss possible applications of such optical nonlinearities in future coherent-control experiments.

Theory of Ultrafast Exciton Motion in Photosynthetic Antenna Systems

To investigate ultrafast exciton motion, exciton relaxation and related nonlinear optical spectra of photosynthetic antenna systems the dissipative multi-exciton theory is utilized. This type of approach accounts for optically induced transitions to the band of delocaliced single-exciton and two-exciton states as well as takes notice of the coupling to the vibrations of the protein matrix. Different applications are mentioned with emphasis on the Fenna Matthews Olson (FRO) photosynthetic antenna complex of Chlorobium Tepidum.

Optical Properties of Excitons in Low Dimensional Systems

We show how to compute the linear and nonlinear optical spectra of Low Dimensional Systems (LDS) in the excitonic energy region using the dynamical density matrix approach. The advantage with respect to the usual quantum mechanical calculation, which makes use of the transition probability, is that we can obtain directly the optical functions and all coherence effects are automatically included. The method is applied to GaAs/Ga1—xAlxAs superlattices (SL), Quantum Wires (QWW) and Quantum Dots (QD), and the optical functions are computed, with inclusion of nonlinear effects in the case of the superlattice. In the case of QDs we present new results which give the properties of scattered optical waves.

Linear and nonlinear optical spectra of polyazomethines fabricated by chemical vapor deposition

We report on the results of third-harmonic generation (THG) measurements on vapor-deposited thin films of two polyazomethines, poly(nitrilo-1,4-phenylenenitrilomethylidyne-1,4-phenylenemethylidyne) (PNPP) and poly(nitrilo-2,5-azinylnitrilomethylidyne-1,4-phenylenemethylidyne) (PNAP) covering pump energies from 0.6 to 1.4 eV. The THG data show the presence of a strong three-photon resonance centered near 0.9 eV for both samples; additional weaker structure is observed for PNPP near 1.2 eV. The peak values of the third-order susceptibility are 2.8 × 10 −11 e.s.u. for PNPP and 1.8 × 10 −11 e.s.u. for PNAP; off-resonance values are nearly an order of magnitude smaller. We have also obtained the χ (3)(3 ω) spectrum of an oriented PNPP film grown on an obliquely evaporated SiO 2 pattern. An enhancement by nearly a factor of three in the peak nonlinear optical susceptibility is observed relative to that of the unoriented PNPP film, demonstrating that a moderate degree of molecular orientation was achieved.

Cooperative Phenomena in Artificial Solids Made from Silver Quantum Dots: The Importance of Classical Coupling

Recent work has shown that metal quantum dots can be treated as “artificial atoms” and crystallized into “artificial solids” that have electronic properties that can be tuned by controlling interparticle coupling through the application of pressure. The interactions between the nanocrystals in such artificial solids are classified as either dipole or exchange.1 Dipole (including many-body) coupling between nanocrystals is treated in a classical manner using an effective medium model that permits the calculation of both the linear and nonlinear optical spectra of the solid as the interparticle separation is continuously varied. We find agreement between the classical model and experimental linear reflectance data until the crystallites are separated by less than 10 Å, after which they sharply diverge. We find that the classical model fails to predict the overall experimental trend in second-harmonic intensities.

2Ag Exciton State below 1Bu Exciton State in Trans-Polyacetylene Observed by Second-Harmonic-Generation Spectroscopy

We measured second-harmonic-generation spectra of centrosymmetric trans-polyacetylene films. A resonance peak was observed and assigned to the electric-quadrupole transition to the 2Ag exciton state at about 0.96 eV, which is located below the 1Bu exciton state at about 1.5 eV. We have also discussed the relative magnitude of the transition dipole between the excited states by comparing the various nonlinear optical spectra.

Time resolved spectroscopy of nonlinear solvation with pulses longer than electronic dephasing

In this study we show that transient four-photon spectroscopy with pulses longer than the electronic transition dephasing, can be used for nonlinear solvation study, i.e., when the linear response for the solvation dynamics breaks down. We derive new equations describing the time evolution of the moments of the nonlinear optical spectra and in particular, time resolved fluorescence in the case of nonlinear solvation. The formulae allow to use stochastic models and the information concerning time resolved Stokes shift is preserved. For broad and featureless electronic molecular spectra they provide simple analytical expressions. We demonstrate how solvation dynamics in the ground electronic state is obtained by time resolved hole-burning spectroscopy. We apply our results to the Debye model of rotational diffusion for the case of nonlinear solvation and obtain a long-time solution for the corresponding density of the conditional probability in a strong electrical field.

Stark profiles of singlet excitons in conjugated polymers

Abstract Electroabsorption (EA) spectra associated with overlapping vibronics of even- and odd-parity excitons are related in the Condon approximation to the linear absorption of discrete localized states. The EA intensity and profile depend strongly on the linewidth, Λ, with Λ −2 scaling of features due to Stark shifts and Λ −1 scaling of induced absorption and bleaching. Static disorder in polymer films broadens the linear absorption and reduces EA intensity compared to polydiacetylene (PDA) crystals, whose resolved spectra indicate overlapping vibrational sidebands. Similar excitations and transition moments in films conspicuously alter EA profiles and allow estimates of symmetry-breaking perturbations. EA spectra of PDA crystals and films are consistent with Pariser-Parr-Pople theory of linear and nonlinear optical spectra. The high sensitivity of EA also provides direct tests of excited-state displacements at induced transitions of even-parity excitons.