M. Lepore

Two-photon absorption coefficient spectrum in CdSSe nanocrystals by nonlinear luminescence technique

Abstract Low-temperature two-photon absorption measurements on semiconductor doped glasses containing CdS 1− x Se x particles of various average sizes have been reported. Such experimental results have been shown to be in good agreement with the effective-mass-approximation calculations in which the contributions of the different valence sub-bands have been taken into account. Furthermore, the two-photon absorption coefficient has been experimentally measured and the imaginary part of the third-order susceptibility has also been evaluated.

Frequency dependence of interband two-photon absorption mechanisms in ZnS and CdI2

The transition mechanism changes involved in direct interband two-photon absorption (TPA) processes have been studied in two II–VI compounds:ZnO and ZnS. The experimental results have indicated that the transition mechanism nature changes when the excitation energy is varied. The transitions are of an allowed-allowed type near the energy gap and become of an allowed-forbidden type far from it. In particular, for ZnO when 2ħw-Ey > ~ 400 meV the first experimental evidence of a dominant mechanism of forbidden-forbidden type has been obtained. Furthermore, the spectral behaviour of the TPA coefficient (α(2)) has been well described by a parametric formula containing terms (with different energy dependences) related to the above-mentioned different transition mechanisms.

Excitonic properties in Zn1−xCdxSe/ZnSe multi‐quantum well structures by one‐ and two‐photon spectroscopy

A detailed investigation of excitonic states by means of one‐ and two‐photon absorption photoluminescence excitation spectroscopy in Zn1−xCdxSe/ZnSe multi‐quantum wells is reported. Ground state and excited heavy‐ and light‐hole excitons associated with the n=1,2 subbands have been selectively probed exploiting the different selection rules governing one‐ and two‐photon absorption processes. Experimental exciton transition energies are found to be in good agreement with theoretical predictions when strain and confinement effects are included. The comparison between experiments and theory allowed us to single out a well defined set of band parameters and to gauge the band‐offsets in these heterostructures. Furthermore, the absorption band edge clearly evident in the two‐photon absorption photoluminescence excitation spectra allowed us to directly measure excitonic binding energies with good accuracy.

Spectral behavior of three-photon absorption coefficient in II–VI compounds: ZnO, CdS and ZnSe

Abstract Three-photon absorption (3PA) spectra has been experimentally investigated in a large excitation energy range above the optical gap for three II–VI semiconductors (ZnO, CdS and ZnSe) with different band structures. The spectral behavior of 3PA coefficient has been demonstrated to be described by a parametric formula containing a varying number of different terms, whose energy dependence being predicted on the grounds of 3PA selection rules. A comparison between experimental and theoretical line-shapes has suggested that almost a linear k -dependence approximation for the optical matrix elements has to be always assumed in order to correctly describe the 3PA coefficient line shape.

Polarization dependence of the excitonic two-photon absorption spectra of GaAs/AlGaAs quantum wells☆

Abstract The excitation spectra of the exitonic two-photon absorption in GaAs/ AlGaAs multiple quantum well structures have been experimentally investigated with the polarization vector of the exciting electromagnetic field parallel or perpendicular to the growth axis of the heterostructure. The selection rules for the excitonic two-photon transitions have been found to strongly change for the two different polarizations. The comparison of linear and nonlinear absorption measurements provides important information on the excitonic states in multiple quantum wells.

Interband two-photon absorption mechanisms in direct and indirect GAP materials

Interband transition mechanisms involved in direct and indirect two-photon absorption (TPA) processes have been investigated in ZnO, CuCl, Bi4Ge3O12, AgCl and CdI2. It has been shown that the transition mechanism nature changes in both direct and indirect processes when the excitation energy is varied. In particular, the transitions are of the allowed-allowed type near the energy gap and become of the allowed-forbidden one far from it. Furthermore, for direct TPA processes, it has been pointed out that the third possible mechanism, i.e., the forbidden-forbidden one, becomes dominant for energies well above the direct gap.

Spectral behaviour of the two-photon absorption coefficient in ZnO, CuCl and Bi4Ge3O12

SummaryThe spectral behaviour of the direct interband two-photon absorption coefficient α(2) in ZnO, CuCl and Bi4Ge3O12 has been investigated in a large excitation energy range. The experimental results have shown that the α(2) spectral behaviour is well described by a parametric formula containing terms with different energy dependence. In particular, for 2ħω-Eg>≈400 meV, the first experimental evidence of 2ħω-Eg)5/2 dependence has been obtained. As a consequence, each of the various models proposed to predict the α(2) dispersion curve gives the correct energy dependence in a limited energy range, due to the poor approximation made in the evaluation of dipole matrix elements. To explain the spectral dependence of the two-photon absorption coefficient, consideration of all the intermediate states is required, with the energy dependence of the dipole matrix elements properly considered. It is shown that the greatest contributions to the oscillator strength come from transitions totally allowed at the critical points.RiassuntoL’andamento spettrale del coefficiente di assorbimento α(2) diretto a due fotoni relativo a transizioni di tipo interbanda è stato studiato in un largo intervallo di energie di eccitazione in diversi materiali (ZnO, CuCl e Bi4Ge3O12). I risultati sperimentali hanno mostrato che l’andamento spettrale è ben descritto da una formula parametrica che contiene termini con differenti dipendenze dall’energia. In particolare, per 2ħω-Eg>≈400 meV, viene riportata la prima evidenza sperimentale della dipendenza (2ħω-Eg)5/2. Di conseguenza, ciascun modello proposto per la valutazione della curva di dispersione di α(2) fornisce una corretta dipendenza dall’energia solo in un limitato intervallo di energie di eccitazione. Questo è dovuto alle pesanti approssimazioni riguardanti l’elemento di matrice di dipolo. Per spiegare la dipendenza spettrale dell’assorbimento a due fotoni è necessario, quindi, considerare tutti gli stati intermedi coinvolti nelle transizioni tenendo conto correttamente della dipendenza dall’energia degli elementi di matrice di dipolo. In questo modo, si può mostrare come il maggiore contributo all’intensità dell’oscillatore sia dovuto a transizioni totalmente permesse nei punti critici.РезюмеВ широкой области энергий возбуждения исследуется спектральное поведение коэффицнента прямого двух-фотонного поглощения (α(2)) в ZnO, CuCl и Bi4Ge3O12. Экспериментальные результаты показывают, что спектраьное поведение хорошо описывается параметрической формулой, содержащей члены с различной энергетической завимостью. В частности, для 2ħω-Eg>−400 мэВ впервые полчена экспериментальная зависимость (2ħω-Eg)5/2. Пюбая из моделей, предложенных для описания α(2) дисперсионной кривой, дает, правильную энергетическую зависимость в огарниченной области, вследствие грубого приближения, сделанного при вычислении липольных матричных элемементов. Для объяснения спектральной занисимости коэффициента двух-фотоннго поглощения необходимо учитывать все промежуточные состояния и энергегескую зависимость дипольных матричных элементов. Показывается, что наиболыший вклад в силу осцилляторов возникает из переходов, полностью разрешенных в критических точках.

On the evaluation of the matrix element of dipole in multiphoton transitions

Abstract The main approximations for the square values of the matrix element of dipole have been reviewed here. In addition a new method to evaluate them has been considered and used to check the validity of the | P ij | 2 k·p evaluations for some semiconductors: ZnO, ZnSe, GaN, GaP, GaS and CdS.

Two-photon absorption processes in GaAs/AlxGa1−xAs quantum wells

SummaryThe two-photon absorption spectra of GaAs/AlGaAs multiple quantum well and superlattice structures have been experimentally investigated by means of the nonlinear luminescence technique in different polarization configurations. A strong excitonic effect overlapping the interband two-photon spectrum has been found and the selection rules for the excitonic transitions have shown to greatly change for different polarizations. The comparison of linear and nonlinear absorption measurements provides important information on the excited states of excitons in multiple quantum wells.

Two-photon absorption coefficient measurements in strained-layer superlattices

The two-photon absorption (TPA) coefficient of strained-layer superlattices has been measured in different samples at excitation energies related to heavy- and light-hole excitons associated with n = 1,2 subbands in the TE polarization. A comparison between the experimental results and the predictions of analytical theories on TPA processes including quasi-bidimensional excitonic effects has been carried out. A satisfactory agreement with the numerical values has been shown by one of these theories. Finally, the imaginary part of the third-order susceptibility has been evaluated from the experimental values.