Two-photon Absorption In Semiconductors Research Articles

Two-photon absorption in semiconductors: A multiband length gauge analysis

The simplest approach to deal with light excitations in direct-gap semiconductors is to model them as a two-band system: one conduction and one valence band. For such models, particularly simple analytical expressions are known to exist for the optical response such as multi-photon absorption coefficients. Here we show that generic multi-band models do not require much more complicated expressions. Our length-gauge analysis is based on the semiconductors Bloch equations in the absence of all scattering processes. In the evaluation, we focus on two-photon excitation by a pump-probe scheme with possibly non-degenerate and arbitrarily polarized configurations. The theory is validated by application to graphene and its bilayer, described by a tight-binding model, as well as bulk Zincblende semiconductors described by k.p theory.

Observation of Nondegenerate Two-Photon Gain in GaAs.

Two-photon lasers require materials with large two-photon gain (2PG) coefficients and low linear and nonlinear losses. Our previous demonstration of large enhancement of two-photon absorption in semiconductors for very different photon energies translates directly into enhancement of 2PG. We experimentally demonstrate nondegenerate 2PG in optically excited bulk GaAs via femtosecond pump-probe measurements. 2PG is isolated from other pump induced effects through the difference between measurements performed with parallel and perpendicular polarizations of pump and probe. An enhancement in the 2PG coefficient of nearly 2 orders of magnitude is reported. The results point a possible way toward two-photon semiconductor lasers.

Measuring photon bunching at ultrashort timescale by two-photon absorption in semiconductors

Photon beams exhibit temporal correlations that are characteristics of their emission mechanism. For instance, photons issued from incoherent sources tend to be detected in bunches. This striking ‘bunching’ behaviour has been observed in the seminal experiment by Hanbury-Brown and Twiss (HBT) in the fifties, who measured the time of arrival of partially coherent photons on two separate photon-counting modules1. Since then, HBT interferometry has become a widespread technique to study photon correlations down to only the nanosecond range, because of the detector-limited bandwidth, preventing the observation of bunching for real thermal sources. It has been suggested later that two-photon absorption (TPA) could measure the photon temporal correlations at a much shorter timescale2,3, as it involves an almost simultaneous absorption of two photons, within a maximum delay given by the Heisenberg principle. Here, for the first time, this prediction is experimentally demonstrated using TPA in a GaAs photon-counting module. We have observed photon bunching in the femtosecond range for real blackbody sources (an enhancement of six orders of magnitude in the time resolution of present techniques), opening the way to monitor optical quantum statistics at the ultrashort timescale.

Cavity-enhanced saturable and two-photon absorption in semiconductors

Two-photon and saturable absorption are studied by placing GaAs and InGaAs materials in a microcavity. We show that field enhancement occurring due to the cavity influences the threshold and dynamic range of rollover in the nonlinear response. This approach can be used in semiconductor laser mirrors with improved capabilities for self-starting passive mode locking with suppressed tendency to Q-switching instability.

Adaptive pulse compression by two-photon absorption in semiconductors

We investigate the adaptive optimization of broadband laser pulses, using a closed-loop learning algorithm in which the merit function is derived from two-photon absorption in semiconductors. Photoluminescence experiments with CdS thin films and photocurrent measurements of a GaAsP photodiode have been performed. The experimental data demonstrate that reliable and accurate pulse compression to the bandwidth limit can be achieved, unperturbed by nontrivial phase effects. Therefore two-photon absorption proves to be an easy-to-implement alternative to second-harmonic generation for the compression of broadband laser pulses.

Electric and magnetic dipole two-photon absorption in semiconductors.

Electric and magnetic dipole two-photon absorption in semiconductors.

Verification of the Scaling Rule for Two-photon Absorption in Semiconductors

The simple parametric scaling rules for the two-photon absorption coefficients, g2, predicted by recent theory, have been experimentally confirmed by the measurement of g2 in ten different semiconductors using carefully characterized picosecond 1 and 0·5 wm pulses. We find g2 (cm/GW) = (3·1 - 0·5) 2 103 i EpF2(2Ey/Eg)n m2E m3 g, where Ep ­ 21 eV for the semiconductors studied, Eg is the energy gap in eV, n the refractive index and F2(x) = (x m 1)3/2/x5 . This relation allows the prediction of g2 in other materials at other wavelengths, which is useful in the design of nonlinear optical elements. The strong self-defocusing observed is consistent with nonlinear refraction by the two-photon-generated free carriers.

Exciton-polaritons by two-photon absorption in semiconductors

The creation of excitonic polariton states by two-photon absorption is theoretically investigated. A semi-classical approach is adopted to compute the two-photon transition probability to polariton states through an intermediate exciton state. The numerical results in CuCl show two peaks corresponding to the longitudinal exciton and transverse polariton states, respectively. These results are in good agreement with the experiment.

Nanosecond and microsecond pulse generation in solid-state lasers (review)

The temporal characteristics of Q-switched laser pulses are considered. The dependences are given of the duration (Δτ) and shape of pulses from solid-state lasers, operating in the giant-pulse regime, on the active-medium and resonator parameters, and the possibilities of varying Δτ over the range 10–9–10–6 sec are considered. Attention is concentrated on the possibilities of increasing the duration of a pulse to 1 μsec and more. Theoretical and experimental results are presented on the subject of increasing the pulse duration by introducing, into the laser cavity, electro-optic negative feedback or nonlinear losses caused by two-photon absorption in semiconductors, stimulated Raman scattering, and second harmonic generation. Methods are also considered of reducing Δτ for a giant pulse to 10–9 sec or less.

Magnetic excitons by two-photon absorption in semiconductors

Magnetic excitons by two-photon absorption in semiconductors

Two-photon absorption in semiconductors with picosecond laser pulses

Single pulses from a well-calibrated mode-locked YAlG:Nd laser have been used to measure the two-photon absorption coefficient at 1.064 \ensuremath{\mu}m in several semiconductors. The materials studied are four direct-gap semiconductors, GaAs, CdTe, ZnTe, and CdSe, and one indirect-gap semiconductor, GaP. The results for the direct-gap semiconductors are interpreted with respect to the imaginary part of the third-order nonlinear susceptibility ${\ensuremath{\chi}}_{1111}^{(3)}(\ensuremath{-}\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega},\ensuremath{-}\ensuremath{\omega})$. An anisotropy of the two-photon absorption coefficient is observed in GaAs.

Effect of a magnetic field on the phonon-assisted two-photon absorption in semiconductors

The theory of the phonon-assisted two-photon transitions in a magnetic field is developed by means of perturbation theory. Three different band models have been introduced. The electronic transitions are defined by (i) interband-interband indirect transitions in the four-band model, (ii) interband-intraband indirect transitions in the three-band model, and (iii) intraband-intraband indirect transitions in the two-band model. These models differ from each other by the number of bands, the type of the photon matrix elements and the Landau selection rules. The result shows that by increasing the value of the magnetic field, the absorption co-efficient adopting the two-band model is strongly increased, but for a fixed value of the field the four-band model gives the largest contribution.

Two-Photon Absorption inCu2O

In the first part, a general theoretical treatment of two-photon absorption in semiconductors is presented. It includes excitonic effects in the intermediate as well as final states. $s$ and $p$ excitonic final states are discussed separately. In the second part, the two-photon absorption of ${\mathrm{Cu}}_{2}$O at 77\ifmmode^\circ\else\textdegree\fi{}K is presented. The spectral region studied includes transitions from the two upper valence bands to the two lower conduction bands. Excitonic structures are observed. The results are discussed within the framework of the theory presented in part one.

Indirect exciton transitions in the two-photon absorption in semiconductors

The theory of indirect two-photon exciton transitions is developed from third-order time-dependent perturbation theory. The problem is treated by introducing three different band models; a four-band, a two-band and an intermediate (three-band) model. Selection rules and numerical estimates show that the three-band model is the most favourable for this type of process. An application for the case of GaP is performed.

Two-photon absorption in semiconductors in a magnetic field

The absorption coefficient for two-photon transitions in a magnetic field is given for the case ofM0-type singularities in second-order perturbation theory. States belonging to the two bands between which the transitions occur and states associated with other bands of the crystal are considered, as well as intermediate states. Correspondingly, two types of transitions, with selection rules Δn = ± 1 (inter-intra) and Δn = 0 (inter-inter), are shown to occur and explicit expressions for the transition probabilities are given. The selection rules are also discussed.