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.     

Excitonic molecule in semiconductors by two-photon absorption

Direct creation of bi-exciton states by two-photon absorption in direct gap semiconductors is investigated theoretically. A numerical application to the case of CuCl shows that the two-photon absorption coefficient for bi-excitonic transitions is larger than that for two-photon interband transitions by three orders of magnitude. It becomes comparable to that for one-photon excitonic transitions for available laser intensities. The main contribution to this enhancement of the absorption coefficient for the transitions to the bi-exciton states is found to be from the resonance effect.     

Exciton molecule in semiconductors by phonon-assisted two-photon absorption

Direct creation of bi-excitonic states by photon-assisted two-photon absorption in indirect-gap semiconductors is investigated theoretically. The symmetry of the indirect bi-exciton states and of the phonon used are given for the case of Ge. A numerical application to the case of Si shows that the indirect two-photon absorption coefficient for bi-excitonic α²_I (bi-ex) transitions is several orders of magnitude larger than both indirect two-photon interband, α²_I (band), and excitonic, α²_I (exc), transitions. It becomes smaller than both indirect one-photon interband, α¹_I (band), and excitonic, α¹_I (exc), transitions for available laser intensities. The essential contribution to this enhancement of α²_I (bi-ex) is found to be from the resonance effect in the first process and from both the resonance effect and matrix elements included in the second process.     

Ultrafast pulse compression by semiconductor two-photon gain

We demonstrate experimentally the compression of femtosecond-scale pulses by two-photon gain in a compact electrically driven AlGaAs waveguide. Dynamic control of the pulse width from 240 to 140 fs is achieved by varying the current injection levels--in good agreement with the calculations. The pulse width is measured by a high-sensitivity intensity autocorrelator based on two-photon absorption in a GaAs photomultiplier tube.     

Feasibility study of pulse compression by two-photon self-induced transparency

The possibility of pulse compression by the process of two-photon self-induced transparency (TPSIT) is considered. Numerical solutions to the two-photon Bloch-Maxwell equations are presented, and limits established for the maximum amount of detuning from exact resonance for which the compression process will still occur.     

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.     

Adaptive femtosecond pulse compression

A practical adaptive method for femtosecond optical pulse compression is demonstrated experimentally for the first time to our knowledge. The method is robust and capable of handling the general case of pulse compression, in which the input pulses are completely uncharacterized or partially characterized.     

Nonparabolicity and exciton effects in two-photon absorption in zincblende semiconductors

The two-photon absorption coefficient is calculated from perturbation theory for direct-gap zincblende semiconductors using exact nonparabolic energies and matrix elements. The enhancement due to exciton effects is included. The results are compared to experiment and to other theories including the tunneling or Keldysh theory.     

Quantitative investigation of the two-photon absorption of Ruby-laser-light in various semiconductors

With aQ-switched Ruby-laser (h v _L =1.785 eV), the two-photon absorption (TPA) coefficient of various semiconductors is determined. The gap energy,E _g , of these substances is lying in the range ofh v _L <E _g <2hv _L . For all measurements the same experimental setup is used, and for the evolution of the data the real, time-dependent intensity of the laser pulse is taken. Thus, the existing discrepancies between the TPA-coefficients as determined by various authors with different experimental techniques can be settled.     

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.     

Contrast ratio variation caused by radiation attenuation in pulse width measurement by two-photon absorption fluorescence

In the technique for measuring pulse width by two-photon absorption fluorescence, theoretical calculations indicate that one can hope to obtain a maximum contrast ratio of three between the point at which the two peaks overlap and the point at which there is no superposition. Nevertheless, this calculation has been carried out assuming that the pulse is not attenuated as it traverses the fluorescent medium. In this paper it will be shown that this attenuation gives rise to a decrease in the contrast ratio. As the fluorescence intensity is proportional to the absorption, an increase in the former will be to the detriment of the contrast ratio. We will attempt to give a criterion for the selection of experimental conditions which lead to maximum intensity without an appreciable decrease in the contrast ratio.     

Phonon-assisted two-photon transitions in semiconductors

The effect of the non-parabolicity of the energy bands on the indirect two-photon transition is theoretically studied. An expression for the absorption coefficient valid both near and far from the energy gap is given. Numerical results, compared with recent experimental data on AgCl, show that the dominant transition mechanisms are of the allowed-allowed type near and far from the edge.     

Energy limits imposed by two-photon absorption for pulse amplification in high-power semiconductor optical amplifiers

We study the combined effects of dynamic gain saturation and two-photon absorption on the amplification of short pulses in semiconductor optical amplifiers and show that two-photon absorption can saturate the amplifier gain and limit the output pulse energies even for amplifiers with large gain saturation energies. We discuss the upper limits for the pulse energies obtainable from semiconductor optical amplifiers in the presence of two-photon absorption and show that for single transverse mode waveguide amplifiers these upper limits can range from values as small as a few picojoules to several hundred picojoules for pulse widths in the 0.5 ps to 20 ps range, respectively.     

UWB monocycle pulse generation using two-photon absorption in a silicon waveguide

We propose and experimentally demonstrate ultrawideband monocycle pulse generation using nondegenerate two-photon absorption in a silicon waveguide. The free-carrier absorption induced pulse tail at the rising edge of inverted probe pulse is largely compensated by the overlapped pump pulse and results in a symmetric negative monocycle pulse. A 143 ps Gaussian monocycle pulse is successfully obtained with a 131.7% fractional 10 dB bandwidth using a 68 ps pulsed pump. The 10 dB bandwidth and center frequency of the RF spectrum for the generated monocycle pulse can be largely tuned using an optical delay line. An operational bandwidth of 30 nm is demonstrated experimentally with stable performance, and larger optical bandwidth is expected.     

Adaptive pulse compression for transform-limited 15-fs high-energy pulse generation

We demonstrate the use of a deformable-mirror pulse shaper, combined with an evolutionary optimization algorithm, to correct high-order residual phase aberrations in a 1-mJ, 1-kHz, 15-fs laser amplifier. Frequency-resolved optical gating measurements reveal that the output pulse duration of 15.2 fs is within our measurement error of the theoretical transform limit. This technique significantly reduces the pulse duration and the temporal prepulse energy of the pulse while increasing the peak intensity by 26%. It is demonstrated, for what is believed to be the first time, that the problem of pedestals in laser amplifiers can be addressed by spectral-domain correction.     

I-scan thermal lens experiment in the pulse regime for measuring two-photon absorption coefficient

We present a new pump-probe mode-mismatched thermal lens method for pulse excitation aimed to the measurement of nonlinear absorption coefficient in optical materials. We develop a theoretical model based on the Fresnel diffraction approximation and their predictions are verified experimentally with samples of Rhodamine 6G and Rhodamine B in ethanol solution. The principal advantage of this technique is that it does not require any mechanical movement during measurement. Below we perform the new type of thermal lens experiment in the pulse regime for the measurement of nonlinear absorption coefficient in transparent samples and we demonstrate the validity of theoretical predictions using an alternative method to the classical thermal lens technique.     

Photorefractive polymers sensitized by two-photon absorption

We demonstrate the recording of holograms and their nondestructive readout in a photorefractive polymer, using two-photon absorption. Sensitivity is provided by the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into the photoconducting poly(N -vinylcarbazole) matrix. The holograms can be fully erased with a pulsed laser source but are insensitive to cw laser beams with the same wavelength. Studies of the field and intensity dependence of the diffraction efficiency indicate that the holograms are formed through the photorefractive effect.     

Photochromism induced by infrared two-photon absorption

An IR laser of 940 nm wavelength induced photochromic reaction in acrylate that contained both spirobenzopyran and rare-earth doped oxide (Gd(2)O(2)S:YbEr). The rare-earth elements were excited by two 940 nm photons and emitted a 550 nm photon, which caused photochromic isomerization of spirobenzopyran. This acrylate turned to its original orange color by either thermal relaxation or ultraviolet irradiation, and was bleached again by IR laser irradiation.