Multiplet structure of P-excitons in CuBr due to valence band degeneracy

The two-photon absorption spectrum of CuBr shows a fourfold energy splitting of the 2P- and 3P-excitons of the Z_1,2 series. The splitting results from d-like contributions to the kinetic energy of the hole (Γ_δ) which lead to a coupling between angular momentum and hole spin. The energy splitting allows to determine the spherical and cubic parameters μ and δ of the reduced exciton mass. The results are compared with corresponding values of an existing band structure calculation.     

Optical properties of semiconductors under strong pumping in the <Emphasis Type="Italic">M</Emphasis>-band region and two-photon probing of the biexciton state

The behavior of the semiconductor dielectric susceptibility under the action of a strong laser pulse in the range of the luminescent M band and two-photon probe of a biexciton level is investigated. It is shown that the pronounced Autler-Towns effect occurs at the two-photon transition. The position of the absorption peaks is essentially determined by the amplitude and frequency of the pump field.     

Intensity of two-photon excitonic absorption in two-band and three-band models

The intensity of two-photon absorption to Wannier excitons is estimated in simplified models. Both two-band and three-band models simultaneously contribute to the optical two-photon spectrum, but it is shown that the two-band model is favoured in crystals with relevant electron-hole interaction. As an example, it is explained why two-photon absorption spectrum in CuCl exhibits 1s, 2p, 3p exciton states.     

Specific features of the two-photon absorption of cationic symmetric polymethine dyes

The two-photon absorption of a number of cationic symmetric polymethine dyes based on 3H-indolium and benzothiazolium is experimentally studied upon excitation by nanosecond radiation from a Nd:YAG laser (1064 nm, 12 ns). The two-photon absorption cross section of dye molecules is determined by the two-quantum standard method. The influence of the spectral-luminescent properties and structure of the polymethine dyes on the two-photon absorption cross section is discussed. It is shown that, upon excitation in the range of the long-wavelength band of the dyes studied (the S ₀ → S ₁ transition), the maximum of their two-photon absorption is blue shifted. The reasons for a considerable increase in the two-photon absorption of symmetric polymethine dyes upon their excitation in the range of the S ₂ state are discussed. Using the data of quantum-chemical calculations, it is shown that, along with changes in the selection rules for two-photon transitions, this increase is connected with an increase in the size of the delocalized π electron cloud of HOMOs involved in the S ₀ → S ₂ transition.     

Creation of spin-oriented 2P excitons in CdS by two-photon magneto-absorption

We report experimental results on the creation of 2P excitons in CdS by the two-photon magneto-absorption process involving a simultaneous absorption of visible and infrared photons. For a magnetic field parallel to the c axis and particularly for a visible beam propagating along the c axis, spin-oriented 2P (A) excitons are created from the A (Γ₉) valence band. We explain these results on the basis of the main contribution of the 1SΓ₅ (A) exciton intermediate state.     

9,10-Anthracene-centered oligo(p-phenyleneethynylene)s with end-capping didecylamines exhibiting enhanced two-photon fluorescence action cross-section

This study reports the synthesis and the two-photon absorption (TPA) properties of a series of 9,10-anthracene-centred oligo(p-phenyleneethynylene)s with end-capping didecylamine. The results show that both the maximal TPA cross-sections (δ_max) and the fluorescence quantum yields (Φ) of the oligomers increase with the extension of phenylethynyl bridges. Interestingly, the Φδ_max per repeating unit to is obviously enhanced by extending linearly conjugation size of the oligomers. Moreover, these oligomers all show the maximal two-photon absorption cross-sections at 800 nm and emit strong green to yellowish-green two-photon excited fluorescence. These materials may find applications in which two-photon excited fluorescence and photo-induced electron transfer are utilized.     

Methods of measuring the nonlinear-optical coefficients of the refractive index of nonlinear media

A cousistent analysis of a method of measuring the coefficient n ₂ of the Kerr nonlinearity and the two-photon absorption coefficient in presented. The method is based on the study of the dependence of the nonlinear transmittance with respect to the far-field axial intensity of the laser beam on the position of the layer of the nonlinear medium on the z (longitudinal) coordinate of this beam (z scanning). The possibility for the measurement of n ₂ and the two-photon (multiphoton) absorption coefficient based on the study of the far-field variations in the spatial profile of the beam having passed through the nonlinear medium is also analyzed. The calculations prove the efficiency of the new method for measuring n ₂ and the two-photon (multiphoton) absorption coefficient.     

Resonance-enhanced multiphoton ionization spectroscopy of CH3 and CD3. Two-photon absorption selection rules and rotational line strengths of the ν3- and ν4-active vibronic transitions

To explore the possibility of K-level resolved, 2+1 resonance-enhanced multiphoton ionization (REMPI) processes of the methyl radical, the two-photon absorption selection rules and rotational line strengths of the 3₀¹ and 4₀¹ vibronic bands of the transition (n=3 or 4) were reported. Stringent selection rules, which were imposed upon these two-photon transitions, are the initial K″=3p (p=0,1,2,…), ΔK=±2, ΔU=±3, and ΔN=0,±1,±2 (O, P, Q, R, and S branches). The previously assigned 2₂² vibronic band of the methyl radical should be studied by the REMPI with a better spectral resolution and analyzed by the newly derived two-photon absorption selection rules and rotational line strength formulas.     

Nonlinear light absorption and luminescence in silicate glasses

The study of luminescence of high-purity multicomponent silicate glasses excited by radiation of a N₂ laser (λ=337 nm, P≈12kW) showed darkening of a sample and a monotone decrease in luminescence intensity and the transmitted radiation intensity from pulse to pulse. Additional illumination with radiation of an Ar⁺ laser (λ=514.5 nm) caused an increase in luminescence intensity. An increase in absorption and a decrease in luminescence intensity were found to be caused by two-photon absorption and electron-hole pair production.     

A transient two-photon–one-photon double resonance on interband transitions in crystals: I. General relations

A theory of the transient absorption of femtosecond light pulses at a two-photon–one-photon double resonance on adjacent interband transitions in the electronic system has been developed. Approximate expressions for the reactive component of the nonlinear polarization of the three-level system, P _s, which determines the absorbed power in the medium, have been obtained, and the dependences of P _s on the light intensity and detunings of resonances have been calculated.     

The break of L-S coupling and the double Stark resonance in the spectrum of 36P → 37P two-photon transition in Rydberg atoms of sodium

New results are presented of an experimental investigation of the spectrum of 36P → 37P two-photon microwave transition in Rydberg atoms of sodium in a constant electric field. Depending on the conditions of excitation of the initial 36P state (the constant electric field is switched on either before or after the exciting laser pulse) and polarization of laser radiation, a strong variation is observed of the amplitudes of individual two-photon transitions between the fine-structure Stark components of the 36P and 37P states. This effect is an analog of the Paschen-Back effect in a strong magnetic field and is due to the break of L-S coupling and to the variation of the wave functions of Rydberg electrons in an electric field. It is also found that the break of L-S coupling affects considerably the shape of double Stark resonance arising upon intersection of the virtual intermediate level of two-photon transition with the real intermediate 37S level.     

Theoretical predictions of recent experiments on multiphoton absorption in gas-dynamically cooled SF6

Theoretical predictions of collisionless multiphoton absorption in low temperature SF₆ are presented and used to interpret the results of two recent experiments. Three major peaks in the experimental multiphoton absorption spectrum at 30 K, measured by Alimpiev and coworkers, are confirmed as being two-photon resonances to the A_1g, E_g and F_2g sub-states of 2v₃. The measurements by Apatin and coworkers of multiphoton-induced depletion of rotational sub-levels of the ground vibrational state show excellent agreement with theory at the P(16) CO₂ laser line, but the present theoretical framework is unable to account for the considerable depletion found experimentally at the P(24) and P(28) lines.     

Selection rules for direct creation of biexcitons by giant two-photon absorption

The selection rules for direct creation of biexcitons by two-photon absorption processes are discussed on symmetry basis. A detailed analysis is performed for CuCl and CdS crystals, with the energy levels given by Bassani et al. It is found that only Г^biex₁ states can be created by giant two-photon absorption when the photons have the same frequency. The polarization dependence is given for the other cases.     

Ultrafast nonlinear optical properties of dye-doped PMMA discs irradiated by 40 fs laser pulses

The two-photon absorption (TPA) characteristics of PMMA discs doped with three different dyes were studied using an fs-pulsed Ti-Sapphire laser as the pump source, and employing the open-aperture Z-scan technique. TPA cross-sections obtained for PMMA doped with the dyes PM597, DCM and rhodamine 6G–rhodamine B (co-doped) were found to be equal to 24.7, 33.3 and 32.3 GM, respectively (1 GM=10^?50 cm⁴ s phot^?1 mol^?1). Furthermore, two-photon fluorescence was measured for the samples containing DCM and rhodamine 6G–rhodamine B (co-doped). Compared to the one-photon fluorescence spectrum, the peaks in the two-photon fluorescence spectrum were red shifted and the extent of red shift increased with increasing doping concentration. We have also observed that the red shift in the two-photon fluorescence peak of the samples in the solid form is much larger than that in the solution state. This phenomenon could be explained by a twisted intra-molecular charge transfer model.     

Two-photon spectroscopy of Hg2Cl2 crystals

Two-photon absorption of Hg₂Cl₂ crystals was investigated using a pulsed dye laser at T ω 8.5 K. Both the indirect method of monitoring the luminescence produced by two-photon absorption and the two-photon attenuation measurements of the laser beam were used. The excitation spectrum of the ∼ 3.13 eV (396 nm) luminescence was measured in the energy range 4.0 ⩽ 2hv ⩽ 5.5 eV. A strong maximum at 2hv ω 5.13 eV dominates the spectrum. Comparison with previous one-photon absorption data allows us to assign the parity of electronic states involved in the transitions. The band gap of Hg₂Cl₂ is found to be direct allowed.     

Multiphoton ionization spectra in lithium vapour

The two-photon absorption spectrum to high-lying states of Li vapour has been obtained using ion-cell detection. ²S states are observed to n = 32 and the ²D series to n = 49. Simple heating techniques circumvented the problem of absorption by the blue-green band system of Li₂ which overlaps the series limit at 4600 Å. The order of the ionization process was examined and found to vary as the square of the laser intensity for two-photon transitions. A conflict with data obtained with a space-charge-limited diode is examined and possible ionization mechanisms are discussed.     

Giant nonlinear absorption in the NiO antiferromagnet

A study of the spectrum of nonlinear two-photon and two-step absorption in NiO single crystals, carried out in the energy region ?ω₁ + ?ω₂ = 2.45–4.575 eV, showed it to have a complex shape and consist of very strong peaks (from 0.05 to 2.7 cm/MW). Within the energy interval 2.45–3.3 eV, the spectrum is due to d-d transitions in the Ni²⁺ ion. The band gap width was determined to be E _g =3.466 eV. The spectral features seen above this energy originate from interband transitions from three valence subbands to the conduction band bottom.     

Landau-Level Quantization of the Yellow Excitons in Cuprous Oxide

Lately, the yellow series of P-excitons in cuprous oxide could be resolved up to the principal quantum number n = 25. Adding a magnetic field, leads to additional confinement normal to the field. Thereby, the transition associated with the exciton n is transformed into the transition between the electron and hole Landau levels with quantum number n, once the associated magnetic length becomes smaller than the related exciton Bohr radius. The magnetic field of this transition scales roughly as n^–3. As a consequence of the extended exciton series, we are able to observe Landau level transitions with unprecedented high quantum numbers of more than 75.     

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.     

Zweiquantenabsorption bei freien Exzitonen in Zinkoxid

The two photon absorption spectrum at the intrinsic absorption edge of ZnO is measured with various directions and modes of polarization of the incident light beams. Several peaks are observed, corresponding to the 2P _0,±1-exciton states of the 3 seriesA, B, C (which occur on account of the threefold upper valence band). The anisotropy of the product of reduced effective mass times dielectric constant determined from the line positions is compatible with theoretical predictions. The lack ofS-exciton states in the two quantum absorption spectrum of ZnO is discussed in detail.