S. M. Dharmaprakash

Two-photon-induced excited-state absorption: Theory and experiment

We develop an open-aperture (OA) Z scan and nonlinear transmission theory of two-photon-induced excited-state absorption, under the excitation of spatial Gaussian laser pulses with temporal Gaussian and hyperbolic secant profiles. The found analytic expressions allow us to straightforwardly fit the OA Z-scan trace and the nonlinear transmission curve, for convenient extraction of the nonlinear absorption coefficients. As a test, the two-photon-induced excited-state absorption in a chalcone derivative of 3,4-dimethoxy-4′-fluorochalcone is explored by performing femtosecond Z-scan measurement and is analyzed by our theory.

Nonlinear optical properties of 2,4,5-Trimethoxy-4-nitrochalcone: observation of two-photon-induced excited-state nonlinearities.

We report experimental investigations of optical nonlinearities and nonlinear dynamics in acetone solution of 2,4,5-Trimethoxy-4- nitrochalcone. By performing Z-scans with femtosecond laser pulses at low excitation intensity, two-photon absorption (2PA) and third-order nonlinear refraction are measured. As laser excitation intensity exceeds a critical value, however, the interplay between third- and fifth-order nonlinearities is observed. It is also confirmed that fifth-order processes mainly originate from 2PA-induced excited-state nonlinearities by conducting femtosecond time-resolved degenerate pump-probe measurements. All the nonlinear parameters are determined unambiguously in the near infrared region of the 2PA cross-section, second-order hyperpolarizability, excited-state absorption cross-section, excited-state refraction cross-section, lifetime of excited states induced by 2PA, and critical population of the excited states in 2,4,5-Trimethoxy-4-nitrochalcone molecule.

Superior characteristics of organic chalcone single crystals as efficient nonlinear optical material

High-quality biaxial single crystals of chalcone derivatives were grown by solution growth technique. Their molecular structures were designed to possess large second-order nonlinearities by choosing proper donor/acceptor groups while retaining a high transparency in the visible and infrared spectral regions. The second-order nonlinear optical coefficients, determined by applying the Maker fringe technique, were found to be much larger than those of LiB3O5, KTiOPO4, KH2PO4, and urea. The advantages, such as easy synthesis and crystal growth, low cutoff wavelength ( 7.2GW∕cm2), make these organic crystals promising for efficient frequency doubling.

Concentration-dependent two-photon absorption and subsequent excited-state absorption in 4-methoxy-2-nitroaniline

By performing both open-aperture Z-scan and transient transmission measurements with femtosecond laser pulses in the near infrared region, the concentration dependence of two-photon absorption (2PA) and subsequent excited-state absorption (ESA) are investigated in acetone solution of 4-methoxy-2-nitroaniline (4M2N). Owing to the high tendency of forming aggregation molecules in solution, a considerable dependence of the nonlinear optical response on the concentration is observable at a 4M2N concentration of 10−3u2002M and beyond. It is found that both effective 2PA and ESA cross sections decrease with increasing concentration, while the effective lifetime of 2PA-induced excited states lengthens.

(2E,2′E)-3,3′-(1,4-Phenylene)bis[1-(3,4-dimethoxyphenyl)prop-2-en-1-one]

In the centrosymmetric title compound, C26H22O4, the dihedral angle between the central and terminal aromatic rings is 13.78u2005(12)°. A weak C—H⋯π interxadaction may help to consolidate the crystal packing.

3‐[(4‐Nitrophenyl)aminocarbonyl]propanoic acid

The title compound, C10H10N2O5, crystallizes in the centrosymmetric space group P21/c. The carboxyl terminus end is almost perpendicular to the rest of the molecule [the dihedral angle between mean planes through the terminal COOH group, the adjacent C atom and rest of the molecule, except for the methylene H atoms, is 82.15u2005(18)°]. This molecule fails to show second-order nonlinear optical property due to the presence of inversion symmetry in the solid state. The crystal structure is stabilized by strong interxadmolecular O—H⋯·O and N—H⋯·O hydrogen bonding.

1-(4-Aminophenyl)-3-(3,4,5-trimethoxyphenyl)prop-2-en-1-one–1-(4-aminophenyl)-3-(3-bromo-4,5-trimethoxyphenyl)prop-2-en-1-one (0.972/0.028)

In the title cocrystal, 0.972(C18H19NO4).0.028(C17H16BrNO3), which arose from an impure starting material, all the atoms are overlapped except for one OMe group and the Br atom. The dihedral angle between the benzene ring mean planes is 18.20u2005(13)°. A weak N—H⋯O hydrogen bond helps to establish the non-centrosymmetric crystal packing.

Design of new organic single crystals for ultrafast optical applications

A new chalcone crystal designed with suitable functional groups exhibits ultrafast response (<90 fs) and very large two-photon absorption (2PA) (β > 8 ×10² cm/GW). Its low intensity threshold (<4 MW/cm²) for 2PA and high damage resistance (>8 GW/cm²) are quite interesting for 2PA-based optical limiting.

1,3-Bis(3,4-dimethoxyphenyl)prop-2-en-1-one

In the title compound, C19H20O5, the dihedral angle between the benzene rings is 5.92u2005(6)°. The molxadecules are linked by two pairs of C—H⋯O hydrogen bonds into a centrosymmetric dimer of R21(6) ring motif. In addition, the crystal structure is stabilized by C—H⋯π interxadactions.

1-(3,4-Dimethoxyphenyl)-3-(4-methoxyphenyl)prop-2-en-1-one

In the title compound, C18H18O4, the molxadecule is slightly twisted with a dihedral angle of 17.49u2005(5)° between the two benzene rings. All of the methxadoxy substituents lie almost in the planes of the benzene rings to which they are attached. The crystal structure is stabilized by interxadmolecular C—H⋯O, C—H⋯π and π–π interxadactions.