Third-Order Nonlinear Optical Properties of Infrared Emitting PbS and PbSe Quantum Dots

The optical properties of small band gap, colloidal quantum dots are presented, with the special emphasis put on the measurements of their nonlinear optical properties in the infrared region of spectra. In particular, two types of colloidal quantum dots (PbS and PbSe), with the first exciton absorption band maxima in the near-infrared region of spectra, were investigated using a tunable femtosecond laser system and the <i>Z</i>-scan technique. The measurements of closed- and open-aperture <i>Z</i>-scan traces allowed for the calculation of real and imaginary parts of cubic nonlinearity, which were presented as appropriate cross sections used to characterize the nonlinear refractive and absorptive properties of the studied quantum dots. The maximum two-photon absorption cross section values taken for a single quantum dot were found to be ∼2400 GM (Goeppert-Mayer units) at 1300 nm and ∼15 500 GM at 1700 nm, for PbS and PbSe QDs, respectively. PbS quantum dots showed two-photon induced emission upon infrared excitation. The obtained results demonstrate the potential of IV–VI group colloidal quantum dots for low-cost photonic devices and two-photon absorbers in the near-infrared and infrared spectral ranges

Enhancement of Two-Photon Absorption Cross Section in CdSe Quantum Rods

Nonlinear optical properties of semiconducting CdSe quantum rods (QRs), with three various aspect ratios, were examined in a wide wavelength range using femtosecond Z-scan technique. The two-photon absorption cross section σ₂ was found to be as large as 164 000 GM at the wavelength of 750 nm: about 4 times larger than that expected for CdSe quantum dots of the same mass. On the basis of the obtained dispersion of the two-photon absorption cross section, we have selected wavelength ranges for optimal excitation of two-photon-induced emission. We have also studied the luminescence kinetics using degenerate pump–probe and time-correlated single-photon counting techniques. A strong influence of semiconducting CdSe rods morphology on their steady-state and time-resolved optical properties was found

Wavelength Dependence of the Complex Third-Order Nonlinear Optical Susceptibility of Poly(3-hexylthiophene) Studied by Femtosecond <i>Z</i>‑Scan in Solution and Thin Film

Comprehensive studies of the third-order susceptibility of regioregular poly­(3-hexylthiophene) in the wavelength range from 530 to 1600 nm for both thin film (with stacked chains interactions) and dilute solution (no chains interactions) were performed with the <i>Z</i>-scan technique. Negative nonlinear refraction was observed in the whole wavelength range of the measurements. The nonlinear absorption exhibits regions of two-photon and three-photon absorption, and on approaching the linear absorption region, a saturable absorption process is seen. It was found that for thin film the effective multiphoton absorption cross-sections σ₂ and σ₃ are 1.5 and 3 times bigger than for solution, respectively. Influence of the elongated conjugation length in the stacked polythiophene chains in thin film on the red shift of the multiphoton absorption cross-section spectra in comparison with those for polythiophene in solution is discussed

Synthesis and Linear and Nonlinear Optical Properties of Three Push–Pull Oxazol-5(4<i>H</i>)‑one Compounds

Three uncharged push–pull oxazol-5­(4<i>H</i>)-ones were synthesized and thoroughly characterized. The examined molecules contained electron-donor and electron-acceptor groups interacting via a π-conjugated bridge. Spectral properties of the oxazol-5­(4<i>H</i>)-ones were studied in detail in three solvents of different polarities. The results indicate a solvatochromic shift toward lower energy for the charge-transfer state. The compounds are weakly fluorescent in polar solvents, but they have high fluorescence quantum yields in nonpolar solvents. Their two-photon absorption (2PA) properties were characterized by the open- and closed-aperture Z-scan technique, by the pump–probe technique, and by the two-photon excited fluorescence method. The dyes exhibit relatively high effective two-photon absorption cross sections ranging from 490 to 2600 GM at ∼100 GW/cm², according to the Z-scan results, which are found, however, to contain significant contribution from higher-order absorption processes. In addition, these compounds display good photostability

Nonlinear-Optical Response of Prussian Blue: Strong Three-Photon Absorption in the IR Region

The nonlinear-optical properties of Prussian Blue nanoparticles have been evaluated with the use of femtosecond Z-scan measurements in the 1350–1750 nm range. This well-known inorganic pigment having interesting magnetic and electrochemical properties was found to be an efficient near-IR three-photon absorber. The maximum of the effective three-photon cross section is as high as 4.5 × 10^–78 cm⁶ s² at 1375 nm. By a comparison of the three-photon molar-mass-normalized merit factors, σ₃/<i>M</i>, we show that this material is a competitive multiphoton absorber, especially in comparison to semiconductor quantum dots