Xiangming Liu

High-order nonlinear optical response of a polymer nanocomposite film incorporating semiconducotor CdSe quantum dots

We report on observation of high-order optical nonlinearities in our recently developed photopolymerizable semiconductor CdSe quantum dot (QD)-polymer nanocomposite films at various volume fractions of CdSe QDs as high as 0.91 vol.% (3.6 wt.%). We performed Z-scan and degenerate multi-wave mixing (DMWM) measurements using a 532-nm picosecond laser delivering single 35 ps pulses at a repetition rate of 10 Hz. Using the uniformly cured polymer nanocomposite films, we observed the third- and fifth-order nonlinear optical effects in closed-aperture Z-scan measurements by which it was found that saturable nonlinear absorption (light-induced transparency) and large negative nonlinear refraction were induced. We also measured dependences of the effective third- and fifth-order nonlinear refraction constants on CdSe QD volume fraction. Based on the Maxwell-Garnett model, we estimated the third- and fifth-order nonlinear optical susceptibilities of CdSe QD and discussed a contribution of the third-order effect to the fifth-order one due to the cascaded (local-field) effect. Coexistence of the third- and fifth-order nonlinear refraction was also confirmed by DMWM.

Holographic assembly of semiconductor CdSe quantum dots in polymer for volume Bragg grating structures with diffraction efficiency near 100

We report on the fabrication of centimeter-size transmission Bragg gratings in semiconductor CdSe quantum dots dispersed 50 μm thick photopolymer films. This was done by holographic assembly of CdSe quantum dots in a photopolymerizable monomer blend. Periodic patterning of CdSe quantum dots in polymer was confirmed by a fluorescence microscope and confocal Raman imaging. The diffraction efficiency from the grating of 1 μm spacing was near 100% in the green with 0.34 vol % CdSe quantum dots, giving the refractive index modulation as large as 5.1×10−3.

Photopolymerizable nanocomposite photonic materials and their holographic applications in light and neutron optics

We present an overview of recent investigations of photopolymerizable nanocomposite photonic materials in which, thanks to their high degree of material selectivity, recorded volume gratings possess high refractive index modulation amplitude and high mechanical/thermal stability at the same time, providing versatile applications in light and neutron optics. We discuss the mechanism of grating formation in holographically exposed nanocomposite materials, based on a model of the photopolymerization-driven mutual diffusion of monomer and nanoparticles. Experimental inspection of the recorded grating’s morphology by various physicochemical and optical methods is described. We then outline the holographic recording properties of volume gratings recorded in photopolymerizable nanocomposite materials consisting of inorganic/organic nanoparticles and monomers having various photopolymerization mechanisms. Finally, we show two examples of our holographic applications, holographic digital data storage and slow-neutron beam control.

Nonlinear optical responses of nanoparticle-polymer composites incorporating organic (hyperbranched polymer)-metallic nanoparticle complex

We report on experimental studies of the nonlinear optical properties of nanocomposites consisting of organic (hyperbranched polymer)-metallic (Au or Pt) nanoparticle complex embedded in polymer films. Open-aperture and closed-aperture z-scan techniques are used to measure the effective third-order nonlinear optical susceptibilities χeff(3) of the composites under picosecond laser radiation at a wavelength of 532 nm. The relative sign of the real and imaginary parts of χeff(3) could be explained qualitatively by the Kramers–Kronig relation. We also determined the third-order nonlinear optical susceptibilities of Au and Pt nanoparticles estimated from the measured values for χeff(3) to be (−5.48+4.76i)×10−8 esu and (4.41−0.65i)×10−6 esu, respectively, at 532 nm with a help of the Maxwell–Garnett model.

Closed-Aperture Z-Scan Analysis for Nonlinear Media with Saturable Absorption and Simultaneous Third- and Fifth-Order Nonlinear Refraction

We present a theory of open- and closed-aperture Gaussian beam Z-scan for nonlinear optical materials with saturable absorption and high-order nonlinear refraction. We show that an approximate expression for a transmitted intensity through the nonlinear optical material is possible by means of the Adomian’s decomposition method and the thin film approximation. The theory is applied to semiconductor CdSe quantum dot-polymer nanocomposite films. It is shown that the theory well explains measured results of open- and closed-aperture transmittances in the Z-scan setup. It is also shown that the nanocomposite film possesses simultaneous third- and fifth-order nonlinear refraction as well as saturable absorption of a homogeneously broadened type.

High-order optical nonlinearities in nanocomposite films dispersed with semiconductor quantum dots at high concentrations

We describe the nonlinear optical properties of inorganic-organic nanocomposite films in which semiconductor CdSe quantum dots as high as 6.8 vol.% are dispersed. Open/closed Z-scan measurements, degenerate multi-wave mixing and femtosecond pump-probe/transient grating measurements are conducted. It is shown that the observed fifth-order optical nonlinearity has the cascaded third-order contribution that becomes prominent at high concentrations of CdSe QDs. It is also shown that there are picosecond-scale intensity-dependent and nanosecond-scale intensity-independent decay components in absorptive and refractive nonlinearities. The former is caused by the Auger process, while the latter comes from the electron-hole recombination process.

Gaussian beam Z-scan analysis for nonlinear optical materials possessing simultaneous third- and fifth-order nonlinear refraction with saturable absorption: an application to semiconductor CdSe quantum dot-polymer nanocomposites

We present a theoretical analysis of the closed-aperture Gaussian beam Z-scan for nonlinear optical materials with both saturable absorption and simultaneous third- and fifth-order nonlinear refraction. We formulate a theoretical expression for the Z-scan transmittance by means of the Adomian’s decomposition method and the thin film approximation. It is applied to the experimental characterization of the nonlinear optical properties of a semiconductor CdSe quantum dot-polymer nanocomposite film. We show that measured results of the open- and closed-aperture Z-scan transmittances of the nanocomposite film are well explained by the theoretical model.

Observation of higher-order optical nonlinearities in photopolymerizable semiconductor CdSe quantum dot-polymer nanocomposites

Semiconductor quantum dots (QDs), also referred to as nanocrystals, that confine optically excited electron-hole pairs in all three space dimensions are also of considerable interest in science and engineering. Tailored patterning of QDs in a thick polymer matrix is of great interest for photonic and biomedical applications. For example, using enhanced third-order nonlinear optical responses of QDs [1], one expects optical multi-stability action from periodic QDs structures embedded in a polymer film [2]. In this work we report on the nonlinear optical properties of semiconductor CdSe QDs dispersed in an optically transparent polymer film. Z-scan and degenerate multiwave mixing (DMWM) measurements were performed to obtain effective nonlinear optical susceptablilities of the film at a wavelength of 532nm for various concentrations of CdSe QDs. It was found that the film exhibited 5th- and 7th-order nonlinearities. We also observed nonlinear Bragg diffraction from holographically patterned [3] CdSe QD-polymer nanocomposite films that exhibited the diffraction efficiency near 100% [4].

Z-scan characterization of nonlinear optical effects in polymer films incorporating hyperbranched polymer-metallic nanoparticle complex

We describe an experimental investigation of the nonlinear optical properties of nanocomposites incorporating organic (hyperbranched polymer)-metallic (Au or Pt) nanoparticle complex embedded in polymer films. Zscan techniques are employed to measure the effective third-order nonlinear optical susceptibilities χ(3) eff of the composites with 35 ps pulses at 532 nm. The relative sign of the real and imaginary parts of χ(3) eff could be explained qualitatively by the Kramers-Kronig relation. The third-order nonlinear optical susceptibilities of Au and Pt nanoparticles from the measured values for χ(3) eff were also determined to be (-5.48 + 4.76i)×10-8 and (4.43 - 0.65i)×10-6 esu, respectively, at 532 nm.