Myoungsik Cha

Two photon absorption of di‐alkyl‐amino‐nitro‐stilbene side chain polymer

The two photon absorption spectrum has been measured in the 780–1600 nm range for a di‐alkyl‐amino‐nitro‐stilbene side‐chain polymer. A single two photon peak centered at 920 nm is observed with a peak two photon coefficient of 5.5 cm/GW. Bleaching via two photon absorption at this wavelength is investigated by measuring a refractive index change after an intensive exposure.

Measurement of the complex nonlinear refractive index of single crystal p‐toluene sulfonate at 1064 nm

Z‐scan at 1064 nm was used with single, 35 ps pulses to measure the nonlinear refraction and absorption in single crystal PTS (p‐toluene sulfonate). Detailed analysis of the Z‐scan data based on Δn=n2I+n3I2 and Δα=α2I+α3I2 yielded n2=5(±1)×10−12 cm2/W, α2=100(±20) cm/GW, n3=−5(±1)×10−21 cm4/W2 and α3=−5(±1) cm3/GW.2 The resulting two photon figure of merit T for PTS is marginal for high throughput, all‐optical waveguide switching at 1064 nm.

Broadband optical parametric amplification at the communication band with periodically poled lithium niobate

We report broadband optical parametric generation (OPG) in a single periodically poled lithium niobate crystal with a picosecond pump pulse at a fixed wavelength. We also demonstrate efficient optical parametric amplification of a broadband seed pulse within the quasi-phase-matched OPG band. The broad parametric gain band is attributed to group-velocity matching and degeneracy between the signal and idler, and the broad spectral width of the pumping source.

Third-order optical spectroscopy of polythiophene

Tunable third-harmonic generation and nondegenerate four-wave mixing experiments were performed on polythiophene thin films to produce six independent nonlinear-optical spectra. An essential states model consisting of three electronic excited states has been compared with the spectra. Inclusion of a Raman state could improve the fit to two of the spectra.

Spectroscopic investigations of the nonlinear response of poly(4-BCMU) films

Two distinct third-order nonlinear optical techniques, third harmonic generator and four-wave mixing, have been used in three independent experiments to probe the frequency dispersion of the nonlinear optical response of thin solid films of a specific x-electron conjugated polydiacetylene polymer, the red form of poly(CBCMU). Comparison with two molecular models, the essential states and the anharmonic oscillator shows a qualitative agreement, possible reasons for the deviations are discussed. Polymers with n-electron conjugated backbones have been proposed as candidate nonlinear materials for all-optical applications because they exhibit some of the highest ultrafast nonlinear optical coefficients [ 11. Therefore understanding the origin of such large nonlinearities is of paramount importance not only in the field of nonlinear optics but also in biology and quantum physics [ 2 1. Modelling of the nonlinear optical response of one-dimensional n-electron conjugated systems has concentrated on isolated molecules and has evolved into a periodic, one-dimensional, Htickel-like Hamiltonian with electron correlations included [ 3-5 1. It has been shown that the electron correlations are sufficiently strong to affect the positions of the one- (B,) and two-photon (A,) active states. It is in the treatment of electron correlation in the ground and excited states that many of the approximations enter the different theories [

Determination of the two first non-trivial orientational order parameters in LB films of rod-like molecules by third-order sum frequency mixing

6 1. Cal

Observation of standing light wave by using fluorescence from a polymer thin film and diffuse reflection from a glass surface: Revisiting Wiener’s experiment

Abstract We demonstrate a new method for investigating orientational order in Langmuir—Blodgett films built from rod-like poly [bis( m -butoxyphenyl) silane] molecules. Two laser beams with different input frequencies ω 1 and ω 2 were collinearly overlapped to generate mixed light at the frequency 2ω 1 +ω 2 using all possible permutations of the beam polarizations. From the independent tensor components of χ( 3 ) (− ω 3 ; ω 1 , ω 1 , ω 2 ) the first non-trivial in-plane order parameters C 2 and C 4 could be deduced with high accuracy. The orientational order could be well described by a two-dimensional diffusion model.

Determination of the orientational order parameters in polysilane LB-films by measuring third-order-susceptibility tensor components

In 1892 Drude and Nernst reported the first observation of a standing light wave by using a fluorescent thin film, which is a real-time version of Wiener’s first experiment on standing light wave. We reproduce the experiment by Drude and Nernst with improved efficiency using modern laser technology and a highly fluorescent π-conjugated polymer. We also used a glass plate scratched slightly on one surface to visualize the same phenomenon using diffuse reflection instead of fluorescence. This method has the advantage of being a wavelength-independent observation compared to the fluorescence method.

Accurate Measurement of Refractive Indices of Optical Wafers by Using Fabry-Perot Type Interference

We demonstrate a new method for investigating the orientational distribution of the rod-like poly[bis(m-butoxyphenyl) silane] molecules in Langmuir-Blodgett (LB) films. The first two non-trivial in-plane orientational order parameters C2 and C4 could be easily deduced by measuring the independent tensor components of (chi) (3)(-(omega) 3;(omega) 1,(omega) 1,(omega) 2). Using this method, we show that the annealing process improves the molecular alignment along the dipping direction. We were also able to get information about the domain morphology of the film from the off-diagonal components of (chi) (3).

Measurement of Birefringence in Nonlinear Crystals by Interferometry

We investigated Fabry-Perot type interference from optical wafers to measure the refractive indices. This method is accurate (~10-5for fused silica), insensitive to environmental perturbation, and simple to implement, compared to the conventional index-measurement methods.