Fumiyo Yoshino

Interplay between self-focusing and high-order multiphoton absorption

We study the distortion of optical beams that is due to the combined effects of strong self-focusing and three- and four-photon absorption, a situation that exists, for example, in the polydiacetylene bis-paratoluene sulfonate (PTS). The characteristic nonlinear distances were defined for each process. Theoretical analysis of the beam propagation leads to two distinct limits, one limit dominated by self-focusing and the other by higher-order absorption. Propagation was studied analytically and numerically for cw and pulsed beams in these two limits and for cases in which both nonlinear effects are important. It was found that beam distortion caused by multiphoton absorption and refraction leads to situations in which diffraction plays an important role, even for input beams whose diffraction length is much larger than the sample length. It was concluded that, for the typical intensities used in Z-scan measurements, nonlinear processes and diffraction contribute simultaneously to beam distortion and must be taken into account.

All-optical multiphoton absorption figures of merit: Polydiacetylene poly (bis para-toluene sulfonate) of 2,4-hexadiyne-1,6diol

The all-optical switching figures of merit are reported for the single crystal PTS (polydiacetylene poly (bis para-toluene sulfonate) of 2,4-hexadiyne-1,6 diol). Included are the effects of the absorption mechanisms from linear up to four-photon over the wavelength range 1.2–2.2μm.

Z-scan and P-scan: new measurements of the nonlinear refractive index of single crystal polymer PTS at 1600 nm

Summary form only given. The poly[bis(p-toluene sulfonate)] of 2,4-hexadine-1,6-diol (PTS) is a conjugated polymer which exhibits strong /spl pi/-electron delocalization and large nonlinear optical effects. Previously, the nonlinear refractive index change was reported to be /spl Delta/n=n/sub 2/I+n/sub 3/I/sup 2/ with n/sub 2/=2.2/spl times/10/sup -3/ cm/sup 2//GW and n/sub 3/=-0.8/spl times/10/sup -3/ cm/sup 4//GW/sup 2/ with 65 ps pulses at 1600 nm using the Z-scan technique. Such a negative n/sub 3/ usually signifies the onset of the saturation of some nearby state. However, no such state was found to exist. A key problem was that the optical quality of the PTS crystals was poor and could have led to misleading measurements. After a successful crystal development program, very good quality PTS crystals have been made. We report new Z-scan measurements on these PTS crystals with 100-fs pulses at 1600 nm and demonstrate a modified Z-scan technique, power-scan, useful for rapid measurements.

MULTIPHOTON EFFECTS IN THE POLYDIACETYLENE POLY BIS(P-TOLUENE SULFONATE) OF 2,4-HEXADIYNE-1,6-DIOL (PTS)

Multiphoton absorption was investigated in single crystals of the polymer poly bis(p-toluene sulfonate) of 2,4-hexadiyne-1,6-diol (PTS). Using modified growth and polymerization techniques, high optical quality crystals were grown and their linear absorption spectrum measured. Very large four-photon absorption was measured using Z-scan with a value of α4=0.3 cm5/GW3 obtained at 1600 nm. From the known one and two-photon absorption spectra, the three- and four-photon spectra are predicted to overlap. This leads to three-photon enhanced, four-photon absorption which is interpreted as the mechanism responsible for such large four-photon absorption.

Nonlinear refraction and absorption from 1300 to 2200 in single crystal polymer poly[bis(p-toluene sulfonate)] of 2, 4-hexadiyne-1, 6-diol (PTS)

Multi-photon absorption and refraction were measured in polydiacetylene-PTS (p-toluene sulfonate) from 1200 to 2200 nm. The four-photon absorption coefficient peaks at 1900 nm and n/sub 2/ decreases monotonically over this range.

Interplay between self-focusing and multiphoton absorption: beam distortion and Z-scan measurements

Summary form only given. Single crystals of polydiacetylene PTS have experimentally shown unique nonlinear optical properties. Four photon absorption (4PA) is very large, (/spl alpha//sub 4/ = 0.25 /spl plusmn/ 0.05 cm/sup 5//GW/sup 3/) at 1600 nm. Furthermore, PTS has a large non-resonant intensity-dependent refractive index coefficient, leading to self-focusing (SF), (n/sub 2/ = [4.0 /spl plusmn/ 0.5] /spl middot/10/sup -4/ cm/sup 2/ /GW). This offers a unique opportunity to study nonlinear beam propagation in regimes previously unavailable experimentally. Large SF occurs already at the distances about 300-500 microns with peak input intensities of several GW/cm/sup 2/. We investigated the interplay between multiphoton absorption and SF, which results in severe beam distortion and affects 4PA Z-scan measurements on relatively thick samples.

Growth and optical characterization of PTS polymer single crystals

The crystal growth technique and associated optical characterizations of polydiacetylene PTS , poly bis(p-toluene sulfonate) of 2,4-hexadiyne-l,6-diol, are reviewed in this paper. The commonly observed defects such as twinning and cracking and their origins are analyzed. Two factors were found to significantly decrease the density of defects and improve the quality. First, the monomer crystal was grown reasonably quick consistent with single crystal growth to reduce polymerization. Second, the polymerization rate was reduced dramatically for the 10% to 90% conversion region for a smooth conversion by reducing the polymerization temperature. The absorption spectrum, typical Z-scans for measuring the optical nonlinearity, and SEM pictures of the surfaces are given as measures of the optical quality of the PTS crystals.

Polydiacetylene PTS: A Molecular Quantum Wire with Exceptional Optical Properties

The conjugated polymer 2,4-hexadiyne-1,6-diol-bis-(para-toluenesuIfonate) or PTS is a classical example of a one-dimensional quantum wire. The π-electrons can move more or less freely in one dimension. There are large repercussions to this effect in the photonics field, including photoconductivity, and the linear and nonlinear optical response. Growing techniques of crystals with high optical quality as well as measurements on a variety of nonlinear optical effects of PTS are reported in this chapter. These include measurements on the large magnitude of the exciton absorption line, the well-defined vibrational side bands at room temperature, massive two and four photon absorption coefficients, very large Raman gain coefficients, minimal excited state absorption and a large nonlinear refractive index.

Linear and Nonlinear Optical Properties of Improved Single Crystal PTS

Improved methods for the growth and polymerization of single crystals of polydiacetylene PTS, poly bis(p-toluene sulfonate) of 2,4-hexadiyne-1,6-diol, have allowed new optical measurements to be made. The absorption spectrum, and typical Z-scans for measuring the optical nonlinearity up to fourth order are described.

Three-photon resonantly enhanced four-photon absorption in single crystal polymer poly [bis(p-toluene sulfonate)] of 2,4-hexadiyne-1,6-diol

Summary form only given. Poly [bis(p-toluene sulfonate)] of 2,4-hexadiyne-1,6-diol (PTS) is a conjugated polymer which exhibits strong /spl Pi/-electron delocalization that leads to a large nonlinear optical effect. We present experimental results on multi-photon absorption from 1800 to 1900 nm. We observed strong three photon enhanced four photon absorption.