J. R. Heflin

Nonlinear-optical processes in lower-dimensional conjugated structures

Reduced dimensionality and quantum confinement in conjugated organic and polymer structures enhance the effects of electron correlation on virtual electronic excitation processes and nonlinear-optical responses. A microscopic many-electron description of the third-order susceptibilities γijkl(−ω4; ω1, ω2, ω3) of conjugated structures is reviewed for one-dimensional chains and extended to two-dimensional conjugated cyclic structures. Electron correlation effects in effectively reduced dimensions result in highly correlated π-electron virtual excitations that lead to large, ultrafast nonresonant nonlinear-optical responses. The increase of dimensionality from linear to cyclic chains is found to reduce the nonresonant isotropic third-order susceptibility γg. Resonant experimental studies of saturable absorption and optical bistability in ultrathin films of quasi-two-dimensional naphthalocyanine oligomers are also presented. In the saturable-absorption studies, the resonant nonlinear refractive index n2 was measured to be 1 × 10−4 cm2/kW in the wavelength range of operating laser diodes. Based on this result, electronic absorptive optical bistability is observed on a nanosecond time scale in a nonlinear Fabry–Perot interferometer employing the saturably absorbing naphthalocyanine film as the nonlinear-optical medium.

Measurement of The Third Order Optical Susceptibility of Quasi-Two Dimensional Conjugated Discs: Silicon Naphthalocyanine

Abstract The isotropically averaged nonresonant microscopic third order susceptibility ⟨γ⟩ has been measured for a quasi-two dimensional metallophthalocyanine at several near infrared wavelengths. An infinite dilution extrapolation method employing the Maker fringe technique for third harmonic generation in the wedge configuration was developed for measurement of ⟨γ⟩ in liquid solutions and is presented in detail. At a fundamental wavelength of 1907 nm, ⟨γ⟩ of silicon naphthalocy-anine is measured to be −3140 ± 250 × 10−36 esu whereas ⟨γ⟩ is less than the experimental sensitivity of ±50 × 10−36 esu for a fundamental wavelength of 1543 nm. These results are discussed in terms of the dispersion of ⟨γ⟩ in the region between the 3ω resonances associated with the Q and B absorption bands.

Resonant and non-resonant third-order optical properties of metallophthalocyanines☆

Abstract Non-resonant third-harmonic generation and resonant saturable absorption measurements of liquid solutions of silicon naphthalocyanine are discussed. The results provide further insight into the non-linear optical properties previously reported for random glassy polymers containing silicon naphthalocyanine.

Recent Studies On The Nonlinear Optical Properties Of Conjugated Linear Chains And Rigid Rod Polymers

One of the major focuses in the study of nonlinear optics in recent years has been the microscopic description of the second and third order π-electron virtual excitation processes in organic and polymeric systems. In particular, the question of correlation in systems of reduced dimensionality has been under intense study. Important evidence for the inadequacy of independent particle theories such as Huckel theory in these systems has been provided by the experimental discovery that below the one-photon 11Bu π-electronic state exists a two-photon 21Ag state in linear chain polyenes, in direct contradiction with these simple theories. It was then demonstrated that a proper account of the electron correlation resulting from electron-electron interactions was required in order to obtain the correct ordering of these two states.

Symmetry-Controlled Electron Correlation Mechanism for Third Order Nonlinear Optical Properties of Conjugated Linear Chains?

Abstract A microscopic many-electron description is reported for the third order de-induced second harmonic susceptibility γ ijkl (-2ωω,ω,0) of all trans-polyenes. Based on results obtained by self consistent field configuration interaction theory that suitably accounts for electron-electron interactions and by direct comparison with experimental gas phase results, we show that symmetry-controlled electron correlation effects determine the properties and behavior of γ ijkl (-2ωω,ω,0) for polyene chains of length 4 through 16 carbon sites. Under well-defined symmetry conditions, the many-electron mechanism involves third order virtual excitations to high-lying, strongly electron correlated two-photon 1 A g states. The major component γ xxxx (-2ωω,ω,0) with all fields aligned along the chain axis exhibits a power law dependence of 5.4 due to three major factors which are described.

Excited-state nonlinear optics and enhancement mechanisms for nonresonant processes

A general enhancement mechanism is reviewed for nonlinear optical processes originating from real population of electronic excited states. The nonresonant third order optical susceptibility y1(-w4;wi,w2,w3) of quasi-one dimensional conjugated linear chains can be enhanced by orders of magnitude, or even change sign, when the first (S1), or second (52), electronic excited state is optically pumped and then populated for times suitably long to perform nonresonant measurements of y1(-w4; w1 , w2,w3) at frequencies different from the resonant pump frequency.

Excited State Nonlinear Optics

As a result of fundamental progress in the field of nonlinear optics, the microscopic origin and mechanism for nonresonant second order and third order optical processes originating from the So ground electronic state of conjugated structures are fairly well understood, especially in terms of highly correlated virtual excitations that are accurately described by electron correlation theory for low dimensional systems. In finite systems, the theoretical description is in excellent agreement with experimental measurements of the magnitude, sign, and dispersion of the microscopic second order sijk S 0(-ω3;ω1,ω2) and third order γijkl S 0(-ω4;ω1,ω2,ω3) optical susceptibilities[1]–[4]. Based on this understanding, the microscopic description can be naturally developed to consider nonlinear optical processes originating not only from the ground state but also from real population of an optically pumped excited state S n.

Third-order optical properties of quasi-two-dimensional conjugated discs: silicon naphthalocyanine

Since the basic phthalocyanine structure intrinsically possesses high thermal and oxidative stabilities, phthalocyanines incorporated into sol-gel glasses provide an attractive approach for realizing highly stable nonlinear optical media. Experimental results recently obtained for the nonresonant and resonant third order optical properties of metallophthalocyanines are reviewed in this paper.

Recent nonlinear optical studies of MNA and conjugated linear chains

There have been a large number of studies concerning the nonlinear optical properties of organic and polymeric materials in recent years [1]. Compared to the usual inorganic nonlinear optical materials, organic and polymeric materials with conjugated bonding structures possess much larger second and third order electronic nonlinear optical susceptibilites [1,2]. It is now well-established that the macroscopic second order χ ijk (2) (−ω3;ω1,ω2) and third order χ ijkl (3) (−ω4;ω1,ω2,ω3) nonlinear optical susceptibilities observed for conjugated organic and polymer structures are due to virtual excitations occurring within the π-electron states of individual molecular, or polymeric chain, sites in condensed assemblies and providing macroscopic sources of nonlinear optical response through the corresponding on-site microscopic second order βijk(−ω3;ω1,ω2) and third order γijkl(−ω4;ω1,ω2,ω3) susceptibilities [3 – 7]. Moreover, since the responses originate from on-site electronic virtual excitations, the nonlinear response time is ultrafast of the order of picosecond to femtosecond time scales. Microscopic descriptions of second order and third order it-electron virtual excitation processes are required in order to understand at a fundamental level the physical properties and behavior of βijk(−ω3;ω1,ω2) and γijkl(−ω4;ω1,ω2,ω3). In addition, they provide important insights for developing guidelines for molecular design of new material structures.

Experimental measurements and electron correlation theory of third-order nonlinear optical processes in linear chains

In this paper, we summarize detailed measurements of the frequency dependence of (gamma) ijkl(-(omega) 4;(omega) 1,(omega) 2,(omega) 3) for the third-order nonlinear optical processes dc-induced second harmonic generation (DCSHG) and third harmonic generation (THG) by wedge Maker fringe techniques recently completed for several important linear polyene structures.