Francois Kajzar

Nonlinear optical properties of thin films of polysilane

Optical third harmonic generation measurements by transmission have been performed on thin films of polysilane deposited by a spinning technique with a thickness varying between 0.15 and 0.45 μm. The films have a good optical quality and support large laser power (∼200 MW/cm2 at 1.064 μm). They are transparent in visible and near infrared. The measured average value of cubic susceptibility 〈χ(3)xxxx (−3ω; ω,ω,ω)〉 is equal to (1.5±0.1)×10−12 esu at 1.064 μm. The technique used for the third harmonic generation measurements allows determination of both modulus and phase of χ(3). It is argued that the large value of χ(3) is due to a three‐photon resonance at 1.064 μm.

Light-induced second-harmonic generation in azo-dye polymers.

The permanent all-optical poling of an azo-aromatic acrylic copolymer is experimentally demonstrated by seeding preparation in a backward phase-conjugation geometry. The microscopic mechanism involves an orientational hole burning followed by orientational redistribution caused by trans-to-cis isomerization of the azo-dye chromophores. The characteristic kinetics of monitored by second-harmonic generation.

Influencing intramolecular motion with an alternating electric field

Analogues of mechanical devices that operate on the molecular level, such as shuttles, brakes, ratchets, turnstiles and unidirectional spinning motors, are current targets of both synthetic chemistry and nanotechnology. These structures are designed to restrict the degrees of freedom of submolecular components such that they can only move with respect to each other in a predetermined manner, ideally under the influence of some external stimuli. Alternating-current (a.c.) electric fields are commonly used to probe electronic structure, but can also change the orientation of molecules (a phenomenon exploited in liquid crystal displays), or interact with large-scale molecular motions, such as the backbone fluctuations of semi-rigid polymers. Here we show that modest a.c. fields can be used to monitor and influence the relative motion within certain rotaxanes, molecules comprising a ring that rotates around a linear ‘thread’ carrying bulky ‘stoppers’ at each end. We observe strong birefringence at frequencies that correspond to the rate at which the molecular ring pirouettes about the thread, with the frequency of maximum birefringence, and by inference also the rate of ring pirouetting giving rise to it, changing as the electric field strength is varied. Computer simulations and nuclear magnetic resonance spectroscopy show the ring rotation to be the only dynamic process occurring on a timescale corresponding to the frequency of maximum birefringence, thus confirming that mechanical motion within the rotaxanes can be addressed, and to some extent controlled, by oscillating electric fields.

Nonlinear spectroscopy in polydiacetylenes

Abstract Wave dispersed third harmonic generation (THG) and electric field induced second harmonic generation (EFISHG) measurements of cubic susceptibilities x 3 (−3ω;ω,ω,ω) and x 3 (−2ω;ω,ω,o) in thin polydiacetylenic films are reported. Two and three photon resonances, different for these two hyperpolarizabilities, are observed in 0,8 – 2 μm wavelength range. This is interpreted in terms of quantum mechanical formulas involving allowed (u) and forbidden (g) excited states. For the “blue” and “red” forms studied the “g” excited states are located 800 cm −1 and 2500 cm −1 below the first “u” excited state.

High gain of light in photoconducting polymer–nematic liquid crystal hybrid structures

Abstract A liquid crystal panel useful for a realtime holography exhibiting also very high exponential gain coefficient is presented. We prepared nematic liquid crystal layer sandwiched between photoconducting polymeric layers (poly( N -vinylcarbazole)/trinitrofluorene). The first-order diffraction efficiency measured in degenerate two-wave mixing experiment amounts to 44%. The most important feature of the presented panel is its ability to switch energy from beam to beam. In two-wave coupling experiment multiple orders of diffraction are present and a very high two-beam coupling gain ratio (12) and a net exponential gain coefficient Γ =3700 cm −1 have been measured. This gain was achieved in samples biased by a dc external electric field and tilted with respect to the beam incidence bisector at 15°. The time constants of grating formation and erasing in the studied system are functions of applied voltage and can be made as short as few milliseconds at favorable conditions.

Grating inscription in picosecond regime in thin films of functionalized DNA

Polymers containing azo-benzene groups are useful holographic recording materials. In these materials the efficient mixed amplitude and phase gratings, frequently accompanied with photoinduced-surface relief gratings, can be inscribed with polarized laser light. The light-induced trans-cis-trans photoisomerization of azo-benzene groups is responsible for optical anisotropy in such systems. The aim of the present work is to study the dynamics of grating inscription in Disperse Red 1 doped deoxyrbonucleic acid- hexadecyltrimethylammonium material (DR1-DNACTMA) using 16 ps laser pulses (532 nm, 1.3 mJ). Results are compared with that obtained for other polymeric matrices loaded with DR1. The dynamics of the grating growth, due to repeated pulses from picosecond laser with 10 Hz repetition rate, was probed by measuring the intensity of the first order of diffraction of a cw He-Ne. We report results in function of the light polarization of writing beams. In this paper we present the first results of the grating inscription in functionalized DNA (in the picosecond pulse regime).

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.

Optical amplification with high gain in hybrid-polymer–liquid-crystal structures

Efficient coherent light amplification with a very high net exponential gain Γ≈2600 cm−1 has been observed in sandwich-type hybrid-polymer–liquid-crystal structures. The amplification is a result of an energy exchange between two beams in a two-beam coupling experiment. The reported gain coefficient has been measured in a tilted sample configuration within a Raman–Nath diffraction regime at electric fields of the order of 1 V/μm.

Second-order optical nonlinearities of azo chromophores covalently attached to a sol-gel matrix

Optically nonlinear disperse red 1 (DR1) was covalently bound into a silica gel network by the coupling of DR1 and 3-isocyanatopropyltriethoxysilane. Organic-inorganic hybrid materials present a large resonant second-order susceptibility (d33 = 55 pm/V for a fundamental wavelength of 1.06 μm) with an excellent room-temperature stability.

Wave-dispersed third-order nonlinear optical properties of C60 thin films

Abstract Results of wave-dispersed third harmonic generation measurements in sublimed C60 thin films are reported and discussed within a three-level model. Two strong resonant enhancements in cubic susceptibility χ(3)(−3ω; ω, ω, ω) are observed. The first one, occurring at a fundamental wavelength of 1.3 μm with a χ(3)(−3ω; ω, ω, ω) maximum value of 6.1×10−11 esu, is interpreted in terms of a two-photon resonance with the one-photon forbidden electronic T1g level. The second resonance at 1.064 μm, with a maximum value of χ(3)(−3ω; ω, ω, ω)=8.2×10−11 esu is interpreted as a three-photon resonance with the lowest one-photon allowed T1u electronic level.