Nonlinear Absorption Spectrum in Perylene Derivatives

Transcription

1 onlinear Absorption Spectrum in Perylene Derivatives S. L. liveira, D. S. orrêa, L. Misoguti, S.. Zilio, and. R. Mendonça Instituto de Física de São arlos-usp.j.l. onstantino Departamento de Física, Química e Biologia, FT-UESP, Brazil R.F. Aroca Materials and Surface Science Group, University of Windsor, anada

2 ABSTRAT Knowledge about nonlinear absorption spectra of materials used in photonic devices is of paramount importance in determining their optimum operation wavelengths. In this work, we have investigated the two-photon absorption (PA) degenerate cross-section spectrum for perylene derivatives using the Z-scan technique with femtosecond laser pulses. All perylene derivatives studied present large PA cross-sections, only comparable to the best ones reported in the literature. The results achieved in the present investigation indicate perylene derivatives as promising materials for twophoton applications. S. L. liveira, D. S. orrea, L. Misoguti,,. J. L. onstantino,, R. F. Aroca,, S.. Zilio,,. R. Mendonça,, Perylene Derivatives with Large Two-photon Absorption ross Sections for Application in ptical Limiting and Upconversion Lasing Advanced Materials, in press (5).

3 TW-PHT ABSRPTI Upon exposure to intense laser pulses, molecules can instantaneously absorb two photons to access an excited state, each of them with half of the energy required to match the electronic transition. r P = χ ( 1 ) r (.E + χ ) r r ( : EE + χ [ ] ( 3 ) Im χ 3 ) r r r MEEE photon absorption (Linear) -photon absorption (onlinear) α α + β I = β: two-photon absorption coefficient Such PA process has interesting characteristics, with direct consequence for applications: (i) improved spatial resolution due to the square dependence on the excitation irradiance 3D optical data storage, micro-fabrication; (ii) negligible linear absorption at the pumping wavelength, with h the resulting increase of penetration depth photodynamic cancer therapy

4 TW-PHT ABSRPTI SPETRUM Accordingly, a great deal of effort was directed to the development ent of various design strategies employed to synthesize new two-photon absorbing materials with large PA cross-sections sections ( (δ ) and increase physical and chemical stability. High optical nonlinearities have been reported in organic materials als at specific wavelengths, only recently the dispersion of the nonlinear absorption over a wide spectral range started to be characterized. Such information is important: (i) Molecular design strategy of a given nonlinear optical material (ii) rigin of the observed nonlinearities (iii) peration wavelength for a given device PA spectrum is obtained from: - Multi-photon excited fluorescence - pen-aperture Z-scanZ discrete wavelengths, tunable sources

5 PE-APERTURE Z-SA EXPERIMETAL SETUP Laser Sample Photodetector Trigger 775 nm λ = 46-6 nm lark-mxr PA Lock-in Sample Spatial Filter ormalized Transmittance: T( z ) ormalized Transmitance ( z) ( z, ) τ [ 1+ q ( z, ) e ] T 1 = = ln dτ LT π q z z = Photodetector (lark-mxr, PA-1) 775 nm; 15 fs.8 mj; 1kHz 46-6 nm; 1 fs; -6µJ Knowing that q ( z,t ) βi ( t )L 1 ( + z / ) 1 = z δ =hνβ M. Sheik-Bahae et al., IEEE J. Quantum Electron. 6, (199). (TPAS, Quantronix)

6 In this context, the present work reports on the degenerate PA cross-section section spectra of perylene tetracarboxylic derivatives (PTD) in the spectral range going from the visible to the near infrared. PTD are organic dyes, readily available, thermal and chemically stable. The perylene moiety presents remarkable electron donor characteristics, although adding lateral groups the molecule could play the role of electron acceptor or donor. Moreover, their strong absorption and emission in the visible spectral range makes them potential candidates for applications as photoconductors, laser materials, etc. MLEULAR STRUTURES S AzoPTD Monothio BZP H 11 5 H H 9 PazoPTD BuPTD

7 σ abs (1-16 cm ) TW-PHT ABSRPTI RSS-SETI SETI AzoPTD BZP PazoPTD BuPTD Wavelength (nm) δ (1 3 GM) FIG. 1. Linear absorption and degenerate PA spectrum of the PTD materials in a solution with 1% trifluoroacetic acid in dichloromethane. - PTD linear absorption bands (4-66 nm), with the vibronic structure superposed to the π-π* transition. - High δ values at several excitation wavelengths. - The increase observed in the PA cross-section section spectra resonance enhancement of the nonlinearity sum-over states (SS) model, assuming that the π π* transition gives the major contribution to the virtual intermediate state and that the excited state corresponds the absorption peaks around 35 nm.

8 FRTIER MLEULAR RBITALS The molecular geometries were optimized by the AM1 method. Frontier molecular orbitals were calculated using ZID/S (Hiperhem( 7.5). onfiguration interaction (I) calculations included single excited configurations from the ground state, 4 (occupied) x 4 (unoccupied) Transition energy ( E( linear.abs ) decreases in a similar way as the π-conjugation length; Bathchromic shift are the result of the HM LUM energy change; By analyzing the PA spectra far from linear absorption edge of the four PTD derivatives it can be established an unambiguous relation between molecular charge delocalization (conjugation length) and the third-order nonlinear optical response (PA): The results suggest that even larger PA cross-sections sections could be obtained by manipulating the perylene derivatives, by positioning donor or acceptor groups symmetrically or by increasing the molecule conjugation length,, in agreement with the molecular design strategies proposed in the literature. M. Albota,, D. Beljone,, J. L. Breda,, J. E. Ehrlich, J. Y. Fu, A. A. Heikal,, S. E. Hess, T. Kogej,, M. D. Levin, S. Marder,, D. Mcord-Maughon Maughon, J. W. Perry, H. Rockel,, M. Rumi,, G. Subramanian, W. W. Webb, X. L. Wu,. Xu, Science 1998, 81,, 1653.

9 AzoPTD HM: LUM:

10 Monothio BZP HM: LUM:

11 PazoPTD HM: LUM:

12 BuPTD HM: LUM:

13 PTIAL LIMITIG The result shows that PTD compounds exhibit effective PA optical limiting action, which is useful to limit very short pulses,, especially at wavelengths near the linear absorption edge. E output (µj) Transmitância ormalizada (a) AzoPTD monothio BZP x (cm) E input (µj) T AzoPTD Monothio BZP PazoPTD BuPTD (b) Irradiance (GW/cm ) FIG.. (a) utput energy as a function of input energy of AzoPTD (1 18 molecules/cm 3 ) in a solution with 1% TFA in DM and placed in a -mm thick quartz cuvette for excitation wavelength at 77 nm. The solid line represents the linear transmittance. The inset t shows open-aperture Z-scan Z signatures for AzoPTD and Monothio BZP; (b) Transmittance T change as a function of irradiance for PTD derivatives

14 ormalized Fluorescence UPVERSI EMISSI IDUED BY PA A strong two-photon excited fluorescent emission, which is a relevant prerequisite for upconversion lasing, was measured in PTD for excitation at 77 nm. It should d be pointed out that the fluorescence is high enough to be seen by naked eyes. AzoPTD Monothio BZP PAzo BuPTD (a) Intensity (arb. units) Wavelength (nm) Incident power (µw) FIG. 3. (a) ormalized two-photon excited fluorescent emission of the PTD compounds in a solution s with 1% TFA in DM pumped at 77 nm; (b) PA fluorescence intensity vs. incident energy slope ~ The observed emission indicates the potential of PTD to be used as active media for PP laser operating at short wavelengths. The efficiency of PP lasing in dyes reported so far has been rather low (overall PP lasing efficiency smaller than.1). (b)

15 LUSIS In summary, large PA cross-sections sections were measured, which are in the same order of the best ones reported for organic compounds specially designed for nonlinear absorption.. The results support that PA cross-section section can be improved by molecular design strategies and the resonance enhancement effect. It is expected that increasing the conjugation and/or adding symmetric donor and acceptor species could further improve e the PA effect. Results on the transmittance change versus excitation irradiance and the strong two-photon induced fluorescence demonstrate that PTD compounds are attractive for application in optical limiting and PP upconversion lasing. Acknowledgments