Organic Thin Films - American Chemical Society

Chapter 24

Optical Dispersion Properties of Tricyanovinylaniline Polymer Thin Films for Ultrashort Optical Pulse Diagnostics 1

1

1

2,3

2,4

Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: August 6, 1998 | doi: 10.1021/bk-1998-0695.ch024

Ph. Prêtre, Ε. Sidick , L . - M .Wu ,A.Knoesen ,D. J. Dyer , and R. J. Twieg 1

NSFCenter on Polymer Interfaces and Macromolecular Assemblies, University of California, Davis, CA 95616 IBM Research Division, Almaden Research Center, San Jose, CA 95120 2

We have investigated a series of tricyanovinylaniline (TCV) polymer thin films for their use in ultrashort optical pulse (USP) diagnostics of femto second Ti:Sapphire lasers. These thin films are ideally suited for USP diagnostics since they eliminate the angle tuning associated with birefringent phase-matched crystals, minimize pulse distortion intro duced by group velocity dispersion, and exhibit excellent photoche mical stability. The linear optical dispersion of these polymers can be tailored over a wide range for efficient and distortionless frequency conversion. Coherence lengths between 420 nm and 54 microns at a wavelength λ = 800 nm have been found for the two extreme cases of dispersion in these materials. Film thicknesses of at least two microns are tolerable without introducing any significant pulse distortion at the same wavelength (λ = 800 nm). The development of lasers producing ultrashort pulses (USP) of 10 fs or less duration has created a need for a simple and complete diagnosis of these pulses. Second Har monic Generation Frequency Resolved Optical Gating (SHG FROG) allows for a direct determination of phase and amplitude of femtosecond pulses (1). Since all the information of the laser pulse is converted onto the second harmonic signal, any distortion by the N L O material will reduce the accuracy of the diagnosis. Pulse distortion occurs due to dispersion of the linear optical properties over the large frequency bandwidth of the fundamental and second harmonic pulses. In phase-matched frequency doubling crystals the group velocity mismatch may cause severe distortion of the generated pulses over long interaction lengths. Even for the thinnest crystals (~ 50 microns), obtaining the correct alignment is iterative and time consuming and presents a significant complication, especially when an unknown pulse shape must be measured. In contrast, because poled nonlinear optical polymer (NLOP) films are highly nonlinear, the interaction length is on the order of a few mic rons, and they do not suffer as much from pulse broadening and the need of precise angle tuning. In addition, films consisting of electric field oriented organic chromo3

Current address: Division of Chemistry and Chemical Engineering, California Institute of Technology,

Pasadena, C A 91125. 4

Current address: Division of Chemistry and the Liquid Crystal Institute, Kent State University, Kent, OH 44242.

328

©1998 American Chemical Society

In Organic Thin Films; Frank, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

329

phores are easily prepared by spin coating and corona poling (2). The critical alignment is eliminated since the angle of incidence onto the NLOP only needs to be set within a few degrees of the Brewster angle. Recently, we reported on SHG FROG measurements of 13 fs pulses from a Ti:Sapphire oscillator using a T C V NLOP (3). We have presented a detailed analysis of USP SHG including effects of group velocity mismatch (GVM) and intrapulse group velocity dispersion (IGVD) elsewhere (4, 5). Here, we demonstrate that NLOPs are uniquely suited for USP applications with advantages not matched by any other material. We examine relevant dispersion properties and compare high conversion with low distortion in N L O P based USP SHG. Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: August 6, 1998 | doi: 10.1021/bk-1998-0695.ch024

Synthesis of N L O Polymers for USP S H G A number of N L O chromophores possess large first hyperpolarizabilities and large ground state dipole moments for high conversion efficiencies and electric field poling, respectively. USP SHG applications require specific properties not found in most common NLOPs. In particular, the material must be sufficiently transparent at both fundamental and second harmonic wavelengths, and photochemically stable as it will be subjected to large peak intensities. We found that the tricyanovinylanilines (TCV) (6, 7) exhibited the requisite characteristics for TirSapphire USP SHG pulse diagnostics. In T C V , the charge transfer (CT)-band is red shifted more than 100 nm relative to the nitroanilines. This shift of the CT-band gives rise to a transparency "window" in the 400 nm region. We investigated spin cast films of the T C V side chain polymers and 5, 10 and 15% by weight guest-host polymers of N,N-diphenyl-4-tricyanovinylaniline (PhTCV) in polymethylmethacrylate (PMMA). PhTCV was prepared from triphenylamine and tetracyanoethylene in dimethylformamide at 80 °C, see also Figure 1. The synthesis of the T C V side chain polymers is shown in Table 1 and described as follows. Table I.

Nomenclature, Substitution Patterns, Molecular Weights and Glass Transition Temperatures of T C V Side Chain Polymers.

Polymer

m

n

R

dd4-ll dd4-14

1

2

CH

1

0

-

dd4-15

1

1

CH

dd4-20

1

1

CH (CF )3CHF 3

Mw 3

3

2

H C >CH

2

dye cone. [wt. %] T [°C] g

38000

44

135

-

70

132

38300

54

137

63700

37

106

53.7

40

159

3

dd4-21

1

1

^p£^ H CH

3

3


330


N-ethyl-N-phenyl-3-[(2-methyl-l-oxo-2-propenyl)oxy]ethylamine (dd4-4). Dicyclohexylcarbodiimide (15.63 g, 75.7 mmol) was added to a stirred solution of the N-ethyl-N-phenyl-ethanolamine (10.0 g, 60.6 mmol), methacrylic acid (5.74 g, 66.6 mmol), and N,N-dimethylaminopyridine (50 mg, 0.41 mmol) in 500 ml dichloromethane. The reaction mixture was stirred for 48 hours and was then filtered. Silca gel ~30ml was added to the solution and the crude was evaporated to dryness. The crude was purified via flash chromatography over silica gel with gradual elutions from 95:5 to 80:20 (hexanes:ethyl acetate). Evaporation of solvent yielded 9.843 g (70%) of a colorless oil: Ή N M R (250 MHz, CDC13) δ 1.17 (t, J=7.03 Hz, 3H), 1.93 (s, 3H), 3.41 (q, J=7.05 Hz, 2H), 3.59 (t, J=6.37 Hz, 2H), 4.30 (t, J=6.40 Hz, 2H), 5.55 (m, 1H), 6.09 (s, 1H), 6.70 (m, 3H), 7.21 (m, 2H). Polymer dd4-ll. Monomer dd4-4 (4.87 g, 20.9 mmol) and methylmethacrylate (4.19 g, 41.8 mmol) in chlorobenzene (25 ml) were degassed under nitrogen for 2 hours at 70 °C. 2,2'-azobisisobutyronitrile (AIBN, -30 mg) as radical initiator for polymerizations was added and the reaction mixture was stirred 12 hours under N2 at 70 °C. More A I B N was added and the reaction mixture was stirred for an additional 12 hours. The reaction mixture was then dissolved in 20 ml dichloromethane and this solution was added dropwise to a vigorously stirred solution of cold methanol (400 ml). The resulting white solid was dissolved in 40 ml dichloromethane and reprecipitated from cold methanol (400 ml) to yield 5.68 g (63%) of a white powder. The resulting polymer (2.0 g, 4.6 mmol) and tetracyanoethylene TCNE (769 mg, 6.0 mmol) were stirred in N,N-dimethylformamide (90 ml) at 70 °C for 20 hours. The reaction mixture was then cooled and added dropwise to a vigorously stirred solution of cold methanol (500 ml). The resulting red solid was dissolved in 50 ml dichloromethane and reprecipitated twice from cold methanol (500 ml) to yield 2.03 g (82%) of a red powder (dd4-ll): M W 38,000; Tg 135 °C (DSC); H N M R (250 MHz, CDC13) δ 0.5-2.0 (m), 3.2-4.2 (m), 6.5-7.0 (m), 8.03 (s). l

Polymer dd4-14. The homopolymer was synthesized according to the procedure for copolymer dd4-11. The crude polymer was dissolved in dimethylformamide (DMF) and reprecipitated once from cold methanol to yield (99%) of a red powder. The polymer was not soluble enough in common organic solvents for GPC or N M R analysis: Tg 132 °C (DSC). Polymer dd4-15. Synthesized according to the procedure for polymer d d 4 - l l to yield (89%) of a red powder: M W 38,300 (GPC); Tg 137 °C (DSC); *H N M R (250 MHz, CD2C12) δ δ 0.64 (s), 0.81 (s), 1.20 (s), 1.55 (s), 3.47 (s), 3.69 (s), 4.04 (s), 5.24 (s), 6.83, (s), 7.97 (s). Polymer dd4-20. Synthesized according to the procedure for polymer d d 4 - l l to yield (89%) of a red powder: M W 63,700 (GPC); Tg 106 °C (DSC); *H N M R (250 MHz, acetone-d6) δ δ 0.90 (s), 1.29 (s), 3.75 (s), 3.90 (s), 4.25 (s), 4.60 (s), 6.70 (t), 7.10, (s), 8.10 (s). Polymer dd4-21. Synthesized according to the procedure for polymer d d 4 - l l to yield (93%) of a red powder: M W 53,700 (GPC); Tg 159 °C (DSC); *H N M R (250 MHz, CDC13) δ 0.5-2.0 (m), 3.2-4.0 (m), 4.0-4.5 (m), 6.9 (s), 8.0 (s). Linear Optical Dispersion and USP SHG It is well known that high SHG conversion efficiencies over long interaction lengths can be achieved for negligible phase mismatch Ak between the center frequencies of fundamental and SH pulses. NLOPs are, in general, not phasematchable and the


331 intensity of the generated SH signal will oscillate along the material with a period given by the coherence length i

a r e

c

=

J L Ak

=

* i

(D 4\ή -η \ 2ω

ω

t n e

where ή and ή incident angle-dependent refractive indices at the fundamental and harmonic frequencies and lambda the fundamental wavelength. Hence, thicknesses in excess of one coherence length will not improve the efficiency. Note that l is entirely determined by the linear optical dispersion in a given material. Consider now a 10 fs fundamental pulse with large spectral bandwidth Δ / of about 70 nm at a wavelength of 800 nm. Over this large bandwidth, group velocity mismatch (GVM) due to phase mismatch between corresponding spectral parts of the fundamental and S H pulse away from the pulse center will act as a band-limiting spectral filter superimposed on the generated SH signal; this unavoidably lengthens the SH pulse. Earlier, we introduced the pulse-width-preservation length, L , as a new reference length to quantify the pulse broadening that occurs during harmonic conversion of USP(4). It is defined as the distance at which the SH pulse width τ (intensity FWHM) becomes equal to the pulse width τ χ of the fundamental, i.e. Tp2 = Τp\. In case of a transform-limited hyperbolic secant fundamental pulse shape we have the relation ω

2ω

c


β 1

T

ρ2

ρ

f

dk d(û

2

ν

2

dk\ dû)

(2)

where dkjdiùi is the incident angle-dependent group velocity at fundamental and harmonic frequency (1,2, respectively). Again, linear optical dispersion determines the maximum thickness of the NLOP so that the fundamental pulse is not distorted. For pulsewidths ~ 10 fs or shorter, group velocity dispersion within the bandwidth of each of the fundamental and the SH pulse (IGVD) introduces additional pulse distortion. The IGVD is a function of the derivative of the group velocity, therefore of the second derivative of the index of refraction. However, in this case a closed form expression for L is not available but can be found in numerical simulations (4) if linear optical properties are known. As seen from the expression for the coherence length and the pulse width preservation length, knowledge of the dispersion of the refractive index as well as the first derivative with respect to the light frequency is needed. T

Determination of the Linear Optical Dispersion Properties of T C V Polymers Most easily, the dispersion properties of thin films are measured directly by means of spectroscopic ellipsometry and derivatives are calculated numerically. However, one would prefer to predict the dispersion and therefore the usefulness of a new NLOP for USP S H G without involved chemistry, film formation and various methods of characterization. We have introduced earlier a method of calculation of optical properties of NLOPs based on simple absorption measurements of the chromophore in solution (8). We demonstrate here the use of the conjugate Fourier series method (CFSM) (9) in order to obtain not only linear optical dispersion properties but also all the needed derivatives. The C F S M is closely related to the Kramers-Kronig approach to obtain refractive indices from measured extinction coefficients. It makes use of the fact that, through the causality condition, the complex dielectric constant and therefore also the


332 complex refractive index, η = V ë , are analytic functions in the upper half complex frequency plane. It can readily be shown that η can be expressed as (9) ή(ω)-\

-> n[-cotfj-\

= n{0)-\-ik{0)

(3)

oo

with

η(θ)-l

= -9- + X « cosmO m

(4)

k(0) = -^a sinmO.

(5)

2

m=l

oo


and

m

m=l

Eq. (4) and (5) can now be recognized as conjugate Fourier series for functions with appropriate symmetry properties. In practice, the Fourier series coefficients are most conveniently determined by a fast Fourier transformation of a finite number of points, Ν (10). Knowing the spectrum of the imaginary part determines all coefficients a except O

= ^+ 2

la =0 m

(6)

m = 1

0)->oo

and hence Ν

a =-2 J^a . 0

m

(7)

m=l

With all coefficients known, Eq. (4) allows then the calculation of the dispersion mode. Note that there are no integral transformations involved in this procedure. Since fast Fourier algorithms operate based on equal spacing for the value of θ in Eq. (4), the spacing on the real (frequency) wavelength axis will be of a (co)tangential functional form according to Eq. (3). In practice, 1024 points equally spaced from 0 < θ < π representing a wavelength range 0 < λ < 500 μιη, will ensure sufficient wavelength resolution in the visible, fc-values outside the measured range are most conveniently set equal to zero. Derivatives of the refractive index are easily obtained from Eq. (4): even/odd -order derivatives are given by the cos/sine series with Fourier coefficients l

a

Organic Thin Films - American Chemical Society

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