Chemistry of Anomalous-Dispersion Phase-Matched Second

Chapter 12

Chemistry of Anomalous-Dispersion Phase-Matched Second Harmonic Generation 1

2

P. A. Cahill and K. D. Singer 1

Sandia National Laboratories, Albuquerque, N M 87185-5800 AT&T Bell Laboratories, Princeton, NJ 08540

Downloaded by UNIV LAVAL on June 15, 2014 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch012

2

The anomalous dispersion associated with a strong absorption in some carefully chosen asymmetric dyes permits efficient phase-matched SHG at a given frequency and concentration. One of these dyes was recently used to demonstrate the validity of the two-state model for β, and leads to a method of enhancing second harmonic coefficients in poled polymer systems by 10 to 10 . The factor that primarily limits the utility of this process is the residual absorbance in a nearly transparent window on the high energy side of a charge transfer band. One figure of merit for comparing dyes for this application is the ratio between this minimum absorbance and ε ; for many dyes this ratio is only 10 to 10 . Synthesis of new dyes has led to ε /ε ratios of 10 to 10 . 1

-1

4

max

-2

-3

min

-4

max

Organic materials for second order nonlinear o p t i c a l (NLO) applications were f i r s t investigated i n the 1960's, and since that time research has become divided along two l i n e s : crystals for second harmonic generation (SHG) and related applications, and thin (aligned) films for integrated optics. The molecular basis of the second order NLO coefficient p is well understood, and the challenges associated with noncentrosymmetric alignment of molecules i n crystals and thin films have been addressed. Our work has focussed on a means of doubling near-IR frequencies by using dyes which absorb between _ the fundamental and second harmonic. This approach leads to a means of e f f i c i e n t , c o l l i n e a r phase-matched second-harmonic generation (SHG) through the anomalous dispersion associated with this electronic t r a n s i t i o n , and results i n an increase i n the useful magnitude of 0, the microscopic second order h y p e r p o l a r i z a b i l i t y . The applications for this approach are i n thin film devices for SHG and electrooptic (E0) modulation. Our recent report(1) on ADPM SHG (Anomalous-Dispersion PhaseMatched Second-Harmonic Generation) addressed the physics of ADPM, a

0097-6156/91/0455-0200$06.00/0 © 1991 American Chemical Society

In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


12.

CAHILL AND SINGER

Phase-Matched Second Harmonic Generation

subject which was f i r s t discussed i n 1962(2) and which has been studied as a means of generating t h i r d (and higher odd order) harmonic l i g h t many times. Phase-matched harmonic generation is obtained by using the anomalous dispersion of an absorbing species to cancel the normal dispersion of a host material, such that at a p a r t i c u l a r concentration of dyes, the indices of r e f r a c t i o n at the fundamental and generated harmonic are equal. This method has been the most successful i n gases where absorption lines are narrow and l i t t l e residual absorption results;(3) i t has been somewhat less useful for t h i r d harmonic generation i n solutions of organic dyes because of problems associated with two photon absorption and residual absorption at the t h i r d harmonic.(4) These problems are absent or less severe with ADPM SHG. Prior to our report, ADPM second order materials had not been been investigated with the exception of one serendipitous discovery of ADPM difference frequency generation i n a noncentrosymmetric semiconductor i n the mid-IR. (5.) However, experimental evidence and theoretical arguments (vide infra) suggest that s i g n i f i c a n t increases (10 to 10 ) i n the effective NLO coefficients i n organic second order materials are possible through this general approach. In addition, this report includes the f i r s t work towards optimizing dyes for these systems. 1

4

Consequences of the Origin of 6 i n Organic Dyes On the microscopic scale, overwhelming evidence suggest that, i n the absence of unusual delocalized excited states, for which there i s very l i t t l e evidence i n organic second order NLO materials, the expression for p for almost a l l dyes is descibed by a simple two state model v i a the following expression:(6) 3 2 [e / i Afi]

2 3w

Q

H

2 "ft

Q

2 2 (co -co Q

2

2 -4w )

where w wo HO A/z

i s the incident fundamental frequency corresponds to the energy of the f i r s t excited state is the t r a n s i t i o n moment, and is the difference between the dipole moments of the ground and excited states. The conventional and very effective approach to increasing f) has been to use dyes that absorb at the longest feasible wavelength and to use wavelengths as close as possible to the dye's absorption edge so that the frequency factor (the second factor i n the above expression) i s favorable. However, i f the wavelength at which the device is required to function is fixed, such as i n the case of doubling a diode laser to 400-450 nm, there is a very severe l i m i t i n the conventional approach to the size of p i n organic dyes because of the inherent nature of chromophores that absorb to the blue of a desired wavelength, i . e . , p is l i m i t e d by the magnitude of the f i r s t factor. Furthermore, once this f i r s t factor i n p i s reasonably optimized, p i s dominated by the second (frequency) factor, which goes as I / W Q ^ .


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MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES


Our alternate approach for SHG is to design a dye which absorbs between to and 2oo but which has a low absorption at both of these wavelengths. (7) Based on the expression for ft, the following points are known: (1) the terms i n the denominator of the frequency factor should change sign and i f WQ is close to either w or 2w , the magnitude of the denominator should decrease, increasing ft; (2) since both the t r a n s i t i o n moment and change i n dipole moment terms also scale with increasing wavelength, ft should further increase; (3) the a>o"2 behavior of p would be expected to y i e l d additional factor of approximately 4. Overall, a t o t a l increase i n j3 of approximately an order of magnitude can be expected from an optimized dye which absorbs between u> and 2to over the conventional approach of using a dye which absorbs only at wavelengths shorter than the second harmonic wavelength.

Consequences of the Origin of y(2) i n Poled Polymers whereas the challenges of second order materials on a microscopic scale l i e primarily i n the nature of the electronic states of isolated molecules, the challenges on a macroscopic scale are associated with the noncentrosymmetric alignment of the NLO molecules. Phase mismatch for harmonic generation occurs due to the natural dispersion i n a l l materials, but a phase-matched condition (long or i n f i n i t e coherence length) is required for e f f i c i e n t transfer of energy from the fundamental to the harmonic. Phasematching is often accomplished v i a the birefringence of c r y s t a l l i n e materials which may be tuned by careful adjustment of the c r y s t a l r e l a t i v e to the beam. In poled polymer systems, which may be the most applicable to integrated optics applications, iproportional to the \L$ product, and phase-matching i n a waveguide might be accomplished by proper modal overlap.(8) The use of a poled polymeric material incorporating a dye with an absorption between uo and 2oo (an "ADPM dye") leads to several advantages. Based on the discussion (above) on f), the macroscopic h y p e r p o l a r i z a b i l i t y , x ^ ) , can be expected to increase by at least an order of magnitude simply because i t is proportional to the HP product. In addition, i f the dye is present i n the proper concentration for ADPM SHG, this c o l l i n e a r process allows coupling to the largest component of the tensor, which results i n a further increase i n Furthermore, because one can phase-match the diagonal components of by propagation along a p r i n c i p a l d i e l e c t r i c axis, geometric problems such as beam walkoff, s p a t i a l dispersion, and beam overlap are reduced or eliminated; and geometric inefficiencies due to p o l a r i z a t i o n changes t y p i c a l of phase-matching i n birefringent crystals are absent. In a waveguide configuration, ADPM SHG allows coupling from/to the lowest order modes and therefore maximizes overlap of the guided waves. In t o t a l , the magnitude of *(2) reasonably be expected to increase by 1()1 to 1()2 over conventional materials, which, combined with the additional e f f i c i e n c i e s gained from geometric considerations, leads to an increase of 10 to 10 or more i n the efficiency of SHG by this technique. Such a large predicted increase i n v(2) j u s t i f i e s a s

c

2

a

n

4



12.

CAHILL AND SINGER


considerable effort i n the synthesis of new dyes and polymers for this approach. These general observations on the size and magnitude of ft were confirmed by using Foron B r i l l i a n t Blue S-R (FBB) i n an EFISH experiment.1 In summary, p changes sign due to the frequency factor, while the frequency independent terms i n j3 are the same within experimental error whether second harmonic l i g h t is generated above or below the f i r s t charge transfer absorption. The increase i n the /i£ product for ADPM dyes i s apparent from Figure 1, i n which the \i$ product for several NLO dyes(9) is plotted against the shortest transparent wavelength (conventional NLO dyes) or the transparent "window" wavelength (for ADPM dyes). In this way, the r e l a t i v e v(2) 's for a given desired (SHG) wavelength can be d i r e c t l y compared. A one order of magnitude increase i n /z/? is apparent even for these nonoptimal ADPM dyes. Even though the requirement for transparency for SHG is less severe than i t is for t h i r d and higher order harmonic generation, the r e a l l i m i t a t i o n for SHG i n FBB was s t i l l found to be absorption of the second harmonic. In FBB, the residual absorption is so great that useful amounts of SHG were not generated. One challenge of ADPM SHG is therefore to design a system (dye(s) plus host) i n which the absorption at the second harmonic is minimized. Two schemes have been devised to accomplish this goal. In the (less preferred) f i r s t scheme, two dyes are used one (generally symmetric) dye with an absorption between oo and 2u> for phase-matching and a conventional 2nd order NLO dye which absorbs to the blue of 2w for SHG. Only the geometric factors leading to an increased y(2) are obtained from this scheme, but a symmetric dye with low residual absorption for phase-matching may be more easily prepared. The preferred scheme, which would p o t e n t i a l l y give r i s e to the greatest SHG efficiency, involves a dye, l i k e FBB, which does both SHG and ADPM. Of course, fine tuning of the system v i a the addition of dyes which either add to or subtract from the dispersion may add considerable f l e x i b i l i t y to both approaches.

Chromophore Design The general constraints for the design of any dyes for ADPM SHG i n poled polymer systems rapidly narrow the choice of chromophores. The dyes should be o v e r a l l charge neutral (to f a c i l i t a t e poling) and highly soluble i n polymer matrices. The f i r s t excited electronic state should be well separated from higher energy states for two reasons: (1) since the vibronic envelope associated with an electronic absorption often t a i l s to the blue, a greater energy separation between excited states may give lower absorption, and (2) the next state's normal dispersion, i f nearby, would subtract from the desired anomalous dispersion of the lower energy t r a n s i t i o n . Dyes that would be expected to show weak, low l y i n g n-?r* transitions above the f i r s t excited state should therefore be avoided. Finally, the molar absorptivity of the dyes should be as large as possible (>50,000 1/mol-cm) i n order to generate the largest possible anomalous dispersion (which is proportional to the area under the absorption curve) at a given concentration.


203


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MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

1

1

1

1

1

10 H — — i — — i — ' — i — — i — — i — — 300 400 500 600 700 800 900 SHG

Wavelength

(nm)

Figure 1. Plot of product vs. absorption edge wavelength (conventional NLO dyes(9)) or transparent "window" wavelength (ADPM dyes, ref. (1) and this work). The lower l i n e is a least squares f i t to the data, the upper (dashed) l i n e is a guide to the eye. An order of magnitude increase i n up for a given wavelength has been observed. This i s 10% of the predicted maximum increase possible for this technique.



12.

CAHILL AND SINGER


The requirement for a strong charge transfer band well separated from other electronic absorptions immediately suggests the use of organometallic or coordination compounds with strong metal-ligand or ligand-metal charge transfer bands. An early attempt v i a this approach f a i l e d due to p h o t o i n s t a b i l i t y of the compound. A l l subsequent work has concentrated on organic dyes.(10) The next l o g i c a l step toward chromophore design was to conduct a spectral survey of commercially available organic compounds i n order to learn some general structure-property relationships for minimization of the residual absorbance. As an e a s i l y measured figure of merit, the r a t i o between the minimum and maximum molar absorptivities has been used. In many cases, this r a t i o (expressed i n percent, or more conveniently, as the minimum molar absorptivity per 100,000 L/mol-cm of maximum absorbance) i s 5-10% (5000-10,000 per 100,000). (The lower the number the better the dye.) An improved figure of merit would take into account the area under the absorption curve as well as the location of the transparent window r e l a t i v e to the peak i n the absorption. This i s tantamount to c a l c u l a t i n g the dispersion from the absorption spectrum, which was too complex for this type of survey. Overall, the spectral survey provided more data on what kinds of dyes should not be used for ADPM, rather than what structural features lead to dyes with improved transparency. Among the symmetrical dyes, the carbo- and dicarbocyanines were among the best dyes surveyed, but these contain fundamentally cationic chromophores. Few of the popular laser dyes - - whether based on open chain merocyanine chromophores ( l i k e DCM), c y c l i c merocyanines ( l i k e the coumarins), or triarylmethine dyes (such as the rhodamines and related compounds) -- were s u f f i c i e n t l y transparent above the f i r s t t r a n s i t i o n to y i e l d insight into structure property relationships. However, among the merocyanine dyes, FBB S-R, a commercial fabric dye, showed the greatest figure of merit among the asymmetric dyes, at approximately 0.75% or 750 per 100,000 (the molar absorptivity of FBB S-R i s approximately 62,000 L/mol-cm) residual absorbance. The wavelength of maximum absorption i s moderately solvatochromatic, which i s consistent with a moderate change i n dipole moment i n the f i r s t excited state. The wavelength of minimal absorbance i s near 445 nm i n CH2CI2, and l i k e most dyes that have been surveyed, A j _ i s less solvatochromatic than A . m

n

m a x

Somewhat related to the (cationic) cyanines are the squarylium dyes which are o v e r a l l charge neutral species derived from squaric acid. They are e a s i l y prepared, have high molar absorptivities (>100,000), but t y p i c a l l y are unstable to hydrolysis i n dipolar aprotic solvents. They are characterized by a sharp strong absorption which l i e s at wavelengths longer than 640 nm, with no other i d e n t i f i a b l e electronic transitions i n the v i s i b l e (Figure 2). The vibronic structure of these dyes may show only one shoulder corresponding to a reasonable value for a C-C or C-0 stretch. We were able to systematically vary the structure of a series of squarylium dyes i n order to test assumptions about the structureproperty relationship for minimization of the residual absorption. An increase i n the r i g i d i t y and symmetry of the structure of the dye was expected to lead to a gradual improvement i n the figure of merit for the dyes i n Figure 3 from diethylaminohydroxy- to


205

206

MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES ^min

e

min


10"

OH

0"

P

e

e r

n

643

480

100

670

508

124

663

502

85

670

421

601

HO

Figure 2. Structure and spectral properties of squarylium dyes synthesized i n this work.

ACB,D

c o o co -O

Chemistry of Anomalous-Dispersion Phase-Matched Second

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