RICHARD S. POTEMBER, ROBERT C. HOFFMAN, KAREN A. STETYICK, ROBERT A. MURPHY, and KENNETH R. SPECK

MOLECULAR MATERIALS FOR NONLINEAR OPTICS

An overview of our recent advances in the investigation of molecular materials for nonlinear optical applications is presented. Applications of these materials include optically bistable devices, optical limiters, and harmonic generators.

INTRODUCTION of potentially important optical qualities and capabilities Organic molecular materials are a class of materials such as optical bistability, optical threshold switching, in which the organic molecules retain their geometry and photoconductivity, harmonic generation, optical para­ physical properties when crystallization takes place. metric oscillation, and electro-optic modulation. A sam­ Changes occur in the physical properties of individual ple of various applications for several optical materials molecules during crystallization, but they are small com­ is shown in Table 1. pared with those that occur in ionic or metallic solids. The optical effects so far observed in many organic The energies binding the individual molecules together materials result from the interaction of light with bulk in organic solids are also relatively small, making organic materials such as solutions, single crystals, polycrystal­ molecular solids mere aggregations of molecules held to­ line fIlms, and amorphous compositions. In these materi­ gether by weak intermolecular (van der Waals) forces . als, each molecule in the solid responds identically, so The crystalline structure of most organic molecular solids that the response of the bulk material is the sum of the is more complex than that of most metals or inorganic responses of the individual molecules. That effect sug­ solids; 1 the asymmetry of most organic molecules makes gests that it may be possible to store and process optical the intermolecular forces highly anisotropic. signals or to exploit nonlinear properties at the molecular Owing to the electronic structure of organic molecular level. The branch of science investigating such possibili­ solids, their optical properties are often much more non­ ties is called "molecular optics." linear than those of inorganic solids. Organic molecular The overview we present here is limited to the three materials exhibit many types of nonlinear optical prop­ research projects undertaken at APL in the field of or­ erties, but three types are of interest in our investigations: ganic molecular materials: optical bistability, optical lim­ optical bistability, excited-state absorption, and second­ iting, and harmonic generation. harmonic generation. There are many more nonlinear optical properties, such as sum-difference frequency gen­ MATERIALS FOR OPTICAL COMPUTING eration, third-harmonic generation, and intensity-depen­ Basic Elements of Optical Computing dent index of refraction, but these are not included in The basic functions of a computer are information our present investigation. processing and information storage. In serial-type com­ Current materials research is centered in three areas: puters (von Neumann architecture) the required arithme­ (1) molecular crystals, including compounds such as tic, logic, and memory operations are performed by de­ urea, 2-methyl-4-nitroaniline (MNA), organic dyes, and vices that can switch reversibly between two states often organic charge-transfer complexes (including organome­ referred to as "0" and "1". The semiconductor devices tallics); (2) natural products such as proteins, lipids, and that enable the computer to perform its complex func­ alkaloids; and (3) organic polymers, including substituted tions must be able to switch between the two states at polydiacetylenes, polypyrroles, and liquid crystals. a very high speed while requiring minimal electrical ener­ Since structural modifications in the individual mole­ gy. The electronic semiconductor device used as the basic cules lead to changes in the bulk properties of the solid, building block in the modern computer is the transistor. techniques known as "molecular engineering" have been It has been proposed that the electronic transistor be developed. By molecular engineering deliberate chemical replaced by an optical counterpart. 2 An optical transis­ modifications are made on the individual molecules to tor, for example, would perform the same operations effect changes in the bulk solid so that a desired optical as its electronic analog, but it would modulate light rath­ property is enhanced. Such modifications simplify devict: er than control an electrical signal. processing technology by locking the optical properties The use of optical beams instead of electrical signals of the device into the molecular structure itself, thus offers several advantages in the design of computer cir­ reducing the number of fabrication steps. cuit elements. First, an optical transistor can operate at Only recently have optical devices been based on or­ a much higher speed (in picoseconds instead of nanose­ ganic single crystals and polymers exhibiting a variety conds) because, unlike the conventional semiconductor f ohns Hopkins A PL Technical Digest, Volume 9, N umber 3 (1 988) 189 Potember et al. - Molecular Materials for Nonlinear Optics

Table 1-Applications of molecular optical materials.

Application Materials Significance Optical display devices Conducting polythiophene complexes Produces colored displays having fast switching times and better viewing ge­ ometry (no polarizing element) than liquid crystal devices. Tetrathiafulvalene or pyrazoline in poly­

methacrylonitrile doped with LiCI04 Poly(3-bromo-N-vinylcarbazole) Very fast «200 ms) photochromic N ,N ' -di(n-heptyl)-4,4' -bypyridinium changes observed dibromide High speed, reversible, and exhibits mem­ ory effects.

Electrophotography Poly(vinyl carbazole)-trinitrofluorenone High quantum efficiency. CuTCNQ + poly(N-vinyl carbazole)­ Charged by halogen lamp, multiduplica­ trinitrofluorenone tion up to 50 copies. Polycarbonate-triphenylalanine N ,N' -diphenyl-N,N' -bis(3-methyl phenyl)­ [1,1 ' -biphenyl]-4,4' -diarnine in polycar­ bonate Optical information storage Metal-TCNQ complexes Erasable, high-contrast media. (optical recording) Photochromic dyes High sensitivity, stable, and erasable. Liquid crystalline polymers Liquid crystal polymers provide high con­ trast and high resolution with high sensitivity.

1,4-dihydroxyanthraquinone Used in amorphous Si02 matrix as fre­ quency-domain storage media; requires cryogenic temperatures.

Photovoltaics Polyacetylene/n-ZnS Used in Schottky barrier configuration; polyacetylene bandgap matches solar spectrum well. Electrochemically doped poly(N-vinyl car­ Conversion efficiencies from 0.015 to bazole) merocyanine dyes 2.000/0.

Optoelectronic devices Metal-TCNQ complexes Electrical switching behavior occurs in conjunction with applied optical field. Photonic devices (solid­ Electrochemically doped polythiophenes Optoelectronic switching accompanied by state optical switching optically induced doping; subsequent elements) undoping occurs electrochemically. Urea Used in optical parametric oscillator; con­ version efficiencies approach 20%; tunable through visible and infrared. Methylnitroaniline Used as frequency-doubling material. 1: 1 copolymer of methylmethacrylate and Used as optical waveguide in integrated glycidilmethylmethacrylate polyphenyl­ optics. syloxane polydiacetylene Liquid crystalline polymers Used as light valve in optical logic networks.

190 Johns Hopkins APL Technical Digest, Volume 9, N umber 3 (1988) Potember et al. - Molecular Materials for Nonlinear Optics

device, electron flow through it is not severely limited (a) by fundamental physical considerations. In the all-optical device, the operating speed is limited only by the switch­ ing time and relaxation time of the material. Second, the all-optical device does not present the disadvantage of the electronic device in which the resistance to electron flow increases power consumption, thus producing heat that tends to retard the operation of densely packed elec­ tron circuits. Third, since interconnecting wires are not used for optical components, there is the possibility that computer architectures can be enhanced for parallel pro­ cessing of information and images. Direct optical pro­ cessing of images in parallel with data is a requirement of future computing that cannot be easily met by con­ ventional electronic systems. Q) Electro-optical Devices > '';:; Several types of electro-optical device are being studied co for possible application in optical computing. These de­ ~ ~ vices are combinations of lasers, detectors, and transis­ '(ii c: tors integrated into single monolithic packages. Such de­ ~ vices provide direct electronic-to-optic interfaces with dig­ , ~ (b) +-' ::l ital and analog integrated circuIts. Because of the extra­ S ::l ordinary properties of the hybrid devices, they are also 0 being considered for use in optical data communications and for the direct optical processing of visual images. One example of the integrated electro-optical device is the electro-optical switch in which the nonlinear prop­ erties of the material are modulated by an applied elec­ trical signal to gate the switching of an optical signal. This kind of electronic switching capability has been re­ ported for several types of devices. Some, like the liquid­ crystal light valve,3 are made of organic and polymeric material. It must be noted, however, that the operating speed of the device, like that of other integrated electro­ optical devices using electrical inputs to gate optical sig­ nals, is limited by the effects of capacitance and electron transit time. All-Optical Logic Devices All-optical devices are switches controlled by the in­ tensity of the incident optical beam. Two basic types of Incident intensity (relative) all-optical switching devices, classified by their functional Figure 1-Representative input-output characteristics of bista­ applications to information processing, are proposed for ble optical devices: (a) single-threshold device, (b) double­ digital optical computing: the single-threshold bistable threshold device, optical device and the double-threshold bistable optical device. In Fig. 1, the intensity of the optical output sig­ nal is plotted as a function of the intensity of the optical lecting the appropriate incident beams supplied to it. An input signal for idealized devices of both systems. all-optical logic device of this type is represented in Fig. 2. For the single-threshold device, the characteristic oper­ The optical characteristics of a double-threshold bista­ ating curve (Fig. la) is that of output intensity increas­ ble optical device (Fig. 1b) are similar to those of the ing linearly as a function of input intensity until a critical single-threshold bistable optical device (Fig. la) because threshold is reached. At that threshold, the output in­ it can also have two stable states for a given set of input tensity rises sharply from the low output regime to a high signals. For the double-threshold device, however, a de­ output regime. As the input intensity is decreased, the crease in the intensity of the input signal when the de­ optical output returns by the same path to the low out­ vice is operating in the high-output regime does not im­ put regime. The two linear regions of the "s" curve rep­ mediately cause a decrease in the output to the low out­ resent the two states of the optical device, namely, the put regime. This resultant hysteresis loop can be used "off" and "on" (or "0" and "1") states. A single­ to add a short-term memory function to the operation threshold device can perform signal amplification and of these devices. the basic logic operations such as analog-to-digital con­ An optical transistor can function as either type of version and logic AND and OR functions by simply se- all-optical device by combining two optical beams at its

Johns Hopkins APL Technical Digest, Volume 9, Number 3 (1988) 191 Potember et al. - Molecular Materials for Nonlinear Optics

Input 1 40 Input 2 Binary 0,1,2, ... ,OR n output ~ 30 (b) Q) u r::: co 20 Input n t:: 'E Binary CIl r::: 10 inputs co Figure 2-Representative n-input binary logic OR gate. t= 500 600 700 800 900 1000 1100 1200 1300 input to produce a single output beam. Combining the Wavelength (nm) input beams is accomplished by adjusting the two beams so that the intensity of one beam is close to, but not ex­ Figure 3-Typical spectrum of optical transmission in (a) un­ switched and (b) switched gel-derived silica glass containing ceeding, the input threshold of the device, while the sec­ AgTCNQ. ond beam, which is of much lower intensity, is added to the fIrst so that the input threshold is exceeded, result­ ing in an optical output signal. By this combining of in­ put signals, a weak beam is effectively amplified by the nonlinearity in the response of the device material. Advantages of Molecular Materials Organic, biological, and polymeric materials are being investigated for application in all-optical processing. Many organic materials, especially polymers, because of The original charge-transfer complex can be reformed by their abundant crosslinking, are often more resistant to heating the complex salt in its solid phase. The changes laser damage than are ionic crystalline materials. Also, observed in the optical properties of AgTCNQ result many organic molecular materials are more transparent directly from the electric-fIeld-induced effect that breaks at certain frequencies than inorganic materials, and the the weak ionic bond between the metal cation and the wavelength-dependent transparency can be controlled by organic-radical anion (an excited state electron-transfer synthetic design to match specific laser frequencies. reaction). Switching times from the simple charge-transfer Finally, it appears that organic materials can be made complex containing neutral molecules is less than 4 ns. capable of storing and processing information at the This effect in AgTCNQ suspended in a matrix of gel­ molecular level. Development of this capability is sup­ derived silica glasses formed at low temperature is de­ ported in recent reports describing voltage-tunable op­ scribed in Ref. 11. The changes in the optical absorption tical data storage that uses persistent spectral hole burn­ by AgTCNQ in the silica matrix were used to demon­ ing4 and reversible multiple-state optical recording that strate optical logic operations by moderate-power con­ uses organic charge-transfer complexes. 5 tinuous or pulsed laser radiation. Organic Photochromics AgTCNQ was prepared in colloidal form, with parti­ cle sizes ranging from 0.22 to 2.50 j.tm, by the aqueous Organic compounds that exhibit distinct optical states reaction of LiTCNQ and AgN0 according to the orig­ are being studied for application in optical storage (e.g., 3 inal procedure described by Melby et al. 12 Monolithic in optical memories and optical data processing). The gate-array processors were prepared from gel-derived reversible photochemical reaction in this class of materi­ glass containing AgTCNQ by hydrolyzing and polymer­ als can convert a single chemical structure alternately be­ izing tetramethyl orthosilicate (TMOS). Typical disks of tween two stable isomers that often exhibit two distinct the finished material were 2.5 cm in diameter by 1 mm absorption maxima. Organic photochromic compounds thick. The AgTCNQ particles remained uniformly dis­ such as spyropyrans and aberchrome dyes are being de­ persed during formation of the glass. The spectrum of veloped for use in reversible optical memories. In addi­ optical transmission in a gel-derived glass containing tion to use in optical memories, these materials are now AgTCNQ is shown in Fig. 3a. Three absorption peaks used for optical data processing, for spatial light modu­ have been identified and are interpreted in reports that lation, and in optical waveguide devices. deal with the optical spectra of powder TCNQ com­ Bistable Optical Switching of Organometallic pounds suspended in potassium bromide pellets. 13 ,14 Materials in Gel-Derived Silica Glasses The disk was then irradiated by aNd: YAG laser at Bistable optical switching is known to occur in the or­ 1060 nm, pulsed at a 10-Hz rate, delivering 200 mW ganometallic compound silver tetracyanoquinodimethane average power, with a beam 6 mm in diameter. The (AgTCNQ) when it is exposed to incident laser radiation beam raster scanned over a I x 1 cm area. The irradi­ 5 1O above a characteristic power threshold. - It has been ation caused the blue color of AgTCNQ to transform postulated that the effect of the applied optical field on rapidly into the pale yellow that is characteristic of the the initial charge-transfer complex is to cause a phase formation of neutral TCNQ, as indicated by curve b in transition induced by an electric field, resulting in the Fig. 3. Because of the great change in the transmission formation of a nonstoichiometric complex salt contain­ spectrum, this material may be useful in high-speed op­ ing neutral TCNQ, as shown in the reaction tical logic devices.

192 Johns Hopkins A PL Technical Digest, Volume 9, Num ber 3 (1 988) Potember et at. - Molecular Materials for Nonlinear Optics

Because of the threshold characteristics of these gel­ Table 2-Time scale of molecular processes. derived glasses containing AgTCNQ, they have also been used to demonstrate analog-to-digital conversion and Time Scale Photochemical Event various logic operations. Samples were irradiated with (s) various combinations of two laser beams, and changes Femto (10 - 15) Electronic motion in the output transmission were monitored. A logic Electron orbital jumps AND-gate function and an optical transistor were dem­ Electron transfer onstrated in concept by using beams from two Nd: YAG Proton transfer lasers (1060 nm), each operating below the switching Vibrational motion threshold. Irradiation from only one of the beams, oper­ (10 - 12) ated below threshold, resulted in minimal output trans­ Pieo Bond cleavages (weak) mission. When the combined power of the two beams Rotational and translational motion (molecules are small and fluid) exceeded the threshold ( =:: 2000 m W in glass) of the or­ ganic complex, the optical transmission through the ir­ Bond cleavages (strong) radiated area to the output increased significantly, cor­ Spin-orbit coupling responding to a decrease in the charge-transfer band cen­ Nano (10 - 9) Rotational and translational motion tered near 1300 nm. The output signal increased by as (molecules are large and fluid) much as ten times. Hyperfine coupling A similar method was used with this material to per­ Micro (10 - 6) form the logic OR-gate function. The intensity of each "Ultrafast" chemical reactions laser beam was set above the 2000-mW (average power) Rotational and translational motion threshold. Irradiation from either or both of the laser Milll (10-3) (molecules are large and/ or very beams resulted in a rapid increase in transmission, just viscous) as it did for the logic AND-gate function. By irradiating the AgTCNQ at a wavelength that is transmitted less in the switched state than in the un­ - (10°) " Fast" chemical reactions switched state (i.e., using the Nd:YAG 532-nm laser line or the 488-nm argon-ion laser line), an Exclusive OR 3 Kilo (10 ) (XOR) function was demonstrated. Below the threshold "Slow" chemical reactions either beam was transmitted equally well, but when th~ 6 combined power of the two exceeded the threshold the Mega (10 ) material became more absorptive in the green spe~tral region, owing to the formation of neutral TCNQ, which has an absorption peak at about 400 nm. The measured output intensity decreased by as much as one half. less than 10 - 9 s. On this basis, Table 2 shows that al­ The reversal of the process in these materials is limited: most all chemical reactions involving the formation of the donor and acceptor species are segregated, owing to an actual product are too slow to meet the prerequisites, the large thermal energy flux during laser exposure. but the events that do meet them are dynamic ones such Work to reduce the optical threshold of the switching as electronic motion, electron orbit hopping, electron or process and to increase the number of cycles of opera­ proton transfer, rotational and translational motion of tion with these materials is continuing, however. small molecules, and bond cleavage. For certain classes of dye chromophores, called reverse EXCITED-STATE PHOTOPHYSICAL saturable absorbers, 15 the excited-state absorption cross PROPERTIES'OF MOLECULAR MATERIALS section (J2 reSUlting from photoinduction is higher than Weare now studying several classes of organic-dye the ground-state absorption cross section (Jl. The macromolecular systems in hopes of developing an all­ ground-state absorption cross section is determined from optical "real-time" threshold logic element based on ex­ the ratio of the monochromatic beams incident on and cited-state photophysical properties. Several critical pre­ emerging from a dye sample, according to the relation requisites must be satisfied before a material having non­ (1) linear optical properties can be used in real-time optical processing or switching applications. They include opti­ where 10 is the intensity of the beam before it impinges cally induced nonlinearity, fast switching and recovery on t~e dye sample, R is the reflection loss at the sample, times, immunity to thermal and laser damage, optical No IS the number of absorbing molecules per unit vol­ activity at common laser wavelengths, low optical power ume, and L is the sample length. The increase in the thresholds, and processability into device structures. excited-state absorption cross section results in an in­ Table 2 is a compilation of the basic photochemical crease in absorption by the dye sample as the intensity events and procesGes that commonly take place in organic of the incident beam is increased, according to the rela­ molecules. Events requiring the least time are listed first, tion followed by those requiring progressively more time. For a photochemcial process to be considered for use in op­ tical processing, the time required for the event must be fohns Hopkins APL Technical Digest, Volume 9, Number 3 (1988) 193 Potember et at. - Molecular Materials for Nonlinear Optics where N2 is the number of molecules in the excited state per unit volume. The excited-state absorption co­ efficient a 2 is determined by 16

assuming NI + N2 = No. We are attempting to deter­ mine the constant a 2 for a variety of laboratory-synthe­ sized materials. It is difficult to describe the exact mechanism of re­ verse saturation because the organic structures are com­ >- plex, but an energy diagram does provide a simple model ~ Q) to illustrate the effect. (Figure 4 is such an energy-level c UJ diagram for an idealized saturable absorber.) At low op­ tical intensities, the optical transparency of the material does not change appreciably, but as the incident intensity increases, the population of the first excited state SI also increases. This population increase is accompanied by nonradiative decay from the vibrationally excited states to the lowest vibrational manifold of the first ex­ cited state. The population of the ground state becomes increasingly depleted at moderate intensity levels. At those moderate intensities, the ground-state absorption al coefficient of the saturable absorber would begin to Ground state (So) decrease and the material would become increasingly transparent, but, at the higher intensities, the first ex­ Nuclear configuration coordinate cited state becomes populated, acting as a trap, and ex­ citation from the fIrst excited state to higher excited states Figure 4-Electronic energy levels in an idealized saturable ab­ S2 begin to dominate the photophysical behavior of the sorber. (From D. J. Harter, M. L. Shand, and Y. B. Band, "Pow­ er/Energy Limiter Using Reverse Saturable Absorption," J. App/. material. In this regime, a 2 becomes significant and the Phys. 23, 865-868 (1984). Reproduced by permission, Am. Inst. material behaves as a reverse saturable absorber and will Phys.) remain in the highly absorbing state as long as the meta­ stable excited state remains heavily populated. Depopula­ tion of state S2 is believed to be caused by a complex relaxation mechanism. We started our investigation by studying a molecular system in which electronic transitions to excited states o" produce nonlinar optical phenomena. The reported non­ "I 1./ linear absorption characteristic of the indanthrone dye chromophore (Fig. 5) provided the reason to choose it ~o H N ~NH0 for our investigation. In an organic dye chromophore, /' I I" the absorption time of visible light and the subsequent " /' electronic motions in the chromophore's 7r-electron sys­ o tem can be on the order of 10 - 12 s. The indanthrone system is known to exhibit reverse saturable absorption Figure 5-Chemical structure of the indanthrone molecule. when it is irradiated by short-duration pulses at high peak power. 15 A typical input-output curve for a 4.5 x 10 - 5 M aqueous solution of indanthrone is shown in Fig. 6. Ob­ Data were obtained (Fig. 8) by directing focused Q­ serve that at 300 mW average power (corresponding to switched pulses from aNd: Y AG laser into a cell con­ about 2.1 x 10 17 W 1m 2 peak power) the transmittance taining the dye. Part of the incident light, proportional at 532 nm drops from 32.2070 to about 0.9%. Note also to the incident power on the cell, was directed into a that very little degradation occurs during irradiation, be­ power meter. The light passing through the cell, propor­ cause the input-output curve follows the same path tional to the transmitted power, was directed into a sec­ whether the power is increasing or decreasing. ond power meter. In most cases, average powers were We have also synthesized three compounds based on measured and the pulse energies were inferred from the the indanthrone chromophore (Fig. 7): oxidized indan­ measured average power. throne, chloroindanthrone, and an indanthrone ladder The highest pulse energy delivered to the cell was ap­ oligomer. 17,18 All three are effective reverse saturable proximately 100 mJ. The near-zero transmittance was absorbers. set at 55070 for each dye (see Fig. 9) by adjusting the con-

194 f ohns Hopkins A PL Technical Digest, Volume 9, Number 3 (1988) Potember et at. - Molecular Materials for Nonlinear Optics centration in an isopropanol suspension. At relatively ~ 0.9% low input levels at 1064 nm, the response of each com­ E 2.0 pound to increasing intensity is linear, because of the intrinsic linear absorption by the dye chromophore. The o~ onset of the nonlinearity observed for each compound a. • Increasing power <1l at higher input levels is due to excited-state absorption. 0> • Decreasing power ~ This highly absorptive state becomes dominant at levels <1l 2 ~ 1.0 of about 10 15 W 1m • The 8 to 30070 drop in transmit­ 2 "0 tance at an incident irradiance of 10 16 W 1m was de­ ~ ·E pendent on the electronic environment of the chromo­ CJ) c phore in the excited state. In all experiments, the tem­ ro .= poral distribution and transverse mode profile were ap­ 100 200 300 proximately Gaussian. The pulse duration was held con­ stant at 10 ns and the nonlinear effect was not polariza­ Incident average power (mW) tion dependent. Figure 6-lnput-output characteristics of a 4.5 x 10 -5 M Although indanthrone is reported to exhibit reverse aqueous solution of indanthrone, illustrating the nonlinear op­ tical properties of the indanthrone molecule. Transmittance at saturable absorption at 532 nm, it is essentially a linear zero power is 32.2%; wavelength of incident beam is 532 nm. absorber at 1064 nm. For the indanthrone structure mod­ ified by chemical oxidation, the nonlinearity at 1064 nm increases in comparison with the nonlinearity of unoxi­ dized indanthrone (Fig. 9). Possibly the oxidized form of indanthrone, with its increased 7I""-electron density, aids (a) in stabilizing the electronic dipole moment in the excited state by increasing the contribution of the excited-state resonance form. This addition to the conjugated 71""­ system may also account for the shift to greater non­ linearity for this molecule at longer wavelengths. Adding a chlorine atom to the basic indanthrone sys­ tem (Fig. 9) increases the excited-state absorption cross section and reduces the power density required for non­ linear behavior. The halogenation of the indanthrone chromophore probably increases the rate of intersystem crossing to the excited state, thus increasing the excited­ (b) state absorption. CI 0 Extending the indanthrone chromophore by adding an aminoanthraquinone subunit to form the ladder I' oligomer (Fig. 9) shifts the absorption maximum toward , I /' the infrared, because conjugation of the molecule is in­ creased. The excited-state absorption cross section for ~o NH'©OOHN0 the ladder oligomer at 1064 nm is higher than for in­ /' I I' danthrone, oxidized indanthrone, or chloroindanthrone. , /' The transmittance versus log irradiance at 532 nm for o the four compounds studied is plotted in Fig. 10. The parent indanthrone molecule exhibits the greatest non­ linearity, followed by oxidized indanthrone, chloroin­ (c) danthrone, and the ladder oligomer. Thus, the experi­ o mental results show that modifying the chemical struc­ ture of a known nonlinear optical material can predict­ ably control the optical properties with respect to wave­ , " ,/ I HN length dependence and changes in optical density. De­ ~o NH veloping this technology of modifying materials having I' nonlinear optical properties can lead to applications in /' nonlinear image processing, optical limiting, and optical pulse compression. HN'©OONH0 /'1 I' ORGANIC MATERIALS FOR SECOND­ , /' HARMONIC GENERATION o Second-harmonic generation (SHG) in organic materi­ Figure 7-Chemical structure of (a) oxidized indanthrone, (b) als (see Table 3) has been known for many years, espe­ chloroindanthrone, and (c) indanthrone ladder oligomer. cially in materials such as MNA (2-methyl-4-nitroani- fohns Hopkins APL Technical Digest, Volume 9, Number 3 (1988) 195 Potember et al. - Molecular Materials for Nonlinear Optics

x-v recorder Power meter and amplifier

I0.o..-....It-----t-e~ (

Figure a-Experimental apparatus Converging for measuring nonlinear absorption lenses in indanthrone and its derivatives. O-switched Nd:YAG laser 532 nm, 1064 nm containing nonlinear absorber

6Or------,------~------_r------_r------~ (4)

50 where P is the macroscopic polarization. The first term on the right is the linear polarizability associated with ~4O linear refraction; the second (which vanishes for optically Q) u isotropic materials) is responsible for SHG in anisotropic § 30 ~ crystals; and the third is responsible for four-wave mix­ 'E • Indanthrone ing effects, photorefraction, and optical bistability. In ~ 20 & Oxidized indanthrone ~ these terms, EO is the permittivity of free space, E is the I- • Chloroindanthrone electric field strength, and X is the dielectric susceptibility . 10 T Indanthrone ladder oligomer For SHG, xl is a third-rank tensor of the form xhf).

o~~~~--~~~--~~~--~~~--~~~ The second-order nonlinear term then takes the form 12 13 14 15 16 17 Log irradiance (W/ m2) p.(2w) = E X (2w) E ·(w) E (w) J' k = 1 2 3 (5) Figure 9-Transmittance plotted as a function of log irradiance I 0 Ijk '} k " ". for indanthrone and indanthrone-based reverse saturable absorb­ ers. Wavelength of incident beam is 1064 nm. The numerical values of the tensor coefficients are usual­ ly represented by the letter d and are functions of fre­ quency and temperature. The units of xbr) are different 70 from those of the first-order linear susceptibility term w xJ) . The first-order term is a dimensionless ratio, 60 whereas the second-order susceptibility tensor has units 50 of inverse electric field strength (meters per volt in the ~ meter-kilogram-second system). 22 Q) u 40 Organic molecules can be adapted to molecular engi­ c co neering. An example of such a molecule is 2-methyl-4- ~ 'E 30 nitro aniline (Fig. lla), which consists of a nitro (N02) CJ) c co • Indanthrone acceptor group and an amino (NH2) donor .group sepa­ .= 20 rated by a conjugated ring system. This molecule is easily 10 • Chloroindanthrone polarized, has a high degree of charge transfer, and is thus an excellent material for SHG. Presence of the methyl group was necessary to assure a noncentrosym­ 13 14 15 16 17 metric crystal structure. Log irradiance (W/ m 2) Another well-known material that has received con­ Figure 10-Transmittance plotted as a function of log irradi­ siderable attention for some time is urea, which has the ance for indanthrone and indanthrone-based reverse saturable structure shown in Fig. 11 b. Urea is often the efficiency absorbers. Wavelength of incident beam is 532 nm. standard against which other organic compounds having nonlinear optical characteristics are compared-a con­ vention we retain here. line)19 and urea_ 20,21 SHG takes place in these materials The potential SHG materials were tested using the because of the highly nonlinear polarizabilty of the 7r­ well-known powder-susceptibility measurements first bond structure and the asymmetrical charge distribution used by Kurtz and Perry. 20 In their technique a pow­ associated with donors and acceptors in the molecule. dered sample is placed in a cuvette and irradiated by The polarizability for any substance is pulses from aNd: YAG laser operating at a wavelength

196 Johns Hopkin s A PL Technical Digest, Volume 9, Number 3 (1988) Potember et at. - Molecular Materials jor Nonlinear Optics

Table 3-0rgan ic materials for second-harmonic generation. (a)

Chemical Name Structure SHG Efficiency Relative to Urea 2-methyl-4-nitroaniline (MNA)

2-nitrobenzaldehyde @( CHO NOz

3-nitrobenzaldehyde t ~ CHO -I x (b) (c) ~C H =N-NH2 NOz Urea 4-nitrobenzaldehyde t OzN -@-CHO -2x O2 N0 4-nitrobenzaldehyde hydrazone 3-cyanobenzaldehyde ~ CHO 0.3 x CN

4-cyanobenzaldehyde NC-@-CHO -Ix

4-nitrobenzaldehyde 4-nitrobenzaldehyde 4-nitrobenzaldehyde OzN-@-CH=N-NHz hydrazone t lO x methyl hydrazone dimethylhydrazone

3-nitrobenzaldehyde ~ CH = N-NH z (e) hydrazone NOz 0.1 x ~C H=N-NH ~ 02N ~ 2-nitrobenzaldehyde @(CH = N-NHz hydrazone 0.001 x 4-nitrobenzaldehyde phenyl hydrazone N01

4-nitorbenzaldehyde (f) Ol N-@-CH= N- NH -@ phenylhydrazone 0.5x

3-nitrobenzaldehyde $ ~ CH = N-NH -@ phenylhydrazone 0.001 x N N02 $ ~ N CH 2-nitrobenzaldehyde @-CH = N- NH -@ 0.001 x ~ phenylhydrazone CH

I-naphthaldehyde phenylhydrazone $-CH=N-NH-@ -5x @@ I-pyrenecarboxaldehyde phenylhydrazone '-CH=N-NH-@ -4.0x 1-naphthaldehyde phenyl hydrazone 1-pyrenecarboxaldehyde I-naphthaldehyde phenylhydrazone hydrazone $-CH=N-NHz 0.001 x Figure 11-Chemical structure of several organic materials used for second-harmonic generation. benzaldehyde @-CH=N-NH2 hydrazone of 1064 run (w) . If the powdered sample is a material 3-cyanobenzaldehyde ~ CH=N-NH-@ suitable for SHG, light at twice the fundamental frequen­ phenylhydrazone 0.002 x CN, cy (2w) will be emitted by the sample (i.e. , wavelength of 532 run). Because the powdered sample contains crys­ 3-nitrobenzaldehyde ~ CH=N-NHCH 3 talline grains that are randomly oriented crystalIographi­ methylhydrazone 0.001 x NOz cally, the emitted second harmonic is weak but easily detectable by electron-multiplier phototubes. 4-nitrobenzaldehyde OzN -@-CH=N-N(CH3)z Efficient SHG requires large biaxial or triaxial crystals dimethylhydrazone 0.05 x so oriented that the fundamental frequency propagates as an o-ray and the harmonic propagates as a e-ray, or 4-nitrobenzaldehyde OzN-@-CH=N-NHCH3 vice versa. (An o-ray, or ordinary ray, is the ray where methylhydrazone 0.001 x the refractive index does not vary with crystallographic 'Only preliminary data are available for these compounds. direction; an e-ray, or extraordinary ray, is the ray where tSee Ref. I. the refractive index varies with crystallographic direc­ tion.) The crystal orientation is adjusted so that the 0- ray (1064 run) and e-ray (532 run) propagate at the same fohns Hopkins APL Technical Digest, Volume 9, Number 3 (1988) 197 Potember et at. - Molecular Materials for Nonlinear Optics speed. Thus, power is coupled from the fundamental to CONCLUSION the harmonic as the two waves propagate in phase Our research is continuing in an effort to synthesize, through the crystal. Meeting this condition is known as modify, and characterize new organic molecular materi­ phase matching. The powder method is also used to test als for nonlinear optical applications that include bistable whether a material can be phase matched by measuring optical switching, using an organometallic complex; op­ the total second-harmonic output (532 nm) while the tical limiting and processing, using organic dyes that ex­ crystallite size of the powdered sample is varied. Phase­ hibit reverse saturable absorption; and second-harmonic matchable materials will show an increase in the second­ generation, using organic crystalline materials. The re­ harmonic output with crystallite size that saturates once sults of our research should provide new materials that the coherence length of the fundamental and harmonic will significantly advance the field of nonlinear molecular frequencies is exceeded. Materials that cannot be phase materials. matched show an increase in second-harmonic output as crystallite size is increased toward the coherence length, and then show a sharp decrease in second-har­ REFERENCES monic output as crystallite size is increased beyond the IF. Nicoud and R. J. Tweig, "Organic SHG Powder Test Data," in Nonlinear coherence length. The crystallite size at which the de­ Optical Properties of Organic Molecules and Crystals, Vol. 2, D. S. Chemla crease occurs is typically a few micrometers. Some ma­ and J. Zyss, eds., Academic Press, Ltd., London, p. 221 (1987) . terials, even though they have acentric electron distribu­ 2J. A. Neff, "Major Initiatives for Opitical Computing," Opt. Eng. 26, 2 (1987). 3E. Marom, "Real-Time Image Subtraction Using a Liquid Crystal Light tions and net dipole moments, often crystallize in a cen­ Valve," Opt. Eng. 25, 274 (1986). trosymmetric crystal system. Despite the centro symmetry 4A . Szabo, "Frequency Selective Optical Memory," U.S. Patent 3,896,420 in such instances, these molecules can be "poled" or (1975). 5R. S. Potember, R. C. Hoffman, and T. O. Poehler, "Molecular Electron­ aligned by an electric field in a polymer that has been ics, " Johns Hopkins APL Tech. Dig. 7, 129-141 (Apr-Jun 1986). heated above the glass transition temperature (Tg) to 6R. S. Potember, T. O. Poehler, and R. C. Benson, "Optical Switching in Semi­ conductor Organic Thin Films," Appl. Phys. Lett. 41 , 548 (1982). produce a noncentrosymmctric solid that, when cooled 7R. C. Benson, R. C. Hoffman, R. S. Potember, E. Bourkoff, and T. O. Poeh­ below Tg , will exhibit SHG. 23 ler, "Spectral Dependence of Reversible Optically Induced Transitions in Or­ We have also investigated a class of compounds that ganometallic Compounds," Appl. Phys. Lett. 42, 855 (1983). 8R. S. Potember, R. C. Hoffman, R. C. Benson, and T. O. Poehler, " Erasa­ has received little attention since the initial investigation ble Optical Switching in Semiconductor Organic Charge-Transfer Complex­ 24 in 1977 by Davydov et al. The prototype compound es ," J. Phys. 44, C3-1597 (1984) . of the series is 4-nitrobenzaldehyde hydrazone (NBAH), 9T. O. Poehler, R. S. Potember, R. Hoffman, and R. C. Benson, "Optical Phase Transitions in Organic Charge-Transfer Complexes," Mol. Cryst. Liq. shown in Fig. 11c. The compound, synthesized by reac­ Cryst. 107, 91 (1984). tion of 4-nitrobenzaldehyde with hydrazine, was first lOR. S. Potember, T. O. Poehler, R. C. Hoffman, K. R. Speck, and R. C. Ben­ reported by Davydov et al. to have a powder efficiency son, " Reversible Electric Field Induced Bistability in Carbon-Based Radical­ Ion Semiconducting Complexes: A Model System Molecular Information Pro­ 2.5 times that of urea. Unexpectedly, we observed much cessing," in Molecular Electronics, F. L. Carter, ed ., Marcel Dekker, Inc., higher powder efficiencies, up to 10 times that of urea. New York, p. 91 (1986). As a result, we synthesized derivatives of NBAH to de­ 11 K. R. Speck, T. O. Poehler, A. A. Burk, C. A. Viands, and R. S. Potember, "Bistable Optical Switching of Organometallic Doped Gel-Derived Silica Glass­ termine whether the SHG powder efficiency could be es, " Proc. 5th Int. Congress on Applications of Lasers and Electro-Optics," enhanced and whether there are similar molecules with C. M. Penney and H. J. Caulfield, eds., Springer-Verlag, New York, p. 243 (1987). high SHG powder efficiencies. The methyl and dimethyl 12L. R. Melby, R. J. Harder, W. R. Hertler, W. Mahler, R. E. Benson, and derivatives of NBAH (shown in Fig. lId) were synthe­ W. E. Mochel, "Substituted Quinodimethans. II. Anion-Radical Derivatives sized in much the same way as the NBAH. Although and Complexes of 7, 7,8, 8-Tetracyanoquinodimethane," J. A m. Chem. Soc. 84, 3374 (1%2). these materials are chemically similar to NBAH, they 13K. Kamaras, G. Gruner, and G. A. Sawatzky, " Optical Absorption in Com­ exhibited lower SHG powder efficiencies that are 0.001 plex TCNQ Salts," Solid State Commun. 27, 1171 (1978). to 0.05 times that of urea. Experiment thus shows that 14 J. B. Torrance, B. A. Scott, and E. B. Kaufman, " Optical Properties of Charge-Transfer Salts of Tetracyanoquinodimethane (TCNQ) , " Solid State minor chemical changes can result in significant changes Commun. 17, 1369 (1975). in the arrangement of molecules within the crystal, often 15c. R. Guiliano and L. D. Hess, "Nonlinear Absorption of Light: Optical Satu­ resulting in decreased SHG powder efficiency. The com­ ration of Electronic Transitions in Organic Molecules with High Intensity La­ ser Radiation," IEEE J. Quantum Electron. QE-3, 358 (1%7). pound 4-nitrobenzaldehyde phenylhydrazone was synthe­ 16M. L. Shand, J. C. Walling, and R. C. Morris, "Excited-state Absorption sized by the same method and has the structure shown in the Pump Region of AIexandrite, " J. Appl. Phys. 52, 953 (1981). 17M. K. Shah and K. H. Shah, "Indanthrones from I-Amino-Anthraquinones: in Fig. lIe, but this compound showed an SHG powder Part I-Studies of Variables in the Preparation," Indian J. Technol. 13, 312 efficiency about 0.5 times that of urea. The efficiency (1975). would be much greater except that the compound is sig­ 18M. C. Clark, " Chemistry of Indanthrone. Part XII. Some Properties of the Dinaphthophenazinediquinones and Their NN ' -Dihydro.derivatives," J. Chem. nificantly absorptive at 532 nm. Soc. (C), 2090 (1966) . We have also made hydrazone derivatives of large 19B. F. Levine, C. G. Bethea, C. D. Thurmond, R. T. Lynch, and J. L. Bern­ conjugated ring systems such as naphthalene and pyrene stein, "An Organic Crystal With an Exceptionally Large Optical Second Har­ monic Coefficient: 2-methyl-4-Nitro Aniline," J. Appl. Phys. SO, 2523 (1979). (Fig. lIt). These compounds do not exhibit much charge 20S. Kurtz and T. T. Perry, "A Powder Technique for the Eval4ation of Non­ transfer, because their aromatic ring systems donate linear Optical Materials," J. Appl. Phys. 39, 3798 (1968). weakly, but both have greater SHG powder efficiencies 21 D. Bauerle, K. Betzler, H. Hesse, S. Kappan, and P. Loose, "Phase-Matched Second Harmonic Generation in Urea," Phys. Status Solidi (A) 42, K119, than urea. Table 3 lists many of the hydrazone deriva­ (1977). tives of various aromatic aldehydes that have been syn­ 22G. C. Baldwin, An Introduction to Nonlinear Optics, Plenum Press, New York, p. 81 (1%9). thesized along with their respective SHG powder effi­ 23K. D. Singer, J. E. Sohn, and S. J. Lalama, "Second Harmonic Generation ciencies referenced to that of urea. in Poled Polymer Films," Appl. Phys. Lett. 49, 248 (1986).

198 Johns Hopkins APL Technical Digest, Volume 9, Number 3 (1988) Potember et at. - Molecular Materials for Nonlinear Optics

24B. L. Davydov, S. G. Kotochshchikov, V. A. Nefedov, "New Nonlinear Or­ KAREN A. STETYICK was born ganic Materials for Generation of Second Harmonics of Neodymium Laser in Darby, Pa., in 1956. She studied Radiation," SOY. 1. Quantum Electron. (Eng. Trans.) 7, 129 (1977). chemistry at Saint Joseph's Univer­ sity in Philadelphia, where she re­ ceived her B.S. degree. As a gradu­ THE AUTHORS ate student at Temple University, she pursued studies in synthetic organic chemistry, with a concentration in RICHARD S. POTEMBER was the area of natural products. In 1987, born in Milton, Mass., in 1953. He she received her Ph.D. in chemistry was educated at Merrimack College, and joined APL as a Postdoctoral where he obtained a B.S. in 1975, Fellow. As a member of the Ad­ and at The Johns Hopkins Univer­ vanced Materials Program, her re­ sity, where he obtained an M.A. in search entails the synthesis and chemistry in 1978, a Ph.D. in chem­ modification of organic materials istry in 1979, and an M.S. in tech­ that exhibit interesting nonlinear op­ nical management in 1986. During tical effects. 1979- 80, he was a JHU/ APL Post­ doctoral Fellow. In 1981, he was em­ ROBERT A. MURPHY was born ployed in APL's Milton S. Eisen­ in Elkridge, Md., in 1930. He re­ hower Research Center as a senior ceived electronics schooling in the chemist in the Quantum Electronics Navy and an A.A. in engineering Group. In 1986, he was appointed studies at Essex Community College. supervisor of the Advanced Materi­ Before joining APL he was em­ als Program. Dr. Potember has been ployed as an electronics manufactur­ visiting professor in the Johns Hopkins Department of Chemistry (1984-85 ing specialist on the technical staff and 1986-87) and in the G.W.c. Whiting School of Engineering, Depart­ at the Electronics Systems and Prod­ ment of Materials Science, since 1985. His research interests include or­ ucts Division, Martin Baltimore. ganic chemistry, nonlinear optics, polymer chemistry, photochemistry, Since joining APL in 1965, he has and the electrical and optical properties of organic solids. worked in the laboratories of the Milton S. Eisenhower Research Cen­ ter, where he has been involved in investigations of plasma contain­ ment, gas laser dynamics, thin ftlms, . and nonlinear optical materials. He IS now an engineering staff associate in the Materials Science Group. ROBERT C. HOFFMAN was born in Bethesda, Md., in 1959 and re­ ceived his B.S. degree in chemistry KENNETH R. SPECK was born in from Loyola College in Baltimore in Baltimore in 1961. He received his 1982. He is now a doctoral candidate B.S. in engineering science from in the Department of Materials Sci­ Loyola College in Maryland in 1983 ence and Engineering, G.W.c. and his M.S.E. in materials science Whiting School of Engineering, The and engineering from The Johns Johns Hopkins University, where he Hopkins University in 1987. He is was awarded an M.S.E. degree in currently a doctoral candidate at 1987. He is also a JHU/ APL Grad­ Johns Hopkins Materials Science uate Student Fellow and is conduct­ and Engineering Department, con­ ing his dissertation research at the ducting his graduate research at Milton S. Eisenhower Research Cen­ APL's Milton S. Eisenhower Re­ ter in the area of organic solids. His search Center. His work at APL in­ work at APL has included research volves the sol-gel chemistry of on novel optical storage materials vanadium dioxide and of gel-derived and nonlinear properties of organic solids. silica glass.

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