March 1, 1996 / Vol. 21, No. 5 / OPTICS LETTERS 357

Picosecond optical bistability in metallophthalocyanine-dopedpolymer film waveguides

Jinhai Si, Yougui Wang, Jiang Zhao, Bingsuo Zou, and Peixian Ye

Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, China

Ling Qiu and Yuquan Shen

Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing, China

Zhigang Cai and Jianying Zhou

Institute for Lasers and Spectroscopy, Zhongshan University, Guangzhou, China

Received August 16, 1995

Ultrafast optical bistability was observed in a planar optical waveguide made with a 2,9,16,23-tetra-octadecanoylamido-substituted vanadyl phthalocyanine-doped polystyrene film. Input–output curves withhysteresis characteristics were measured with laser pulses of 60-ps pulse width at 532 nm. The switchup andswitchdown times were less than 10 and 20 ps, respectively. The origin of the nonlinearity giving rise tooptical bistability was predominantly electronic effects. 1996 Optical Society of America

Organic and polymeric materials have attracted muchattention for nonlinear optical applications, based pri-marily on their large and ultrafast nonlinearities.1 – 3

Although the magnitudes of optical nonlinearities oforganic polymers are large, they are still small in anabsolute sense. Thus nonlinear optical devices thatincorporate organic polymers will likely be based onwaveguide structures because of the large power densi-ties achievable in thin guiding f ilms. Phthalocyanineswith large two-dimensional p-electron delocalizationcan be prepared as high-quality f ilms with a large, fastthird-order nonlinear optical response.4 – 7 In addition,they are environmentally stable and easily processable,have minimal scattering and absorptive losses in thenear IR, and have methods available for patterning thematerials to produce channel waveguides. They are,therefore, emerging as a new class of optical materialfor waveguide applications.

Optical bistabilities observed in a planar quasi-waveguide and a hybrid waveguide made with apolydiacetylene f ilm have been reported.8,9 Opticalbistability with low input power and slow temporalresponse in organic polymer waveguides was ob-served by us recently.10 However, to our knowledge,study of ultrafast optical bistability in organic poly-mer waveguides has not yet been reported. In thisLetter we present our observation of ultrafast opti-cal bistability in a planar optical waveguide madewith a 2,9,16,23-tetra-octadecanoylamido-substitutedvanadyl phthalocyanine- (ODVPC-) doped polystyrenefilm. Input–output curves with hysteresis character-istics were measured with laser pulses of 60-ps pulsewidth at 532 nm. The switchup and switchdowntimes were less than 10 and 20 ps, respectively. Thenonlinearity resulting in optical bistability was dueprimarily to electronic effects.

Optical bistability in waveguides arises from thevariation of coupling eff iciency with incident power

0146-9592/96/050357-03$6.00/0

through the variation of the power-dependent nonlin-ear refractive index of the spatial layer in the couplingregion. In a prism-coupling setup, the optimum cou-pling angle at a high incident power is different fromthat at a low incident power.

The coupling equations in the prism-coupling regioncan be written as10

≠Asxd≠x

­ g0E exphifkbx 2 csxdgj 2 gRAsxd 2 glAsxd ,

(1)

≠csxd≠x

­ kfn0jAsxdj2 1 b0g , (2)

where Asxd and csxd are the amplitude and the phase,respectively, of the guided wave at x, E is the ampli-tude of the incident light, k is the propagation constantin vacuum, kb0 is the linear mode propagation constantof the guided wave, kb is the wave-vector component ofthe incident light along the x direction, g0 is the inputcoupling (to the guided wave) coeff icient, gR is the cou-pling coefficient from the guided wave to the radiationfield that is related to g0 as a reciprocal coupling, gl isthe dissipative loss coeff icient, and n0 is the nonlinearrefractive index.

Using coupling equations (1) and (2), we can obtainthe switchup and switchdown thresholds (IH and IL) ofoptical bistability:

IH ­2Dbn0fsDbd2 1 9g2g 1 2hn02fsDbd2 2 3g2g3j1/2

27kn02g20

,

(3a)

IL ­2Dbn0fsDbd2 1 9g2g 2 2hn02fsDbd2 2 3g2g3j1/2

27kn02g20

,

(3b)

1996 Optical Society of America

358 OPTICS LETTERS / Vol. 21, No. 5 / March 1, 1996

where g ­ gR 1 gl and Db sDb ­ b 2 b0d can beexpressed by the detuning angle da sda ­ a 2 a0d as

Db ­sb0 cos w 2 sin a0dcos a0

b0 2 cos w sin a0da , (4)

where w is the base angle of the prism and a and a0are the incident angle and the optimum incident angleon the prism, respectively. From Eqs. (3) and (4) wecan conclude that IL, IH , and DI (the width of thebistable loop, DI ­ IH 2 IL) increase with an increasein the detuning angle da. Nevertheless, it should bepointed out that the above theoretical analysis is onlyan approximate treatment, in which decoupling of theguide wave was ignored.

We constructed the optical waveguide used in ourexperiments by spin coating a thin f ilm of polysty-rene, doped with ODVPC, upon the clean surface ofa fused-quartz substrate. ODVPC was prepared bythe reaction of tetraamino vanadyl phthalocyanineand stearoyl chloride in pyridine solution. Thiscompound has extremely large off-resonant third-ordernonlinearities, because of the extensive p-electrondelocalization. The absorption spectrum of ODVPC isshown in Fig. 1. At a wavelength of 532 nm, the linearabsorption of the molecule is weak. Polystyrene andODVPC (2% ratio) were dissolved in chloroform for spincoating. The doped number density of ODVPC wasapproximately 3 3 1018 moleculesycm3. The filmthickness and the refractive index were measured to be3 mm and 1.583, respectively. The losses in the filmfrom scattering and absorption were estimated to beless than 1 dBycm by measurement of the intensity ofthe scattered light along the path of the guided beam.

The experimental setup for the measurement isshown in Fig. 2. A Nd:YAG mode-locked laser (1-Hzrepetition rate) producing 60-ps pulses at a 1.06-mmwavelength was used as the source in the experi-ments. Polarized light from the Nd:YAG laser, whosefrequency was doubled to 532 nm, was divided into twobeams; one was used as the reference light, and theother was focused into the film by a lens with a 20-cm focal length and used to excite the TM modes ofthe waveguide. A pair of prisms made of ZF1 glasswas used to realize the input and output coupling ofthe waveguide. The diameter of the light spot at thefilm was approximately 200 mm. The laser pulse en-ergy varied up to 200 mJ, with a good Gaussian spa-tial and temporal profile. The maximum energy usedin the experiments was limited by the damage thresh-old of the polymer film (approximately 50 mJ over a3 3 1024 cm2 area). The pulse shapes of the refer-ence light and the output light coupled out from thewaveguide by the decoupling prism were monitoredsimultaneously by a streak camera. By measurementwith very low power of incident light, several modesallowed into the waveguide were observed. However,all measurements for optical bistability were made ina TM0 mode, the optimum coupling angle a0 of whichwas approximately 31.5±.

When the incident angle was set at positive de-tuning, bistability was observed in the input–outputcharacteristics of the waveguide. Typical input andoutput pulse shapes recorded at a positive detuning

angle are shown in Fig. 3. The input–output charac-teristics of the waveguide for different detuning anglesare shown in Fig. 4. It can be seen from Figs. 4(a)and 4(b) that both the width of the bistable loop (i.e.,the difference between the switchup and switchdownthresholds) and the switchup threshold increase withincreasing detuning angle. These characteristicsare in good agreement with the theoretical analysismentioned above. From the bistable loop [Fig. 4(a)]and the observed pulse shapes (Fig. 3), the switchupand switchdown times can be estimated to be less than10 and 20 ps, respectively. In addition, no bistabilitywas observed with negative detuning [Fig. 4(c)]. Thisindicates that the sign of the nonlinear refractive

Fig. 1. Absorption spectrum of ODVPC.

Fig. 2. Experimental setup for the measurement of opticalbistability in waveguides. M’s, mirrors; R, rotation stage.

Fig. 3. Observed input (dashed curve) and output (solidcurve) pulse shapes at a positive detuning angle da ­ 960.

March 1, 1996 / Vol. 21, No. 5 / OPTICS LETTERS 359

Fig. 4. Observed input–output characteristics for dif-ferent detuning angles: (a) da ­ 960, (b) da ­ 1560,(c) da ­ 21350. , and s indicate data measured for theupper and lower sides of the input pulses, respectively.

index giving rise to this behavior is positive. A Z-scanstudy conducted on the ODVPC-doped polystyrenefilm with a cw Ar-ion laser reveals that dnydT of this

film is negative. This fact illustrates that the non-linearity giving rise to the bistabilities in our experi-ments does not originate from thermal effects.Previous studies conducted on ODVPC using femtosec-ond time-resolved experiments have indicated that thelarge nonlinearity and the ultrafast response of thiscompound at the resonant wavelength of 647 nm areattributed to excited-state population and the inter-action between neighboring excited molecules (suchas exciton–exciton interaction), respectively.11 Butsince our experiments were performed at a fairlynonresonant wavelength, the nonlinearity does notappear to be due to excited-state population andexciton–exciton interaction. However, we believethat the nonlinearity giving rise to the opticalbistability arises predominantly from electroniceffects.

In summary, ultrafast optical bistability in aplanar optical waveguide made with a ODVPC-doped polystyrene f ilm was demonstrated. Theswitchup and switchdown times were less than 10 and20 ps, respectively. The origin of the nonlinearityresulting in optical bistability was predominantlyelectronic.

This research was carried out with the sup-port of the National 863 High-Technology Bond ofChina.

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