Experimental investigations and numerical simulations of spatial solitons in planar polymer waveguides

__ __ i!!iE!l

15January

pi&

1997

OPTICS COMMUNICATIONS

ELSEVIER

Optics Communications

134 (1997) 49-54

Experimental investigations and numerical simulations of spatial solitons in planar polymer waveguides U. Barhuzh a.1,U. Peschel a, Th. Gabler b, R. Waldhiiusl a Instituie

ofApplied

b Fruunhofer ’ Institute

of Orgunic

Chemistry

Physics. Friedrich-Schiller-Uniuersity

Institute for Applied

Optics und Precision

und Mucromoleculor

Chemistry.

b, H.-H. H&-hold ’

Jenu. 07743 Jew. Engineering.

Germuny

Jenu. Germuny

Friedrich-Schiller-Uniuersity

Jew.

07743 Jew.

Cermcmy

Received15April 1996;revised version received 18June 1996; accepted 23 July 1996

Abstract We investigated the beam propagation in a planar polymer waveguide. The conjugated polymer used in our experiments is a solution processable polyarylene-vinylene derivative. the DP-PPV/DP-PFV copolymer, which exhibits both a high focusing Kerr-like nonlinearity and an outstanding light and air stability. We report the first experimental observation of spatial solitons in a poiymer waveguide. The corresponding spectral broadening could be detected. To obtain a deeper understanding investigation.

of the experimental results we carried out numerical simulations of the spatial-temporal The numerical simulations agree quite well with the experimental findings.

1. Introduction Optical solitons existing in Kerr-like materials are one of the most interesting phenomena of nonlinear optics. Temporal solitons in optical fibers, where nonlinear self phase modulation compensates dispersion, were studied in many experimental and theoretical groups. In the last few years significant progress has been achieved in transmission experiments at dispersionless propagation. Spatial solitons, which propagate in planar waveguides, were not investigated in such dimensions up to now. They are confined perpendicularly to the layer structure, but form

’ E-mail: [email protected]. 0030-40 I8/97/$17.00 Copyright Pff SOO30-4018(96)00500-7

objects

under

a stable equilibrium state between diffraction and nonlinearly induced self focusing parallel to the interfaces. Spatial solitons were observed in some materials which exhibit a focusing nonlinearity such as carbon disulfide CS, [ 11, inorganic glasses [2] and different semiconductors like AlGaAs [3]. Our aim is to demonstrate the propagation of spatial solitons in a polymer waveguide. In the last few years polymeric materials became more and more popular in optics. This approach has been motivated by the fact that polymeric materials offer many advantages in comparison to inorganic glasses and crystals, including large optical nonlinearities. ease of device fabrication and tailoring their linear and nonlinear behavior by molecular engineering (chemical synthesis). Thus, polymers are ideal materials for nonlinear optics at all.

0 1997 Elsevier Science B.V. All rights reserved.

50

U. Bartuch et al./ Optics Communications

134 (1997) 49-54

2. Experimental i

,

l

8 1.70. .E s ‘S ks ’ 1.66

1

_~DP-PPVIDP-PFV-Copolymers .

. .

-

.

. .

.

.

l

.

.

. .

I

,

,&l

~.

-_

__~_~._~

800

600 wavelength

1000

[nm]

Fig. 1. Refractive index dispersion of DP-PPV/DP-PFV-cope lymer, the inset shows the molecular structure.

Because the power levels required are rather high, short pulses have to be used in the experiments. Therefore the soliton formation in space is accompanied by a pulse evolution in time. We made first attempts to study the resulting spatial temporal object in more detail. To illustrate the dynamics and to obtain a deeper understanding of the underlying physics we simulate the experiment. We solve the corresponding 2 + 1-dimensional nonlinear Schriidinger equation numerically and compare the results with the experimental data.

Ar’ - laser

Our experiments reveal, that the amorphous copolymer DP-PPV/DP-PFV, a conjugated polymer of the soluble DP-PPV-type, is a particularly suitable nonlinear optical material for the generation and propagation of spatial solitons. Its systematic name is poly[l,4-phenylene-l,2-diphenylvinylene-co-2,7-fluorenylene-1,2_diphenylvinylene (Fig. 1, inset [4]). It is one of the several derivatives of PPV, which are well suited for investigation of nonlinear optical effects in waveguides [5-71. The DP-PPV/DP-PFV copolymer is a molecular material, which exhibits besides high optical nonlinearities good filmforming properties. This polymer is a high molecular weight material CM,,= 25000 g/mol>. It is very stable against the effects of air. light and temperature because of its high glass transition temperature (T, = 292°C) and because of the total phenyl substitution of the vinylene double bonds. The absorption spectrum, measured in solid state, shows a maximum at A,,, = 375 nm. The waveguides have been produced by spin coating and have excellent waveguiding properties. The waveguide losses (scattering + absorption) measured with a special two prism method are less than 1 dB cm-’ at the wavelength of A = 893 nm. The disper-

1

Fig. 2. Experimental set-up.

Ii. Barruch

er (II./

Optics Communications

sion n(h) of the DP-PPV/DP-PFV film was measured by m-line spectroscopy for both, TE- and TM-polarization of the incident light, respectively (Fig. 1). The birefringence of the polymer is caused by molecule ordering in the spin coating process. From the fitted curve for TE-polarization we deduced a normal dispersion of D = 350 ps*/km CD = a2p/aw2). For the determination of the sign and the magnitude of the nonlinearity we used a “semiintegrated” Mach-Zender interferometer set-up with high accuracy. We deduced from these investigations a high real part for the focusing nonlinearity of n, = 2.7 X 10-l’ cm’/W at the wavelength of 900 nm [71. Power dependent transmission measurements result in a rather small amount of the two photon absorption of about o2 = 0.18 cm/GW in this wavelength region [7]. To investigate spatial soliton propagation we used a 720 nm thick DP-PPV/DP-PFV polymer film, spin coated on a 2.7 km thick thermically oxidized silicon wafer (nsioJh = 893 nm) = 1.4519). The experimental set-up used for our investigations is shown in Fig. 2. Pulses of 100 fs duration were generated by a tunable argon-pumped modelocked Ti-sapphire laser at a wavelength of A = 893 nm. A pulse picking system reduced the repetition rate from 76 MHz to 9 kHz. To excite the TE,-mode we used a highly refractive index glass prism mounted on the waveguide surface (ATR-geometry). The beam was focused with a microscope objective onto the basis of the prism. The corresponding beam waist was about WRvHM = 30 pm. The waveguide together with the prism were mounted on a motorized rotary stage. After 9 mm of propagation in the waveguide the lateral field distribution at the end-face was imaged onto a CCD-camera. In order to measure the spectral broadening versus the guided power we used a removable mirror to image outcoupled pulses at the end-face of a multimode fiber mounted on the entrance split of a spectrum analyzer.

3. Numerical

51

134 (1997) 49-54

rise to a diffraction length of 3.8 mm. Hence a linear beam will be broadened by a factor of 2.6 during propagation through the sample. We have to take into account dynamical effects because we used short pulses to perform the experiment. The normal dispersion of the polymer (D = 350 ps’/km) results in a dispersion length of a 100 fs pulse of about 8.4 mm. Subsequently the linear pulse broadening is rather weak and amounts to a factor of 1.5. The main temporal effects expected are due to the variation of the beam intensity in time. While the leading and the trailing edges of the pulse behave more or less linearly the center can show strong nonlinear effects. The propagation of those spatial temporal objects is well described by a 2 + l-dimensional nonlinear Schriidinger equation [8]

a

‘a+----: 4rrn,ff +++-

a*

D a*

ax2

2 at?

A

2?rnz Iu(x. *

I, Z)l’

eff

u(x,

t, z)=O,

1

(1) where neff = 1.67 and neff = 670 nm are the effective index and the effective thickness of the waveguide respectively. Linear and nonlinear losses are represented by (Y and Im(n,). 1u I 2 accounts for the power per unit width. Note that because of the normal temporal dispersion the signs in front of the second derivatives in Eq. (1) are opposite. Due to this unequality the onset of the nonlinearity will cause the pulse to narrow in spatial direction but to spread in temporal one. To estimate the field distributions and the power levels required we first neglect the time derivative and all losses in Eq. ( 1). The well known bright soliton is a solution of the resulting equation:

analysis

To get some impression of the scales of the device we first analyse the linear pulse propagation. The initial beam width of W,,, = 30 pm gives

xexp(

i 4an,::s(

$

) 3

(2)

52

II. Bartuch et al./Optics

Communications

134 (1997) 49-54

where soliton width W, and power P, are related in the following way:

P,(*) =

4ffh2 2772%ffn2

1 W,(f)

'

WFWHM W,(t) =

2 ln( 1 + fi)

’

For a pulse like power evolution P,(t), Eqs. (2) and (3) lead to a butterfly like intensity distribution in space and time. To recover the initial beam width of W wHM = 30 pm at the end face the incident peak power must exceed at least the soliton power P, = 350 W. To obtain a significant beam narrowing with respect to the time averaged power distribution recorded at the end face a much higher power is necessary. Linear and nonlinear losses together with the weak dispersive broadening also increase the power levels required.

0.2.

x(m) 1.0,

A”’

“’

0

200

‘I

4. Results and discussion Fig. 3a and b show experimentally observed time averaged lateral beam profiles of the TE,-mode imaged onto a CCD-camera. They were recorded on two different positions of the polymer film. Differences between Fig. 3a and b with respect to peak power levels and field distributions may be caused by a varying incoupling efficiency or by disturbances in the polymer film. In Fig. 3a nonlinear beam compression is demonstrated. For incident peak powers above 1000 W a significant narrowing of the beam could be observed. While the incident beam has approximately a Gaussian shape the field distribution recorded at the end face is sech like. Both the beam compression as well as the transformation of the shape hint to an excitation of a one soliton state. The measured distribution did not change in a wide peak power range between 1000 W and 1400 W. On this special point of the waveguide no higher power levels could be used without causing an irreversible destruction of the polymer. To demonstrate higher soliton states we changed the incoupling position and increased the incident pulse energy further (see Fig. 3b). If the incident

-400

-200

4cil

x(rm)

Fig. 3. Experimentally detected field distribution at the end-face for different incident peak power levels. (a) Formation of a one soliton state. (b) Formation of a two soliton state.

peak power exceeded 1500 W dramatic changes were observed. The beam broadened again and a formation of new maxima could be detected. We attribute those maxima to a two soliton state which was created in the center of the pulse. Two soliton solutions are known to react to perturbations very sensitively [9]. Spatial inhomogeneities in the polymer film may cause an asymmetric decay of those two hump solutions as it was observed for the highest power level displayed in Fig. 3b. The results obtained by a numerical solution of Eq. (1) agree quite well with the experimental findings (see Fig. 4). A detailed investigation of the whole beam propagation shows that in the power range which we have attributed to a one soliton state the peak power of the final field distribution is rather constant. The dispersive pulse broadening compen-

U. Bartuch

et al./Optics

Communications

linear 6OOW .. ... 1200w -

I

53

134 (1997) 49-54

z=2.25mm

z=4.5mm

I

I1

iaoow ----

r

z=6.75mm

z=9mm

I

~,, ; 0 _.: .:-I-:-

-100

0 x (pm)

100

-200

0

200

‘I

I

-200

0

200

t (fs)

at the end face for different Fio

6. Simulation of the spatial temporal evolution of a tw”d soliton state during propagation through the sample ( P ,” = 1800 W).

sates for the increase of the incident pulse energy. A further spatial compression cannot be obtained. From this point of view the spatial one soliton solution is surprisingly stable with respect to the temporal pulse evolution (see Fig. 5). This picture is changed if satellite structures due to a two soliton state emerge (see Fig. 6). But even for high pulse energies one soliton states at the leading and at the trailing edges of the pulse coexist with a two soliton state situated at the pulse center. The whole spatial temporal object has four humps, but leads to a time averaged power distribution at the end face with three local maxima (compare with the highest power levels in Fig. 4 and 3b).

The temporal dynamics can be detected by measuring the spectral intensity distribution of the outcoming beam (see Fig. 7). The nonlinear dynamics should manifest itself by a spectral broadening. While the spectral width of the initial pulse was about 13 nm the temporal spectrum of a spatial one soliton state was broadened up to 30 nm. A breakup of the spectrum as it is known from experiments in waveguiding structures [lo] could not be observed but a plateau like structure was found for high peak power levels above 1000 W, which corresponds to a spatial one soliton state. From the numerical simulations (see Fig. 8) we deduce that this flat part of the spectrum is due to a spatial average of different parts

z=9mm

z=6.75mm r----l

L---J -200

-50

.: I

t (fS) Fig. 4. Simulated field distribution incident peak power levels.

50

c,::c;~‘,’ ___~,O x (pm)

0

t (W

200

50

I-I

50

-5o

I

-200

0

200

-50

t Us)

Fig. 5. Simulation of the spatial temporal evolution of a one soliton state during propagation through the sample (P, = 1200 W).

870

a80

890 wavelength

900

910

920

(nm)

Fig. 7. Experimentally detected spectrum for different peak power levels.

of the outcoming

beam

54

U. Bartuch et al./ Optics Communicutions linear -

:'

s

4oow 8OOW 12oow

Acknowledgements

.. .. -

This research was supported by the DFG in the framework of the Sonderforschungsbereich 196 (projects Bl, B2 and BS>. The authors are grateful to Dr. G. Onichshukov and Dr. A. Shipulin from the Institute of Applied Physics and Dr. Th. Peschel from the Institute of Solid State Theory and Theoretical Optics at the Friedrich-Schiller-University of Jena for stimulating discussions.

0.8

P $_ .C F c .-

0.6

0.4

m

880

Fig. 8. Numerical power levels.

simulations

134 (1997) 49-54

890 900 wavelength (nm)

of the spectrum

910

920

for different

peak

of the beam giving rise to different nonlinear phase shifts. The experimentally observed spectral asymmetry of the nonlinearly induced broadening could be caused by a spectral variation of the nonlinear absorption. Highly dispersive contributions to the nonlinear absorption can be expected because the operating wavelength is close to half the band gap of the polymer.

5. Conclusions

We have demonstrated spatial soliton formation in a polymer waveguide for the first time. The polymer we used exhibits remarkable optical properties as low linear damping, a high focusing nonlinearity and a rather small amount of two photon absorption. The formation of one and two soliton state could be experimentally observed. The numerical results agree quite well with our experiments. The measured spectral broadening of the outcoming beam demonstrates considerable nonlinear dynamics accompanied by soliton formation. Experimental investigations of soliton collisions are in preparation.

References

ill

S. Maneuf, R. Desailly and C. Foehly, Optics Comm. 65 (1988) 193; S. Maneuf and F. Reynaud, Optics Comm. 6.5 (1988) 325. RI J.S. Aitchinson, A.M. Weiner, Y. Silberberg, M.K. Oliver, J.L. Jackel, D.E. Leaird, E.M. Vogel and PW.E. Smith, Optics Lett. 15 (1990) 471. [31 J.S. Aitchinson, K. Al-Hemiari, C.N. Ironside, R.S. Grant and W. Sibbett, Electron. Lett. 28 (1992) 1879. [41 H.-H. Horhold and M. Helbig, Makromol. Chem., Macromol. Symp. 12 (1987) 229 (review article on polyarylene vinylenes). 151 U. Bartuch, A. BrIhter, P. Dannberg, H.-H. H&bold and D. Raabe, Int. J. Optoelec. 7 (1992) 275. l61T. Gabler, U. Bartuch, F: Michelotti, A. Brker, R. Waldhausl and H.-H. HBrhoId, in: Nonlinear Guided Waves and their Application, Vol. 6, OSA Technical Digest Series (Optical Society of Amerika, Washington DC, 1995) pp. 216-218. [71 Th. Gabler, R. Waldhausl, U. Bartuch, H.-H. H&-hold, R. Stockmann and A. Brkter, Proc. EOS Top. Meeting on Materials for Nonlinear Optics, Val Thorens, Jan. 1996. ts1 J.H. Marburger, Prog. Quantum Electron. 4 (1975) 35; Y. Silberberg, Optics Lett. 15 (1990) 1282; P. Chemev and V. Petrov, Optics Lett. 17 (1992) 172; J.E. Rothenberg, Optics Lett. 17 (1992) 583. 191 V.V. Afanasjev, Y.S. Kivshar and J.S. Aitchinson, in: Nonlinear Guided Waves and their Application, Vol. 6, OSA Technical Digest Series (Optical Society of America, Washington DC, 1995) pp. 82-84. [lOI G.P. Agrawal, Nonlinear Fiber Optics (Academic Press, New York, 1995) p. 94.