Polymers for Second-Order Nonlinear Optics - American Chemical

Chapter 36

Periodic Domain Inversion in Thin Nonlinear Optical Polymer Films for Second Harmonic Generation

Downloaded by UNIV QUEENSLAND on October 13, 2014 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch036

G. Khanarian and M. A. Mortazavi Hoechst Celanese Research Division, 86 Morris Avenue, Summit, NJ 07901

This paper reviews recent progress in the fabrication of periodic structures in poled polymer films for quasi phase matched frequency doubling. In the case of waveguides a new two step poling process is demonstrated for periodic domain 2

inversion, such that the sign of χ changes every coherence length. Quasi phase matched frequency doubling is also demonstrated in periodic free standing films with picosecond -2

lasers resulting in efficiencies of 10 %.

Methods for efficient second harmonics generation(SHG, frequency doubling) have been studied in nonlinear optical(NLO) poled polymers because of the ease of fabrication of thinfilmstructures either as waveguides on silicon wafers or as free 2

standingfilms,and also because of their potentially high NLO susceptibilities χ . A key element to obtaining high SHG efficiencies is phase matching the fundamental to the second harmonic wave(l). We have explored the use of the periodic modulation of the nonlinear susceptibility of poled polymers for quasiphase matching(QPM), and have reported efficiencies as high as 0.01%/W over a distance of 5 mm in a slab waveguide(2,3,4). However the periodic poling was unidirectional consisting of poled and unpoled regions and so only half the length of the waveguide was effectively phase matched. In this paper we describe a new poling technique for periodic bidirectional poling in a waveguide, i.e. periodic 2

reversal of the sign of χ which would utilize the whole NLO medium, and report QPM SHG from a slab waveguide.

0097-6156/95/0601-0498S12.00/0 © 1995 American Chemical Society In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

36. KHANARIAN & MORTAZAVI

Periodic Domain Inversion in Thin Films

We have also demonstrated efficient frequency doubling in free standing periodic NLO films(5,6). Such films would be useful for intracavity SHG devices and for frequency doubling femtosecond lasers. Single free standing poled NLO thin films with thicknesses of the order of 1 pm have been used in the frequency doubling of femtosecond lasers because they exhibit low dispersion(12). Recently, Knoessen et al.(13) have shown theoretically that the same is also true for periodic X films. We describe the fabrication of bidirectionally poled free standing films and demonstrate the enhancement of SHG using picosecond and nanosecond pulsed lasers. Downloaded by UNIV QUEENSLAND on October 13, 2014 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch036

2

Bidirectionally Poled Waveguides The principles of QPM in polymer waveguides have been previously(2,3,4) described in detail. The key is the periodic modulation of the NLO susceptibility every coherence length Lc,

C

4[N(2G»-N(CO)]

(1)

where N(o)) and N(2oo) are the effective indices of the zeroth order modes of the fundamental and harmonic waves, respectively, and X is the wavelength of the fundamental beam. Typically the coherence length in polymer waveguides is between 3 and 10 pm for X = 0.8 pm. Previously we fabricated periodic unidirectionally poled waveguides via a one step poling process(2,3). The fabrication of the waveguide started with a silicon wafer coated with a metallic electrode. Then we spun coated cladding buffer layers and an N L O polymer guide layer. The N L O polymer MO3ONS/MMA 50/50 consisted of the dye 4oxy^'nitrostilbene attached as a sidechain to a polymethylmethacrylate(PMMA) backbone with a 50:50 mole percent composition .The buffer layers consisted either of PMMA or MO3ONS/MMA 30/70. Finally we deposited a top electrode which was patterned into a periodic electrode pattern using lithographic techniques. The whole structure was heated to near the glass transition temperature Tg of the polymer (115 C) and poled with an electric field( 100 V/pm). The result was the periodic modulation of % i periodic regions of poled and unpoled N L O film. Phase matched SHG from such waveguides were reported(2-4) earlier. 2

# e

In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

500

New process for bidirectional poling.The most efficient structure for frequency doubling is one where there is a periodic reversal of the sign of the domains o f / every coherence length(4). This is accomplished by a novel two step poling process where a waveguide is first poled uniformly, a periodic electrode structure is patterned at room temperature and then the waveguide is poled again with the reverse polarity at a lower temperature. This new approach(ll) is based on our observation that the alignment of molecules in an electric field occurs at a faster rate than it's decay in the absence of an electric field, when the temperature of poling is below the Tg of the polymer. Thus the regions underneath the periodic electrodes undergo a reversal in the sign of X while the regions where there is no electrode undergo a slow decay of x . 2


2

2

In order to verify this new approach to fabricating bidirectionally poled waveguides we characterized the dynamics of poling and the resultant modulation depth of periodic poling. Figure 1 shows the dynamics of poling and it's decay for the polymer M O 3 O N S / M M A 50/50. We report the normalized x versus poling time when the electric field for poling was 140 V/pm and the temperature was 95 °C (20 °C below Tg of the polymer).The characteristic rise time for poling was only 20 minutes whereas the characteristic times for poling decay was 3 minutes(initial decay) and 110 minutes(long term decay) at the same temperature. The results of figure 1 verify that NLO polymers pole more rapidly than they decay when the temperature is below the Tg of the polymer. Recently, Kuzyk et al. (7,8) have reported similar observations. 2

Modulation depth of periodic poling.We have quantified the improvement in modulation depth by the two step poling process versus the one step poling process reported earlier (2-4). Let us suppose that the normalized x as a result of poling with positive polarity is 1. In the one step periodic poling process only 1/2 of the waveguide is periodically poled and so the modulation depth is 1/2. In the two step poling process, uniform poling with positive polarity is followed by periodic poling with the opposite negative polarity, and so the maximum theoretical modulation depth is [1 - (-l)]/2 = 1. In practice, the modulation depth as a result of the two step poling process is a function of the rise time of the second poling step and the decay time of the originally poled region. Figure 2 shows the modulation depth obtained as a function of poling time for the polymer M O 3 O N S / M M A 50/50 at 2






501


502


95 °C and 140 V/pm. Initially the modulation depth is zero because there is a uniformly poled substrate as a result of the first poling step. Then the modulation depth rapidly increases with time as the second poling step takes effect. Interestingly, the maximum modulation depth is reached 50 minutes after the beginning of the second poling step and has a value of 0.65 for this polymer and these poling conditions. It is clear that the two step bidirectional poling process gives greater modulation depth than the unidirectional poling process(0.5) described earlier(2-4) but does not reach the theoretically maximum value of 1. One could optimize the modulation depth by adjusting the poling temperature which in turn influences the poling rise and decay times. When the two step poling process is carried out for very long times the modulation depth approaches 0.5 because the originally poled regions are completely decayed to zero and only those regions underneath the periodic electrodes are now poled. Yilmaz et al .(9,10) have proposed and demonstrated an alternative approach to creating periodic bidirectionally poled NLO films. The NLO film was sandwiched between two electrodes with the top electrode transparent to visible radiation. A laser beam was focussed tightiy onto the film to locally heat it to near Tg while an electric field was applied. They demonstrated that poledfilmsof lateral dimensions of about 7 pm could be written. By moving the film under the laser beam and periodically reversing the poling field, they claimed that they could fabricate a film with periodic domain reversal. Both our and Yilmaz's approach should be evaluated critically in terms modulation depth of poling, waveguide loss and SHG output efficiency from waveguides. Fabrication of waveguides.The fabrication of a bidirectionally poled SHG waveguide is shown schematically infigure3. A waveguide structure is fabricated as described above but with one difference. A thin layer of polyimide polymer(SIXEFF, Hoechst) is spun coated on top of the upper cladding layer. The purpose of the polyimide layer is to stop any deformation of the waveguide/cladding layer during the periodic poling process due to electrostriction. The polyimide has a Tg=250 C and so it does not deform at the poling temperatures of 95 °C used in this work. Then the two poling steps are carried out. The waveguide is first uniformly poled near Tg(l 15 °C) for a time t = 1-5 minutes at 140 V/pm and cooled down with the electric field on. The electrodes are periodically etched using a room temperature lithographic process so that no poling decay occurs. Then the second


36.

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503


poling process takes place. The polarity is reversed and the waveguide structure is heated up to a temperature 20 °C below Tg. The electric field (-140 V/pm) is applied for t = 30 minutes to the periodic electrodes and then cooled down with the field on. According to the results of figure 2 the modulation depth should be near 0.5 for that time period of poling. We did not measure the optical waveguide losses. The periodic waveguides were tested for phase matched SHG using a tunable Q switched nanosecond laser with a wavelength near 1.3 pm.The laser beam was coupled into the slab waveguide with cylindrical lenses and the output SHG signal measured as a function of wavelength. SHG versus fundamental wavelength is shown in figure 4. One notes the narrow tuning bandwidth showing that phase matching has occured over a distance greater than 3 mm. These results verify that a two step poling process is a viable approach to creating periodic domain reversal in polymer films. More work needs to be done to find the optimum temperature of poling, the optimum time for poling and characterizing the modulation depth and waveguide losses in these bidirectionally poled films. The principle of the two step poling process described here may also have applications wherever % regions of opposite sign are needed, for example, in a push pull Mach Zehnder modulator. 2

Bidirectionally Poled Free Standing Films NLO polymers have also been studied for through the plane applications such as modulators for optical interconnects(14), photorefractive elements(15) for image processing and frequency doubling(5,6). The advantage of polymers is that large area mechanically robust NLO elements can be fabricated either by spin coating or by a film extrusion process. In contrast inorganic and organic crystals are difficult to grow and generally small in size. In the case of frequency doubling of femtosecond lasers there is another advantage in so far that very thin NLO films can be fabricated which have low group velocity dispersion(12). Recently we proposed and demonstrated quasi phase matched SHG from a periodic free standing film(5,6) depicted in figure 5. The thickness of each film equaled a coherence length at the optimum phase matching angle of 50 °. Knoessen et al.(13) analysed theoretically the transmission of a femtosecond pulse and the buildup of a femtosecond SHG pulse through the periodic structure depicted in figure 5 and concluded that under certain conditions, no pulse broadening should occur.


504


1ST STEP

Pole with positive polarity


electrodes

High Tg polymer buffer waveguide

t t t t t t t t t t t t

Lithography to form periodic electrodes Pole with reverse polarity at lower temperature

2ND STEP

High Tg polymer ^buffer waveguide

electrodes

Figure 3. Schematic of fabrication steps of bidirectionally poled SHG waveguides.

o N

O

z

(0

1294

1296

1298

1300

1302

Wavelength (nm)

Figure 4. SHG versus fundamental wavelength from bidirectionally poled waveguide




Fabrication of free standing films. The fabrication of these periodic structures was as follows. The poled polymer used was MO3ONS/MMA with molar composition of 10/90. A silicon wafer was first vacuum deposited with 500 A of gold(Au). This metallic layer served as the poling electrode and as the liftingoff layer for making free standing films. The polymer was spun coated and dried to give a thickness of 5.9 pm. The entire film surface was deposited with 1000 A of Au and poled at a temperature of 110 °C and at a field of 70 V/pm. After the Au upper electrode was etched off, small squares (0.7x0.7cm) were cut into the film using an ablation process with a LPX 100 Excimer laser (X = 193 nm). The wafer was then immersed in Au etchant for a few minutes until the Au dissolved and the square pieces floated to the surface. The films were picked up and placed on a Teflon sheet making certain that the poling direction was alternating between adjacent layers, resulting in a structure depicted in figure 5. Samples were prepared with N= 1, 5, 10,20, 30, and 43 layers. 0


0

By reversing the direction of poling one obtains a periodic structure whose X changes sign. When fundamental radiation is incident at angle 0, the mismatch in propagation vectors is 2

AK(0) = K

2(D

10

- 2K «

* Lc V 1 - (sinO/n®)

2

G)

(2)

0)

where K®= 27in cose A, (G3=27CC/A.) is the wave vector and 0® is the internal angle of incidence, Lc is the coherence length of the nonlinear optical material, co,2co ^ the refractive indices at the fundamental and harmonic frequencies, X is the fundamental wavelength and c is the speed of light, respectively. When the film thickness is A = jc/AK(0 ) then the condition for quasi phase matching at 0 is satisfied and the second harmonic adds up coherently. The optimum angle 0 for obtaining maximum SHG is determined by a tradeoff between the projection of the X2 tensor on the incident optical field and the linear t* and nonlinear T"2G> Fresnel transmission factors, and is near 50-60 degrees. A pulsed Y A G laser (A,=1.06 pm) was incident on a periodic stack and the SHG was measured as a function of incident angle. The results are shown in figure 6. One notes that as the film thickness increases, the angular dependence of the SHG signal becomes sharper indicating a longer phase matching length. Figure 7 depicts the relative efficiency n

m

m

m

0


505


506


Incident Radlatlon(p polarized)

Figure 5. Schematic of periodic free standing film

Angle of Incidence (degrees)

Figure 6. Normalized SHG from periodic free standing films for different film thicknesses.



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KHANARIAN & MORTAZAVI


507

Figure 7. Relative efficiency of S H G versus fundamental power for a periodic free standing film of thickness 160 μπι.

of S H G from a periodic stack of thickness 160 μπι as a function of fundamental intensity.

One observes bright green light

from the polymer stack with an

2

efficiency of 10~ %. Further improvements will come from using nonlinear optical polymers with larger χ 2 , more efficient poling and combining it with mechanical stretching(5). Therefore it is possible to fabricate polymer "synthetic crystals" for efficient frequency doubling and parametric processes.

Conclusions We have demonstrated experimentally in this paper that periodic structures with domain reversal are possible in both waveguides and free standing films, and that efficient quasi phase matched SHG is observed. In the case of waveguides we describe a novel two step poling process with a reversal in the sign of the poling field resulting in periodic domain reversal along the length of the waveguide. In the case of the free standing films, we pole a film and then physically reverse the direction of the film during the fabrication of the multilayer film.

References 1. Zyss, J. and Chemla, D.S. in " Nonlinear Optical Properties of Organic Molecules and Crystals", Zyss, J. and Chemla, D.S.(eds.)(Academic Press, Orlando, 1987) chapter II-1,


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2. Khanarian, G.; Norwood,R.A.; Haas, D.; Feuer, B . and Karim, D. Appl. Phys. Lett. 1990 ,57, 977 3. Norwood,R. A . and Khanarian,G. Elec. Lett. 1990, 26, 2105 4. Khanarian,G. and Norwood,R. A . in "Nonlinear Optics: Fundamentals,Materials and Devices" S. Miyata (ed.)(Proc. Fifth Toyota Conf.,Elsevier, 1992) p.461 5. Khanarian, G . ; Mortazavi, M . A . and East, A . J. Appl. Phys. Lett. 1993, 63, 1462 6. Mortazavi, M . A . and Khanarian,G., Optics Lett. 1994, 19, 1290


7. Zimmerman, K.; Ghebremichael, F.; Kuzyk, M . G . and Dirk, C. W. , J. Appl. Phys. 1994, 75,1267 8. Ghebremichael, F. and Kuzyk, M . G., J. Appl. Phys. (in press, 1995) 9. Yilmaz, S.; Bauer, S.;Wirges, W. and Gerhard-Multhaupt, R., Appl. Phys. Lett. 1993, 63, 1724 10. Yilmaz, S.; Bauer, S. and Gerhard-Multhaupt, R., Appl. Phys. Lett. 1994, 64, 2770 11. Khanarian, G . , to be published 12. Mortazavi, M . A . ; Yankelvich, D. ;Dienes, A.;Knoesen, A . ; Kowel, S. T. and Dijali,S., Appl. Opt. 1989, 28, 3278 13.Sidick, E . ; Knoesen, A . and Dienes, A . , Opt. Lett. 1994, 19, 266 14. Yankelvich,D.R.; Hill, R. A . ; Knoesen, A . ; Mortazavi, M.A.;Yoon, H . N . and Kowel, S.T., Photonics Lett. 1994 , 6 15. Ducharme, S.; Scott, J. C.; Tweig, R. J. and W. E . Moerner, Phys. Rev. Lett., 1991, 66, 1846 RECEIVED March 10, 1995


Polymers for Second-Order Nonlinear Optics - American Chemical

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