Second Harmonic Generation by Counter-Directed Guided Waves in

Chapter 34

Second Harmonic Generation by Counter-Directed Guided Waves in Poled Polymer Waveguides 1

1,3

2

2

A. Otomo , G. I. Stegeman , W. Horsthuis , and G. Mohlmann Downloaded by GEORGETOWN UNIV on June 4, 2018 | https://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch034

1

Center for Research and Education in Optics and Lasers, University of Central Florida, Orlando, FL 32826 Akzo Electronic Products, Arnhem, Holland 2

Doubling the frequency of laser light has been investigated since the early days of nonlinear optics. Until recently the goal has been to extend the frequency range of sources, mostly for scientific investigations. With the advent of inexpensive semiconductor lasers in the near infrared, interest has been strong in generating milliwatts of blue light for data storage and xerography. Still relatively unexplored are applications to signal processing, for example correlation, frequency demultiplexing etc. Again signal levels in the milliwatts range are required. When dealing with sub-watt power inputs, either guided wave geometries or doubling in highly resonant cavities isrequired.(1,2) In fact most of the work has concentrated on interaction geometries in which the fundamental and second harmonic waves travel in essentially the same direction. In terms of waveguides, the best results to date have been obtained with ferroelectric materials such as LiNbO , K T P etc. (1,3,4) Organic materials, including poled polymers, have been shown to have large nonlinearities, but to date the loss in the blueregionof the spectrum has been too large for these materials to be truly competitive with the oxide materials for blue light generation.(5) The problem has been that the harmonic has to travel multimillimeter distances to achieve large enough signal intensities. 3

We have been pursuing a different sum frequency generation geometry in waveguides. Although the original goal was for signal processing, demultiplexing in our case, our approach may also prove useful for efficient blue light generation. (7) In our scheme, two oppositely propagating beams are mixed in a nonlinear (x^ active) waveguide, and the resulting signal is radiated out of the waveguide, normal to the surface. Because the two input fields pass through (sample) each other, the timedependence of the output signal contains information about the convolution of the input signals.^ A nonlinear polarization source field polarized in the plane of the surface is required. Counter-propagating SHG has been demonstrated previously in ferroelectrics and semiconductors.(6,9) The best conversion efficiencies have been obtained in modulated AlGaAs layered structures. (10) Recently we have been investigating poled 3

Corresponding author 0097-6156/95/0601-0469$12.00/0 © 1995 American Chemical Society Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

470

POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

polymers and LB films for this purpose. (11,12) In this paper we discuss poled polymers for counter-propagating SHG. The Counter-Propagating Geometry The slab waveguide interaction geometry is shown in Figure 1. We write the incident guided wave fields as Efr,t) • ± * / x ) a ( z ) « p * * *

M

• c.c.

Downloaded by GEORGETOWN UNIV on June 4, 2018 | https://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch034

±

(1)

where the + and - signs refer to guided waves propagating along the + and -z axes respectively. For example, for TE polarized input fields, the case for poled polymer films, i = y. The fields are normalized so that | a (z) | is the guided wave power per unit distance along the y-axis. Note that we have chosen the case in which the input frequencies and propagation wavevectors, a>and P respectively, are not necessarily equal Assuming that the waveguide consists of a non-centrosymmetric material, the nonlinear polarization induced is 2

±

±

±

2 (2) +

i

[

P

f

a

(

x

>

z

)

^-(M>]

e

t

C

c

]

The first two terms correspond to co-propagating second harmonic generation (SHG), one for each of the input waves. The third term, 9 , is a polarization source oscillating at the frequency co + G>_ with wavevector parallel to the surface of P P_. It leads to the sum frequency signal of interest here. The fourth term at a> - a> oscillates at a low frequency and has too large a spatial wavevector P + P_ to lead to radiation terms for o> - co . For the case of interest ±

+

+

+

+

+

P (x,z) = 2^(-[( ) oJ; o ,)< c

It proves more convenient to express the result of this nonlinear process in terms of input and output powers as

1

P ( 2 o ) = A *—.P( * a>_ has potential applications to real time wavelength demultiplexing. As indicated in Figure 1, the sum frequency is radiated at some angle away from the surface normal Conservation of wavevector in the plane of the surface gives +

Lindsay and Singer; Polymers for Second-Order Nonlinear Optics ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

472


sin [AO]

(P. - p > ( )

(8)

+

In these equations, when we write co as the argument for the refractive indices we mean co - (G>+G>_)/2. Note that if there is a cladding above the film, but the signal is detected in air, then from Snell's law the angle in air is obtained by replacing n(2o>) by 1. The key point is that the angle is effectively linear in the frequency (wavelength) difference between the two input signals. Therefore if one is a reference beam, then the different wavelength components of the input signal beam are separated spatially, with resolution A A/A - L/W. This is one of the applications which we are pursuing. +


c

Fabrication of DANS Poled Polymer Waveguides The waveguide fabrication process consists offivedistinct steps. First, gratings are milled into the substrate glass surface to facilitate coupling into guided modes. Second, parallel electrodes are deposited on the glass surface for poling of the polymer. Next, the polymer film is spun on to a predetermined thickness. Fourth, the waveguide structure is made by photobleaching through an appropriate mask. And finally, thefilmis poled at high temperatures. Our technology did not initially allow us to couple high powers into channel waveguides by focusing the input light onto the end facet of the waveguide. Thus we adopted a grating coupler approach to excite the guided waves, coupled with a tapered section to decrease the beam width down to single mode channel dimensions. The grating couplers, the taper sections, and the single mode channel waveguide region are shown in Figure 3. The grating was fabricated in the glass substrate prior to spinning on the DANSfilmin the usual way. That is, photoresist was spun onto the substrate and exposed to the interference pattern of a He-Cd laser to produce a period of about 500 nm. The exposed photoresist was "developed" and the grating ion milled into the substrate to a depth of approximately 100 nm. The particular interaction being used here requires large effective nonlinearities in the plane of the waveguide, i.e. CL( )(2CI>;G),CO). Thisrequiresinplane poling of polymers. Although transverse poling, primarily corona poling, has been by far the most frequently used approach, there are priorreportsof successful inplane poling. (13-16) Typically the problem has been arcing (electrical discharge) which has limited in many cases the maximum applied voltages to 50 to 100 2

Vl\xm.(17)

We used the side-chain polymer DANS which has been investigated extensively for its application to electro-optic devices. (18,19) The DANS molecule had about a 50% loading and was attached to the backbone polymer as a side-chain. Parallel aluminum electrodes were deposited with a spacing of approximately 20 pm. Sub-micron thickfilmswere spun on. Using standard techniques we measured slab waveguide losses as low as 0.27 db/cra Values of 0.7 db/cm were quite typical of high quality waveguides. Channel-like waveguide structures were formed by photobleaching through an


OTOMO ET AL.

Second Harmonic Generation by Guided Waves 473


Figure 1. The waveguide counter-propagating SHG geometry for both equifrequency and unequal frequency inputs (CD'*» CO).

DANS sin(/*

(2fl,)

/(*)

substrate

*): -4-

1/ I -

V V

I

v

+d

u

-d

Figure 2. The variation in the polarization source field (solid line) and the generated second harmonic electricfield(dashed line) along the transverse direction (along the normal to the surface).

Figure 3. The SHG device structure viewed from the direction into which the SHG signal appears. Note the grating couplers at either end, the tapered transition region and the central narrow channel waveguide.


474



appropriate mask Because photobleaching reduces the retractive index, it is necessary to block out the waveguiding region when iUuminating the sample . In order to design the required structures it was necessary to first calibrate the photobleaching process. There is some debate about whether the photobleaching of DANS is due to cis-trans isomerization or due to other mechanisms such as oxygenation of one of the DANS bonds, or both. (20-22) Independent of the process, we begin our modelling by assuming that the number of unchanged (unbleached) molecules decreases exponentially with the energy deposited in a film small on the scale of the maximum inverse absorption coefficient. Thus the contribution to the linear susceptibility, Ax, due to the DANS molecules with an intact charge transfer state is given by (23)

J0

(9) Jo

where P is the bleaching constant and the single and double primes refer to the real and imaginary parts respectively. The refractive index, real [n(t)] and imaginary [ic(t)] b

"(0 • - ^ / [ v A x ' ( ' ) ]

J

ff

• [e

2

b

• Ax"(01 • [vAx'COH

V2

parts evolve as

where e refers to the background dielectric constant due to all of the other resonances in the UV part of the spectrum The evolution of the photobleached species with distance into a film is obtained by modelling thatfilmas a series of very thin slabs, thin enough so that the illumination intensity can be considered constant over each slab. The problem is then solved on a computer to calculate the index distribution in the film The transmission through a DANS film (absorption maximum at k = 430 nm) was measured as a function of time. The source was the k = 442 nm line from a HeCd laser. The results are shown in Figure 4. Thefitto the data is within the spread b


34. OTOMO ET AL.

Second Harmonic Generation by Guided Waves

475

of the experimental data, yielding a value of P = 7.5x10" m /J. This allowed us to calculate the refractive index as a function of depth into a thick film, as shown in Figure 5 for an illumination intensity of 40mW/cm . This information is needed to obtain the photobleaching times etc. for the formation of channel waveguides. The standard effective index method was then used to model the photobleached channel waveguides.(24) The conditions used to form effectively single mode waveguides in both dimensions are shown in Figure 6. Note that the inplane mode structure allows a higher order mode, but it is difficult to excite because it is very near cut-off. Using grating couplers only one mode at a time is excited so that the higher order mode plays no significant role in our devices. The taper section, shown in Figure 7, was used to increase the intensity in the channel waveguide by a factor of 12.5 over that of the coupling region, which increases the SHG signal by a factor of 150. The form of the taper is chosen to minimize radiation losses out of the waveguide, i.e. an adiabatic taper for which 0 < A/[N (z)W(z)] where N = P(z)/kQ is the effective index.(25,26) It is given by b


2

eff

e f f

W(z) -

2 — * * wL

(12)

In our case L = 1.9mm, W = 50pm, W = 4pm, X = 1064nm and N = 1.62. The taper was fabricated by photobleaching, leaving the region outside the taper with a lower refractive index. The additional propagation loss due to the taper and channel waveguide structure was measured to be only 24% (relative to a slab waveguide on the same substrate). The total throughput, including input coupling, taper transitions, channel propagation and output coupling was about 10%. The effective channel device length was 1 cm. Over a period of one year we made various improvements to increase the maximum poling voltage from the frequently reported 50-100 Vl\xm.(27) Initially we found arcing to occur at 100 V/pm or less, typical of results reported in the literature. We were poling just below the glass transition temperature of 142°C, using glass slides as substrates with air above the film. The current flow into one of the electrodes was monitored and found to be tens of pA just before film damage occurred. This was improved by using high purity fused silica substrates, eliminating the charge injection and migration, and increasing the breakdown voltage to over 100 V/pm. Most of the experiments reported here were performed with these samples. At higher poling fields, we found that arcing occurred along the air-film interface. In order to eliminate this problem we added a cladding layer of lower refractive index, spun on to a thickness of about 15 pm. As a result, poling voltages up to 370 V/pm were recently achieved before breakdown occurred The films were poled for about 30 minutes and then the temperature was decreased slowly (

Second Harmonic Generation by Counter-Directed Guided Waves in

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