DYNAMIC OPERATION OF A NONLINEAR WAVEGUIDE FOR OPTICAL LOGIC

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DYNAMIC OPERATION OF A NONLINEAR WAVEGUIDE FOR OPTICAL LOGIC

A. Niepceron, A. Koster, N. Paraire, M. Carton, S. Laval

To cite this version:

A. Niepceron, A. Koster, N. Paraire, M. Carton, S. Laval. DYNAMIC OPERATION OF A NON-

LINEAR WAVEGUIDE FOR OPTICAL LOGIC. Journal de Physique Colloques, 1988, 49 (C2),

pp.C2-289-C2-292. �10.1051/jphyscol:1988267�. �jpa-00227684�

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DYNAMIC OPERATION OF A NONLINEAR WAVEGUIDE FOR OPTICAL LOGIC

A. NIEPCERON, A. KOSTER, N. PARAIRE, M. CARTON and S. LAVAL

Institut dlElectronique Fondamentale, CNRS UA-22, Universite Paris-Sud, Bdt. 220, F-91405 Orsay Cedex, France

Resume : Un guide d'onde non lineaire de silicium sur saphir, ayant subi un recuit est dtudid expdrimentalement en regime impulsionnel. L'dtude met en evidence l'existence de deux commutations subnanosecondes consdcutives, permettant l'usage de ce dispositif en porte logique optique.

Abstract : Experimental study of an annealed silicon on sapphire nonlinear waveguide in the pulsed regime can show two consecutive subnanasecond switchings allowing its operation as an optical logic gate.

The system most widely studied leading to all-optical switching is the nonlinear (NL) Fabry Perot interferometer. However a growing interest appears for nonlinear waveguide structures /1-4/. In this paper, we?report the experimental study of a waveguide structure operating in a pulsed regime and we mainly concentrate on its tempera behaviour. Owing to the relative values of the excitation pulse duration we have used (bt c 20 ns) and the NL material response.time ^{( T}^sa few hundred picoseconds) two opposite switchings can occur during the excitation pulse due to the competition between electronic and thermal nonlinearities and their characteristics be optimized in order to be used in a optical logic device.

The basic element under study is a silicon on sapphire waveguide whose dispersive nonlinearities are associated with the indirect band edge absorption of silicon. Such a device can be considered as a resonant cavity with a very high Q factor leadihg to a large enhancement of the EM field in the nonlinear medium and consequently to a great sensitivity.

This also yields sharp variations in the reflection and transmission coefficients of the structure for small refractive index variations. A diffraction grating whose groove spacing and depth modulation can be chosen to optimize the coupling efficiency /5/ is etched a t the silicon surface.

In a first device /6/ (figure 1) the grating is covered with a silver layer to improve the coupling efficiency. The recombination time of the photogenerated e-h pairs evaluated from photoconduction measurements is T c 200 ps.

As the device is 2nlightened by a low power c.w. Md-Yag laser ( h = 1.064 ~ r n ) and the incidence angle 0 is varied, 'measurements performed on the reflected beam exhibit a sharp resonance for a given value 8 0 . This resonance corresponds to the TEo rode of the guide and presents a half minimum width A 8 = 0.16O.

silver

1 = c.oepm ^{T I M E}

-

_10"s

Fig.1-a Structure of device 1 : Ag-Si-Sapphire

b Nonlinear regime operation : temporal variations of 1. the incident intensity, 2. the reflected intensity I R ,

3. the guided light intensity IG for P I C = 500 kW/cmZ, 0

-

80 = 0.04O

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988267

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C2-290 JOURNAL DE PHYSIQUE

As the device is excited by a Q-switched Nd-Yag laser delivering 20 ns light pulses nonlinear effects appears. However a single subnanosecond switching, can be observed for 8 > 80

(Fig. 1-b), none for 5 < 80 /6/.

In order to understand and improve the operation of this device for optical logic a theoretical analysis of the dynamic phenomena occuring in the structure under study has been carried out / 7 / , assuming that those phenomena evolve in such a way that the carrier density and the temperature can be considered as uniform in the silicon film depth. Furthermore, calculations are performed in the plane wave approximation : these two assumptions, valid if the excitation pulse is longer than a few nanoseconds and for large diameter ( @ t 1 mm) incident light beams,would fail in other circumstances. In our model, photoabsorption of light in silicon associated with e-h pair generation has been considered : this is an instantaneous phenomenon inducing a refractive index variation. This is followed by nonlinear photoabsorption by free carriers, their thermalization, and e-h pair recombination (with a characteristic time T I . The carrier de-excitation which is mainly non radiative in silicon heats first silicon, then the whole device, inducing modifications of the refractive index and of some of the geometrical parameters of the device. These various factors have been taken into account to describe our experiments : Fig.2-a gives the theoretical results, to be compared with experimental ones given in Fig.1-b. In Fig. 2-b the time variations of the refractive index, its electronic (6ne) and thermal ( 6 n ~ ) contributions are reported. They show that, for the device under study and using 20 ns excitation pulses, electronic (6ne ⁽0) and thermal ( 6 n ~ > 0 ) effects first cancel one another, then thermal effects predominate, yielding for 8 > 80 a switching whose duration is shorter than that induced by, any one of these 2 effects.

I I l , l , I ,

0. 50. t (ns) 0. 50. t (ns)

Fig.2-a Theoretical results giving the temporal variations of the incident intensity (I), IR(~), I G ( ~ ) for P i c = 500 kW/cm2 and 8

-

80 = 0.04O

b Temporal variations of silicon refractive index (an), with its electronic (6ne) and thermal ( 6 n ~ ) contributions in the same experimental conditions.

According to the agreement between theory and experiment, it is possible from our model, to foresee the potentialities of the presented structure. In particular, calculations show that observation of one or two switchings during the exciting pulse depends on the relative value of T and At : if thermal effects are delayed, a first switching from electronic origin can precede a second one, similar to the previously described one : this has been widely studied experimentally using a second device in which the recombination time has been increased, using a chemical annealing treatment, to T = 800 ps, At being held constant At = 20 ns.

In this device, represented in Fig.3-a the grating has a period d = 0.31 pm and a depth modulation h s 400 nm ; an antireflection coating has been deposited on the sapphire. No silver layer is used. As an excitation beam is sent onto the device with an incidence angle

8 , one can detect the transmitted (IT) reflected (IR) or guided (IG) beam. The use of a

shield, limiting the incident beam allows observation of the guided one in the shadowed region of the device where the guided wave intensity exponentially decreases.

As the incident beam is not a plane wave but a gaussian beam (with @ = 1.2 mm at l/e2) in order to discriminate temporal from spatial factors, light issued from a 100 pin spot of the device is observed using an optical system (Fig.3-b) to avoid spatial integrating effects.

Measurements are performed using either the TMo mode resonance (8 M = 2.33O b 8 ~

=

0.08O) or the TEo mode one (BE = 4.20°, b 8 ~

=

0.05O). Indeed experiments are easier on the TMo mode as

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grating d. o.31pm h.40nm

0.7pm (silicon) 32s pm (sapphire)

-

itld

.

excitation

I

guided-mode deexcitation

Fig.3-a Structure of device 2 : Air -Si

-

Sapphire b Experimental set-up

In the linear regime, a spatial study of the TMo growth has been performed on the guided beam, the incident beam being translated with respect to the device and the detection system.

Results for 8 = en are reported in Fig.4-b which show that the guided wave amplitude maximum is shifted b y A x = 300 um with respect to the incident beam. Indeed, this value depends on O and on the incident power. However, in the following, all the results will be presented for A x = 300 Ipm.

Fig.4-a Transmission coefficient and guided light intensity versus the incidence angle

-

recorded with a c.w Nd-Yag laser

0 *measured with a low power Q-switched Nd-Yag laser

b Spatial dependance of the incident and of the guided intensities (for 8 = en) in the linear regime.

In the nonlinear regime, nonlinear effects can be seen for 8

5

^{B M , E}and small values of the incident power but two consecutive switches only occur for 8 < ~ M , E :

For 8 < ~ M , E , as the incident peak power Pic increases, a first switching appears,

simultaneously on any detected beam (IT, IR, IG), corresponding to a switch down

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on the guided beam (Fig.5-a). Its duration decreases as PIC increases for a given 8 to reach a value which is within the detection limit time ( t ~ : 1.3 ns). For large values of Pic, this switch occurs for a given instant ti independent of e (Pic being constant) or independent of Pic

(for 8 constant) (Fig.5-b,c).

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C2-292 JOURNAL

DE

PHYSIQUE

As the first switch is already present, for increasing values of Pic, a second switch appears, corresponding to a switch up

( t )

of the guided beam. This switch occurs at an instant t ( tl and is longer than the first one

.

For a given value of 8, the larger the incident peak power Pic (or for a given Pic the smaller the detuning 8-&.E, ie the greater the initial coupling efficiency) , the earlier the switching appears and the longer the low level transmissivity lasts. Between the two switchings, the transmission and reflection coefficients keep a constant value.

As in the case of the first device, these observations are qualitatively well explained using our theoretical model, that allows further generalizations /7/ :

Fig.5 Experimental results

a Temporal variations of the incident (1) and guided intensities for 8 - 8~ = -0.06O and Pic = 300 kW/cm2 (2), 500 kW/cm2 ( 3 ) , 700 kW/cm2 (4)

b Temporal variations of the incident (1) and guided intensities, for Pic = 1 MW/cm2 and 8

-

8n =: -.10°(4), -.12O(3), -.14O(2)

c Temporal variations of the transmission coefficient for Pic = 1,7 MW/cm2 and 8

-

BE =: -.12O(4), -.10°(3), -.08O(2), -.06O(I).

In order to obtain two well contrasted and fast switchings with an adjustable delay, ie a manageable optical logic device, thermal effects must be attenuated acting on the ratio A ~ / T or on the device heat capacity, and a better coupling efficiency must be achieved. In these circumstances, as thermal effects are delayed, the switch up has a purely electronic origin and the larger Pic, the shorter its duration. Moreover, the switch down occurs later, after the incident power has reached its maximum Pic, allowing a NOR gate operation for the device.

At last, accurate power thresholds appear for the switching emergence.

Let us note however that in any device where nonlinearities are associated with absorption, which concern any semiconductor device, the refractive index does not depend only on the instantaneous intensity but on its temporal variation : in the short time range, perfect bistability cannot be reached.

References :

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44,

164 (1984)

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/4/ CHEN Y.J., CARTER G.M., SONEK G.J., BALLANTYNE J.M., Appl.Phys.Lett.,

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272 (1986) /5/ VINCENT P., PARAIRE N., NEVIERE M., KOSTER A., REINISCH R., J.Opt.Soc.Amer.,

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(1985)

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