GaAs-Bragg reflectors

Microelectronic Engineering 19 (1992) 53-56 Elsevier

53

Experiments on Optoelectronic Bistability in Distributed A1As/GaAs-Bragg Reflectors S. Knigge a, S. Zumkley% G. Wingen a, O. Humbach% C. Chaix b and D. JS~gera FG Optoelektronik, Universit/it Duisburg, Kommandantenstr. 60, D-4100 Duisburg, Germany. b Riber, 133/137 Bd. National, F-92503 Rueil Malmaison, Cedex, France.

Abstract Vv'e present measurements of optoelectronic bistability in hybrid distributed A1As/GaAs Bragg reflectors. A threshold optical input power of 40 #W is observed leading to an estimated switching energy of 0.1 fJ#m -2. Using a voltage shift of 60 V and an optical input power of 100 nW. an electrooptical modulation of 40 % is obtained.

1. I N T R O D U C T I O N Because of their potential applications to optical information processing optically bistable memory, switching and logic devices have received great interest in the past. The most promising device structures for 2-D arrays are monolithic microresonators, i. e. epitaxially grown Fabry Perot cavities [1, 2, 3, 4] or monolithic distributed Bragg reflectors (DBR), where optical bistability as well as optical solitons have been predicted [5, 6, 7] and observed both in model experiments [8] and for short optical pulse excitation [9].

2. E X P E R I M E N T S In this paper, we present experimental results of quasi static optoelectronic bistability in a hybrid 20 pair A1As/GaAs DBR structure as shown in Fig. 1. The nominal thicknesses of the GaAs and AlAs layers are 58 nm and 69 nm, respectively. In Fig. 2 the measured optical reflectivity is plotted as a function of the optical wavelength. When applying a voltage V0 at the device, the reflectivity can be changed which has been shown recently [10]. In the present case, the reflectivity change is 40 % at V0 = 60 V and a wavelength of )~ = 885 nm for small signal operation, i.e. for an optical input power of 0.1 #W. Increasing the optical input power, optical, electrooptical, optoelectronic and electrical bistabitity for both wavelengths t = 880 nm and ~ = 885 nm are observed. Fig. 3 shows the measured optical bistability of the device for an applied voltage of V0 = 60 V. It is further concluded from Fig. 3 that a threshold voltage for optical bistability exists. In particular, below V0 = 50 V no optical bistability has been detected. It should also be noted, that for increasing Pin switching up occurs at A = 885 nm and 0167-9317/92/$05.00 © 1992 - Elsevier Scienc'ePublishers B.V~ All rights reserved.

54

S. Knigge et al. / Optoelectronic bistability

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switching down at A = 880 nm corresponding to different slopes in the reflection spectrum of Fig. 2. In Fig. 4 electrooptical bistability is shown. As can be seen at A = 880 nm the reflected power switches down whereas at A = 885 nm {not shown in the Figure), the reflected power switches up according to the behaviour in Fig. 3. On the other hand the plot of experimental optoelectronic bistability in Fig. 5 shows switching up of the current. The same behaviour occurs at A = 885 nm (not plotted here), i. e. independently of the slope in the reflection spectrum of Fig. 2. In this case tile necessary optical input power for switching decreases with increasing applied voltage V0. In the present device the threshold voltage for optoelectronic bistability is not influenced if the wavelength is varied between 880 nm and 885 nm. A further experimental observation, not shown here, should also be mentioned, i. e. an electronic bistability where with decreasing optical input power Pin the switching point

S. Knigge et al. / Optoelectronic bistability

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Figure 5. Current versus optical input power for different, voltages shifts to higher values of V0. Using a fixed optical input power Pi, the current switches from lower to higher values independently from the slope in Fig. 2 but at the same value of the applied voltage V0. Because a voltage V0 of more than ca. 80 V would most probably destroy the sample, no evidence could be made whether the lowest measured input power for this bistability of Pin = 40/zW is a threshold power of the effect or not.

3. D I S C U S S I O N Our quasi static measurements demonstrated, that optical bistability occurs in periodic devices as predicted by Chen and Mills [5]. Moreover, in the present hybrid structure electrooptical, optoelectronic and electronic bistability have also been observed. Because tile value of the threshold optical input power is extremely small, cooperative optoelectronic and electrooptical effects at the edge of the stopband seem to be responsible for the observed optical bistability [11]. Using the measured threshold optical input power for optical bistability of 40 #W from Fig. 3 with a spot diameter of 60 #m and a switching time of 10 ns as measured in a setup using the sample as an electrooptical modulator Ill], we estimate a switching energy of only 0.1 fJ#m -2. This is one of the lowest values ever reported in the literature.

4. S U M M A R Y In summary, we have measured for the first time quasi static optical, electrooptical, optoelectronic and electrical bistability in a periodic A1As/GaAs superlattice with applied voltage. The bistability shows an extremely small optical switching energy of only 0.1 fJ/tm -2. This is to our knowledge the first experimental proof of the predicted bistabtity and the first quasi static observation of optical bistability in nonlinear periodic structures.

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S. Knigge et al. / Optoelectronic bistability

Acknowledgment This study is part of a project which is financially supported by the VolkswagenStiftung under contract No. 4150.10/165.

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