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OPTICAL SWITCHES WITH COMBINED BRAGG REFLECTORS AND DOPING SUPERLATTICES
A. Kost, E. Garmire, M. Kawase, A. Danner, H. Lee, P. Dapkus
To cite this version:
A. Kost, E. Garmire, M. Kawase, A. Danner, H. Lee, et al.. OPTICAL SWITCHES WITH COM- BINED BRAGG REFLECTORS AND DOPING SUPERLATTICES. Journal de Physique Colloques, 1988, 49 (C2), pp.C2-201-C2-204. �10.1051/jphyscol:1988247�. �jpa-00227664�
JOURNAL DE PHYSIQUE
Colloque C 2 , Supplhment au n e 6 , Tome 4 9 , juin 1988
OPTICAL SWITCHES WITH COMBINED BRAGG REFLECTORS AND DOPING SUPERLATTICES
A . KOST, E . GARMIRE, M . KAWASE, A . D A N N E R * , H.C. LEE* and P . D . DAPKUS*
Center f o r Laser S t u d i e s , Department o f E l e c t r i c a l Engineering, U n i v e r s i t y o f Southern C a l i f o r n i a , Los Angeles, CA 90089-1112, U . S . A .
' c e n t e r f o r Photonic Technology, Department of E l e c t r i c a l Engineering, U n i v e r s i t y o f Southern C a l i f o r n i a , Los Angeles, C A 90089-0483, U. S . A .
A b s t r a - We discuss new studies of a multiple quantum well hetero n-1-p-I structure which combines the advantages of multiple quantum wells and doping superlattices The structure exhibits large changes i n i t s absorption coefficient and refractive index for intensities of only a few m ~ / c m Z . We propose incorporating the structure into a semiconductor dielectric mirror for use as an all optical switch with very low operating intensities.
It has been demonstrated that the absorption coefficient i n doping superlattices or "n-i-p-i"
structures can be modulated with very low intensity light.l12$ Unfortunately the changes have been relatively modest. On the other hand, semiconductor multiple quantum well structures have large optical nonlinearities (but require somewhat larger excitations). A structure which combines the advantages of both n-i-p-i structures and multiple quantum wells i s the hetero n-i-p-i, a periodic arrangement of n-type and p-type layers with undoped layers of smaller band-gap material inserted?
In a conventional hetero n-i-p-i, single quantum wells are separated by doped wide band gap regions of comparable width. We have designed a structure in which several quantum wells are seperated by doped regions much wider than each quantum well. We call this structure a Multiple Quantum Well Hetero n-l-p-l (MQW H nipi)S.
The structure i s pictured i n the Fig. 1. The wide band-gap material i s A10.32Ga0.68A~. The quantum wells are undoped GaAs. P-type and n-type regions have nominal dopant concentrattons of 2 x 1018 cm-3. The material was grown by metalorganic chemical vapor deposition. For light incident on the MQW H nipi, with wavelengths longer than 700 nm, absorption takes place primarily in the quantum wells. Photogenerated electron - hole pairs are spatially separated by the built-in electric fields
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988247
JOURNAL DE PHYSIQUE
I
v,
, a Ii_ ,
i
932.42 875.70 819.08 762.50 705.90 649.50 WAVELENGTH (nm)
Fig. 1
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The band diagram f o r a Multiple Fig. 2 - Absorption coefficient f o r pump intensities Quantum Well Hetero n-i-p-i structure. of 40 j.tW/crnZ (smallest changes), 250 j.tW/cm2,1 mW/cm2,5 m ~ / c m * , and 50 m ~ / c m ?
into the the doped AIGaAs regions. In the doped regions, the excess carriers compensate the space charge, and thus, modify the electric fields and the absorption coefficient of the quantum wells. The photogenerated carriers have increased recombination lifetimes because of the electron-hole spatial separation; so that, even weak lllumlnation can be sufflcient t o generate a large number of excess carriers.
2
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NONLINEAR ABSORPTION AND REFRACTIONIn these experiments, nonlinear transmission of the MQW H nipi was measured for wavelengths between 650 nm and 930 nm using a microscope lamp as low intensity broad band probe, the 8 15 nm CW output of a Styryl 9M dye laser as a pump source, and an optical multichannel analyzer as a detector. The intensity of the probe (100 pW/cm2) was kept as small as possible, while s t i l l retaining a reasonable signal to noise ratio, i n order t o minimize i t s pumping effects. Fig. 2 shows the change i n the absorption coefficient a of the quantum wells, as a function of wavelength and pump intensity, deduced from the transmission measurements. The spectral dependence of the changing absorption can be understood i n the following way. In the absence of strong illumination the built-in f i e l d which appear!; across the quantum wells i s quite strong, which broadens the excitonic absorption resonances and shifts them t o lower energies. The pump photoexcites carriers which screen the built i n field. At wavelengths that correspond to a zero field excitonic resonance, the absorptlon coelf iclent Increases. At slightly longer wavelengths, the absorptlon decreases. Features which correspond t o the n= l and n=2 exci tonic resonances are visible i n the spectra.
We have found that the maximum absorption change (the magnitude of the long wavelength dips i n Fig.
2) lncreases most rapidly for small intensities (less than 1 mW/cm2); and that the absorption does not change as the purnp intensity i s increased beyond 5 0 mW/cm2. Saturation occurs when the space charge, i n the depleted n-type and p-type reglons i s almost completely compensated by photogenerated carriers. A Kramers-Kronig analysis of the absorption changes indicates that we
should expect changes i n the index of refraction of the MQW H nipi as large as r 0.02 near the absorption edge of the material. From the intensity dependence of the wavelength for maxfmum absorption change, we have deduced the lifetimes of photogenerated carriers. They range from = 1 mlllisecond t o s 1 microsecond as the pump intensity i s increased from 40 p ~ / c m 2 to 50 m ~ / c m 2 . 3
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NONLINFAR BRAGG RFFl ECTORSDistributed feedback structures (DFB) w i t h a nonlinear index of refraction have been lnvesitgated theoretically6; and optical bistability has been demonstrated i n dlstributed feedback semiconductor laser amplifiers7. As of yet, however, the full potential of the passive nonlinear DFB has not been realized. Recent developments i n the design and fabrication of semiconductor heterostructures have made the passive nonlinear DFB an at tractive candidate for a l l opt ical switches. These developments include: ( 1 ) The epitaxlal growth of high reflectivlty all semiconductor mlrrors, called Bragg reflectors (BR), and (2) fabrication of hetero n-i-p-i structures such as the one discussed above. We propose combining a MQW H nipi w i t h a BR as indicated i n Fig. 3. Layer thicknesses are chosen such that the edge of the central reflectivity maximum for the BR i s at a wavelength at which the MQW H nlpl has a large nonlinear refractive index. In thls case, a few milliwatts per square centimeter of optical power w i l l be sufficient to shift the reflectivity edge, producing large changes i n the reflectivity at certain wavelengths. Fig. 4 shows that a 2% change i n the coupling constant KL = AnkL, where An i s the difference i n the refractive index of adjacent BR layers and L i s the total BR length, can change the reflectivlty from almost zero to near t OO%, i f the proper detuning i s chosen. A change in the refractive Index of the multiple quantum well layers equal to 2% of ~n w l l l change the coupling constant by 2%. Since An s 0.5 for a AlxGal-xAs BR, the required nonlinearities are within the range of the MQW H nipi.
MULTIPLE QUANTUM WELLS
m
COUPLING CONSTANT
Fig. 3
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A Multiple Quantum Well Hetero n-i-p-i Fig. 4 - Reflectivity as a function of cor~piing andBraggref1ectorcornbined.Thelayerthick- constantforawavelengthnearaRRreflectivitynesses are quarter wavelength. edge.
C2-204 JOURNAL DE PHYSIQUE
There are two main advantages t o such a configuration as a nonlinear switching element (in addition t o the fact that it Incorporates the very sensitive MQW H nipi). First, as compared t o the all semiconductor nonlinear Fabry-Perot (two Bragg mirrors with the nonlinear medium inserted), the stucture requires fewer layers. Second, an analysis indicates that the nonlinear Bragg reflector can switch at lower thresholds than the corresponding nonlinear Fabry-Perot. Specifically, we have shown that a requirement for switching in the nonlinear Fabry-Perot is:
where 8n i s the maximum refractive index change, a i s the corresponding absorption coefficient and k i s the corresponding wavenumber. On the other hand, the requirement for the nonlinear Bragg reflector is:
Thus, the nonlinear Bragg reflector w i l l switch at a lower threshold i f KL ) 6.
4
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CONCLUSIONSWe have observed absorption coefficient changes in the MQW H nipi as large as 2500 cm-1 in response to a 50 m ~ / c m 2 optical pump. We have caiculated corresponding refractive index changes of f: 0.02.
Incorporated directly into a Bragg reflector, the MQW H nipi could operate as a very low power, all optical switch.
This work was supported by the Army Research Off ice, the National Science Foundation, and the National Aeronautics and Space Administration Lewis Research Center.
REFERENCES
/ 1 / G. H. Dohler, H. Kimel, and K. Ploog, Phys. Rev. B25, 26 16 ( 1982).
/ 2 / 'T. 6. Simpson, C. A. Pennise, 8. E. Gordon, J. E. Anthony, and T. I?. AuCoin, Appl. Phys. Lett. 49, 590
( 1986).
/3/ A.
D.
Danner, P. D. Dapkus, A. Kost, and E. Garmire, submitted t o J. Appl. Phys../4/ R. A. Street, G. H. Dohler, J. N. Miller, and P. P. Ruden, Phys. Rev. 833, 7043 ( 1986).
/5/ A. Kost, E. Garmire, A. Danner, and P. D. Dapkus, Appl. Phys. Lett. 52,
/ 6 / Herbert G. Winful, J. H. Marburger, and E. Garmire, Appl. Phys. Lett. 35,379 (1979).
/7/ M. .1. Adams and FI. Wyatt, IEE PROCEEDINGS 134, Pt. J , No. I , 35 ( 1987).
