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1274 OPTICS LETTERS / Vol. 26, No. 16 / August 15, 2001 Fixing multiple waveguides induced by photorefractive solitons: directional couplers and beam splitters Aqiang Guo, Michael Henry, and Gregory J. Salamo Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701 Mordechai Segev Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel, and Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544 Gary L. Wood U.S. Army Research Laboratory, AMSRL-SE-EO, Adelphi, Maryland 20783-1 197 Received April 11, 2001 We show how to transform multiple real-time photorefractive solitons into permanent two-dimensional single-mode waveguides impressed into the crystalline lattice of the host material. We experimentally demonstrate two specific configurations of such fixed multiple waveguides: directional couplers and multiple beam splitters. O 2001 Optical Society of America OCIS codes: 190.5530, 190.5330. Photorefractive solitons1 have been observed at low light powers and exhibit robust trapping in both transverse dimensions.'-E Among the numerous types of photorefractive soliton, the most commonly used type is screening soliton^,^-'^ which form when an electric field applied to a photorefractive crystal is partially screened within the incident light beam as a result of transport of photoexcited charges. As a result, an internal electric field exists around the optical beam and modifies the refractive index through the Pockels effect. The parameters of soliton self-induced waveguides can be engineered by use of the soliton existence curve." Such solitoninduced waveguides can be used in various waveguide applications and in multiple configurations, such as directional couplers1' and beam splitters (Y juncti~ns),l~-~~ (as was demonstrated with other kinds of solitons),16 and more recently were used to obtain a waveguiding environment for efficient nonlinear frequency conver~ion.~~~~~ However, these real-time waveguides induced by photorefractive solitons decay and disappear when the applied field is turned off while the waveguides are guiding a light beam. This decay and disappearance occur because the trapped electrons that have screened the applied electric field are re-excited and undergo transport, giving rise to a charge distribution that cannot support solitons. Although the self-induced and easily erased nature of photorefractive soliton-induced waveguides is attractive for dynamic applications, for many applications it is advantageous to impress waveguides into the crystalline structure permanently, that is, to have the induced waveguide last indefinitely without an applied field, even under intense illumination. Recently, Klotz et al.'' and DelRe et a1.20 demonstrated how to transform the "real-time" screening soliton into a permanent waveguide by means of ferroelectric domain rever~al.'~~'~ Here, we experimentally demonstrate the fixing of multiple photorefractive solitons into permanent two-dimensional single-mode waveguides that also act as directional couplers and multiple beam splitters. We start by demonstrating the process of fixing two soliton-induced waveguides into a directional coupler. A directional coupler consists of two waveguides in close proximity, which are coupled to each other by evanescent fields. The separation between the waveguides determines the coupling efficiency. The closer the waveguides are to each other, the larger the coupling is. In principle, in a directional coupler consisting of two completely identical waveguides, as much as 100% of the energy injected into one waveguide can be transferred into the second waveguide after a specific propagation distance L. For lengths less than L the coupling is less than loo%, whereas for lengths greater than L, energy is coupled back into the original waveguide and the coupling is also less than 100%. A directional coupler made by using real-time soliton-induced waveguides was demonstrated in Ref. 12. In this Letter we generate such a directional coupler and the convert it into a permanent structure, using the technique for fixing photorefractive solitons demonstrated in Ref. 19. We use the standard setup for forming and fixing screening soliton^,^^^' combined with the technique for generating a soliton-induced directional coupler.'' Beam splitters divide the output of a 514.5-nm argon-ion laser into two 20-pW soliton beams and a 100-mW background beam. The crystal is a 1-cm cube of SBN:75 doped with 0.02% cerium by weight. The soliton beams each have a 12-pm FWHM input diameter [Figs. l(a)- l(c)l, and the 100-mW background beam is expanded to fill the entire crystal. Normally, the incident beams, which propagate along an a axis, diffract to -100 pm at the exit face. When an electric field is applied along the c-axis, the beams self-trap, as the output beam diameters are reduced to 12 pm. Once the beam self-focuses to its initial width of 3 kV/cm, we switch off the laser light and the applied electric field sequentially. After 1 min, the 0146-9592/01/161274-03$15.00/0 O 2001 Optical Society of America
APPLIED PHYSICS LETTERS VOLUME 77, NUMBER I8 3n 0('1'0[31 R lr100 Thermal annealing recovery of intersubband transitions in proton-irradiated . . .a GaAsIAIGaAs multiple quantum wells F. Hegeler and M. 0. ~anasreh~) Deparlmenl of'Electriccr1 and Computer Engineering, Universiy qf'iVew h.le,rico, Albuquercpe, :Vew Mexico 87/31 C. Morath Air Force Research Lab (.4FRL/VSSS), 3550 Aber-&en Avenue SE, El& 426, Kirtland AFB. New .blexico 871 17 P. Ballet, H. Yang, and G. J. Salarno Depczrtment of Physics, Crniversiy 0/'.4rkansas. Fqetteville, drk[~nsas 72701 H. H. Tan and Chennupati Jagadish . Departnrrtit qf'Elsctronic hlaterials Engirreer-irlg, Reser~rch School of Physictrl Sciences ond Engineering, .4ustraliun rVutionc11 Universit): Cunberra ACT OOO. Austrcrlic~ (Received 12 May 2000; accepted for publication I September 2000) Intersubband transitions in 1 MeV proton-irradiated GaAs/AIGaAs multiple quantum wells were studied using an optical absorption technique and isochronal thermal annealing. The intersubband transitions were completely depleted in sa~nplcs irradiated with doses as low as 4 X l0I4 cm-I. Morc than 80% recovery of these depleted transitions was achieved after the samples were thermally anncaled at teniperatures lcss than 650 "C. The total integrated areas and peak position energies of the intersubband transitions in irradiated and unirradiated samples were monitored as a hnction of annealing temperature. It was noted that the recovery of the depleted intersubband transitions in irradiated samples depend on the irradiation dose and thermal annealing temperature. O 2000 A4merican Itrstitzite of Physics. [S0003-695 1 (00)00444-71 . Irradiation induced atomic displacement in semiconductors affccts both material properties and devicc performance. Rcccntly, the proton irradiation effect on the intersubband transitions in GaAs/AIGaAs multiple quantum wells was reported.' It was shown that the intensity of the intersubband transitions is decreased as the proton irradiation dose is increased. This was explained in terms of trapping of the twodimensional electron gas in the GaAs quanh~m wells by irradiation-induced defects such as vacancies, antisites, and more complex defects. A reduction of the intensity of the intersubband transitions in electron irradiated GaAsIAIGaAs multiple quantum wells was also observed.' In this letter, we report on the themla! recovery of depleted intersubband transitions in proton irradiated GaAs/ AlGaAs ~nultiple quantum well samples. The intersubband transitions were nieasured bcforc and after proton irradiation and it was observed that the intcrsubband transitions wcre co~npletcly washed out in samples irradiated with 1 MeV protons and doses higher than 4 X 10'~cm-'. Upon isochronal thermal annealing, these transitions were observed to recover at annealing temperatures (To) as low as 250°C in samples that received low irradiation doses. Both the total integrated areas and the peak position energies of the intersubband transitions in irradiated samples and in onereference sample were measured as a function of To. The T, two-dimensional electron gas. The behav~ui , . ' ":' rvdl ;n~ugrated areas and the peak position energies of tl-.A: !.ILL rs~; 3- band transitions will be explained in terms of energy lcvel shifts due to interdiffusion. Two nlultiple quantum well structures used in !his study were grown by the molecular-beam epitaxy technique on a semi-insulating GaAs substrate with a 0.5 pm thick GaAs buffer layer and an - 200 A thick GaAs cap layer.' he structures of the two wafers are shown in Table I. Thc barriers of the wafer labeled "A" are bulk AlGaAs. Wllilc t!~c barricks of the wafer labeled "B" are made of five periods AICin!\s/ GaAs superlattices. The well regions were Si-dopcil {[Si] = 2X lot8 ~rn-~}. Several samples were cut and ~rradiated with diffcrcnt doscs of 1 MeV proton beams. Ttrc infrared absorption spectra were recorded at the Brewsterfs anglc ~f GaAs (73") from the normal using a B0ME.M 1:ouriiir: transform interferometer in conjunction with n ~ont~nutirs flow cryostat. The te~npcraturc was controlled ~\.itl!in I .O'K and the spectra were measured at either 77 or 300 K. Furnace TABLE 1. Structures of the wafcrs used in the present stu~);. ~ iwater l wcre Si doped in the well ([Si]=3.0X 1O1%rn-I). The bariicr inarcrinl of wafer "H" is made of AlGaAslGaAs superlanice. . . -- Wafer A 11 , Wcll thickness (A) 75
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