THERE is an increasing amount of experimental interest

Transcription

1 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 4, NO. 4, JULY/AUGUST Modulation of the Second-Order Nonlinear Tensor Components in Multiple-Quantum-Well Structures J. Stewart Aitchison, Member, IEEE, M. W. Street, N. D. Whitbread, D. C. Hutchings, Member, IEEE, John H. Marsh, Senior Member, IEEE, G. T. Kennedy, and Wilson Sibbett (Invited Paper) Abstract In this paper, we present experimental results which demonstrate that quantum-well intermixing techniques can be used to modulate the magnitude of the second-order nonlinear coefficient (2). Impurity-free vacancy disordering with SiO 2 and Ga 2O 3 caps was used to modulate the position of the band edge and hence, the magnitude of (2) e. Using a coupled quantum-well structure we were able to demonstrate modulation of the d33 tensor components associated with the asymmetric structure and of the d14 component associated with the bulk crystal structure. Index Terms Nonlinear optics, quantum wells, semiconductor waveguides. I. INTRODUCTION THERE is an increasing amount of experimental interest in the use of optical frequency conversion techniques for the generation of new wavelengths. In particular, in the mid-infrared (mid-ir), there are limited sources of coherent radiation and a rapidly increasing demand from sensing, photomedical and automotive applications. Currently, the only viable sources are based on quantum cascade lasers and narrow bandgap semiconductor lasers [1]. However, these sources typically operate at low temperatures and offer limited tunability. In contrast, sources based on optical frequency conversion offer room-temperature operation and extended tunability. Frequency conversion techniques also have potential applications in future wavelength-division-multiplexed (WDM) networks, where wavelength-agile components will be required, which can convert signals from one channel to another [2]. The second-order nonlinear coefficients available in III V semiconductors are considerably larger that those available in ferroelectric materials such as LiNbO and KTP. Combined with the mature fabrication technology and the extended transparency into the mid-ir, these materials offer the prospect of efficient frequency conversion devices. However, their application has been limited by the difficulties associated with phase-matching. The cubic structure of III V semiconduc- tors means that there is no natural birefringence in these crystals. There are two principal approaches to solving this phase matching problem, namely birefringent phase matching [3] and quasi-phase matching [4]. Recent experiments have reported birefringent phase matching in two-dimensional (2-D) waveguides using the selective oxidation of high Al content AlAs, and this approach has been used to demonstrate mid- IR generation by difference frequency mixing [5]. A tunable output was also obtained by tuning the temperature of the nonlinear crystal [6]. Quasi-phase matching has been demonstrated in samples grown on prepatterned substrates [7]. This method shows promise, however problems associated with scattering at the domain interfaces and the corrugated nature of the waveguide still need to be solved [8], [9]. An alternative approach to QPM is based on creating an asymmetry in the structure. In this case, quasi-phase matching can be achieved by periodically destroying this asymmetry using quantum-well intermixing (QWI) [10]. More recently the use of QWI has been extended to the modulation of the bulk tensor element [11], [12]. In this paper, we report the use of QWI as a mechanism for modulating the nonlinear optical properties of MQW structures. The QWI process has previously been used to control both the resonant [13] and nonresonant [14] thirdorder nonlinear coefficients in AlGaAs MQW structures. On intermixing, the QW becomes shallower and wider, with a resulting increase in the energy of the band-edge. This technique offers an alternative to the etch and regrowth approach to optoelectronic integration, and has been used to demonstrate a range of extended cavity semiconductor laser structures incorporating passive sections [15]. In the following sections, we review the intermixing process used in this work and describe the mechanisms which give rise to a change in.in addition to the realization of QPM structures, the use of QWI offers the prospect of integrating semiconductor pump sources directly on to the same chip as the frequency conversion element. Manuscript received May 5, 1998; revised July 1, J. S. Aitchison, M. W. Street, N. D. Whitbread, D. C. Hutchings, and J. H. Marsh are with the Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K. G. T. Kennedy and W. Sibbett are with the Department of Physics and Astronomy, University of St. Andrews, St. Andrews Fife, KY16 9SS Scotland, U.K. Publisher Item Identifier S X(98) II. IMPURITY-FREE VACANCY DISORDERING Several techniques have been used to intermix quantum well materials, including impurity induced disordering (IID) [16] and impurity-free vacancy disordering (IFVD) [17]. The IID technique involves the implantation, or diffusion, of an X/98$ IEEE

2 696 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 4, NO. 4, JULY/AUGUST s. Photoluminescence (PL) measurements indicated that a differential bandgap shift of 23 nm had been induced. Identical, 2-mm-long single-mode waveguides were then fabricated from both the intermixed and suppressed materials. Fig. 1. Schematic of the AlGaAs asymmetric coupled QW waveguide structure. The inset shows the detail of the coupled well structure. impurity into the layer structure containing the MQW. This is followed by a high-temperature annealing stage. The presence of the dopant leads to the formation of group-iii vacancies, or interstitials, which migrate during annealing through the crystal to the MQW layer where they facilitate intermixing. The IFVD technique requires the deposition of a dielectric cap onto the surface of the sample prior to annealing. To promote intermixing, a silica cap is normally used. This allows Ga to diffuse into the cap material leaving group-iii vacancies at the interface between semiconductor and the SiO, which, in turn, diffuse down to the QW and cause intermixing. To suppress the intermixing process a different capping material is deposited, e.g., P:SiO or Ga O. Under the second cap Ga out diffusion process is inhibited, and hence, the intermixing is suppressed. The wafer structure used in the following experiments consisted of an undoped waveguide structure grown on a semi-insulating substrate by molecular beam epitaxy (MBE). The lower waveguide cladding was a 4- m-thick layer of Al Ga As and the upper cladding a 0.8- m-thick layer of the same material. The MQW waveguide layer consisted of an asymmetric coupled well structure, shown in Fig. 1. Room-temperature (RT) optical absorption measurements performed on this sample indicated light- and heavy-hole exciton resonances at 692 and 699 nm, respectively. The intermixing process used relies on the high temperature, preferential absorption of Ga atoms into a SiO cap. To prevent intermixing, a hydrogen plasma treatment was used to promote the growth of a Ga O cap [18]. The GaAs surface initially contains a mixture of GaAs, As O, GaO, and free As. Hydrogen radicals generated in the plasma reduce the As O to form AsH and oxygen. The oxygen is then free to react with gallium atoms to form Ga O leaving free As. The reaction proceeds until the surface is covered with Ga O and can be summarized by the following solid-state reaction As O GaAs H Ga O AsH. The Ga O cap prevents the absorption of Ga atoms from the surface and hence suppresses the QWI process. To demonstrate QWI, a sample was cleaned and coated with 2000 Å of plasma enhanced chemical vapor deposited SiO. A second sample was exposed to a H plasma for 30 min, at 40 C with a dc bias of 80 V, a radio-frequency (RF) power of 70 W and a pressure of 900 mt. This process has previously been reported to lead to the formation of Ga O. Both samples were then annealed in a rapid thermal processor at 950 C for III. SPATIAL RESOLUTION In order to realize a QPM structure, it is necessary to know what the spatial resolution of the QWI process is. If the resolution is too poor, it will be impossible to define the periodic phase-matching gratings required. Simple calculations of the first-order phase matching period for second-harmonic generation (SHG) from a m fundamental input, indicate that a QPM period of 3 m will be required. Therefore, the spatial resolution of the intermixing process must be better than 1.5 m. It has previously been demonstrated that the IFVD process can be applied to selected areas by opening windows in the SiO cap and exposing the sample to the hydrogen plasma prior to the annealling stage [10]. This results in Ga O over the exposed GaAs regions. Resolution tests were performed using a waveguide epitaxial structure consisting of a 4- m-thick Al Ga As lower cladding layer, a m GaAs Al Ga As MQW guiding region and a 0.8- m Al Ga As upper cladding. For these resolution studies, a 28-Å-wide rectangular well was used. The test samples were coated with a 200-nm layer of SiO and 2 mm by 2 mm squares were defined in which different resolution test patterns were defined. In each square, the period of the pattern was 30 m, while the window, over which intermixing was suppressed, ranged from 16 m down to 1.5 m. A schematic of the sample is shown in Fig. 2. Vacancies created under the thin SiO stripes diffuse both down toward the MQW and in the horizontal direction. Two control areas were also defined on the chip, one in which the SiO had been completely removed and the other where the cap had been left intact. The sample was exposed to a hydrogen plasma for 40 min at 900 mtorr and 40 C, after which it was annealed at 900 C for 90 s. The normalized PL spectra for three different regions on the sample are shown in Fig. 3. The region capped with the SiO showed a bandgap shift of 45 nm (solid curve) compared with a 12-nm bandgap shift for the regions, which had been exposed to the hydrogen plasma (dotted curve). The PL signals from the areas with window widths from 16 m down to 1.5 m all exhibited two peaks (dashed curve), the long wavelength peak corresponding to the suppressed shift and the short wavelength peak to the intermixed region. The laser spot used in the PL measurements overlapped both the suppressed and the intermixed regions, hence the double peaked PL signature. The long dashed curve shows the PL signal for a strip of Ga O, which was 1.7 m wide. The spreading of the Ga vacancies from the SiO stripes (28.3 m wide) on either side is insufficient to intermix all of the area under the Ga O cap. This indicates that this method of suppression of the QW intermixing process has a spatial resolution of 1.5 m, or better and is thus suitable for the realization of first-order QPM gratings.

3 AITCHISON et al.: MODULATION OF SECOND-ORDER NONLINEAR TENSOR COMPONENTS 697 Fig. 2. Schematic diagram showing the lateral diffusion of Ga vacancies during the selective-area IFVD process. Fig. 3. Normalized PL spectra (77 K) for the IFVD spatial resolution tests. IV. ASYMMETRIC QUANTUM WELLS By designing a suitable asymmetric quantum-well structure it is possible to break the symmetry of the cubic III V crystal and induce additional nonlinear tensor coefficients [19]. This can also be achieved by applying an electric field across the MQW [20], however, in this paper, we confine our discussion to the asymmetric coupled QW structures, though other forms of asymmetric QW also lead to induced second-order nonlinear coefficients. The magnitude of these nonlinear coefficients has been the subject of a great deal of theoretical and experimental interest [21] [23]. Calculations have predicted values as large as several hundred pm/v in the near-ir. However, corresponding experiments have only reported values of the order of 10 pm/v. Early predictions were based on the use of an E.r dipole perturbation. The calculations are usually truncated so that only the most resonant terms are included; this approach is over-simplistic and commonly leads to an over-estimate in the magnitude of the second-order nonlinearity. More recent calculations are based on A.p perturbation for a three-level system and a bandstructure model using two conduction and five valence subbands [24]. This model has been used to estimate that the nonlinearities associated with the structure shown in Fig. 1 which are a few pmv in accord with experimental observations [9]. It should be noted that these structures were not optimized to maximize the nonlinearity associated with the asymmetric QW, but rather to ensure high-quality MBE growth. However, the model used to predict these values has also resulted in a number of simple design rules for producing optimized asymmetric structures. In particular, short period superlattice structures, with large compositional changes, are predicted to produce the largest values of. This enhancement is due to the increase in the density-of-states and the increased subband energy separation. Indeed the coefficient associated with the bulk AlGaAs crystal can be thought of as arising from the asymmetric structure of one monolayer period in the [111] direction. When the induced nonlinear tensor coefficient is related to the asymmetry of the structure there is an intriguing possibility for modulating the nonlinearity using QWI. During the QWI process, group-iii vacancies diffuse down into the QW and cause the well to broaden and become shallower. Due the diffusion processes that underlie QWI, it follows that an asymmetric QW will tend to become more symmetric on intermixing. Since the nonlinearity depends on the degree of asymmetry in the QW, it follows that intermixing will correspondingly reduce the value of. This process can be demonstrated by solving a one dimensional diffusion equation, of the form Assuming the initial Al profile of the wafer structure as the diffusion equation was solved for a system of three QW s, typical results are shown in Fig. 4. Results are plotted for different diffusion lengths, where. By plotting the Al profile for different diffusion lengths it is possible to simulate different anneal times and temperatures, since depends on temperature. It is clear from the figure that as the intermixing proceeds the degree of asymmetry decreases.

4 698 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 4, NO. 4, JULY/AUGUST 1998 Fig. 6. Nonphase-matched SHG signal for a TM polarized second-harmonic signal as a function of the TE polarized fundamental power. The dark circles show the as-grown and the light circles the intermixed response. Fig. 4. of ld = 20 Å. The Al profile for the coupled-well structure for a diffusion length Fig. 5. Nonphase-matched SHG signal for a TM polarized input and a TM polarized second harmonic signal. The dark circles show the as-grown and the light circles the intermixed response. To characterize the effect of QWI on the magnitude of the nonlinearity associated with the asymmetric coupled quantum well, nonphase-matched second harmonic generation experiments were carried out. The samples used were 2-mmlong waveguides formed from disordered and undisordered material. A KCl:T1 color center laser, producing 660-fs pulses at 1.52 m, was end-fire coupled into the waveguides. The wavelength was chosen to lie below the half-bandgap so that two-photon absorption of the fundamental and single photon absorption of the second harmonic could be neglected. The measured nonphase-matched SHG signals for both the disordered and nondisordered waveguides are shown in Fig. 5 as a function of the average transmitted fundamental power. Care was taken to measure the output power levels and work back to the input powers using the measured propagation losses, facet reflectivities and output coupler losses. This allowed us to accurately determine the both input and second harmonic powers and hence, the relative change in the nonlinear coefficient. It can been seen that the second harmonic signal has been reduced by a factor of 4.8 as a result of the intermixing step. This translates to a reduction in by a factor of at least 2.2. V. MODULATION OF It is also possible to use the much larger coefficient associated with the bulk III V crystal. The second-order nonlinear tensor element associated with the bulk GaAs crystal, has a resonance at the bandgap energy (e.g., for SHG when ). Since the intermixing process results in an increase in the bandgap it follows that the detuning of the waves involved in the nonlinear interaction also increases and hence, the magnitude of the effective nonlinearity decreases. Thus, is should also be possible to apply intermixing techniques to modulate the value of. The crystal symmetry of the [100] grown crystal implies that the coefficient produces a TM polarized SHG signal (parallel to the [001] direction) for a TE polarized fundamental input (parallel to the [110] direction). This allows us to differentiate the modulation in the bulk component from the modulation in the asymmetric components. Nonphase-matched SHG experiments were performed on the same two samples described above. The results for the TM polarized second harmonic signal as a function of the TE polarized fundamental input are shown in Fig. 6 for both the intermixed and nonintermixed samples. The reduction in the measured SHG signal corresponds to a reduction in of approximately 17%. Assuming that 150 pm/v, this translates into a modulation in the effective nonlinearity of 25 pm/v. This figure is comparable to the coefficient of LiNbO 33 pm/v yet is the result of a relatively modest bandgap shift. To show that the observed reduction in is consistent with the bandgap widening induced by the QWI process, one can assume that, to a first approximation, the densities of states for both bulk and AlGaAs MQW material give rise to similar values of nonlinear coefficient. The reduction in can then be estimated by calculating the reduction in the resonant contribution for a 23-nm blue-shift in the band edge. The calculation is based on a k.p model consisting of the highest valence-band triplet, and the lowest conduction band singlet, and triplet [25]. Including the conduction band breaks the inversion symmetry and results in a nonzero second-order nonlinearity. This band-structure model describes the states in the vicinity of the -point well, but is less accurate for the edge of the Brillouin zone. Hence, calculating the value for for GaAs provides a good description of the halfbandgap resonant feature. However, it should be noted that there is a significant background that is strongly dependent on the chosen integration volume in -space in this model. Fig. 7 shows the calculated reduction in for GaAs, assuming a 23 nm bandgap shift, for three different integration volumes. It can be seen that the reduction in is relatively insensitive to the integration volume and that a reduction of

5 AITCHISON et al.: MODULATION OF SECOND-ORDER NONLINEAR TENSOR COMPONENTS 699 Fig. 8. Schematic diagram showing the modulation of the second-order nonlinearity possible when both the asymmetric quantum well (AQW), d15 or d31, and the bulk d14 components are present. Fig. 7. Calculated reduction in (2) xyz(!;!) as a function of detuning from the half-bandgap for a 23-nm blueshift in the fundamental absorption edge. The three curves correspond to different integration regions in k space. a few tens of pm/v are typical. Given the approximations of this model, it can be concluded that the experimental values are consistent with those predicted. VI. IMPLEMENTATION AND APPLICATIONS The previous choice of investigating the coefficient in an asymmetric heterostructure was made on the basis of eliminating the bulk coefficient from the measurement. In practice, unlike ferroelectric materials, the on-diagonal coefficient is smaller than the off-diagonal and coefficients. The normal orientation for semiconductor waveguides results in the TM polarization parallel to the [001] crystalline direction and TE parallel to [110]. Therefore the coefficient can be employed in type I interactions (TE TE TM) whereas the largest asymmetric coefficient,, will be employed in type II interactions (TE TM TE). Although the magnitude of these nonlinear coefficients are small in the structure studied here, it is anticipated that by reducing the period to a few monolayers these coefficients will be enhanced to several tens of pmv. The bulk coefficient can be employed in either type-i or type-ii configuration. The bandgap shift that was obtained here was quite modest and hence it is anticipated that for optimized structure the degree of modulation may be enhanced to 30% 40% of the coefficient itself. For comparison, the observed modulation in at the half-bandgap exceeded 50% [14]. By choosing the appropriate crystal orientation it should be possible to achieve quasi-phase-matching with cooperative contributions from asymmetric and bulk coefficients to give a modulation in coefficient 100 pmv. This is shown schematically in Fig. 8. For efficient wavelength conversion in semiconductor waveguides, it is important to avoid spectral regions of highoptical loss, i.e., applications will normally be restricted to optical frequencies less than the fundamental bandgap. Hence, it is not anticipated that SHG will be relevant for GaAs based materials, although II VI s or nitrides may be employed in a similar fashion for generation of visible wavelengths. Since integrated diode lasers will produce radiation at the bandedge, the principal application will be the generation and amplification of light at longer wavelengths. Two spectral regions of interest are m for communications applications and 2 7 m for spectroscopically sensing gases. Difference frequency generation has been employed in the alternative techniques of birefringent phasematching by selective oxidation [5] and for WDM channel shifting by patterned substrate QPM [7]. The same configuration can also be employed for parametric amplification, which has particular advantages due to its coherent nature. If the parametric gain can be made large enough, then by placing the nonlinear element in a cavity, optical parametric oscillation can occur. The fabrication possibilities in GaAs-based structures would allow an integrated format e.g., intracavity or coupled-cavity with the pump laser. It is important to reiterate that the output wavelength is determined by postgrowth processing and hence, it is possible to have several wavelength sources on one chip. Furthermore, with all the elements fabricated on one chip the problems of alignment of OPO s will be eliminated. VII. CONCLUSION We have demonstrated that IFVD techniques can be used to modulate the second-order nonlinear coefficients of GaAs AlGaAs MQW structures. Two approaches to realizing quasi-phase matched structures were investigated. The first was based on the use of an asymmetric QW which, by breaking the material symmetry, resulted in additional nonlinear tensor elements. The IFVD process was used to reduce the asymmetry in the QW and hence reduce the nonlinear coefficient. The second approach was based on the use of IFVD to increase the bandgap of the MQW. This results in an increase in the detuning of the wavelengths involved in the interaction and, as a consequence, a reduction in the nonlinear coefficient. The IFVD technique offers an attractive approach to realizing quasi-phase matched structures in semiconductor waveguides. In addition, the technique is regrowth free and offer the prospect of integration of semiconductor pump sources together with nonlinear frequency conversion element. REFERENCES [1] J. Faist, F. Capasso, D. L. Sivco, A. L. Hutchinson, C. Sirtori, and A. Y. Cho, Quantum cascade laser A new optical source in the midinfrared, Infrared Phys. Tech., vol. 36, pp , [2] S. B. J. Yoo, Wavelength conversion technologies for WDM network application, J. Lightwave Technol., vol. 14, pp , 1996.

6 700 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 4, NO. 4, JULY/AUGUST 1998 [3] A. Fiore, V. Berger, E. Rosencher, S. Crouzy, N. Laurent, and J. Nagle, Delta n = 0.22 biregringence measurement by surface emitting second harmonic generation in selectively oxidized GaAs/AlAs optical waveguides, Appl. Phys. Lett., vol. 71, pp , [4] S. B. J. Yoo, R. Bhat, C. Caneau, and M. A. Koza, Quasiphase-matched 2nd-harmonic generation in AlGaAs waveguides with periodic domain inversion achieved by wafer-bonding, Appl. Phys. Lett., vol. 66, pp , [5] A. Fiore, V. Berger, E. Rosencher, P. Bravetti, N. Laurent, and J. Nagle, Phase matched mid-infrared difference frequency generation in GaAs based waveguides, Appl. Phys. Lett., vol. 71, pp , [6] P. Bravetti, A. Fiore, V. Berger, E. Rosencher, J. Nagle, and O. GauthierLafaye, m source tunable by frequency conversion in GaAs based waveguides, Opt. Lett., vol. 23, pp , [7] S. B. J. Yoo, C. Caneau, R. Bhat, M. A. Koza, A. Rajhel, and N. 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Springthorpe, Enhancement of second harmonic generation at 1.06 m using a quasiphase-matched AlGaAs/GaAs asymmetric quantum well structure, Appl. Phys. Lett., vol. 65, pp , [22] R. Atanasov, F. Bassani, and V. M. Agranovich, Second order nonlinear optical susceptibility of asymmetric quantum wells, Phys. Rev. B., vol. 50, pp , [23] C. Kelaidis, D. C. Hutchings, and J. M. Arnold, Asymmetric two-step GaAlAs quantum well for cascaded second order processes, IEEE J. Quantum Electron., vol. 30, pp , [24] D. C. Hutchings and J. M. Arnold, Determination of second-order nonlinear coefficients in semiconductors using pseudospin equations for three level systems, Phys. Rev. B., vol. 56, pp , [25] P. Pfeffer and W. Zawadzki, Conduction band electrons in GaAs- 5-level K. P theory and polaron effects, Phys. Rev. B, vol. 41, pp , J. Stewart Aitchison (M 96), for a biography, see this issue, p M. W. Street, photograph and biography not available at the time of publication. N. D. Whitbread, photograph and biography not available at the time of publication. D. C. Hutchings (M 98), photograph and biography not available at the time of publication. John H. Marsh (M 91 SM 91), for photograph and biography, see this issue, p G. T. Kennedy, photograph and biography not available at the time of publication. Wilson Sibbett received the B.Sc. degree in physics from the Queen s University, Belfast, Ireland, in 1970, and the Ph.D. degree studies in laser physics at Queen s University and Blackett Laboratory, Imperial College, London, U.K. He became a Lecturer and Reader in physics at Imperial College. In 1985, he became Professor of Natural History and Head of the Department of Physics and Astronomy, University of St. Andrews, Scotland, U.K. Since 1994, he has been Director of Research at St. Andrews. His main research interests include ultrashort-pulse lasers, nonlinear/waveguide optics, diode-pumped minilasers, and applications in photomedicine. He has coauthored over 300 papers in technical journals and conferences. Prof. Sibbett is a Fellow of the Royal Society (London), a Fellow of the Royal Society of Edinburgh, and has fellowships from the U.K. Institute of Physics and the Optical Society of America. In 1993, he was awarded the Institute of Physics C. V. Boys Prize and Medal for experimental physics, and in 1997, the Rank Prize for Opto-Electronics.