Monolithically Integrated All-optical Switch using Quantum Well ...

Optical and Quantum Electronics (2006) 38:567–573 DOI 10.1007/s11082-005-4700-9

© Springer 2006

Monolithically integrated all-optical switch using quantum well intermixing j o n g b u m n a h∗ a n d p a t r i c k l i k a m wa College of Optics and Photonics, University of Central Florida, Orlando, FL 32816-2700, USA (∗author for correspondence: E-mail: [email protected]) Received 26 July 2005; accepted 2 November 2005 Abstract. We report the realization of a compact monolithically integrated all-optical switch using selective area bandgap tuning of a multiple quantum well structure by impurity-free vacancy induced disordering technique. A 3-dB waveguide coupler was fabricated in the disordered section of the switch device. All-optical wavelength conversion at 1Gb/s was demonstrated.

1. Introduction There is increasing demand for the integration of semiconductor photonic devices that can provide a higher level of functional performance. To integrate photonic elements by optical circuits, spatial control of the optical and electrical properties of the underlying materials is required. Several approaches, such as hybrid integration, selective regrowth, selective area epitaxy (Aoki et al. 1991; Kotaka et al. 1993), and postgrowth modification of the optical properties of quantum wells, have been investigated to achieve this goal. Quantum well intermixing (QWI) changes the effective transition energies of the electrons from the valence band to the conduction band and shifts the position of the absorption edge (Marsh 1993; He et al. 1996). Selective-area QWI is a very promising technique for the realization of monolithically integrated devices. To date, a number of QWI techniques have been reported, including impurityinduced disordering (Ooi et al. 1994), impurity-free vacancy-induced disordering (IFVD) (Helmy et al. 1999), ion implantation-induced inter-diffusion (Wan et al. 1997), and several laser-induced disordering processes (Ralston et al. 1987). IFVD is usually implemented by the deposition of a dielectric film coating followed by rapid thermal annealing (RTA). InP-based structures have attracted increasing interest for use with electronic, optoelectronic, and photonic devices for optical communication in the 1.55 µm wavelength band. In this study, the intermixing of an InGaAsP multiple quantum well (MQW) laser structure annealed at various temperatures has been investigated by photoluminescence (PL) measurements. The samples were coated

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with thin SiO2 dielectric films using a plasma-enhanced vapor phase deposition (PECVD) and were then subjected to RTA process. As the vacancies diffuse from the InGaAs cap layer into the MQW structure, the constituent elements of the quantum wells and barriers interdiffuse, resulting in a shift of the electronic levels in the quantum wells. Area-selectivity was achieved by photolithographically removing the SiO2 film in the regions where QWI was not needed. The PL measurements showed that highly uniform selective area quantum well intermixing was achieved in the sections covered with SiO2 film after the samples had undergone RTA. This paper reports on an all-optical switch that exploits the large carrier-induced nonlinearities of a MQW structure (Chemla et al. 1984) and that consists of elements with different absorption spectra that are monolithically integrated by the post-growth modification of the MQW structure. These techniques (Kan’an et al. 1996; Ooi et al. 1997) for selective area QWI provide a simple, reliable, and cost-effective approach for a monolithic integration that bypasses the need for a selective area epitaxy. An ultrafast, monolithically integrated, all-optical Mach-Zehnder interferometric (MZI) switch device was realized using selective area disordering.

2. Experiment A quantum well laser structure grown on n-type InP substrate by MOCVD is used. The active layer consists of seven 1% compressively strained InGaAs quantum wells, each with a thickness of 7 nm and lattice-matched InGaAsP barriers, each with a thickness of 14 nm. The upper and lower confinement layers are 70 nm undoped InGaAsP layers with an optical bandgap corresponding to a wavelength of 1.18 µm. The 1.5 µm-thick InP upper cladding layer is doped with zinc at a concentration of 2 × 1018 cm−3 . The whole structure is capped with a 100 nm thick InGaAs layer. A schematic drawing of the sample structure coated with a 200 nm thick SiO2 film is shown in Fig. 1. The samples were processed by RTA at 800, 830, and 850◦ C for 40 s in a flowing N2 ambient. The PLs of the samples were measured at room temperature, with a continuous wave (CW) Nd:YAG laser used as the optical pump. The normalized PL spectra in Fig. 2a indicate that the degree of the blue-shift in the position of the PL peak increases as the annealing temperature is increased. However, the increase in bandgap shift due to the increase in annealing temperature is also accompanied by a weaker and noisier PL spectrum, which is attributed to an increase in the level of defect generation.

MONOLITHICALLY INTEGRATED ALL-OPTICAL SWITCH

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Fig. 1. Schematic representation of the multiple quantum well structure.

Fig. 2. (a) Photoluminescence results of annealed samples with silica caps at different temperatures. (b) Multiple superimposed spectra from several intermixed regions with a silica cap and un-intermixed regions without a silica cap of a sample annealed at 830◦ C for 40 s.

In order to characterize the area selectivity of the intermixing process, a sample was coated with SiO2 and parts were removed using conventional photolithography and wet chemical etching by buffered HF acid. The sample was then annealed at 830◦ C for 40 s in flowing nitrogen ambient. It was found that good selectivity between the covered and uncovered parts was produced. In order to verify the areal uniformity of the QWI, PL spectra were measured at many different randomly selected locations (both intermixed and non-intermixed). Fig. 2b shows that the superimposed PL data taken from several intermixed and non-intermixed locations across the sample displayed good areal uniformity and selectivity. The PL data for the as-grown sample shows that only a small degree of intermixing occurred in the region that was not covered by the SiO2 cap.

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3. All-optical device and its operation A schematic drawing of the all-optical switch is shown in Fig. 3. In this investigation, a symmetric Mach-Zehnder type all-optical switching device was fabricated on a MQW sample that was selectively disordered using the IFVD. Photolithography and wet chemical etching were used to open 200 µm×750 µm windows in the SiO2 film. These regions correspond to the nonlinear phase sensitive sections of the MZI switch. The sample was then subjected to RTA to intermix the quantum wells. The spectra of the device sample showed that the peaks of ground state transitions of the quantum wells in the silica capped region and in the uncapped region were 1.451 and 1.528 µm, respectively, at room temperature. The 2 µm-wide ridge waveguides that form the integrated MZI were delineated using photolithography and wet chemical etching. The wet chemical etching was stopped at the surface of the InGaAsP core guiding layer. The two symmetric waveguide arms in the nonlinear phase sensitive sections were each 750 µm long, and the 3-dB directional coupler, which consisted of two waveguides separated by a gap of 1 µm, was 280 µm long. Fig. 4 shows a schematic representation of the experimental setup that was used to measure the all-optical switching performance. In this device test for all-optical switching, a wavelength conversion was demonstrated using a harmonically modelocked erbium-doped fiber laser that produced 50 ps pulses at a repetition frequency of 1GHz. The laser was operated at a wavelength of 1567 nm and provided the source for the signal pulses (λ1 ). A CW laser diode operating at a wavelength of 1550 nm (λ2 ) was also employed to provide the source for the probe beam. The beam from the CW laser diode was split into two separate beams that were launched into

Fig. 3. Schematic representation of the device structure.


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Fig. 4. Schematic of an experimental setup.

the two input arms (A & B) of the device. By carefully adjusting the phase of the two input beams, the output of the coupled lights was maximized at the lower output port D. The signal pulses from the modelocked laser were also split into two beams and a temporal delay was introduced between the two pulses before being launched into the two input arms (A & B) of the device. In this setup the preset delay was about 100 ps. The polarization of the signal pulses (λ1 ) was aligned along the quantum well layers so that the TE mode could maximize the absorption of the signal photons in the nonlinear sections of the device. The polarization of the CW laser beam (λ2 ) was oriented perpendicular to the quantum wells so that the TM mode could minimize photon absorption. Owing to the orthogonal polarizations of the signal beams and the CW laser beams, a single polarization beam splitter/combiner was used to combine the beams prior to launching them into the two input arms of the device. In our setup, a ×40 microscope objective lens was used to launch the laser beams into the device. The output beam from the upper output port C was collected using another ×40 objective lens and was amplified with an erbium-doped fiber amplifier (EDFA). The signal light (λ1 ) and the amplified spontaneous emission background were blocked using a band pass filter prior to sending it onto a high speed photoreceiver connected to a sampling oscilloscope. The average optical power in each of the signal beams (λ1 ) was about 10 mW at the polarization beam splitter/combiner. In the absence of the signal pulses, the two beams from the CW laser recombine at the 3-dB coupler by adjusting the phase difference so that all of the lights appear at the lower output port D. When a signal pulse arrives at input port A, the photons are absorbed in the nonlinear arm and electron-hole pairs are generated. These free carriers induce a phase modulation on the CW beam. If the phase change of the CW beam is equivalent to π-radians, then the recombination of the CW beams causes them to be switched into the upper output port C. In this measurement, after the preset time delay of 100 ps, the signal pulse arrives at input port B

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and generates carriers with density equal to that remaining in the adjacent arm, the induced phase changes in the two arms cancel each other out, and the CW laser beam is switched back into output port D. Consequently, the signal pulses λ1 arriving at the input ports A & B switch output λ2 into the output port C. Although the switching mechanism is due to carrier-induced nonlinearities in the MQW layer, the device geometry allows for a switching operation that is not restricted by the slow relaxation time of the nonlinearities (Nakamura et al. 1994). Fig. 5 shows the temporal variations of the output intensities at port C, when the pulses were blocked from entering port A and when the pulses were blocked from port B, respectively. The dynamic behaviour of the switching of the CW light by only one set of signal pulses is directly related to the carrier lifetime in the nonlinear section. From these measurements it can be inferred that the carrier lifetime in the quantum wells is about 1 ns. However, when the pulse beams are allowed to enter both ports A & B, the input signal pulses are converted into 100 ps duration pulses with a wavelength of 1550 nm, as shown in Fig. 6. The temporal width of the gating window is fully adjustable and its lower limit (50 ps in this case) is indeed set by the switching pulse duration, but the upper limit (hundreds of picoseconds), which for some applications is more important, has more to do with the carrier lifetime. In this investigation, all-optical wavelength conversion has been achieved using the carrier-induced nonlinear refractive index change in a MQW layer. It is expected that the device should be capable of performing switching on a 1 ps time scale if control pulses with duration of 1 ps are employed (Nakamura et al. 2001).

Fig. 5. Output signal at output port C using either set of signal pulses at port A or port B.


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Fig. 6. Output signal using both signal input ports.

4. Conclusion We have demonstrated the post-growth modification of the bandgap energy of quantum wells by impurity-free vacancy-induced disordering intermixing technique. The intermixing process is highly reproducible and produces uniform areal bandgap alterations. Simple photolithographic etching of the top SiO2 film is employed to achieve the area-selectivity that is so important for photonic integration. A monolithically integrated MZI all-optical switch has been realized using a selective area impurity-free vacancy disordering method. The temporal gating width must be limited to 50 ps because it mirrors the temporal width of the signal pulses. References Aoki, M., H. Sano, M. Suzuki, M. Takahashi, K. Uomi and A. Takai. Electron. Lett. 27 2138, 1991. Chemla, D.S., Miller, D.A.B., Smith, P.W., Gossard, A.C. and W. Wiegmann. IEEE J. Quantum Electron. 20 265, 1984. He, J.J., S. Charbonneau, P.J. Poole, G.C. Aers, Y. Feng, E.S. Koteles, R.D. Goldberg and I.V. Mitchell. Appl. Phys. Lett. 69 562, 1996. Helmy, A.S., S.K. Murad, A.C. Bryce, J.S. Aitchison, J.H. Marsh, S.E. Hicks and C.D.W. Wilkinson. Appl. Phys. Lett. 74 732 (1999) Kan’an, A.M., P. Likamwa, Mitra-Dutta and I. Pamulapati. J. Appl. Phys. 80 3179, 1996. Kotaka, I., K. Wkita, M. Okamoto, H. Asai and Y. Kondo. IEEE Photon. Technol. Lett. 5 61, 1993. Marsh, J.H. Semicon. Sci. Technol. 8 1136, 1993. Nakamura, S., K. Tajima and Y. Sugimoto. Appl. Phys. Lett. 65 283, 1994. Nakamura, S., Y. Ueno and K. Tajima. IEEE Photon. Technol. Lett. 13 1091, 2001. Ooi, B.S., A.C. Bryce, J.H. Marsh and J. Martin. Appl. Phys. Lett. 65 85, 1994. Ooi, B.S., A.C. Bryce, J.H. Marsh and J. Martin. IEEE J. Quantum Electron. 33 1784, 1997. Ralston, J.D., A.L. Moretti, R.K. Jain and F.A. Chambers. Appl. Phys. Lett. 52 1817, 1987. Wan, J.Z., J.G. Simmson and D.A. Thompson. J. Appl. Phys. 81 765, 1997.