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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 15, Iss. 19 — Sep. 17, 2007
  • pp: 12123–12130
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Low-saturation-energy-driven ultrafast all-optical switching operation in (CdS/ZnSe)/BeTe intersubband transition

G.W. Cong, R. Akimoto, K. Akita, T. Hasama, and H. Ishikawa  »View Author Affiliations


Optics Express, Vol. 15, Issue 19, pp. 12123-12130 (2007)
http://dx.doi.org/10.1364/OE.15.012123


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Abstract

The authors report their latest results on II–VI intersubband all-optical switches in which the 10 dB absorption saturation energy is lowered to ~2.0–2.2 pJ for 1.55–1.58 µm by decreasing the thickness of the active layer and increasing the refractive index difference between the core layer and the cladding layers in waveguides. Such low saturation energies greatly improve the switching performance. <7 pJ pump energy at 1520 nm is sufficient for realizing 10 dB switching operation at 1566 nm (switching energy: ~0.7 pJ/dB). To the best of our knowledge, these switching energy and saturation energy values are the lowest reported ones for such ultrafast intersubband all-optical switches at telecommunication wavelengths.

© 2007 Optical Society of America

1. Introduction

High-speed all-optical switch modules (>100 Gbps) are fundamental components in optical multiplexing and demultiplexing devices for next-generation fiber communication (λ~1.55 µm) networks. Such ultrafast nature can be satisfied in the intersubband transition (ISBT) of semiconductor quantum wells (QWs) due to its subpicosecond carrier relaxation [1

1. S. Noda, T. Uemura, T. Yamashita, and A. Sasaki, “All-optical modulation using an n-doped quantum-well structure,” J. Appl. Phys. 68, 6529–6531 (1990). [CrossRef]

3

3. J. Hamazaki, S. Matsui, H. Kunugita, K. Ema, H. Kanazawa, T. Tachibana, A. Kikuchi, and K. Kishino, “Ultrafast intersubband relaxation and nonlinear susceptibility at 1.55 µm in GaN/AlN multiple-quantum wells,” Appl. Phys. Lett. 84, 1102–1104 (2004). [CrossRef]

]. Therefore, quantum systems with sufficiently large conduction band offsets can be fabricated into ultrafast ISBT waveguide switches for the light of the transverse-magnetic (TM) polarization. So far, such switches have been reported in InGaAs/AlAsSb [4

4. S. Sekiguchi, T. Simoyama, H. Yoshida, J. Kasai, T. Mozume, and H. Ishikawa, “Waveguide design of InGaAs/AlAs/AlAsSb intersubband transition optical switch,” Technical Digest 2005 Optical Fibre Communication Conference (Optical Society of America, Washington, DC, 2005) paper OFE4.

,5

5. T. Simoyama, S. Sekiguchi, H. Yoshida, J. Kasai, T. Mozume, and H. Ishikawa, “Absorption dynamics in all-optical switch based on intersubband transition in InGaAs-AlAs-AlAsSb coupled quantum wells,” IEEE Photo. Technol. Lett. 19, 604–606 (2007). [CrossRef]

], GaN/AlN [6

6. N. Iizuka, K. Kaneko, and N. Suzuki, “Sub-picosecond modulation by intersubband transition in ridge waveguide with GaN/AlN quantum wells,” Electron. Lett. 40, 962–963 (2004). [CrossRef]

9

9. Y. Li, A. Bhattacharyya, C. Thomidis, T.D. Moustakas, and R. Paiella, “Nonlinear optical waveguides based on near-infrared intersubband transitions in GaN/AlN quantum wells,” Opt. Express 15, 5860–5865 (2007). [CrossRef] [PubMed]

], and (CdS/ZnSe)/BeTe [10

10. R. Akimoto, B.S. Li, K. Akita, and T. Hasama, “Subpicosecond saturation of intersubband absorption in (CdS/ZnSe)/BeTe quantum-well waveguides at telecommunication wavelength,” Appl. Phys. Lett. 87, 181104 (2005). [CrossRef]

,11

11. K. Akita, R. Akimoto, T. Hasama, H. Ishikawa, and Y. Takanashi, “Intersubband all-optical switching in submicron high-mesa SCH waveguide structure with wide-gap II-VI-based quantum wells,” Electron. Lett. 42, 1352–1353 (2006). [CrossRef]

] systems. The switching energy (Es) is the characteristic parameter used for evaluating the switching performance, which currently is still not low enough for above switches (32 pJ [4

4. S. Sekiguchi, T. Simoyama, H. Yoshida, J. Kasai, T. Mozume, and H. Ishikawa, “Waveguide design of InGaAs/AlAs/AlAsSb intersubband transition optical switch,” Technical Digest 2005 Optical Fibre Communication Conference (Optical Society of America, Washington, DC, 2005) paper OFE4.

], 100 pJ [7

7. N. Iizuka, K. Kaneko, and N. Suzuki, “Sub-picosecond all-optical gate utilizing an intersubband transition,” Opt. Express 13, 3835–3840 (2005). [CrossRef] [PubMed]

], and 11.3 pJ [11

11. K. Akita, R. Akimoto, T. Hasama, H. Ishikawa, and Y. Takanashi, “Intersubband all-optical switching in submicron high-mesa SCH waveguide structure with wide-gap II-VI-based quantum wells,” Electron. Lett. 42, 1352–1353 (2006). [CrossRef]

], respectively) to achieve a 10 dB switching extinction ratio (SER). Therefore, a further reduction in Es is necessary for practical switching operation at >100 Gbps.

2. Waveguide fabrication

3. Measurement setup

Figure 1 schematically shows the main components of the laser system for the absorption saturation and pump-probe measurements. A Ti:sapphire modelocked laser with a repetition rate of ~76 MHz and output power of ~1.2 W pumps an optical parametric oscillator (OPO) that generates two pulse beams. One beam can be varied within a large wavelength region by changing the output wavelength of the Ti:sapphire laser, while the other is normally maintained at ~1566 nm. The pulse durations of both beams can be adjusted to 0.3–0.4 ps by selecting suitable filters. A high-resolution stage and OD filters are controlled by a computer to gradually change the pump-probe delay time and the pulse power coupled to the fiber by a freespace-to-fiber (FF) coupler, respectively. Optical pulses are injected into the waveguide and are received through two lensed dispersion-shifted polarization-maintaining fibers (DS-PMF). For the input fiber between the FF coupler and the waveguide, we introduce a coupler that draws out a small fraction (~one eighth) of the pulse power to monitor the remaining power in the DS-PMF. Therefore, the input pulse energy is defined as the power that flows into the DS-PMF connecting the coupler and waveguide. Considering the fiber-to-waveguide loss, the real saturation energy as well as the real switching energy should be lower than our measured values. The input pulse energy is always monitored by an optical powermeter, while the output pulse has two path selections for power analysis. During the absorption saturation measurement, the output pulse energy is also analyzed by a powermeter to calculate the insertion loss, whereas for the pump-probe measurement, the output pulse in the fiber is coupled to a spectrometer by mirrors, which monitors the probe wavelength. Meanwhile, the probe beam is chopped, and a lock-in amplifier is used to increase the signal-to-noise ratio for the intensity analysis of the probe pulse. Except for aforementioned time-resolved system, a Fourier transform infrared spectrometer was used to measure the static intersubband absorption spectrum at room temperature. For this case, the samples were mechanically polished to the multiple-pass geometry.

Fig. 1. Time-resolved ultrafast laser system for the absorption saturation and pump-probe measurements.

4. Results and discussion

4.1 Beam propagation simulation

Fig. 2. Schematic tapered-waveguide model: (a) cross-section view and (b) top view with main parameters set up in BPM simulation. The inset table lists the real parameters of the waveguides in this work and our previous work. (h: height, w: width).

Figure 3(a) shows two simulated absorption saturation curves, from which an apparent improvement of saturation efficiency is observed due to the increase in RIC and the decrease in h AL. For quantitative comparisons, the half-saturation intensity (Ish) is defined in Fig. 3(a) as the input power that increases the insertion loss from the unsaturated value (T0) to 0.5T0. Here we use Ish instead of the 10 dB saturation energy since the maximum increase in transmittance is occasionally less than 10 dB. As seen, the increase in RIC and the decrease in h AL by two-thirds results in a ~67% reduction in Ish. In order to separate the effects of these two changes and further optimize the waveguide parameters, we map Ish against h OCL and w mesa for three situations with different h AL, n CL, and n OCL values, as shown in Figs. 3(b)–3(d). By comparing Fig. 3(b) and 3(c), we can understand the role of the increase in RIC, which enlarges the structure region allowing the existence of the optical mode and moves the valley path to a smaller h OCL value, although Ish at the valley paths exhibits no obvious difference. A comparison of Figs. 3(c) and 3(d) reveals a >50% decrease in Ish for any w mesa and h OCL values, which shows the remarkable effectiveness of decreasing h AL in lowering Ish. Ish exhibits a weak dependence on h OCL for w mesa >1.5 µm (Fig. 3(b)) and w mesa >1 µm (Fig. 3(c)). Unlike that in Figs. 3(b) and 3(c), the decrease in h OCL in Fig. 3(d) results in an apparent decrease in Ish for each w mesa value. Moreover, the valley path shown in Fig. 3(d) continues to move down to h OCL=0.3 for w mesa >1 µm. In all these three figures, Ish always decreases with decreasing w mesa for any h OCL value due to the improved optical confinement. Therefore, we can make the following conclusions to guide future waveguide design: (1) decreasing h AL can significantly decrease the saturation energy; (2) decreasing h OCL is more effective for thinner ALs; and (3) the lowest saturation intensity occurs close to the edge of the mode-inhibited region that depends on RIC and w mesa. The numerical results indicate a lower saturation energy in our current waveguide with thinner AL, which is confirmed in the following experimental parts.

Fig. 3. (a) Two representative simulated saturation curves for w mesa=1 µm. The contrast between the black and red lines corresponds to that between the previous and current waveguides. The Ish value of each line is shown. (b)–(d) show the contour mappings of Ish versus h OCL and w mesa. The optical mode does not exist in the black shaded regions and the dashed lines (valley paths) plot h OCL at the minimum Ish along w mesa. The two triangles in (b) and (d) indicate the approximate positions for the previous work and this work, respectively.

Our purpose of the simulation is to find a guidance direction to adjust waveguide parameters in order to increase the pulse power per unit area in AL cross section and to understand how the waveguide structure influences saturation behavior. By relative comparison with experiments, we can know how helpful the simulation is. However, it is necessary to mention that we cannot directly compare the simulated saturation curves with the experimental ones because the continuous wave light is used in simulation. In addition, the nonlinear parameters are different from the real values since it is difficult to know such parameters for a part of AL. So to make the results of different AL thicknesses comparable, we set constant nonlinear parameters in simulation for a thickness unit and only discuss what differences the changes in waveguide structure result in.

Next, we discuss the mechanism for optimizing structural parameters in reducing saturation energy. Generally, the saturation energy depends on the basic properties of quantum wells such as relaxation time and transition moment and waveguide structures. If assuming the nonlinear properties of AL is constant, the difference in the saturation energy should result from the difference in the waveguide structure. As we know, the waveguide structure determines the optical mode profile and the relative position of AL in the mode profile influences the saturation energy. We discuss two cases: (1) vary h AL and h OCL with a constant wmesa, and (2) vary wmesa with constant hAL and hOCL. (1) There should exist an optimum h OCL for each h AL. If h OCL is too large, the fraction of the pump power to excite AL will decrease. If h OCL is too small, either the optical mode will disappear, or the upper and lower areas of AL’s cross-section will approach the edges of the mode profile where the power is low. As a result, these areas will be unsaturated. Therefore, h AL should be decreased for AL to feel the power as large as possible, and meanwhile h OCL should be optimized to increase the pulse power per unit area in AL. As seen in Fig. 3(c) and 3(d), Ish at valley paths decreases by more than a half after optimizing h AL and h OCL. But the decrease in h AL will result in the decrease of on-off ratio. So we may need to adjust the doping contration. (2) Only if the optical mode exists, more smaller w mesa, more lower the saturation energy. This is because the pulse power per unit area in AL increases when horizontal light confinement becomes better with decreasing w mesa. In our case, RIC can not be increased more to get an obvious decrease in the saturation energy due to the composition limitation of CL. So it could be a feasible method to improve waveguide structure by the method introduced above.

Fig. 4. Static intersubband absorption spectra under p and s polarization measured by Fourier transform infrared spectrometer. The inset enlarges the ISBT region.

4.2 Absorption saturation

Before measuring the absorption saturation, the static intersubband absorption spectra were measured to check its peak position. We use the same thick epitaxial wafer as that for waveguide fabrication and polish a piece of wafer into multiple-pass geometry, as seen in Fig. 4. The absorption peak at ~1565 nm is observed in Fig. 4 for p-polarization, but absent from s-polarization, which is sufficient to attribute it to ISBT due to the quantum well confinement direction. Except for this peak, other peaks and valleys result from interference due to the existence of optical path difference when the light passes epilayers. Such interference does not exist in waveguide samples since the light is parallel to the surface of epilayers. When we change the incident angle, the interference phenomenon shows the same varied behavior for both p and s polarization, however, the ISBT peak position does not change. This difference further supports its intersubband origin. In the inset of Fig. 4, we can determine its peak position and full width at half maximum (FWHM) as ~1565 and 100 nm, respectively.

Fig. 5. (a) The waveguide absorption saturation curves of TM polarization at 1565 nm with a definition of 10 dB saturation energy. (b) Wavelength-dependent 10 dB saturation energy and TM insertion loss.

4.3 Switching performance

Fig. 6. Temporal transmitted probe intensity versus pump-probe delay-time under different pump energies (increase along y axis). The inset shows the pump-energy dependent SER.

5. Conclusion

Based on the composition optimization of the cladding layers and the introduction of a new ZnSe/BeTe MQW optical confinement layer in II–VI-based ISBT waveguide switches, the refractive index contrast between the core layer and the cladding layers was increased to ~0.2 and the thickness of the active layer was reduced to 0.072 µm. Beam propagation simulation revealed that the above changes could greatly improve the saturation properties. We experimentally confirmed that the 10 dB absorption saturation energy of such ultrafast switches was further decreased to 2.0–2.2 pJ for 1.55–1.58 µm. Further, the 10 dB switching operation at 1566 nm was achieved under only <7 pJ pump energy by using a control pulse wavelength of 1520 nm. The above switching energy (<0.7 pJ/dB) is the lowest reported value in ISBT ultrafast all-optical switches. These results are promising for packing waveguides into all-optical signal processing devices.

References and links

1.

S. Noda, T. Uemura, T. Yamashita, and A. Sasaki, “All-optical modulation using an n-doped quantum-well structure,” J. Appl. Phys. 68, 6529–6531 (1990). [CrossRef]

2.

N. Suzuki and N. Iizuka, “Feasibility study on ultrafast nonlinear optical properties of 1.55-µm intersubband transition in AlGaN/GaN quantum wells,” Jpn. J. Appl. Phys. 36, L1006–1008 (1997). [CrossRef]

3.

J. Hamazaki, S. Matsui, H. Kunugita, K. Ema, H. Kanazawa, T. Tachibana, A. Kikuchi, and K. Kishino, “Ultrafast intersubband relaxation and nonlinear susceptibility at 1.55 µm in GaN/AlN multiple-quantum wells,” Appl. Phys. Lett. 84, 1102–1104 (2004). [CrossRef]

4.

S. Sekiguchi, T. Simoyama, H. Yoshida, J. Kasai, T. Mozume, and H. Ishikawa, “Waveguide design of InGaAs/AlAs/AlAsSb intersubband transition optical switch,” Technical Digest 2005 Optical Fibre Communication Conference (Optical Society of America, Washington, DC, 2005) paper OFE4.

5.

T. Simoyama, S. Sekiguchi, H. Yoshida, J. Kasai, T. Mozume, and H. Ishikawa, “Absorption dynamics in all-optical switch based on intersubband transition in InGaAs-AlAs-AlAsSb coupled quantum wells,” IEEE Photo. Technol. Lett. 19, 604–606 (2007). [CrossRef]

6.

N. Iizuka, K. Kaneko, and N. Suzuki, “Sub-picosecond modulation by intersubband transition in ridge waveguide with GaN/AlN quantum wells,” Electron. Lett. 40, 962–963 (2004). [CrossRef]

7.

N. Iizuka, K. Kaneko, and N. Suzuki, “Sub-picosecond all-optical gate utilizing an intersubband transition,” Opt. Express 13, 3835–3840 (2005). [CrossRef] [PubMed]

8.

N. Iizuka, K. Kaneko, and N. Suzuki, “All-optical switch utilizing intersubband transition in GaN quantum wells,” IEEE J. Quantum Electron. 42, 765–771 (2006). [CrossRef]

9.

Y. Li, A. Bhattacharyya, C. Thomidis, T.D. Moustakas, and R. Paiella, “Nonlinear optical waveguides based on near-infrared intersubband transitions in GaN/AlN quantum wells,” Opt. Express 15, 5860–5865 (2007). [CrossRef] [PubMed]

10.

R. Akimoto, B.S. Li, K. Akita, and T. Hasama, “Subpicosecond saturation of intersubband absorption in (CdS/ZnSe)/BeTe quantum-well waveguides at telecommunication wavelength,” Appl. Phys. Lett. 87, 181104 (2005). [CrossRef]

11.

K. Akita, R. Akimoto, T. Hasama, H. Ishikawa, and Y. Takanashi, “Intersubband all-optical switching in submicron high-mesa SCH waveguide structure with wide-gap II-VI-based quantum wells,” Electron. Lett. 42, 1352–1353 (2006). [CrossRef]

12.

http://web1.rsoftdesign.com/products/component_design/BeamPROP/

13.

H.C. Liu and F. Capasso, Intersubband Transitions in Quantum Wells: Physics and Device Applications I (Academic Press, 2000), Chap. 1.

14.

P. Harrison, Quantum Wells, Wires and Dots: theoretical and computational physics (John Wiley & Sons Ltd, 2000), Chap. 3.

15.

R. Akimoto, K. Akita, F. Sasaki, and T. Hasama, “Sub-picosecond electron relaxation of newar-infrared intersubband transitions in n-doped (CdS/ZnSe)/BeTe quantum wells,” Appl. Phys. Lett. 81, 2998–3000 (2002). [CrossRef]

16.

C.V.-B. Tribuzy, S. Ohser, S. Winnerl, J. Grenzer, H. Schneider, M. Helm, J. Neuhaus, T. Dekorsy, K. Biermann, and H. Künzel, “Femtosecond pump-probe spectroscopy of intersubband relaxation dynamics in narrow InGaAs/AlAsSb quantum well structures,” Appl. Phys. Lett. 89, 171104 (2006). [CrossRef]

OCIS Codes
(190.5970) Nonlinear optics : Semiconductor nonlinear optics including MQW
(230.7370) Optical devices : Waveguides
(320.7080) Ultrafast optics : Ultrafast devices

ToC Category:
Ultrafast Optics

History
Original Manuscript: July 25, 2007
Revised Manuscript: August 16, 2007
Manuscript Accepted: August 16, 2007
Published: September 10, 2007

Citation
G. W. Cong, R. Akimoto, K. Akita, T. Hasama, and H. Ishikawa, "Low-saturation-energy-driven ultrafast all-optical switching operation in (CdS/ZnSe)/BeTe intersubband transition," Opt. Express 15, 12123-12130 (2007)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-15-19-12123


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References

  1. S. Noda, T. Uemura, T. Yamashita, and A. Sasaki, "All-optical modulation using an n-doped quantum-well structure," J. Appl. Phys. 68, 6529−6531 (1990). [CrossRef]
  2. N. Suzuki and N. Iizuka, "Feasibility study on ultrafast nonlinear optical properties of 1.55-µm intersubband transition in AlGaN/GaN quantum wells," Jpn. J. Appl. Phys. 36, L1006−1008 (1997). [CrossRef]
  3. J. Hamazaki, S. Matsui, H. Kunugita, K. Ema, H. Kanazawa, T. Tachibana, A. Kikuchi, and K. Kishino, "Ultrafast intersubband relaxation and nonlinear susceptibility at 1.55 µm in GaN/AlN multiple-quantum wells," Appl. Phys. Lett. 84, 1102−1104 (2004). [CrossRef]
  4. S. Sekiguchi, T. Simoyama, H. Yoshida, J. Kasai, T. Mozume, and H. Ishikawa, "Waveguide design of InGaAs/AlAs/AlAsSb intersubband transition optical switch," Technical Digest 2005 Optical Fibre Communication Conference (Optical Society of America, Washington, DC, 2005) paper OFE4.
  5. T. Simoyama, S. Sekiguchi, H. Yoshida, J. Kasai, T. Mozume, and H. Ishikawa, "Absorption dynamics in all-optical switch based on intersubband transition in InGaAs-AlAs-AlAsSb coupled quantum wells," IEEE Photon. Technol. Lett. 19, 604−606 (2007). [CrossRef]
  6. N. Iizuka, K. Kaneko, and N. Suzuki, "Sub-picosecond modulation by intersubband transition in ridge waveguide with GaN/AlN quantum wells," Electron. Lett. 40, 962−963 (2004). [CrossRef]
  7. N. Iizuka, K. Kaneko, and N. Suzuki, "Sub-picosecond all-optical gate utilizing an intersubband transition," Opt. Express 13, 3835−3840 (2005). [CrossRef] [PubMed]
  8. N. Iizuka, K. Kaneko, and N. Suzuki, "All-optical switch utilizing intersubband transition in GaN quantum wells," IEEE J. Quantum Electron. 42, 765−771 (2006). [CrossRef]
  9. Y. Li, A. Bhattacharyya, C. Thomidis, T.D. Moustakas, and R. Paiella, "Nonlinear optical waveguides based on near-infrared intersubband transitions in GaN/AlN quantum wells," Opt. Express 15, 5860−5865 (2007). [CrossRef] [PubMed]
  10. R. Akimoto, B.S. Li, K. Akita, and T. Hasama, "Subpicosecond saturation of intersubband absorption in (CdS/ZnSe)/BeTe quantum-well waveguides at telecommunication wavelength," Appl. Phys. Lett. 87, 181104 (2005). [CrossRef]
  11. K. Akita, R. Akimoto, T. Hasama, H. Ishikawa, and Y. Takanashi, "Intersubband all-optical switching in submicron high-mesa SCH waveguide structure with wide-gap II-VI-based quantum wells," Electron. Lett. 42, 1352−1353 (2006). [CrossRef]
  12. http://web1.rsoftdesign.com/products/component_design/BeamPROP/
  13. H.C. Liu and F. Capasso, Intersubband Transitions in Quantum Wells: Physics and Device Applications I (Academic Press, 2000), Chap. 1.
  14. P. Harrison, Quantum Wells, Wires and Dots: theoretical and computational physics (John Wiley & Sons Ltd, 2000), Chap. 3.
  15. R. Akimoto, K. Akita, F. Sasaki, and T. Hasama, "Sub-picosecond electron relaxation of newar-infrared intersubband transitions in n-doped (CdS/ZnSe)/BeTe quantum wells," Appl. Phys. Lett. 81, 2998-3000 (2002). [CrossRef]
  16. C.V.-B. Tribuzy, S. Ohser, S. Winnerl, J. Grenzer, H. Schneider, M. Helm, J. Neuhaus, T. Dekorsy, K. Biermann, and H. Künzel, "Femtosecond pump-probe spectroscopy of intersubband relaxation dynamics in narrow InGaAs/AlAsSb quantum well structures," Appl. Phys. Lett. 89, 171104 (2006). [CrossRef]

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