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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 25 — Dec. 7, 2009
  • pp: 23247–23253
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Integration of GaN/AlN all-optical switch with SiN/AlN waveguide utilizing spot-size conversion

Norio Iizuka, Haruhiko Yoshida, Nobuto Managaki, Toshimasa Shimizu, Sodabanlu Hassanet, Chiyasit Cumtornkittikul, Masakazu Sugiyama, and Yoshiaki Nakano  »View Author Affiliations


Optics Express, Vol. 17, Issue 25, pp. 23247-23253 (2009)
http://dx.doi.org/10.1364/OE.17.023247


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Abstract

Spot-size converters for an all-optical switch utilizing the intersubband transition in GaN/AlN multiple quantum wells are studied with the purpose of reducing operation power by improving the coupling efficiency between the input fiber and the switch. With a stair-like spot-size converter, the absorption saturation of 5 dB is achieved with a pulse energy of 25 pJ. The switch is integrated with a SiN/AlN waveguide and spot-size converters, and the structure provides the possibility of an integration of the switch with other functional devices. To further improve the coupling loss between the waveguide and the switch, triangular-shaped converters are investigated, demonstrating losses as low as 2 dB/facet.

© 2009 OSA

1. Introduction

The intersubband transition (ISBT) in GaN/AlN multiple quantum wells (MQWs) has been of great interest for optical devices, such as detectors and switches that operate at optical communication wavelengths, since the conduction-band offset between GaN and AlN is sufficiently large and they can provide a widely tunable wavelength range with a simple quantum well (QW) structure [1

1. D. Hofstetter, S.-S. Schad, H. Wu, W. J. Schaff, and L. F. Eastman, “GaN/AlN-based quantum-well infrared photodetector for 1.55 μm,” Appl. Phys. Lett. 83(3), 572–574 ( 2003). [CrossRef]

, 2

2. N. Iizuka, K. Kaneko, and N. Suzuki, “All-optical switch utilizing intersubband transition in GaN quantum wells,” IEEE J. Quantum Electron. 42(8), 765–771 ( 2006). [CrossRef]

]. In addition, owing to the strong interaction with longitudinal optical phonons, the absorption recovery time is extremely short for the ISBT in GaN/AlN MQWs. It is therefore anticipated that ultra-fast all-optical switches could be realized that can be operate at a bit rate of 1 Tbps or higher.

Since Suzuki et al. proposed applying GaN ISBTs to all-optical switches [3

3. N. Suzuki and N. Iizuka, “Feasibility study on ultrafast nonlinear optical properties on 1.55-μm intersubband transition in AlGaN/GaN quantum wells,” Jpn. J. Appl. Phys. 36(Part 2, No. 8A), L1006–L1008 ( 1997). [CrossRef]

], there have been many achievements in this field of research. Iizuka et al. verified ultrafast absorption recovery at a wavelength of 4.6 μm [4

4. N. Iizuka, K. Kaneko, N. Suzuki, T. Asano, S. Noda, and O. Wada, “Ultrafast intersuband relaxation (≤150 fs) in AlGaN/GaN multiple quantum wells,” Appl. Phys. Lett. 77(5), 648–650 ( 2000). [CrossRef]

]. The first ISBT at optical communication wavelengths was reported by Ng et al. in 2000 [5

5. H. M. Ng, C. Gmachl, S. N. G. Chu, and A. Y. Cho, “Molecular beam epitaxy of GaN/AlxGa1-xN superlatttice for 1.52-4.2 μm intersubband transitions,” J. Cryst. Growth 220(4), 432–438 ( 2000). [CrossRef]

]. The characteristics of the absorption recovery were investigated by several groups [6

6. J. D. Heber, C. Gmachl, H. M. Ng, and A. Y. Cho, “Comparative study of ultrafast intersubband electron scattering times at = 1.55 μm wavelength in GaN/AlN heterostructures,” Appl. Phys. Lett. 81(7), 1237–1239 ( 2002). [CrossRef]

8

8. J. Hamazaki, H. Kunigita, K. Ema, A. Kikuchi, and K. Kishino, “Intersubband relaxation dymnamics in GaN/AlN multiple wuantum wells studied by two-color pump-probe experiments,” Phys. Rev. B 71(16), 1–5 ( 2005). [CrossRef]

]. Ultrafast all-optical modulation was demonstrated with an extinction ratio of 2.4 dB by the authors in 2004 [9

9. N. Iizuka, K. Kaneko, and N. Suzuki, “Sub-picosecond modulation by intersubband transition in ridge waveguide with GaN/AlN quantum wells,” Electron. Lett. 40(15), 962–963 ( 2004). [CrossRef]

], and a gate switch with an extinction ratio of more than 10 dB was reported in 2005 [10

10. N. Iizuka, K. Kaneko, and N. Suzuki, “Sub-picosecond all-optical gate utilizing GaN intersubband transition,” Opt. Express 13(10), 3835–3840 ( 2005). [CrossRef] [PubMed]

]. Absorption saturation at a wavelength as short as 1.43 μm was reported for a waveguide with AlN cladding layers [11

11. C. Kumtornkittikul, T. Shimizu, N. Iizuka, N. Suzuki, M. Sugiyama, and Y. Nakano, “AlN waveguide with GaN/AlN quantum wells for all-optical switch utilizing intersubband transition,” Jpn. J. Appl. Phys. Lett. 46(15), L352–L355 ( 2007). [CrossRef]

, 12

12. T. Shimizu, C. Kumtornkittikul, N. Iizuka, N. Suzuki, M. Sugiyama, and Y. Nakano, “Fabrication and measurement of AlN cladding AlN/GaN multiple-quantum-well waveguide for all-optical switching devices using intersubband transition,” Jpn. J. Appl. Phys. 46(No. 10A), 6639–6642 ( 2007). [CrossRef]

]

At present, lowering the operation energy is one of key issues for the application of ISBT in GaN/AlN MQWs to practical devices. Li et al., proposed a novel waveguide design to reduce the excess transmission loss with the purpose of decreasing in the switching energy [13

13. Y. Li, A. Bhattacharyya, C. Thomidis, T. D. Moustakas, and R. Paiella, “Ultrafast all-optical switching with low saturation energy via intersubband transitions in GaN/AlN quantum-well waveguides,” Opt. Express 15(26), 17922–17927 ( 2007). [CrossRef] [PubMed]

]. Another effective way for reducing the operation energy would be to reduce the cross section of the waveguide. To establish a high coupling efficiency between the input fiber and the narrow waveguide, it is necessary to employ a spot-size conversion (SSC).

In this study, SSC structures and their integration with the GaN/AlN ISBT switch were investigated. There are edge dislocations in the nitride semiconductors grown on a sapphire substrate, and the edge dislocations cause a propagation loss for transverse magnetic (TM) -polarized light [14

14. N. Iizuka, K. Kaneko, and N. Suzuki, “Polarization dependent loss in III-nitride optical waveguide for telecommunication devices,” J. Appl. Phys. 99(9), 1–5 ( 2006). [CrossRef]

]. In general, the edge dislocation density is higher near the substrate. Then, the downsizing the waveguide thickness for the SSC may lead to the increase in the excess propagation loss due to the higher dislocation density. On the other hand, SiN is proved to be a low-loss material at optical communication wavelengths [15

15. J. Kageyama, K. Kintaka, and J. Nishii, “Transmission loss characteristics of silicon nitride waveguides fabricated by liquid source plasma enhanced chemical vapor deposition,” Thin Solid Films 515(7-8), 3816–3819 ( 2007). [CrossRef]

]. Therefore, in this study, SiN instead of AlN was used as an upper cladding.

The SSC consists of the horizontal conversion and the vertical conversion. The horizontal conversion seems to be achieved by a simple lithography technology. Then, designing the vertical conversion has been a key for an efficient SSC. In this study, two types of SSC were proposed and examined. First, stair-like SSC were proposed and fabricated with an ISBT switch. With this structure, the absorption saturation due to the ISBT was verified and characterized. Then, for further reduction of the conversion loss, a triangularly-tapered structure was proposed and the light propagation through the SSC was simulated. Finally, the SiN/AlN waveguides with the triangularly-tapered SSC were fabricated and the low-loss coupling was verified.

2. Simulation of a tapered spot-size converter

First, tapered SSCs were examined with three dimensional finite-difference time-domain (3D-FDTD) method. In the simulation, the waveguides were assumed to consist of AlN (n = 2.03 [16

16. R. Hui, S. Taherion, Y. Wan, J. Li, S. X. Jin, J. Y. Lin, and H. X. Jiang, “GaN-based waveguide devices for long-wavelength optical communications,” Appl. Phys. Lett. 82(9), 1326–1328 ( 2003). [CrossRef]

]) and SiN (n = 1.85) on a sapphire substrate (n = 1.74). The refractive index of SiN was determined by the separate experiment. In Fig. 1
Fig. 1 Simulation results for the coupling efficiency. The waveguide consisted of AlN (n = 2.03) and SiN (n = 1.85) on a sapphire substrate (n = 1.74). The cross section of the input facet is 3.5 x.2.2 μm2. The beam diameter is 3 μm. (a) Comparison for smooth slopes and step-like slopes. The cross section of narrow waveguide is 0.8 x 1.2 μm2. (b) Simulation for the actual fabricated device.
, the calculated coupling efficiencies of the SSCs with TM-polarized light are shown as a function of the taper length. The simulations were performed for three-step stair-like SSCs as well as smooth-slope SSCs, because sophisticated technique will be necessary for fabrication of the smooth slope. In the simulations, input facet dimensions of 3.5 (width) x 2.2 (height) μm2 were investigated, with input beam diameters of 3 μm. The cross section of the narrow waveguides was 0.8 x 1.2 μm2. The results are plotted as a function of the taper length. For the slope-type SSC, the coupling loss is less than 1 dB even when the taper length is as short as 10 μm. The loss is almost unchanged for taper length, beam diameter and cross section of the waveguide (dependency on the beam diameter (1.6 – 3.2 μm) and the cross section of the input facet (2.4 x 2.4 – 3.2 x 3.2 μm2) are not shown in the figure), which indicates that the error tolerance of this structure is excellent. For the stair-like SSC, on the contrary, the coupling loss varies with the taper length. This dependency on the taper length is caused by the reflection at the stairs.

3. Fabrication of an ISBT switch with a stair-like tapered SSC

As shown in Fig. 1, the coupling efficiency for the slope-type SSC is better than that of the stair-type SSC. To realize such a smooth-slope structure, however, would require rather sophisticated fabrication techniques. On the other hand, the stair-like SSC could be achievable with simpler fabrication and the coupling efficiency is still expected to be more than 60%. Therefore, an ISBT switch was fabricated with the stair-like SSC to examine the applicability of the structure to an actual device. In addition, Si3N4 was utilized for a cladding layer with the following anticipated advantages: (1) quality of the dry etching will be better than that for AlN so that smooth sidewall is expected, and (2) there is no transmission loss caused by defects, whereas the edge dislocations in AlN layer lead to the loss. The device structure is schematically shown in Fig. 2
Fig. 2 Schematic view of the fabricated device. (a) Top view. (b) Side view.
.

After the growth of 0.4-μm thick AlN on a sapphire substrate by metalorganic vapor phase epitaxy (MOVPE), 100-nm-thick GaN, MQWs, and 100-nm-thick GaN were grown with molecular beam epitaxy (MBE). The MQWs consisted of two GaN (1.5 nm) /AlN (1.5 nm) QWs with Si doping in the wells of 2 x 1020 cm−3. The GaN and MQW layers were in part etched off, which formed the SSC and the outer waveguide. Then, the semiconductor layers were covered by 1.8-μm-thick Si3N4 deposited by sputtering method. The use of SiN made it possible to reform the waveguide rather than re-growing a thick AlN layer. This is a further advantage for using SiN as an upper cladding. The refractive index of the Si3N4 layer was 1.85 for a wavelength of 1.55 μm. Vertical SSC was realized by etching of the Si3N4 layer three times, which led to stair-like shape. Then, horizontal tapers were formed by etching the SiN layer and the semiconductor layers. Finally the wafer was thinned by polishing and cleaved to chips.

The lengths of the switches were 104 - 424 μm and the lengths of the tapers and the total device lengths were 36 and 1050 μm, respectively. The simulation indicated that a length of 8 μm would be sufficient for a good coupling. It also suggests, however, that the coupling become drastically worse as the taper length is shorter. Then, for the first fabrication, a long taper length was adopted in expectation of a sufficiently safe margin. The width of the input and output facets was 3.5 μm. Figure 3
Fig. 3 SEM images for the ISBT switch device. (a) Cross section of the switch. (b) Top view of the SSC.
shows images of scanning electron microscopy (SEM) for the switch. The cross section is shown in Fig. 3(a). The width was 2 μm at the top layer and 2.8 μm at the MQWs. Because the processing technique was not well-matured, the waveguide width was unintentionally wider than designed one (1.6 μm), and narrower waveguides could not be fabricated well. The improvement of the fabrication processing is a subject to be resolved. The total thickness was approximately 1.2 μm. Figure 3(b) shows a top view of the switch. It can be observed that the sidewall roughness is smaller at the SiN layer than at the AlN layer.

The devices were characterized by measurements of the insertion loss, absorption spectrum and absorption saturation. The spectra were measured with a wavelength range from 1.3 to 1.75 μm using a super continuum light that was produced with a femtosecond fiber laser and a photonic crystal fiber. The absorption saturation was characterized with an optical parametric oscillator excited by a mode-locked Ti:sapphire laser with a repetition rate of 80 MHz. The pulse width was nominally 130 fs. The polarization of the pulses were controlled with a polarizer and a wide-band waveplate. The light was input to the sample with a polarization-maintaining dispersion-shifted fiber.

Figure 4(a)
Fig. 4 (a) The ratio of the transmittance spectra for TM-polarization to that for TE-polarization for the sample with the length of 104 μm. The blue line represents the Gaussian fit. (b) Characteristics of the absorption saturation. Saturation by 5 dB was achieved with a pulse energy of 25 pJ.
shows the polarization dependent spectrum, that is, the ratio of transmittance for TM-polarization to that of transverse electric (TE) polarization for the sample with a switch length of 104 μm. The fringe oscillations observed in the trace of Fig. 4(a) were a result of the reflections at the step interfaces of the SSC. A Gaussian fit to this trace indicates an absorption peak wavelength of 1.48 μm and a full width at half maximum (FWHM) of 110 meV. These results verify that large and narrow ISBT absorption spectrum can be realized for a waveguide as short as 100 μm.

Since there is no ISBT for TE polarization it was possible to estimate the insertion loss for the device from the losses for TE polarization, which was found to be in the range of 12-13 dB. The dependence of the insertion loss on the length of the switch indicated that the loss was approximately 3 dB/100 μm for the narrow waveguide. Consequently, the loss is 9-10 dB except the marrow waveguide. The causes for the loss may be attributable to the coupling loss at the stair-like SSC. To estimate the coupling loss for the fabricated structure, simulations were performed for TM- and TE-polarized light as shown in Fig. 1(b). According to the results, the coupling efficiency is high for both polarizations. Therefore, the reason for the high insertion loss is unclear at present and it is a subject that needs further investigation.

The device structure has two advantages from the viewpoint of applications. Firstly, the switch is integrated with passive waveguides that mainly consist of SiN. Since the MQWs are removed, the optical loss should be very low in the passive waveguide regions. It will therefore be possible to fabricate additional integrated components such as splitters, filters and reflectors to expand the functionality of this device. Thus, the results shown in Fig. 4 open the way for integration of the ISBT switch with functional waveguide devices. Secondly, a switch as short as 100 μm could be realized owing to the integration with waveguides. So far, ISBT switches have been long due to handling requirements during the experiments and to facilitate the formation of facets by cleaving. Due to the integration process, however, these issues were overcome. By increasing the number of the wells, the switch can be further miniaturized. Excess losses caused by defects in the crystal or by surface roughness of the switch can be reduced for a shorter switch, which leads to further decrease in the operation energy.

4. Improvement of SSC

The structure of the stair-like SSC was a little troublesome due to the multiple etches required to make the steps. An alternative SSC structure with a simpler fabrication process was considered. This triangularly-tapered structure is schematically shown in Fig. 5(a)
Fig. 5 (a) Schematics of the devices structure designed for the fabrication. SEM images ware shown (b) for bird’s eye view of the SSC and (c) for cross section of the narrow waveguide.
. Simulations with three-dimensional beam propagation method (3-D BPM) suggested that a coupling loss of less than 1 dB could be expected.

5. Summary and conclusion

Structures of spot-size converters and their integration with GaN/AlN ISBT switches were studied with the purpose of reducing the operation energy by improving the coupling efficiency between the input fiber and the switch. SiN/AlN waveguide structure was also examined for low propagation loss. With a stair-like SSC, the absorption saturation of 5dB was achieved with a pulse energy of 25 pJ for a switch length of as short as 100 μm. Moreover, to establish a simpler structure and a better coupling efficiency, triangular-shaped converters were investigated. SiN/AlN waveguide with the triangular-shaped converter was fabricated and the coupling loss was as low as 2 dB/facet. These results led us to the conclusion that the switching power of GaN/AlN ISBT switches can be reduced by utilizing SSC, and, furthermore, the switches can be integrated with functional devices employing SiN/AlN waveguide.

Acknowledgements

This work was supported by Strategic Information and Communications R&D Promotion Programme (SCOPE) of the Ministry of Internal Affairs and Communications (MIC).

References and links

1.

D. Hofstetter, S.-S. Schad, H. Wu, W. J. Schaff, and L. F. Eastman, “GaN/AlN-based quantum-well infrared photodetector for 1.55 μm,” Appl. Phys. Lett. 83(3), 572–574 ( 2003). [CrossRef]

2.

N. Iizuka, K. Kaneko, and N. Suzuki, “All-optical switch utilizing intersubband transition in GaN quantum wells,” IEEE J. Quantum Electron. 42(8), 765–771 ( 2006). [CrossRef]

3.

N. Suzuki and N. Iizuka, “Feasibility study on ultrafast nonlinear optical properties on 1.55-μm intersubband transition in AlGaN/GaN quantum wells,” Jpn. J. Appl. Phys. 36(Part 2, No. 8A), L1006–L1008 ( 1997). [CrossRef]

4.

N. Iizuka, K. Kaneko, N. Suzuki, T. Asano, S. Noda, and O. Wada, “Ultrafast intersuband relaxation (≤150 fs) in AlGaN/GaN multiple quantum wells,” Appl. Phys. Lett. 77(5), 648–650 ( 2000). [CrossRef]

5.

H. M. Ng, C. Gmachl, S. N. G. Chu, and A. Y. Cho, “Molecular beam epitaxy of GaN/AlxGa1-xN superlatttice for 1.52-4.2 μm intersubband transitions,” J. Cryst. Growth 220(4), 432–438 ( 2000). [CrossRef]

6.

J. D. Heber, C. Gmachl, H. M. Ng, and A. Y. Cho, “Comparative study of ultrafast intersubband electron scattering times at = 1.55 μm wavelength in GaN/AlN heterostructures,” Appl. Phys. Lett. 81(7), 1237–1239 ( 2002). [CrossRef]

7.

R. Rapaport, G. Chen, O. Mitronov, C. Gmachl, H. M. Ng, and S. N. Chu, “Resonat optical nonlinearityies from intersubband transitions in GaN/AlN wuamtum wells,” Appl. Phys. Lett. 83(2), 263–265 ( 2003). [CrossRef]

8.

J. Hamazaki, H. Kunigita, K. Ema, A. Kikuchi, and K. Kishino, “Intersubband relaxation dymnamics in GaN/AlN multiple wuantum wells studied by two-color pump-probe experiments,” Phys. Rev. B 71(16), 1–5 ( 2005). [CrossRef]

9.

N. Iizuka, K. Kaneko, and N. Suzuki, “Sub-picosecond modulation by intersubband transition in ridge waveguide with GaN/AlN quantum wells,” Electron. Lett. 40(15), 962–963 ( 2004). [CrossRef]

10.

N. Iizuka, K. Kaneko, and N. Suzuki, “Sub-picosecond all-optical gate utilizing GaN intersubband transition,” Opt. Express 13(10), 3835–3840 ( 2005). [CrossRef] [PubMed]

11.

C. Kumtornkittikul, T. Shimizu, N. Iizuka, N. Suzuki, M. Sugiyama, and Y. Nakano, “AlN waveguide with GaN/AlN quantum wells for all-optical switch utilizing intersubband transition,” Jpn. J. Appl. Phys. Lett. 46(15), L352–L355 ( 2007). [CrossRef]

12.

T. Shimizu, C. Kumtornkittikul, N. Iizuka, N. Suzuki, M. Sugiyama, and Y. Nakano, “Fabrication and measurement of AlN cladding AlN/GaN multiple-quantum-well waveguide for all-optical switching devices using intersubband transition,” Jpn. J. Appl. Phys. 46(No. 10A), 6639–6642 ( 2007). [CrossRef]

13.

Y. Li, A. Bhattacharyya, C. Thomidis, T. D. Moustakas, and R. Paiella, “Ultrafast all-optical switching with low saturation energy via intersubband transitions in GaN/AlN quantum-well waveguides,” Opt. Express 15(26), 17922–17927 ( 2007). [CrossRef] [PubMed]

14.

N. Iizuka, K. Kaneko, and N. Suzuki, “Polarization dependent loss in III-nitride optical waveguide for telecommunication devices,” J. Appl. Phys. 99(9), 1–5 ( 2006). [CrossRef]

15.

J. Kageyama, K. Kintaka, and J. Nishii, “Transmission loss characteristics of silicon nitride waveguides fabricated by liquid source plasma enhanced chemical vapor deposition,” Thin Solid Films 515(7-8), 3816–3819 ( 2007). [CrossRef]

16.

R. Hui, S. Taherion, Y. Wan, J. Li, S. X. Jin, J. Y. Lin, and H. X. Jiang, “GaN-based waveguide devices for long-wavelength optical communications,” Appl. Phys. Lett. 82(9), 1326–1328 ( 2003). [CrossRef]

17.

N. Iizuka, T. Shimizu, C. Kumtornkittikul, and M. Sugiyama, and Y. Nakano, “Absorption saturation of AlN-based waveguide utilizing intersuuband transition in GaN/AlN quantum well,” presented at Joint Conference of the Opto-Electronics and Communications Conference and the Australian Conference on Optical Fibre Technology, Sydney, Australia, 7–10 July, 2008, TuH-6.

OCIS Codes
(190.5970) Nonlinear optics : Semiconductor nonlinear optics including MQW
(230.4320) Optical devices : Nonlinear optical devices
(230.7370) Optical devices : Waveguides
(250.6715) Optoelectronics : Switching

ToC Category:
Optical Devices

History
Original Manuscript: October 7, 2009
Revised Manuscript: November 16, 2009
Manuscript Accepted: November 22, 2009
Published: December 3, 2009

Citation
Norio Iizuka, Haruhiko Yoshida, Nobuto Managaki, Toshimasa Shimizu, Sodabanlu Hassanet, Chiyasit Cumtornkittikul, Masakazu Sugiyama, and Yoshiaki Nakano, "Integration of GaN/AlN all-optical switch with SiN/AlN waveguide utilizing spot-size conversion," Opt. Express 17, 23247-23253 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-25-23247


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References

  1. D. Hofstetter, S.-S. Schad, H. Wu, W. J. Schaff, and L. F. Eastman, “GaN/AlN-based quantum-well infrared photodetector for 1.55 μm,” Appl. Phys. Lett. 83(3), 572–574 (2003). [CrossRef]
  2. N. Iizuka, K. Kaneko, and N. Suzuki, “All-optical switch utilizing intersubband transition in GaN quantum wells,” IEEE J. Quantum Electron. 42(8), 765–771 (2006). [CrossRef]
  3. N. Suzuki and N. Iizuka, “Feasibility study on ultrafast nonlinear optical properties on 1.55-μm intersubband transition in AlGaN/GaN quantum wells,” Jpn. J. Appl. Phys. 36(Part 2, No. 8A), L1006–L1008 (1997). [CrossRef]
  4. N. Iizuka, K. Kaneko, N. Suzuki, T. Asano, S. Noda, and O. Wada, “Ultrafast intersuband relaxation (≤150 fs) in AlGaN/GaN multiple quantum wells,” Appl. Phys. Lett. 77(5), 648–650 (2000). [CrossRef]
  5. H. M. Ng, C. Gmachl, S. N. G. Chu, and A. Y. Cho, “Molecular beam epitaxy of GaN/AlxGa1-xN superlatttice for 1.52-4.2 μm intersubband transitions,” J. Cryst. Growth 220(4), 432–438 (2000). [CrossRef]
  6. J. D. Heber, C. Gmachl, H. M. Ng, and A. Y. Cho, “Comparative study of ultrafast intersubband electron scattering times at = 1.55 μm wavelength in GaN/AlN heterostructures,” Appl. Phys. Lett. 81(7), 1237–1239 (2002). [CrossRef]
  7. R. Rapaport, G. Chen, O. Mitronov, C. Gmachl, H. M. Ng, and S. N. Chu, “Resonat optical nonlinearityies from intersubband transitions in GaN/AlN wuamtum wells,” Appl. Phys. Lett. 83(2), 263–265 (2003). [CrossRef]
  8. J. Hamazaki, H. Kunigita, K. Ema, A. Kikuchi, and K. Kishino, “Intersubband relaxation dymnamics in GaN/AlN multiple wuantum wells studied by two-color pump-probe experiments,” Phys. Rev. B 71(16), 1–5 (2005). [CrossRef]
  9. N. Iizuka, K. Kaneko, and N. Suzuki, “Sub-picosecond modulation by intersubband transition in ridge waveguide with GaN/AlN quantum wells,” Electron. Lett. 40(15), 962–963 (2004). [CrossRef]
  10. N. Iizuka, K. Kaneko, and N. Suzuki, “Sub-picosecond all-optical gate utilizing GaN intersubband transition,” Opt. Express 13(10), 3835–3840 (2005). [CrossRef] [PubMed]
  11. C. Kumtornkittikul, T. Shimizu, N. Iizuka, N. Suzuki, M. Sugiyama, and Y. Nakano, “AlN waveguide with GaN/AlN quantum wells for all-optical switch utilizing intersubband transition,” Jpn. J. Appl. Phys. Lett. 46(15), L352–L355 (2007). [CrossRef]
  12. T. Shimizu, C. Kumtornkittikul, N. Iizuka, N. Suzuki, M. Sugiyama, and Y. Nakano, “Fabrication and measurement of AlN cladding AlN/GaN multiple-quantum-well waveguide for all-optical switching devices using intersubband transition,” Jpn. J. Appl. Phys. 46(No. 10A), 6639–6642 (2007). [CrossRef]
  13. Y. Li, A. Bhattacharyya, C. Thomidis, T. D. Moustakas, and R. Paiella, “Ultrafast all-optical switching with low saturation energy via intersubband transitions in GaN/AlN quantum-well waveguides,” Opt. Express 15(26), 17922–17927 (2007). [CrossRef] [PubMed]
  14. N. Iizuka, K. Kaneko, and N. Suzuki, “Polarization dependent loss in III-nitride optical waveguide for telecommunication devices,” J. Appl. Phys. 99(9), 1–5 (2006). [CrossRef]
  15. J. Kageyama, K. Kintaka, and J. Nishii, “Transmission loss characteristics of silicon nitride waveguides fabricated by liquid source plasma enhanced chemical vapor deposition,” Thin Solid Films 515(7-8), 3816–3819 (2007). [CrossRef]
  16. R. Hui, S. Taherion, Y. Wan, J. Li, S. X. Jin, J. Y. Lin, and H. X. Jiang, “GaN-based waveguide devices for long-wavelength optical communications,” Appl. Phys. Lett. 82(9), 1326–1328 (2003). [CrossRef]
  17. N. Iizuka, T. Shimizu, C. Kumtornkittikul, and M. Sugiyama, and Y. Nakano, “Absorption saturation of AlN-based waveguide utilizing intersuuband transition in GaN/AlN quantum well,” presented at Joint Conference of the Opto-Electronics and Communications Conference and the Australian Conference on Optical Fibre Technology, Sydney, Australia, 7–10 July, 2008, TuH-6.

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