Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Carrier plasma shift in GaInAsP photonic crystal point defect cavity observed at l ¼ 1.536 mm with a full width at half maximum of 3.8 nm and the corresponding Q factor of 410. T. Baba, M. Shiga, K. Inoshita and F. Koyama input light A 1.43-mm-GaInAsP photonic crystal slab with a point defect cavity and line defect waveguides has been fabricated. A 1.5 mmwavelength light is inserted into the waveguide and the cavity resonant spectrum observed. A 5.6 nm blue shift in the resonant peak arising from the carrier plasma effect by photopumping is also observed. These results will be applicable to all-optical switches and tunable devices. cavity output spot air PC slab output spot 10 µm substrate Fig. 2 Near field pattern of light at l ¼ 1.504 mm, propagating in waveguide Top view (left) and side view of output end (right) Introduction: In recent years, applications of semiconductor photonic crystal (PC) slabs have been actively explored, e.g. point defect lasers [1], line defect waveguides [2], and resonant filters composed of point and line defects [3, 4]. However, there have been no reports of experiments on light control devices in PC slabs. Even apart from PC slabs, opal PCs infiltrated with liquid-crystals [5] are limited examples that show the possibility of such devices. A tunable PC with the carrier plasma effect has been discussed theoretically [6]. Normally, the response time of this effect is no shorter than nanoseconds. However, in a point defect cavity in a PC slab, the carrier relaxation is accelerated by the Purcell effect [7], so a response time of the order of picoseconds is possible. In this Letter, we report the fabrication of a filter structure of a point defect cavity and line defect waveguides in a GaInAsP PC slab, and the evaluation of the transmission characteristics with the carrier plasma effect by photopumping. Fabrication: An epiwafer was prepared with a 0.28 mm-thick 1.43 mm GaInAsP layer on InP substrate. The PC had a triangular lattice structure of airholes with a lattice constant a of 0.40 mm and an airhole diameter of 0.17–0.28 mm. The fabrication process flowed as (i) Ti=Ni evaporation, (ii) electron beam lithography, (iii) pattern transfer by CF4 inductively coupled plasma (ICP) and Ar electron cyclotron resonance plasma etchings, (iv) hole opening by Cl2=Xe-ICP etching, (v) formation of input=output ends by cleavage, and (vi) formation of the airbridge PC slab by HCl wet etching. A fabricated device is shown in Fig. 1. The point defect cavity of four missing airholes is sandwiched by PC photonic bandgap mirrors with four airholes and singleline defect waveguides. The total length of the device depended on cleaved positions and was typically 100 mm. Fig. 1 Top-view of fabricated device Measurements: Tunable laser light was controlled by transverseelectric polarisation and focused on the input end of the waveguide. The near field pattern (NFP) of light propagation from the top and lateral side of the sample was observed using vidicon cameras. The NFP for a resonant wavelength l ¼ 1.504 mm is shown in Fig. 2. Only around this wavelength did we observe scattered light at the cavity and a light spot at the output end. Then we shadowed the sample except for the output end using an aperture and detected the light output using an optical power meter. The spectrum of the light output observed by changing the source wavelength is shown in Fig. 3a. (Here, the sample is different from that for Fig. 2.) A resonant peak is Fig. 3 Spectra of light output (sample is different from that for Fig. 2) a Effective pump power of 850 mW b 13 mW c No excitation Next, CW laser light of 0.98 mm wavelength was irradiated in to the cavity from the top using a lensed fibre with a spot diameter of 10 mm. Considering the slab thickness, an absorption coefficient of 20 000 cm1 for irradiated light and a surface reflectivity of 30%, the absorbed power was estimated to be 30% of the irradiated power. Spectra near l ¼ 1.536 mm under such light irradiation are shown in Fig. 3b and c. The resonant peak blue-shifted by <1 nm for an absorbed power of 13 mW and 5.6 nm for 850 mW. Considering the surface recombination at airholes [7] and assuming a carrier lifetime of 4 ns, a pump power of 850 mW corresponds to a carrier density of 5.7 1017 cm3. It is expected to provide an index change Dn of 3.9 103 due to the carrier plasma effect. For this Dn, we calculated the resonant wavelength shift in the cavity to be 2.0 nm by the finite-difference time-domain method. Thus the measured value is 2.8 times larger than the theoretical value. One reason considered, which can explain this difference, is the resonant absorption of irradiated light in the PC slab. According to a three-dimensional photonic band calculation of the PC slab [8], the G point of the second band locates at a normalised frequency a=l 0.41, which corresponds to l 0.98 mm for a ¼ 0.4 mm. Therefore, the guided resonance effect [9] can strongly enhance the absorption of irradiated light. If it realises the perfect absorption, the wavelength shift will be 6.6 nm, which is much closer to the measured value. Conclusions: We have fabricated a filter structure of a point defect cavity and line defect waveguides in a GaInAsP PC slab and observed a resonant transmission spectrum. In addition, we have confirmed the carrier plasma shift of the resonant peak by photopumping, which can be explained by the resonant absorption effect. This result will be ELECTRONICS LETTERS 16th October 2003 Vol. 39 No. 21 applicable to fast optical switches and tunable devices based on PC slabs. 2 3 Acknowledgments: This work was supported by Nano-Photonic and Electron Devices Technology Project, and 21st Century COE program of the Ministry of Education, Culture, Sports, Science and Technology, and by CREST project of Japan Science and Technology Corporation. # IEE 2003 Electronics Letters Online No: 20030930 DOI: 10.1049/el:20030930 4 July 2003 T. Baba, M. Shiga and K. Inoshita (Department of Electrical and Computer Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogayaku, Yokohama, 240-8501, Japan) 4 5 6 7 8 E-mail: [email protected] F. Koyama (P&I Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama, 226-8503, Japan) 9 References 1 BABA, T., FUKAYA, N., and YONEKURA, J.: ‘Observation of light propagation in photonic crystal optical waveguides with bends’, Electron. Lett., 1999, 35, pp. 654–655 NODA, S., CHUTINAN, A., and IMADA, M.: ‘Trapping and emission of photons by a single defect in a photonic bandgap structure’, Nature, 2000, 407, pp. 608–610 LIN, S.Y., CHOW, E., JOHNSON, S.G., and JOANNOPOULOS, J.D.: ‘Direct measurement of the quality factor in a two-dimensional photoniccrystal microcavity’, Opt. Lett., 2001, 26, pp. 1903–1905 JOHN, S., and BUSCH, K.: ‘Photonic bandgap formation and tunability in certain self-organizing systems’, J. Lightwave Technol., 1999, 17, pp. 1931–1943 VILLENEUVE, P.R., ABRAMS, D.S., FAN, S., and JOANNOPOULOS, J.D.: ‘Singlemode waveguide microcavity for fast optical switching’, Opt. Lett., 1996, 21, pp. 2017–2019 BABA, T., and SANO, D.: ‘Low threshold lasing and Purcell effect in microdisk lasers at room temperature’, IEEE. J. Sel. Top. Quantum Electron., 2003 (to be published) BABA, T., MOTEGI, A., IWAI, T., FUKAYA, N., WATANABE, Y., and SAKAI, A.: ‘Light propagation characteristics of straight single-line-defect waveguides in photonic crystal slabs fabricated into a siliconon-insulator substrate’, IEEE. J. Quantum Electron., 2002, 38, pp. 743– 752 FAN, S., and JOANNOPOULOS, J.D.: ‘Analysis of guided resonances in photonic crystal slabs’, Phys. Rev. B., Solid State, 2002, 65, pp. 235112 PAINTER, O.J., HUSAIN, A., SCHERER, A., O’BRIEN, J.D., KIM, I., and DAPKUS, P.D.: ‘Two-dimensional photonic band-gap defect mode laser’, Science, 1999, 284, pp. 1819–1821 ELECTRONICS LETTERS 16th October 2003 Vol. 39 No. 21