- Email: [email protected]
Optical properties of asymmetric photonic crystals
Physica E 7 (2000) 681–685
www.elsevier.nl/locate/physe
Optical properties of asymmetric photonic crystals T. Fujita a , T. Kitabayashi a , A. Seki a , M. Hirasawa b , T. Ishihara a;b;∗ a
Department of Physical Electronics, Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan b CREST, Japan Science and Technology Corporation, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan
Abstract Transmission and photoluminescence spectra are investigated in periodic waveguides of tris(8-hydroxyquinoline) aluminum (Alq 3) with asymmetric pro le. Spontaneous emission pattern was found to be asymmetric due to the asymmetric coupling between the guided-wave mode and the radiation modes. The samples are fabricated by evaporating Alq 3 obliquely to quartz grating substrate and can be regarded as one-dimensional asymmetric photonic crystals. ? 2000 Elsevier Science B.V. All rights reserved. PACS: 42.82.Et; 42.70.Qs; 78.55.m Keywords: Asymmetric photonic crystal; Grating structure; Waveguide; Spontaneous emission
Photonic crystals are dielectric structures modulated periodically on the scale of optical wavelength. Under speci c conditions, they can exhibit a forbidden band of frequencies, a photonic band gap (PBG), in its electromagnetic dispersion relation [1,2]. The existence of a PBG gives rise to a number of interesting and useful properties, including the localization of light at defects [3,4], and the inhibition of the spontaneous emission [1], which would facilitate the advancement of optics and optoelectronics. Investigation on the exciton– photon strong coupling state in distributed feedback waveguide microcavities has been initiated by us [5 –7]. These microcavities were fabricated by spin coat∗ Corresponding author. Present address: Frontier Research System, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan. Tel.: +81-48-467-9604; fax: +81-48-465-8048. E-mail address: [email protected] (T. Ishihara)
ing an inorganic–organic semiconductor with large exciton oscillator strength on a quartz grating substrate with period in the order of optical wavelength. They can be regarded as one-dimensional photonic crystals consisting of a resonant dispersive material. Apart from the strong coupling, new phenomena may show up if one can endow asymmetry on such a periodic structure. The asymmetry of the radiation modes in the grating structure with an asymmetric pro le had been investigated long ago [8]. Introduction of the asymmetry to the photonic crystals is an interesting method for controlling of spontaneous emission: for example, emission pattern concentrated to speci c directions is expected. In this paper, we report on the transmission of and the spontaneous emission from periodically arranged molecular aggregates with asymmetric pro le. The setup of the oblique evaporation is schematically shown in Fig. 1. The one-dimensional
1386-9477/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 0 ) 0 0 0 3 7 - 0
682
T. Fujita et al. / Physica E 7 (2000) 681–685
Fig. 1. Schematic representation of oblique evaporation.
asymmetric photonic crystals are fabricated simply by evaporating molecules obliquely at angle to the quartz grating substrate. Since the aim of this particular work is to investigate the optical properties of the asymmetric photonic crystals in a wide range of spectrum, we used tris(8-hydroxyquinoline) aluminum (Alq3 for short) as an evaporation material, which has a broad emission band with large Stokes shift. Alq3 gives very ecient uorescence even at room temperature and is used for organic electroluminescent devices. Alq3 was deposited by thermal evaporation in a vacuum chamber at a pressure of 10−6 Torr. The substrate was kept at room temperature. The deposi and the total amount deposited tion rate was 5 A=s was 270 nm. The grating structures are fabricated on a quartz substrate by the electron beam lithography and the reactive ion etching techniques. Typical period , depth and line-and-space ratio of the grating structures are 700, 270 nm and 1 : 2, respectively. Fig. 2(a) shows the cross-sectional view of the one-dimensional asymmetric photonic crystal. Let us x the Cartesian coordinate so that the direction of the grating lines and the normal of the substrate are the y- and z-axis, respectively. The x-axis is the direction of the periodic modulation. We de ne +x direction as shown in the gure. Fig. 2(b) represents schematic dispersion relation (E versus kx ) for the
Fig. 2. (a) Cross-sectional view of the one-dimensional asymmetric photonic crystal (not to scale). (b) Schematic dispersion relation for guided-wave modes (PBG at crossing points and the guided-wave dispersion are neglected for simplicity). Solid and dashed lines represent original dispersion curves and their replicas. ˜!m are the mth-order resonant energies.
guided-wave modes in the one-dimensional photonic crystal, where we neglect PBG at the crossing points and the guided-wave dispersion for simplicity. By virtue of the reciprocal lattice vector, Gm = m(2=)xˆ where m is an integer, the original dispersion curves (solid lines) are folded to form replicas (dashed lines). The shadowed region represents the light cone in which the guided-wave modes are converted into the radiation modes satisfying kx conservation. An optical standing wave mode is formed at the photon energy that satis es kx − (−kx ) = Gm . Let us refer this energy to the mth-order resonant energy hereafter ˜c m; n∗ where n∗ = ckx =! is the e ective index of refraction for the waveguide. We performed two measurements at room temperature: one is a transmission measurement for investi-
˜!m =
T. Fujita et al. / Physica E 7 (2000) 681–685
Fig. 3. Transmission dip position as a function of kx in an asymmetric photonic crystal with the grating period of 640 nm. The size of closed circles stands for the relative coupling strength into each radiation mode.
gation of the asymmetry of the coupling strength into the radiation modes, and the other is PL for the emission pattern. These measurements were performed for a series of kx = k sin Â, in which  is the rotation angle of the substrate or the observation angle measured from the normal, for the case of transmission and PL measurements, respectively. In these experiments, we measured transmission (PL) spectra on the substrate (air) side. We performed these measurements ◦ ◦ ◦ ◦ for = 0 , 10 , 30 , and 60 samples. The following results were obtained for the polarization paral◦ lel to the grating lines of a = 10 sample which exhibits the strongest asymmetry among four samples. Fig. 3 shows a plot of the dip position on the transmission spectra for = 640 nm as a function of kx , which represents the in-plane dispersion relations of the radiation modes along the x direction. The size of
683
closed circles in Fig. 3 stands for the relative coupling strength between the guided-wave modes and the radiation modes in the substrate region. The fourth- and the third-order resonances are located at 2.50 and 1.95 eV, respectively. While the symmetric photonic crystals show a symmetric behavior of the coupling strength into each radiation mode with respect to kx = 0, the asymmetric photonic crystals show an asymmetric one as shown in Fig. 3. Fig. 4 shows PL spectra in the air region from the same asymmetric photonic crystal for a series of observation angle Â’s. To estimate the effects of the asymmetric periodic structure on the coupling eciency, each spectrum is normalized by the one without the grating structures (see the inset). The largest enhancement factor is 13. The observed peaks correspond to the dips in the transmission. In Fig. 4, we nd that the PL spectra are asymmetric with respect ◦ to  = 0 (kx = 0), as in transmission: the PL intensity is suppressed (enhanced) for a mode folded by +Gm (−Gm ). In both measurements, two modes folded by the same |Gm | in the asymmetric photonic crystals have di erent intensity: while it is weaker (stronger) for −kx + Gm (+kx − Gm ) radiation modes than the ones in the symmetric structures in the air region, it is stronger (weaker) in the substrate region. The largest PL intensity ratio between −kx + Gm and +kx − Gm radiation modes is 1 : 5 for m = 1 in the present case. Let us consider the origin of the asymmetric optical response in the asymmetric crystal. The dispersion relation for the guided-wave modes in the photonic crystal is determined by the Bragg di raction (Fig. 2(b)). The guided-wave mode with kx in the light cone are converted into two radiation modes. These radiation modes are tilted with respect to the z-axis at angle Âj given by √ k0 j sin Âj = kx ; where j = a, s designates the air or substrate regions, respectively, j is the dielectric constant of the jth medium and k0 is the wave number in vacuum. The radiation modes we observe in the transmission or the PL measurements are the ones propagating in the substrate or air regions, respectively. In the one-dimensional symmetric photonic crystal, the coupling strength from ±kx guided-wave modes folded by the same |Gm | into the radiation modes is symmetric with respect to kx = 0. Therefore, the emission pattern is also symmetric with respect to kx = 0. In
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T. Fujita et al. / Physica E 7 (2000) 681–685
Fig. 4. Normalized PL spectra for a series of observing angle Â’s in the asymmetric photonic crystal. The inset shows a PL spectrum of Alq3 without the grating structures.
the one-dimensional asymmetric photonic crystal, however, there arises a discrimination between the air and substrate regions: while the coupling into the radiation mode in the air (substrate) region is stronger (weaker) for +kx guided-wave mode, it is contrary to that for −kx mode. By introducing the asymmetry, the emission pattern in the air or substrate regions becomes asymmetric. Although we investigated the optical properties in the region of the third- and the fourth-order Bragg resonance due to the limit of spectral range of Alq 3 emission, the above phenomena are expected to be more pronounced for the lower resonance. Adoption of a material with a narrow emission band such as a free exciton emission may lead to highly ecient and unidirectional spon-
taneous emission owing to the limitation of the radiation modes. Period, asymmetry, and contrast of the dielectric function in the one-dimensional photonic crystal are related to the emission angle, and the coupling asymmetry and strength into the radiation mode, respectively. The sample we used is not an optimized asymmetric structure. We can expect more remarkable e ects by optimizing the structure appropriately. In conclusion, we have realized one-dimensional asymmetric periodic structures by means of oblique evaporation on a quartz grating substrate. They can be regarded as an array of the emitter with the asymmetric pro le. We have observed the asymmetric spontaneous emission pattern due to the asymmet-
T. Fujita et al. / Physica E 7 (2000) 681–685
ric coupling between the guided-wave mode and the radiation modes. Acknowledgements The electron-beam lithography and dry etching processes in this experiment were carried out at the Research Center for Nanodevices and Systems, Hiroshima University. This work was partly supported by CREST (Core Research for Evolutional Science and Technology) of JST (Japan Science and Technology Corporation) and Ministry of Education, Science, Sports, and Culture.
685
References [1] E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. [2] J.D. Joannopoulos, P.R. Villeneuve, S. Fan, Nature 386 (1997) 143. [3] S. John, Phys. Rev. Lett. 58 (1987) 2486. [4] E. Yablonovitch, T.J. Gmitter, R.D. Meade, A.M. Rappe, K.D. Brommer, J.D. Joannopoulos, Phys. Rev. Lett. 67 (1991) 3380. [5] T. Fujita, Y. Sato, T. Kuitani, T. Ishihara, Phys. Rev. B 57 (1998) 12 428. [6] T. Fujita, T. Ishihara, Y. Sato, M. Yamanishi, Tech. Dig. CLEO/Paci c. Rim’97, IEEE, NJ, p. 41, 1997. [7] T. Fujita, T. Ishihara, J. Phys. Soc. Japan 68 (1999) 2918. [8] S.T. Peng, T. Tamir, Opt. Commun. 11 (1974) 405.
www.elsevier.nl/locate/physe
Optical properties of asymmetric photonic crystals T. Fujita a , T. Kitabayashi a , A. Seki a , M. Hirasawa b , T. Ishihara a;b;∗ a
Department of Physical Electronics, Faculty of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan b CREST, Japan Science and Technology Corporation, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan
Abstract Transmission and photoluminescence spectra are investigated in periodic waveguides of tris(8-hydroxyquinoline) aluminum (Alq 3) with asymmetric pro le. Spontaneous emission pattern was found to be asymmetric due to the asymmetric coupling between the guided-wave mode and the radiation modes. The samples are fabricated by evaporating Alq 3 obliquely to quartz grating substrate and can be regarded as one-dimensional asymmetric photonic crystals. ? 2000 Elsevier Science B.V. All rights reserved. PACS: 42.82.Et; 42.70.Qs; 78.55.m Keywords: Asymmetric photonic crystal; Grating structure; Waveguide; Spontaneous emission
Photonic crystals are dielectric structures modulated periodically on the scale of optical wavelength. Under speci c conditions, they can exhibit a forbidden band of frequencies, a photonic band gap (PBG), in its electromagnetic dispersion relation [1,2]. The existence of a PBG gives rise to a number of interesting and useful properties, including the localization of light at defects [3,4], and the inhibition of the spontaneous emission [1], which would facilitate the advancement of optics and optoelectronics. Investigation on the exciton– photon strong coupling state in distributed feedback waveguide microcavities has been initiated by us [5 –7]. These microcavities were fabricated by spin coat∗ Corresponding author. Present address: Frontier Research System, RIKEN, 2-1 Hirosawa, Wako 351-0198, Japan. Tel.: +81-48-467-9604; fax: +81-48-465-8048. E-mail address: [email protected] (T. Ishihara)
ing an inorganic–organic semiconductor with large exciton oscillator strength on a quartz grating substrate with period in the order of optical wavelength. They can be regarded as one-dimensional photonic crystals consisting of a resonant dispersive material. Apart from the strong coupling, new phenomena may show up if one can endow asymmetry on such a periodic structure. The asymmetry of the radiation modes in the grating structure with an asymmetric pro le had been investigated long ago [8]. Introduction of the asymmetry to the photonic crystals is an interesting method for controlling of spontaneous emission: for example, emission pattern concentrated to speci c directions is expected. In this paper, we report on the transmission of and the spontaneous emission from periodically arranged molecular aggregates with asymmetric pro le. The setup of the oblique evaporation is schematically shown in Fig. 1. The one-dimensional
1386-9477/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 6 - 9 4 7 7 ( 0 0 ) 0 0 0 3 7 - 0
682
T. Fujita et al. / Physica E 7 (2000) 681–685
Fig. 1. Schematic representation of oblique evaporation.
asymmetric photonic crystals are fabricated simply by evaporating molecules obliquely at angle to the quartz grating substrate. Since the aim of this particular work is to investigate the optical properties of the asymmetric photonic crystals in a wide range of spectrum, we used tris(8-hydroxyquinoline) aluminum (Alq3 for short) as an evaporation material, which has a broad emission band with large Stokes shift. Alq3 gives very ecient uorescence even at room temperature and is used for organic electroluminescent devices. Alq3 was deposited by thermal evaporation in a vacuum chamber at a pressure of 10−6 Torr. The substrate was kept at room temperature. The deposi and the total amount deposited tion rate was 5 A=s was 270 nm. The grating structures are fabricated on a quartz substrate by the electron beam lithography and the reactive ion etching techniques. Typical period , depth and line-and-space ratio of the grating structures are 700, 270 nm and 1 : 2, respectively. Fig. 2(a) shows the cross-sectional view of the one-dimensional asymmetric photonic crystal. Let us x the Cartesian coordinate so that the direction of the grating lines and the normal of the substrate are the y- and z-axis, respectively. The x-axis is the direction of the periodic modulation. We de ne +x direction as shown in the gure. Fig. 2(b) represents schematic dispersion relation (E versus kx ) for the
Fig. 2. (a) Cross-sectional view of the one-dimensional asymmetric photonic crystal (not to scale). (b) Schematic dispersion relation for guided-wave modes (PBG at crossing points and the guided-wave dispersion are neglected for simplicity). Solid and dashed lines represent original dispersion curves and their replicas. ˜!m are the mth-order resonant energies.
guided-wave modes in the one-dimensional photonic crystal, where we neglect PBG at the crossing points and the guided-wave dispersion for simplicity. By virtue of the reciprocal lattice vector, Gm = m(2=)xˆ where m is an integer, the original dispersion curves (solid lines) are folded to form replicas (dashed lines). The shadowed region represents the light cone in which the guided-wave modes are converted into the radiation modes satisfying kx conservation. An optical standing wave mode is formed at the photon energy that satis es kx − (−kx ) = Gm . Let us refer this energy to the mth-order resonant energy hereafter ˜c m; n∗ where n∗ = ckx =! is the e ective index of refraction for the waveguide. We performed two measurements at room temperature: one is a transmission measurement for investi-
˜!m =
T. Fujita et al. / Physica E 7 (2000) 681–685
Fig. 3. Transmission dip position as a function of kx in an asymmetric photonic crystal with the grating period of 640 nm. The size of closed circles stands for the relative coupling strength into each radiation mode.
gation of the asymmetry of the coupling strength into the radiation modes, and the other is PL for the emission pattern. These measurements were performed for a series of kx = k sin Â, in which  is the rotation angle of the substrate or the observation angle measured from the normal, for the case of transmission and PL measurements, respectively. In these experiments, we measured transmission (PL) spectra on the substrate (air) side. We performed these measurements ◦ ◦ ◦ ◦ for = 0 , 10 , 30 , and 60 samples. The following results were obtained for the polarization paral◦ lel to the grating lines of a = 10 sample which exhibits the strongest asymmetry among four samples. Fig. 3 shows a plot of the dip position on the transmission spectra for = 640 nm as a function of kx , which represents the in-plane dispersion relations of the radiation modes along the x direction. The size of
683
closed circles in Fig. 3 stands for the relative coupling strength between the guided-wave modes and the radiation modes in the substrate region. The fourth- and the third-order resonances are located at 2.50 and 1.95 eV, respectively. While the symmetric photonic crystals show a symmetric behavior of the coupling strength into each radiation mode with respect to kx = 0, the asymmetric photonic crystals show an asymmetric one as shown in Fig. 3. Fig. 4 shows PL spectra in the air region from the same asymmetric photonic crystal for a series of observation angle Â’s. To estimate the effects of the asymmetric periodic structure on the coupling eciency, each spectrum is normalized by the one without the grating structures (see the inset). The largest enhancement factor is 13. The observed peaks correspond to the dips in the transmission. In Fig. 4, we nd that the PL spectra are asymmetric with respect ◦ to  = 0 (kx = 0), as in transmission: the PL intensity is suppressed (enhanced) for a mode folded by +Gm (−Gm ). In both measurements, two modes folded by the same |Gm | in the asymmetric photonic crystals have di erent intensity: while it is weaker (stronger) for −kx + Gm (+kx − Gm ) radiation modes than the ones in the symmetric structures in the air region, it is stronger (weaker) in the substrate region. The largest PL intensity ratio between −kx + Gm and +kx − Gm radiation modes is 1 : 5 for m = 1 in the present case. Let us consider the origin of the asymmetric optical response in the asymmetric crystal. The dispersion relation for the guided-wave modes in the photonic crystal is determined by the Bragg di raction (Fig. 2(b)). The guided-wave mode with kx in the light cone are converted into two radiation modes. These radiation modes are tilted with respect to the z-axis at angle Âj given by √ k0 j sin Âj = kx ; where j = a, s designates the air or substrate regions, respectively, j is the dielectric constant of the jth medium and k0 is the wave number in vacuum. The radiation modes we observe in the transmission or the PL measurements are the ones propagating in the substrate or air regions, respectively. In the one-dimensional symmetric photonic crystal, the coupling strength from ±kx guided-wave modes folded by the same |Gm | into the radiation modes is symmetric with respect to kx = 0. Therefore, the emission pattern is also symmetric with respect to kx = 0. In
684
T. Fujita et al. / Physica E 7 (2000) 681–685
Fig. 4. Normalized PL spectra for a series of observing angle Â’s in the asymmetric photonic crystal. The inset shows a PL spectrum of Alq3 without the grating structures.
the one-dimensional asymmetric photonic crystal, however, there arises a discrimination between the air and substrate regions: while the coupling into the radiation mode in the air (substrate) region is stronger (weaker) for +kx guided-wave mode, it is contrary to that for −kx mode. By introducing the asymmetry, the emission pattern in the air or substrate regions becomes asymmetric. Although we investigated the optical properties in the region of the third- and the fourth-order Bragg resonance due to the limit of spectral range of Alq 3 emission, the above phenomena are expected to be more pronounced for the lower resonance. Adoption of a material with a narrow emission band such as a free exciton emission may lead to highly ecient and unidirectional spon-
taneous emission owing to the limitation of the radiation modes. Period, asymmetry, and contrast of the dielectric function in the one-dimensional photonic crystal are related to the emission angle, and the coupling asymmetry and strength into the radiation mode, respectively. The sample we used is not an optimized asymmetric structure. We can expect more remarkable e ects by optimizing the structure appropriately. In conclusion, we have realized one-dimensional asymmetric periodic structures by means of oblique evaporation on a quartz grating substrate. They can be regarded as an array of the emitter with the asymmetric pro le. We have observed the asymmetric spontaneous emission pattern due to the asymmet-
T. Fujita et al. / Physica E 7 (2000) 681–685
ric coupling between the guided-wave mode and the radiation modes. Acknowledgements The electron-beam lithography and dry etching processes in this experiment were carried out at the Research Center for Nanodevices and Systems, Hiroshima University. This work was partly supported by CREST (Core Research for Evolutional Science and Technology) of JST (Japan Science and Technology Corporation) and Ministry of Education, Science, Sports, and Culture.
685
References [1] E. Yablonovitch, Phys. Rev. Lett. 58 (1987) 2059. [2] J.D. Joannopoulos, P.R. Villeneuve, S. Fan, Nature 386 (1997) 143. [3] S. John, Phys. Rev. Lett. 58 (1987) 2486. [4] E. Yablonovitch, T.J. Gmitter, R.D. Meade, A.M. Rappe, K.D. Brommer, J.D. Joannopoulos, Phys. Rev. Lett. 67 (1991) 3380. [5] T. Fujita, Y. Sato, T. Kuitani, T. Ishihara, Phys. Rev. B 57 (1998) 12 428. [6] T. Fujita, T. Ishihara, Y. Sato, M. Yamanishi, Tech. Dig. CLEO/Paci c. Rim’97, IEEE, NJ, p. 41, 1997. [7] T. Fujita, T. Ishihara, J. Phys. Soc. Japan 68 (1999) 2918. [8] S.T. Peng, T. Tamir, Opt. Commun. 11 (1974) 405.