Toward Small Size Waveguide Amplifiers Based on Erbium Silicate for ...
IEICE TRANS. ELECTRON., VOL.E91–C, NO.2 FEBRUARY 2008
138
INVITED PAPER
Special Section on Silicon Photonics Technologies and Their Applications
Toward Small Size Waveguide Amplifiers Based on Erbium Silicate for Silicon Photonics Hideo ISSHIKI†a) , Nonmember and Tadamasa KIMURA† , Member
SUMMARY Integration of light sources on a Si chip is one of milestone to establish new paradigm of LSI systems, so-called “silicon photonics.” In recent years remarkable progress has been made in the Si wire waveguide technologies for optical interconnection on a Si chip. In this paper, several Er embedded materials based on silicon are surveyed from the standpoint of application to the light emission and amplification devices for silicon photonics. We have concentrated to investigate an erbium silicate (Er2 SiO5 ) as a light source medium for silicon photonics. To mention the particular features, this material has a layered structure with 0.86-nm period and a large amount of Er (25at%) as its constituent. The single crystalline nature gives several remarkable properties for the application to silicon photonics. We also discuss our recent studies of Er2 SiO5 and a possibility of the shorter waveguide amplifier. key words: erbium, waveguide amplifier, upconversion, silicate
1.
Introduction
CMOS compatible photonics technologies, so-called “silicon photonics,” have developed rapidly and their achievements are attracting much attention to realize the optical interconnection in computing systems [1]. Silicon wire waveguide can play the core of silicon photonics because of low optical propagation loss and small bending radius for infrared lights due to the high refractive index contrast [2]. Taking optical signal processing into consideration, it is urgent to develop optical amplifiers those are shorter length than 1 mm. Optical signal processing devices are basically passive components and the signal decreases in each of the signal processes. This is a different situation from electrical signal processing, because the electrical signal is converted to an enough signal level in each of the electrical signal processes. In optical communication systems, development of erbium doped fiber amplifiers (EDFA) has been epoch-making at the end of last century [3], [4]. Erbium is used as an optically active element in the amplifiers because of its intra-4 f transition corresponding to the standard wavelength of 1.54μm. It can be considered that 1.5 μm light is also suitable for silicon photonics to combine photonic network. Nowadays growing the wavelength division multiplex (WDM) technologies, the success of EDFA has stimulated a great deal of interest in planar light circuits (PLC) with Er-doped waveguide amplifiers (EDWA), which can provide many signalprocessing functions to work together on a single chip of silManuscript received September 6, 2007. The authors are with The University of Electro-Communications (UEC), Chofu-shi, 182-8585 Japan. a) E-mail: [email protected] DOI: 10.1093/ietele/e91–c.2.138 †
ica (or silicon) platform [5], [6]. In order to achieve enough gain on the centimeter-scale PLC, higher erbium concentration of atomic percent range are needed. In this paper, several Er embedded materials based on silicon are surveyed from the standpoint of applying the light emission and amplification to silicon photonics. Recently we have reported to find a novel crystalline phase of erbium silicate formed on Si [7], [8]. So far we have investigated the erbium silicate (Er2 SiO5 ) as a novel light source medium for silicon photonics [9]–[12]. The single crystalline nature gives several remarkable properties for the application to silicon photonics. We also discuss our recent studies of Er2 SiO5 and a possibility of the shorter waveguide amplifier. 2.
Si Based Materials Embedded with Er Ions
Silicon based materials embedded with Er can be categorized as shown in Fig. 1. So far several kinds of Er doped materials have been frequently reported. In 1983, H. Ennen reported 1.54 μm PL emission from Er-doped crystalline Si fabricated by using ion implantation method (Fig. 1(a)) [13]. Er ions are excited by energy transfer through electron-hole pair recombination. He also demonstrated carrier injection excitation in an Er doped Si light emitting diode fabricated by molecular beam epitaxy (MBE) [14]. However crystalline Si doped with Er of about 1018 cm−3 shows strong thermal quenching of the Er-related emission resulting in very weak emission at room temperature. This is caused by Auger deexcitation and energy back flow from the excited 4f electron to carriers in the host. Furthermore nonradiative transition of the excited 4f electrons is clearly noticeable in high Er doping above 1019 cm−3 , which is caused by defects and clustering of Er due to low solubility of Er in crystalline Si. Several studies show that the oxygen codoping enhances the Er-related emission and improves the thermal quenching (Fig. 1(b)) [15], [16]. Oxygen co-doping also increases the solubility of Er in Si [17] and inhibits Er clustering during solid phase epitaxy [17], [18]. However the heavy doping still can induce defects that increase the non-radiative process. These materials make electrically pumping of Er possible, but the Er concentration is limited to about 1019 cm−3 . EDFA has been successfully realized by using Er doped SiO2 glass (Fig. 1(c)) [3], [4]. It can be considered that there is no effect of the solubility because of a glass media, however the influence of clustering still remains in the
c 2008 The Institute of Electronics, Information and Communication Engineers Copyright
ISSHIKI and KIMURA: TOWARD SMALL SIZE WAVEGUIDE AMPLIFIERS BASED ON Er2 SiO5 FOR SILICON PHOTONICS
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Fig. 1
Er embedded materials based on silicon. N is the total population of Er ions.
tra corresponding to the Er3+ intra 4f-shell transitions of I13/2 → 4 I15/2 . PL spectra of Er doped materials show inhomogeneous broadening due to effects of various crystalline fields as shown in Fig. 2(a). However Er2 SiO5 crystallites exhibit the PL spectrum fine structure with line width of less than 4 meV as shown in Fig. 2(b) [7], [8]. This result indicates that a large number of optically active Er is on one certain site in the crystalline matrix. We discuss in the next secession our recent studies of Er2 SiO5 crystalline films.
4
3.
Er2 SiO5 Crystalline Thin Film
3.1 Preparation and Structural Characterizations of Er2 SiO5 Thin Film
Fig. 2 PL spectra of Er-related emission from Er-doped nc-Si (a) and erbium silicate (Er2 SiO5 ) (b).
high doping above 1020 cm−3 . On the other hand, Er doped Si rich SiO (SRSO) has shown a large excitation crosssection after a proper annealing [19]. Nanocrystalline Si (nc-Si) with about 3-nm radius is formed by the annealing. The pumping energy can be transferred to Er through the electron-hole pair recombination in nc-Si. Then nc-Si functions as a sensitizer because of its large absorption crosssection. However the effective Er concentration is limited to about 1019 cm−3 by a number of nc-Si [20]. In contrast, crystalline erbium silicates contain a few ten percent of Er as a constitutional element. The single crystalline nature gives suppression of the Er clustering and defects, resulting in realization of huge active Er density (∼1022 cm−3 ). Therefore crystalline erbium silicates can be expected to become a medium for small size and high optical gain amplifiers. Er3+ -related emission is very sensitive to detrimental interactions with the surrounding matrix. Figure 2 shows room temperature PL emission spec-
In our first report, Er2 SiO5 crystallites were formed by wet chemical synthesis using spin-coating of ErCl3 /ethanol solution and the following two-step annealing of 600◦ C in O2 and 1200◦ C in Ar [7], [8]. After that we have proposed the thin film formation processes, which are using porous Si [9], sol-gel method [10] and metalorganic molecular beam epitaxy (MOMBE) [11], [12]. We describe here about solgel method, since there is an accumulation of data acquired through many observations. Sol-gel solutions mixed with Er-O and Si-O precursors were prepared using our own recipe by Kojundo Chemical Lab. CO., LTD. The sol-gel solution was spin-coated on Si(100) substrate and the samples dried at 140◦ C for 30 min in air. Then the annealing process was performed at 600◦ C for 30 min in Ar atmosphere to obtain an ErSiO amorphous preform. This procedure was repeated 5 times to obtain sufficient thickness. Finally the heat-treatment at about 1100◦ C was performed for 30 min in Ar atmosphere for the crystallization. Figure 3 shows cross sectional TEM image of ErSiO crystalline compound [8]. The ErSiO crystallite has a layered structure with 0.86-nm period and belongs to the monoclinic system, as shown in the TEM image. D-spacing of 0.86-nm is also observed a diffraction peak at about 10 degree in XRD results, as shown in Fig. 4. Figure 4 (a) shows
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Fig. 3 TEM cross-sectional photograph of an ErSiO crystalline compound prepared by a wet-chemical method using ErCl/ethanol solution [8]. Fig. 5 Er-related PL emission spectra of Er2 SiO5 thin film at 20 K and 300 K. The line width is less than 1 meV at 20 K and 8-peaks correspond to Stark splitting of the ground state in Er ions.
tails will be presented elsewhere. The phase separation exhibits highly oriented crystallization especially in the higher Si-O mole fraction. Figure 4(b) shows an XRD pattern of the highly oriented Er2 SiO5 synthesized from a sol-gel solution of Er:Si = 1:2. Before annealing, there is no peak but Si(200) originated from the Si substrate. As shown in Fig. 4(b), each XRD peaks oriented to Si [100] corresponds to (n00) diffraction (n = 1–5) of Er2 SiO5 . We have also demonstrated the possibility of Er2 SiO5 hetero-epitaxy on Si by using MOMBE [12]. 3.2 PL Characterizations of Er2 SiO5 on SiO2
Fig. 4 XRD patterns of ErSiO crystalline thin films prepared by sol-gel method. Filled circles in upper side indicate the peaks of Er2 SiO5 from a JCPDS file of #52-1809. Lower side is of a highly oriented Er2 SiO5 Each peaks can be assigned (n00) diffraction peaks. The peak at around 10 degree corresponds to d-spacing of 0.86-nm shown in TEM photograph.
typical XRD pattern of the ErSiO crystalline thin film. At the beginning, the ErSiO crystalline compound structure could not be determined only by the TEM result. Then we have reviewed JCPDS files published after our earlier reports [7], and found a diffraction file on Er2 SiO5 (#52-1809) in partially agreement with the XRD pattern. Filled circles in the figure indicate the corresponding peaks. An RBS result also indicates the composition of Er2 SiO5 . From these results, we can determine that the ErSiO crystalline compound is an erbium silicate of Er2 SiO5 . Peak positions of the XRD pattern are in good agreement with the reference, however the relative intensities are not in agreement among them. Er2 SiO5 crystalline thin films can be obtained, however the crystallization basically does not depend on the mole fraction of Er-O and Si-O precursors in the sol-gel solution. Our recent result shows phase separation growth of Er2 SiO5 in the Si-O rich media. The de-
Refractive index of Er2 SiO5 thin film was estimated to be 1.8 by ellipsometry and reflection spectroscopy measurements. In order to achieve high optical confinement in Er2 SiO5 waveguide, SiO2 is suitable as the cladding materials for silicon photonics applications. From the XRD result, it is confirmed that the sol-gel procedure makes possible to reproduce the typical Er2 SiO5 thin film on SiO2 . Figure 5 shows PL spectra at 20 K and 300 K of Er2 SiO5 on SiO2 . A 650 nm-LD with 30 mW was used as the pumping light. The PL spectrum fine structure at 20 K indicates a crystal field splitting of the 4 I15/2 manifold in Er3+ ions due to the local environment. In the figure, mainly 8 peaks are clearly seen. The 4 I15/2 level splits into 8 lines by crystalline field, in spite of cubic symmetry. These results suggest that the Er-related emission at low-temperatures comes from one type of Ercenter caused by a certain crystal field in a homogeneous medium. On the other hand, the linewidth of the 1528-nm peak broadens to 4 meV (7-nm) at 300 K and many other peaks are observed as the hot lines. Then thermal-quenching of the PL peak intensity is small as about 20%, and the integrated intensity are almost same. The Stark splitting is still clearly seen at room temperature and few types of Er-centers may be appeared.
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Fig. 7 Decay rate of the first excited state (4 I13/2 ) in Er ion in Er2 SiO5 as a function of the SOI interlay thickness, which corresponds to the optical mode density. Also the dependence of PL intensity is shown. The PL intensity is proportional to radiative transition rate.
Fig. 6 Decay curves of 1528-nm PL emissions at 20 K from various Er2 SiO5 /Si/SiO2 structures. System response is less than 2 μs. Schematic of the structure is also shown on upper side.
3.3 Fast Decay Time of PL Emission from Er2 SiO5 PL decay curves were corrected by time-gated photon counting method using an InGaAs photomultiplier. The system time response is less than 2 μs. A pulse-modulated laser diode (650-nm, 30 mW) was used as a pumping light source, which corresponded to the transition from 4 I15/2 to 4 F9/2 . Under this condition, any upconversion emission was not observed. Figure 6 shows PL decay curves of 1.528 μm peak emission at 20 K. The PL decay time of a 220-nm thick Er2 SiO5 thin film formed on Si is 19 μs. Remarkable temperature quenching of the decay rate is not observed. The decay time τ f is about three orders faster than that of Er ion doped into SiO2 . In order to investigate the extreme fast decay, we have attempted modulation of the radiative transition rate wrad [21]. The total decay rate w f is given by w f = τ−1 f = wrad + wnon−rad
(1)
then wrad and wnon−rad are radiative and non-radiative transition rates respectively. Also PL emission intensity at the steady state IPL is proportional only to the radiative transition rate wrad IPL = N2 · wrad
(2)
then N2 is population of the excited state. According to Fermi’s golden rule, the radiative transition rate is proportional to the optical mode density and can be modified by changing the layer structure. Schematics of the sample
structure and the profile of refractive indexes are also shown in Fig. 6. Then the mode density increases with increasing thickness of SOI inter layer. Er2 SiO5 layer with 220nm thick is formed at the same time on each SOI sample. The SOI thickness dependence of the decay curve is clearly seen in Fig. 6. With increasing the SOI thickness, the decay time become shorter and the PL intensity increases. Figure 7 shows the transition rate w f and PL intensity at the steady state as a function of the SOI thickness. Then the PL intensity is normalized in consideration of a multiple reflection effect for the excitation of the SOI structure. Behavior of the transition rate is due to the change of the radiative transition rate wrad because the non-radiative processes are considered to be independent of the structure. The PL intensity is also proportional to the radiative transition rate wrad . From the figure, it is seen that the two resemble each other very much. These results suggest that the radiative process is dominant in the decay rate (w f ∼ wrad ) and the extreme fast decay is mainly due to increasing the radiative transition rate. A result of the optical mode density calculation also supports this consideration. 3.4 Upconversion Emission from Er2 SiO5 Waveguide From results of the decay time measurements, we have concluded that the extreme fast decay is mainly due to the radiative transition at the low power excitation. Upconversion is frequently observed at the high power excitation, and becomes the non-radiative process. We have also investigated the upconversion emission in an Er2 SiO5 waveguide [22], [23]. An SiO2 /Er2 SiO5 /polymer slab waveguide was prepared. Polymer film (n = 1.54) with 4 μm thickness was used as the over cladding layer. To obtain lateral optical confinement, strip-loaded structures with 5-μm width polymer stripe were formed by focused ion beam (FIB) etching. Facets of the Er2 SiO5 waveguide were also formed by FIB
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guide is also given by φ = φ0 exp (−αΓx) ,
(3)
then α and Γ are the absorption coefficient of Er ions for 1480-nm light and the optical confinement factor. Then scattering loss is not considered. The populations of the states are labeled N1 (4 I15/2 ), 4 N2 ( I13/2 ) and N3 (4 S 3/2 and 2 H11/2 ). In this excitation condition, N2 is proportional to the pumping light φ, and the upconversion emission intensity is proportional to N3 . Considering cooperative upconversion from 4 I15/2 to 4 S 3/2 or 2 H11/2 , three ions excited to the 4 I13/2 state couple with each other. In the rate equation of the three exciton system, green ∝ N3 ∝ N23 . IPL
Fig. 8 Top view of upconversion emission profile. The pumping light comes from left side through a lensed fiber. Line profile along the waveguide is also shown. Filled circles indicate the emission from Er2 O3 clusters. Dash line is extrapolation of the emission decay.
etching. The optical confinement factor Γ of the Er2 SiO5 waveguide layer is estimated to be 0.42, assuming the lateral confinement factor is 1 because of the sufficient wide waveguide for the light. In addition a λ/4 air gap (λ = 1.5 μm) as a photon trap for the pumping light was formed on the waveguide at a position of 86 μm from the left facet by FIB. The waveguide was coupled with a lensed fiber. Optical pumping was performed by 1480-nm light coming from the left side through the lensed fiber. The pumping power is 30 mW at the fiber input. Figure 8 shows a top view of the waveguide. The photograph was taken by a conventional CCD system. As shown in Fig. 8, a visible (green) emission along the waveguide can be seen. The green emission corresponds to 4f intra-shell transitions from 4 S 3/2 and 2 H11/2 to 4 I15/2 in Er3+ ions, which is due to stepwise upconversion of Er3+ ions. It is indicated that the 1480-nm propagating light is well confined into the lateral and vertical directions of the waveguide. Line profile of the upconversion emission along the waveguide is also shown in Fig. 8. The emission intensity gradually decays and then rapidly goes down at a position around 30 μm from the input facet, and the upconversion process exhibits a certain criterion for the excitation. The upconversion emission appears again at a position around 85 μm from the input facet, which corresponds to the photon trap for 1480-nm light. It is indicated that the 1480-nm propagating light still survives beyond the upconversion emission decay. Moreover some small green spots are observed beyond the decay. These spots correspond to upconversion emissions from Er clusters (Er2 O3 ). In the line profile, the spots are also indicated by filled circles. This result suggests that the upconversion hardly occurs in Er2 SiO5 in comparison with Er2 O3 . A dash line in Fig. 8 shows an extrapolated line of the decay, which is fitted by an exponential curve of exp(−ax). The 1480-nm pumping light propagation along the wave-
(4)
Then using these relations, the upconversion emission line profile corresponding to the gradually decay is expressed by green ∝ φ3 = φ30 exp (−3αΓx) IPL
(5)
From the fitting of this part, 3αΓ is estimated to be 135.8 cm−1 . Then the absorption coefficient of Er ions for 1480-nm light α is estimated to be 107.7 cm−1 . The absorption coefficient becomes approximately σabs N, then σabs is the absorption cross-section and estimated to be ∼10−20 cm2 . This value is comparable with values found in an earlier report [21]. Optical gain G is given by G (dB) = 4.43 × (σem N2 − σabs N1 ) ΓL
(6)
then σem is the emission cross-section and comparable with the absorption cross-section. When the pump intensity is very strong, such that the erbium ions are all inverted, N2 becomes approximately N. Then the gain factor is estimated to be 47.7 dB/mm. This value seems to be a little bit overestimated but it can be expected to be sufficient to realize micro waveguide amplifiers (L
138
INVITED PAPER
Special Section on Silicon Photonics Technologies and Their Applications
Toward Small Size Waveguide Amplifiers Based on Erbium Silicate for Silicon Photonics Hideo ISSHIKI†a) , Nonmember and Tadamasa KIMURA† , Member
SUMMARY Integration of light sources on a Si chip is one of milestone to establish new paradigm of LSI systems, so-called “silicon photonics.” In recent years remarkable progress has been made in the Si wire waveguide technologies for optical interconnection on a Si chip. In this paper, several Er embedded materials based on silicon are surveyed from the standpoint of application to the light emission and amplification devices for silicon photonics. We have concentrated to investigate an erbium silicate (Er2 SiO5 ) as a light source medium for silicon photonics. To mention the particular features, this material has a layered structure with 0.86-nm period and a large amount of Er (25at%) as its constituent. The single crystalline nature gives several remarkable properties for the application to silicon photonics. We also discuss our recent studies of Er2 SiO5 and a possibility of the shorter waveguide amplifier. key words: erbium, waveguide amplifier, upconversion, silicate
1.
Introduction
CMOS compatible photonics technologies, so-called “silicon photonics,” have developed rapidly and their achievements are attracting much attention to realize the optical interconnection in computing systems [1]. Silicon wire waveguide can play the core of silicon photonics because of low optical propagation loss and small bending radius for infrared lights due to the high refractive index contrast [2]. Taking optical signal processing into consideration, it is urgent to develop optical amplifiers those are shorter length than 1 mm. Optical signal processing devices are basically passive components and the signal decreases in each of the signal processes. This is a different situation from electrical signal processing, because the electrical signal is converted to an enough signal level in each of the electrical signal processes. In optical communication systems, development of erbium doped fiber amplifiers (EDFA) has been epoch-making at the end of last century [3], [4]. Erbium is used as an optically active element in the amplifiers because of its intra-4 f transition corresponding to the standard wavelength of 1.54μm. It can be considered that 1.5 μm light is also suitable for silicon photonics to combine photonic network. Nowadays growing the wavelength division multiplex (WDM) technologies, the success of EDFA has stimulated a great deal of interest in planar light circuits (PLC) with Er-doped waveguide amplifiers (EDWA), which can provide many signalprocessing functions to work together on a single chip of silManuscript received September 6, 2007. The authors are with The University of Electro-Communications (UEC), Chofu-shi, 182-8585 Japan. a) E-mail: [email protected] DOI: 10.1093/ietele/e91–c.2.138 †
ica (or silicon) platform [5], [6]. In order to achieve enough gain on the centimeter-scale PLC, higher erbium concentration of atomic percent range are needed. In this paper, several Er embedded materials based on silicon are surveyed from the standpoint of applying the light emission and amplification to silicon photonics. Recently we have reported to find a novel crystalline phase of erbium silicate formed on Si [7], [8]. So far we have investigated the erbium silicate (Er2 SiO5 ) as a novel light source medium for silicon photonics [9]–[12]. The single crystalline nature gives several remarkable properties for the application to silicon photonics. We also discuss our recent studies of Er2 SiO5 and a possibility of the shorter waveguide amplifier. 2.
Si Based Materials Embedded with Er Ions
Silicon based materials embedded with Er can be categorized as shown in Fig. 1. So far several kinds of Er doped materials have been frequently reported. In 1983, H. Ennen reported 1.54 μm PL emission from Er-doped crystalline Si fabricated by using ion implantation method (Fig. 1(a)) [13]. Er ions are excited by energy transfer through electron-hole pair recombination. He also demonstrated carrier injection excitation in an Er doped Si light emitting diode fabricated by molecular beam epitaxy (MBE) [14]. However crystalline Si doped with Er of about 1018 cm−3 shows strong thermal quenching of the Er-related emission resulting in very weak emission at room temperature. This is caused by Auger deexcitation and energy back flow from the excited 4f electron to carriers in the host. Furthermore nonradiative transition of the excited 4f electrons is clearly noticeable in high Er doping above 1019 cm−3 , which is caused by defects and clustering of Er due to low solubility of Er in crystalline Si. Several studies show that the oxygen codoping enhances the Er-related emission and improves the thermal quenching (Fig. 1(b)) [15], [16]. Oxygen co-doping also increases the solubility of Er in Si [17] and inhibits Er clustering during solid phase epitaxy [17], [18]. However the heavy doping still can induce defects that increase the non-radiative process. These materials make electrically pumping of Er possible, but the Er concentration is limited to about 1019 cm−3 . EDFA has been successfully realized by using Er doped SiO2 glass (Fig. 1(c)) [3], [4]. It can be considered that there is no effect of the solubility because of a glass media, however the influence of clustering still remains in the
c 2008 The Institute of Electronics, Information and Communication Engineers Copyright
ISSHIKI and KIMURA: TOWARD SMALL SIZE WAVEGUIDE AMPLIFIERS BASED ON Er2 SiO5 FOR SILICON PHOTONICS
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Fig. 1
Er embedded materials based on silicon. N is the total population of Er ions.
tra corresponding to the Er3+ intra 4f-shell transitions of I13/2 → 4 I15/2 . PL spectra of Er doped materials show inhomogeneous broadening due to effects of various crystalline fields as shown in Fig. 2(a). However Er2 SiO5 crystallites exhibit the PL spectrum fine structure with line width of less than 4 meV as shown in Fig. 2(b) [7], [8]. This result indicates that a large number of optically active Er is on one certain site in the crystalline matrix. We discuss in the next secession our recent studies of Er2 SiO5 crystalline films.
4
3.
Er2 SiO5 Crystalline Thin Film
3.1 Preparation and Structural Characterizations of Er2 SiO5 Thin Film
Fig. 2 PL spectra of Er-related emission from Er-doped nc-Si (a) and erbium silicate (Er2 SiO5 ) (b).
high doping above 1020 cm−3 . On the other hand, Er doped Si rich SiO (SRSO) has shown a large excitation crosssection after a proper annealing [19]. Nanocrystalline Si (nc-Si) with about 3-nm radius is formed by the annealing. The pumping energy can be transferred to Er through the electron-hole pair recombination in nc-Si. Then nc-Si functions as a sensitizer because of its large absorption crosssection. However the effective Er concentration is limited to about 1019 cm−3 by a number of nc-Si [20]. In contrast, crystalline erbium silicates contain a few ten percent of Er as a constitutional element. The single crystalline nature gives suppression of the Er clustering and defects, resulting in realization of huge active Er density (∼1022 cm−3 ). Therefore crystalline erbium silicates can be expected to become a medium for small size and high optical gain amplifiers. Er3+ -related emission is very sensitive to detrimental interactions with the surrounding matrix. Figure 2 shows room temperature PL emission spec-
In our first report, Er2 SiO5 crystallites were formed by wet chemical synthesis using spin-coating of ErCl3 /ethanol solution and the following two-step annealing of 600◦ C in O2 and 1200◦ C in Ar [7], [8]. After that we have proposed the thin film formation processes, which are using porous Si [9], sol-gel method [10] and metalorganic molecular beam epitaxy (MOMBE) [11], [12]. We describe here about solgel method, since there is an accumulation of data acquired through many observations. Sol-gel solutions mixed with Er-O and Si-O precursors were prepared using our own recipe by Kojundo Chemical Lab. CO., LTD. The sol-gel solution was spin-coated on Si(100) substrate and the samples dried at 140◦ C for 30 min in air. Then the annealing process was performed at 600◦ C for 30 min in Ar atmosphere to obtain an ErSiO amorphous preform. This procedure was repeated 5 times to obtain sufficient thickness. Finally the heat-treatment at about 1100◦ C was performed for 30 min in Ar atmosphere for the crystallization. Figure 3 shows cross sectional TEM image of ErSiO crystalline compound [8]. The ErSiO crystallite has a layered structure with 0.86-nm period and belongs to the monoclinic system, as shown in the TEM image. D-spacing of 0.86-nm is also observed a diffraction peak at about 10 degree in XRD results, as shown in Fig. 4. Figure 4 (a) shows
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Fig. 3 TEM cross-sectional photograph of an ErSiO crystalline compound prepared by a wet-chemical method using ErCl/ethanol solution [8]. Fig. 5 Er-related PL emission spectra of Er2 SiO5 thin film at 20 K and 300 K. The line width is less than 1 meV at 20 K and 8-peaks correspond to Stark splitting of the ground state in Er ions.
tails will be presented elsewhere. The phase separation exhibits highly oriented crystallization especially in the higher Si-O mole fraction. Figure 4(b) shows an XRD pattern of the highly oriented Er2 SiO5 synthesized from a sol-gel solution of Er:Si = 1:2. Before annealing, there is no peak but Si(200) originated from the Si substrate. As shown in Fig. 4(b), each XRD peaks oriented to Si [100] corresponds to (n00) diffraction (n = 1–5) of Er2 SiO5 . We have also demonstrated the possibility of Er2 SiO5 hetero-epitaxy on Si by using MOMBE [12]. 3.2 PL Characterizations of Er2 SiO5 on SiO2
Fig. 4 XRD patterns of ErSiO crystalline thin films prepared by sol-gel method. Filled circles in upper side indicate the peaks of Er2 SiO5 from a JCPDS file of #52-1809. Lower side is of a highly oriented Er2 SiO5 Each peaks can be assigned (n00) diffraction peaks. The peak at around 10 degree corresponds to d-spacing of 0.86-nm shown in TEM photograph.
typical XRD pattern of the ErSiO crystalline thin film. At the beginning, the ErSiO crystalline compound structure could not be determined only by the TEM result. Then we have reviewed JCPDS files published after our earlier reports [7], and found a diffraction file on Er2 SiO5 (#52-1809) in partially agreement with the XRD pattern. Filled circles in the figure indicate the corresponding peaks. An RBS result also indicates the composition of Er2 SiO5 . From these results, we can determine that the ErSiO crystalline compound is an erbium silicate of Er2 SiO5 . Peak positions of the XRD pattern are in good agreement with the reference, however the relative intensities are not in agreement among them. Er2 SiO5 crystalline thin films can be obtained, however the crystallization basically does not depend on the mole fraction of Er-O and Si-O precursors in the sol-gel solution. Our recent result shows phase separation growth of Er2 SiO5 in the Si-O rich media. The de-
Refractive index of Er2 SiO5 thin film was estimated to be 1.8 by ellipsometry and reflection spectroscopy measurements. In order to achieve high optical confinement in Er2 SiO5 waveguide, SiO2 is suitable as the cladding materials for silicon photonics applications. From the XRD result, it is confirmed that the sol-gel procedure makes possible to reproduce the typical Er2 SiO5 thin film on SiO2 . Figure 5 shows PL spectra at 20 K and 300 K of Er2 SiO5 on SiO2 . A 650 nm-LD with 30 mW was used as the pumping light. The PL spectrum fine structure at 20 K indicates a crystal field splitting of the 4 I15/2 manifold in Er3+ ions due to the local environment. In the figure, mainly 8 peaks are clearly seen. The 4 I15/2 level splits into 8 lines by crystalline field, in spite of cubic symmetry. These results suggest that the Er-related emission at low-temperatures comes from one type of Ercenter caused by a certain crystal field in a homogeneous medium. On the other hand, the linewidth of the 1528-nm peak broadens to 4 meV (7-nm) at 300 K and many other peaks are observed as the hot lines. Then thermal-quenching of the PL peak intensity is small as about 20%, and the integrated intensity are almost same. The Stark splitting is still clearly seen at room temperature and few types of Er-centers may be appeared.
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Fig. 7 Decay rate of the first excited state (4 I13/2 ) in Er ion in Er2 SiO5 as a function of the SOI interlay thickness, which corresponds to the optical mode density. Also the dependence of PL intensity is shown. The PL intensity is proportional to radiative transition rate.
Fig. 6 Decay curves of 1528-nm PL emissions at 20 K from various Er2 SiO5 /Si/SiO2 structures. System response is less than 2 μs. Schematic of the structure is also shown on upper side.
3.3 Fast Decay Time of PL Emission from Er2 SiO5 PL decay curves were corrected by time-gated photon counting method using an InGaAs photomultiplier. The system time response is less than 2 μs. A pulse-modulated laser diode (650-nm, 30 mW) was used as a pumping light source, which corresponded to the transition from 4 I15/2 to 4 F9/2 . Under this condition, any upconversion emission was not observed. Figure 6 shows PL decay curves of 1.528 μm peak emission at 20 K. The PL decay time of a 220-nm thick Er2 SiO5 thin film formed on Si is 19 μs. Remarkable temperature quenching of the decay rate is not observed. The decay time τ f is about three orders faster than that of Er ion doped into SiO2 . In order to investigate the extreme fast decay, we have attempted modulation of the radiative transition rate wrad [21]. The total decay rate w f is given by w f = τ−1 f = wrad + wnon−rad
(1)
then wrad and wnon−rad are radiative and non-radiative transition rates respectively. Also PL emission intensity at the steady state IPL is proportional only to the radiative transition rate wrad IPL = N2 · wrad
(2)
then N2 is population of the excited state. According to Fermi’s golden rule, the radiative transition rate is proportional to the optical mode density and can be modified by changing the layer structure. Schematics of the sample
structure and the profile of refractive indexes are also shown in Fig. 6. Then the mode density increases with increasing thickness of SOI inter layer. Er2 SiO5 layer with 220nm thick is formed at the same time on each SOI sample. The SOI thickness dependence of the decay curve is clearly seen in Fig. 6. With increasing the SOI thickness, the decay time become shorter and the PL intensity increases. Figure 7 shows the transition rate w f and PL intensity at the steady state as a function of the SOI thickness. Then the PL intensity is normalized in consideration of a multiple reflection effect for the excitation of the SOI structure. Behavior of the transition rate is due to the change of the radiative transition rate wrad because the non-radiative processes are considered to be independent of the structure. The PL intensity is also proportional to the radiative transition rate wrad . From the figure, it is seen that the two resemble each other very much. These results suggest that the radiative process is dominant in the decay rate (w f ∼ wrad ) and the extreme fast decay is mainly due to increasing the radiative transition rate. A result of the optical mode density calculation also supports this consideration. 3.4 Upconversion Emission from Er2 SiO5 Waveguide From results of the decay time measurements, we have concluded that the extreme fast decay is mainly due to the radiative transition at the low power excitation. Upconversion is frequently observed at the high power excitation, and becomes the non-radiative process. We have also investigated the upconversion emission in an Er2 SiO5 waveguide [22], [23]. An SiO2 /Er2 SiO5 /polymer slab waveguide was prepared. Polymer film (n = 1.54) with 4 μm thickness was used as the over cladding layer. To obtain lateral optical confinement, strip-loaded structures with 5-μm width polymer stripe were formed by focused ion beam (FIB) etching. Facets of the Er2 SiO5 waveguide were also formed by FIB
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guide is also given by φ = φ0 exp (−αΓx) ,
(3)
then α and Γ are the absorption coefficient of Er ions for 1480-nm light and the optical confinement factor. Then scattering loss is not considered. The populations of the states are labeled N1 (4 I15/2 ), 4 N2 ( I13/2 ) and N3 (4 S 3/2 and 2 H11/2 ). In this excitation condition, N2 is proportional to the pumping light φ, and the upconversion emission intensity is proportional to N3 . Considering cooperative upconversion from 4 I15/2 to 4 S 3/2 or 2 H11/2 , three ions excited to the 4 I13/2 state couple with each other. In the rate equation of the three exciton system, green ∝ N3 ∝ N23 . IPL
Fig. 8 Top view of upconversion emission profile. The pumping light comes from left side through a lensed fiber. Line profile along the waveguide is also shown. Filled circles indicate the emission from Er2 O3 clusters. Dash line is extrapolation of the emission decay.
etching. The optical confinement factor Γ of the Er2 SiO5 waveguide layer is estimated to be 0.42, assuming the lateral confinement factor is 1 because of the sufficient wide waveguide for the light. In addition a λ/4 air gap (λ = 1.5 μm) as a photon trap for the pumping light was formed on the waveguide at a position of 86 μm from the left facet by FIB. The waveguide was coupled with a lensed fiber. Optical pumping was performed by 1480-nm light coming from the left side through the lensed fiber. The pumping power is 30 mW at the fiber input. Figure 8 shows a top view of the waveguide. The photograph was taken by a conventional CCD system. As shown in Fig. 8, a visible (green) emission along the waveguide can be seen. The green emission corresponds to 4f intra-shell transitions from 4 S 3/2 and 2 H11/2 to 4 I15/2 in Er3+ ions, which is due to stepwise upconversion of Er3+ ions. It is indicated that the 1480-nm propagating light is well confined into the lateral and vertical directions of the waveguide. Line profile of the upconversion emission along the waveguide is also shown in Fig. 8. The emission intensity gradually decays and then rapidly goes down at a position around 30 μm from the input facet, and the upconversion process exhibits a certain criterion for the excitation. The upconversion emission appears again at a position around 85 μm from the input facet, which corresponds to the photon trap for 1480-nm light. It is indicated that the 1480-nm propagating light still survives beyond the upconversion emission decay. Moreover some small green spots are observed beyond the decay. These spots correspond to upconversion emissions from Er clusters (Er2 O3 ). In the line profile, the spots are also indicated by filled circles. This result suggests that the upconversion hardly occurs in Er2 SiO5 in comparison with Er2 O3 . A dash line in Fig. 8 shows an extrapolated line of the decay, which is fitted by an exponential curve of exp(−ax). The 1480-nm pumping light propagation along the wave-
(4)
Then using these relations, the upconversion emission line profile corresponding to the gradually decay is expressed by green ∝ φ3 = φ30 exp (−3αΓx) IPL
(5)
From the fitting of this part, 3αΓ is estimated to be 135.8 cm−1 . Then the absorption coefficient of Er ions for 1480-nm light α is estimated to be 107.7 cm−1 . The absorption coefficient becomes approximately σabs N, then σabs is the absorption cross-section and estimated to be ∼10−20 cm2 . This value is comparable with values found in an earlier report [21]. Optical gain G is given by G (dB) = 4.43 × (σem N2 − σabs N1 ) ΓL
(6)
then σem is the emission cross-section and comparable with the absorption cross-section. When the pump intensity is very strong, such that the erbium ions are all inverted, N2 becomes approximately N. Then the gain factor is estimated to be 47.7 dB/mm. This value seems to be a little bit overestimated but it can be expected to be sufficient to realize micro waveguide amplifiers (L
