Ultrabroadband Mirror Using Low-Index Cladded ... - Semantic Scholar
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 2, FEBRUARY 2004
Ultrabroadband Mirror Using Low-Index Cladded Subwavelength Grating Carlos F. R. Mateus, Student Member, IEEE, Michael C. Y. Huang, Student Member, IEEE, Yunfei Deng, Andrew R. Neureuther, Fellow, IEEE, and Connie J. Chang-Hasnain, Fellow, IEEE
Abstract—We report a novel subwavelength grating that has a very broad reflection spectrum and very high reflectivity. The design is scalable for different wavelengths. It facilitates monolithic integration of optoelectronic devices at a wide range of wavelengths from visible to far infrared. Index Terms—Infrared imaging sensor, infrared surveillance, mirror, optical communication, optical microelectro-mechanical (MEM), optical sensors, reconfigurable architecture, tunable filters, tunable optoelectronic devices. Fig. 1. Scheme of the subwavelength grating reflector. The low index material under the grating is essential for the broadband mirror effect.
I. INTRODUCTION
B
ROADBAND mirrors ( ) with very high re) are essential for numerous applicaflectivity ( tions, including telecommunications, surveillance, sensors, and imaging, ranging from 0.7–12- m wavelength regimes. Metal mirrors have larger reflection bandwidths but lower reflectivities ( ), limited by absorption loss. As a result, they are not suitable for fabricating transmission-type optical devices such as etalon filters. Dielectric mirrors have a low loss and, thus, can achieve a higher reflectivity. However, the deposition methods are often not precise enough to lead to very high reflectivities. Furthermore, the typical material combinations often have a rather small bandwidth, limited by the refractive index difference of the materials used. For tunable etalon type devices, such as microelectro-mechanical (MEM) vertical cavity surface emitting lasers (VCSEL), filters [1], and detectors [2], the tuning range is often limited by semiconductor-based distributed Bragg to . The challenge of dereflectors (DBRs) to signing a mirror with broadband reflection, low loss and compatibility with optoelectronic processing has not been overcome yet. Subwavelength gratings have been used either to create structures with either sharp highly reflective peaks at resonance wavelengths [3], [4] or broad antireflective bands [5]. The final spectral characteristic of the grating can be further tailored by the materials used and parameters chosen. In this letter, we report a novel subwavelength grating that has a very broad reflection spectrum and very high reflectivity. with Two examples are shown for and with . The design is scalable for
Manuscript received July 22, 2003; revised October 4, 2003. This work was supported in part by the Defense Advanced Research Projects Agency Center for Bio-Optoelectronic Sensor Systems (BOSS) under Grant MDA9720010020 and in part by the CAPES Foundation and Brazilian Air Command. The authors are with the Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA, 94720 USA (e-mail: [email protected]). Digital Object Identifier 10.1109/LPT.2003.821258
different wavelengths. It facilitates monolithic integration of optoelectronic devices at a wide range of wavelengths from visible to far infrared. II. SUBWAVELENGTH GRATING AS BROADBAND REFLECTOR The proposed structure has a large refractive index difference among materials, resulting in a very broadband reflector. Fig. 1 shows the scheme of such a mirror that consists of lines of high/low index material surrounded by two low index layers. The larger the difference between high and low indexes is, the larger the reflection band. The low index layer under the grating is critical for the mirror effect. Design parameters for the structure include the materials involved (index of refraction), thickness of the low index layer under the grating ( ), grating period ( ), grating thickness ( ), and fill factor. Fill factor is defined as the ratio of the width of the high index material to . Fig. 2 shows reflected power for light polarized perpendicular to the grating lines. The simulation was based on rigorous coupled wave analysis (RCWA) [6] and confirmed by finite-difference time-domain electromagnetic propagation using TEMPEST [7]. The two methods are in excellent , with agreement. A very broadband mirror , was obtained around 1.55 m, over the range 1.33–1.80 m, as depicted by Fig. 2(a). The mirror is also very (1.40–1.67 m or ). broad for The parameters used in the simulation were: Si sub), m, (Poly-Si.), strate ( (air), low index material in and above the grating (SiO ), m, and fill factor . The index of refraction was considered constant along the covered range, which is a very good approximation, since most semiconductor materials such as Si, GaAs, and ZnSe have index of refraction practically independent of wavelength in the considered ranges. It is interesting to note that the broadband reflectivity does not result from a resonance, as the period of the grating is subwavelength but not half-wavelength. Furthermore, the reflec-
1041-1135/04$20.00 © 2004 IEEE
MATEUS et al.: ULTRABROADBAND MIRROR
Fig. 2. Reflected power for light polarized perpendicularly to the grating lines. (a) Thick line was obtained based on RCWA [6], while the dashed line was obtained with TEMPEST [7]. (b) A simple scaling factor (6.5) applied to the dimensions gives completely overlapped traces. The thick line is centered at 1.55 m, while the dashed line is at 10 m.
tivity spectrum can be scaled with wavelength, as shown in Fig. 2(b). By simply multiplying the dimensions by a constant, in this case 6.5, while keeping the other parameters, the reflection band shifts to the 8.6–11.7- m wavelength range with all features and values being identical. Hence, any different wavelength regime can use the same design. Note that the same constant has also multiplied the horizontal scale in order to make the comparison easier. Although it may be obvious that any periodic structure should be wavelength scalable, the scaling here is easily manufacturable, since it only requires changing the layer dimensions. Physical origins of the broadband reflection phenomenon are under investigation. The low index material layer under the grating is essential to obtain the high broadband reflection. This is shown in Fig. 3, which consists of contour plots of reflectivity as a function of and . Keeping all the other parameters the wavelength, m. Above this same, there is no reflection band for thickness, the structure has low sensitivity to the low index layer, but this parameter can be used to optimize the reflection band. . If Si N ( ) The mirror also does not exist if is used instead of SiO , the result would be a much smaller reflection band, ranging from 1.7–1.8 m. III. PARAMETER ANALYSIS The various design parameters play interesting roles on the final reflectivity spectrum. Any material system with a large
519
Fig. 3. Effect of the low index layer under the grating. (a) Reflectivity as function of wavelength and t . There is no reflection band when t < 0:1 m, and above this value, the structure has low sensitivity to this parameter. (b) Reflectivity as function of wavelength and n . The mirror also does not exist if n > 2:5.
difference in index of refraction can be used as a base for this broadband mirror, and the larger this difference, the larger the band. Our simulations show results for Poly-Silicon–air–SiO , but GaAs–Al O , GaN–air or ZnSe–CaF would be comparable. Thus, this grating is a potential candidate for several active and passive devices such as visible and infrared wavelength VCSELs [8] and MEM tunable devices [1], [2]. In the following discussion, we show design tolerance by varying one of the parameters, while keeping the others constant. The grating period determines the location of the center wavelength of the reflection band, and this effect is shown in Fig. 4. The band shifts to longer wavelengths proportionally to , and for m, the band is the broadest. The period can be controlled very accurately by lithographic methods, and thus, the reflection band can be precisely fabricated. Grating thickness and fill factor determine the intensity of modulation, or grating strength. However, this strength cannot increase indefinitely and there is an optimum point where the grating effect is strongest with respect to reflectivity. Fig. 5 shows the effect of . For a very thin grating, the mirror is sharp and the optimized bandwidth occurs for m. Above this value, the mirror gets sharp again. As this parameter can be precisely controlled by epitaxial growth or plasma deposition techniques, the optimized design can be easily fabricated.
520
Fig. 4. Reflectivity as function of wavelength and 3. The reflection band shifts to longer wavelengths proportionally to the period, and for 3 = 0:7 the band is the broadest.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 2, FEBRUARY 2004
1.6 m. As the fill factor increases, the two peaks merge to form one broad and flat reflection band. This parameter is probably the most critical in fabrication as small variations in lithography can change the final value. It may slightly affect the flatness of the band (if the fill factor gets smaller, the two peaks tend to separate) or its coverage (if fill factor gets larger, mirror bandwidth decreases). In this design, where lines are used, reflection is polarization dependent. This can be advantageous to control the polarization on a VCSEL, e.g., if the grating design is used for the mirrors. If a two-dimensional grating is chosen instead, reflectivity would be polarization independent. The grating sensitivity to all these parameters can be optimized iteratively. If the application has , most a less stringent requirement on reflectivity, i.e., of the parameters have a large tolerance range, sometimes up to 10% variation. Experimental results are being carried on and will be presented soon. IV. CONCLUSION We have presented a subwavelength grating that under normal incident light has very broad reflection spectrum ( and ). The mirror can be easily scaled by simply multiplying the dimensions by a constant. This design has potential application on micro-electro-mechanical tunable devices, VCSELs and reconfigurable focal plane arrays. It is insensitive to lateral position on cascaded structures and can be easily fabricated monolithically with optoelectronic devices.
Fig. 5. Reflectivity as function of wavelength and t . The optimized bandwidth occurs for t = 0:45, and it gets sharper when it is further increased. This parameter can be precisely controlled by epitaxial growth or plasma deposition techniques.
ACKNOWLEDGMENT Authors would like to acknowledge Dr. L. Chen (Raytheon Company) for support. REFERENCES
Fig. 6. Reflectivity as function of wavelength and fill factor. When fill factor is increased, two reflection peaks merge to form one broad and flat reflection band.
Fig. 6 shows the effect of fill factor. There are two reflection peaks for a fill factor of 0.5, one at 1.1 m and the other at
[1] C. F. R. Mateus et al., “Widely tunable torsional optical filter,” Photon. Tech. Lett., vol. 14, no. 6, pp. 819–21, June 2002. [2] G. S. Li, W. Yuen, and C. J. Chang-Hasnain, “Wide and continously tunable (30 nm) detector with uniform characteristics over tuning range,” Electronics Letters, vol. 33, no. 13, pp. 1122–4, 1997. [3] R. Magnusson and S. S. Wang, “New principle for optical filters,” Appl. Phys. Lett., vol. 61, no. 9, pp. 1022–4, Aug. 1992. [4] B. Cunningham, P. Li, B. Lin, and J. Pepper, “Colorimetric resonant reflection as a direct biochemical assay technique,” Sensors Actuators B, vol. 81, pp. 316–28, 2002. [5] Y. Kanamori and K. Hane, “Broadband antireflection subwavelength gratings for polymethyl methacrylate fabricated with molding technique,” Opt.Rev., vol. 9, no. 5, pp. 183–5, Sept.-Oct. 2002. [6] M. G. Moharam and T. K. Gaylord, “Rigorous coupled-wave analysis of planar-grating diffraction,” J. Opt. Soc. Amer., vol. 71, no. 7, pp. 811–18, July 1981. [7] A. K. Wong, R. Guerrieri, and A. R. Neureuther, “Massively parallel electromagnetic simulation for photolithographic applications,” IEEE Trans.Computer-Aided Design, vol. 14, pp. 1231–1240, Oct. 1995. [8] D. I. Babic´ , J. Piprek, and J. E. Bowers, “Long-wavelength vertical-cavity lasers,” in Vertical-Cavity Surface-Emitting Lasers, C. W. Wilmsen, Ed. Cambridge, U.K.: Cambridge Univ. Press, 1999, pp. 303–25.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 2, FEBRUARY 2004
Ultrabroadband Mirror Using Low-Index Cladded Subwavelength Grating Carlos F. R. Mateus, Student Member, IEEE, Michael C. Y. Huang, Student Member, IEEE, Yunfei Deng, Andrew R. Neureuther, Fellow, IEEE, and Connie J. Chang-Hasnain, Fellow, IEEE
Abstract—We report a novel subwavelength grating that has a very broad reflection spectrum and very high reflectivity. The design is scalable for different wavelengths. It facilitates monolithic integration of optoelectronic devices at a wide range of wavelengths from visible to far infrared. Index Terms—Infrared imaging sensor, infrared surveillance, mirror, optical communication, optical microelectro-mechanical (MEM), optical sensors, reconfigurable architecture, tunable filters, tunable optoelectronic devices. Fig. 1. Scheme of the subwavelength grating reflector. The low index material under the grating is essential for the broadband mirror effect.
I. INTRODUCTION
B
ROADBAND mirrors ( ) with very high re) are essential for numerous applicaflectivity ( tions, including telecommunications, surveillance, sensors, and imaging, ranging from 0.7–12- m wavelength regimes. Metal mirrors have larger reflection bandwidths but lower reflectivities ( ), limited by absorption loss. As a result, they are not suitable for fabricating transmission-type optical devices such as etalon filters. Dielectric mirrors have a low loss and, thus, can achieve a higher reflectivity. However, the deposition methods are often not precise enough to lead to very high reflectivities. Furthermore, the typical material combinations often have a rather small bandwidth, limited by the refractive index difference of the materials used. For tunable etalon type devices, such as microelectro-mechanical (MEM) vertical cavity surface emitting lasers (VCSEL), filters [1], and detectors [2], the tuning range is often limited by semiconductor-based distributed Bragg to . The challenge of dereflectors (DBRs) to signing a mirror with broadband reflection, low loss and compatibility with optoelectronic processing has not been overcome yet. Subwavelength gratings have been used either to create structures with either sharp highly reflective peaks at resonance wavelengths [3], [4] or broad antireflective bands [5]. The final spectral characteristic of the grating can be further tailored by the materials used and parameters chosen. In this letter, we report a novel subwavelength grating that has a very broad reflection spectrum and very high reflectivity. with Two examples are shown for and with . The design is scalable for
Manuscript received July 22, 2003; revised October 4, 2003. This work was supported in part by the Defense Advanced Research Projects Agency Center for Bio-Optoelectronic Sensor Systems (BOSS) under Grant MDA9720010020 and in part by the CAPES Foundation and Brazilian Air Command. The authors are with the Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA, 94720 USA (e-mail: [email protected]). Digital Object Identifier 10.1109/LPT.2003.821258
different wavelengths. It facilitates monolithic integration of optoelectronic devices at a wide range of wavelengths from visible to far infrared. II. SUBWAVELENGTH GRATING AS BROADBAND REFLECTOR The proposed structure has a large refractive index difference among materials, resulting in a very broadband reflector. Fig. 1 shows the scheme of such a mirror that consists of lines of high/low index material surrounded by two low index layers. The larger the difference between high and low indexes is, the larger the reflection band. The low index layer under the grating is critical for the mirror effect. Design parameters for the structure include the materials involved (index of refraction), thickness of the low index layer under the grating ( ), grating period ( ), grating thickness ( ), and fill factor. Fill factor is defined as the ratio of the width of the high index material to . Fig. 2 shows reflected power for light polarized perpendicular to the grating lines. The simulation was based on rigorous coupled wave analysis (RCWA) [6] and confirmed by finite-difference time-domain electromagnetic propagation using TEMPEST [7]. The two methods are in excellent , with agreement. A very broadband mirror , was obtained around 1.55 m, over the range 1.33–1.80 m, as depicted by Fig. 2(a). The mirror is also very (1.40–1.67 m or ). broad for The parameters used in the simulation were: Si sub), m, (Poly-Si.), strate ( (air), low index material in and above the grating (SiO ), m, and fill factor . The index of refraction was considered constant along the covered range, which is a very good approximation, since most semiconductor materials such as Si, GaAs, and ZnSe have index of refraction practically independent of wavelength in the considered ranges. It is interesting to note that the broadband reflectivity does not result from a resonance, as the period of the grating is subwavelength but not half-wavelength. Furthermore, the reflec-
1041-1135/04$20.00 © 2004 IEEE
MATEUS et al.: ULTRABROADBAND MIRROR
Fig. 2. Reflected power for light polarized perpendicularly to the grating lines. (a) Thick line was obtained based on RCWA [6], while the dashed line was obtained with TEMPEST [7]. (b) A simple scaling factor (6.5) applied to the dimensions gives completely overlapped traces. The thick line is centered at 1.55 m, while the dashed line is at 10 m.
tivity spectrum can be scaled with wavelength, as shown in Fig. 2(b). By simply multiplying the dimensions by a constant, in this case 6.5, while keeping the other parameters, the reflection band shifts to the 8.6–11.7- m wavelength range with all features and values being identical. Hence, any different wavelength regime can use the same design. Note that the same constant has also multiplied the horizontal scale in order to make the comparison easier. Although it may be obvious that any periodic structure should be wavelength scalable, the scaling here is easily manufacturable, since it only requires changing the layer dimensions. Physical origins of the broadband reflection phenomenon are under investigation. The low index material layer under the grating is essential to obtain the high broadband reflection. This is shown in Fig. 3, which consists of contour plots of reflectivity as a function of and . Keeping all the other parameters the wavelength, m. Above this same, there is no reflection band for thickness, the structure has low sensitivity to the low index layer, but this parameter can be used to optimize the reflection band. . If Si N ( ) The mirror also does not exist if is used instead of SiO , the result would be a much smaller reflection band, ranging from 1.7–1.8 m. III. PARAMETER ANALYSIS The various design parameters play interesting roles on the final reflectivity spectrum. Any material system with a large
519
Fig. 3. Effect of the low index layer under the grating. (a) Reflectivity as function of wavelength and t . There is no reflection band when t < 0:1 m, and above this value, the structure has low sensitivity to this parameter. (b) Reflectivity as function of wavelength and n . The mirror also does not exist if n > 2:5.
difference in index of refraction can be used as a base for this broadband mirror, and the larger this difference, the larger the band. Our simulations show results for Poly-Silicon–air–SiO , but GaAs–Al O , GaN–air or ZnSe–CaF would be comparable. Thus, this grating is a potential candidate for several active and passive devices such as visible and infrared wavelength VCSELs [8] and MEM tunable devices [1], [2]. In the following discussion, we show design tolerance by varying one of the parameters, while keeping the others constant. The grating period determines the location of the center wavelength of the reflection band, and this effect is shown in Fig. 4. The band shifts to longer wavelengths proportionally to , and for m, the band is the broadest. The period can be controlled very accurately by lithographic methods, and thus, the reflection band can be precisely fabricated. Grating thickness and fill factor determine the intensity of modulation, or grating strength. However, this strength cannot increase indefinitely and there is an optimum point where the grating effect is strongest with respect to reflectivity. Fig. 5 shows the effect of . For a very thin grating, the mirror is sharp and the optimized bandwidth occurs for m. Above this value, the mirror gets sharp again. As this parameter can be precisely controlled by epitaxial growth or plasma deposition techniques, the optimized design can be easily fabricated.
520
Fig. 4. Reflectivity as function of wavelength and 3. The reflection band shifts to longer wavelengths proportionally to the period, and for 3 = 0:7 the band is the broadest.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 2, FEBRUARY 2004
1.6 m. As the fill factor increases, the two peaks merge to form one broad and flat reflection band. This parameter is probably the most critical in fabrication as small variations in lithography can change the final value. It may slightly affect the flatness of the band (if the fill factor gets smaller, the two peaks tend to separate) or its coverage (if fill factor gets larger, mirror bandwidth decreases). In this design, where lines are used, reflection is polarization dependent. This can be advantageous to control the polarization on a VCSEL, e.g., if the grating design is used for the mirrors. If a two-dimensional grating is chosen instead, reflectivity would be polarization independent. The grating sensitivity to all these parameters can be optimized iteratively. If the application has , most a less stringent requirement on reflectivity, i.e., of the parameters have a large tolerance range, sometimes up to 10% variation. Experimental results are being carried on and will be presented soon. IV. CONCLUSION We have presented a subwavelength grating that under normal incident light has very broad reflection spectrum ( and ). The mirror can be easily scaled by simply multiplying the dimensions by a constant. This design has potential application on micro-electro-mechanical tunable devices, VCSELs and reconfigurable focal plane arrays. It is insensitive to lateral position on cascaded structures and can be easily fabricated monolithically with optoelectronic devices.
Fig. 5. Reflectivity as function of wavelength and t . The optimized bandwidth occurs for t = 0:45, and it gets sharper when it is further increased. This parameter can be precisely controlled by epitaxial growth or plasma deposition techniques.
ACKNOWLEDGMENT Authors would like to acknowledge Dr. L. Chen (Raytheon Company) for support. REFERENCES
Fig. 6. Reflectivity as function of wavelength and fill factor. When fill factor is increased, two reflection peaks merge to form one broad and flat reflection band.
Fig. 6 shows the effect of fill factor. There are two reflection peaks for a fill factor of 0.5, one at 1.1 m and the other at
[1] C. F. R. Mateus et al., “Widely tunable torsional optical filter,” Photon. Tech. Lett., vol. 14, no. 6, pp. 819–21, June 2002. [2] G. S. Li, W. Yuen, and C. J. Chang-Hasnain, “Wide and continously tunable (30 nm) detector with uniform characteristics over tuning range,” Electronics Letters, vol. 33, no. 13, pp. 1122–4, 1997. [3] R. Magnusson and S. S. Wang, “New principle for optical filters,” Appl. Phys. Lett., vol. 61, no. 9, pp. 1022–4, Aug. 1992. [4] B. Cunningham, P. Li, B. Lin, and J. Pepper, “Colorimetric resonant reflection as a direct biochemical assay technique,” Sensors Actuators B, vol. 81, pp. 316–28, 2002. [5] Y. Kanamori and K. Hane, “Broadband antireflection subwavelength gratings for polymethyl methacrylate fabricated with molding technique,” Opt.Rev., vol. 9, no. 5, pp. 183–5, Sept.-Oct. 2002. [6] M. G. Moharam and T. K. Gaylord, “Rigorous coupled-wave analysis of planar-grating diffraction,” J. Opt. Soc. Amer., vol. 71, no. 7, pp. 811–18, July 1981. [7] A. K. Wong, R. Guerrieri, and A. R. Neureuther, “Massively parallel electromagnetic simulation for photolithographic applications,” IEEE Trans.Computer-Aided Design, vol. 14, pp. 1231–1240, Oct. 1995. [8] D. I. Babic´ , J. Piprek, and J. E. Bowers, “Long-wavelength vertical-cavity lasers,” in Vertical-Cavity Surface-Emitting Lasers, C. W. Wilmsen, Ed. Cambridge, U.K.: Cambridge Univ. Press, 1999, pp. 303–25.
