Tunable photonic bandgap structures for optical interconnects

Transcription

1 Optical Materials 27 (05) Tunable photonic bandgap structures for optical interconnects S.M. Weiss a, *, M. Haurylau b, P.M. Fauchet a,b a Institute of Optics, University of Rochester, Rochester, NY 14627, USA b Department of Electrical and Computer Engineering, University of Rochester, Rochester, NY 14627, USA Available online 12 October 04 Abstract As continued progress towards faster, low power consuming microelectronic devices becomes increasing difficult due to the scaling challenges of electrical interconnects, it becomes even more critical to explore alternative technologies. Tunable porous silicon photonic bandgap structures are viable building blocks for optical interconnects, which present a possible long term solution to the interconnect problem. Forming the structures on a silicon platform provides the advantage of easier integration with current semiconductor processing techniques. In this work, tuning of the optical properties is controlled by liquid crystals (LCs) that are infiltrated into the silicon matrix. Active tuning is demonstrated both out-of-plane, with one-dimensional porous silicon photonic bandgap microcavities, and in-plane, using two-dimensional porous silicon photonic bandgap structures. Ó 04 Elsevier B.V. All rights reserved. 1. Introduction * Corresponding author. Tel.: ; fax: address: sweiss@optics.rochester.edu (S.M. Weiss). Over the last three decades, there has been rapid improvement of computer chip performance and downscaling of individual components. However, as computational power continues to increase, it is becoming increasingly difficult to provide the necessary communication between computer boards, chips and even individual chip components. This creates an interconnect bottleneck, which is predicted to become a grand challenge for the semiconductor industry by the year 18 [1]. Consequently, investigation of unconventional interconnect technologies, such as optical interconnects, is required. Optical interconnects for applications in VLSI technology were first proposed two decades ago [2]. However, their realization still remains extremely challenging due to the monolithic integration problems of intrinsically bulky optical components. Silicon-based photonic bandgap (PBG) structures have emerged in the research community as an established technology for fabricating high-performance ultracompact optical elements [3,4]. PBG structures have the advantage of providing strong light confinement in a compact geometry compared to other optical interconnect alternatives, such as Mach-Zehnder interferometers [5]. Furthermore, by having the capability for active control and multiplexing of the propagating light, tunable silicon-based PBG structures offer an exciting opportunity to investigate a potentially viable solution to the interconnect bottleneck. 2. Experimental The porous silicon PBG structures are fabricated by electrochemical etching in a hydrofluoric acid-based electrolyte. One-dimensional porous silicon photonic bandgap microcavities are formed on 0.01 X cm p-type (100) silicon in a solution of 15% hydrofluoric acid in ethanol. In order to create the periodic refractive index profile necessary for PBG structures, an alternating current density of 5mA/cm 2 for 39s and 50mA/cm 2 for 9s is applied to form a Bragg mirror with layers of 50% and 75% porosity. A microcavity is created by etching a /$ - see front matter Ó 04 Elsevier B.V. All rights reserved. doi: /j.optmat

2 S.M. Weiss et al. / Optical Materials 27 (05) defect layer between two Bragg mirrors using a current density of 50mA/cm 2 for 19s. The porous silicon created by this etching procedure is mesoporous silicon with an average pore diameter of 50 nm. Therefore, each layer of the microcavity with a different porosity has a distinct refractive index. This refractive index is calculated by the Bruggeman effective medium approximation. Near the resonance wavelength, the refractive indices of the 50% and 75% porosity layers are 2.15 and 1.43, respectively. Fabrication of two-dimensional porous silicon PBG structures is slightly more complicated and requires pre-structuring of the silicon surface prior to electrochemical etching. Interferometric (or holographic) lithography is used to define a square or triangular lattice of nucleation pits on the surface of moderately doped ( 25 X cm) p-type silicon. Electrochemical etching of such pre-structured silicon in dimethylformamidebased electrolyte forms an ordered array of straight cylindrical macropores with a micrometer size scale. Details of the fabrication can be found in [6]. After fabrication, the porous silicon structures are partially oxidized. E7 nematic LCs are infiltrated inside the pores under vacuum. Prior to infiltration, some of the structures are treated with surface agents in order to initiate a particular initial LC orientation. Optical characterization is done in the reflection mode using a Lambda 900 spectrophotometer for the one-dimensional PBG structures and a Digilab FTS-40 FTIR microscope for the two-dimensional photonic crystals in-plane of the PBG layer. Simulations are performed using the Translight FDTD PBG simulation package [7] as well as a custom-built transfer matrix simulation program. Parameters for the simulations were taken directly from the SEM micrographs of the fabricated structures. The polarization conventions are as follows: TE polarized light has an electrical field vector perpendicular to the pore growth direction; TM polarized light has a magnetic field vector perpendicular to the pore growth direction. 3. Results and discussion An interconnect consists of three major components source (with internal or external modulation), transmission medium (waveguide) and receiver (detector). While there are working prototypes of waveguides and detectors available for optical interconnects, none of the developed sources are currently suitable for commercial applications. Development of a silicon-based modulator is considered to be the main challenge due to the size as well as power consumption of available devices. This is explained by the small value of the electrooptic effect in silicon, which increases the interaction length of light in the medium, thus also increasing the dimensions of the modulator. Confinement of light in photonic crystals offers a solution to this problem by reducing the size and power consumption of the device. Additionally, the PBG platform offers a number of new optical devices which are not achievable in other systems [8 10]. One-dimensional porous silicon PBG microcavities can be used to modulate optical signals out-of-plane for interboard communication while two-dimensional PBG structures can provide in-plane light modulation for interchip communication Photonic crystals as building blocks for optical interconnects Fig. 1a shows an SEM image of the one-dimensional porous silicon microcavity. The defect layer in the center of the structure breaks up the perfect periodicity of the refractive index profile and creates a passband within the PBG, as shown in Fig. 1b. A good agreement is achieved between simulation and experiment for the Fig. 1. (a) SEM image of porous silicon one-dimensional PBG microcavity. The image is rotated by 45 in order to view the entire microcavity and maintain a sufficient magnification to reveal the mesoporous morphology. The air interface is at the upper left and the silicon interface is at the lower right of the image. The microcavity consists of a 5.5 period upper Bragg mirror with alternating 50% and 75% porosity quarter wavelength optical thickness layers, a central 75% porosity half wavelength optical thickness defect layer, and a 6 period lower Bragg mirror. (b) Experimentally measured (black) and simulated (gray) reflectance spectra of porous silicon one-dimensional PBG microcavity infiltrated with LCs. The measured and simulated spectra match well, indicating a high quality of fabrication and uniform LC infiltration throughout the microcavity. The amplitude of reflectance at the resonance and stopband frequencies determines the overall contrast of the optical signal to be modulated.

3 742 S.M. Weiss et al. / Optical Materials 27 (05) LC infiltrated microcavity. The contrast between the optical signal level at the resonance wavelength and the stopband is critical to the efficient operation of the out-of-plane modulator. The sharper the resonance, the easier it is to transition between the off- and onstates of the device. The plan view of the two-dimensional PBG structure is shown in Fig. 2a. The periodicity of the triangular lattice of air cylinders in silicon creates a photonic bandgap, which can be measured by either transmission or reflection in the plane of the photonic crystal layer. The measured and simulated reflection spectra for TE polarized light are shown in Fig. 2b and a reasonable agreement between experimental and calculated PBG properties is achieved. While the two-dimensional bandgap is located in the mid-infrared region, the principles discussed in this paper can be directly applied to the PBG structures in the near infrared region [11]. For practical device operation, it is essential to actively control the optical properties of the photonic crystals. Liquid crystals are a good choice of active optical material because of their large birefringence and compatibility with back-end silicon processing. LCs are infiltrated into the porous silicon one-dimensional and twodimensional PBG structures for this purpose. The refractive index of LCs depends upon their orientation with respect to incident light. Therefore, changing the LC alignment leads to a modulation of the optical properties of the PBG structures. Fig. 3a shows an example of the effect that can be achieved by simply heating the one-dimensional PBG microcavity. As the device is heated, the LCs experience a phase transition at approximately 59 C from an ordered nematic state to a disordered isotropic state. As the LCs rotate, the resonance frequency of the microcavity shifts to shorter frequencies. Consequently, for incident light near the resonance frequency, a contrast in reflectance out-of-plane from 24% to 56% can be achieved. A similar analysis of the two-dimensional PBG structures can be followed where the photonic band edge shifts with temperature and light is modulated in-plane. A comparison of the tunable oneand two-dimensional structures is given in Fig. 3b. Note that these frequency shifts are larger than those produced by the silicon PBG structure itself [12]. Fig. 2. (a) Plan view of the porous silicon two-dimensional photonic bandgap structure. The photonic layer consists of a triangular lattice of air cylinders surrounded by a silicon host. (b) Experimentally measured (black) and calculated (gray) reflectance in plane of the photonic crystal layer for TE-polarized light. The measured and simulated spectra show a good match for the first order bandgap C C (a) Wavenumber (cm -1 ) (b) D T c 2-D Temperature ( C) Fig. 3. (a) Influence of heating on the optical spectrum of porous silicon one-dimensional PBG microcavity with LCs. Near the resonance frequency, a large contrast in reflectance occurs for a temperature change of 33 C. (b) Comparison of out-of-plane (diamonds) and in-plane (squares) optical signal contrast for tunable one-dimensional and two-dimensional PBG structures, respectively. For the two-dimensional PBG structure, the optical signal contrast on the high frequency edge of the first order bandgap is shown. The measurement was performed near 60cm 1 for the onedimensional microcavity and 1140cm 1 for the two-dimensional PBG structure. The largest modulation occurs near the LC phase transition temperature (T c ) at approximately 59 C.

4 S.M. Weiss et al. / Optical Materials 27 (05) Influence of liquid crystal alignment on PBG modulation The slightly larger amplitude of out-of-plane modulation compared to in-plane modulation can be explained in part by the degree of LC rotation in each structure upon heating. The LC rotation governs the refractive index change and, therefore, magnitude of spectral shift and contrast in optical signal intensity at a particular frequency. The refractive index difference of LCs in the nematic and isotropic phases depends largely on the initial LC alignment. In the case of one-dimensional porous silicon photonic bandgap structures, small pore diameters lead to strong confinement of liquid crystal molecules. Thus, it is reasonable to expect sufficient surface-lc interaction energy to create a uniform initial orientation of the LCs over the entire pore diameter. Comparison of experimental and simulated reflectance spectra of microcavities with and without LCs suggests that the initial alignment of the LCs is axial, where the LC directors are parallel to the pore axis [4]. Fig. 3a supports this conclusion since the spectral shift to shorter frequencies upon heating indicates that the refractive index increases as the LCs transition from the ordered nematic phase to the disordered isotropic phase. Fig. 4a schematically illustrates the alignment for each LC phase. The origin of the initial axial orientation can be explained by the LC infiltration method, where the directional flow of (a) (b) nematic nematic isotropic isotropic LCs in the confined pore geometry aligns LC directors parallel to the pore walls. In the case of the two-dimensional PBG structures, the orientation of the LCs depends on two main factors. First, surface anchoring defines the orientation of the LC layer attached to the surface. Second, the surface- LC interaction energy determines the orientation of the LCs as a function of distance from the pore walls [13]. Analysis of the magnitude and direction of the photonic band edge shift suggests an escaped radial alignment of LC molecules inside the pores, as illustrated schematically in Fig. 4b. Additional investigation of the LC alignment is in progress to confirm this conclusion. Enhanced optical signal modulation may be accomplished by using surface agents to achieve a more uniform LC alignment throughout the pores [6] Comparison of optical interconnect technologies Telecommunication technology has led to the development of a number of superior interconnect platforms. For example, modulation speeds up to 70Gbits/s are achievable using lithium niobate or III V compound semiconductors based devices [14,15]. However, these platforms are difficult to introduce into interchip or intrachip interconnects, where compact and cost-efficient devices are needed. On the other hand, silicon microelectronic technology offers a unique possibility of large-scale integration and low-cost manufacturing of electronic and optical devices. Unfortunately, the magnitude of the electro-optic effect in silicon is very small, which significantly complicates the design of compact and high-performance optical devices. For example, the state-of-the-art electrooptic modulator has dimensions of 10 mm and needs more than 5W of power to operate at 1GHz [5]. Infiltration of silicon-based photonic crystals with active optical materials offers a way to significantly reduce the dimensions of existing active optical devices while maintaining compatibility with microelectronic technology. The demonstrated tunable PBG structures are suitable for a limited number of communication applications, such as data switching and routing, where low modulation frequencies are required. For most interconnect applications, however, the modulation speed must be significantly increased. Electrical switching of LCs, as well as infiltration of faster electro-optic polymers, is currently under investigation. 4. Conclusions Fig. 4. Schematic illustration of LC orientation inside (a) a mesopore of the one-dimensional PBG microcavity and (b) a macropore of the two-dimensional PBG structure for nematic (left) and isotropic (right) LC phases. For the one-dimensional PBG structure, light is incident from the top while for the two-dimensional PBG structure, light is incident from the side. Tunable silicon-based PBG structures have been demonstrated for optical interconnect applications. Significant optical signal modulation was achieved after infiltration of LCs into the PBG structures. Onedimensional PBG microcavities enable out-of-plane

5 744 S.M. Weiss et al. / Optical Materials 27 (05) modulation and routing while two-dimensional photonic crystals allow for fabrication of active in-plane interconnect devices. The effect of LC alignment in each of the PBG structures is explained and advantages over other optical interconnect technologies are shown. Acknowledgements The authors would like to thank the University of Rochester Laboratory for Laser Energetics and the University of Buffalo South Campus Instrumentation Center for use of their optical characterization equipment. Financial support from the Air Force Office of Scientific Research (Grant No. F ) and the Semiconductor Research Corporation (Grant No. 01-NJ- 967) is also acknowledged. References [1] [2] J.W. Goodman, F.I. Leonberger, S.Y. Kung, R.A. Athale, Proc. IEEE 72 (1984) 850. [3] A. Birner, R.B. Wehrspohn, U.M. Gösele, K. Busch, Adv. Mater. 13 (01) 377. [4] S.M. Weiss, P.M. Fauchet, Phys. Status Solidi A 197 (03) 556. [5] A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, M. Paniccia, Nature 427 (04) 615. [6] M. Haurylau, A.R. Shroff, P.M. Fauchet, Optical properties and tunability of macroporous silicon 2-D photonic bandgap structures, Phys. Status Solidi C, in press. [7] [8] S.H. Fan, S.G. Johnson, J.D. Joannopoulos, C. Manolatou, H.A. Haus, J. Opt. Soc. Am. B 18 (01) 162. [9] S.H. Fan, P.R. Villeneuve, J.D. Joannopoulos, Phys. Rev. Lett. 80 (1998) 960. [10] S.J. Johnson, C. Manolatou, S.H. Fan, P.R. Villeneuve, J.D. Joannopoulos, H.A. Haus, Opt. Lett. 23 (1998) [11] S. Rowson, A. Chelnokov, J.-M. Lourtioz, J. Lightwave Technol. 17 (1999) [12] S.M. Weiss, M. Molinari, P.M. Fauchet, Appl. Phys. Lett. 83 (03) [13] G.P. Crawford, S. Žumer, in: G.P. Crawford, S. Žumer (Eds.), Liquid Crystals in Complex Geometries Formed by Polymer and Porous Networks, Taylor & Francis, London, 1996 (Chapter 1). [14] E.L. Wooten, K.M. Kissa, A. Yi-Yan, E.J. Murphy, D.A. Lafaw, P.F. Hallemeier, D. Maack, D.V. Attanasio, D.J. Fritz, G.J. McBrien, D.E. Bossi, IEEE J. Select Topics Quan. Elect. 6 (00) 69. [15] J.S. Cites, P.R. Ashley, J. Lightwave Technol. 12 (1992) 1167.