Multi-channel Si-liquid crystal filter with fine tuning capability of individual channels for compensation of fabrication tolerances
© Baldycheva et al.; licensee Springer. 2012
Received: 2 May 2012
Accepted: 5 June 2012
Published: 12 July 2012
In this study, a technique for the optimization of the optical characteristics of multi-channel filters after fabrication is proposed. The multi-channel filter under consideration is based on a Si photonic crystal (PhC), tunable liquid crystal and opto-fluidic technologies. By filling air grooves in the one-dimensional, Si-Air PhC with a nematic liquid crystal, an efficiently coupled multi-channel filter can be realised in which a wide stop band is used for channel separation over a wide frequency range. By selectively tuning the refractive index in various coupled cavities, continuous individual tuning of the central channel (or edge channels) up to 25% of the total channel spacing is demonstrated. To our knowledge, this is the first report on the electro-optical solution for the compensation of fabrication tolerances in an integrated platform.
KeywordsMulti-cavity photonic crystal Coupled Fabry-Pérot resonators Liquid crystal devices Integrated optics devices Wavelength filtering devices Lithography error Line-width variation Critical dimension variation 42.60.Da Resonators Cavities Amplifiers Arrays Rings 42.60.Fc Modulation Tuning Mode locking 42.70.Df Liquid crystals
Although Si fabrication technology has significantly developed over the last 20 years, one of the main problems for optical, nano-scale periodic structures, such as Fabry-Perot interferometers and multi-channel photonic crystal (PhC) filters, remains in defining of the critical dimensions precisely in the system [1–3]. In general, structural deviations and non-uniformities present in the patterned features on the wafer occur for three principal reasons. First of all, there is the fundamental diffraction limit of the projection optics. Secondly, the mask pattern differs from the original design due to limitations in the mask fabrication process. Finally, random and systematic variations inevitably occur in a multitude of lithographic process parameters, such as focus and exposure . Fabrication tolerances for modern e-beam lithography are usually assumed to be a minimum of about 5% to 10% of the nominal target dimensions. This crucially affects the optical characteristics of multi-channel filter devices [5–7]. It changes the precise wavelength position of individual channels; it increases the out-of-band reflection and causes an attenuation of the maximum intensity, resulting in a lowering of the quality factor, Q. One of the most promising solutions to these problems may be a filter device with low-power and low-loss capability to compensate for the optical filter deviations. This would allow fine tuning of individual channels in the filter system by varying the temperature or by applying an electric field. However, one of the main challenges in the realisation of tunable multi-channel devices remains the fixed channel spacing or free spectral range which cannot be easily tuned due to the strong coupling between channels. One of the more successful attempts to tackle this problem was presented in , where the authors demonstrated a solution for an integrated platform. This approach requires the incorporation of metallic micro-heaters into large 245 μm resonators, demanding precise temperature control of the device during operation.
To our knowledge, there have been no reports in the literature on addressing this problem using electro-tuning for the individual channels in a multi-channel system that is compatible with complementary metal oxide semiconductor technology. The design of the tunable multi-channel filter proposed in this work is based on a one-dimensional (1D) Si photonic crystal (PhC) using opto-fluidics and liquid crystal (LC) technologies [2, 5, 9–14]. An LC is one of the most attractive tuning material for Si-based integrated devices, enabling tuning of the resonance modes using low applied voltages (from 1.5 V) with negligible absorption during device operation. By creating optical cavities within the periodic structure by infiltration with LC, the PhC mirror can be transformed into a highly efficient coupled multi-channel filter . A coupled multi-cavity PhC system has significant advantages over other types of coupled resonators in terms of device simplicity, ease of integration on a Si chip, and power consumption. We extended the continuous fine-tuning capability of LC single microcavity to a system of individually tunable coupled multi-cavities. The application of an electric field to each cavity was done individually, and its continuous selective variation across all cavities allows to overcome the problems related to the strong coupling between channels. Using an example of a coupled triple-cavity PhC filter operated using the first SBs, we have developed a simple model for easier manipulation of the LC within individual cavities, enabling the independent fine tuning of each channel in the overall system. We note that the model suggested can be extended to a higher number of coupled cavities (defects) and, therefore, to a higher number of resonances, with an improved Q value.
The creation of a microcavity resonator requires a doubling of the optical thickness of certain grooves, , where i is the number of the groove. This can be achieved, for example, by infiltration of the groove with filler with a suitable refractive index. By varying the optical thicknesses of the defects simultaneously, the resonance (or channel) peak positions, λ c , and the Q factor of the coupled resonances obtained can be continuously tuned within the stop band (SB) [5, 18, 19]. For this purpose, the LC is a promising candidate material, with its capability of continuous reverse tuning of the refractive index over a wide range of values by the application of a voltage, V. In this study, we use the commercially available nematic LC E7 available by Merck KGaA, Darmstadt, Germany, which demonstrates a high anisotropy of the refractive index under applied voltages, V = 1.5…15 V [11–14]. For clarity, we consider an initial switching position that can achieve the same intermediate orientation of the LC molecules in all cavities. That is, it produces the average refractive index at a switching voltage of approximately 10 V.
Optimal design parameters and channel characteristics of the first, second, third order SBs of triple-cavity PhC
a = dSi + dair(nm)
λ s = λs12 = λs 23(nm)
Results and discussion
Deviation of the Si wall thicknesses and channel characteristics for the fabricated triple-cavity PhC
dSi + dair(nm)
dSi ± δ dSi(nm)
±Δ λc 1(nm)
±Δ λc 2(nm)
±Δ λc 3(nm)
λ s ± Δ λs 12(nm) λ s ± Δ λs 23(nm)
325 ± 35
153.3 − 24.3
(± 10.8 %)
153.3 + 27.7
360 ± 20
43 − 6
(± 5.6 %)
43 + 6
486 ± 20
20 − 1
20 + 2
(± 4 %)
The simplest way to compensate for these errors, post-fabrication (dair ± δdSi), is to adjust the refractive indices of the coupled cavities individually to their optimised values, . This can be achieved by fabrication of the device on SOI platform and by applying a different voltage to each individual cavity. Then, by increasing or decreasing the applied voltage by ΔV in the various cavities, different variations in the LC refractive indices can be obtained, . For the example of a triple-cavity PhC, we focus on the two most obvious relationships between , allowing precise manipulation of the central channel position, λc 2, or the edge-channel positions, λc 1 and λ3.
The opposite manipulation of the refractive indices, i.e. reducing while increasing , results in an individual red shift of the central channel (Figure 3b, red dashed dotted and dashed lines). Note that the maximum intensity of T = 100% and the initial bandwidths are unaffected for all switching positions of the triple-channel system.
As in the previous model, the overall increase in the refractive index, Δn up to 0.052, for the example considered, leads to a linear change in the channel spacings of up to 25% or λs 12/λs 23 = 1.5 (Figure 4a, b). Although the out-of-band reflection between the central and right edge channel is decreased, it still reaches 95%. Further increases in Δn up to the limiting case of Δn = 0.11, will result in a rapidly growing difference between channel spacings, λs 12/λs 23 of up to 2.7 with a decrease of the out-of-band reflection between the central and left-edge channel to 90% (Figure 4b, blue dashed dotted line).
For significantly higher values of Δn > > 0.11 than those considered in this paper, the central cavity acts as a single cavity, independent of the edge cavities. The left-edge channel will be shifted out of the SB, and the right edge channel will be merged with the central channel, thus changing the resonator mode from triple mode to single mode. Obviously, the opposite manipulation of the refractive indices, i.e. increasing the and fixed = , results in the individual red shift of the edge channels with the same Δλ s value (Figure 4c). Again, as for the central tuning combination, the maximum intensity of T = 100% is not affected for all switching positions of the triple-channel system.
To summarise, an approach based on the individual tunability of multi-channel filters has been proposed for the compensation of optical parameter deviations caused by structural fluctuations. Electro-optical tuning of individual central (or edge) channels was demonstrated by up to 25% of the channel spacings, which is sufficient for the optimization of devices with fabrication tolerances of up to 20%. The approach suggested here can be utilised for the optimization of multi-channel silicon devices over a wide infrared range.
AB is a final year PhD student doing research under supervision of TS-P, professor and director of Microelectronics Technology Group in Trinity College Dublin, Ireland. VA-T is a senior researcher at Ioffe Physical Technical Institute, Russia, and KB is a lecturer at Dublin Institute of Technology, Ireland.
scanning electron microscopy
transfer matrix method.
This work has been supported by the ICGEE Programme (Ireland) and NAP-368 (Science Foundation Ireland). AB wishes to express the appreciation to Alan Blake (Tyndall Institute Ireland) for the useful discussions.
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