Tunable optical filters with wide wavelength range based on porous multilayers
© Mescheder et al.; licensee Springer. 2014
Received: 30 April 2014
Accepted: 12 August 2014
Published: 22 August 2014
A novel concept for micromechanical tunable optical filter (TOF) with porous-silicon-based photonic crystals which provide wavelength tuning of ca. ±20% around a working wavelength at frequencies up to kilohertz is presented. The combination of fast mechanical tilting and pore-filling of the porous silicon multilayer structure increases the tunable range to more than 200 nm or provides fine adjustment of working wavelength of the TOF. Experimental and optical simulation data for the visible and near-infrared wavelength range supporting the approach are shown. TOF are used in spectroscopic applications, e.g., for process analysis.
KeywordsTunable optical filter Porous silicon Photonic crystal Dual tunability Silicon anodization MEMS MOEMS Biosensor
Tunable optical filter (TOF) is used in spectroscopic applications e.g., for process analyses. Over the last few years, research has been focusing on miniaturizing TOF for applications in microoptical electromechanical systems (MOEMS). For example, TOF systems based on MOEMS Fabry-Perot interferometers (FPI) have been reported, where wavelength tuning results from changing the gap between the involved mirrors and thus requires an extremely precise control of the micromechanical movement [1–4]. In  a system with thermal actuation for changing the refractive medium inside the FPI was presented, which provides relatively small tuning range and low frequency response. A tunable optical filter using porous silicon and sub-surface electropolishing was developed by Lammel et al. . In that work, the flip-up optical filter was tilted and tuned by two sophisticated thermal bimorph microactuators where tilt position could not be controlled exactly. Change of spectral response of photonic crystals based on porous multilayers using pore-filling, including fabrication and characterization aspects, and application of this method for sensing were reported by different research groups [7–10]. In a similar approach, Ruminski et al.  demonstrated spectral wavelength shifts of porous-silicon-based photonic crystals due to tilting and irreversible pore-filling with polystyrene as optical reference.
In our paper an approach for a tunable micromechanical TOF system based on porous silicon 1D photonic crystal is presented. This MOEMS TOF system, in contrast to the above mentioned examples, can be tuned over a wide wavelength range based on a dual tuning principle: by tilting the photonic crystal and by reversible filling the pores of the photonic crystal with liquids or gases.
where d is the thickness of a period of the two layers with low and high refractive indices (d = dL + dH), and n is the effective refractive index of the porous layer.
According to Equation 3, fast tuning of some hundreds of nanometers to shorter wavelengths (blue shift) of the resonant peak position can be achieved by a relatively large rotation (up to 20° to 40°) of the photonic crystal in respect to the incident light.
By pore-filling of the porous optical filter with different gases or liquids (organic or aqueous solutions), shift to longer wavelengths (red shift) of the central wavelength can be achieved. This shift is due to increase of the effective refractive index of the porous silicon during pore-filling. It is important to note that the response times for this tuning principle are limited by the transport processes in nanostructured layers .
The photonic crystals used for the demonstration of tuning principles in this paper have been fabricated from p-type boron-doped one-side-polished silicon wafers (10 to 20 Ω cm). The backside (not polished side) was doped additionally with boron by ion implantation to achieve low sheet resistance about 24 Ω/□ in order to provide good electrical contact of the wafer's backside to the electrolyte during the anodization process. Silicon samples have been processed from 4-in. wafers by cleaving the wafers to quarters. The area exposed to the electrolyte was 28 × 28 mm2. The samples were anodized at room temperature in a double-tank cell (AMMT GmbH, Frankenthal, Germany) with two platinum electrodes operated under current control. Electrolyte mixture of 1:1 volume ratio of 50 wt.% HF and pure ethanol was used. Two types of photonic crystals were realized - DBR and rugate filters. The DBR filters comprised 20 porous layers with alternating low and high refractive indices. The rugate filters were fabricated by sinusoidal modulation of refractive index with 16 and 32 periods. The time-dependent current profiles for anodization were calculated based on experimentally determined dependencies on current density of the effective refractive index (calculated using the Bruggeman model  from porosity values) and of porous silicon formation rate. The power supply for the anodization process was provided with NI LabView™ controlled Gossen Metrawatt PSP-1500 power source (Gossen Metrawatt, Nürnberg, Germany). The current density for all filters fabricated in this work was set between 20 and 70 mA/cm2. All photonic crystals were designed and fabricated to have a central wavelength λ0 in the visible spectrum.
In order to measure the dual tunability with the pore-filling and the tilting, a closed chamber with dedicated inlet and outlet orifices for vapor or liquid, an anti-reflection glass window, and a holder for the porous Si photonic crystal was constructed. Ethanol vapor was pumped into the closed chamber by a self-designed circulating system through the inlet orifice and left through the outlet orifice. The spectrometer detector fiber was synchronized to the rotation in such a way that this detector fiber was always aligned to the light reflected from the crystal. In order to characterize the dual tunability, the spectrum of the photonic crystal was measured for each tilting angle for two states. First, the spectrum of the photonic crystal in the empty chamber (pores filled with air) was recorded. Afterwards, the chamber was filled with vapor, which resulted in capillary condensation of vapor in the pores of the photonic crystal. Then the spectrum was recorded again.
From the simulation (Figure 3) and the experimental results (Figure 5), it is clearly demonstrated that tilting the photonic crystal causes a shift of the central wavelength to a lower wavelength, i.e., a blue shift of the spectrum. The tunability range of a low-doped porous silicon photonic crystal by tilting was found to be wider than that of the high-doped photonic crystal (Figure 3). This effect can be explained by a difference in refractive index contrast nH/nL for the two doping levels, where the low-doped porous silicon photonic crystal has a lower refractive index contrast. The measured spectral shift of the central wavelength as function of tilt angle for the low-doped photonic crystal was found to be in good agreement with the simulation (Figure 5). The experiment showed that the shift of the central wavelength as a result of tilting is instantaneous without any noticeable delay. Tunability by the tilting worked well in a narrow wavelength range limited by tilting angles up to 50°. For higher tilting angles, the integrity of the spectrum tended to fade away due to total internal reflection.
A concept of miniaturized MOEMS system with the integration of both tuning principles has been developed. The tilting angle of photonic crystals is limited by the phenomenon of total internal reflection; therefore, angles up to 20° to 40° are required from the system. For a miniaturized actuation system, this tilting range is challenging. Various actuation principles for tilting such as electrostatic, electromagnetic, piezoelectric, and thermoelectric have been evaluated.
A novel MOEMS-based concept for tunable optical filter is presented. Combining fast micromechanical tilting and pore-filling of the porous-silicon-based photonic crystal, a tunable range of ±20% around the working wavelength of the TOF was realized. The tunability range for photonic crystals made out of low-doped p-type silicon was found to be wider than for photonic crystals made from high-doped p-type silicon. The feasibility of the concept was demonstrated experimentally. Experimental results confirmed the optical simulation results.
distributed Bragg reflector
microoptical electromechanical system
scanning electron microscope
tunable optical filter.
The authors would like to thank Ms. A. Malisauskaite for her support in the measurements and simulation. Mr. B. Müller supported the preliminary analytical study of tilting effect on wavelength shift. Dr. W. Kronast, Mr. J. Liu, and Mr. L. Pemmasani are acknowledged for developing the concept of micromirror for large deflection angles. Mr. L. Kajdocsi helped with the LabView control system during the fabrication of the photonic crystals. The work was financially supported by German Ministry for Education and Research (BMBF) in frames of the project 'Mini-Refraktometer’ (FKZ 17020X11).
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