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.
Porous-silicon-based 1D photonic crystals forming Bragg filters, rugate filters, microcavities, or other optical components show a pronounced resonant peak of the stop band or a sharp resonant fall-off within the stop band. For a distributed Bragg reflector (DBR) with layers of alternating high and low refractive indices nL
, the position of the resonance peak (central wavelength λ0
) is given by
are the thicknesses of low and high refractive index layers, respectively. The bandwidth (Δλ
) of the so-called stop band around the central wavelength (λ0
) can be selected by the proper adjustment of nL
and is given for DBR by [12
The shift of the central wavelength λ0
in the transmission or reflection spectrum as function of incidence angle (
) can be described with the Bragg's law [6
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 .