The authors report a compact and highly selective tunable filter using a Fabry-Perot resonator combining a bottom micromachined 3-pair-InP/air-gap Bragg reflector with a top photonic crystal slab mirror. It is based on the coupling between radiated vertical cavity modes and waveguided modes of the photonic crystal. The full-width at half maximum (FWHM) of the resonance, as measured by microreflectivity experiments, is close to 1.5nm (around 1.55μm). The presence of the photonic crystal slab mirror results in a very compact resonator, with a limited number of layers. The demonstrator was tuned over a 20nm range for a 4V tuning voltage, the FWHM being kept below 2.5nm. Bending of membranes is a critical issue, and better results (FWHM≡0.5nm) should be obtained on the same structure if this technological point is fixed.
© 2006 Optical Society of America
Photonic crystals (PC) have become an active research area, especially in the prospect of future planar Photonic Integrated Circuits (PICs) in the waveguided configuration. When operating above the light line, waveguided modes suffer from out-coupling losses because of their coupling with radiated modes , which appears as an important drawback for the development of PICs. However, whenever it is meant to operate above the light-line and not only in the waveguided regime, coupling of waveguided and radiated modes is requested and may be used for the development of new class of devices, where PCs are deliberately opened to the third dimension of space .
For example, instead of attempting to confine light entirely within the PC, the coupling between waveguided modes and radiated modes can be exploited to design very efficient Photonic crystal membrane (PCM) reflectors. Such PCM reflectors have already been demonstrated. They use either 1D [3,4] or 2D [5,6] gratings, and present very broadband and high-efficiency characteristics, being thus quite comparable and competitive with usual multilayer Bragg stacks.
They may so replace multilayer Bragg stacks in devices like MOEMS filters [7,8], VCSELs [9,10], etc. To demonstrate this potentiality, we have designed, fabricated, and characterized a new tunable filter associating a bottom 3-pair-multilayer-InP/air Bragg stack with a top PCM reflector. This device is part of a potential new class of devices, named PCMOEMS , in which MOEMS are associated to PCs; the 1D vertical high index contrast structuration being combined with the 2D (in-plane) lateral high index contrast structuration (PCM reflector) leads to a quasi-3D control of the electromagnetic environment.
Figure 1 shows a true scale representation of the PCMOEMS filter designed, compared with an equivalent traditional double-Bragg MOEMS filter with a 2λ-thick cavity air-gap, operating at the same wavelength λ (λ=1.55um). We will show in this paper that the PCMOEMS filter, owing to the presence of the PCM, is theoretically able to provide the same quality factor of resonance as the MOEMS one, with a much better vertical compactness.
It should, however, be noted that the combination of PCMs and MOEMS have other advantages than compactness. Here, for example, it enables the introduction of polarization effects in the filtering response. In other configurations, it can lead to a wide range of innovative optical functions like, for example, in-plane insertion of externally incident light [2,11].
2. Design of the filter
A PCM may present several guided modes, whose resonant wavelength depend on the k//-vector in the first Brillouin zone. When illuminating a PCM embedded in air by a plane wave at a wavelength λ, and considering that a waveguided mode exists for this wavelength and for the k//-vector corresponding to the incidence angle, a total reflection can be achieved. Incident light is coupled to the waveguided mode, and then reemitted in third direction by constructive interferences, because of the coupling from the waveguided mode to the radiated modes, with the corresponding time τc.
In order to be useful for integrated photonics, such reflectors should operate on a limited area. This can be achieved by exploiting extremes of the dispersion characteristics, like the Γ-point (normal incidence), where the group velocity tends to zero. Indeed, around the Γ-point, a parabolic approximation may apply for the dispersion characteristics:
where α is the second derivative of the dispersion function ω(k) at k=0, that is the curvature of the corresponding band around the Γ-point.
For a PCM illuminated by a normal incidence beam of area S, the mean group velocity can be expressed as:
Thus, the time τg related to the escape of photons from the illuminated area is given by :
We see that, in order to achieve a good lateral confinement of photons, a low curvature of the band is needed around Γ. τg should be maximized, in such a way as τc/τg is made low enough for photons of the waveguided mode to be reemitted vertically before they can laterally escape out of the illuminated area. Typically, for a reflection of 99.8%, τc/τg should be on the order of 10-3, as can be derived from the coupled mode theory .
Such low-curvature bands can be obtained in high index contrasts PCMs, formed in high index contrast semiconductor materials. In this contribution, we use InP-based materials, which are very attractive for the development of active optical devices operating in the 1.3μm-1.55μm range. The InP/air index contrast is 3,17/1.
The filter was designed in order to present a high selectivity within a large wavelength range, which is interesting for optical communications (especially for wavelength division multiplexing). It was so necessary to design broadband high-efficiency reflectors at both sides of the cavity. For the multilayer Bragg stack, broadband high-efficiency reflectivity can be easily obtained by a suitable choice of index contrast, optical thicknesses, and number of pairs of layers. As the index contrast is quite high for InP/air multilayer stacks, we made the choice of a 3-periods (3λ/4 InP-λ/4 air) one, which is sufficient to get a high reflectivity (>99.8%) over a 200nm wavelength range around λ.
For the PCM reflector, however, achieving such a broadband reflectivity is not straightforward. Two resonant modes of the PCM are used; the overlap between the resonances may thus lead to a broadband high-efficiency reflectivity .
The PCM used in our approach is a 1D InP/air grating with a thickness h=255nm, a period p=1.15μm, and an air filling factor f=65% (Fig. 2(a)). RCWA simulations  showed for this PCM a broadband and high-efficiency reflection centered around 1.55um (Fig. 2(b)), for light polarization parallel to the slits. The reflection presents two maxima of 100% at 1.48 μm and 1.54 μm, and is superior to 99.8% in the 1.45-1.6 μm wavelength range for these nominal parameters.
The two guided modes responsible for the broadband reflection can be made visible by slightly modifying the air filling factor; for a 75% air filling factor, the overlap between the two resonances is not optimal, and a small dip appears in-between (dashed curve in Fig. 2(b)). For light polarization perpendicular to the slits, only one mode can be excited in Γ in the spectral range of interest, and thus broadband reflection cannot be achieved.
The nominal parameters of this PCM were chosen in order to allow the best tolerance as possible on the geometrical parameters (membrane thickness, air filling factor). By plotting the reflection as a function of both wavelength and air filling factor for three different membrane thicknesses (255nm+/-20nm), we see in Fig. 3 that broadband high-efficiency reflection around 1.55μm remains possible for a quite large range of air filling factors (the two modes are related to the two dark branches in Fig. 3(a), Fig. 3(b), and Fig. 3(c)). Indeed, for these membrane thickness values, the energy related to each mode is not significantly affected by the air filling factor, which gives a great flexibility for achieving a flat reflection. So, tolerances on epitaxy and fabrication processes are acceptable. Similar broadband reflectors could have also been obtained using 2D PCMs. The principal difference lies in the polarization behavior, as reported in [5,6].
The potential compactness of the PCM reflector was then validated by 3D FDTD calculations, with a 30μmx30μm PCM illuminated by a 13μm-wide circular gaussian beam. The reflection characteristics are represented in Fig. 4(a). It shows a high reflection, up to 99.9%, over a 350nm wavelength range around 1.55μm. As a comparison, the maximum reflection achieved on the same PCM with a 6 μm-wide circular Gaussian beam is 99%, due to larger lateral losses induced by the narrowing of the beam area.
A mapping of the Ey-field amplitude in the middle-plane of the PCM is shown in Fig. 4(b). It confirms that the critical direction for lateral confinement is the direction perpendicular to the slits. Indeed, the filter yield is expected to depend critically on the lateral confinement of the resonant mode of the filtering structure, which is principally controlled by the ability of the PC to retain the waveguided photons (whose k-vector is perpendicular to the slits) in the PCM within the incident beam area, and during the lifetime of the resonance. This allowed us to restrict our analysis to 2D-FDTD calculations for later simulations of the complete structure.
Once both reflectors are designed, the adequate cavity air-gap needs to be chosen, for operation around λ, (λ=1.55um). Considering the phase shift at the reflection on the PCM mirror, the phase matching condition calculated for the λ wavelength corresponds to a nearly -quarter wavelength cavity air-gap. We made the choice of a 3λ/4-thick air-gap in order to prevent any unwanted significant evanescent coupling of the waveguided modes of the PCM with the Bragg membranes.
2D-FDTD calculations were performed for the two filter configurations presented in Fig. 1, with 30μm-wide membranes, and a 13μm-wide gaussian beam, both air cavities being adjusted for operation around 1.55μm (Fig. 5). A similar low FWHM of about 0.5nm is obtained for both filters. The MOEMS (Fig. 5(a)) and the PCMOEMS (Fig. 5(b)) filters are theoretically equivalent, in terms of quality factor of the resonance (Q=λ/FWHM around 3000). Since the bottom mirror is the same for both structures, and the cavity air-gap much larger for the MOEMS filter (3300nm) than for the PCMOEMS filter (1300nm), we can deduce that the lifetime of photons inside the cavity is enhanced by the presence of the PC. This can be explained by the higher reflectivity achieved by the PC membrane. In fact, the reflectivity of the Bragg mirror is limited by the number of periods, whereas with a single PCM reflector, a 100% reflection can be obtained at the resonance for a normal incident plane wave illuminating an infinite grating. When taking into account the finite size of the incident beam, the reflection is deteriorated by lateral losses due to the multiple angular components contained by the beam. However, these lateral losses are very low, owing to the low mean group velocity around the Γ point. Therefore, the single PCM behaves as a better mirror than the 3-layers top Bragg stack, for these wavelengths.
For the PCMOEMS filter, the transmittivity is not optimal because of unbalanced reflectivities of cavity mirrors. For the MOEMS filter, the configuration is also asymmetric, since the presence of the substrate leads to an additional InP/air interface which accounts for a better reflectivity of the bottom mirror. Moreover, the transmittivity is also deteriorated by lateral losses, which have approximately the same value for both structures (around 55%).
3. Experimental results
The PCMOEMS filter is based on an InP/InGaAs heterostructure, which consists of successive InP/InGaAs layers epitaxially grown on an InP substrate by low-pressure Metal Organic Vapor Phase Epitaxy. The InGaAs layers are meant to be sacrificial and are subsequently removed by surface micromachining, resulting in InP membranes suspended in air. The complete structure includes a three-InGaAs (387nm-thick) / InP(367nm-thick) bottom stack for the later Bragg mirror, a 1450 nm-thick InGaAs layer for the later cavity air-gap, and a 255nm-thick InP top layer for the later PCM. The electrostatic actuation of the cavity air-gap is allowed by a n-type doping of substrate and Bragg layers, and a p-type doping of the top membrane during the epitaxy. This results in the formation of a p-i-n junction, which can be reversibly biased. The electrostatic tuning voltage is applied to the layers adjacent to the Fabry-Perot cavity, thus reducing its thickness and tuning the resonant wavelength. The 1450nm-thick cavity has been designed for a center wavelength of 1600 nm (with no tuning bias), in order to operate in the range 1500-1600 nm. Pd-AuGe alloys were formed for the top ohmic electrostatic tuning contacts of the filter. The filter fabrication is based on our PCMOEMS procedure (see  for extra details).
Figure 6 shows the scanning electron microscope (SEM) views of the fabricated structures. The PCMs are 50μm in diameter (the diameter of the PC being 35μm), suspended by 10μm-wide arms. The maximum length of each arm is 55μm.
The reflectance of the structure was then characterized. The optical characterizations presented in Fig. 7 were applied to a two-55μm-long-arms filter. For measurements, the incident beam from a monomode fiber was coupled to the device using a lens collimator (waist diameter of 13μm). The excitation light was provided by a superluminescent emitting diode (EDFA/SLED) as a large band light source connected to the collimator through a 50/50 directional coupler, followed by a polarizer and a section of polarization maintaining fiber. The reflected beam was collected back by the collimator, separated on the directional coupler, and analyzed by a spectrum analyzer. The reference for 100% reflectance was taken onto the metal contacts of the device under test.
Quality factors of the resonance as high as 1000 have been achieved (FWHM≅1.5nm around 1.57um), with no tuning bias. When applying a bias, the resonant wavelength has been shifted from 1.57μm to 1.55 μm, for a tuning bias voltage of 4V. The FWHM was kept below 2.5nm over this 20nm tuning range.
Two observations can be pointed out from these experimental results. On one hand, the quality factors are not as high as expected. On the other hand, when no tuning bias is applied, the filtered wavelength is located at 1.57μm instead of 1.6μm designed.
This is due to the downward bending of the top PCM, which has been shown by interferometric microscopy measurements (Fig. 8). This bending results not only in a lower air-gap cavity thickness towards the center of the platform (around 1350nm instead of 1450nm) which accounts for a 1.57μm filtered wavelength, but also in a variable thickness of the air-gap cavity by an amount of 20 nm from the center to the edges of the platform, which critically increases lateral losses. 2D-FDTD simulations performed with such a curved top PCM showed, indeed, a FWHM of 1.3nm (instead of 0.5 nm), which is in good agreement with experimental results. When increasing the tuning voltage, the quality factor of the resonance decreases (from 1000 to 600 for a 4V tuning voltage). Indeed, the curvature of the top PCM getting more important with the tuning voltage, the lateral escape of photons increases, which results both in a lower lifetime of photons inside the cavity and a drop of the reflectivity.
We have demonstrated that PCMs can potentially replace multilayer Bragg stack mirrors in micromachined MOEMS filters, and more generally, in any devices where Bragg stacks are traditionally used as mirrors. The PCMOEMS filter proposed is based on the interplay between radiated modes of the cavity and guided modes of the PCM, resulting in a quasi-3D control of photons. Such PCMs may allow not only for more compact and functional devices, but also for tunable devices with high speed and low actuation voltage.
The tunable filter presented in this paper has shown a quality factor up to 1000 (FWHM of 1.5nm around 1.55um), and thus presents characteristics quite similar to usual tunable filters based on two 3-periods-InP/air Bragg stacks. It has been tuned over a 20nm-wavelength range, keeping a FWHM below 2.5nm, but it could even present better performances, if structural technological deficiencies are fixed.
The mismatch between the experimental (1.5nm) and the theoretical (0.5nm) FWHM has been attributed to bending effects due to unwanted residual stresses in the top PC membrane, and resulting in degraded selectivity of the filter. This drawback could be circumvented by using a thicker membrane for the Photonic crystal reflector, which would require a new design (lattice period, air filling factor). One could also try to get a very fine control over the compressive as well as tensile stress distribution in the top membrane. It would be of course at the expense of its flexibility, with an increase of the tuning voltage needed for a given displacement of the PCM.
At this point, one should notice that we studied a PCM based on a 1D grating, which results in a polarization sensitive filter. Associated with a gain medium, such a structure could be very interesting for the realization of compact polarized VCSELs. Nevertheless, polarization-independent filters could also be envisaged, using 2D photonic crystals that exhibit a 90-degree rotational symmetry.
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