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Advances in Optical Technologies

Volume 2008 (2008), Article ID 780784, 12 pages

http://dx.doi.org/10.1155/2008/780784

## Tuning of the Optical Properties in Photonic Crystals Made of Macroporous Silicon

^{1}Department of Chemistry, Faculty of Science, University of Paderborn, 33095 Paderborn, Germany^{2}Institute of Physics, Martin-Luther-University Halle-Wittenberg, 06099 Halle, Germany^{3}Department of Physics and Institute for Optical Sciences, University of Toronto, Toronto, ON, Canada M5S 1A7

Received 9 January 2008; Accepted 11 April 2008

Academic Editor: D. Lockwood

Copyright © 2008 Heinz-S. Kitzerow et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

It is well known that robust and reliable photonic crystal structures can be manufactured with very high precision by electrochemical etching of silicon wafers, which results in two- and three-dimensional photonic crystals made of macroporous silicon. However, tuning of the photonic properties is necessary in order to apply these promising structures in integrated optical devices. For this purpose, different effects have been studied, such as the infiltration with addressable dielectric liquids (liquid crystals), the utilization of Kerr-like nonlinearities of the silicon, or free-charge carrier injection by means of linear (one-photon) and nonlinear (two-photon) absorptions. The present article provides a review, critical discussion, and perspectives about state-of-the-art tuning capabilities.

#### 1. Introduction

Artificial structures exhibiting a spatially periodic structure with lattice constants comparable to the wavelength of light [1, 2] are extremely promising materials for integrated optical devices. These structures, referred to as photonic crystals [3–11], are characterized by an unusual dispersion relation which might show a photonic band gap (PBG), that is, a frequency range in which the propagation of light is not permitted. Properly designed defects within these structures may serve, for example, as optical waveguides, frequency filters, optical switches, or resonant microcavities with high quality factor, which can be used for low-threshold lasing. Many excellent works have demonstrated the potential capabilities of these structures, but their fabrication is still elaborate.

Electrochemical etching of silicon turned out to be a very reliable and versatile technique to fabricate two-dimensional periodic arrays of macropores [12–15]. Similarly to the method of electrochemical polishing, silicon wafers are dipped into hydrofluoric acid and a DC voltage between the wafer and a counter-electrode is applied. The generation of free charge carriers in the doped silicon is assisted by exposure to infrared radiation. However, in contrast to the common polishing process, the parameters are chosen in a narrow range where the etching process does not lead to flattening of the surface, but instabilities cause the self-organized growth of pores. Spontaneously, the pores tend to arrange in a two-dimensional hexagonal lattice with spacings between 0.5m and 10m. Additional pretreatment of the surface by photolithography can be used to alter the symmetry of the lattice, to create extremely high correlation lengths, and to design the arrangement of defects with very high precision. The depth-to-width ratio of the pores can be as large as 500. In addition, it is possible to modulate the width of the pores along the pore axes by controlling the current through the sample during the growth process, thereby creating even three-dimensional structures [13–15].

In addition
to utilizing the benefits of the unusual dispersion relation, it is highly
desirable to change the dielectric properties and to control them by external
parameters, thereby achieving even a tunable dispersion relation [16–18]. This
might be necessary in order to compensate for fabrication tolerances and to
fine-tune the properties of the produced photonic crystal, or be motivated by
targeting active switching devices. An obvious way to achieve tunable
properties is altering the dielectric susceptibility of the silicon. As usual,
the dielectric susceptibility can be expanded in a power series as follows: Thus, linear [] or nonlinear effects [] may be considered. In
addition, the macroporous silicon structures can be filled with a dielectric
compound. If the silicon () is infiltrated with a different
compound (), the most fundamental change is a
shift of the average dielectric constant , which is approximately given by
the Maxwell-Garnett relation [19] where *f _{2}* is the volume fraction of component 2. If the
denominators in (2) are not too different, the effective refractive index of
the composed material is approximately given by where

*f*is the volume fraction of component

_{i}*i*of the heterogeneous system. Thus, changing the refractive index of one of the two components has an effect on the properties of the entire structure. However, the influence on the dispersion relation and on the linear and nonlinear photonic properties is much more subtle than a simple change of the average refractive index. Due to their sensitivity to external parameters, liquid crystals proved to be very efficient as a dielectric liquid yielding addressable photonic properties.

In this paper, we would like to review the methods of tuning by means of liquid crystal infiltration (Section 2) as well as all-optical effects that are due to the properties of the silicon (Section 3). The latter effects can be based on charge carrier injection due to one- or two-photon absorption or on Kerr-like nonlinearities (the latter being described by the real part of ).

#### 2. Tuning by Means of Liquid Crystal Infiltration

Liquid
crystals [20–24] exhibit very sensitive electro- and thermo-optical properties.
Filled into the pores of a photonic crystal, they provide the opportunity of
adjusting the effective refractive index by external parameters. This method
was proposed by Busch and John [25–28] and experimentally demonstrated for
colloidal crystals [29–34] before being applied to macroporous silicon [35–46]
and other PBG semiconductor structures, including tunable light sources
[47–50]. The sensitivity of liquid crystals is due to a preferred uniform
alignment of their typically rod-like molecules, which in turn leads to
birefringence. The local structure of the least complicated liquid crystalline
mesophase, the nematic phase (Figure 1), can be described by the director **n** (a pseudovector) and a scalar
order parameter *S*, which indicate the local molecular alignment (i.e., the
optical axis) and the degree of orientational order, respectively. External
fields can rotate the optical axis, while an increasing temperature results in
a decreasing order parameter and thus a decreasing birefringence . Typically, the difference between the extraordinary
refractive index *n _{e}* and the ordinary refractive index is of the order if the temperature is several Ks below the
nematic-isotropic phase transition (clearing temperature). No birefringence
appears above the clearing temperature. Thus, relatively large thermally
induced changes of the effective refractive index are observed at this phase
transition.

Leonard et al. [35]
were the first to infiltrate a two-dimensional structure made of macroporous
silicon with a liquid crystal and observed changes of the photonic band edge
for light propagating in the plane of the silicon wafer. This effect might be
very useful for integrated optical waveguides in silicon. Subsequent
experiments were focused on three-dimensional
(3D) structures consisting of macroporous silicon that are filled with a liquid
crystal. The latter structures show also a stop band for light propagating
perpendicular to the plane of the wafer. Two-dimensional hexagonal or
rectangular arrays of pores with an extremely high aspect ratio (diameter ≤1m,
depth ≥100m)
were fabricated by a light-assisted electrochemical etching process using HF
[12, 13], and a periodic variation of pore diameter was induced by variation of
the electric current during the etching procedure, thereby yielding in a
three-dimensional photonic crystal (PhC) [14, 15]. The macroporous structure was
evacuated and filled with a liquid crystal. The photonic properties for light
propagation along the pore axis
were studied by Fourier transform infrared (FTIR) spectroscopy [36–38].
Deuterium-nuclear magnetic resonance (^{2}H-NMR) [36, 37] and
fluorescence confocal polarizing microscopy (FCPM) [39–42] were used in order to
analyze the director field of the liquid crystal inside the pores.

For example,
Figure 2 shows the infrared transmission of samples that show a two-dimensional
hexagonal array of pores with a lattice constant m. Along the pore
axis, the diameter of each pore varies periodically between m and m
with a lattice constant m. The pores were filled with the nematic
liquid crystal 4-cyano-4^{′}-pentyl-biphenyl (5CB, Figure 1) which shows a
clearing temperature of C. For light propagation along the
pore axes, the FTIR transmission spectrum of the silicon-air structure shows a
stop band centered at m. Filling the pores with 5CB decreases the dielectric contrast to
silicon and results in a shift of the stop band to m. The band edge was found to be sensitive
to the state of polarization of the incident light. For linearly polarized
light, rotation of the sample with respect to the plane of polarization was
found to cause a shift of the liquid crystal band edge by nm (1.61 meV). This effect can quantitatively
be explained by the square shape of the pore cross-section, which brakes the
threefold symmetry of the hexagonal lattice. Due to the presence of the liquid
crystal, the band edge at lower wavelengths (“liquid crystal band” edge) can be
tuned by more than 140 nm (1.23 meV) by heating the liquid crystal from 24^{°}C
(nematic phase) to 40^{°}C (isotropic liquid phase).

The shift of
the photonic band edge towards larger wavelengths indicates an increase of the
effective refractive index with increasing temperature. This effect can be
explained by a predominantly parallel alignment of the optical axis (director)
of the nematic liquid crystal along the pore axis. For a uniform parallel
alignment, the effective refractive index of the nematic component corresponds
to the ordinary refractive index *n _{o}* of 5CB. Increasing the
temperature above the clearing point causes an increase to the isotropic value
,
where

*n*is the extraordinary refractive index of the liquid crystal (). For a very crude approximation, the average dielectric constant of the heterogeneous structure can be calculated from the respective dielectric constants [ and ] using the Maxwell-Garnett relation (2) [19]. The relative shift of the stop band edge towards larger wavelengths corresponds approximately to the relative increase of the average refractive index by 0.65%. However, more precise analysis of the data shown in Figure 2 indicates that the shift of the two band edges is not the same (the shift of the left band edge is larger). The reason for the difference is that the overlap of the electric field with the pores is different at the two band edges (it is larger at the short wavelength edge), therefore changes of the refractive index in the pores translate into different shifts of the band edge. This is correctly predicted by the calculated dispersion relation shown in the right part of Figure 2, but cannot be explained by the Maxwell-Garnett relation [36].

_{e}Planar microcavities inside a 3D
photonic crystal appear when the pore diameter is periodically modulated along
the pore axis, stays constant within a defect layer, and is continued to vary
periodically. Figure 3 shows a structure where a defect layer is embedded between five periodic modulations of the pore
diameter. The pores are arranged in a 2D square lattice
with a lattice constant
of m. The pore width varies along the pore axis between m and m. The length of a modulation is m. The defect has a length
of m and pore diameters m. Within the defect
layer, the filling fraction of the liquid crystal is . For infrared
radiation propagating along the pore axes, a
fundamental stop gap at around 13m and a second stop gap at around 7m are expected from calculations using the plane wave approximation [51]. The
experiment shows a transmission peak at m in the center of the second stop band, which can be attributed to a
localized defect mode. Filling the structure with the liquid crystal
4-cyano-4^{′}-pentyl-biphenyl (5CB, Merck) at 24^{°}C causes a spectral red-shift of
the stop band. Together with the stop band, the wavelength of the defect state
is shifted by 191 nm to m. An additional shift of nm
to m is observed when the liquid crystal is heated from 24^{°}C (nematic phase) to
40^{°}C (isotropic liquid phase). Again, the shift towards larger wavelengths
indicates an increase of the effective refractive index of the
liquid crystal with increasing temperature and can be attributed to the
transition from an initially parallel aligned nematic phase () to the isotropic state ().
During continuous variation of the temperature, a distinct step by 20 nm is
observed at the phase transition from the nematic to the isotropic phase. The
quality factor *Q* of the investigated structure, , is rather small and thus the shift by 20 nm appears to be small compared to the spectral width of the defect mode. However,
the same order of magnitude of the temperature-induced wavelength shift can be
expected for structures with a much higher quality factor and might be quite
large compared to the band width of the defect mode.

Comparison of
experimental ^{2}H-NMR results and calculated spectra (Figure 4)
confirms a parallel (P) alignment of the director along the pore axis for
substrates that were treated like the samples described above. However, also an
anchoring of the director perpendicular to the silicon surfaces (“homeotropic”
anchoring) can be achieved if the silicon wafer is cleaned with an ultrasonic
bath and a plasma-cleaner and subsequently pretreated with
*N*,*N*-dimethyl-*n*-octadecyl-3-aminopropyl-trimethoxysilyl chloride (DMOAP). NMR
data indicate the appearance of an escaped radial (ER) director field in the
latter case.

For the first time,
optical microscopic studies of the director field in pores with a spatially
periodic diameter variation could be achieved by means of a nematic liquid crystal polymer that shows a glass-like nematic state at room
temperature [39, 40]. For fluorescence polarizing microscopy, the polymer was
doped with *N*,*N ^{′}*-bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylene-carboximide
(BTBP). After filling the photonic crystal in vacuum, the sample was annealed
in the nematic phase at 120

^{°}C for 24 hours and subsequently cooled to room temperature, thereby freezing the director in the glassy state. The silicon wafer was dissolved in concentrated aqueous KOH solution and the remaining isolated polymer rods were washed and investigated by fluorescence confocal polarizing microscopy (FCPM). The transition dipole moment of the dichroic dye BTBP is oriented along the local director of the liquid crystal host. The incident laser beam (488 nm, Ar

^{+}) and the emitted light pass a polarizer, which implies that the intensity of the detected light scales as for an angle

*α*between the local director and the electric field vector of the polarized light. Thus, the local fluorescence intensity indicates the local orientation of the liquid crystal director with very high sensitivity. For a template with homeotropic anchoring and a sine-like variation of the pore diameter between 2.2m and 3.3m at a modulation period of 11m, the FCPM images of the nematic glass needles (Figure 5) indicate an escaped radial director field. Comparison with numerical calculations based on a tensor algorithm [52, 53] reveals some characteristic features that differ from nonmodulated pores. In the cylindrical cavities studied previously, point-like hedgehog and hyperbolic defects appear at random positions and tend to disappear after annealing, due to the attractive forces between defects of opposite topological charges. In contrast, the modulated pores stabilize a periodic array of disclinations. Moreover, disclination loops appear instead of point-like disclinations.

#### 3. All-Optical Tuning

Altering the optical properties by optical irradiation has been the subject of intense research efforts related to the potential development of active photonic crystal components. Here, the impact of an optical pump beam on a photonic crystal consisting of a two-dimensional array of macropores in silicon [54–59] is reviewed. Figure 7(a) shows a sketch of such a crystal with photo-electrochemically etched straight pores with an aspect ratio of 100 [8, 12, 14]. If the photon energy () of the pump beam is larger than the electronic band gap of silicon, absorption causes a free charge carrier generation in the semiconductor which in turn changes the dielectric constant due to the Drude relation [54] (Section 3.1). These free carriers, generated by photon absorption, can be injected either by a single photon absorption or, in the presence of very high pump intensities, by two-photon absorption. In contrast to the changes achieved by liquid crystal reorientation, this direct optical addressing of the silicon is very fast. Whereas the former occurs on time scales ranging from milliseconds to seconds, the optical tuning takes place in the subpicosecond regime.

Because of the centrosymmetric space group of silicon, bulk-contributions corresponding to the second-order nonlinear susceptibility are ruled out, but surface effects and third-order, that is, Kerr-like nonlinearities, corresponding to can be found. In addition, two-photon absorption of photons with low energy can cause a charge carrier injection like the one-photon absorption of photons with high energy. Silicon has an indirect band gap of 1.1 eV (m) at 295 K and a direct band gap of 3.5 eV ( nm). Thus, a relatively weak phonon-assisted linear absorption or a two-photon one occurs across the visible and near infrared. If the pump intensity is sufficiently high, nonlinear optical effects can cause changes of the dielectric constant even for photon energies that are smaller than the electronic band gap of silicon [55].

##### 3.1. Charge Carrier Injection by One-Photon Absorption

To explore the influence of an
electron-hole-plasma, a high density of carriers in a native macroporous
silicon sample had been optically injected using optical techniques and by this
monitoring the shift of a stop-gap edge [54]. As the (indirect band gap)
absorption edge of silicon is at 1.1m for room temperature, electron-hole
pairs can be efficiently produced at shorter wavelengths. The presence of the
high density (*N*) carriers is expected
to alter the real part of the dielectric constant
at probe frequency *ω* through
the expression where (11.9) is the quiescent dielectric constant of silicon, is the permittivity of free space,
and is the optical effective mass
of the electrons and holes. The carrier relaxation rate (~6 THz) has been
neglected in comparison with the probing frequencies of interest. In the
infrared region of the spectrum for wavelengths between 1m and 5m, this is a
reasonable approximation. Similarly, the influence of the imaginary part of the
dielectric constant that would contribute to loss was neglected. From the
frequency dependence in (1) one observes that for probing wavelengths in the
near infrared region, changes in the dielectric constant of the order of 10%
can be obtained for our peak carrier densities. These substantial changes are
expected to modify the location of the band gaps. For the experiments, the
lowest stop band was investigated, which occurs in the photonic crystal
described above between 1.9m and 2.3m for *E*-polarized light propagating
along the -*M* direction. In particular, the shift of the
shorter wavelength band edge was measured. Similar shifts are expected for the
longer wavelength band edge. The experimental results were obtained with a
parametric generator pumped by a 250 kHz repetition rate Ti-sapphire
oscillator/regenerative amplifier which produces 130 fs pulses at 800 nm at an
average power of 1.1 W. The signal pulse from the parametric generator is
tunable from 1.2m to 1.6m and the idler pulse is tunable from 2.1m to 1.6m. Reflection measurements were made using 150 fs pulses with center wavelength of 1.9m and of sufficient bandwidth to probe
the dynamical behavior of this edge. The pulses were focused onto the silicon
photonic crystal with fluence per pulse up to ~2 m J cm^{−2}. Simple estimates based on anticipated absorption
properties of the photonic crystal at this wavelength indicate that the peak
density of electron hole pairs is >10^{18} cm^{−3}. The focal spot diameter of the probe beam was
30m, reasonably small compared to the ~100m spot diameter of the
H-polarized pump beam. Figure 6 shows how the probe reflection characteristics
change with the fluence of the pump beam.

The major effect of the optical pumping is to shift the band edge to shorter wavelengths as expected since the Drude contribution decreases the dielectric constant of the silicon. Detailed calculations based on absorption characteristics of the photonic crystal at the pump wavelength and the variation of the photonic crystal dispersion curves with injected carrier density are in agreement with the maximum shift of about 30 nm (at the 3 dB point) that is observed here. Indeed, the shift of the edge scales linearly with the pump fluence, or injected carrier density, as expected theoretically. It should be noted however that the shift of the edge is not rigid. The shift is less for higher values of the reflectivity. This is presumably related to the fact that the 800 nm pump radiation is inhomogeneously absorbed, with an absorption depth of a few microns. In the spectral range near the peak value of the reflectivity associated with a stop gap, the reflectivity originates from lattice planes over a considerable depth within the crystal. In contrast, Fresnel reflectivity of the surface region dictates the reflection characteristics in the spectral range with higher transmission. To overcome this problem, other pumping schemes have to be used.

It was also possible to time resolve the reflectivity behavior by monitoring the probe reflectivity of the band edge as a function of delay between the probe and pump beams. Not surprisingly, the probe reflectivity change is virtually complete within the duration of the pump beam as charge carriers accumulate in the silicon. However, the recovery of the induced change occurs on a much longer time scale (at least nanoseconds) in our photonic crystal reflecting the electron-hole carrier recombination characteristics.

##### 3.2. Tuning by Kerr-Like Optical Nonlinearities

The Kerr effect was used to tune the short wavelength edge of a photonic band gap. In these experiments, a 2D photonic crystal was used to demonstrate the all optical tuning. Both the short wavelength edge (1.3m) and the long wavelength edge (1.6m) could be redshifted by the Kerr effect (). But for high pump intensities, the two-photon absorption was significantly generating free carriers, leading to a blueshift of the photonic band edge via the Drude contribution to .

The
2D silicon PhC sample has a triangular lattice arrangement of 560 nm diameter,
96m deep air holes with a pitch, *a*, equal to 700 nm. Figure 7(a) shows
a real space view of the sample while Figure 7(b) illustrates the photonic band
structure for the -*M* direction, which is normal to a face of the PhC.
Of particular interest is the third stop gap for *E*-polarized (*E*-field
parallel to the pore axis) light. Lying between 1.3m and 1.6m,
this gap falls between two dielectric bands that are sensitive to changes in
the silicon refractive index. The purpose was to optically induce changes to
the two edges with idler pulses from the parametric generator and probe these
changes via time-resolved reflectivity of the signal pulses. Note that, because
of the link between the signal and idler wavelengths, different pump
wavelengths (2.0m for a 1.3m probe; 1.76m for a 1.6m probe) must be
used when the probe wavelength is changed. However, as will be shown in what
follows, small changes in the pump wavelength can lead to significant changes
in the induced optical processes.

Figure 8 shows the time-dependent change in reflectivity at 1.3m for a 2.0m pump
pulse and the cross-correlation trace of both pulses. The pump and probe
intensities are 17.6 and 0.5 GW/cm^{2}, respectively. The decrease in
reflectivity is consistent with a redshift of the band edge due to a positive nondegenerate Kerr index. The FWHM of the reflectivity trace
is 365 ± 10 fs which is 1.83 times larger than the pump-probe cross-correlation
width as measured by sum frequency generation in a beta-barium borate (BBO) crystal. This difference
can be explained in terms of pump and probe beam transit time effects in the
PhC as discussed above. Indeed, from the pump group velocity and probe spot
size, one can deduce that the reflected probe pulse is delayed by 110 fs within
the PhC sample. After these effects are taken into account, the intrinsic
interaction times are essentially pulse width limited, consistent with the Kerr
effect.

One
can estimate a value for the nondegenerate Kerr coefficient *n _{2}* in
the silicon PhC from the relation [55] where

*I*is the incident intensity,

_{0}*f*is the filling fraction, and

*R*is the reflectivity of the sample. The experimental values of the steepness of the band edge reflectivity, nm

_{u}^{−1}, and the differential change in band edge wavelength with refractive index, nm, are relatively large. Thus, induced reflectivity changes in the vicinity of the 1.3m band edge are found to be 70 times more sensitive than that in bulk materials for the same refractive index change, a degree of leverage also noted by others [54, 60]. Indeed, when the PhC is replaced by bulk crystalline silicon,

*no change*in reflectivity is observed for the range of pump intensity.

The
inset to Figure 9 shows there is good correlation between the change in probe
reflectivity and the steepness of the band edge reflectivity (measured
separately) at different wavelengths and for a range of pump intensities.
Figure 9 shows the change in reflectivity with pump intensity at zero time
delay. The linear dependence is consistent with the Kerr effect and the
nondegenerate Kerr index is estimated to be cm^{2}/W. This is within an order of magnitude of the degenerate Kerr index reported
[61, 62] at 1.27m and 1.54m and represents reasonable agreement
considering uncertainty in the lateral position (*x*) of the pump pulse and its intensity at the probe location. It should also be
noted that linear scattering losses as the pump pulse propagates through the
PhC along the pore axis have not been taken into account.

##### 3.3. Tuning by Kerr-Like Nonlinearities and Two-Photon Absorption

In general, overall pulse-width limited response can only be
achieved using nonresonant, nonlinear induced changes to material optical
properties such as the optical Kerr effect (a third-order nonlinearity). In
this case, the change in refractive index for a probe beam is given by where *I* is the intensity of the pump beam and *n _{2}* is the Kerr coefficient associated with the
pump and probe frequencies. If the probe light intensity is limited to values
like in the experiment of Leonard et al. [54], the imaginary terms in the
dielectric function arising from free-carrier absorption and intervalence-band
absorption are very small.

Results
from experiments used to probe the 1.6m band edge when the sample is pumped
with 1.76m pulses are illustrated in Figure 10, which shows the temporal response
of the change in probe reflectivity at different pump intensities for a probe
intensity of 0.13 GW/cm^{2}. There is an initial increase and decrease
in probe reflectivity on a subpicosecond time scale followed by a response that
decays on a time scale of 900 picoseconds and partially masks the Kerr effect
near zero delay. At this band edge, the subpicosecond behavior is consistent
with a Kerr effect similar to the previous experiments. The long time response
could possibly be due to thermal or Drude contributions to the dielectric
constant due to the generation of free carriers. Using a peak pump intensity of
120 GW/cm^{2} and a 0.8 cm/GW two-photon absorption coefficient [61, 62] for 1.55m as an upper limit, one can estimate the surface peak carrier density
to be <10^{19} cm^{−3} and the maximum change in temperature
to be <0.15 K. From the thermo-optic coefficient K^{−1} at the probe wavelength [63],
the change in silicon refractive index is on the order of 10^{−5} and
the (positive) induced change in reflectivity is expected to be about the same.
From free carrier (Drude) contributions to the refractive index at the probe
wavelength, changes to the imaginary part of the dielectric constant are about
2 orders of magnitude smaller than that of the real part [64], which is about
−10^{−3}. Hence, free carrier absorption of the probe pulse as well as
thermally induced changes can be neglected in what follows and the change in
reflectivity is ascribed to changes in the real part of the dielectric constant
due to Drude effects.

At
low pump powers, the change in probe reflectivity scales quadratically with
pump intensity. This can be explained by free carrier generation due to
two-photon absorption, with the charge carrier density *N* being given by where
*β* is the
two-photon absorption coefficient, is the temporal FWHM pulse width
of the pump pulses, and is the pump frequency. However, at
higher pump intensities, there is an apparent deviation from this quadratic
dependence (see inset in Figure 10) due to pump saturation effects, since the
pump intensity *I* varies along the *z*-direction as according
to attenuation by two-photon absorption. With increasing intensity in a
two-photon absorption process, an increasing fraction of the carriers are
created closer to the surface where the pump pulse enters and the probe region
develops a reduced and increasingly nonuniform carrier density. It can be
estimated that at a depth of 60m, the expected saturation pump intensity is
about an order of magnitude larger than the maximum pump intensity used in this
setup. The carrier lifetime of 900 picoseconds is most likely associated with
surface recombination within the PhC sample with its large internal surface
area.

The
reflectivity change due to the Drude effect is given by where
is the probe frequency, is the
effective optical mass of the electrons and holes (), and is the permittivity of free space.
Thus, from the low intensity behavior in the inset to Figure 10, the two-photon
absorption coefficient, *β*, can be estimated to be 0.02 cm/GW, which is
within an order of magnitude but smaller than that reported [61, 65] for
wavelengths near 1.55m. For the 2m pump wavelength, the upper limit for *β* is
estimated to be 2×10^{−3} cm/GW from the signal to noise and the fact
that no measurable long-lived response at the highest pump intensity used is
observed. This value is an order of magnitude smaller than what is determined
at 1.76m and it is not a surprise since *β* is
expected to decrease rapidly with increasing wavelength as the indirect gap
edge is approached.

#### 4. Summary

In conclusion, either the
infiltration of macroporous silicon with liquid crystals and subsequent control
of the thermodynamic variables or the use of light absorption of Kerr-like
optical nonlinearities can be used to achieve tunable properties in photonic
crystals made of macroporous silicon. For both methods, the relative frequency
shift of photonic bands, band edges, or resonance frequencies of microcavities
is roughly of the order of 1% of the absolute frequency (Table 1). The effect
is limited, but can nevertheless be much larger than the linewidth of modes to
be tuned, since microresonators with very large *Q* factors can be fabricated.
The methods summarized in Sections 2 and 3 may find different applications. The
use of liquid crystals has the advantage that the control parameters
temperature and electric fields are easily available. However, the greatest
disadvantage is probably the limited speed of director reorientation, which
corresponds to time constants in the millisecond range. In contrast, absorption and nonlinear effects
lead to very fast changes of the photonic properties (with time constants below
1 picosecond) and can be used for all-optical switching. However, very large
intensities are required for the nonlinear optical effects.

Besides further technical developments that make use of the effects studied, so far, a couple of novel, fundamentally interesting systems deserve to be explored in more detail:

(1) Chiral liquid crystals: cholesteric phases and blue phases [20–24] show a helical superstructure of the local alignment, thereby leading to a spatially periodic director field . This intrinsic periodicity can be combined with two-dimensional arrays of pores, thereby leading to novel three-dimensional heterogeneous structures [41]. As an example, Figure 11 shows the FCPM image of a sample which shows an inherent periodicity within the pores. Such structures may show enhanced nonlinear optical effects or may be used for switching between a three-dimensional and a two-dimensional periodicity of the optical density. Additional work on these systems is in progress.

(2) Liquid crystals can exhibit both second- and third-order optical nonlinearity. Thus, infiltration of photonic crystals with liquid crystals that exhibit large - or -values may be used for frequency conversion or all-optical switching, respectively. A considerable enhancement of second harmonic generation (SHG) intensity is known to appear in spatially periodic structures, where both the fundamental frequency and the second harmonic are close to photonic stop bands [66, 67]. In addition, suitable liquid crystals are known for their giant optical nonlinearity (GON) [68], that is, a huge Kerr effect which is due to collective reorientation of the liquid crystal molecules induced by the optical electric field strength.

Fundamental studies of these effects in the environment of a silicon photonic crystal appear to be challenging. In conclusion, the development of tunable photonic crystals based on silicon is still in progress.

#### Acknowledgments

This work was supported by the company E. Merck (Darmstadt) with liquid crystals. Also, the funding by the German Research Foundation (KI 411/4 and SPP 1113) and the European Science Foundation (EUROCORES/05-SONS-FP-014) is gratefully acknowledged.

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