In this work, a technique for precise position control of individual transmission channels in a triple-cavity resonator
device is proposed. The resonator design is based on Si photonic crystal (PhC) and liquid crystal technologies. By filling
of the particular air grooves in one-dimensional, Si-Air PhC with nematic liquid crystal, an efficient coupled Fabry-
Pérot resonator can be realized in which a wide stop band is used for broad frequency channel separation and high out-of-
band reflection. By random tuning of the refractive index in all coupled cavities, a continuous individual tuning of the
central channel (or edge channels) up to 25% of the total channel spacing is demonstrated. Additionally, an approach
for precise controllable improvement of transmission up to 100% is demonstrated for the edge channels with decrease of
the channel spacing 1%. Based on the proposed design, a prototype triple-channel filter was fabricated on Silicon-On-Insulator platform and optimized to the desired operational mode.
Two-dimensional (2D) photonic crystal (PC) bars with 6 and 21 periods were fabricated by simultaneous photoelectrochemical
etching of macropores and trenches in a pre-patterned silicon wafer. The structures had square
lattice of cylindrical pores and were terminated by nonmodulated silicon pre-layers. The infrared reflection
spectra of the PC bars have been simulated using scattering matrix method. In order to take into account the
roughness of pore inner surface an additional silicon layer around the pores was introduced with a fitted complex
refractive index. A comparison between the simulated refection spectra and those obtained experimentally
demonstrates a satisfactory agreement in the region of secondary photonic band gaps.
The model of transformation of one-dimensional, high-contrast silicon photonic crystal (PC) into a Fabry-Pérot resonator
is considered. This transformation is achieved either by decreasing the number of periods up to 1.5 or the introduction of
optical defect in the ordinary multi-period PC while retaining a high modulation of the resonance peaks up to ~0.95. The
simultaneous use of maps of photonic band gaps (PBG) and transmission bands can predict the appearance or
disappearance of PBGs in the optical spectra as well as to determine their position and width depending on the order of
the band-gap (or stop-band) and the value of the filling fraction. The variation of the refractive index by 0.2 results in
significant shift of the resonance peaks of high order up to 10% of the frequency corresponding to the center of the peak.
This paper reports on investigation and possible applications of the optical elements based on one-dimensional (1D)
multi-component photonic crystal (PCs). The gap map approach and the transfer matrix method were used in order to
mathematically describe multi-component 1D PC structures. We have found that the introduction of the additional
regular layer into PC affects the properties of high-order PBGs, resulting in their vanishing in the certain range of the
wavelengths and the formation of wide regions of total transparency instead. Tuning the number, position and width of
these regions of total transparency in Si PCs has been demonstrated using the map of transmission bands. By analogy
with multilayer dielectric coatings the additional component in multi-component Si PCs can be considered as an
antireflection layer. The experimental results for the high-contrast multi-component PCs based on SiO2-Si-SiO2-Air
structure with wide transmission bands are demonstrated in this study. The suggested approach can also be applied to the
design of any micro- and nano- structured semiconductor or dielectric materials for application across wide
electromagnetic spectrum.
The results of a simulation of the optical properties of a silicon Fabry-Pérot resonator (with liquid crystal filler in the
cavity), operated on the shift of the interference bands in the infrared range are presented. The possibility of tuning the
reflection coefficient from 0 to 0.95 (or transmission coefficient from 1 to 0.05) by changing the refractive index by 0.1
in the cavity and using the stop-bands and resonance peaks of high order is demonstrated. The prototype Fabry-Pérot
resonators were fabricated by dry and wet etching of (100)Si and (110)Si. Some of the resonators were fabricated on a
silicon-on-insulator platform. A superposition of transmission peaks with reflection maxima, predicted from calculations,
was confirmed experimentally, using infrared microspectroscopy, with a temperature variation from 20 oC to 65 oC and
an applied electric field from 0V to 10V.
In this study, three-component One-Dimensional (1D) Photonic Crystal (PC) structures were investigated by modeling
them as two-component PCs with an additional regular layer. The Gap Map approach and the Transfer Matrix Method
were used in order to mathematically describe these structures. The introduction of a third component to a 1D PC allows
manipulation of the optical contrast to a high degree of precision by varying the thickness and refractive index of the
additional layer. It also partially reduces the area of the photonic band gaps (PBGs) on the gap map, leaving the
remainder of the PBG area unchanged from that of the gap map for the original, two-component, PC. Using this
approach to decrease the optical contrast in photonic crystals allows omni-directional bands to be obtained in highcontrast
periodic structures constructed from, for example, an array of silicon and air.
Three methods, namely 2×2 and 4×4 transfer matrix methods as well as scattering matrix method, for simulation
of the transmission and reflection spectra of the layered structures are described in this paper. The advantages of each of these methods for simulation of the optical spectra of one-dimensional photonic crystals are analyzed. The modified 2×2 transfer matrix method is suggested for calculation of the reflection and transmission coefficients of the layered structures in situation when the incident light beam has a cone-like shape.
In this paper, the forbidden Photonic Band Gaps (PBGs) of a
one-dimensional Photonic Crystal (1D PC) with additional
regular layer, t for the constant value of the lattice constant A and at normal incident of light beam were investigated.
The additional regular layer was formed from both sides of the
high-refractive index layer H. The gap map approach and
the Transfer Matrix Method were used for numerical analysis of this structure. The limitation of filling fraction values
caused by the presence of t-layer was taking into account during calculations of the Stop-Band (SB) regions for threecomponent
PC. The red shift of SBs was observed at the introduction of t-layer to conventional two-component 1D PC
with optical contrast of N=3.42/1. The blue edge of the first PBG occupied the intermediate position between the blue
edges of SBs regions of conventional PCs with different optical contrast N. This gives the opportunity of tuning the
optical contrast of PC by introduction of the additional layer, rather than using the filler, as well as fine tuning of the SB
edge. The influence of the number of periods m and the optical contrast N on the properties of SBs was also
investigated. The effect of the PBG disappearance in the gap map and in the regions of the PBGs of high order was
revealed at certain parameters of the additional layer.
One dimensional periodic and non-periodic silicon photonic structures have been designed and fabricated on silicon-on-insulator
substrate for the investigation of the electro-tuning effect in composite system Photonic Crystal - Liquid
Crystal. The reflection spectra registered for non-periodic structures demonstrate the phase polarisation shift for bands
of high reflection, while for the periodic structure the shift of the photonic band gap edge was observed. Under an
applied electric field in the range from 2V to 10V, the shift of the polarised reflection spectra, caused by reorientation of
the LC director from planar to homeotropic alignment, has been obtained. A significant change in the refractive index
close to Δn=0.2, which is a characteristic feature for LC E7, has been achieved due to LC reorientation in all structures
just after LC infiltration. It was found that after switching-off the applied electric field the initial planar orientation of LC
molecules is not restored. This effect is related to weak anchoring of LC molecules to the silicon side-walls which
results in the transition of LC to the pseudo-isotropic alignment after the applied voltage is off. A relatively smaller (with
Δn=0.07), but highly reproducible electro-tuning effect was revealed during the LC reorientation from pseudo-isotropic
to homeotropic alignment. The shift of the edge of PBG by Δλ=0.16 or by Δλ/λ=1.6% in relative shift units was
observed in this case. The response time estimated under applied square shaped ac pulses of various frequencies was
found to be around 30 ms.
This paper is focused on the design, fabrication and characterization of the conventional and tunable photonic devices
based on grooved silicon, serving as one-dimensional (1D) photonic crystal. The advantages of these photonic structures
are as follows: the large refractive index contrast, in-plane moulding of the light flow, the possibility to fabricate a
composite photonic structures by filling the grooves with a different compounds and compatibility with current
semiconductor processing techniques. The optical properties of grooved Si structures were simulated using a transfer
matrix method and gap map method and have been verified experimentally using FTIR microscopy. The air spaces in
the basic silicon-air matrices were infiltrated with nematic liquid crystal E7. It is shown that the optical properties of the
obtained composite 1D photonic crystals can be tuned by means of electro- and thermo-optical effects. Such a structures
suit well for the various elements of the integrated optics and can serve as a building blocks for optical interconnects.
The orientation of nematic liquid crystal (LC) E7 on the surface of (111) silicon wafers and in the channels of grooved silicon structure has been investigated. Grooved Si is a periodical structure obtained by wet anisotropic etching of deep and narrow grooves in (110) Si using an alkaline solution. This structure can serve as a one-dimensional (1D) photonic crystal. Composite structures obtained from grooved Si infiltrated by LC are promising candidates for electro-optic application. LC E7 was infiltrated into the channels of grooved Si and into the bulk Si cells. IR spectroscopy and capacitance-voltage characteristics under DC electric field were applied to investigate LC orientation. It is shown that the E7 liquid crystal in grooved silicon exhibits a weak planar orientation with respect to the silicon walls.
Reflection spectra and photonic band gaps (PBGs) for periodic structures consisting of grooved Si infiltrated with nematic liquid crystals (LCs) E7 have been obtained experimentally and by simulation. Periodically grooved Si matrixes, with lattice periods ranging from 2 to 6 μm, were fabricated using wet anisotropic etching of (110) Si in an alkaline solution. It is shown theoretically that a substantial shift of PBGs can be obtained when there is a homogeneous alignment of the liquid crystal molecules with respect to the Si walls and, therefore, a change in the LC's refractive index from no to ne during elctro-tuning can be expected. This effect is smaller during thermo-tuning effect when refractive index changed from no to, ni (isotropic LC phase). The simulated reflection spectra are in good agreement with experimental data obtained with a Digilab FTS 6000 FTIR spectrometer in conjunction with a UMA 500 infrared microscope. A shift of the PBG's edges by a factor Δλ/λ=7% was predicted as a result of a thermo-tuning effect and ~10% as a results of electro-tuning effect.
Periodic structures of grooved Si with lattice constants A = 2 - 4 µm, having the horizontal modulation of the refractive index, n, and acting as the 1D Photonic Crystal (PC) in the middle and far IR range (5-50 µm), have been fabricated by means of wet anisotropic etching of (110) oriented single crystalline silicon. The “gap maps” for both the empty matrices of periodically grooved Si and the composites infiltrated with liquid crystal (LC) of the average refractive index n=1.6 have been calculated. These maps display the frequency regions of photonic band gaps (PBGs) as a function of the filling factor Dsi/A and simplify the design of 1D PC structures for user-specified wavelength range. FTIR measurement revealed wide stop bands in the reflection and transmission spectra. Infiltration of the periodically grooved Si with the nematic liquid crystal E7 results in a shift of the main band gap to the higher wavelengths and in the consequent changes in the secondary stop bands. It is shown that polarized spectra of grooved Si infiltrated with LC allows determination the refractive index of the LC and the orientation of its molecules.
The reflection spectra of composite materials on the base of grooved silicon and grooved silicon infiltrated with nematic liquid crystal (LC) have been calculated using the optimal parameters of a grooved silicon matrix suitable for the infrared range. The grooved silicon structures with different lattice constants (A=16, 12, 8 and 4 mm) have been designed and prepared. An important parameter of these structures is the thickness of the silicon walls (DSi). This has
been obtained using simulations of the spectra. This parameter was used for further analysis of the spectra of composite material grooved Si-LC. The experimental reflection is reaching of 65% in maximum (with signal modulation from maximum to minimum up to 55%) for the composite structures with a small number of lattice periods that makes these structures very perspective with a potential applications. The analysis of the polarised infrared spectra of Si structures infiltrated with LC allows one to determine the orientation and the refractive index (NLC) of the liquid crystal. For the samples with a distance between Si walls of 6-10 mm, it was found that the refractive index was NLC=~ 1.5 for polarised light and NLC 1.5 for s-polarised light. This leads to the conclusion on the planar orientation of liquid crystal molecules with respect to the Si walls. For the samples with distance between Si walls less than 3 mm, a homeotropic alignment of liquid crystal molecules has been found. Micro-Raman spectroscopy has been applied for analysis of stress in such Si structures. The maximum stress of about 2 GPa was obtained on the top of Si walls (under Si dioxide layer).
Planar and homeotropic orientations of nematic liquid crystals have been obtained using novel alignment layers which were prepared by plasma-activated chemical vapor deposition (CVD) of a hydrocarbon vapor. Optical constants and porosity of the layers were characterized ellipsometrically. Layer surface energies were estimated from measuring the contact angles. Molecular structure of the alignment layers and orientation of the liquid crystals on their surfaces were studied using the IR spectroscopy. The layers are transparent in the visible region and their refractive index is very close to that of the liquid crystal. The determination of a dichroic ratio and of an order parameter of the IR absorption bands of the aligned liquid crystal shows the planar and homeotropic alignment of cyanobiphenyl molecules to be stable on the alignment layers.
The results of using the inhomogeneous models as well as technique for their error determining are demonstrated. An inhomogeneous profile error was presented as set of 'error profiles'. The homogeneous, linear and exponential models of profiles of N and K were used for searching. Initial colloid SiO2 film, after its heating and exposition in air as well as after reactivating was studied. The layers with decreasing PbO component were found on surface of PbO-SiO2 glass. Graded N and K profiles of In2O3-SnO2 film, used as transparent conducting electrode, were determined by multiangle ellipsometry at wavelength of 632.8 nm.
Using two measurements in vacuum and under condition for condensing water, one might more correctly determine the true refractive index Ntrue and porosity Q with Bruggeman's EMA. The characteristics of chamber-attachment to ellipsometer, algorithm and application samples of new method for SiO2, ZrO2, HfO2, a-C:H, and In2O3-SnO2 films are presented. It is found that the differences between measurements of an effective refractive index Nef in air, in vacuum and with condensed water may be up to 0.09. The void fraction for these films are range from 0 to 0.277, Difference between Ntrue and N in air may be up to 0.036.
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