Macroporous Silicon Photonic Crystals Research Articles

Absorption mechanisms in macroporous silicon photonic crystals

We investigate the large differences classically found in literature between theoretical and empirical results in the optical characterization of macroporous silicon photonic crystals. Mainly, we study the impact of two optical absorption mechanisms, free carrier absorption and scattering. Optical measurements with different membranes and previously reported data have been compared with numerical simulations. Results show that free carrier absorption is irrelevant for the used bulk doping. Nevertheless, the implanted N+ ohmic contact imposes a high absorption in the long-infrared region, leading to a 40 % decrease in transmission. Concerning scattering, we demonstrate that error/deviations in the etching of the photonic crystals –such as branching or spiking–, with sizes comparable to the modulation radius, are responsible for Lambertian light trapping. This absorption mechanism is dominant for wavelengths shorter than the bandgap ones, regardless of the horizontal periodicity used, due to the size dependence between error/deviation and modulation radius.

Empirical demonstration of CO2 detection using macroporous silicon photonic crystals as selective thermal emitters.

This study describes the detection of CO2 using macroporous silicon photonic crystals as thermal emitters. It demonstrates that the reduction of structural nonhomogeneities leads to an improvement of the photonic crystals' emission. Narrow emission bands (Q∼120) located within the R-branch of carbon dioxide were achieved. Measurements were made using a deuterated triglycine sulfate photodetector and the photonic crystals, heated to 400°C, as selective emitters. A gas cell with a CO2 concentration between 0ppm and 10,000ppm was installed in the center. Results show high sensibility and selectivity that could be used in current nondispersive infrared devices for improving their features. These results open the door to narrowband emission in the mid-infrared for spectroscopic gas detection.

Macroporous Silicon Filters, a Versatile Platform for NDIR Spectroscopic Gas Sensing in the MIR

This paper describes the spectroscopic detection of gases using macroporous silicon photonic crystals as narrow filters. The study begins by demonstrating the feasibility of photoelectrochemical etching to fabricate narrow filters along the mid infrared band. Next, we focus on the filter centered on the carbon dioxide fingerprint. The filter response is described for three different cell lengths and concentrations below 1%. Results show a concordance with the reformulated Beer-Lambert law. This can be used to predict the response of the filter for longer path lengths and higher concentrations, showing broad working ranges and compact sizes for CO2. In addition, optical robustness to external variations and long-term stability are also reported. Results are extrapolated to other macroporous silicon filters centered on the absorption spectra of N2O, OCS, NO2 and SO2. Finally, high sensitivity and selectivity is demonstrated by comparing them with some commercial filters.

Bandgap widening in macroporous silicon photonic crystals by multiperiodic structures

Large bandgaps with low transmission in 3D macroporous silicon photonic crystals have been proved as an interesting technology for the development of optical filters and spectroscopic MIR gas sensors. The aim of this study is the investigation of different bandgap widening methods based on multiperiodic structures for 3D macroporous silicon photonic crystals. To do so, chirped modulations and structures with different periodicity groups have been modelled and theoretically analysed by means of 3D FDTD simulations. They have revealed that by using different decreasing periodicity groups, bandgaps with null transmission and widths as high as 1800 nm, 4 times the original single periodicity photonic crystal bandgap, can be obtained. Furthermore, it has been shown that a resonant cavity with a 20% transmission can be placed in a 1 μm wide bandgap. The results open a way to use this type of structures not only for gas sensing but also for other purposes such as wide stop-band filters, selective filters or broadband mirrors.

Photonic molecules for improving the optical response of macroporous silicon photonic crystals for gas sensing purposes.

In this paper, we report the benefits of working with photonic molecules in macroporous silicon photonic crystals. In particular, we theoretically and experimentally demonstrate that the optical properties of a resonant peak produced by a single photonic atom of 2.6 µm wide can be sequentially improved if a second and a third cavity of the same length are introduced in the structure. As a consequence of that, the base of the peak is reduced from 500 nm to 100 nm, while its amplitude remains constant, increasing its Q-factor from its initial value of 25 up to 175. In addition, the bandgap is enlarged almost twice and the noise within it is mostly eliminated. In this study we also provide a way of reducing the amplitude of one or two peaks, depending whether we are in the two- or three-cavity case, by modifying the length of the involved photonic molecules so that the remainder can be used to measure gas by spectroscopic methods.

Effect of fabrication tolerances in macroporous silicon photonic crystals

Macroporous silicon photonic crystals show a great potential for a range of applications such as optical sensing or signal processing. These applications require tight fabrication tolerances. In particular, the effect of process variability in 3-d photonic crystals and out of plane propagation has been seldom studied in literature. In this paper we report the effect fabrication imperfections on the spectral response of macroporous silicon photonic crystals. To quantify fabrication disorder and its influence, several 3-d macroporous silicon structures were fabricated consisting of modulated pores arranged in a square lattice. The pore modulation is similar to a stretched sinusoidal waveform. Lattice pitch is 700nm, pore diameter is in the range from 250nm to 520nm, and modulation period is 1.2μm. The samples were characterized by SEM inspection and the actual etched pore profiles extracted. The statistical analysis of the profiles reveals two main sources of randomness: radius variability and modulation period irregularity. Surface roughness and asymmetry do not seem to play a major role. Several FDTD simulations have been performed based on the statistical parameters extracted, and the results are compared to actual FT-IR measurements of the fabricated samples. The obtained results show that, in general, the dispersion in z period has the most severe effect in the structure’s optical response.

Enhanced geometries of macroporous silicon photonic crystals for optical gas sensing applications

A macroporous silicon photonic crystal is designed and optimized theoretically for its use in gas sensing applications and IR optical filters. Light impinges perpendicularly onto the sample surface (vertical propagation) so a three-dimensional (3d) structure is used. For gas sensing, a sharp resonance is desired in order to isolate an absorption line of the gas of interest. The high Q-factors needed mandate the use of a plane defect inside the PhC to give rise to a resonant mode inside the bandgap tuned to the gas absorption line. Furthermore to allow gas passage through the device, an open membrane is required. This can affect the mechanical resilience. To improve the strength of the photonic crystal the pores are extended after the “active” 3d part. The number of modulations, and the extension length have been optimized to obtain the largest Q-factor with reasonable transmitted power. These proposed structures have been experimentally performed, probing an enhancement of almost an order of magnitude in the Q-factor in respect with the basic case. Simulations considering CO2 have been performed showing that the proposed structures are promising as precise optical gas sensors.

Study of resonant modes in a 700 nm pitch macroporous silicon photonic crystal

In this study the modes produced by a defect inserted in a macroporous silicon (MP) photonic crystal (PC) have been studied theoretical and experimentally. In particular, the transmitted and reflected spectra have been analyzed for variations in the defect’s length and width. The performed simulations show that the resonant frequency is more easily adjusted for the fabricated samples by length tuning rather than width. The optimum resonance peak results when centered in the PC bandgap. The changes in the defect geometry result in small variations of the optical response of the PC. The resonance frequency is most sensitive to length variations, while the mode linewidth shows greater change with the defect width variation. Several MPS photonic crystals were fabricated by the electrochemical etching (EE) process with optical response in the range of 5.8μm to 6.5μm. Results of the characterization are in good agreement with simulations. Further samples were fabricated consisting of ordered modulated pores with a pitch of 700nm. This allowed to reduce the vertical periodicity and therefore to have the optical response in the range of 4.4μm to 4.8μm. To our knowledge, modes working in this range of wavelengths have not been previously reported in 3-d MPS structures. Experimental results match with simulations, showing a linear relationship between the defect’s length and working frequency inside the bandgap. We demonstrate the possibility of tailoring the resonance peak in both ranges of wavelengths, where the principal absorption lines of different gases in the mid infrared are placed. This makes these structures very promising for their application to compact gas sensors.

The Effect of Absorption Losses on the Optical Behaviour of Macroporous Silicon Photonic Crystal Selective Filters

The effect of losses in photonic crystal structures on transmittance and reflectance has been studied by numerical techniques, and compared to fabricated macroporous silicon samples. Material absorption and tolerances of the pores play an essential role in determining performance in optical applications. The relevance of the structure geometry has been investigated through the porosity. The considered photonic crystals in this paper are 3-d macroporous silicon structures fabricated by electrochemical etching with pore profile modulation, and defect inclusion. The examined structures are intended to work with quasi-normal light incidence as selective filters. The effect of losses on the forbidden band and resonance is studied and reported for both transmittance and reflectance. The reflectance peak is less affected by losses, while transmitted light experiences a noticeable reduction even for small absorption factors. This has critical implications for the performance of these structures as transmission filters. Furthermore, it is shown that an optimum can be found for a given absorption coefficient by choosing the appropriate porosity.

Thermal emission of macroporous silicon chirped photonic crystals

In this Letter we report on the thermal properties of macroporous silicon photonic crystals with the unit cell gradually varied along the pore axis. We show experimentally that arbitrarily large omnidirectional total-reflectance bands can be produced with such structures. We also demonstrate that those bands can be effectively used to reduce thermal radiation in large spectral bands.

Characterization of photonic structures using visible and infrared polarimetry

Photonic Crystals are materials with a spatial periodic variation of the refractive index on the wavelength scale. This confers these materials interesting photonic properties such as the existence of photonic bands and forbidden photon frequency ranges, the photonic band gaps. Among their applications it is worth mentioning the achievement of low-threshold lasers and high-Q resonant cavities. A particular case of the Photonic Crystals is well-known and widely studied since a long time: the periodic thin film coatings. The characterization of thin film coatings is a classical field of study with a very well established knowledge. However, characterization of 2D and 3D photonic crystals needs to be studied in detail as it poses new problems that have to be solved. In this sense, Polarimetry is a specially suited tool given their inherent anisotropy: photonic bands depend strongly on the propagation direction and on polarization. In this work we show how photonic crystal structures can be characterized using polarimetry equipment. We compare the numerical modeling of the interaction of the light polarization with the photonic crystal with the polarimetry measurements. With the S-Matrix formalism, the Mueller matrix of a Photonic Crystal for a given wavelength, angle of incidence and propagation direction can be obtained. We will show that useful information from polarimetry (and also from spectrometry) can be obtained when multivariate spectra are considered. We will also compare the simulation results with Polarimetry measurements on different kinds of samples: macroporous silicon photonic crystals in the near-IR range and Laser-Interference-Lithography nanostructured photoresist.

Linear and non‐linear optical experiments based on macroporous silicon photonic crystals

AbstractMacroporous silicon is a model system for 2D silicon photonic crystals. In this review, we describe our recent theoretical and experimental advances in the macroporous silicon technology as well as in novel near‐field optical experiments concerning modified light emission from photonic crystal cavities and waveguides. We also review our recent work on tuning silicon photonic crystals by free‐carriers or non‐linear effects and compare the two tuning mechanisms. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterization of 2D macroporous silicon photonic crystals: Improving the photonic band identification in angular-dependent reflection spectroscopy in the mid-IR

We report the experimental characterization of two-dimensional (2D) macroporous silicon photonic crystals using angular-dependent reflectance spectroscopy in the mid-IR region. We have investigated different sample structures and we have shown that an adequate post-processing of the measured data is crucial in order to recognize the photonic bands and to achieve a good agreement of the measured data with the theoretical predictions for the studied structures.

Photonic band structure of oxidized macroporous silicon

AbstractWe investigated theoretically the effect of surface silicon oxide layer on the photonic band structure of a macroporous silicon photonic crystal. Using the plain wave method we have shown that the bandgap in oxidized structure is shifted to the higher frequencies relative to non-oxidized case. We also demonstrate that comparatively wide absolute bandgap can be obtained for low air filling fractions by using thick SiO2 layer. As an example of possible application of such three-component systems, we have shown the concept of a selector of electromagnetic modes based on our calculations.

Three-dimensional macroporous silicon photonic crystal with large photonic band gap

Three-dimensional photonic crystals based on macroporous silicon are fabricated by photoelectrochemical etching and subsequent focused-ion-beam drilling. Reflection measurements show a high reflection in the range of the stopgap and indicate the spectral position of the complete photonic band gap. The onset of diffraction which might influence the measurement is discussed.

Complete three-dimensional band gap in macroporous silicon photonic crystals

The prospect of obtaining a complete three-dimensional band gap in macroporous silicon photonic crystals is investigated theoretically. Band structure calculations indicate that a modified form of the simple cubic lattice of air spheres in silicon exhibits a complete band gap, with a bandwidth of up to 4%. It is further shown that this quasispherical crystal may be fabricated using standard etching procedures. These results provide a practical route to obtaining large-scale, high-quality, silicon photonic crystals with a complete band gap.

Lateral confinement in macroporous silicon photonic crystal waveguides

Macroporous silicon photonic crystals (PCs) are known as easy and low-cost two-dimensional PC demonstrators with patterns of very high aspect ratios. Slab waveguides can be created in these crystals by modulating the pore diameter during the etching process. In this work, we demonstrate the feasibility of light confinement in the periodicity plane by using an in-filling technique. In-filling of ~0.6μm period macroporous silicon PCs with polymer is achieved over a depth approaching 15μm. The optical properties of in-filled crystals are compared experimentally with those of the original crystals. An excellent agreement is found between the results of measurements and numerical simulations. The combination of the in-filling technique with the modulation of the pore diameter during the etching process opens the way toward linear PC waveguides buried in macroporous silicon. The guiding properties of this new type of PC waveguide are analysed using a full three-dimensional plane-wave model with the supercell method.

Spectroscopy of photonic bands in macroporous silicon photonic crystals

Variable-angle reflectance performed on macroporous silicon photonic crystals yields the dispersion of two-dimensional photonic bands. A comparison with calculated optical spectra identifies the spectral structures, whichmark the onset of a propagating photonic mode, as one-dimensional critical points. The experimental results agree with theoretical determinations of the reflectance and of the photonic-hand dispersion in a wide energy range. A symmetry analysis yields the selection rules for excitation of photonic modes, which can be treated like elementary excitations of the crystal.

Tuning of 2-D Silicon Photonic Crystals

AbstractWe demonstrate two ways in which the optical band-gap of a 2-D macroporous silicon photonic crystal can be tuned. In the first method the temperature dependence of the refractive index of an infiltrated nematic liquid crystal is used to tune the high frequency edge of the photonic band gap by up to 70 nm as the temperature is increased from 35 to 59°C. In a second technique we have optically pumped the silicon backbone using 150 fs, 800 nm pulses, injecting high density electron hole pairs. Through the induced changes to the dielectric constant via the Drude contribution we have observed shifts up to 30 nm of the high frequency edge of a band-gap.

Optical characterisation of 2D macroporous silicon photonic crystals with bandgaps around 3.5 and 1.3 μm

Transmission measurements were performed on thin 2D silicon photonic crystals (PCs) with 1–4 crystal rows in order to investigate the effect of a finite structure and to obtain an estimate of the crystal thickness necessary to minimize crosstalk between adjacent waveguides. For wavelengths deep within the H-bandgap a strong exponential decay revealing an attenuation constant of 10 dB per crystal row was measured. For opto-electronic applications, the lattice constant of macroporous Si was successfully downscaled from a pitch of 1.5 to 0.5 μm. Reflection measurements performed at these structures show good agreement with corresponding bandstructure calculations exhibiting a complete bandgap around λ=1.3μm.