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Experimental study of the sensitivity of a porous silicon ring resonator sensor using continuous in-flow measurements


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A highly sensitive photonic sensor based on a porous silicon ring resonator was developed and experimentally characterized. The photonic sensing structure was fabricated by exploiting a porous silicon double layer, where the top layer of a low porosity was used to form photonic elements by e-beam lithography and the bottom layer of a high porosity was used to confine light in the vertical direction. The sensing performance of the ring resonator sensor based on porous silicon was compared for the different resonances within the analyzed wavelength range both for transverse-electric and transverse-magnetic polarizations. We determined that a sensitivity up to 439 nm/RIU for low refractive index changes can be achieved depending on the optical field distribution given by each resonance/polarization.
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Experimental study of the sensitivity of a
porous silicon ring resonator sensor using
continuous in-flow measurements
1Nanophotonics Technology Center, Universitat Politècnica de València, Camino de Vera s/n, 46022
Valencia, Spain
2Micro- and Nanoelectronics Department, Belarusian State University of Informatics and
Radioelectronics, P. Brovka str. 6, 220013 Minsk, Belarus
Abstract: A highly sensitive photonic sensor based on a porous silicon ring resonator was
developed and experimentally characterized. The photonic sensing structure was fabricated
by exploiting a porous silicon double layer, where the top layer of a low porosity was used to
form photonic elements by e-beam lithography and the bottom layer of a high porosity was
used to confine light in the vertical direction. The sensing performance of the ring resonator
sensor based on porous silicon was compared for the different resonances within the analyzed
wavelength range both for transverse-electric and transverse-magnetic polarizations. We
determined that a sensitivity up to 439 nm/RIU for low refractive index changes can be
achieved depending on the optical field distribution given by each resonance/polarization.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
OCIS codes: (130.3120) Integrated optics devices; (130.6010) Sensors; (230.5750) Resonators.
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Journal © 2017
Received 6 Sep 2017; revised 2 Nov 2017; accepted 4 Nov 2017; published 5 Dec 2017
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1. Introduction
Optical ring resonators (RRs) are probably the planar photonic structures mostly employed
for the development of sensing devices due to their structural simplicity, compact size and
high sensitivity. Many research groups and commercial companies have demonstrated strong
activity in the development of RR arrays for the highly sensitive and multiplexed detection of
analytes for different applications [1–6]. Nowadays, a traditional Silicon-on-Insulator (SOI)
RRs can provide sensitivities up to around 160 nm/RIU (Refractive Index Units) [7].
However, the performance of traditional planar photonic sensing structures based on a
configuration with a high refractive index (RI) contrast is limited by the fact that only the
evanescent field propagating outside of the photonic structure is used for sensing purposes,
while the majority of the optical field distribution associated with the guided mode propagates
within the structure itself. Novel RR configurations have been proposed to overcome this
limitation, as those based on slot waveguides [8], on ultra-thin SOI substrates [9] or on sub-
wavelength waveguides [10]. Another option to increase the sensitivity, which is considered
in this work, is the use of porous silicon (PS) for the development of the sensing structures.
The high potential of porous silicon for sensing purposes, what has been demonstrated in
several works [11–18], comes from the possibility of infiltrating the target analytes directly
into the pores in order to obtain a significantly improved sensitivity [19]. Additionally, PS is
characterized by a very high surface-to-volume ratio [20], what allows immobilizing a
significantly larger amount of bioreceptors over the inner walls of the pores for the better
detection of biorecognition events. Moreover, PS can be formed simply, quickly and
inexpensively since it is the result of the electrochemical etching of a silicon substrate.
In this work, we study the sensing performance of a porous silicon ring resonator (PSRR)
sensor for transverse-electric (TE) and transverse-magnetic (TM) polarizations. In case of
each polarization, we measured the sensitivity for every resonance within the analyzed
wavelength range, which is determined by the optical field distribution of the PSRR resonant
mode and its interaction with the medium filling the pores. Experimental results obtained by
flowing several ethanol-water dilutions over the porous structure provided a sensitivity up to
439 nm/RIU for the low ethanol concentrations when working with TE-polarized light.
2. Fabrication of the PSRR
First, we selected the RI profile of the double-layered PS to be used, aiming at having a
strong light confinement in the top layer, where the photonic sensing structures will be
patterned. To reach this goal, the RI contrast between the top and bottom layers of PS needs
to be as high as possible when water is infiltrated into the pores. However, there is a
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limitation in the maximum reduction of the RI of the bottom layer since it implies increasing
the porosity of this PS layer, what compromises the stability of the photonic structure. Taking
these considerations into account, we selected porosities of the top and bottom PS layers of
35% and 65%, respectively. By using the Bruggeman equation [21], we determined the RI of
the PS layers for these porosities, being ntop-water = 2.67 and nlc-water = 1.95 when considering
water-filled pores, and ntop-air = 2.56 and nlc-air = 1.7 when considering air-filled pores.
The designed double-layered PS was fabricated by an electrochemical etching of a single-
side polished boron doped (resistivity of 0.01 – 0.02 cm) <100> Si wafer, in a solution of
48% hydrofluoric acid and 99% ethanol mixed in a 1:2 volume ration. Just before the etching
process, the silicon wafer was cleaned from possible organic residues using a piranha solution
(volume ration H2SO4:H2O2 = 3:1) and then immersed into aqueous solution of HF (5%) to
remove SiO2 from the surface. After each cleaning step, the wafer was rinsed in deionized
water (DIW). The top layer with high RI was formed by applying a current density of 5
mA/cm2 for 100 seconds and the bottom layer of 45 mA/cm2 for 110 seconds. The cleaning
procedures and the electrochemical etching were performed at room temperature. The
fabricated PS sample was washed with acetone, isopropyl alcohol and DIW to remove
residual surface impurities. Finally, an O2 plasma was applied for 6 minutes, in order to
improve the hydrophilicity of the PS surface.
The fabricated PS sample was cleaved into two parts: one for the physical characterization
of the PS layers and the other one for the fabrication of the photonic structures. Field
emission scanning electron microscope (FE-SEM) was used to study the thickness and the
uniformity of both PS layers as well as the average pore diameter of the top PS layer. Figure
1(a) shows a FE-SEM image of the cross section of the double-layered PS, where the
different porosities of both layers are clearly distinguished. The measured thicknesses for the
top and bottom PS layers were around 800 nm and 4350 nm, respectively. Refractive indices
of the double-layer were calculated from Fresnel coefficients under the Transfer Matrix
Method approach using reflection measurements of the porous structure [22, 23]. In this
respect, the refractive indices obtained were ntop-air = 2.48 and nlc-air = 1.75 for the air-filled
pores, what corresponds to porosities of 38% and 63%, respectively. These porosities are
practically identical to those previously selected in the design phase, thus indicating a good
control of the PS fabrication procedure. Additionally, the ImageJ software [24] was used to
analyse a top-view FE-SEM image, shown in Fig. 1(b), in order to obtain the average pore
diameter of the sensing structure, which was determined to be around 12 nm.
Fig. 1. FE-SEM images of (a) cross section and (b) top-view of the PS structure.
Finally, a lithographic process was used to create the planar photonic structures in the top
PS layer. Designs were patterned into a PMMA positive resist layer using e-beam lithography
with an acceleration voltage of 10 keV and a dose of ~87 μC/cm2. Then, after developing the
resist, the pattern was transferred to the top PS layer by inductively coupled plasma (ICP)
etching. The fabricated design consisted of a 20µm-radius RR coupled to a coupling
waveguide. Several sets of this structure were fabricated using different widths of the
Vol. 25, No. 25 | 11 Dec 2017 | OPTICS EXPRESS 31653
coupling and the RR waveguides (800, 1000 and 1200 nm) and different coupling gaps
between the coupling waveguide and the RR (150, 200, 250, 300 and 350 nm). In order to
ease the butt-coupling characterization of the photonic structures, the coupling waveguide
was tapered to a width of 10 µm at the edges of the chip. Figure 2 shows several FE-SEM
images of the fabricated structures.
Fig. 2. FE-SEM images of the fabricated PSRR chip. (a) 60°-sectional image of the 10µm-
wide access waveguide. Thanks to the vertical cut of this sample edge, the vertical orientation
of the pores can be observed. (b) Cross-sectional image of the 10µm-wide access waveguide.
Thanks to the non-vertical cut of this sample edge, the sponge-like morphology of the PS and
the boundary between the two PS layers can be clearly observed. (c) Top-view image of the
top and bottom PS layers in the access waveguide region, where the porosity difference
between them can be clearly observed. (d) Top-view image of the RR and the coupling
3. Experimental setup and static characterization of the PSRR
An opto-fluidic setup, which is shown in Fig. 3, was developed to carry out the optical
characterization of the PSRR and the RI detection experiments. The optical part of the setup
consisted in a horizontal coupling interrogation platform where the light from a continuous
sweep tunable laser (Keysight 81980A) was coupled to the access waveguides in the PSRR
chip using a lensed fiber. The polarization of the input light was adjusted using a polarization
controller (Thorlabs FPC562) before injecting it into the chip. The light coming out from the
PSRR chip was collected with an objective (20X Olympus Plan Achromat, 0.4 NA) and
passed through a polarizer (Newport RM25A) in order to measure the signal level for the
selected polarization using an infrared (IR) camera (Xenics Xeva-1.7-320). The interrogation
platform was controlled using a software programmed in LabVIEW able to synchronize the
continuous sweep of the laser with the image acquisition of the IR camera via a trigger signal
in order to obtain the spectrum of the photonic structure with the desired spectral resolution.
Regarding the fluidic part of the setup, a microfluidic delivery system made in
polydimethylsiloxane (PDMS) was placed on top of the chip in order to flow all the solutions
over the sensing structure. A Finetech flip-chip tool was used to properly align the fluidic
microchannel with the PSRR structures. When the experiments were carried out, the target
Vol. 25, No. 25 | 11 Dec 2017 | OPTICS EXPRESS 31654
solutions were flowed using a syringe pump working in withdraw mode and set to a constant
flow rate of 20 μl/min. By working in withdraw mode, the liquid flowed through the PDMS
channel can be easily changed by simply changing the vial at the end of the tubing.
Fig. 3. Pictures of the fiber-to-camera horizontal interrogation setup. The PDMS microfluidic
flow cell on top of the photonic chip can be also observed.
After characterizing in water environment the TE and TM spectra for all the
configurations included in the photonic chip (i.e., different waveguide widths and coupling
gaps), we determined that the best spectral responses in terms of resonances quality and
optical losses were obtained for the configuration having a coupling waveguide width of 1000
nm and a coupling gap of 200 nm. Figure 4 shows the measured TE and TM spectra, in water
environment, for that PSRR configuration. The average quality factor of the measured
resonances is quite reduced (453 and 512 for TE and TM polarizations, respectively) due to
the higher losses that are produced in the PS-based waveguides compared to typical solid core
ones. The inset images in Fig. 4 represent the light spots acquired with the IR camera at the
10µm-wide output waveguide for each polarization. It is possible to appreciate that a better
confined mode is obtained for TE polarization, while a higher amount of light is going into
the lower cladding for TM polarization.
Fig. 4. Spectra of the PSRR for (a) TE and (b) TM polarization. The amplitude is represented
in terms of the analog-to-digital units (ADU) measured by the camera (in logarithmic scale).
The inset in each graph shows the measured optical profile at the output waveguide for each
The selected waveguide configuration was simulated using the software FemSIM in order
to determine the modes existing on it for a water environment. Two fully propagating modes
were obtained for that waveguide configuration for each polarization, while a third mode also
begins to propagate for both polarizations. The field profiles of those modes are shown in Fig.
5. From the optical profiles measured in the experiments, which were depicted in Fig. 4 we
can see that only the fundamental mode is properly excited and propagated for TE
polarization, while higher order modes are excited and propagated for TM polarization,
leading to a higher delocalization of the optical field.
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Fig. 5. Simulations of the optical modes propagating in the selected PS waveguide
configuration (height = 800 nm and width = 1000 nm) for (a) TE and (b) TM polarization. The
modes are ordered from left to right.
4. PSRR sensitivity characterization
Several RI sensing experiments were performed to determine the sensitivity of the photonic
structure. To this aim, different ethanol-water dilutions were flowed over the sensor while the
transmission spectrum was continuously acquired in order to determine the shift of the PSRR
resonances. The ethanol (EtOH) concentrations in DIW used in the experiments were 10%,
5% and 1%, which correspond to RI changes of 6.6·103, 3.3·103 and 6.6·104 RIU respect
DIW, respectively. For the spectra acquisition, the tunable laser was swept from 1520 to 1620
nm with a sweeping speed of 10 nm/s (the total time for each sweep is 10 seconds) and the
synchronization with the IR camera provided a spectral resolution of 20 pm. A Lorentzian
fitting of each resonance was then performed in Matlab to determine its position with a higher
accuracy. All the experiments were carried out for TE and TM polarizations.
Figures 6(a) and 6(b) show the time evolution for the most and least sensitive PSRR
resonances within the sweeping range for TE and TM polarizations, respectively. As it can be
observed in these figures, the resonances presented a similar behavior for both polarizations
and only a slightly higher sensitivity is obtained for TE polarization resonances. This is
related to the fact that the sensing occurs mainly inside the guiding structures, where the field
distribution is similar in both cases. However, since a better light confinement is obtained for
TE polarization, as it was shown in the spot images presented in Fig. 4, a higher sensitivity is
provided by this polarization. This is the opposite to what typically happens for evanescent
wave sensors, where TM modes are generally more sensitive because of the presence of a
higher amount of evanescent field over the top surface of the structure. The average
sensitivity of the PSRR sensor (i.e., for all the resonances within the measured range) is
around 350 nm/RIU for TE and 320 nm/RIU for TM when considering the whole range of
refractive index variations that has been measured. The maximum sensitivity value, around
380 nm/RIU, was provided by the resonance located at ~1607.5 nm for TE polarization,
whose quality factor is 513. Such values are more than twice higher than the sensitivity
achieved for a traditional SOI RR. The noise level measured for all the resonances is below 1
pm, reaching values even in the range of 0.1 pm for some resonances as it is depicted in the
inset of Fig. 6(a). Finally, note that the time required by the resonances to reach the plateau
Vol. 25, No. 25 | 11 Dec 2017 | OPTICS EXPRESS 31656
for each flowed substance (around 1 minute) is determined by the diffusion that is produced
in the input tubing at the interface between the liquids.
Figure 7 represents the spectral shift measured for each resonance for both polarizations,
where an oscillatory-like behavior can be observed as we move towards higher order
resonances. This sensitivity variation indicates that a different interaction between the
resonance optical modes and the target substance is taking place, which might be determined
by the local variations of the pores properties and distribution of the fabricated PS layer
where the RR is created. Besides that oscillatory-like behavior, the average shift also
increases as a function of the wavelength, due to the larger evanescent field of the modes as
wavelength is increased.
Fig. 6. Time evolution of the resonance shift for (a) TE polarization and (b) TM polarization
when flowing three cycles of EtOH in DIW with concentrations 10%, 5% and 1% (meaning RI
changes of 6.6·103, 3.3·103 and 6.6·104 RIU respect DIW, respectively) with cycles of DIW
flow between them. The different colors indicate the different resonances within the measured
wavelength range being tracked.
Fig. 7. Wavelength shift of each resonance for each RI variation for (a) TE and (b) TM
In Fig. 8, the sensitivity curves of each resonance for TE and TM polarizations are shown.
As it can be observed, the sensitivity behavior of the PSRR sensing structure was not totally
linear. In fact, the sensor exhibited a higher sensitivity for lower RI variations, specifically,
for RI variation of 6.6·104 RIU corresponding to the EtOH 1% concentration. This is related
with the non-linear variation of the RI of the top porous layer that is produced when changing
Vol. 25, No. 25 | 11 Dec 2017 | OPTICS EXPRESS 31657
the RI of the solution infiltrated into the pores, as it is shown in Fig. 9. We can see that a
higher variation of the effective refractive index of the top layer is obtained for the EtOH 1%
concentration than for the EtOH 5% and 10% concentrations. For this reason, the PS structure
showed a higher sensitivity for such EtOH concentration. Thanks to such behavior, the
highest sensitivity value provided by the PSRR sensor was 439 nm/RIU for the TE resonance
located at ~1607.5 nm when considering a RI variation of 6.6·104 RIU.
Fig. 8. Sensitivity curve of each resonance for (a) TE and (b) TM polarizations.
Fig. 9. Top PS layer RI variation as a function of the EtOH solution RI.
5. Conclusions
A highly sensitive PSRR was developed thanks to the formation of a PS double layer with a
high RI contrast and its sensing performance was characterized both for TE and TM
polarizations. The characterization was performed by monitoring in continuum the evolution
of the sensing structure spectrum. The experimental results indicate that the sensitivity of the
PSRR was slightly better for the TE polarization than for the TM polarization. This is because
the sensing occurs in the core of the structure, where the TE polarization plays a major
influence due to the better confinement of the light in the photonic structure. Working with
such polarization, a sensitivity of 439 nm/RIU was achieved for the detection of low RI
variations. The higher light-matter interaction that is produced due to the fact that the sensing
occurs directly inside the structure, together with the possibility of immobilizing the
bioreceptors on the inner surface of the pores, make PS a suitable platform for the
development of new biosensing devices exhibiting a high sensitivity. Finally, note that the
combination of PS substrates with some of the configurations that have been proposed to
increase the sensitivity of photonic structures as slot or sub-wavelength waveguides might
provide even higher sensitivities.
Vol. 25, No. 25 | 11 Dec 2017 | OPTICS EXPRESS 31658
European Commission through the project H2020-644242 SAPHELY; Spanish government
through the projects TEC2013-49987-EXP BIOGATE and TEC2015-63838-C3-1-R-
OPTONANOSENS; Generalitat Valenciana through the Doctoral Scholarship
Vol. 25, No. 25 | 11 Dec 2017 | OPTICS EXPRESS 31659
... A.M. Author is with Department of Informatics, Center for Interdisciplinary Research and Innovation, Aristotle University of Thessaloniki, 10 th km upon a change in the refractive index of the surrounding environment and increases for waveguides that support an enhanced evanescent field, like TM-operated photonic [3], slotbased [4], [5], sub-wavelength grating (SWG) [6], porous silicon [7], and plasmonic waveguides [8]. On the other hand, device sensitivity is determined by the overall photonic circuit architecture that is responsible for transforming the inherent transducer waveguide sensitivity into an interpretable and, most often, magnified device sensitivity. ...
... This becomes even more essential when regular low-cost measurement instruments (e.g., low resolution OSA) are used. Several types of structures have been investigated so far for the implementation of photonic integrated sensors, including strip or slot waveguides incorporated either in resonant structures like ring resonators (RRs) [3], [6], [7], [9], [10] and photonic crystal cavities (PhCs) [11], [12], or in interferometric layouts like Mach-Zehnder [13]- [15] and Young configurations [16], [17]. ...
We demonstrate a novel theoretical framework for refractive index Mach-Zehnder interferometric (MZI) sensors that can accurately calculate the sensor FSR and sensitivity while taking into account waveguide effective index dispersion and dip splitting effects. In contrast to the state-of-the-art mathematical equation that relates sensitivity with FSR, our analysis concludes to a mathematical expression that retains its validity and accuracy both in low and large FSR sensor layouts, suggesting its suitability for use in optimizing sensor performance even in the high-sensitivity, high-FSR configurations. This is validated by applying our theory to integrated plasmo-photonic MZI sensors with FSR values up to hundreds of nm, confirming our theoretical results through accurate numerical and circuit-level simulations and demonstrating how sensitivity can be boosted to >10 <sup xmlns:mml="" xmlns:xlink="">5</sup> nm/RIU values exploiting dispersion engineering of the waveguides. To this end, our analytical formula that relates sensitivity with FSR and waveguide effective index dispersion can lead to reliable designs and well-matched fabricated modules when targeting high FSRs and high sensitivity MZI photonic integrated sensors.
... Generally, high bulk responsivity R bulk , narrow FWHM and high FoM are desired. Efforts have been made to increase the responsivity or narrow the FWHM with plasmonic sensors based on surface plasmon resonance (SPR) [2], [3], [6], long-range SPR [11], photonic sensors such as photonic crystal sensors based on Bloch surface waves [12] and whispering gallery mode sensors such as micro-ring resonators [13]. These approaches allow label-free detection of proteins, viruses and various molecules found in human body that are indicative in disease diagnosis and treatment with high precision and sensitivity [14]- [17]. ...
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This paper presents a high aspect-ratio open grating Fabry-Perot resonator theoretically and experimentally for aqueous refractive index sensing with high figure of merit. Transmission line modelling and associated Smith charts are utilized to give a better intuitive understanding of the physical sensing mechanism. The analysis aids understanding of the relative contribution to refractive index changes both within the grating fingers and external to them. Measurements of refractive index are carried out with Kretschmann coupling configuration for both transverse magnetic (TM) and transverse electric (TE) polarization. The results show an improvement of 10- and 44-fold in figure of merit over conventional SPR for TM and TE, respectively. When the designed system is analyzed for sensitivity, ${{1}.{874}\times }{{{10}}}^{-{8}}{\textit {RIU}}$ is achieved with a single frame even without a reference channel for noise cancellation. With 794 frames averaging (approximately 1-second acquisition time), the sensitivity of the system can be further improved to ${{1}.{609}\times }{{{10}}}^{{-{9}}}{\textit {RIU}}$ . The simulations and experimental results demonstrate strong potential for biosensing applications, e.g., kinetic molecular binding, ultrasensitive refractive index sensing and photoacoustic detection.
... Their small footprint enables multiplex array format, which not only improves the sensitivity of the detection, but also allows the simultaneous detection of an array of samples using the same sensor chip. Several integrated-optical RI sensing schemes have been investigated so far, including Mach-Zehnder interferometer configurations [5,6], microcavity resonators [7][8][9][10][11][12] and subwavelength grating waveguides [13,14]. Among them, microcavity resonators have the advantage of high quality factors and small mode volumes. ...
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We propose a new type of self-referencing and multiplexed refractive index (RI) sensor based on a compound optical microresonator structure consisting of Fabry-Pérot (FP) resonators coupled with microring resonators. The transmission spectra shows resonant features that are superimposed on a background defined by FP oscillations. The resonances have asymmetric Fano-like non-Lorentzian shapes, which are used as sensing peaks, while the FP oscillations are used as reference peaks for internal self-referencing. The sensing peaks shift linearly with the increased RI of the cladding in the microring resonator, while FP peaks stay constant. When the temperature is increased, both the FP peaks and the Fano resonances shift linearly at the same rate, which eliminates the temperature effect on RI measurements. We theoretically analyzed that the two-mirror FP resonator coupled with a single microring resonator and optimized its sensing performance through finite-difference time-domain simulations. A sensitivity value of 220 nm/RIU and a maximum figure of merit of 4400 RIU ⁻¹ were achieved. We also proposed two possible multiplexing schemes consisting of two-mirror and three-mirror FP resonators coupled with two microring resonators of different radii. The proposed sensor concept is simple, easy-to-fabricate, self-calibrating and can be used for simultaneous measurements of different samples.
... For a resonator based on strip WG, this value is 90% of the maximum realizable sensitivity. Lately, an extremely responsive photonic sensor based on porous silicon RR has been proposed which offers a sensitivity of 439 nm/RIU for low RI variations [41]. ...
In this paper, the modal characteristics of silicon-based optical waveguides (hereafter represented as WGs) such as ridge, rib, slot and the hybrid plasmonic WGs are discussed. For sensing applications, the geometric parameters of the WGs are optimized to evaluate the mode sensitivity (Smode) and evanescent field ratio (EFR). For highly sensitive devices, these two parameters should be as high as possible to facilitate strong light-matter interaction. The analysis is performed via the finite element method (FEM) which demonstrate that EFR and Smode follow the following order: Hybrid WG ≥ Slot WG > Ridge WG @ TE mode > Rib WG @ TE-mode > Ridge WG @ TM mode > Rib WG @ TM mode. Furthermore, refractive index sensor established on a ridge, slot and hybrid plasmonic WGs are studied. The numerical calculations have revealed that the best sensitivity of 167 nm/RIU, 233.3 nm/RIU and 333.3 nm/RIU is delivered by ring resonator designs constructed on ridge, slot and the hybrid plasmonic WG, respectively. We believe that these findings are quite beneficial to have a detailed analysis of different WG types to make a judicious selection of optical WG structure for sensing applications.
... From the results obtained, we demonstrate that the combination of PhC/SWG structures and bimodal interferometry opens the door to the creation of ultra-sensitive and ultra-compact photonic sensing devices. We expect that the broad design possibilities of periodic structures (i.e., different shapes, periodicities, dimensions, materials, defect types, etc.), together with the combination with other concepts such as the use of porous materials 14 , can lead to novel configurations having even better sensing performances. ...
... The miniature silicon piezoresistive pressure sensor adopts the advanced miniaturization manufacturing process to integrate the silicon diaphragm as the sensitive element. The four resistors on the diaphragm are connected through the screen-printing circuit, and the delicate micro-packaging is carried out with the metal shell [26,27]. A schematic diagram of the sensor packaging structure is shown in Figure 3, and the picture of the sensor is shown in Figure 4. ...
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In order to study the penetration mechanism of jacked piles in viscous soil foundation, the stress variation law of the pile-soil interface was obtained by installing silicon piezoresistive earth pressure and pore water pressure sensors, and fiber Bragg grating (FBG) sensors in a model pile body, and the penetration characteristics of jacked piles in homogeneous viscous soil were defined. The test results show that: Fiber Bragg grating and silicon piezoresistive sensing technology can better meet the requirements of testing the characteristics of jacked pile in viscous soil. The ratio of pile lateral resistance to pile end resistance varies when pile is jacked in homogeneous viscous soil. In the early stage of pile jacking, the ratio of pile lateral resistance is small, and in the later stage of pile jacking, the ratio of pile lateral resistance increases, but the ratio of pile end resistance is still higher than that of pile lateral resistance. The ratio of the effective stress to the total radial stress is high, and the variation law of the two is consistent with the depth. The total radial stress, pore water pressure, and effective radial stress all exhibit the degradation phenomenon, and the degradation degree decreases gradually with the increase in penetration depth at the same depth. The ratio of excess pore water pressure to overburden weight decreases with the increase in depth, and the maximum value is 87%. The research results can provide a reference for the engineering practice of jacked pile in viscous soil foundation.
Direct laser writing (DLW) of mesoporous porous silicon (PS) films is shown to selectively create spatially separated nitridized and carbonized features on a single film. Nitridized or carbonized features are formed during DLW at 405 nm in an ambient of nitrogen and propane gas, respectively. The range of laser fluence required to create varying feature sizes while avoiding damage to the PS film is identified. At high enough fluence, nitridation using DLW has been shown as an effective method for laterally isolating regions on the PS films. The efficacy in preventing oxidation once passivated is investigated via energy dispersive X-ray spectroscopy. Changes in composition and optical properties of the DL written films are investigated using spectroscopic analysis. Results show carbonized DLW regions have a much higher absorption than as-fabricated PS, attributed to pyrolytic carbon or transpolyacetylene deposits in the pores. Nitridized regions exhibit optical loss similar to previously published thermally nitridized PS films. This work presents methods to engineer PS films for a variety of potential device applications, including the application of carbonized PS to selectively engineer thermal conductivity and electrical resistivity and of nitridized PS to micromachining and selective modification of refractive index for optical applications.
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Silicon photonics is a key emerging technology in next-generation communication networks and data centers interconnects, among others. Its success relies on the ability of using CMOS-compatible platforms for the integration of optical circuits into small devices for a large-scale production at low-cost. Within this field, integrated interferometers play a crucial role in the development of several on-chip photonic applications such as biological sensors, electro-optic modulators, all-optical switches, programmable circuits or LiDAR systems, among others. However, it is well known that optical interferometry usually requires very long interaction paths, which hinders its integration in highly compact footprints. To mitigate some of these size limitations, several approaches emerged including sophisticated materials or more complex structures, which, in principle, reduced the design area but at the expense of increasing fabrication process steps and cost. This thesis aims at providing general solutions to the long-standing size problem typical of optical integrated interferometers, in order to enable the densely integration of silicon-based devices. To this end, we combine the benefits from both bimodal waveguides and periodic structures, in terms of high-performance operation and compactness to design single-channel interferometers in very reduced areas. More specifically, we investigate the dispersive effects that arise from subwavelength grating and photonic crystal structures for their implementation in different bimodal interferometric configurations. Furthermore, we demonstrate various potential applications such as sensors, modulators and switches in ultra-compact footprints of a few square microns. In general, this thesis proposes a new concept of integrated interferometer that addresses the size requirements of current photonics and open up new avenues for future bimodal-operation-based devices.
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Porous silicon (PSi) has been widely used as a biosensor in recent years due to its large surface area and its optical properties. Most PSi biosensors consist in close-ended porous layers, and, because of the diffusion-limited infiltration of the analyte, they lack sensitivity and speed of response. In order to overcome these shortcomings, PSi membranes (PSiMs) have been fabricated using electrochemical etching and standard microfabrication techniques. In this work, PSiMs have been used for the optical detection of Bacillus cereus lysate. Before detection, the bacteria are selectively lysed by PlyB221, an endolysin encoded by the bacteriophage Deep-Blue targeting B. cereus. The detection relies on the infiltration of bacterial lysate inside the membrane, which induces a shift of the effective optical thickness. The biosensor was able to detect a B. cereus bacterial lysate, with an initial bacteria concentration of 105 colony forming units per mL (CFU/mL), in only 1 h. This proof-of-concept also illustrates the specificity of the lysis before detection. Not only does this detection platform enable the fast detection of bacteria, but the same technique can be extended to other bacteria using selective lysis, as demonstrated by the detection of Staphylococcus epidermidis, selectively lysed by lysostaphin.
It is a remarkable and straightforward approach to customize the dispersion and nonlinear properties of the photonic devices without varying the composition of the material by employing periodic segmented waveguide structures at the subwavelength level of the operational wavelength. This method addresses the diffraction limit and it is likely to engineer the waveguides as a uniform optical medium with an effective refractive index that relies on the waveguide geometry. In recent years, advances in lithographic technology in the semiconductor-on-insulator platform providing sub-100-nm patterning resolution have been renowned with many useful devices based on subwavelength structures. At the beginning of the paper, the modal characteristics of the sub-wavelength grating (SWG) waveguides are presented. And in afterwards, we provide an insight into noteworthy progress in the subwavelength grating (SWG) waveguides based devices for signal processing and sensing applications such as ring resonators for surface and bulk sensing, couplers, suspended membrane waveguides for mid-infrared applications, filters, and fiber-to-chip couplers.
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This work presents simulation and experimental results of ultra-thin optical ring resonators, having larger Evanescent Field (EF) penetration depths, and therefore larger sensitivities, as compared to conventional Silicon-on-Insulator (SOI)-based resonator sensors. Having higher sensitivities to the changes in the refractive indices of the cladding media is desirable for sensing applications, as the interactions of interest take place in this region. Using ultra-thin waveguides (<100 nm thick) shows promise to enhance sensitivity for both bulk and surface sensing, due to increased penetration of the EF into the cladding. In this work, the designs and characterization of ultra-thin resonator sensors, within the constraints of a multi-project wafer service that offers three waveguide thicknesses (90 nm, 150 nm, and 220 nm), are presented. These services typically allow efficient integration of biosensors with on-chip detectors, moving towards the implementation of lab-on-chip (LoC) systems. Also, higher temperature stability of ultra-thin resonator sensors were characterized and, in the presence of intentional environmental (temperature) fluctuations, were compared to standard transverse electric SOI-based resonator sensors.This paper was published in Optics Express and is made available as an electronic reprint with the permission of OSA. The paper can be found at the following URL on the OSA website: Systematic or multiple reproduction or distribution to multiple locations via electronic or other means is prohibited and is subject to penalties under law
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Large wavelength shifts have been measured in the reflectivity spectra of Bragg mirrors etched in porous silicon after exposure of the mirrors to vapor from organic solvents. The shift in the Bragg wavelength of the mirror arises from refractive index changes, induced by capillary condensation of the vapor in the mesoporous silicon, in the layers of the mirrors. Modeling of the reflectivity changes shows that the layer liquid volume fraction occurring in the measurements was 0.29 for acetone and 0.33 for chlorobenzene. Time-resolved measurements show that condensation occurs on the time scale of tens of seconds. © 1999 American Institute of Physics.
While silicon photonic resonant cavities have been widely investigated for biosensing applications, enhancing their sensitivity and detection limit continues to be an area of active research. Here, we describe how to engineer the effective refractive index and mode profile of a silicon-on-insulator (SOI) waveguide using sub-wavelength gratings (SWG) and report on its observed performance as a biosensor. We designed a 30 μm diameter SWG ring resonator and fabricated it using Ebeam lithography. Its characterization resulted in a quality factor, Q, of 7 · 10<sup>3</sup>, bulk sensitivity Sb = 490 nm/RIU, and system limit of detection sLoD = 2 · 10<sup>-6</sup> RIU. Finally we employ a model biological sandwich assay to demonstrate its utility for biosensing applications.
A ring resonator is patterned on a porous silicon slab waveguide to produce a compact, high quality factor biosensor with a large internal surface area available for enhanced recognition of biological and chemical molecules. The porous nature of the ring resonator allows molecules to directly interact with the guided mode. Quality factors near 10,000 were measured for porous silicon ring resonators with a radius of 25 μm. A bulk detection sensitivity of 380 nm/RIU was measured upon exposure to salt water solutions. Specific detection of nucleic acid molecules was demonstrated with a surface detection sensitivity of 4 pm/nM.
The use of porous silicon (PSi) as a sensor for detection of various analytes is reviewed. The optical or electrical properties of PSi are key sensing parameters that have been used in many chemical and biological sensing applications. PSi is a promising candidate due to ease of fabrication, large surface area, various accessible pore sizes and morphologies, controllable surface modification and its compatibility with conventional silicon processing technology. The adsorption of chemical or biological molecules into the pores modifies the electrical and/or optical properties, allowing convenient and sensitive measurement approach. In this review, we provide a critical assessment of the development of PSi as a promising material for chemical and biosensing applications. Formation procedures of PSi with various pore sizes and morphologies are firstly given. Surface properties and structural characteristics of the material are briefly described. The recent progress on utilization of such porous structures in chemical and biosensing applications is then addressed in the context of surface chemistry effects and nanostructures, measuring approaches, operating concepts and device sensitivity and stability. Finally, concluding remarks with existing challenges that hinder the material for commercial use are highlighted.
Being able to manipulate light and confine it to small length scales have a multitude of applications in modern technology. Predicting the behavior of nanophotonic devices and the realization of new ones will greatly benefit from insights offered by analytical calculations and numerical modeling. In this paper, we elucidate the fundamental electromagnetic responses of materials and introduce a versatile technique, called transfer matrix method, in modeling the behavior of nanoscale heterostructures. Its application in novel photonic devices such as semiconductor microcavities and surface plasmon resonance sensors will be demonstrated.
Optical waveguides comprised of nanoporous materials are uniquely suited for on-chip sensing applications, because they allow for a target chemical or analyte to directly infiltrate the optical material that comprises the core of the waveguide. We describe here the fabrication and characterization of nanoporous waveguides, and demonstrate their usefulness in measuring small changes in refractive index when exposed to a test analyte. We use a process of electrochemical etching and laser oxidation to produce channel waveguides and integrated on-chip Mach-Zehnder structures, and we compare the responsivity and interferometric stability of the integrated sensor to that of a fiber-based interferometer. We quantify the detection capability by selectively applying isopropanol to a 200 μm length waveguide segment in one arm of the interferometer, which produces a phase shift of 9.7π. The integrated interferometer is shown to provide a more stable response in comparison to a comparable fiber-based implementation.
Silicon-on-insulator (SOI) wafer is one of the most appealing platforms for optical integrated circuit with the potential to realize high performance Ultra Large Scale Integration (ULSI) and device miniaturization. In this work, based on simulations to obtain appropriate optical properties of a porous silicon microcavity (PSM), we successfully fabricated a highly efficient PSM on SOI wafer by electrochemical etching for DNA detection at optical wavelength 1555.0nm. The narrow resonance peak with a full width at half maximum about 26.0nm in the reflectance spectrum gives a high Q factor which causes high sensitivity for sensing performance. The sensitivity of this sensor is investigated through 19-base pair DNA hybridization in the PSM by surface modification using a standard cross link chemistry method. The red shift of the reflectance spectra shows a good linear relationship with complementary DNA concentration, ranging from 0.625 to 12.500μM, and the detection limit is 43.9nM. This optical PSM on SOI is highly sensitive, fast responsive, easy to fabricate and low-costly, that will broadly benefit to develop a new optical label-free biosensor on SOI wafer and has a great potential for biochips based on integrated optical devices.