We report development, testing, and in vivo characterization of a multichannel optical probe for continuous wave (CW) functional near-infrared spectroscopy (fNIRS) that relies on silicon photomultipliers (SiPMs) detectors. SiPMs are cheap, low voltage, and robust semiconductor light detectors with performances analogous to photomultiplier tubes (PMTs). In contrast with PMTs, SiPMs allow direct contact with the head and transfer of the analog signals through thin cables greatly increasing the system flexibility avoiding optical fibers. The coupling of SiPMs and light-emitting diodes (LEDs) made the optical probe lightweight and robust against motion artifacts. After characterization of SiPM performances, which was proven to provide a noise equivalent power below 3 fW, the apparatus was compared through an in vivo experiment to a commercial system relying on laser diodes, PMTs, and optical fibers for light probing and detection. The optical probes were located over the primary sensorimotor cortex and the similarities between the hemodynamic responses to the contralateral motor task were assessed. When compared to other state-of-the-art wearable fNIRS systems, where photodiode detectors are employed, the single photon sensitivity and dynamic range of SiPMs can fully exploit the long and variable interoptode distances needed for correct estimation of brain hemodynamics using CW-fNIRS.
Functional Near Infrared Spectroscopy (fNIRS) uses near infrared sources and detectors to measure changes in
absorption due to neurovascular dynamics in response to brain activation. The use of Silicon Photomultipliers (SiPMs) in
a fNIRS system has been estimated potentially able to increase the spatial resolution. Dedicated SiPM sensors have been
designed and fabricated by using an optimized process. Electrical and optical characterizations are presented. The design
and implementation of a portable fNIRS embedded system, hosting up to 64 IR-LED sources and 128 SiPM sensors, has
been carried out. The system has been based on a scalable architecture whose elementary leaf is a flexible board with 16
SiPMs and 4 couples of LEDs each operating at two wavelengths. An ARM based microcontroller has been joined with a
multiplexing interface, able to control power supply for the LEDs and collect data from the SiPMs in a time-sharing
fashion and with configurable temporal slots. The system will be validated by using a phantom made by materials of
different scattering and absorption indices layered to mimic a human head. A preliminary characterization of the optical
properties of the single material composing the phantom has been performed using the SiPM in the diffuse radial
reflectance measurement technique. The first obtained results confirm the high sensitivity of such kind of detector in the
detection of weak light signal even at large distance between the light source and the detector.
The Silicon Photomultiplier (SiPM) is a novel pixelated photon detector able to detect single photon arrival with good timing resolution and high gain. In this work we present a complete study of the performances of different SiPMs produced by STMicroelectronics. Their potentialities and limits were identified using experimental measurements and electrical simulations performed both on single pixel and SiPM having up to ∼4000 pixels. SiPM was tested in different experiments such as photoluminescence emission measurement, lifetime measurement of semiconductor materials and light diffusion in highly scattering material in the near infrared spectrum showing its aptitude to replace PMT in those applications.
KEYWORDS: Luminescence, Silicon photomultipliers, Sensors, Biosensors, Absorption, Single photon, Signal to noise ratio, Temperature metrology, Pulsed laser operation, Monochromators
A novel Si-based detector, having a low noise and a high sensitivity, up to a single photon detection, was used for a
biosensor application. It is a Silicon photomultiplier (SiPM), a device formed by avalanche diodes operating in Geiger
mode, in parallel connections. Arrays with different dimensions were electro-optically characterized (5×5; 10×10 and
20×20 pixels) in order to identify the best geometry to be used in terms of signal-to-noise ratio, for our purposes. The
SiPM array was used to study both traditional and innovative fluorophores. CY5 was chosen as “reference” marker. It
has an absorption peak at 649 nm and an emission peak at 670 nm. Ru(bpy)3[Cl]2 was identified as innovative fluorophore, since it has absorption and emission peaks at 455 and 630 nm, respectively. Measurements were carried out in both functional regimes: continuous and pulsed. Emission spectra in the range 550–750 nm were measured with both traditional photomultipliers tubes (PMT) and SiPM operating at room temperature in continuous mode. More
interestingly, fluorophore lifetimes were monitored showing that SiPM can measure lifetimes as short at 1 ns (CY5
lifetime), well below the lowest PMT limit (23 ns). Ru(bpy)3[Cl]2 lifetime characterization was performed with both
PMT and SiPM (being in the hundreds of ns range), as a function of the solvent and after deposition and drying on glass
substrates.
KEYWORDS: Silicon, Temperature metrology, Sensors, Optical testing, Quenching (fluorescence), Diodes, Data modeling, Single photon, Silicon photomultipliers, Surgery
We electrically and optically tested both single pixels and complete arrays of Silicon Photomultipliers, from 5×5 to
64x64, fabricated by STMicroelectronics. Single cell devices operation was studied as a function of the temperature from
-25°C to 65°C varying the voltage over breakdown, from 5% up to 20% of the breakdown voltage. Optical
characterization was performed using a laser at 659 nm and opportunely chosen filters to vary the optical power. We
determined the single pixel gain by using both the time resolved dark count signal and the current under controlled
illumination. Typical gain values above 1×105 and above were obtained for operation times of 10 ns, while higher gains
are obtained for longer integration times and lower photon flux.
Single photon Si detectors were fabricated by STMicroelectronics and fully characterized in standard operation
conditions and after irradiations. Both single cells and arrays, of dimensions ranging from 5x5 up to 64x64, were
electrically tested. The devices operation was studied as a function of the temperature from -25°C to 65°C varying the
voltage over breakdown, from 5% up to 20% of the breakdown voltage before and after irradiation using both light ions,
10 MeV B ions to doses in the range 3×107 - 5×1010 cm-2, and X-rays irradiations in the range 0.5 - 20 krad(Si). Optical
characterization was performed using a laser at 659 nm and opportunely chosen filters to vary the optical power. A
strong difference in the radiation damage effect is observed for the two different irradiation sources. Ion irradiation, or
better implantation, produces a damage preferentially sitting in the active device region, hence even at the lowest
irradiation dose the device functionality is compromised, while at the highest dose the device is completely blind. On the
other hand, X-rays produce damage in a low concentration, in fact it does not significantly affect the device dark current,
only an increase in the leakage current under breakdown is observed. Hence the device functionality is preserved to
doses up to 20 Krad(Si).
We studied the enzyme glucose oxidase (GOx) immobilization on silicon oxide surfaces. In particular, we optimized the immobilization protocol and verified that it fulfills both requirements of enzyme preservation (measured by enzymatic activity) and VLSI compatibility. The immobilization consists of four steps: oxide activation, silanization, linker molecule deposition and GOx immobilization. It is crucial to form an uniform linker layer on the sample surface in order to maximize the sites available for enzyme bonding and achieving the best enzyme deposition. In this study, utilizing glutaraldehyde as bifunctional reagent, we monitored its uniformity on the surface through X-ray Photoelectron Spectroscopy (XPS). Once optimized, the same protocol was used to anchor the enzyme in a porous silicon dioxide matrix. Gold labeled GOx molecules were monitored by electron diffraction X-ray (EDX) measurements coupled with scanning electron microscopy (SEM). The enzymatic activity was also monitored to confirm the goodness of the proposed immobilization method. Finally, the electrical characterization of MOS capacitors, showing a shift of about 1 V in the flat band voltage, demonstrated the possibility to use this approach for electrical detection.
In this paper we present a general methodology for the design of resonant cavity enhanced (RCE) photodetectors based on the internal photoemission effect. In order to estimate the theoretical quantum efficiency we take advantage of the analytical formulation of the internal photoemission effect (Fowler theory), and its extension for thin films. In particular, the absorptance is numerically determined by means of an approach based on the transfer matrix method. Finally, we apply the proposed methodology to the design of a silicon RCE photodetector operating at 1.55μm, based on the internal
photoemission effect at an Au-Si schottky barrier.
Interest in silicon as a material for optoelectronics has increased year after year. We propose numerical analysis of an integrated waveguide-vanishing-based modulator realized by ion implantation in SOI wafer. The active region is 3×3 μm2 and the lateral confinement is guaranteed by two highly-doped As (8×1019cm-3) and B (2×1019cm-3) implanted regions 1-μm-deep. This type of structure allows to obtain a planar device, avoiding structural steps which are harmful for photolithography processes. The resulting channel waveguide shows single mode operation and propagation losses of about 1.8 dB/mm, which are acceptable for short structures.
The modulation is based on a lateral p-i-n diode, which injects free carriers into the rib volume between the doped regions. We have optimized the device for maximum injection efficiency for a given applied voltage. The resulting optical behavior can be explained by the lateral confinement vanishing that transforms the rib waveguide in a slab waveguide, once the rib is full of free carriers. This phenomenon occurs at driving voltage of about 1.0 V, with electrical power consumption below 1 mW, and implies a rapid variation of the propagating characteristics, and as consequence an optical beam lateral redistribution into the structure. Results show that an optical modulation depth close to 100% can be reached with a switching time of about 30 ns. A set of numerical simulations has been performed in order to evaluate the thermal response of the device and thus to estimate the thermo-optic effect related to the biasing of the device itself. The main advantages of this device are the low cost and full integrability with electronic devices; thus the device can be suitable in many application fields.
Silicon optical receivers, operating at the optical communication wavelengths in the 1.3-1.55 μm range, have attracted much research effort. Unfortunately, the performance of the devices proposed in literature are poor because this wavelength range is beyond the absorption edge of silicon. In order to extend the maximum detectable wavelength, the most common approach, in the realization of Si-based detectors, is the use of silicon-germanium layers on silicon, anyway, requiring processes non compatible with standard CMOS technology. In this paper, with the aim to extend the operation of silicon-based photo-detectors up to the 1.3-1.55 μm range, an alternative approach is investigated: we propose the design of a resonant cavity enhanced Schottky photodetector based on the internal photoemission effect. The device fabrication is completely compatible with standard silicon technology.
We fabricated and characterized an electro-optic Si-based amplitude light modulator working at 1.5 μm. It is a Bipolar Mode Field-Effect transistor (BMFET) integrated within a Si rib waveguide. The devices, 100 μm long, were fabricated using epitaxial Si wafers and standard clean room processing. The light is absorbed during its travel in the device optical channel when a plasma of free carriers, electrically driven, is generated and placed inside the channel. We experimentally monitored the plasma formation and localization in the device using standard Emission Microscopy analysis. The optical characterization in static conditions provides a modulation depth of ~ 90%, well above the 25% minimum required to consider a device a modulator. Furthermore, dynamical measurements show a modulation depth of 65% at 100KHz of operation frequency. Finally, an experimental evidence of a frequency threshold, at about 500 KHz, is observed in the plasma behavior. Theoretical considerations and experimental data suggest that at frequencies below threshold the dominant phenomenon is the plasma generation/recombination, while above threshold the carrier drift leads the plasma motion and redistribution in the device channel.
Optical interconnects are, nowadays, considered a promising alternative to electrical ones and monolithic integration in Si is the only choice when high volumes, low fabrication costs and reduced spaces are needed. We fabricated an electro-optic Si-based modulator working at 1.5 um using a Bipolar Mode Field-Effect transistor integrated within a Si rib waveguide. The principle of operation is the light absorption by a plasma of free carrier that can be opportunely moved inside or outside the device optical channel by properly changing the bias. The optical channel of the modulator is embodied within its vertical electrical channel.
The devices were fabricated using epitaxial Si wafers and standard clean room processing. The optical characterization in static conditions shows a modulation depth, defined as M=(POff-POn)/POff , of ~ 90 %. It was measured at 1.48 um using a laser diode source coupled with the modulator through a silica optical fiber. The dynamic electrical characterization provides an electrical switching time of ≈10 ns. A modulation depth of 72 % is observed at 100kHz electrical modulation frequency.
Biosensors are a very useful tool to produce drugs or to monitor chemical species through their product of reaction. The sensor is fabricated bounding on its surface specific enzymes that can accomplish the synthesis function. We studied the possibility to fabricate Si-based micro-biosensors to detect glucose in water solutions using porous Si (PS) as surface to bound the specific enzyme. We ideated and fabricated a novel biosensor structure based on a PS membrane that can be used for glucose monitoring and for drug production, by properly choosing the enzyme to immobilize in the reactor. The fabrication details of the structure, having a suspended and auto-supporting PS membrane, through surface micromachining processes, ULSI compatible, are shown. Micro channels localised below the membrane will allow the buffer solution flow through the porous matrix. Moreover, in this work we acquired the know-how on the enzyme manipulation, bonding and detection on Si-based surfaces. The enzyme that accomplish the synthesis function is the glucose oxidase. We deposited it on different substrates: PS, bulk Si and on glass. On these samples photoluminescence, absorbance and optical microscopy measurements were performed.
The implementation of efficient Si optical functions has attracted a considerable interest in the last years since it would allow the use of Si technology for the realisation of integrated optoelectronic (OE) devices. We have fabricated and characterised a novel Si-based light modulator working at the standard communication wavelength of 1.54μm. It consists of a three terminal Bipolar Mode Field Effect Transistor (BMFET) integrated in a silicon rib waveguide realised on epitaxial (epi) Si wafers. The optical channel of the modulator is embodied within its vertical electrical channel. Light modulation is obtained through the formation of a plasma of carriers, inside the optical channel, that produces an increase of the absorption coefficient. Modulation is achieved by moving the plasma inside and outside the optical channel by properly changing the bias of the control electrode. The devices have been fabricated using clean room processes fully compatible with ULSI technology. Electrical characterisation shows a strong channel conductivity modulation. Optical measurements confirm the plasma formation in the channel. The distribution of the plasma under different bias conditions has been directly derived from Emission Microscopy analysis. The devices exhibit modulation depths ranging from 68% up to 83% depending on the bias conditions.
Biosensors are a very useful tool to produce drugs or to monitor chemical species through their product of reaction. The sensor is fabricated bounding on its surface specific enzymes that can accomplish the synthesis function. We studied the possibility to fabricate Si-based micro-biosensors to detect glucose in water solutions. We used porous Si (PS) as surface to bound the glucose oxidase enzyme. We ideated an fabricated a novel biosensor structure based on a PS membrane that can be used for glucose monitoring and for drug production, by properly choosing the enzyme to immobilize in the reactor. The fabrication details of the structure, having a suspended and auto-supporting PS membrane, through surface micromachining processes, ULSI compatible, are shown. Micro channels localised below the membrane will allow the buffer solution flow through the porous matrix. Moreover, in this work we acquired the know-how on the enzyme manipulation, bonding and detection on Si-based surfaces. The glucose oxidase was deposited in PS, on bulk Si and on glass to perform photoluminescence, absorbance and optical microscopy measurements.
The implementation of efficient Si-based optical functions has attracted a considerable interest in the last years since it would allow the use of the Si technology for the realization of integrated optoelectronic devices. We have fabricated and characterized a novel Si-based light modulator working at the standard communication wavelength of 1.54 micrometers . It consists of a three terminal Bipolar Mode Field Effect Transistor integrated with a silicon RIB waveguide on epitaxial Si wafers. The optical channel of the modulator is embodied within its vertical electrical channel. Light modulation is obtained through the formation of a plasma of carriers, inside the optical channel, that produces an increase of the absorption coefficient. Fast modulation is achieved by moving the plasma inside and outside the optical channel by properly biasing the control electrode. The devices have been fabricated using clean room processing. Detailed electrical characterization and device simulation confirm that strong conductivity modulation and plasma formations in the channel are achieved. The plasma distribution in the device under different bias conditions has been directly derived from Emission Microscopy analysis. The expected device performances in terms of modulation depth and speed will be presented and discussed.
During the last decade, many studies have addressed the use of synthetic diamond as sensing material for deep UV photon detection. Solar missions, as the Solar Orbiter and the Solar Probe will require UV photon detectors with unprecedented sensitivity at wavelengths (lambda) <200 nm and radiation hardness. Diamond appears to be the ideal photosensitive material for such applications, as it is very solar blind (105 rejection ratio (lambda) >230 nm and 107 above 400 nm), its very low dark current level avoids the use of cooling systems, has a high UV responsivity and it is radiation hard and chemically inert. This paper is a review of our recent results on the electro- optical properties of diamond and a comparative analysis and characterization of the performances of different diamond- based photodetectors in the vacuum UV. In addition, we will describe some of the photodetectors that are under development for space applications and that will exploit the properties of diamond.
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