We discuss and present preliminary experimental evidence of three novel add-on functionalities that can make Optical Feedback Imaging a truly small footprint and label-free bioimaging technology. The first is single-pixel compressed sensing. Here we report on scanless optical feedback imaging in free space by spatially modulated illumination of the target. The second is chemical sensitivity. Here, we report the identification of several pigments by selective spectral discrimination at three different wavelengths. The third functionality is the integration of OFI in silicon photonic chips. Here we identify the building blocks necessary to implement a scanless imaging system in an integrated photonic chip and show evidence of laser modulation through optical feedback provided by the emitted radiation after passing through a silicon passive integrated waveguide.
Numerous biological surfaces exhibit intricate micro- and nano-structures, which fulfill various functions such as anti-reflective properties, structural coloration, anti-fouling capabilities, and pro- or anti-adhesive characteristics. These features have inspired a plethora of industrial applications. In recent years, there has been a significant surge in research in this domain, largely attributable to the growing interdisciplinary nature of the approaches applied to the investigation of structured biosurfaces.
The convergence of classical zoology and botany with advances in genetics and molecular biology is noteworthy, as biologists increasingly collaborate with nanotechnologists, materials scientists, and engineers. This collaborative effort contributes significantly to expanding the horizons of research on micro- and nano-structured biological surfaces, fostering biomimetic and bioengineering applications in various industries (Fig.1). Our proposal seeks to capitalize on this momentum and align with the current developments in the field.
The primary objective of the COST Action titled "Understanding interaction light – biological surfaces: possibility for new electronic materials and devices" is to unite scientists from diverse disciplines within this dynamic research realm. The emphasis of this collaborative effort is placed on exploring the photonic effects arising from the nano- and micro-structuring of biological surfaces, along with their potential bionic applications. Through our consortium, we aim to facilitate cross-inspiration among participants from distinct research fields, fostering an environment conducive to innovation in research and eventual industrial advancements. Our initiative seeks to ride the wave of these scientific developments, propelling forward the exploration of the intricate world of micro- and nano-structured biological surfaces.
Various biological surfaces are known to be covered by elaborated micro- and nano-structures, serving a number of functions (e.g. anti-reflective, structural coloration, anti-fouling, pro- or anti-adhesive, etc.) and inspiring numerous industrial applications. Recent years have witnessed a remarkable boost in research in this field. To a large extent, this boost owes to the increasing interdisciplinary of approaches being applied to the study of structured biosurfaces. Sciences as different as classical zoology and botany are inseminated with the advances in genetics and molecular biology; biologists collaborate more and more with nanotechnologists, materials scientists and engineers – all these contribute to the widening of the horizons of research on micro- and nano-structured biological surfaces, and to biomimetic and bioengineering applications of these surfaces in industry. We aim at ‘riding the wave’ of these developments with our proposal. In our talk I will present the main goal of the COST Action “Understanding interaction light – biological surfaces: possibility for new electronic materials and devices”.
In this article, we present and discuss a board game called QTris that uses a concise set of rules to describe the quantum mechanics of nine qubits through the conceptual framework of preparation, operations, and measurement. Indeed, there is no quantum experiment on nine qubits that is not a game of QTris. Similarly, every game of QTris describes a quantum mechanical experiment. We show how, through its gameplay mechanics and strategies, QTris clearly emphasises the differences between statistical mixtures and coherent superpositions. As a result, the game mechanics of QTris provide an interpretation of quantum mechanics in the form of game strategies. This interpretation is clear-cut and free of metaphors.
Optical Feedback Imaging is the most recent advancement of the traditional Laser Self-Mixing. It is a coherent scanning imaging technique with the unique feature of having the light source acting also as the signal detector. Optical Feedback Imaging is being increasingly applied to both inorganic and organic samples, especially in the infrared and Terahertz spectral region. The technique relies on the analysis of the back-scattered (or reflected) radiation, thus it is contactless, detectorless and suited to be applied also in-vivo. As a coherent technique in reflection, it is sensitive to both phase (index) contrast and amplitude (absorption) contrast. We demonstrate selective contrast imaging of different organic pigments at three different visible wavelengths allowing for unambiguous identification of absorption features in back-scattering geometry. However, a wealth of information about the chemical composition of tissues is more readily available at mid-infrared wavelengths, where molecular bonds have their spectral signatures. We also present Optical Feedback images collected in organic (agar-agar) tissue phantoms at mid infrared wavelengths. 1% sensitivity to potato starch concentration included in the agar-agar matrix was detected at 6.26 μm, in resonance to the amide-I band.
We demonstrate superresolution in position tracking sensing based on feedback interferometry in quantum cascade lasers
(QCLs). QCLs with optical feedback make highly compact sensors since they work as mixer oscillator and detector of
infrared radiation. Additionally, QCL continuous-wave emission remains stable at steady state in strong feedback
regimes, permitting to gain control on the nonlinearity of the QCL active medium. Here, nonlinear frequency mixing in a
QCL-based common-path interferometer is exploited to unveil object’s position with nanometer-scale resolution, far
beyond the intrinsic limit of half-wavelength. Experimental results are in excellent agreement with simulations based on
Lang-Kobayashi model encompassing multiple-target dynamics.
We study the dynamics of a Quantum Cascade Laser subject to strong optical feedback in the framework of the
Lang-Kobayashi model. In particular, we demonstrate that the continuous wave instability may lead to coherent
multimode oscillations that indicate spontaneous phase-locking among external cavity modes. We recently
predicted that this unique behavior is linked to the absence of relaxation oscillations in unipolar semiconductor
lasers, which are characterized by a fast carriers recombination time (class-A lasers). These theoretical evidences
may help understanding the mechanisms possibly leading to spontaneous mode-locking and pulse generation in
QCLs.
We demonstrate a common-path optical interferometer based on a quantum-cascade laser (QCL), in which the QCL acts
both as source and detector of the infrared radiation. The collinear arms of the interferometer are terminated by a plastic
surface (acting as the beam splitter) and by a metallic one (acting as the mirror). The different reflectivity of the surfaces
allows for high contrast feedback-interferometry fringes exhibited on the laser-emitted power and revealed by voltage
compliance measurement at the QCL terminals. The displacement of each surface can be identified and measured with
sub wavelength resolution. The experimental results are in excellent agreement with the numerical simulations based on
the Lang-Kobayashi model for multiple cavities. Applications to microfluidics and resonant chemical detection can be
envisaged.
We consider a multi-transverse mode Vertical Cavity Surface Emitting Laser (VCSEL) subject to optical feedback.
When the field profile can be described in terms of few Gauss-Laguerre modes we show that the self-mixing
interferometric signal exhibits features peculiar of the spatial distribution and/or polarization state of the re-injected
field. Based on these results we provide both theoretically and experimentally the proof-of-principle of
an operational scheme for a sensor that can be used to simultaneously measure target translations along the
optical axis and target rotations in the orthogonal plane.
The recent development of ultrafast laser ablation technology in precision micromachining has dramatically increased
the demand for reliable and real-time detection systems to characterize the material removal process. In particular, the
laser percussion drilling of metals is lacking of non-invasive techniques able to monitor into the depth the spatial- and
time-dependent evolution all through the ablation process. To understand the physical interaction between bulk material
and high-energy light beam, accurate in-situ measurements of process parameters such as the penetration depth and the
removal rate are crucial. We report on direct real time measurements of the ablation front displacement and the removal
rate during ultrafast laser percussion drilling of metals by implementing a contactless sensing technique based on optical
feedback interferometry. High aspect ratio micro-holes were drilled onto steel plates with different thermal properties
(AISI 1095 and AISI 301) and Aluminum samples using 120-ps/110-kHz pulses delivered by a microchip laser fiber
amplifier. Percussion drilling experiments have been performed by coaxially aligning the diode laser probe beam with
the ablating laser. The displacement of the penetration front was instantaneously measured during the process with a
resolution of 0.41 μm by analyzing the sawtooth-like induced modulation of the interferometric signal out of the detector
system.
Direct real-time measurements of the penetration depth during laser micromachining has been demonstrated by
developing a novel ablation sensor based on laser diode feedback interferometry. Percussion drilling experiments have
been performed by focusing a 120-ps pulsed fiber laser onto metallic targets with different thermal conductivity. In-situ
monitoring of the material removal rate was achieved by coaxially aligning the beam probe with the ablating laser. The
displacement of the ablation front was revealed with sub-micrometric resolution by analyzing the sawtooth-like induced
modulation of the interferometric signal out of the detector system.
We report on the development of an all-interferometric sensor based on the laser-self-mixing for the simultaneous
detection of multi-degrees-of-freedom displacement of a remote target. The prototype system consists only of a laser
head, equipped with 6 diode lasers and a properly designed reflective target. Information on a single degree-of-freedom
motion is extracted by the differential measurement of two linear displacements by means of two nominally identical
self-mixing interferometers. The sensor has been experimentally tested to measure yaw, pitch, roll, straightness and
flatness corrections over a continuous linear range of 1 m, with resolutions of 0.7 μm (longitudinal), 20 μm (straightness
and flatness), 0.001° (yaw and pitch) and 0.015 °(roll).
The development of a contactless sensor based on the Laser-Self-Mixing effect for the simultaneous measurement of
linear and transverse degrees-of-freedom (DOFs) of a moving target is described in this paper. The sensor is made of
three laser diodes with integrated monitor photodiodes, and a properly designed reflective target attached to the moving
object. The proposed technique exploits the differential measurement of linear displacements by two identical self-mixing
interferometers (SMIs) and makes the system more compact and easier to align with respect to traditional
interferometric systems, thus providing an effective low-cost motion control system. The feasibility of the proposed
sensor is experimentally demonstrated over a range of 1 m for linear motion and ± 6 mm for transverse displacements,
with resolutions of 0.7 μm and 20 μm, respectively.
A new fiber strain sensor based on the Laser-Self-Mixing effect in Distributed Feedback Laser diodes is presented.
Compared with existing fiber strain sensors based on Fiber Bragg Gratings, that are sensitive to local strain, our device
achieves comparable sensitivity (~ 1 με) distributed along the whole fiber length and requires a much simpler
electronics. The sensor is based on the interferometric principle of the laser-self-mixing in the moderate feedback
regime, whereby the fiber strain results in a variation of the optical path-length seen by the radiation reflected by the end
facet of the fiber itself. Increasing or decreasing strain recognition is directly provided by the sign of the sawtooth-like
fringes derivative.
We report the experimental observation of transverse optical patterns in optically injected phase-coupled VCSEL arrays. The devices consist of a single VCSEL with a metallic top contact defining a 8x8 pixels array. The different semiconductor/air - metal/air index step determines the periodic variation of the cavity transmission, that enables the selection of a single laser supermode above threshold. The patterns were observed in the near field at driving currents below the laser threshold, while injecting a nearly resonant optical field. At large frequency detuning hexagonal patterns were observed, whereas rolls appeared closer to the cavity resonance. The experimental observations are explained according to a well established model (Spinelli, Tissoni, Brambilla, Prati, Lugiato, Phys.Rev.A 58, (1998) 2542) accounting for diffraction and nonlinear effects in optically injected VCSELs, and modified to allow for the spatially modulated cavity transmission.
It has been proposed that the temperature of the active layer in VCSELs could be inferred from the wavelength shift of the laser line. However, in VCSELs the laser emission wavelength is primarily selected by the cavity resonance, instead of the material gain peak. Hence the shift of the laser emission only provides an estimate of the temperature averaged over the whole cavity volume. We present a non-invasive microprobe technique for the temperature mapping of operating VCSELs, based on the analysis of the spontaneous electroluminescence emission transmitted through the DBR mirrors. While the sample is temperature stabilized and held onto a xy piezo stage, it is scanned across with an optical microscope (achieving ~2 um spatial resolution). The signal is spectrally resolved and analyzed by a CCD. By comparing the spectra taken under cw and pulsed current injection, the temperature contribution to the emission lineshape can be extracted straightforwardly. We demonstrate the capability of the proposed technique by mapping the temperature rise of a broad area proton implanted oxide VCSEL. Our results clearly demonstrate that the temperature rise is not uniform across the device cross-section, in contrast to the uniform temperature distribution measured by the laser wavelength shift method.
Thermal effects are of fundamental importance in the cw operation of both gain- and index-guided VCSELs. At today, the actual temperature of operating devices is usually estimated from the spectral shift of the laser emission. This method only probes the temperature distribution averaged over the whole cavity volume and cannot provide spatially resolved information across the VCSEL cross- section. One single experiment has been performed to locally measure the temperature distribution in VCSEL by using a thermal scanning microscope. However, such technique required the cleaving and re-processing of the device. We present a new non-invasive technique to map the temperature of operating VCSELs that can be used to test devices at the wafer level. The method is based on the analysis of the spontaneous emission transmitted through the DBR mirrors. While the sample is temperature stabilized and held onto a xy piezo stage, it is scanned across with an optical microscope (achieving 1 micrometers spatial resolution). The signal is spectrally resolved and analysed by a CCD. By comparing the spectra taken under cw and pulsed current injection, the temperature contribution to the emission lineshape can be extracted straightforwardly. We demonstrate this technique by mapping the temperature rise of a broad area proton implanted VCSEL.
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