Albert Redo-Sanchez
@redo-sanchez.net
Investigador Distinguido DIIS
Universidad de Zaragoza
RESEARCH, TEACHING, or OTHER INTERESTS
General Physics and Astronomy
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Albert Redo-Sanchez, Pablo Luesia-Lahoz, Diego Gutierrez, and Adolfo Muñoz
Optica Publishing Group
The non-line-of-sight (NLOS) imaging field encompasses both experimental and computational frameworks that focus on imaging elements that are out of the direct line-of-sight, for example, imaging elements that are around a corner. Current NLOS imaging methods offer a compromise between accuracy and reconstruction time as experimental setups have become more reliable, faster, and more accurate. However, all these imaging methods implement different assumptions and light transport models that are only valid under particular circumstances. This paper lays down the foundation for a cohesive theoretical framework which provides insights about the limitations and virtues of existing approaches in a rigorous mathematical manner. In particular, we adopt Dirac notation and concepts borrowed from quantum mechanics to define a set of simple equations that enable: i) the derivation of other NLOS imaging methods from such single equation (we provide examples of the three most used frameworks in NLOS imaging: back-propagation, phasor fields, and f-k migration); ii) the demonstration that the Rayleigh-Sommerfeld diffraction operator is the propagation operator for wave-based imaging methods; and iii) the demonstration that back-propagation and wave-based imaging formulations are equivalent since, as we show, propagation operators are unitary. We expect that our proposed framework will deepen our understanding of the NLOS field and expand its utility in practical cases by providing a cohesive intuition on how to image complex NLOS scenes independently of the underlying reconstruction method.
Pablo Luesia, Miguel Crespo, Adrian Jarabo, and Albert Redo-Sanchez
Optica Publishing Group
Non-line-of-sight (NLOS) imaging aims to reconstruct partially or completely occluded scenes. Recent approaches have demonstrated high-quality reconstructions of complex scenes with arbitrary reflectance, occlusions, and significant multi-path effects. However, previous works focused on surface scattering only, which reduces the generality in more challenging scenarios such as scenes submerged in scattering media. In this work, we investigate current state-of-the-art NLOS imaging methods based on phasor fields to reconstruct scenes submerged in scattering media. We empirically analyze the capability of phasor fields in reconstructing complex synthetic scenes submerged in thick scattering media. We also apply the method to real scenes, showing that it performs similarly to recent diffuse optical tomography methods.
Christos Melios, Nathaniel Huang, Luca Callegaro, Alba Centeno, Alessandro Cultrera, Alvaro Cordon, Vishal Panchal, Israel Arnedo, Albert Redo-Sanchez, David Etayo,et al.
Springer Science and Business Media LLC
AbstractGraphene has become the focus of extensive research efforts and it can now be produced in wafer-scale. For the development of next generation graphene-based electronic components, electrical characterization of graphene is imperative and requires the measurement of work function, sheet resistance, carrier concentration and mobility in both macro-, micro- and nano-scale. Moreover, commercial applications of graphene require fast and large-area mapping of electrical properties, rather than obtaining a single point value, which should be ideally achieved by a contactless measurement technique. We demonstrate a comprehensive methodology for measurements of the electrical properties of graphene that ranges from nano- to macro- scales, while balancing the acquisition time and maintaining the robust quality control and reproducibility between contact and contactless methods. The electrical characterisation is achieved by using a combination of techniques, including magneto-transport in the van der Pauw geometry, THz time-domain spectroscopy mapping and calibrated Kelvin probe force microscopy. The results exhibit excellent agreement between the different techniques. Moreover, we highlight the need for standardized electrical measurements in highly controlled environmental conditions and the application of appropriate weighting functions.
Alessandro Cultrera, Danilo Serazio, Amaia Zurutuza, Alba Centeno, Oihana Txoperena, David Etayo, Alvaro Cordon, Albert Redo-Sanchez, Israel Arnedo, Massimo Ortolano,et al.
Springer Science and Business Media LLC
Electronic applications of large-area graphene films require rapid and accurate methods to map their electrical properties. Here we present the first electrical resistance tomography (ERT) measurements on large-area graphene samples, obtained with a dedicated measurement setup and reconstruction software. The outcome of an ERT measurement is a map of the graphene electrical conductivity. The same setup allows to perform van der Pauw (vdP) measurements of the average conductivity. We characterised the electrical conductivity of chemical-vapour deposited graphene samples by performing ERT, vdP and scanning terahertz time-domain spectroscopy (TDS), the last one by means of a commercial instrument. The measurement results are compared and discussed, showing the potential of ERT as an accurate and reliable technique for the electrical characterization of graphene samples.
Alvaro Cordon, M. Castrillo, A. G. Miguel Laso, Israel Arnedo, Luis Miranda, Cristian Martinez, Andrea Ines, David Etayo, Montserrat Fernandez, Pablo Rodriguez,et al.
IEEE
In this paper, we present a system that provides meso-scale characterization of thin film materials, covering the gap between nano-scale and macro-scale methods. Nano-scale methods are slow and cannot characterize large surfaces. Macroscale methods generate characterization that averages the magnitudes and, thus, cannot provide localized information. Our system works in reflection as opposed to state-of-the-art methods and provides mobility, carrier density, and conductance maps in the THz range. Moreover, it can be integrated with reactors and enables monitoring of the fabrication of materials in real-time, supporting, for instance, the production of graphene at industrial scale.
L. Callegaro, C. Cassiago, A. Cultrera, V. D'Elia, D. Serazio, M. Ortolano, M. Marzano, O. Kazakova, C. Melios, F. Raso,et al.
IEEE
GRACE - Developing electrical characterisation methods for future graphene electronics is a 2016 Normative Joint Research Project of the European Metrology Programme for Innovation and Research. The project focuses on the measurement of the electrical properties of graphene. Its objectives are to develop validated measurement methods and protocols, including fast-throughput examples. The work is performed in collaboration with international standardisation committees, with an aim to initiate and develop dedicated documentary standards.
David M. A. Mackenzie, Patrick R. Whelan, Peter Bøggild, Peter Uhd Jepsen, Albert Redo-Sanchez, David Etayo, Norbert Fabricius, and Dirch Hjorth Petersen
The Optical Society
We present a comparative study of electrical measurements of graphene using terahertz time-domain spectroscopy in transmission and reflection mode, and compare the measured sheet conductivity values to electrical van der Pauw measurements made independently in three different laboratories. Overall median conductivity variations of up to 15% were observed between laboratories, which are attributed mainly to the well-known temperature and humidity dependence of non-encapsulated graphene devices. We conclude that terahertz time-domain spectroscopy performed in either reflection mode or transmission modes are indeed very accurate methods for mapping electrical conductivity of graphene, and that both methods are interchangeable within measurement uncertainties. The conductivity obtained via terahertz time-domain spectroscopy were consistently in agreement with electrical van der Pauw measurements, while offering the additional advantages associated with contactless mapping, such as high throughput, no lithography requirement, and with the spatial mapping directly revealing the presence of any inhomogeneities or isolating defects. The confirmation of the accuracy of reflection-mode removes the requirement of a specialized THz-transparent substrate to accurately measure the conductivity.
Chi Lee
CRC Press
Albert Redo-Sanchez, Barmak Heshmat, Alireza Aghasi, Salman Naqvi, Mingjie Zhang, Justin Romberg, and Ramesh Raskar
Springer Science and Business Media LLC
Spatial resolution, spectral contrast and occlusion are three major bottlenecks for non-invasive inspection of complex samples with current imaging technologies. We exploit the sub-picosecond time resolution along with spectral resolution provided by terahertz time-domain spectroscopy to computationally extract occluding content from layers whose thicknesses are wavelength comparable. The method uses the statistics of the reflected terahertz electric field at subwavelength gaps to lock into each layer position and then uses a time-gated spectral kurtosis to tune to highest spectral contrast of the content on that specific layer. To demonstrate, occluding textual content was successfully extracted from a packed stack of paper pages down to nine pages without human supervision. The method provides over an order of magnitude enhancement in the signal contrast and can impact inspection of structural defects in wooden objects, plastic components, composites, drugs and especially cultural artefacts with subwavelength or wavelength comparable layers.
Alireza Aghasi, Barmak Heshmat, Albert Redo-Sanchez, Justin Romberg, and Ramesh Raskar
The Optical Society
Heavy sweep distortion induced by alignments and inter-reflections of layers of a sample is a major burden in recovering 2D and 3D information in time resolved spectral imaging. This problem cannot be addressed by conventional denoising and signal processing techniques as it heavily depends on the physics of the acquisition. Here we propose and implement an algorithmic framework based on low-rank matrix recovery and alternating minimization that exploits the forward model for THz acquisition. The method allows recovering the original signal in spite of the presence of temporal-spatial distortions. We address a blind-demodulation problem, where based on several observations of the sample texture modulated by an undesired sweep pattern, the two classes of signals are separated. The performance of the method is examined in both synthetic and experimental data, and the successful reconstructions are demonstrated. The proposed general scheme can be implemented to advance inspection and imaging applications in THz and other time-resolved sensing modalities.
Victor Lopez-Dominguez, Antoni Boix-Montañes, Albert Redo-Sanchez, and Javier Tejada-Palacios
Oxford University Press (OUP)
Abstract Background Drug permeation through skin, or a synthetic membrane, from locally acting pharmaceutical products can be influenced by the permeation behaviour of pharmaceutical excipients. Objective Terahertz time-domain technology is investigated as a non-invasive method for a direct and accurate measurement of excipients permeation through synthetic membranes or human skin. Methods A series of in-vitro release and skin permeation experiments of liquid excipients (e.g. propylene glycol and polyethylene glycol 400) has been conducted with vertical diffusion cells. The permeation profiles of excipients through different synthetic membranes or skin were obtained using Terahertz pulses providing a direct measurement. Corresponding permeation flux and permeability coefficient values were calculated based on temporal changes of the terahertz pulses. Results The influence of different experimental conditions, such as the polarity of the membrane and the viscosity of the permeant, was assessed in release experiments. Specific transmembrane flux values of those excipients were directly calculated with statistical differences between cases. Finally, an attempt to estimate the skin permeation of propylene glycol with this technique was also achieved. All these permeation results were likely comparable to those obtained by other authors with usual analytical techniques. Conclusion Terahertz time-domain technology is shown to be a suitable technique for an accurate and non-destructive measurement of the permeation of liquid substances through different synthetic membranes or even human skin.
Guy Satat, Barmak Heshmat, Nikhil Naik, Albert Redo-Sanchez, and Ramesh Raskar
SPIE
Ultrafast imaging has been a key enabler to many novel imaging modalities, including looking behind corners and imaging behind scattering layers. With picosecond time resolution and unconventional sensing geometries, ultrafast imaging can fundamentally impact sensing capabilities in industrial and biomedical applications. This paper reviews the fundamentals, recent advances, and the future prospects of ultrafast imaging-based modalities.
Albert Redo-Sanchez, Norman Laman, Brian Schulkin, and Thomas Tongue
AIP Publishing LLC
This paper illustrates the non-destructive application of a compact and portable Terahertz (THz) system to analyze the structure of an old painting and to measure the layers of composite plastic samples. THz images from the painting reveal features that resemble the signature of the artist that is not visible in the optical or X-ray channels, which support the authenticity of the painting. On the other hand, data from a composite plastic sample is analyzed to measure the thickness of each layer and determine the presence or absence of adhesive bonding between them. The presence and position of the adhesive is clearly visible in the THz images and the measured thickness shows an excellent agreement with nominal thickness. These applications demonstrate the capabilities of THz technology for unique non-destructive inspection applications. Furthermore, available compact and portable THz systems enable to perform these inspections onsite without the need to bring the sample to the laboratory, increasing the u...
Albert Redo-Sanchez, Norman Laman, Brian Schulkin, and Thomas Tongue
IEEE
This paper describes an example of using a compact Terahertz (THz) system to analyze the layered structure of a composite plastic samples in a non-destructive manner. Time-domain data is analyzed to measure the thickness of each layer of the sample and determine the presence or absence of adhesive bonding the plastic parts. The presence and position of the adhesive is clearly visible in the THz images and the measured thickness shows an excellent agreement with nominal thickness.
Albert Redo-Sanchez, Norman Laman, Brian Schulkin, and Thomas Tongue
Springer Science and Business Media LLC
Technological progress in Terahertz (THz) instrumentation in recent years has produced commercial THz systems with excellent performance, smaller footprint, easier to use operation and more reliable than their homemade laboratory predecessors. Form factor, weight, and data rate are, perhaps, the parameters that have shown the highest improvements in recent years. These parameters also have a major impact on practical application outside the laboratory environment. However, gaps still exists between proof of concepts demonstrated in the laboratory and application requirements in a real environment. The readiness of a technology can be assessed using the Technology Readiness Level (TRL) criterion, which considers nine readiness levels starting from basic concepts at TRL=1 up to full deployment at TRL=9. Applications of THz technology in spectroscopic characterization score high in TRL (7-9) because most of the progress of THz technology has been mainly focused in developing THz instrumentation for spectroscopy. Applications of THz for non-destructive evaluation applications score lower (TRL 5-6) due to higher requirements in terms of performance, especially data rate and form factor in imaging applications. Applications in the medical field have been studied with promising results but they are still in early stages of development, thus, TRL is low (1-4). The progress in THz technology is generating systems with better performance (faster acquisition rates, higher signal-to-noise ratio, bandwidth), broader availability of form factors and configurations, and tighter integration with particular applications. This progress will reduce the gap between the capabilities of the technology and the high-demanding requirements of applications in environments such as quality control and in-line production control in the manufacturing industry.
C. Seco-Martorell, V. López-Domínguez, G. Arauz-Garofalo, A. Redo-Sanchez, J. Palacios, and J. Tejada
The Optical Society
In this paper we use a Terahertz (THz) time-domain system to image and analyze the structure of an artwork attributed to the Spanish artist Goya painted in 1771. The THz images show features that cannot be seen with optical inspection and complement data obtained with X-ray imaging that provide evidence of its authenticity, which is validated by other independent studies. For instance, a feature with a strong resemblance with one of Goya's known signatures is seen in the THz images. In particular, this paper demonstrates the potential of THz imaging as a complementary technique along with X-ray for the verification and authentication of artwork pieces through the detection of features that remain hidden to optical inspection.
Michael Riley, Albert Redo-Sanchez, Panagiotis Karampourniotis, Joel Plawsky, and Toh-Ming Lu
IOP Publishing
A simple technique is reported to create 31 and 45 μm thick, graded-index Si films in the form of nanospirals on a Si substrate using a dynamic, oblique angle deposition technique. We show that the success in producing such a thick, nanostructured film without delamination from the Si substrate is primarily due to the nano-porous nature of the film which effectively eliminates the stress generated during growth. Effective refractive indices of 1.9 and 2.1 were extracted from the terahertz time-domain reflectivity data, which correspond to 57% and 51% porosity for the 31 and 45 μm thick films, respectively. The gradient of porosity through the film was modeled to describe quantitatively the terahertz reflectance data in the 0.2-2.0 THz regime.
Albert Redo-Sanchez and Xi-Cheng Zhang
The Optical Society
The method allows retrieval of the absorbance of a sample without the need for a reference measurement. The method measures the dynamic variation of frequency resolution as the waveform is being acquired. In terahertz wave time-domain spectroscopy, the frequency resolution increases as the temporal window increases. Therefore, narrow absorption peaks will appear in the spectrum when the temporal window is long enough to resolve the peak. By measuring the dynamic values of each frequency component at specific points in time, a reference value and a peak value are extracted and, hence, the self-referenced is achieved. In addition, the method provides a mechanism to remove the effects of echoes, which enables arbitrary temporal window length and, thus, achieves high-resolution frequency. Examples of extraction of the water vapor lines and resonant features in gas and semiconductors are demonstrated in transmission and reflection geometries.
Albert Redo-Sanchez, Gerard Salvatella, Regina Galceran, Eva Roldós, José-Antonio García-Reguero, Massimo Castellari, and Javier Tejada
Royal Society of Chemistry (RSC)
We report the use of terahertz (THz) spectroscopy to explore the spectral properties of eleven antibiotics commonly used in livestock production. Eight of the eleven antibiotics showed specific fingerprints in the frequency range between 0.1 and 2 THz. The main spectral features of two antibiotics (doxycycline and sulfapyridine) were still detectable when they were mixed with three food matrices (feed, milk, and egg powder). These preliminary results indicate that THz spectroscopy could be suitable for screening applications to detect the presence of antibiotic residues in the food industry, with the prospect to allow inspections directly on the production lines. THz spectroscopy is a non-destructive, non-contact, and real-time technique that requires very little sample preparation. Moreover, THz radiation can penetrate plastic and paper, which enables the detection of antibiotics in packaged food.
Qian Song, Yuejin Zhao, Albert Redo-Sanchez, Cunlin Zhang, and Xiaohua Liu
Elsevier BV
Continuous terahertz wave (CW THz) has been widely used in imaging field. However, the speed of imaging calls for an improvement for security screening since the speed of previous CW imaging systems which scan point to point is too slow to be applied in security field. To increase the imaging speed, we proposed a fast CW-THz imaging system in which a galvanometer is introduced. The galvanometer makes the beams reflected in different angles by vibrating at a certain frequency which can significantly decrease the image acquisition time compared to traditional CW-THz imaging system. Furthermore, the system is compact due to source and detector of small size. Examples of measurements of concealed weapons are presented and discussed. Ideal results of better resolution are obtained.
A. Redo-Sanchez, G. Kaur, Xi-Cheng Zhang, F. Buersgens, and R. Kersting
Institute of Electrical and Electronics Engineers (IEEE)
We report on 2-D acoustic phase imaging with millimeter-wave radiation for locating concealed objects. Active adaptation of the imaging interferometer provides enhanced images. We demonstrate that the method allows for finding metallic as well as dielectric materials. Even objects having similar optical properties as the background can be located.
Qian Song, Albert Redo-Sanchez, Yuejin Zhao, and Cunlin Zhang
SPIE
Continuous THz wave (CW THz) has been widely used in imaging field. But for security screening such as inspection at the airport, the speed of the imaging calls for an improvement since the former CW image systems which scan point to point could not satisfy. To increase the image speed, we proposed a fast CW THz image system in which a galvanometer is introduced for the first time. The galvanometer makes the coming beam reflected in different angles by vibrating at a certain frequency which can significantly decrease the image acquisition time compare to point scan THz imaging. A big hyperbolic polyethylene lens is also used in the system to collect all the beams on to the target. A Gunn oscillator and a corresponding Schottky diode are the source and detector respectively. The image we get has ideal resolution. And after image processing, the images looked not only clear but also realistic. The system has more practicality because it is designed in reflection geometry instead of transmission geometry. Moreover, the source and detector in our system do not as ponderous as gas laser which has been used in many THz imaging system previously. Example of measurements of weapons concealed behind the cloth and box are presented and discussed. A compact high speed THz imaging system is expectable which will have a widely application in security field.
Albert Redo-Sanchez and Xi-Cheng Zhang
Institute of Electrical and Electronics Engineers (IEEE)
The recent progress in terahertz science and technology (THz-S&T) opens up a range of potential research opportunities. Historically, THz technologies were mainly used by the astronomy community for searching far-infrared radiation (cosmic background), and by the laser fusion community for the diagnostics of plasmas. Since the first demonstration of THz wave time-domain spectroscopy in the late 1980s, there has been a series of significant advances (particularly in recent years) as intense THz sources and more sensitive detectors provide new opportunities for understanding the basic science in the THz frequency range. THz radiation can penetrate through many nonpolar dielectric materials and can be used for nondestructive/noninvasive sensing and imaging of targets under nonpolar, nonmetallic covers or containers. An immediate application of THz wave technology is in nondestructive testing or inspection. Short-term applications (within three to five years) are expected in spectroscopic sensing and imaging for homeland security. Biomedical applications are expected in the long term (five to ten years). By comparing the publication record trend of THz-S&T related papers with the publication record of proxy fields (laser, microwave, Raman, and infrared), it is possible to anticipate that the number of publications in the THz-S&T arena will increase and also the impact in other research areas. We compare the publication pattern (number of papers versus time) with searching keywords in title or abstract. We found that all the publication trends share a common pattern with four periods defined by discovery, acceptance, adoption, and maturity. From this pattern trend, THz-S&T seems to be in an acceptance period. The unique properties of THz-S&T suggest that its applications will grow.