Absorption coefficients determine how deeply the light of a particular wavelength can penetrate a material before being absorbed. When the absorption coefficient of a material is low, meaning when a material is semitransparent, light is poorly absorbed, and if the material is thin enough, it will appear transparent to that wavelength. These coefficients depend not only on the material but also on the wavelength of the absorbed light. They are of fundamental importance in multiple contexts, providing crucial information about the optical behavior of transparent materials that can aid in understanding how these materials interact with light and what their optical properties are. However, achieving high accuracy in measuring low values (such as 10^(-3)-10^(-4)) is challenging and represents a significant technical hurdle.
A new study conducted by five researchers, all of whom are affiliated with ICTP, introduces an innovative setup enabling unprecedented accuracy in measuring the absorption coefficients of transparent materials. This new research, titled “Self-referencing photothermal digital holographic microscope for the characterization of low-loss liquids,” has been published in Applied Physics Letters, a prestigious journal covering a wide range of topics in the fields of Physics and Astronomy, which is well-regarded for sharing impactful research in applied science.
Currently, conventional methods for measuring absorption coefficients rely on UV-VIS spectrophotometers, but these struggle to accurately measure these coefficients of transparent samples. More sensitive alternatives include photothermal spectroscopy and the digital holographic microscope (DHM). In photothermal spectroscopy, a special type of light is directed at the material being studied, causing absorption. This absorbed energy leads to slight heating of the material, altering its interaction with light. When another beam of light passes through the material, it bends differently due to this temperature and refractive index change. By analyzing this altered bending, it is possible to determine the extent of absorption, offering insights into the material's properties. DHM, on the other hand, is composed of a glass plate, a probe beam, and a CMOS camera and it captures detailed images by examining how objects influence light.
In this new research, scientists introduce a “compact photothermal lateral shearing digital holographic device,” a combination of photothermal spectroscopy and digital holography. This innovative setup, complemented by an excitation laser, enables the precise measurement of absorption coefficients in low-loss solutions -- something that was not achievable with previous instruments. The resulting accuracy of this technique underscores its utility and wide-ranging potential applications. What adds to its appeal is that it combines high technology with affordability, making it stand out as a practical solution. “This method permits studying pollution in drinking water, rivers, and sea,” says Humberto Cabrera, first author of the paper and senior postdoctoral researcher at MLab, STI Unit, ICTP. Beyond its immediate applications, the measurement of absorption coefficients for semitransparent liquids holds great significance for the characterization and identification of the samples under study. This approach could prove valuable in diverse fields, including biology and medicine, as it enables the extraction of absorption coefficients of biological cells. “Compared to other methods, this can be used for real-time measurements, because it is very fast and it provides the results right away,” highlights Masoomeh Dashtdar, a co-author of the paper.
A Shared Affiliation
However, this is not just the story of a scientific publication. The five co-authors of this paper have a background that needs to be told. They come from diverse corners of the world, all united by their origins in developing countries and their participation in programs offered by the ICTP.
Humberto Cabrera, a native of Cuba, emerges as a pivotal figure in this story. A former senior Associate at ICTP from 2011 to 2017, he dedicated his efforts to maintaining an optics research program at ICTP, including providing support for ICTP's annual Winter Colleges on optics by directing the hands-on experimental sessions and the College itself in 2017. Today, he is a postdoctoral researcher in ICTP’s MLab, Science, Technology, and Innovation (STI) section.
Co-author Behnaz Abbasgholi-NA is a researcher hailing from Iran. Presently, she is a TRIL fellow at the NanoInnovation Laboratory of Elettra Sincrotrone Trieste. During her academic journey, she was enrolled in the ICTP’s Training and Research in Italian Laboratories (TRIL) programme in 2021, guided by Cabrera’s mentorship. The TRIL programme is a bridge between the Italian scientific sphere and developing countries.
Abdul Rahman, another co-author of this study, emerges as further evidence of this global collaboration. Hailing from Pakistan, he participated in ICTP’s TRIL programme in 2021, under Cabrera’s guidance. At ICTP's laboratories, Rahman found the opportunity to work on the experimental part of his PhD thesis. Presently, he is a postdoctoral fellow at the University of Szeged in Hungary.
Now we move on to India, where co-author Subhash Utadiya comes from. Pursuing his PhD at the Maharaja Sayajirao University of Baroda, he enriches his academic journey by joining ICTP’s Sandwich Training Educational Programme (STEP): he has the opportunity to study in his home country but has the financial support to visit ICTP. Utadiya has a co-advisor at ICTP, Humberto Cabrera, and a co-advisor at his home institution, Arun Anand.
Arun Anand is another co-author coming from India. He too engaged with ICTP’s TRIL programme and currently, he is a senior professor at Sardar Patel University, in India.
Finally, the last co-author is Masoomeh Dashtdar, an academic hailing from Iran. She was enrolled in the TRIL programme with Cabrera in 2022. Today, she is a professor at Shahid Beheshti University in Iran.
“We provide them the opportunity to be here and to develop their own ideas, supported by ICTP and importantly through a long-term partnership with SPIE, the International Society for Optics and Photonics, providing critical laboratory support through the joint Anchor Research in Optics Program. They can devise low-cost methods, as in this case. This way, they could return to their home countries and succeed in developing that specific technology there as well,” states Cabrera. “When they come back to their origin country, they come back with a higher level of knowledge. We are guiding the future of these people, giving them opportunities. This is the mission of ICTP.”
As Cabrera says, the mission of ICTP is to promote scientific collaboration and scientific development in developing countries, by providing advanced training, research opportunities, and exchange programs to scientists from around the world. These scientific papers, such as the one just published by these five authors, are indeed the tangible results of collaborations made possible by ICTP, a real bridge connecting advanced technology and developing countries.