ASIAN JOURNAL OF PHYSICS

                When I was offered the task to guest edit a special issue of the Asian Journal of Physics, dedicated to Prof Robert R Alfano, by the Editor-in-Chief, Prof Vinod Rastogi, I gladly accepted it. It is indeed an honour to do this job, largely because Prof Alfano was my doctoral supervisor and then, because of his tremendous contributions to the area of photonics. Let me begin by writing about Prof Alfano, who, of course, needs no introduction in the photonics community.

                Prof Alfano hailed from Teaneck, New Jersey from a family of Italian origin that believed in the transforming power of higher education. He went to the nearby T Dickinson University (TDU), planning to pursue electrical engineering. The boredom of tedious laboratory work pushed him towards physics, a subject requiring less laboratory hours. His natural liking of mathematics made him take a large number of mathematics courses, compensating for the liberal arts requirements. He excelled in physics and got a job in Sylvania, a part of General Telephone (GT), after his first rejection from a company that would become Verizon later.  At GT, he met several researchers, who went on to have a lifelong influence on him, the prominent one being Stanley Shapiro, with whom he discovered the supercontinuum at GT and obtained his first patent in 1974, out of the 129 patents he has to his name till date. Professor Alfano and S Shapiro published their discovery of the broad emission between 400nm and 700nm in Physical Review Letter (PRL) and subsequently published a series of PRLs dealing with self phase modulation and four photon coupling in glass.

                Prof Alfano is a pioneer in the applications of light and photonics technologies to the study of biological, biomedical and condensed matter systems. In the late 1970’s and early 80s, his research focus was on ultrafast laser pulses and production of picosecond pulses by mode-locking. His laboratory, which eventually became the Institute for Ultrafast Spectroscopy (IUSL), could boast of home-built picosecond Nd-Glass lasers, which, as graduate students, many of us handled in the 1980s. Schiller and Alfano studied the broadening of ultrafast pulses passing through a spectrograph using the super ultrafast temporal and spectral measuring tool, the streak camera. In fact, Alfano’s lab was literally “littered” with streak cameras at a time when it was almost unheard of in many labs.

                 The 1980s was the period when the IUSL was booming with activity. Alfano had already initiated his pioneering work in biomedical optics by developing the method and apparatus for detection of caries in teeth using luminescence in 1981. He held 14 patents in that decade with developments, ranging from methods to calibrate the streak camera, to developing picosecond gated light detection tube, to producing new Emerald Laser, Chromium Doped Beryllium Aluminum Silicate Laser Systems and in the area of biophotonics. The next decade of the 90’s began with the lasing action in Chromium-doped Forsterite, which brought in a new excitement in IUSL. The decade also established the era of biophotonics with patents on laser based cancer diagnosis. Prof Alfano became a pioneer in various domains of biophotonics. Use of fluorescence spectroscopy and lifetimes for cancer diagnosis, determining key wavelengths and signatures from native fluorophores, associated with tumor detection were his initiatives in the late 1980s and early 1990. The idea of utilising the property of specificity of Raman spectroscopy to distinguish the various types of atherosclerotic plaque was one of his initiatives. Subsequently, Raman signatures have been exploited as a biological diagnostic tool in the biomedical research arena. Analysing fluorescence spectroscopic data in biological tissue led to the understanding the effects of scattering and absorption. A series of path breaking research evolved, on imaging in random media and detection of hidden objects in highly scattering media, again initiated by Prof. Alfano. Synchronous fluorescence (Stokes shift spectroscopy or S3) is known to decouple overlapping fluorophores. Exploiting this for better diagnostics was a neat idea for complex systems like biological tissue. A natural offshoot of his seminal work on supercontinuum generation was to develop sources for techniques such as S3, which led to several patents. His pioneering work in 1981, using fluorescence spectroscopy to detect dental cavities, paved the path to non-invasive detection of cancer and ultimately created the vast research area of biomedical optics known as biophotonics. Optical Coherence Tomography, which is now a commonly used imaging instrument found in most ophthalmology clinics, is based on white light supercontinuum source that he discovered. Deep tissue imaging with multiphoton excitation, Resonance Raman based instrument, Stimulated Raman vibration microscope, spatial frequency spectrometer were all his patented ideas. His intuitive scientific prowess has given rise to several original contributions in photonics research, new discoveries and novel instrumentation. He has always worked in forefront areas, reflected by his recent work, where he has launched the use of quantum entangled photons for brain imaging. He coined words like ‘Optical Biopsy’ in the early days of his research in biomedical optics and subsequently, ‘snake-like photons’ while attempting to understand light propagation in turbid media. The area of biophotonics became a major research area of his Institute for Ultrafast  Lasers and Spectroscopy in the City College of the City University of New York, since 1982. Prof. Alfano can well be known as the ‘father of biophotonics’, pioneering the use of
light for disease diagnostics and cure.

                It is thus an honour for me to dedicate this issue to Prof Robert R Alfano. After accepting the task of being the Guest Editor, my first thoughts for contributions to this special issue, were to invite ex-students and well wishers of Prof. Alfano. The goal of this issue then becomes a representation of his work through the research developments of students he trained or his well-wishers and subsequently, the next generation in the area of photonics. The issue contains 15 invited articles by experts in photonics, spanning areas such as Biophotonics, Supercontinuum light, 2D materials in photonics, fiber optics, Nonlinear optics, high intensity laser physics, nanophotonics and optical tomography. It essentially offers a glimpse of a large area of photonics, especially focussing on the research initiated by Prof. Alfano.

                 The issue begins with an article on quantum entanglement for tissue diagnosis. The phenomenon of quantum entanglement originated from the famous EPR paradox (1935), where the effect was thought to be impossible, thereby pointing towards incompleteness of the quantum theory. However, the validity of quantum mechanics has since been established through experiments with polarization or spin of entangled photons, measured at separate locations. Light matter interaction is generally treated classically or semi-classically. The last few decades have seen the emergence of the use of superposition of quantum states, primarily for the purpose of conveying information. This has led to the growth of a new interdisciplinary field of science technology, known as quantum information, which does not focus on the physical medium but on its quantum preparation and evolution. The question that one asks is then the following: can decoherence of entanglement of two photons be used for the purpose of medical diagnosis? Prof Enrique Galvez and co-authors have elaborated on this technique and how it can be applied to tissue diagnostics, a forefront research development. Their work showed that in passage through thin slices of tissue, entanglement seems quite robust against decoherence. A second ongoing effort investigates whether entanglement can distinguish between healthy or diseased brain tissue.

                Enrique Galvez is professor at Colgate University, in Hamilton, New York, USA. His association with Prof Alfano has been for over 20 years. One of his students was a graduate student with Prof Alfano. In his words,

                “in 2015 I spent a sabbatical at City College, which became the genesis of the project involving my submission, on using quantum entanglement to diagnose brain disease. Since then we have been research collaborators”.

                An issue dedicated to Prof Alfano would be incomplete without an article on supercontinuum of light So the next article by Dr Lingyan Shi is a review on supercontinuum, which may interest readers, especially since it focuses on the applications of the supercontinuum source. Dr Lingyan pursued her postdoctoral research with Prof Alfano. In her words,

                 “It was such a fortunate event after I asked Dr Giovanni Milione a question about laser-tissue interaction, he took me to Prof Alfano’s office and told me: “This Professor is extremely knowledgeable and will give you a good explanation.” At that time, I was a second year Ph D student at City College of New York, where I had the opportunity to continue discussions with Prof Alfano’s on various optical imaging problems and later become a postdoc in his Lab to explore multiple biophotonics applications. I was lucky to have seen and experienced the diligent and creative daily research activities at IUSL under Prof Alfano’s leadership. His vision and insightfulness, and the environment he created made it possible for all of us who worked under him to ‘go beyond’ and ‘dig deep’. He gave every one of us the opportunity and “led us to water”, and using his own words: “You drink it!” He highly impacted on my career path in academia. His curiosity, passion, creativity, and hard-working will always positively influence us and encourage us to explore and go beyond.”

                Biophotonics research in Prof. Alfano’s laboratory began as a hobby and a high school project of his daughter. However, in the next 20 to 30 years, it became the major research area of IUSL. Thus it is not surprising that in this issue one third of the articles deal with light based research on biological systems. The first article on Biophotonics deals with fluorescence spectroscopy for solid tumor detection. As mentioned earlier, Prof. Alfano and his group have worked extensively on tissue diagnostics, using fluorescence, Raman and elastic scattering techniques. This paved the way for biophotonics research in diagnostics of various diseases. Fluorescence spectroscopy, due to its sensitivity, is a preferred tool utilised by several researchers. In the very early phase of this work, the focus was on determining the dominant native fluorophores and the key excitation wavelengths for diagnosis. Over the past 30 years the research has moved towards developing novel techniques for early diagnosis, instrument designing and fabrication for clinical testing and development of classification algorithms. However, the precision of diagnosis is still an area which is quite important and the article by Laura Sordillo and Peter Sordillo deals with determining the aggressiveness of solid tumors of the breast by correlating with the contributing native fluorophores. The study is performed using a portable, miniature, easy to use UV LED ratiometer unit, built in Prof Alfano’s lab. Laura Sordillo was a graduate student of Prof Alfano is continuing as a Research Assistant Professor at the Institute for Ultrafast Spectroscopy and Lasers. She published many important papers with Professor Alfano, including the use of the short wavelength infrared (SWIR) windows for biomedical applications (including the brain, cancerous tissue and bone fractures). They are now investigating the existence of quantum effects in photosynthesis and in the human brain using ultrafast light techniques. 

                An offshoot of fluorescence spectroscopy is the Stokes Shift Spectroscopy (S3) or synchronous fluorescence spectroscopy, where, if an offset between the excitation and emission wavelengths matches the difference between the absorption and fluorescence peaks of a certain fluorophore, it produces a sharp peak. This brilliant technique, introduced in 1971 helps decouple contributing fluorophores in a multicomponent system, making it both a specific and a sensitive tool. Prof. Alfano was again instrumental in bringing this method to the forefront for understanding complex tissue fluorescence in disease development. Interestingly, this issue contains three articles dealing with synchronous fluorescence, displaying the recent applications of S3 in biophotonics and nanophotonics. One such novel study is by Prof Masilamani, in using this technique to test the presence of purity of a quantum dot of a certain size by ascertaining the presence or absence of a quantum dot of a different size whose signature spectrum shows up in the synchronous spectrum. Prof Masilamani has just retired as distinguished professor from King Saud University, Riyadh KSA. He had been following Prof Alfano’s work since 1980, when he was a graduate student at IIT Madras, India. In his words,

                “I was fascinated  and longing to work with him for a long time and finally, I had the opportunity to spend three months in his Lab in New York  on time-resolved spectra of benign and malignant prostate tissue.  All through the time, he was very cordial, friendly and easy to access. He had a soft corner for all those coming from developing countries and who were able to access the very modern advanced facilities because of his broad mind. For me, he is a fine scientist and a cultured gentleman. I have cherished memories of New York because of him.”

                The next article is a short review on 2D semiconductors, which are increasingly becoming important in photonic applications as optical or optoelectronic devices. Over the past decade, graphene has been extensively studied as a 2D material for such devices. Currently, transition metal dichalcogenides (TMDs) are being investigated for 2D semiconductor based optoelectronic devices due to their remarkable linear and nonlinear properties. Nanophotonics based investigations, to understand the strong light matter interactions in such materials, is the focus of this review by Prof Vinod Menon. Prof Menon joined CUNY as part of the Photonics Cluster hires that was initiated  through the vision of Prof Alfano back in 2004. In his words

                “ In my early days as an assistant professor, I greatly enjoyed the photonics meet ups he organized and the support he provided to junior faculty - and I was one of them. Prof. Alfano has played a key role in putting CUNY on the photonics world map and this is another aspect that has benefited all of us in the field at CUNY. Finally, as a colleague and collaborator, he is great to work with and is always bubbling with new ideas. I consider myself lucky to have a colleague like him at CCNY!”

                The next article is again based on supercontinuum, dealing with one of the processes responsible for its generation, self phase modulation (SPM). As mentioned earlier, supercontinuum generation has been investigated in various nonlinear media and seen to be better controlled in optical fibers. Wider spectral broadening of SPM in optical fibers can be generated using picosecond laser pulses. The fine structures observed in the wide SPM spectra have garnered a lot of interest. In this study, the authors, Dr Q Z Wang and Prof P P Ho reviewed the theoretical and experimental measurements of SPM spectra generated with picosecond laser pulses propagating in optical fibers. Both authors have worked for several years with Prof. Alfano, Dr. Wang as a graduate student and Prof. Ho as a collaborating researcher in IUSL.

                Another of Prof. Alfano’s 57 Ph.D students, Dr B B Das, who has contributed to the Biophotonics research from its infancy has written a short review on the applications of nonlinear processes in biological systems. In the article, he has provided a flavour of second harmonic generation and other multiphoton processes in imaging of biological tissue, beginning with an introduction to the basics of nonlinear processes. Having been closely associated with Prof Alfano for over 25 years, he is an ideal candidate for writing about his contribution to nonlinear processes in biophotonics. Dr Das has provided lively descriptions of the work at IUSL on SHG imaging in cancer detection, two-photon excitation imaging of tryptophan distribution and an interesting study on Alzheimer’s disease time resolved fluorescence using multi-photon excitation. Combining these two powerful techniques, Dr Das compared the decay times of one and two photon processes to understand the underlying physical phenomenon. With his long association with Prof Alfano, Dr Das writes,

                 “Professor Alfano has a child-like curiosity about various scientific problems with a matching tenacity to pursue it incessantly once he makes up his mind. Whenever I send him a manuscript around midnight the reply with corrections would be back before early morning. Many times I wondered if he slept at all. I have never seen him saying no to an undergraduate student looking for research experience in his lab. He is an institution at CCNY.”

                The subsequent articles were by researchers who have done forefront work in photonics and others who are ‘the second generation products’ of Alfano!

                Prof Ravindra Kumar needs no introduction to the photonics community, having contributed enormously in the area of high intensity laser matter interactions which create plasmas of mega ampere currents in the laboratory. Such plasmas generate the highest terrestrially available magnetic fields. Studying the mega gauss magnetic fields helps understand the complex physics involved in ultrafast light and the electron transport properties inside the plasma. Prof. Ravindra Kumar and his team have used two-colour, pump-probe reflectivity and Doppler spectrometry on the critical surface of hot, dense laser produced plasma to understand its rapid motion and generation of shock waves. Crucial information is gathered from the temporal studies of the mega gauss magnetic fields, comparing such results with measurements using low contrast pulse. A final note by the authors on generation of an efficient supercontinuum laser probe is to obtain information of the critical density surface at various depths that shows the influence of Prof. Alfano’s seminal work in various applications. Prof Ravindra Kumar mentions,

                 “I never met Prof Alfano in person either in India or abroad, but  to me the influence of his work in many areas of NLO was evident once I started  working for a Ph D. His work on supercontinuum  was of course path breaking and  spectacular but I came across his papers in other areas of nonlinear optics too and was grateful to learn from those. My actual areas of research  did not overlap with his but in my teaching his work was very useful.”

                An issue on photonics would be incomplete without an article on photonic crystals which are extensively studied due to their amazing property of being able to control light. An illuminating article by Prof. H.Wanare, describes how light flow is controlled by weak perturbations by adding a very small dielectric rod or impinging a femtosecond pulse, which alter the refractive index locally. Weak perturbations are compared with strong ones and various spatial modes are studied through numerical simulations using the finite difference time domain tools. Prof Wanare has been greatly influenced by Prof Alfano’s work during his graduate and postgraduate days and in his words,

                 “His pioneering studies on time-resolved imaging in random media were extremely exciting to us. These studies attracted us immensely as we were developing then (late 1990s) the split-operator technique to solve Maxwell’s equation is exactly in random media. It was inspirational to test for ourselves the validity of the diffusion approximation that he had talked about in the early 1990s.”

                Globally, a significant amount of time has been invested in nanophotonics due to its long term implications in revolutionizing technology. Fabrication of nanoparticles and other nanostructures is thus an area of great interest. Prof. Narayan Rao’s article in this issue deals with nanostructure fabrication, specifically Laser Induced Periodic Surface Structures. Fabrication is done on different metals, using a simple and single step experimental technique of Laser Direct Writing (LDW). An analysis based on the decay lengths of surface Plasmon polaritons is performed to find out which metals can be potential candidates as perfect absorbers by suppression of total and specular reflections over a large wavelength regime.

                The next 5 articles are devoted to Biophotonics, an area that Prof Alfano has pioneered since early 1980s and still continues to bring in novel ideas to the domain with his keen insight. The ‘next generation’ Biophotonics researchers are also working on innovative ideas as can be seen in the next article by Dr Nrusingha Biswal. His team has worked on a neat idea of utilising the imperfections in optical filters to extract the absolute phase lag maps of biological tissue in frequency domain measurements. This has led to improvements in radiative transport model based reconstructions.

                Use of nanoparticles in biomedical applications, majorly in sensing and drug delivery, is a well established area of research. Dr Sharad Gupta’s article here discusses a simple chemical technique of synthesizing ZnO nanoparticles using tulsi (holy basil) extract as a reducing and stabilizing agent. This green chemistry technique was found to produce pure, crystalline, spherical nanoparticles, with uniform size distribution and biocompatibility.

                An area extensively researched is noninvasive disease diagnostics through optical spectroscopy. Over the last 35 years, studies have been conducted on human tissue, on different organs in patients to detect signatures of the disease. However, very little attention has been given to body fluids, which can reflect similar changes as in tissues and yet be completely noninvasive. Two of the next few articles focus on this aspect. The article by Prof. Ganesan investigates through fluorescence and excitation spectra, the efficacy of blood plasma as a diagnostic medium for detection of different stages of cervical cancer. A statistical analysis based on linear discriminant analysis showed that blood plasma is a potential candidate as a diagnostic medium.

                The fourth article on tissue diagnostics by Dr Ebenezer Jeyasingh uses synchronous fluorescence spectroscopy (SFS) to examine early tissue transformation in DMBA treated mouse skin carcinogenesis models. With a ratio mapping algorithm, the study indicates that early transformations can be detected by SFS.

                The final article by Pavan Kumar and Asima Pradhan exploits the specific nature of SFS in probing another body fluid, saliva for early developments of oral cancer. The efficacy of saliva as a diagnostic medium is displayed by comparing the results with those of oral tissue SFS.

                As can be seen now, the fifteen articles provide a flavour of the various fields in photonics and laser matter interaction.

                I am particularly pleased and grateful to have the foreword written by Peter Delfeyett, Pegasus Professor and Trustee Chair Professor of Optics, EE & Physics in CREOL, The College of Optics and Photonics, University of Central Florida who is currently serving as the Director of the Townes Laser Institute. Peter was a graduate student at IUSL and so it is appropriate for someone who has had a long association with Prof. Alfano to write the foreword.

                I am also extremely grateful to Prof G P Agrawal, James C Wyant Professor of Optics of the Institute of Optics at University of Rochester, NY, USA and Prof Arjun Yodh. James M Skinner Professor of Science at the University of Pennsylvania (Penn) and the Director of Penn’s Laboratory for Research on the Structure of Matter (LRSM) and its NSF-Supported Materials Science and Engineering Center (MRSEC), for sparing valuable time from their other commitments (due to which they could not contribute articles) to write two pleasant letters of appreciation on their association with Prof Alfano.

                I would also like to thank all the authors and co-authors for their efforts in making this special issue possible. I am indebted to them, more so because of the terrible times we are in this year. They could manage to write articles in State of the Art subjects in photonics, which will surely interest readers, especially during a period when students and staff could assist only remotely. I am quite certain that readers will benefit from these insightful articles.

                Finally, I would like to thank the editorial team for their tremendous efforts during this period. I am grateful to Er Manoj, Ms A Singh and Ms. Laxmi for their tireless work, day in, day out. Last but not the least, a big thanks to Prof Vinod Rastogi for being a pillar of support in this debut editorial venture of mine.

Asima Pradhan