If you play a guitar string, you will hear a note, whose tone will depend on the string diameter and on the string tension. This sound is not just made of a pure (“single”) vibration, which would sound rather ugly and boring, but rather to the overlap of several acoustic frequencies playing simultaneously, which makes the note “round” and pleasant. More than that: it makes the note unique. You will be able, in fact, to tell it’s a guitar and not a piano, or even which kind of guitar. The very same concept applies to the voice of people: you can easily distinguish two persons pronouncing the same sentence at the phone because they have a different tone. The ensemble of the vibrating frequencies that form the sound and the voice is called the timbre. In physics, we call it spectrum, and it not only applies to sound but also to light, indicating its various frequency components, i.e. its colors!
If you hit a molecule (which is a billionth time smaller than a guitar string) with a special hammer made of an extremely short light pulse, you can make it vibrate like a guitar string (but at a billion times higher frequency!). If you listen to the sound it makes (now you need a very special ultrasonic ear!), you can uniquely identify it and distinguish it from another molecule because it has a different oscillating pattern that we call vibrational spectrum (see figure). The basic concept, enabling very accurate chemical analysis of specimens, brings the name of its inventor Raman, Nobel laureate in Physics in 1930. Its recent implementation, using ultrashort laser pulses, is called coherent Raman process and enables higher sensitivity and thus higher acquisition speed, so that it is possible to apply it to microscopy. Coherent Raman Microscopy can visualize in real time the spatial distribution and concentration of several chemical species. Applied to biology, it can provide insights on the spatial arrangement of proteins, lipids, DNA, water and other constituents of cells. It is a so-called label-free technique, in the sense that it does not require any preparation of the sample by addition of fluorescent markers or other kind of stains, which could perturb the sample under examination or even contaminate it, altering the biological function. Furthermore, this technique does not require contact (it is stand-off) and it is nondestructive, because it employs infrared light, which is not absorbed by the sample, so that thermal load is minimized.
The European Research Council has recently financed the Politecnico di Milano in Italy to develop a next-generation microscopy system capable of rapidly visualizing in two dimensions the chemical content of a biological specimen using the coherent Raman process (see: www.vibra.polimi.it). If successful, the instrument will be applied to identify tumors in human biopsies with a higher accuracy and reproducibility than the actual practice (based upon the visual inspection of the tissue under a microscope and by the ensuing subjective opinion of a pathologist). In this way neoplasms can be located and their borders with normal tissue traced for surgery. This will pave the way towards future “virtual histopathology”: intraoperative non-invasive evaluation of cancerous tissue. The vision of this project is to allow researchers and doctors without a specific knowledge in lasers and optics to routinely visualize functional properties of cells and tissues in vivo.
Dario Polli is the Group Leader of the “VIBRA” project. He was born in Milan (Italy) on September 5th, 1976. In 2001 he graduated in Electronics Engineering at the Politecnico di Milano and in Physics Engineering at the Ecole Centrale de Paris (France). In 2005 he received a Ph.D. degree in Physics at the Politecnico di Milano. In January 2005 he became “Ricercatore di ruolo” (Assistant Professor) at the Physics Department of the Politecnico di Milano. Since November 2014 he is Associate Professor in Physics at the Politecnico di Milano. His research activity is mainly focused on the generation and characterization of ultrashort laser pulses and their application to time-resolved pump-probe spectroscopy and Coherent Raman microscopy. This activity is witnessed by the publication of more than 78 scientific papers on international journals.