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Photophysics and photochemistry of light harvesting complexes in the vicinity of optical antennas

Periodic Reporting for period 1 - NanoLight (Photophysics and photochemistry of light harvesting complexes in the vicinity of optical antennas)

Reporting period: 2015-09-01 to 2017-08-31

Many physical processes cannot be fully understand when we are probing huge amounts of the chemical or biochemical substances. In some situations, the problem can be solved much easier if the process would be studied on only one molecule level. For example, when the same process occurs in billions of adjacent molecules, but its beginning is shifted in time a little bit in every molecule, then what we see looking at a large amount of substance is averaged and in many cases impossible to figure out how this process starts and how it ends. Such problem can be solved in two ways: by preparing all molecules to start the process in very the same moment of time; or by looking at many molecules one by one but at only one molecule at a time. This second option has more advantages because even if all molecules start a process at the same time, their slightly different environment can cause that after a while they will behave completely different and we will end up at square one. Furthermore, understanding of the process is the first step toward its optimization and application in technology. Therefore, in this project, we decided to optimize one of the optical methods (so-called surface-enhanced coherent anti-Stokes Raman scattering, SE-CARS) looking at single molecules and solve some technical problems related to the application of ultrashort laser pulses for this purpose.

The methods developed in the project can be applied to for studies of chemical reactions, especially those of high importance for medicine and technology. At the same time, further development of optical single molecule detection will allow developing much more efficient detectors of minimal amounts of chemicals, with sensitivity down to just single molecules. That can have extream significance if one wants to detect very slight amounts of danger or unknown substances.
The overall scientific objective of the project was to expand the level of understanding what is happening when chemical molecules located nearby the metallic nanoobjects are illuminated by laser light. Such a metallic nanoobject (about 1000 times smaller than the human hair thickness) acts as an optical antenna. Commonly used in TV, radio, and smartphones antennas are the tools allowing to collect and radiate the electromagnetic energy efficiently. Optical antennas do the same but with electromagnetic waves of optical frequencies (visible light, near infrared or ultraviolet light). Light collected by optical antennas is concentrated to tiny spots, just ten times bigger than the size of the atom and comparable in size to the size of many chemical molecules. Such strongly concentrated energy can be used to generate a response of the chemical molecule which again can be enhanced by the antenna and detected much easier than the similar response of the same molecule without antenna (or located far away from the antenna). For some kinds of optical responses, like so-called Raman scattering, the excitation and registration of the signal can be boosted by more than billion times. The studies performed in the project helped to understand this process and optimize optical antennas for single molecule detection with the help of optical antennas and with ultrashort laser pulses.
Our work during the project was focused on the optimization of the optical detection by Raman scattering process enhanced by optical antennas for both continuous and pulsed laser excitation conditions. Therefore, we tried to develop more efficient optical antennas and controlling of these antennas itself. The efficiency of the optical antenna depends on the number of parameters, like its size, shape, material, and the local environment. It also strongly depends on the orientation of the optical antenna relative to the chemical molecules and the laser light used for generation of Raman scattering signal by the molecule. We have studied the efficiency of different antennas (different shapes and different environment) experimentally and by computer simulations. Mainly we were focused on antennas working with near-infrared light, due to some advantages over visible range, as the much lower probability of interference of the Raman scattering signal by other molecules which can fluoresce when they are excited by visible light.

On the other side, the equipment and methodology of generating optimal illumination were developed. Particularly, two different methods were tested, based on laser source continuously emitting light and more difficult but providing much more possibilities, ultrashort laser pulses. The first method allows to detect and identify single molecules, but it can be used to watch only very slow processes (on the scale of milliseconds or longer). The method based on pulsed lasers can be used even for observation of very fast processes like chemical reactions.

Finally, the directivity of the Raman scattered light enhanced by single optical antenna was studied. The results show strong correlation of the directionality of light scattered by molecules located in the vicinity of the optical antenna and the light scattered by the antenna itself. That means, we can further improve the detectability by the proper designing of the antenna shape and its orientation.
The single molecule detection based on Raman scattering process with near infrared excitation was developed. The results can be applied immediately in the field of ultrasensitive optical detectors of chemicals. In long perspective, the project results can be utilized for understanding and control of chemical reactions, end then resulting with more efficient technologies of producing the chemicals. That will allow saving resources, what will translate not only into lower prices of products but also into the much lower impact on the environment.