The group uses photothermal techniques and electron spin resonance to investigate properties of materials and various physical phenomena - from the conduction of electrical current or heat conduction to biophysical phenomena as photosynthesis. Academic studies and applied investigations (production of electronic devices, for example), are both developed in the group.
A common way to explore the physical properties of new materials – i.e., to characterize them - is to focus laser light on them and see what happens. The laser may, for example, be partially reflected, or - if the sample is sufficiently transparent - to be transmitted. Furthermore, the material may undergo heating, and these phenomena contain important information about the material.
Photothermics is a technique that analyzes the heating of a material caused by light (laser) incident on it, in order to characterize it.
An advantage of this technique over the observation of reflected or transmitted light by the sample is that the reflection and transmission are greatly affected if the material surface is highly irregular, or if it is in powder form. This does not happen with the produced heat, which makes photothermics quite appropriate for these cases.
In fact, there is a set of photothermal techniques, since there are several ways to observe the heating of the sample. The idea is not to observe it directly, but its effects on the material and the surrounding environment. And there are many effects that we can observe: acoustic waves generated by the sample, deviation of light passing near it ("mirage effect" and "lens effect"), changes in their ability to reflect light and conduct heat, changes in dielectric constant.
One of the interesting effects for scientific research is that the heating of the sample also heats the surrounding air - which then expands. If the laser striking the sample is interrupted several times quickly to make it blink, the air will expand several times in sequence, producing a series of "pulses" that propagate through space as acoustic waves. This is nothing more than a sound wave, which can be captured by a microphone.
The situation is analogous to that of a circular wave that propagates along the surface of the water of a swimming pool when one dips a finger in it. If we plunge the finger several times, multiple circles will grow in sequence - a wave in water. In the case of photothermics we have an acoustic wave through the air, and the analysis of these waves with appropriate equipment and software reveals important properties of the material. This technique is called photoacoustics.
Photoacoustics can be used to investigate the samples through the heat produced not only by the incidence of laser, but by various other forms - for example, by the presence of magnetic fields (magnetocaloric effect) and also through the photosynthesis of the plants, in studies of biophysics of the plants, which has also been done by the group (see figure below).
Another possibility is to observe the temperature variation in the sample at a distance from the heating point. Thus, one can study the propagation of heat - and the characteristics of the sample related to the conduction of heat. It is a field called thermal characterization.
Mirage effect and lens effect
You can also pass a second laser (other than the laser used to heat the material) skimming the sample surface (laser shown in red in the figure below). This laser is called "probe" (the first one is called "excitation"). The variation of the air density around the sample - because of the heating caused by the excitation laser - causes a slight deviation in the probe laser. This is called "mirage effect", because the mechanism is the same as mirages in the desert. A detector measures the deviation of the probe laser and, and from these measurements one can determine properties of the sample. An appropriate configuration of the experimental apparatus can also explore the probe laser deviation to produce a "lens-like effect".
The magnetocaloric effect and magnetic resonance imaging
The group also associates photothermics with electron spin resonance technique. In the magnetic resonance technique, the sample is placed in a magnetic field that increases slowly. The spins of electrons in the material align with the applied magnetic field (like a compass needle aligns with the Earth's magnetic field). At the same time, the sample, which is in a cavity, is irradiated with microwaves. For a given value of the magnetic field, microwave absorption takes place, with a consequent change in the alignment of the spins of the electrons, and this is detected by the equipment. The value of magnetic field for which this change occurs depends on the arrangement of electrons in the atoms forming the material. This technique is similar to magnetic resonance tomography apparatus used in medicine - but the later deals with the interaction between magnetic field and the nuclei of atoms instead of the interaction between magnetic field and electrons.
The group has been using magnetic resonance with photoacoustics in the investigations of magnetic materials, studying mainly the magnetocaloric effect (see figure below). Several materials are heated when subjected to a magnetic field. Again, the sample is placed in a constant magnetic field, and a small modulation is added to it (a second magnetic field that is oscillating in time). The heating will occur at the frequency modulated, and, in the same way that in photoacoustics, a microphone placed in sealed tube will detect the pressure changes induced by heat (acoustic waves).
Collaborations with other research centers
All these studies are used to characterize materials from the standpoint of academic and also within the scope of applications, both in genetic improvement of plants, or in the production of electronic devices etc. In the latter case, photothermics allows to study the performance of devices - transistors, solar cells, lasers - at the same time as they are running.
In the case of the study of lasers, for example, the industry's interest is based on the fact that, if these devices are too hot, they may degrade and fail to function properly. And there are cases where you need to ensure that they work well for a very long time (optical amplifiers placed on the seabed for the transmission of information through optical fibers).
The interaction with INPE - There was also collaboration with INPE to study solar cells (devices that transform sunlight into electrical energy). Members of the group studied the layers of adhesive used to glue the glass shield of the cells used in solar panels for satellites, with which they collect the sun's energy to work (see figure above).
In this case, what was observed was another parameter, other than those already mentioned, the pictures produced by the interference of light reflected by the multilayered material. The adhesive and protective glass form thin layers, and the incident laser is reflected in different interfaces. Therefore, the "total" light reflected by the adhesive is a superposition of each of these reflected beams. These various "components" interfere with each other, forming figures with light-dark or colored patterns (see the figure above the bubble).
It turns out that the interference pattern changes with the variation of the sample temperature, since the temperature changes a number of sample properties, including thickness of each layer. The proper interpretation of the variations of the interference figures reveals important information about the properties of the material.
History of the group
The group was founded in 1980 by Prof. Helion Vargas, who worked until then, in what is currently the Group of Optical and Magnetic Properties of Solids. In the 1970s, research in solid samples with photoacoustics had effervescence in the world. Vargas spent time in England researching this subject and brought it to the IFGW.
At the beginning of the group, the main line was photoacoustic spectroscopy and electron spin resonance. Therefore, researches were diversified by other areas of photothermics: mirage effect, thermal lens effect, thermal characterization, thermal reflectance, magnetocaloric effect. Research also included collaborations with centers and companies producing electronic devices and other samples.