Ultra sound operates on the principle of detecting sound waves reflected from surfaces. The ultrasound comes in two types, one measures the Doppler shift in the frequency due to structural movement and the other measures the time duration it takes for the wave to come back. Usual practice is to use ultrasonic transducers tuned at 5 to 18MHz. The new generation of ultrasonic sound generators can produce sounds with frequency as high as 10GHz. This is several times higher than the widely used 20MHz. The [tp lang=”en” only=”y”]latest technology innovations[/tp][tp not_in=”en”]latest technology innovations[/tp] keep pushing the limits of how sharp images we can achieve using sound.
Such ultra high frequency sound waves are produced with the help of lasers which can naturally operate in frequency range of several hundred gigahertz. Using laser pulses and nanostructures, scientists were able to produce ultra high frequency acoustic phonons. Phonons are quasi-particles consisting of vibrational energy and they can move through the atomic structure in form of sound waves.
The sound waves become shorter in duration as the frequency increases. Lower frequency implies a larger wavelength and such wavelengths can easily penetrate different objects and suffer less attenuation from object material. As we increase the frequency, attenuation also increases and there is more reflection from the surface. But going in to the ultra high frequency range, behavior is very different. Similar behavior is also exhibited by electromagnetic microwaves. Light has the limitation of not being able to penetrate objects and this has rendered it useless for imaging internal structures of material. Sound waves in GHz range find it easy to penetrate objects and the high frequency also enables high resolution imaging because of the sub pico second wavelength.
Phonons in such as frequency range can be pass through materials which are opaque to photons of same frequency. Such short wavelength will yield unprecedented resolution from the ultra sound imaging technology.
The laser light produces very short duration pulses of less than a pico second. The pulse excites the plasmons which then dissipates energy in form of heat. This heat causes expansion of the nanostructure and as a result generates acoustic phonons. The phonons are highly coherent in nature, meaning that they oscillate at a fixed frequency or an extremely narrow band of frequencies.
Ultrasound technology relies on the transducer as well as the detector. We talked about the transducer and now let us look at how detectors would operate. When electrons travel through any surface they produce wave which rolls out with the conduction on surface of metal. These waves are known as plasmons. By making phonons and plasmons interact with each other, complex phonon mode properties can be detected which will help in imaging the nano mechanical dynamics using polarization-resolved transient absorption spectroscopy.
In order to detect motion using this technology, the polarization of the transducer output needs to be changed. This is done by changing the symmetry of the nanostructure. The nanostructure appears in the shape of a Swiss cross. It is about 35 nanometers high, and has length of 120 nanometers and 90 nanometers for vertical and horizontal arm respectively. The two arms when made to oscillate in phase with each other, a symmetric mode of phonons is created and when both the arms oscillate out of phase, the mode produces is anti-symmetric.
Not only does ultrasound gel help guide the transducer during the procedure, it also improves the quality of the images the technician is able to see on the screen.