February** Dynamic Incentive Program
An article published in Physical Review Letters** implements a new cooling mechanism that can cool sound waves in an optical fiber to 219 K below room temperature. This is a very remarkable achievement, as it not only demonstrates the powerful potential of the photoacoustic effect, but also opens up the possibility of exploring the quantum behavior of macroscopic objects and developing new quantum technologies. So, how do they do it? Let's take a look.
The photoacoustic effect, or Brillouin scattering, is an interactive phenomenon between light and sound waves. When a beam of light passes through a medium, it is affected by the sound waves in the medium, resulting in scattered light that varies in frequency and direction. This process can be seen as scattering between photons and phonons.
According to the principle of conservation of energy and momentum, the scattering between photons and phonons can be divided into two cases: one is that the photons lose energy and the phonons gain energy, which is called Stokes scattering; The other is when the photons gain energy and the phonons lose energy, which is called anti-Stokes scattering. Both of these scattering results in a fixed difference between the frequency of the scattered light and the frequency of the incident light, and this difference is the frequency of the phonon, also known as the Brillouin shift. The magnitude of the Brillouin frequency shift depends on the nature of the medium and the speed of the sound wave, and is generally in the range of tens to several gigahertz.
We know that temperature is a measure of the average energy of the molecular motion of a substance, and the higher the temperature, the more intense the molecular motion and vice versa. Sound waves are the propagation of molecular vibrations, the energy of sound waves is the energy of molecular vibrations, and the frequency of sound waves is the frequency of molecular vibrations. Therefore, if we want to cool the sound waves, we need to make the sound waves lose energy and reduce the frequency. So, is there a way to have sound waves and light waves interact to achieve sound wave cooling? The answer is yes, and that is to use the anti-Stokes scattering in the photoacoustic effect.
We can imagine that if there is a laser beam of very high intensity, propagating along an optical fiber, there is a special structure inside the fiber that can produce ultrasound waves. In this way, the laser will be scattered by Stokes with the ultrasonic waves, and a part of the energy of the laser will be transferred to the ultrasonic waves, resulting in a lower frequency scattered light. This process is equivalent to the laser "pumping" energy to the ultrasound, so that the energy and frequency of the ultrasound increase. However, if we adjust the frequency of the laser to a little lower than the frequency of the ultrasound, then the Stokes scattering will become anti-Stokes scattering, and the laser will absorb some energy from the ultrasound, resulting in a higher frequency scattered light. This process is equivalent to the laser "extracting" energy from the ultrasound, reducing the energy and frequency of the ultrasound. In this way, we achieve the goal of cooling ultrasonic waves with the photoacoustic effect.
Of course, this process is not a one-time success, because the interaction between the ultrasound and the laser is very weak and requires many repetitions to achieve noticeable results. And we also need to take into account the loss and noise in the fiber, as well as the heat exchange between the ultrasonic waves and the environment, which can affect the efficiency and limits of cooling. Therefore, in order to achieve effective ultrasonic cooling, some technical optimization and innovation are also needed.
The authors of this article did just such an experiment, using a 50-centimeter-long tapered photonic crystal fiber as the medium of the photoacoustic effect. A photonic crystal fiber is a special type of optical fiber that has a periodic structure in its core that selectively transmits and reflects light and sound waves. In this way, the photonic crystal fiber can be directed simultaneously to light and sound waves, thereby enhancing the intensity of the photoacoustic effect.
They used a 1550-nanometer laser, which was used as a pump light, along one end of the fiber, and with an optical modulator, they added a 10 to the laser8 GHz frequency offset as a condition for anti-Stokes scattering. They used another 1550-nanometer laser beam as the detection light, which was input along the other end of the fiber, and with an optical detector, they measured the scattering spectrum of the detection light, thus obtaining the frequency and energy of the ultrasound waves. They used a piezoelectric crystal, which acts as an exciter for ultrasound, to produce 11 gigahertz of ultrasound by applying periodic pressure to the optical fiber, which serves as a target for cooling.
The results of their experiments showed that when the power of the pump light reached 1At 5 watt-hours, the frequency of ultrasound is reduced from 11 gigahertz to 108 gigahertz, the energy of ultrasound from 05 nanojoules reduced to 01 nanojoule, the temperature of the ultrasonic wave is reduced from 300 K to 81 K, enabling effective cooling of the ultrasonic. They also demonstrated the feasibility and stability of this cooling mechanism, as well as the dependence on the fiber structure and laser parameters, through theoretical simulations. They point out that the extreme temperature of this cooling mechanism depends on the thermal noise and photon rate in the fiber, and can theoretically reach below 1 K or even close to absolute zero.