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The wonderful connection between light and sound: How is the optical acoustic effect produced?
The optical acoustic effect, also known as the photoacoustic effect, describes the phenomenon of sound waves generated by light absorption inside a material sample. The phenomenon relies on changes in light intensity, whether it is periodically modulated light or a single flash of pulsed light. With a suitable detector, such as a microphone or a piezoelectric sensor, we can measure the changes in sound waves, i.e. the changes in pressure, and quantify them as photoacoustic signals. This measurement technique has proven its value in many applications, especially in analyzing the properties and concentration of samples, allowing scientists to draw precise absorption spectra and use intense pulses of light provided by lasers to detect trace concentrations. Increase sensitivity.
The study of photoacoustic effect not only has a profound impact on spectroscopy, but also plays a role in photochemical reactions and plant photosynthesis.
The phenomenon dates back to 1880, when Alexander Graham Bell first experimented with the photoacoustic effect and discovered that when a beam of light was rapidly interrupted, a solid sample would emit sound waves as it absorbed the light energy. At that time, Bell used his "photophone" to transmit sound over long distances; in an experiment, he observed that the interruption of a light beam would lead to the generation of sound waves, which inspired the later exploration of the photoacoustic phenomenon. Over time, the study of this effect has become more mature with the technological advances in sensitive sensors and strong light sources, and its application scope has become increasingly broad.
Physical Mechanisms
The main mechanism of the photoacoustic effect is the photothermal effect, which is the thermal expansion caused by the absorption of light by matter. The process can be roughly divided into several stages: first, the pulsed or modulated radiation of light is converted into heat energy; then, the temperature changes at the site of absorption; then, due to the rise and fall in temperature, the material expands or contracts, causing pressure. changes. These pressure changes can be detected by sensors coupled directly to the sample.
In the case of condensed phase samples, pressure changes are usually measured in the surrounding gas phase and these changes are extremely small due to thermal diffusion.
In addition to the photothermal effect, other mechanisms related to photophysical and photochemical reactions may also contribute significantly to the photoacoustic signal, especially gas production and molecular organization changes occurring during photosynthesis. This demonstrates that an understanding of multiple mechanisms is crucial when applying photoacoustic techniques.
Detection technology
In order to apply the photoacoustic effect, scientists have developed a variety of measurement modes. For gas samples or condensed phase samples, a microphone is typically used to measure pressure changes in the surrounding gas phase. In this case, the stimulus light source is usually switched continuously at a certain frequency, and the simulated photoacoustic signal is then analyzed using a lock-in amplifier. Measuring the pressure inside the condensed phase uses piezoelectric sensors mounted on the sample, which requires high-precision equipment to capture rapid pressure changes.
The characteristics of a photoacoustic signal depend on many factors, including the physical properties, mechanism of signal formation, and the light-absorbing material used. To make accurate measurements, researchers faced challenges in signal separation and time-dependency analysis, which required a combination of multiple techniques and methods.
Applications: From spectroscopy to life sciences
The photoacoustic effect has a wide range of applications. Photoacoustic spectroscopy is particularly useful in light absorption measurements of transparent samples because it overcomes the limitations of conventional spectroscopy and is directly related to light absorbance. This allows accurate diagnosis between both high and low concentrations of material. In the case of opaque samples, by placing the sensor above the sample for measurement, the signal obtained is directly related to the surface of the sample and its light absorption.
Currently, photoacoustic technology is used for targeted analysis and quantitative measurement of other macroscopic molecules such as proteins, showing great potential.
In addition, the photoacoustic effect also plays an important role in photosynthesis research, providing information about the effectiveness of converting light energy into chemical energy and the generation of oxygen. This allows scientists to gain mechanistic insights into photosynthesis and determine the health of plants through their responses to environmental changes.
Overall, the connection between light and sound waves continues to yield profound scientific insights that are changing the way we understand and study the natural world. With the advancement of science and technology, can future photoacoustic technology reveal more mysteries of the universe to us?
