1
From Photoacoustic Effect to Laser-generated Ultrasound Technology

Ultrasound is a mechanical wave characterized by a frequency beyond the upper threshold of human hearing (20 kHz) and wavelengths typically falling within the millimeter to micrometer range. Methods for generating ultrasonic waves primarily include the piezoelectric method and the magnetostrictive method. The piezoelectric method represents the most widely adopted mainstream technology, operating on the principle of the inverse piezoelectric effect exhibited by piezoelectric crystals such as piezoelectric ceramics and quartz [1]. When subjected to a high-frequency alternating electric field, the crystal lattice of the piezoelectric material undergoes periodic deformation, thereby inducing mechanical vibrations. These vibrations are subsequently transmitted into the propagation medium via a coupling layer to form ultrasonic waves. Conversely, the magnetostrictive method relies on the periodic volumetric changes of ferromagnetic materials-including nickel alloys and ferrites-under an applied alternating magnetic field to excite mechanical vibrations [2].

Despite the established advantages of both piezoelectric and magnetostrictive approaches in ultrasound generation, devices based on electric or magnetic actuation generally exhibit limitations such as narrow operating frequency bands and susceptibility to electromagnetic interference. In recent years, laser-generated ultrasound (LGUS) devices, which operate on the photoacoustic effect, have gained increasing attention due to their unique advantages [3, 4]. As a novel approach for ultrasound generation, LGUS is governed by the fundamental principle of mutual conversion between light energy and acoustic energy. The key operational sequence comprises the absorption of light energy, followed by localized thermoelastic expansion, which ultimately leads to the emission of ultrasonic waves [5].

Figure 1.1 The development history from photoacoustic effect to LGUS.

LGUS devices are relatively straightforward to fabricate and possess notable characteristics including broad bandwidth, elimination of the need for direct electrical connections, and inherent immunity to electromagnetic interference [6]. These attributes confer immense promise upon LGUS within the biomedical field, while simultaneously underscoring its versatility for a broad spectrum of applications across diverse scientific and industrial sectors, particularly in scenarios demanding noninvasiveness, high-resolution imaging, or operation in electromagnetically sensitive environments [7].

To fully understand the technological trajectory of LGUS, it is necessary to look back at the origins of its fundamental mechanism. The photoacoustic effect, a distinct physical phenomenon that bridges optical and acoustic principles, has followed an evolutionary path spanning over a century. From its discovery in the late 19th century, through cycles of resurgence and theoretical development in the 20th century, to its active adoption in areas such as biomedicine in the 21st century, its progression illustrates both the persistence and complexity of scientific inquiry, as well as the dynamic potential of cross-disciplinary collaboration [8]. This chapter will track the chronological development of the photoacoustic effect, offering a systematic overview of its full historical trajectory (Figure 1.1) and examining critical innovations, technical evolutions, and expanding applications across distinct periods.

1.1 Discovery of the Photoacoustic Effect

The latter half of the 19th century was a golden era of rapid advancement in modern physics. Progress in electromagnetic theory, innovations in optical technology, and deepening studies in acoustics together established a robust scientific foundation for the eventual discovery of the photoacoustic effect. During this period, scientific exploration often grew from a dynamic interplay between serendipity and deliberate inquiry. It was within this context that the work of Alexander Graham Bell provided the direct catalyst for the emergence of the photoacoustic phenomenon. Motivated by the discovery in 1873 of selenium's photosensitivity-its electrical resistance varies with light intensity-he conceived the idea of using selenium to transform optical signals into electrical ones, thereby achieving wireless vocal communication [9]. After his groundbreaking invention of the telephone in 1876, Bell turned his focus to the possibility of transmitting speech using light [10, 11].

After two years of experimental effort, Bell and his assistant Charles Sumner Tainter succeeded in June 1880 in transmitting wireless voice communication over a distance of approximately 213 meters. Their apparatus, named the "photophone," consisted of a transmitter and a receiver [11]. In operation, sound waves induced vibrations in a glass mirror, thereby modulating the intensity of the reflected light. A selenium cell then converted this modulated optical signal back into an electrical signal, which was ultimately transformed into audible sound via a telephone receiver circuit. Widely recognized as the world's first practical wireless communication device, this invention also laid essential technical groundwork for the subsequent advancement of optical communications [12-14].

Of particular note is a serendipitous discovery made during the research on the photophone: when a solid material was irradiated with a rapidly interrupted beam of light, sound waves were generated at the same frequency as the light modulation [11]. Bell described this phenomenon in a letter to his father, and this observation represents the earliest recognized form of what is now known as the photoacoustic effect.

1.2 The Emergence of Photoacoustic Technology

Following Bell's discovery of the photoacoustic phenomenon, a considerable number of scientists dedicated themselves to exploring its fundamental mechanisms. In 1881, Lord Rayleigh introduced the "solid vibration theory," proposing that intermittent illumination induced non-uniform heating in diaphragms or plate-like structures within objects, resulting in vibrations that generated sound. This theory garnered endorsement from Bell [13, 14]. Concurrently, based on their experimental investigations, Mercadier and Preece advanced the "gas expansion theory," which attributed the origin of sound to the periodic expansion and contraction of air in contact with heated objects [15-17]. Subsequent contributions by Röntgen and Tyndall independently corroborated and further developed the gas expansion theory [18, 19]. Experimental evidence confirmed that the phenomenon also manifested in gases and vapors: Röntgen examined the photoacoustic effect in coal gas and ammonia, while Tyndall systematically studied various gases and vapors, even proposing that the effect held potential for detecting trace amounts of combustible gases in mining environments [19]. Bell further showcased the "spectrophone," an instrument capable of reflecting a sample's absorption spectrum, thereby establishing new pathways for spectral analysis [14]. However, constrained by limitations such as the lack of detection methods capable of precise quantification, research into the photoacoustic effect entered a phase of stagnation. It was not until the period spanning the 1930s to the 1960s that the effect underwent its first substantial revival, notably within the domain of gas analysis.

In the 1930s, Veingerov built upon the earlier work of Röntgen and Tyndall in the study of gas-phase photoacoustic effects [20], particularly emphasizing Tyndall's concept of "utilizing this effect for trace gas detection." In 1938, he introduced a photoacoustic gas analysis method: employing a capacitive microphone as the detector and a Nernst glower as a high-intensity blackbody infrared source, then he successfully measured the concentration of CO2 in a nitrogen (N2) matrix [21]. This achievement marked the formal transition of the photoacoustic effect from theoretical exploration to practical application. Over the subsequent years, related technologies progressed rapidly and eventually reached the stage of commercialization [22]. As the range of applications broadened, the theoretical framework of the photoacoustic effect continued to be refined. Beyond its use in gas concentration analysis, it was also applied to investigate gas vibrational relaxation rates and molecular energy transfer processes [23-26]. By the mid-20th century, however, the dominance of photoacoustic methods in gas analysis was supplanted by gas chromatography, owing to the latter's superior detection sensitivity.

The year 1960 marked a pivotal moment with the invention of the laser, which profoundly revolutionized the field of optics and, in turn, reinvigorated the research momentum behind the photoacoustic effect [27]. Lasers, characterized by their unique advantages-including high power, exceptional spectral purity, superior stability, and outstanding repeatability-provided an effective solution to the intrinsic limitations of conventional light sources in photoacoustic studies, such as inadequate energy output and suboptimal spectral properties.

The emergence of laser technology catalyzed the first major resurgence in photoacoustic gas analysis. In 1968, Kerr and...