Abstract
A brief historical account of photoacoustic (PA) effect is followed by a simple mathematical model for the generation of PA signals due to nonradiative transitions in atoms and molecules. Some experimental setups, with microphone and piezoelectric transducers, are described for recording PA spectra of gaseous, solid, and liquid samples. Applications of PA spectroscopy in the investigation of harmful chemicals are presented with illustrative examples. The principle of photoacoustic imaging (PAI) is discussed along with examples of molecular imaging of biological tissue and internal organs in small animals.
Keywords
- photoacoustic effect
- nonradiative decay
- microphone
- piezoelectric transducer
- photoacoustic cell
- aerosols
- explosives
- actinides
- hyperspectral imaging
- molecular imaging
- photoacoustic microscopy (PAM)
- photoacoustic tomography (PAT)
1. Introduction
Photoacoustic imaging (PAI) is a novel method of obtaining spectral images of chemical constituents of a sample or a scene, to gain valuable insight into its structure and dynamics. It is based on the technique of photoacoustic spectroscopy (PAS) and covers the entire spectral range from the ultraviolet to the infrared. When light is incident on a sample, photons can be either absorbed, transmitted, or reflected, and the PAS technology focuses on the amount of absorption and its subsequent release as heat. PAS is an extremely sensitive detection technique as it can detect molecular concentrations below the parts per billion (ppb) level. This technique emerged from the discovery of the photoacoustic effect by Graham Bell in 1880 during his attempt to transmit sound over a beam of sunlight [1]. However, it remained dormant for almost a century until the advent of tunable lasers in the 1970s and was successfully used by Kreuzer and Patel for the trace detection of atmospheric pollutants [2].
1.1 Photophone and the spectrophone
Bell succeeded in wireless audio communication about two decades before the radio transmission. He used the newly discovered selenium cell in the receiver in view of selenium’s property to react to modulated intensity of sunlight incident on it, as the resistance of selenium crystal depends on the incident light. A flexible mirror was attached at the speaking end of the photophone that created slight deviation of the beam of light reaching the receiver end. This led to variation of intensity at the selenium receiver, which acted like an optical version of the electric coil in the telephone receiver, converting the intensity modulated light back into sound. Bell performed many experiments and observed that sound waves were also produced directly from a solid sample when exposed to a periodically modulated beam of sunlight as illustrated in Figure 1. A hearing tube, whose other end was tightly attached to the open end of a transparent glass test tube with sample placed at its closed end, could be used as a photophone. When a beam of sunlight focused on the sample was rapidly interrupted with a rotating slotted wheel at an audible frequency, the intensity of sound in the hearing tube was dependent on the type of material. The loudest sound was heard when the sample was carbon black, leading to the conclusion that photoacoustic effect was caused by the absorbed light energy which subsequently heats the sample.
During Bell’s visit to England in 1880, John Tyndall performed the photoacoustic experiment in gases, and although the photoacoustic effect was confirmed, Tyndall was of the view that it was caused mainly by the radiant heat [3]. Bell was driven by rare intellectual curiosity to learn, and it led him to invent the spectrophone to find out the wavelengths that were more efficient for the radiant heat [4]. For this purpose he converted a prism spectroscope into a spectrophone by replacing the eyepiece of the telescope with a hearing tube in which a thin wire mesh coated with lampblack was fitted in the position of the cross wires (see Figure 2). When the incident sunlight was interrupted by a mechanical chopper, the hearing tube produced sound whose frequency was equal to the periodic intermittence of light. The loudness of sound, however, varied in accordance with the intensity of the solar spectrum, being maximum in the green-yellow region and decreasing at both the red and violet ends, and observations were made by fixing the position of the telescope in different spectral regions of the solar spectrum. These observations confirmed the fact that the photoacoustic effect is due to optical absorption, since lampblack totally absorbs light at each wavelength. On the basis of his observations, Bell made the prophetic statement about the great importance of photoacoustic spectroscopy in the infrared.
The interaction between light and matter giving rise to photoacoustic effect has three distinct features. (1) The absorbed energy of optical radiation is converted into heat. (2) At the site of optical absorption, there is a temporal rise and fall of temperature. (3) The expansion and contraction following these temperature changes lead to periodic pressure variation to generate sound.
2. Nonradiative transitions and PA signal generation
The heat generation following optical absorption is caused by internal motions in molecules or those of the matrix in which atoms are imbedded in condensed matter. In the quantum mechanical description, an excited molecular state reached by the optical absorption has two channels of relaxation. The radiative decay leads to optical emission, whereas the nonradiative decay causes heating. Thus a molecule, optically excited to a vibronic or a rovibrational state, loses a part of its excitation energy as heat leading to the photoacoustic signal. The photoacoustic spectrum is similar to the absorption spectrum, but its intensity at the exciting wavelength is proportional to the product of the absorption coefficient and the probability of nonradiative decay of the excited state.
Figure 3 shows the energy level diagram, in the Born-Oppenheimer approximation, of a typical organic molecule with an even number of electrons where total internal energy
2.1 Generation of photoacoustic signal in gases
The rate of radiative transition
where
Suppose the number density of molecules in the ground and excited states of Figure 4 is
We define the radiative and nonradiative lifetimes to be
In a similar manner, we find the following expression for the rate of change of ground state population density:
Hence from Eqs. (3) and (4), we get
In a photoacoustic experiment, we assume the incident light intensity I to vary slowly so that we may consider the upper and lower state population density changes to be an adiabatic interchange. Under this approximation we can set the left-hand side of Eq. (5) to zero. Since the total molecular density for the two-level system is
The spectral radiant energy density is directly proportional to the intensity I of the light source, and we define a constant
Mechanical chopping of the light source at a frequency ω can be expressed as the periodic “on” and “off” of the intensity by
If the gas is very weakly absorbing, we assume that most of the molecules are in state
For a molecular gas, its kinetic energy (K) has to be considered to get the total internal energy density
The rate of change of energy is given by
This change of energy is equal to the difference between the absorbed and radiated optical energy so that
We know that
Since the volume of the photoacoustic cell does not change in the experiment, and thermodynamic evaluation of the change in kinetic energy at constant volume gives
Since the specific heat capacity of gas
where
The pressure of the ideal gas
Since
Substituting for
Integrating Eq. (19) we get the following expression for photoacoustic signal, which is detected by a sensitive microphone:
2.2 Generation of photoacoustic signal in solids
Let us consider a cylindrical photoacoustic cell shown in Figure 5 where the light-absorbing solid sample is surrounded by optically transparent gas (air) on the front side and the backing material is a poor conductor of heat. The absorption of light, of a particular wavelength in the sample, generates heat by nonradiative transitions. The acoustic signal produced in the coupling gas is due to periodic heat flow from the sample. Rosencwaig-Gersho model for
where
The temperature variation in the gas dies out within a thickness of
3. Experimental methods in photoacoustic spectroscopy (PAS)
The temperature and pressure changes involved in the process of PA signal generation are extremely small, typically a micro- to millidegree and nano- to microbar, respectively. This was the reason that the field of PAS remained dormant till the advent of tunable laser sources and of sensitive audio detectors. A piezoelectric transducer or a sensitive microphone serves as the acoustic detector in photoacoustic cells used during the laboratory experiments. Recent developments in the field of miniaturization and the related progress in computer software have made it possible to use tiny quartz-based acoustic detection devices like cantilever and crystal tuning forks. The photoacoustic spectrometer using miniature lasers fitted with these novel detectors can be easily packed in a small box and can be used in the laboratory or in the field for standoff detection of hazardous materials. The heat generated by the photoacoustic effect produces density changes caused by temperature fluctuations in liquid and gaseous samples. In such cases, photoacoustic spectroscopy is carried out by detection of thermal lens formation using a probe laser. In the present case, however, we will confine to the detection of acoustic vibrations in the analysis of PA spectra.
3.1 Photoacoustic cells for gaseous samples
One of the simplest PA cells for measurements on gases and vapors is made from a Pyrex tube fitted with quartz windows at the two ends. The length of the tube and location of the microphone are chosen to maximize the PA signal using the resonance of sound generated by the modulated light beam. The design of such a cell made from 62.5-cm-long Pyrex tube of 2.5 cm diameter and fitted with quartz windows at Brewster’s angle is shown in Figure 6(a). The cell was resonant at 335 Hz with maximum signal in the middle port for the microphone, and it was resonant at 669 Hz, rendering maximum signal at two ports symmetrically located on either side of the middle port. The microphone ports correspond to the positions of three possible antinodes of the stationary acoustic waves formed at the two resonant frequencies. Acoustic isolation was achieved by putting the cell in a wooden box filled with sand. Two of the ports, not in use during the measurement, were sealed using O-rings and flat Teflon disks. PA measurements were carried out with iodine vapor at room temperature in the presence of air at atmospheric pressure using 20 milliwatt Argon laser light at 514.5 nm. The chopping frequency of the laser light was varied between 27 and
The photoacoustic spectrum of iodine vapor recorded using a Nd:YAG laser pumped tunable dye laser in the presence of atmospheric air and at 15 Torr is shown in Figure 7. Dye laser pulses used for the measurements were of 7 ns duration, 0.05 nm bandwidth, and
To make photoacoustic measurements on a flowing gas sample, such as in the case of pollution monitoring, one needs a different type of acoustic resonant PA cell as schematically illustrated in Figure 8. The U-shaped cell has a total length
The airborne particulate matter (aerosols) remains suspended in air inside the PA cell shown in Figure 8 and has enough time to absorb radiation from the tunable laser beam. The absorbed optical energy is transferred as heat to the surrounding air, before the next laser pulse arrives, to build the stationary pressure wave in the PA cell. An instrument of this type can directly measure light absorption by aerosols over the entire range of sunlight entering the atmosphere. This type of PA cell has been used with a single laser as well as with two lasers of different wavelengths impacting the same gaseous sample [10, 11].
3.2 Photoacoustic cells for solid samples
PA cells fitted with microphone, for recording PA spectra of solid samples, have been routinely used in the laboratory for almost four decades. One of the important aspects of homemade cells is to choose a material for effective shielding from extraneous sound. The design of a nonresonant PA cell is schematically shown in Figure 9. The main body of the cell has been constructed from a single block of aluminum with a cavity made from the bottom side for fixing the microphone along with its preamplifier. A cavity is made on the top of the block to put the sample cuvette whose open end is in the same horizontal plane as the microphone surface. A flat aluminum plate with double quartz windows in front of the sample cuvette is tightly fixed at the top of the main body with a very thin, suitably cut rubber sheet to make the chamber airtight. The thickness of the air duct connecting the sample and microphone is about 1 mm, and its total volume is less than 1 cc. The exterior dimensions of the stainless steel sample cuvettes are identical so as to tightly fit into their designated cavity. The sample cuvettes are, however, of varying depths to make measurements on powder samples of different thicknesses. Carbon black is used as the standard sample for recording the power spectrum of the excitation source of light to normalize the PA signals of the sample under investigation.
The PA spectra of powder samples of RDX and TNT recorded with a PA cell of the above type are shown in Figure 10. The source of excitation used in these experiments was a rotational line tunable cw
3.3 PA spectroscopy of contaminated water
PA instrumentation for detection of liquid samples is somewhat complicated. A schematic diagram of the experimental setup for detection of harmful and dangerous pollutants in water is shown in Figure 11. The tunable dye laser beam, pumped by an excimer laser or a Nd:YAG laser, is focused into a 600 micron core multimode optical fiber for investigation on remotely located samples. The polluted water is kept in a quartz cuvette which is acoustically coupled to a piezoelectric transducer. The light exiting from the optical fiber is collimated into the quartz cuvette by means of a
3.4 Quartz tuning fork for PA detection
In a PA cell, the acoustic energy is accumulated in a resonant cavity, but the principle of PA detection by a quartz tuning fork (QTF) involves the accumulation of the acoustic energy in a sharply resonant acoustic transducer [16, 17, 18]. Crystal quartz is an easy material for such a transducer because of its low loss piezoelectric property, and QTFs can be designed to resonate at any frequency between 4 Hz and 200
When a laser beam is focused at the center between the two prongs of the QTF placed in a gaseous sample, the absorbed optical energy converted into heat generates a weak acoustic pressure wave. When the laser beam is modulated at half the QTF resonant frequency (f), the pressure wave makes the two prongs move apart two times during each acoustic cycle. In this situation the QTF detects sound oscillations at the second harmonic of the modulation frequency due to two absorption events during each modulation period. The laser light is modulated at “
The use of QTF for solid phase PA detection in the laboratory requires a very thin film of the molecular sample to be adsorbed on the outer surface of one of the prongs. The absorbed laser light heats the sample, generating an acoustic wave at the prong’s surface interface with air. When the frequency of repetition of the incident laser pulse coincides with the mechanical resonant frequency of the QTF, the localized pressure variation sets the latter into vibration. The amplitude of this vibration and the resulting piezoelectric voltage are proportional to the amount of heat produced by optical absorption at the surface.
An experimental arrangement using the above concept is schematically shown in Figure 13 using a quantum cascade laser (QCL). The large wavelength coverage in the mid-IR region combined with narrow linewidth and powering up to tens of
4. Photoacoustic detection of harmful chemicals
PA spectroscopy has been widely used in chemical sensing applications in environmental science and medical diagnostics. It is useful in rapid detection of illicit drugs, nerve agents, and hazardous biological materials. In a typical hospital environment, there is a need for evaluation of anesthetic gaseous components. Although hospital staff are exposed to much lower anesthetic concentration than the patients, this exposure extends over many years. Under inadequate hygiene conditions, people working in hospitals or factories often complain of headaches and fatigue due to traces of harmful gases in the environment. Illicit drug trafficking poses many challenges for detection of dangerous chemicals that threaten life and property. In the following sections, we will present examples of point detection as well as standoff detection of chemical compounds using PA spectroscopy.
4.1 PA spectroscopy of ethylene
Ethylene
4.2 PA spectroscopy of gases emanating from the human body
The smells emanating from various parts of the body are unique to an individual, made up of specific chemical compounds that vary depending on age, diet, metabolism, and health. Near-IR diode laser at
Trace level detection of nitric oxide has many applications in medicine, biology, and environmental science. CO laser was the first to be used by Kreuzer and Patel [2] for PA detection of NO concentrations of 0.01 ppmV. Since its first detection in exhaled air [23], NO has been found to be a sensitive marker for asthmatic airway inflammation [24]. A QCL-based PA cell has been developed by Elia et al. [25], while Spagnolo et al. have reported a minimum NO concentration limit of 15 ppbV [26].
A
4.3 PA spectroscopy of dangerous drugs
Morphine is the prototype narcotic drug, and it is the standard against which all other opioids are tested. An acetylated form of morphine, almost two times more potent than morphine itself, is known as heroin. Animal and human studies and clinical experience back up the contention that morphine is one of the most euphoric drugs on earth. Both morphine and heroin are used for pain medication, but both are addictive and identified as illegal drugs.
Microgram quantities of powders of morphine and heroin were used in a PA cell shown in Figure 9 and fitted with
5. Photoacoustic imaging
Photoacoustic imaging is an emerging technique that combines the high resolution of light and deep imaging capability of ultrasound. It is similar to hyperspectral imaging except for the fact that optical sensors are replaced by ultrasonic detectors that convert the sound waves into images. It has many applications as a noninvasive technique in medicine to produce molecular images of internal organs. It is based on the rapid production of heat, when the optical energy from a nanosecond laser pulse is absorbed in the tissue, causing thermal expansion and the generation of ultrasonic waves. The processes involved in the image formation are schematically illustrated in Figure 15, which show that it is a hybrid technique making use of optical absorption and ultrasonic wave propagation.
There are two basic conditions for efficient generation of the PA signal for imaging. The condition of “thermal confinement” requires the laser pulse duration
It can be shown that a nanosecond laser pulse impacting a biological tissue sample satisfies the conditions for PA imaging. Thermal diffusion length during the laser pulse is given by
5.1 Photoacoustic microscopy and photoacoustic tomography
Photoacoustic tomography (PAT) and photoacoustic microscopy (PAM) are the two methods of PA imaging. In the PAT mode, an expanded laser beam illuminates the whole sample, and laser photons are absorbed at various points in the sample generating ultrasonic waves. PAT acquires depth-dependent information by time-of-flight measurements of the acoustic waves. An ultrasonic transducer placed outside the sample detects the PA signal, which is measured either by moving a single transducer around the sample or by using an array of transducers. PA image is obtained from the data set of PA signals by using appropriate reconstruction algorithms in the computer. In the case of PAM, the laser beam is focused into a tiny volume, and ultrasonic waves from this localized region are imaged by the detector. To obtain a
5.2 Photoacoustic microscopy of zebra fish
Fluorescence microscopy is an effective tool in thin biological samples like single-celled organisms, but with slightly thicker samples, it becomes difficult to know where exactly the fluorescence originates. In a complex organism, like zebra fish, it is crucial to image deeper and deeper while the organism is kept alive. Fluorescent light emerging from the point of absorption suffers multiple scattering in the tissue on its way to the optical detector. This leads to loss of information on the origin and propagation path of the fluorescent light, giving rise to a blurred image and destruction of the spatial resolution. PA detection of optical absorption circumvents these limitations, because the sound waves travel through the diffuse biological media with much less distortion than light.
The experimental components used in PAM by Harrison et al. [30] are shown in Figure 16a. The laser beam, from a tunable source (L), is diverted down the
In the recording of images of Figure 16b, the zebra fish was held on a rotating platform immersed in water. The position of the laser focus was fixed at a particular depth inside the body of the sample, and the platform was rotated through 360° to record the two-dimensional sections [31]. The location of fluorescent protein mCherry (in red) is clearly seen in the image of the zebra fish brain at the top, and transverse image slices of the zebra fish hindbrain are shown in the lower half of Figure 16b, where each slice is separated by a depth of 0.5 mm inside the tissue.
5.3 Photoacoustic imaging of prostate cancer
PA imaging is emerging as a new diagnosis technique with specificity, high resolution, and enough imaging depth for early detection of prostate cancer. Ex vivo multispectral PA imaging has been carried out to differentiate between malignant prostate tissue, benign prostatic hyperplasia (BPH), and normal human prostate tissue. The preliminary results of investigations carried out by Dogra et al. [32] show that there was a significant difference in the mean PA intensity of dehydroxy hemoglobin (dHb) and lipid between malignant and normal prostate. There was also a significant difference in the mean intensity of dHb between malignant prostate and BPH. There was, however, no significant difference in HbO2, dHb, and lipid between normal prostate tissue and BPH. Laser radiation at 1064 nm and 1197 nm has been used to obtain PAT images, corresponding to optical absorption of hemoglobin and lipid, to determine the clustering prostate cancer tissue at each wavelength [33]. It was found that 1064 nm PAT in conjunction with ultrasound image is more effective in identifying prostate cancer biopsy targets than the PAT at 1197 nm.
6. Conclusion
This chapter starts with a brief history of photoacoustic effect and photoacoustic spectroscopy. A simple mathematical derivation for the generation of PA signal in gaseous and solid samples is followed by experimental methods. The design and construction of a variety of PA cells and detectors have been described along with their use in the investigation of gaseous, solid, and liquid samples. Some illustrative examples of trace detection of explosives and harmful chemicals have been discussed. A brief account of the principle and application of the emerging technique of PA imaging is discussed at the end of the chapter.
Acknowledgments
I am grateful to all the authors and scientists whose results have been cited to make the presentation meaningful. I am thankful to Dr. Punam Rai for taking care of my health, Sudheer for the help with the computer, and to my grandchildren, Leo and Mia, for their innocent inquiries during the course of my writing.
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