Trace Gas Detection at the parts-per-quadrillion Level
Typically, the photoacoustic effect, which was discovered by Alexander G. Bell in 1881, is generated by tuning a laser to the absorption of a substance which, as a result of heat deposition causes a temperature increase followed by thermal expansion of the irradiated material. The thermal expansion is a mechanical motion that launches sound which can be detected by a microphone or other acoustic transducer. Owing to the inherent high sensitivity of microphones and the typically high power of lasers, the photoacoustic effect can be a remarkably sensitive detector of trace gases such as atmospheric pollutants.
Almost universally, the photoacoustic effect, when used for purposes of trace gas detection, is generated by modulating the laser beam and using a resonator to detect the acoustic wave. It is possible, however, to generate sound by moving a light beam rather than modulating its amplitude. A feature of the photoacoustic effect that has received little attention is that if a laser beam moving in a one-dimensional geometry is translated at the sound speed, the acoustic amplitude increases without bound—at least according to linear acoustics. We have used this principle by creating a moving optical grating using a carbon dioxide laser. The transducer is a highly sensitive BiBO3 crystal that has a resonance at roughly 400 kHz. The device was found to have a sensitivity of 750 parts-per-quadrillion for SF6 in Ar. We are continuing this work to develop instruments that use quantum cascade lasers so that the whole instrument can be miniaturized and made field portable.
Cryogenic Ultrasensitive Temperature Detection
In principle, it is possible determine the rate of cell growth by monitoring the rate at which it a cell or a group of cells emits infrared radiation relative to the background. While there have been advances in recording temperature using optically probed nanoparticles, none of the methods reported have the sensitivity to detect the small temperature changes that accompany cell growth as the temperature changes can be calculated to be on the order of 10 μK.
The Diebold group is working on a solution to the problem of determining cell or tissue growth through the use of temperature sensors such as the HgCdTe detector, superconducting bolometers, and cryogenic detectors. The first of these operates at 77K, whereas the other two typically are designed to run at the temperature of liquid He, 4K. We are presently investigating the use of heterodyning techniques to increase the sensitivity of the HgCdTe detector. The instruments that are under development use position modulation where an oscillating mirror directs radiation alternately from a reference at room temperature to the cell culture. The idea is to carry out the amplification at a high frequency, on the order of 1 MHz, where low frequency (1/f) noise is small. Since the increase in sensitivity can be estimated from a knowledge of the noise power in the amplifier versus frequency we are optimistic about achieving sensitivities in the μK range.
Superconducting edge detectors and Si cryogenic detectors as a result of their remarkably high sensitive have been used in astronomy for looking at distant sources of radiation and for detection of teraherz radiation in imaging experiments. We plan to use either of these detectors to provide higher sensitivity than is possible with the heterodyned HgCdTe detector.
Successful development of the instrument should have numerous applications in biology and medicine. For instance, determination of cell growth rate would make possible rapid determination of the effects of chemotherapy agents for individual patients. Whether a cell is a rapidly growing cancer cell, or a healthy cell could be determined. Outside of the field of biology, the rate of chemical reaction or the evaluation of catalysts could be carried out. There are possibly many more applications since temperature fluctuations are present in so many processes that accompany physical and chemical change.
At present the author's research group is investigating optical trapping as a method for determination of the masses of optically levitated particles. In experiments it was found that at pressures of approximately 2 torr , large fluctuations in the position of levitated quartz spheres took place so that they were ejected from the trap. Ashkin and coworkers in the 1970's, showed that micron size particles could be levitated by a focused laser beam as a result of optical radiation forces. The authors calculated that in a high vacuum, the oscillation decay time of levitated spheres displaced from their equilibrium position in the beam would be 2.2 x 10⁷s, (0.7 year ), making it "one of the lowest-loss mechanical oscillators known." Levitation has been observed for glass, quartz, plastic, and liquid spheres as well as spherical shells.
We are investigating a mass spectrometer based on Mathieu instabilities that result when a laser is amplitude modulated with a known modulation depth and focused onto a particle resulting in optical levitation and oscillation of the particle about its equilibrium position. The equation of motion of the particle is shown to obey the Mathieu equation which has stable and unstable regions. Note that quadrupole mass spectrometers use Mathieu instabilities for mass selection. It is shown here that measurement of the points of instability of the oscillations in the optical trap determines the ratio of the radiation force to the gravitational force on the particle. With the mass of the particle known, the device becomes what might be called a "radiation force" spectrometer. By adding a gas flow with a known flow rate to the trap, a Stokes force is added to the particle so that it can be used to determine particle masses. The apparatus involves the use of a continuous laser to levitate the particles, a light modulator to induce oscillations, and a high speed camera to observe the particle motion. The properties of the device are unique as a spectrometer.
The gas phase photoacoustic effect can be generated by exposing a photoacoustic cell filled with an infrared absorbing gas to a body whose temperature is lower than that of the cell. The effect
follows from the principle that for any two bodies there is a dynamic exchange of radiation with each body emitting and absorbing radiation at their respective absorption and emission wavelengths. Both the emission and absorption of radiation takes place over a range of infrared wavelengths depending on the Planck distribution at the temperature of the cell and the energy levels of the gas. In the work carried out in the author's laboratory some time ago, a photoacoustic cell filled with SF₆ viewed a liquid nitrogen bath through a chopping wheel. Thus, the SF₆ was periodically exposed to the room temperature surface of the chopping wheel and then to the bottom surface of the Dewar flask filled with liquid N₂ with the alternating pressure signal from the microphone recorded with a lock in amplifier.
Since the infrared radiation flux emitted from the cell is greater than that it receives from the Dewar flask, an "inverse" photoacoustic effect is generated. It was found, as expected, that when the liquid nitrogen bath was replaced by a heated body, a photoacoustic signal with the opposite phase as was produced with a heated body replacing the Dewar flask was also generated. The phase of the signal, however, was reversed.
Currently, research on the use of the photoacoustic effect as a pyrometer for remote temperature determination is under investigation. As shown in the figure, a photoacoustic cell equipped with two infrared transmitting windows views a remote surface whose temperature is to be determined and a reference surface whose temperature is known. A chopping wheel at the temperature of the cell interrupts the paths to both surfaces.
Immediately outside of the cell, a pair of identical chopping blades placed on single shaft but positioned to be 180 degrees out of phase, periodically block the field of view of the photoacoustic cell. The photoacoustic effect will be generated in a non resonant mode of the cell, typically 100Hz . The pyrometer operates at a null of the photoacoustic effect, that is, if the temperatures of the two surfaces are identical, the signal from the microphone vanishes . The phase of signal from the lock-in amplifier will indicate which surface is hotter, and has been found to be a more sensitive indicator of the equality of the temperature than observation of the signal magnitude. A typical plot of phase versus temperature difference, as seen in the figure shows a rapid transition in phase at the null point. It is the rapidity of this change that determines the sensitivity of the pyrometer. When a very slow temperature ramp is given to the remote surface, variations in the phase are found; that is, the phase jumps back and forth between its extreme values. A measurement of this region of variation gives the uncertainty in the null point, and hence the minimum detectable temerature difference.
In the ongoing research, investigation of the accuracy and sensitivity of the device is being carried out. The goal of the research is to do fundamental research on the device, that is, to demonstrate the principle of operation of a photoacoustic pyrometer and to explore the factors that govern its sensitivity and accuracy.
We have investigated a new tissue imaging technique, invented here, using the ultrasonic vibration potential as a means of detecting tumors. The work is motivated by the search for more sensitive methods for detection breast tumors—methods to detect the presence of tumors at their earliest stages of development. The primary difficulty in using x-ray mammography is that the contrast mechanism for distinguishing tumors from healthy tissue depends on density differences, which are not so large as to allow early tumor detection.
The vibration potential offers the possibility of high sensitivity detection precisely as a result of its contrast mechanism which is sensitive to the presence of colloidal objects. Blood is a colloidal suspension as a result of the presence of red blood cells. Since tumors are highly vascularized, that is, they are surrounded by a large network of blood vessels, a detector of blood concentration becomes a detector of tumors.
Colloids are suspensions of charged particles in a liquid with a counter charge distributed in the fluid around each particle. The counter charge, which is normally a spherical distribution around the particles, gives the solution overall charge neutrality and stabilizes the suspension against particle agglomeration. When sound propagates through a suspension where the particles have either a higher or lower density than that of the surrounding fluid, the amplitude and phase of the particle motion, owing to the difference in inertia between the particle and the volume of fluid it displaces, differs from that of the fluid so that fluid flows back and forth relative to the particle on alternating phases of the acoustic cycle. Since the counter charge is carried by the fluid, the oscillatory motion of the fluid relative to the particle distorts the normally spherical counter charge distribution creating an oscillating dipole at the site of each particle, which, added over a half wavelength of the sound wave, results in a macroscopic voltage that can be recorded by a pair of electrodes placed in the solution.
Similar considerations of particle inertia show that vibration potentials are generated in ionic solutions as well. We have found that the voltage signal measured across a pair of electrodes attached to a body with a colloidal or ionic region inside can be inverted to calculate an image of the presence of blood.