1. Introduction

Manipulation and guidance of sound waves is of critical importance to many different scientific and engineering fields, including: acoustic design engineering, secure communications, stealth technology and photo-acoustic sensing. Traditionally manipulation of sound waves has been performed through the use of physical materials interfaces, with the difference in compressibility of the dissimilar acoustic transport media and the shape of the interface (relative to the incident acoustic wave) controlling the magnitude and direction of the reflected, refracted and transmitted portions of the incident sound waves. While such methods provide an effective and predictable means of manipulating sound, they rely on different physical material barriers and can require extensive construction times and efforts.

Alternatively, in the 1860’s John Tyndall demonstrated that the amplitude of sound waves propagating through a flame were significantly decreased on the other side, revealing that the differences in air density associated with the hot air through which the sound had passed dramatically affected them [1]. Although Tyndall’s early work in sound propagation demonstrated reduced transmission through hot gases, along with the fact that passing the sound through a larger number of thinner slot burners resulted in more efficient sound suppression than a single larger burner, the mechanism by which this acoustic suppression occurred (i.e., refraction of the incident sound, reflection of the incident sound, etc.) was not characterized beyond a brief phenomenological description of the effect [1, 2]. In the 1880’s, the relationship between sound, heat and light became somewhat clearer with Alexander Graham Bell’s discovery of the photoacoustic effect, in which the absorption of modulated light from the sun generated photo-thermal expansion and subsequent rarefaction of the absorbing material thereby creating an audible sound [3, 4]. Largely unexploited until the 1970s, with the advent of intense laser sources, photoacoustic spectroscopy [5, 6] and its related family of photothermal spectroscopies (e.g., photothermal lensing) have since been employed for numerous applications [7], ranging from medical diagnostics and imaging [8–12] to trace chemical monitoring [13–16]. Using these photothermal techniques, localized density differences generated in the samples being illuminated are then measured either by the generation of sound waves (i.e., photoacoustic spectroscopy) or the deviation of optical beams associated with the local difference in refractive index between the molecularly dense regions and the rarefied regions (i.e., photothermal lensing/photothermal deflection). Due to the potentially large optical absorptivity of the analyte molecules being detected as well as the efficient nature of non-radiative relaxation in most molecules, these photothermal effects can result in extremely sensitive detection methodologies.

In this report, we demonstrate and experimentally characterize, for the first time, how optically-induced photothermal barriers can be used to reflect and waveguide external acoustic waves, without the need for different physical material interfaces. Infrared excitation is used to rapidly generate localized air density barriers, along the optical beam path, that act as efficient acoustic reflection interfaces for any incident externally generated sound waves. Since these barriers are optically-induced, the potential for rapid alteration of acoustic propagation direction and magnitude is possible. Furthermore, by shaping the laser beam both spatially and temporally, it is possible to not only generate acoustic reflection barriers, but also acoustic waveguides capable of extending the propagation distance of sound waves in air with dramatically reduced amplitude losses, as compared to non-confined acoustic waves. This ability to optically guide sound without the need for rigid physical structures offers a new paradigm in the manipulation and use of sound for potential future advances in standoff and guided photoacoustic sensing, secure communications, acoustic suppression and many other fields.

2. Results and discussion

2.1. Optically-induced acoustic reflection

Generation and characterization of optically-induced acoustic barriers in air was performed using a waveguide-based CO₂ laser (Laser Photonics; Model CL55WTVO) operating at 9.6 μm and 3.0 W [see Fig. 1]. The laser was modulated at various frequencies from 0 – 3.1 kHz using an optical chopper (Scitech; Model 300CD) with a 50% duty cycle. The modulated laser beam was then passed through an adjustable diameter iris to create a sharp outside edge to the laser, providing a well-defined beam with a 9-mm beam diameter. An external acoustic source (i.e., earbud; JLabs; Model J6M), was placed orthogonal to the direction of laser propagation and at the same height as the laser. A variable frequency function generator (GW Instek; Model GFG-3015) was connected to the acoustic source to produce controlled monotonic frequency sound waves ranging from 2 kHz to 20 kHz (the audible frequency range). Measurement of the acoustic signal amplitude was performed using two matched hearing aid microphones (Knowles; Model FG-23629-P16), placed on either side of the laser beam. One microphone (i.e., microphone #1; the transmission microphone) was placed on the other side of the laser beam directly opposite of the earbud and behind the iris to prevent the potential introduction of any noise in the measured signal due to the laser striking themicrophone. The second microphone (microphone #2; the reflection microphone) was then placed adjacent to the earbud (at an equal distance from the laser as the earbud) to monitor the sound emitted from the earbud as well as any reflected sound. The output of the microphones and the optical chopper were then monitored using a 500-MHz digital oscilloscope (LeCroy; Model Waverunner LT342).

 

Fig. 1 Schematic diagram depicting the optical system employed for optical reflection of acoustic waves. A CO₂ laser (9.6 μm) is modulated by an optical chopper before passing through an iris to provide a defined beam diameter. On one side of the laser beam an earbud is located that emits a constant amplitude and frequency acoustic tone based on a sinusoidal voltage applied by a frequency generator. On the opposite side the laser beam from the earbud is a microphone (microphone #1) for measuring the acoustic signal transmitted through the optically excited barrier and a second microphone (microphone #2) is located on the same side as the earbud to monitor acoustic reflections from the optically excited barrier.

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To provide an optically absorbing species in the air that is capable of being excited via the CO₂ laser employed, the entire optical system, including the microphones and acoustic source, was surrounded by a rectangular plexiglass enclosure (48 inches long x 18 inches wide x 18 inches tall), inside of which the air was saturated with methanol and ethanol vapor. To provide a constant source of alcohol vapor, a recrystallization dish containing a reservoir of the alcohol mixture was placed in the bottom of the chamber and allowed to evaporate for continuous saturation over extended periods of time (i.e., 12-15 hours).

Following saturation of the air in the plexiglass chamber with alcohol vapor, which was verified via photoacoustic analysis to occur within 45 minutes of closing the chamber (at an ambient room temperature of 21°C), measurement of the efficiency of the optically-induced reflection of acoustic waves was performed. A fixed frequency (measured between 2 kHz - 20 kHz) and amplitude sine wave was applied to the earbud to generate a constant pure acoustic tone, while the amplitude of the acoustic signal was measured on the transmission and/or reflection microphone(s). Simultaneously the transistor-transistor logic (TTL) signal from the optical chopper was monitored to correlate the acoustic signals to the presence or absence of laser.

Figure 2(a), shows characteristic acoustic responses of both the transmission and reflection microphones for an externally applied 5.85 kHz acoustic tone incident on a photothermally depleted acoustic barrier optically modulated at a frequency of 2.85 kHz. Following absorption of the 9.6 μm radiation from the CO₂ laser by the alcohol vapor present, spatially localized differences in air density are rapidly generated, resulting in a sharp acoustic impedance boundary along the laser excitation pathway [see Fig. 2(a) inset]. In the presence of the optically-induced boundary (high TTL signal from optical chopper), asignificant reduction in the amplitude of the 5.85 kHz acoustic tone transmitted through the optically excited air volume is observed (solid purple curve). When compared to the amplitude of a reference acoustic tone measured on the same microphone in the absence of the laser (solid black curve) or during times corresponding to periods of the laser being blocked (low TTL signal from optical chopper), a 28% suppression of transmitted acoustic signal is observed. Simultaneously, a corresponding 29% increase in acoustic amplitude is measured by the reflection microphone (dashed red curve) when the laser beam is present (high TTL signal), demonstrating that suppression of the transmitted acoustic signal is due to reflection from the optically-induced interface as opposed to refraction losses associated with traversing the photothermally excited region.

 

Fig. 2 Efficiency of optically-induced acoustic reflection. (A) Optical excitation and photo-thermal expansion generates a transient acoustic impedance barrier in the air that results in suppression in the amplitude of acoustic waves passing through this barrier as well as a corresponding increase in acoustic amplitude reflected off of the optically-induced barrier. (B) The efficiency of the optically-induced barrier for suppression/reflection of acoustic waves of different audible frequencies is nearly constant with the exception of enhanced acoustic suppression/reflection when the optical modulation frequency of the barrier is one half of the acoustic frequency transmitted. (C) Barrier stability and acoustic wave suppression efficiency is stable over incident acoustic waves ranging from 0 dB to 70 dB. Error bars represent ± one standard deviation of the average (N = 15).

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Acoustic reflection efficiencies at these optically generated interfaces were found to be largely independent of the acoustic frequency monitored, over the entire audible frequency range measured (2 kHz – 15 kHz; range determined by optimal microphone response range), with acoustic suppression/reflection efficiencies of 29 ± 6%. Figure 2(b) shows the acoustic suppression efficiency for two additional acoustic frequencies (i.e., 6.0 kHz, and 12.0 kHz) as a function of the laser modulation frequency (i.e., optical chopper frequency). As previously observed, a statistically consistent acoustic suppression of approximately 30% is observed for each acoustic frequency evaluated and for every optical modulation frequency employed. An exception to this consistent 30% acoustic suppression/reflection efficiency occurs when the optical modulation frequency corresponds to exactly half that of the acoustic frequency incident on the optically-induced barrier. Under such conditions, acoustic suppression is dramatically increased, to approximately 43 ± 5% [see Fig. 2(b)]. Although the reason for this enhanced signal suppression under the conditions corresponding to the acoustic tone being twice the optical modulation frequency are unclear at this point, this effect was found to be consistent for all acoustic frequencies evaluated in which these conditions could be achieved, with the enhanced suppression ranging between 37 – 46%.

In addition to evaluating the efficiency of the optically-induced acoustic barrier as a function of incident acoustic frequency, the effect of the optical modulation frequency on acoustic suppression efficiency was studied by varying the optical chopper frequency from 0 Hz to its maximum limit of 3.1 kHz. From these studies, it was found that at optical modulation frequencies greater than 500 Hz, consistent suppression and reflection efficiencies of approximately 28 – 29% were observed as previously reported [see Fig. 2(b)]. Since acoustic reflection in these studies is based on a molecular depletion barrier approximately 9-mm in thickness (i.e., laser beam diameter), increases in optical modulation frequency to even greater than 3.1 kHz should also provide similar acoustic reflection efficiency, assuming sufficient time for molecular depletion is allowed to exist. However, since constant acoustic signal reflection/suppression efficiency occurs for all optical modulation frequencies over 500 Hz, and the slower the modulation frequency the longer the barrier generated (both temporally and spatially), little is gained for most applications, beyond spatial localization of the reflecting barrier, by modulating at higher frequencies.

Alternatively, at optical excitation frequencies less than 500 Hz, a dramatic decrease in acoustic reflection efficiency occurs; with no statistically measurable reflection or suppression observed at 480 Hz or less for room temperature (i.e. 21 - 23°C) studies. At optical modulation frequencies lower than 500 Hz, the optically heated air evacuated from the beam path has greater than 2-ms to equilibrate with its surroundings, potentially resulting in a more diffuse interface and a more gradual thermal/density gradient instead of an abrupt barrier. This density gradient, unlike an abrupt barrier, provides an acoustic impedance matching with the surrounding air, allowing efficient acoustic propagation, analogous to refractive index matching gradients employed in anti-reflection coatings on optics.

Following characterization of the acoustic reflection and suppression efficiencies of these optically-induced barriers, evaluation of their stability as a function of incident acoustic signal was investigated [see Fig. 2(c)]. In these studies, the output voltage of the variable frequency function generator driving the acoustic source was varied, generating fixed frequency acoustic tones ranging in magnitude from 1 dB – 70dB, corresponding to quiet whispers to loud conversation or yelling. In fact, as shown in Fig. 2(c), over the 1dB – 70 dB acoustic source range investigated (upper limit capped by the maximum output of the earbud), no statistically significant difference in acoustic suppression was observed despite the dramatically different incident pressure wave on the optically-induced acoustic barrier. Although the average acoustic suppression values shown in Fig. 2(c) appear to be slightly greater at the lower amplitude acoustic source outputs than those measured at the higher amplitude source outputs they are statistically equivalent and the perceived decrease in suppression efficiency as the incident sound amplitude increases is due to the significantly smaller signal-to-noise ratios associated with measuring the weaker sound amplitudes (i.e., 0 – 25 dB) and the associated increased uncertainty in the ratio of these smaller signal values.

2.2. Source distance dependence

In order to achieve efficient reflection of incident acoustic waves from an optically-induced acoustic barrier, it was found that the acoustic source must be located at least one acoustic wavelength (e.g., 8.41 cm for a 4.05 kHz acoustic wave at room temperature) away from the optical excitation zone (i.e., optically-induced barrier). Acoustic sources located closer than this critical one acoustic wavelength distance, provide minimal to no measureable acoustic reflection or suppression, with an abrupt transition occurring between no reflection and reflection. Figure 3 shows this phenomenon for two different acoustic frequencies (i.e.,4.05 kHz and 5.05 kHz), with the vertical dashed line in each plot representing the distance corresponding to one acoustic wavelength for the specific frequency employed. For the two frequencies shown in Fig. 3, as well as all others measured across the audible spectrum, the percent decrease in acoustic amplitude recorded on the transmission microphone and the percent increase measured on the reflection microphone rapidly transitioned between no measurable percent change to their respective maxima (~30%) immediately following their respective acoustic wavelengths (i.e., 6.73 cm for 5.05 kHz and 8.41 cm for 4.05 kHz). Beyond one acoustic wavelength distance between the acoustic source and the optically-induced barrier, the percent signal suppression and percent signal reflection levels off at a maximum (as shown by the dashed trend lines).

 

Fig. 3 Spatial dependence of the acoustic source from the optically induced barrier for different frequency acoustic tones. (A) Acoustic signal suppression of a 5.05 kHz acoustic wave transmitted through an optically-induced barrier as a function of distance between the acoustic source and the barrier. (B) Acoustic signal reflection of the same 5.05 kHz acoustic source off of an optically-induced barrier as a function of distance between the source and the barrier. (C) Acoustic signal suppression of a 4.05 kHz acoustic wave transmitted through an optically-induced barrier as a function of distance between the acoustic source and the barrier. (D) Acoustic signal reflection of the same 4.05 kHz acoustic source off of an optically-induced barrier as a function of distance between the source and the barrier. Data points correspond to five averaged measurements for each distance with the dashed sigmoidal trend lines revealing that in order to achieve significant reflection/suppression from the barriers, the acoustic source must be located at least one acoustic wavelength from the optically depleted acoustic barrier zone.

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2.3. Multipass signal suppression

Since this acoustic reflection phenomenon is due to the presence of sharp, discreet density barriers generated in air at the location of the excitation laser beam and each barrier results in greater than 25% signal suppression, the effect of employing multiple barriers [see Fig. 4(a)] was evaluated for enhanced acoustic signal suppression. From these studies, it was found that the use of multiple barriers can provide nearly complete suppression of the transmission of incident acoustic signals [see Fig. 4(b)]. By reflecting the CO₂ laser that induces the optical barrier off of multiple gold mirrors and ensuring sufficient spacing (i.e., 3.8 cm) between the laser beams was provided to allow the necessary temperature and density differences to be achieved, it was found that each optically-induced interface could provide approximately the same 29% signal suppression of transmitted acoustic signals as previouslydemonstrated. After transiting through four isolated barriers, no acoustic signal corresponding to the fixed frequency external tone was observed, resulting in complete suppression of the incident acoustic tone prior to reaching the transmission microphone (microphone #1), located beyond the final optically-induced barrier. Figure 4(b) shows the amplitude of the acoustic signal measured on the transmission microphone in such a multi-pass geometry. As can be seen, when the laser is not present (low TTL square wave signal from the optical chopper), the 5 kHz acoustic tone being emitted by the acoustic source is easily measured by the microphone (solid black line). However, when the laser beam is present (high TTL square wave signal from the optical chopper), the 5 kHz acoustic tone signal is rapidly and completely suppressed and only small fluctuations in acoustic amplitude are measured. These small fluctuations are not associated with the incident externally applied acoustic tone, but instead correspond to the photoacoustic signal being generated by the non-radiative relaxation of the excited alcohol vapor in the air following absorption of the laser. The photoacoustic nature of this signal was confirmed by monitoring the same signal in the absence of an external constant frequency acoustic tone from the source (i.e., earbud). While complete suppression of external acoustic signals at the microphone (up to 70 dB) was achieved in this study, it employed four randomly aligned, optically-induced interfaces that resulted in reflection of scattered sound in different directions for each interface. Although it is beyond the scope of this proof-of-principle demonstration work, it may be possible to provide comparable suppression of external acoustic signals with fewer optically-induced interfaces by employing alternative optical geometries that take advantage of the reflected portions of the acoustic signals for destructive interference of the incident sound waves.

 

Fig. 4 Optical multipass barrier for enhanced acoustic suppression. (A) Schematic diagram of the multiple optical barriers employed sequential dampening of the amplitude of the emitted acoustic tone. The reflected laser beam generated four non-overlapping barriers in space arranged in a single z-plane with the acoustic source located on the opposite side of the barriers than the transmission microphone. (B) Acoustic amplitude of the fixed frequency acoustic tone measured by the transmission microphone in the presence (solid black line) and absence (dashed blue line) of the laser used to generate the acoustic barrier (high square wave = laser on; low square wave = laser off).

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2.4. Optical waveguiding and channeling of sound

In addition to generation of transient, optically-induced acoustic barriers for reflection and/or suppression of acoustic signals, the generation of optically-induced acoustic waveguides in free space is also possible by employing a doughnut shaped laser beam for photothermal excitation of the air with the acoustic source located within the central region of the doughnut. Following absorption of the laser beam localized heating results in rapid and transient regions of rarefied air along the optical beam path with increased air density in the center of the doughnut and immediately outside of the optical excitation region. When an acoustic source is launched within the central dense region, wave guiding and enhanced acoustic propagation occurs inside the channel due to acoustic reflection off of the barrier as well as enhanced propagation thorough the increased density zone, where an effective change in the elasticity of the air has been generated [see Fig. 5(a)].

 

Fig. 5 Optical channeling of acoustic waves. (A) Cross sectional schematic representation of the optically-induced channel acoustic channel. The acoustic source is surrounded by a donut shaped optical beam that generates a cylindrical acoustic barrier for enhanced propagation of the sound. (B) A schematic diagram of the system employed for optical channeling of acoustic signals. A CO₂ laser is modulated by an optical chopper before passing through the ZnSe window of on a plexiglass chamber saturated with alcohol vapor. A metallic earbud (driven by the sinusoidal output of a variable function generator) is suspended by its cord in the center of the laser beam, masking the central region and generating a donut shaped laser beam. Two movable microphones are placed downfield of the earbud (one inside the channel and one outside) to allow for monitoring of the acoustic amplitude at different distances.

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Demonstration and characterization of this optically-induced acoustic waveguide was performed using the optical system shown in Fig. 5(b). A CO₂ laser operating at 9.6 μm was modulated with a variable speed, 50% duty cycle, optical chopper (described previously) before passing through a ZnSe window into the plexiglass chamber saturated with methanol and ethanol vapor. To generate an acoustic channel, a metal earbud (described previously) was suspended by its cable (~2 mm in diameter) into the center of the laser beam. To prevent damage to the plastic cable coating by the laser, a layer of aluminum foil was wrapped around the cable where it passed through the beam. The metal earbud then blocked the center of the laser beam generating a donut shaped beam approximately 1.7 cm in diameter, with a laser-free center region 1.0 cm in diameter. To generate the 1.7 cm diameter collimated beam prior to striking the earbud, a ZnSe beam expander (not shown; II-VI Inc.; Model BEC210.6C1.05-D2) was placed between the optical chopper and the ZnSe window. Acoustic tones were generated by driving the earbud with fixed frequency sine waves (between 2 - 15 kHz) from a frequency generator (described previously). Although a larger diameter channel would allow for better propagation of audible frequency acoustic waves, based on their acoustic wavelengths, laser power density constraints for efficient barrier generation (see Appendix 1) limited the maximum diameter of the channel for these proof-of-principle studies.

To demonstrate the potential of optical channeling (i.e., waveguiding) of the acoustic waves, two microphones were placed equal distances downfield from the earbud (ranging from 1.27 cm – 97.79 cm) while the amplitude (i.e., peak-to-peak) of the acoustic signal was measured at each location. One microphone was placed inside the laser-free portion of the optical channel [see microphone #1 in Fig. 5(b)] and the second was located just outside of the optical channel at an equal distance from the acoustic source. Following placement of the microphones at each specific distance, the plexiglass chamber was closed and allowed to equilibrate for 45 minutes, to ensure saturation of the alcohol vapor, prior to measurement. The amplitude of the resulting acoustic signals for microphone #1 both with the laser on and with the laser off were then plotted as a function of distance from the acoustic source.

Figure 6 shows the resulting signals for microphone 1 (inside the optically-induced acoustic channel) at various distances, for both when the laser is firing and in the absence of the laser. Although the signals from microphone 2 provided a means of simultaneously monitoring acoustic suppression outside the channel when the laser was firing and provided similar responses to microphone 1 in the absence of a laser-induced channel (in terms of decay profile), only results from microphone 1 are displayed for comparison in Fig. 6 to prevent any potential concerns associated with variations in positioning distances and microphone response characteristics between microphones 1 and 2.

 

Fig. 6 Acoustic amplitude decay profile as a function of distance from the source; with (red triangles) and without (blue circles) an optical channel present. The solid blue and the dashed red curves represent power function fits to the data revealing the expected 1/r distance dependent signal decay for non-channeled sound and a slower 1/r ^0.6 decay for the optically channeled sound.

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A clear increase in the amplitude of the transmitted signal can be seen in the presence of the optically-induced channel [Fig. 6; triangles and dashed line] as compared to in the absence of the laser [Fig. 6; circles and solid line]. Employing a fixed frequency acoustic tone as the source, it was possible to ensure that any increases in signal amplitude in the presence of the optically-induced channel were associated with the acoustic tone emitted from the earbud and not a combination of these signals with photoacoustic waves generated from the non-radiative relaxation of the alcohol vapor. Deconvolution of any potential photoacoustic signals was achieved via the process described in Materials and Methods.

Analysis of the distance dependence of the acoustic signal reveals that in the absence of the laser, the acoustic amplitude decreases with distance from the source at the expected rate of 1/r [17]. However, in the presence of the optically-induced waveguide, the amplitude of the acoustic transmission is significantly greater at any distance along the channel and decays at a significantly slower rate, 1/r^0.6. As with the acoustic reflection/suppression results, this enhanced propagation of acoustic signals with distance, when confined to an optically-induced acoustic channel, was found to exist for all audible frequencies measured as long as the diameter of the optical channel was large enough to support the sound wave and the power density of the laser beam used to generate the barrier was sufficiently large enough (and modulated at > 500 Hz) to generate a sufficient photo-thermal depletion region.

3. Conclusions

In summary, we have demonstrated in this study that photothermally-induced reflection and confinement of acoustic signals in air, saturated with alcohol vapor, is possible over the entire audible frequency range. In order to achieve efficient acoustic reflection from these optically induced barriers, it is critical that the acoustic source be located at least one acoustic wavelength away from the barrier and that the optical barrier be modulated at a minimum of 500 Hz to ensure an abrupt thermal barrier is generated. Furthermore, the ability to waveguide acoustic signals with significantly reduced losses in amplitude has also been demonstrated in this work using alcohol doped air samples. Employing higher power laser sources as well as different optical geometries should allow for even greater improvements in opto-acoustic waveguiding efficiency over the proof-of-principle demonstration in this work, opening the potential application of this phenomenon to secure communications, guided ultrasound and photoacoustic imaging in tissues to standoff photoacoustic sensing.

4. Materials and methods

4.1. Chemical reagents

Ethanol and methanol were purchased from Sigma-Aldrich with purities of greater than 90%. The plexiglass enclosure was constructed in house from 1/8 inch thick plexiglass sheets from Online Metal Supply. The ZnSe window on the chamber was 3 inches in diameter and purchased from Edmund Optics Inc.

4.2. Minimization and deconvolution of photoacoustic and external acoustic tones

In addition to measuring the acoustic signal from the microphones in the presence of an external acoustic tone from the earbud, photoacoustic signals associated with the alcohol vapor absorption were also measured and used to correct the acoustic reflection/transmission data for photoacoustic distortions when significant (as determined by their magnitude being larger than the noise of the microphone signal associated with the acoustic tone from the earbud). To minimize the convolution effect of the photoacoustic signal (due to alcohol absorption and subsequent non-radiative relaxation) on the acoustic reflection (and/or transmission) measurements of the externally applied pure tone, the optical modulation frequency of the chopper was tuned, whenever possible, to a frequency that results in minimal/negligible photoacoustic signals at the specific location of the microphone within the chamber. Due to destructive interference of photoacoustic reflections within the chamber, altering the optical modulation frequency provides a simple means of reducing any acoustic effects associated with the direct photoacoustic emission from the alcohol vapor on the externally applied acoustic tone. When it was not possible to reduce photoacoustic signals to levels below the noise of the microphone by tuning the optical modulation frequency, deconvolution of the externally applied tone and the photoacoustic signal was performed by subtracting the photoacoustic signal measured in the absence of an applied external tone from the signal containing the external tone plus any photoacoustic results.

4.3. Calculation of suppression and reflection efficiencies

To provide quantitative measures for acoustic suppression efficiency, the difference between the peak-to-peak amplitude of the sinusoidal acoustic wave when the laser is off and the photo-acoustically corrected sine wave when the laser is on is divided by the peak-to-peak amplitude of the same wave when the laser is off and multiplied by 100 to provide the measure referred to in this work as percent suppression. Similarly, the acoustic reflection efficiency of the optically-induced barrier is calculated by measuring the peak-to-peak amplitude of the photo-acoustically corrected reflected waveform when the laser pulse is on then subtracting the peak-to-peak amplitude of the same signal when the laser is off and dividing that by the peak-to-peak amplitude of the same wave when the laser is off.

Appendix 1

Laser power dependence of the optically-induced barrier

To further verify that the suppression of acoustic signals through the optically generated barrier was not simply due to a convolution of the photoacoustic signal from the alcohol absorption and the external acoustic signal, laser power dependence studies of both the photoacoustic signal generated from the alcohol absorption and the acoustic suppression efficiency were performed [see Fig. 7]. Control of the laser power was achieved by inserting CaF windows of various thicknesses (ranging between 0 - 6 mm) between the laser and the plexiglass chamber to attenuate the laser to the desired level. For each measurement, the air inside the plexiglass chamber was saturated with both methanol and ethanol vapor as described previously. By varying the power of the CO₂ laser from approximately 1 W - 3.5 W, a distinct difference in the power dependence of these two signals was observed, with the photoacoustic signal providing a linear response relative to laser power (as expected) and the acoustic suppression efficiency providing a distinct sigmoidal response with a sharp on/off transition. The optical modulation frequency used for both sets of measurements was 2.54 kHz and was chosen to provide a significant and measurable photoacoustic response (measured as the peak-to-peak maximum of the photoacoustic waveform). In the case of the acoustic suppression efficiency, no significant acoustic suppression is observed until the laser power is greater than 1.5 W, for the saturated alcohol vapor. Between 1 – 2 W of laser power a rapid increase in acoustic suppression is observed that continues to increase slightly before leveling off at laser powers greater than 2.5 W. The acoustic suppression data shown in Fig. 7 correspond to data taken with an external acoustic tone frequency of 5.27 kHz and an optical modulation frequency of 2.54 kHz. A frequency of 5.27 kHz was chosen for this study due to the large microphone response at this acoustic frequency. Similar data has also been obtained for numerous other acoustic and optical modulation frequencies, revealing the same sigmoidal response.

 

Fig. 7 Laser Power Dependence of Acoustic Barrier Efficiency. The percent acoustic suppression as a function of CO₂ laser power is shown by the red dots (left axis) with a dashed red trend line revealing the non-linear power dependence of the barrier efficiency. The photoacoustic background signal from the alcohol vapor used to generate the acoustic barrier as a function of laser power (right axis) is shown by black + , with a solid black trend line showing the linear power dependence.

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Funding

Battelle Scientific Services Program (SSP) TCN 15031 subcontract number 488904.

Acknowledgments

Dr. Cullum would like to thank the U.S. Army Research Laboratory for facilities and support of these efforts while on sabbatical leave from the University of Maryland Baltimore County. Support for Dr. Cullum was received from the Scientific Services Program (SSP) TCN 15031 administered through Battelle for the U. S. Army Research Laboratory under subcontract number 488904.

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