Experimental design and numerical investigation of a photoacoustic sensor for a low-power, continuous-wave, laser-based frequency-domain photoacoustic microscopy

Abstract

We have developed a photoacoustic (PA) sensor using a low-power, continuous-wave laser and a kHz-range microphone. The sensor is simple, flexible, cost-effective, and compatible with commercial optical microscopes. The sensor enables noncontact PA measurements through air, whereas most current existing PA techniques require an acoustic coupling liquid for detection. The PA sensor has three main components: one is the chamber that holds the sample, the second is a resonator column used to amplify the weak PA signals generated within the sample chamber, and the third is a microphone at the end of the resonator column to detect the amplified signals. The chamber size was designed to be 8 mm × 3 mm as the thermal diffusion length and viscous-thermal damping of air at room pressure and temperature are 2 and 1 mm, respectively. We numerically and experimentally examined the effect of the resonator column size on the frequency response of the PA sensor. The quality factor decreased significantly when the sample chamber size was reduced from 4 mm × 3 mm to 2 mm × 3 mm due to thermos-viscous damping of the air. The quality factor decreased by 27%, demonstrating the need for optimal design for the sample chamber and resonator column size. The system exhibited noise equivalent molecular sensitivity (NEM) per unit bandwidth (NEM∕ p Δf) of ∼19;966 Hz −1∕2 or 33 × 10 −21 mol or 33 zeptomol, which is an improvement of 2.2 times compared to the previous system design. This PA sensor has the potential for noncontact high-resolution PA imaging of materials without the need for coupling fluids. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Development of a frequency-modulated photoacoustic microscope system for thermal imaging using a picosecond laser

“…The system measures a PA signal with a maximum amplitude of 2x10 -4 V for a modulation frequency of 760 Hz. The signal is about 4 times lower than those obtained (8 x10 -4 ) by a 532 nm CW laser of power 3 mW 1,4. The SNR of the system measured using the USAF chart is 5.5 dB.…”

“…The signal is about 4 times lower than those obtained (8 x10 -4 ) by a 532 nm CW laser of power 3 mW. 1,4 The SNR of the system measured using the USAF chart is 5.5 dB. The SNR is calculated by measuring the PA signal with and without a chromium film (a bare area of a glass substrate) of the USAF chart.…”

“…The thickness of the sample can be measured by integrating a high-frequency transducer (350 MHz) into the FDPAM. 1 Thus, using the same picosecond laser, the PA signal as a function of modulation frequency can be obtained using an FDPAM 4 , and the thickness of the sample by using a high-frequency transducer (350 MHz) 1 can be obtained simultaneously. The present configuration needs a slight modification to adapt a 350 MHz transducer to achieve this.…”

“…The PA system is composed of an optical imaging system, a laser for PA excitation, a motorized translation stage to raster scans the sample, and the PA sensor. 3,4 The optical imaging system was developed in an inverted microscope configuration. The system contains an infinitycorrected 40X and 0.25 NA Olympus objective and a Ximea xiQ camera (Ximea, NorPix, Inc., Montreal, Canada).…”

“…The PA sensor is made of polyacrylamide/PDMS material. 3,4 The sensor has a cylindrical sample chamber (SC) with a small tunnel (T) leading to the microphone (M) (Type: 4189, Brüel & Kjaer Sound & Vibration Measurement, Pointe-Claire, Quebec, Canada). The microphone has an almost flat frequency response from 50 Hz to 20 kHz with a sensitivity of 50 mV∕Pa.…”