Evanescent Field Based Photoacoustics: Optical Property Evaluation at Surfaces

The overall goal of this set of photoacoustic techniques is to characterize the optical properties of liquids, solids, and transparent thin films in one consolidated device. This method can help answer key questions in the materials characterization field, such estimated thickness and refractive index. The main advantage of this technique is that bulk refractive index, thin form refractive index, thickness, and optical absorption can be measured in a single device.

Some equipment must be prepared to begin the experiment. First, the experiment requires two ultrasonic transducers like these 10 megahertz models used for this protocol. Each has a nine millimeter diameter, one millimeter thick, red latex rubber cylinder, epoxied on its front face.

The reference transducer is also epoxied to an acrylic block, which acts as an acoustic spacer. Another piece of equipment to prepare is the evanescent field-based photoacoustic, or EFPA prism holder. The EFPA prism holder consists of a prism mount and a transducer mount.

A prism is seated in the prism mount. The transducer mount holds one of the prepared ultrasonic transducers so that the latex is exposed. With the prism and the transducer in place, put the two parts of the prism holder together, so that the latex faces a prism across a gap that will accommodate a sample.

Secure the pieces to one another with screws. The remaining apparatus for this experiment is already in place on an optical bench. The beam is produced by a cue switch neodymium dome deuterium aluminum garnet laser.

It's output goes into a beam expander. This is followed by a manually adjustable aperture. The next element is a polarizing beam splitter cube.

The beam then enters a non-polarizing beam splitter. One output goes to a mounted ultrasonic transducer that will serve as a reference. The other output from the beam splitter goes to the mounted EFPA prism holder and its transducer.

The beam exiting the prism holder goes into a beam block. The EFPA prism holder is mounted to an a XY theta translation stage to allow it to be properly positioned. It's angle can be changed by a computer-controlled stepper motor.

To prepare for the experiment, return to the laser and beam expander. Use lenses to create a beam expansion of at least a factor of three. The beam should be oversized compared to the latex absorber on the transducers.

After working with the expander, use a digital level to align the beam and the prism holder. The flat side of the prism mount should be at zero degrees with respect to the beam. Connect and power on the transducers, oscilloscopes, and computer-controlled monitors.

Use 50 ohm BNC cables to connect the reference transducer to channel zero of the oscilloscope. Connect to the transducer in the EFPA prism mount to channel one. For the alignment steps, be certain to have on the appropriate laser safety goggles.

At the adjustable aperture, reduce the beam to one millimeter diameter. At the computer, start the software that will control the execution of the experiments. Use the software to rotate the prism mount to 70 degrees.

Look into the prism perpendicular to the laser beam from the side, and observe whether the laser spot is visible on the latex. Manually adjust the XY theta translation stage until the laser spot is correctly positioned on the transducer. The goal is for the spot to be at the center of the latex.

Return to the aperture, and expand it to its maximum opening. On the computer, review the energy measurements from the transducers. In this case, the red line is from the direct laser energy measurement, the reference.

The blue line is from the transducer in the EFPA prism holder. Check that the two measurements have approximately the same magnitude. When all is in order, stop the measurement, which will automatically reset the prism to zero degrees before continuing.

The total internal reflection photoacoustic spectroscopy technique will be used to analyze the liquid sample. The sample is direct red dye 81 in water. In addition, the technique requires immersion oil that is index-matched to the prism, as well as a microscope slide as a substrate to cover the prism surface.

Have the EFPA prism holder out of the setup and opened in order to put the sample in place. Begin with a prism, and place 2.5 microliters of the immersion oil at its center. Then, place the substrate on top of the layer of oil that is spread.

Next, move to work with the latex on the EFPA ultrasonic transducer. There, place 25 microliters of the liquid sample without producing any bubbles. Now, reassemble the EFPA holder by putting the prism oil substrate combination on top of the sample-covered latex.

Compress the prism holder, and tighten the assembly with mounting screws. When the EFPA holder is ready, return it to the experiments setup. Reintroduce the EFPA holder to its position among the optical elements.

Ensure all components are ready before initializing data collection. Employ lab software to measure and display the acoustic signal generated by the sample. The data show the peak to peak voltage as a function of time.

To prepare for POSS TUR POSS refractometry, again work with the EFPA prism holder from the setup. Separate the holder to expose the prism and the latex from the transducer. Have ready the sample, immersion oil that is index-matched to the prism, and a substrate to cover the prism surface.

Place 2.5 microliters of immersion oil on the center of the prism. Sandwich the oil by placing a substrate on top of the layer of oil. Move to the transducer in its mount and put 25 microliters of sample dye on its latex pad.

Once this is done, put the prism mount with its oil and substrate on the latex or the transducer. Compress the two mounts together and secure with mounting screws. With the EFPA prism holder in place in the setup, turn to the computer-controlled software.

Input the range of angles of incidents over which the spectrum should be measured and the step size. Run the program to measure the spectrum and plot it as the peak to peak voltage as a function of angle. Starting with the measured spectrum, use software to find and graph the numerical derivative with respect to angle.

Use smooth data from the numerical derivative to manually identify local minima and their corresponding angles of incidents. The minima indicate a transition from POSS to TUR POSS regimes. Work with the EFPA prism holder to ready it for optical tunneling photoacoustic spectroscopy.

Have the prism and transducer mounts separated and immersion oil index-matched to the prism at hand. The sample is a thin film of magnesium fluoride deposited on a glass slide. At the center of the prism, place 2.5 microliters of immersion oil.

After the oil is in place, orient the sample to have the thin film away from the prism, and put the sample on the oil. At the transducer mount, place 25 microliters of immersion oil on the latex pad, so that it coats the entire surface without forming bubbles. Put the latex in contact with the sample, and tighten the mounting screws to compress the assembly.

Return the EFPA holder to the optical setup, and prepare to run the experiment. At the computer, enter the parameters for taking an angular spectrum and begin the measurement. This data from the total internal reflection photoacoustic spectroscopy, is representative of the acoustic wave generated from an absorbing sample.

In this case, red dye 81 in water. The bipolar nature of the wave occurs due to acoustic reflection at the interface between the sample and the glass substrate, where this is a large difference in acoustic impedance. The POSS TUR POSS technique generates an angular spectrum.

When the numerical derivative is taken in smooth, the local minimum corresponds to the measured critical angle. Knowing this angle, it is possible to calculate the bulk index of refraction at the laser wavelength of the experiment. This is angular spectrum of laser energy normalized signal is for a magnesium film on an an NBK-7 optical glass substrate.

The percentage of optical tunneling as a function of the angle of incidents can be found from these values. This information makes estimation of the refractive index and thickness possible. While attempting this procedure, it's important to remember to prevent bubble formation in the oil or sample.

Having watched this video, you should now have a good understanding of how to use EFPA methods to estimate the optical properties of materials.