This method can help answer key questions in fluorescence enhancement, such as surface plasmon-mediated fluorescence. The main advantage of this technique is that it differentiates the excitation and the decay processes of surface plasmon-mediated fluorescence entirely in frequency domain. Prepare a substrate for the periodic array.
For this experiment, use a one square centimeter piece of glass. Clean it in ultrasonic baths of methanol and acetone for 10 minutes each. When done, place it on a 200 degree Celsius hotplate for one hour.
For the spin coating steps, dilute native photoresist with a thinner, and have adhesion solution ready. Move the glass substrate to a spin coater, and dispense three to five drops of adhesion solution onto it. Spin the substrate at 600 rpm for 10 seconds, then 3, 600 rpm for one minute.
Next, place five to seven drops of diluted photoresist on the substrate. Spin coat the substrate at 600 rpm for 10 seconds, then 3, 600 rpm for one minute. When done, move the spin coated sample to a 65 degree Celsius hotplate, and leave it there for one minute.
Then, transfer the sample to a 95 degree Celsius hotplate, and leave it for another minute. From the hotplate, take the sample to mount on a Lloyd's interferometer. The set-up has a prism for a sample holder, with a mirror perpendicular to it.
Use a drop of refractive index matching oil on the sample to attach it to the prism. Position the sample as close to the mirror as possible. Expose the sample in this initial orientation.
Then, to create a two dimensional square lattice, rotate the sample 90 degrees on the prism. Use the same exposure time for a second exposure. Return the sample to the 65 degree Celsius hotplate for two minutes.
Then, place it on the 95 degree Celsius hotplate for two minutes. Next, immerse the sample in developer for two minutes with continuous agitation. Retrieve the sample and immerse it in isopropyl alcohol for one minute, to rinse residuals.
Work with an RF sputtering deposition system to put a 100 nanometer gold film on the sample. Once the sample is ready, take it and a styryl 8 dye preparation to a spin coater. Dispense 0.2 milliliters, or three to five drops, of the solution onto the sample.
Spin coat this to produce a thickness of about 80 nanometers. For the reflectivity measurements, use a goniometer. This system uses broadband white light from a quartz lamp.
A multi-mode fiber bundle transmits the light to face-to-face objective lenses for collimation. A polarizer and shutter are next in the path. The light illuminates a sample on a sample stage with three independent rotation stages.
A detection analyzer is after the sample stage. Another multi-mode fiber collects the specular reflection from the sample, to transmit it to a spectrometer and CCD camera. After aligning and calibrating the set-up, set up an automated system to collect data.
Use the automation to step through different incident angles and collect the reflection spectra. For these measurements, the step size of the incident angle is 0.5 degrees. The wavelength resolution is 0.66 nanometers.
To measure the orthogonal reflectivity, alter the set-up. Set the incident polarizer to 45 degrees with respect to the incident plane. Set the analyzer to negative 45 degrees with respect to the incident plane.
Proceed with another automated measurement. Photoluminescence measurements require another change to the set-up. Replace the broadband light source with a 633 nanometer helium neon laser.
Place a laser line filter before the polarizer. Also, place a half wave plate after the shutter. Complete the set-up by substituting a notch filter for the analyzer before the spectrometer.
Conduct automated incident scans by fixing the detection angle, and varying the incident angle with respect to the sample normal. The surface plasmon polaritons are incident angle dependent, and are selectively excited. Next, conduct a detection scans by fixing the incident angle with respect to the sample normal, and varying the detection angle.
This is the P-polarized reflectivity mapping taken along the gamma X direction of the gold array in the inset. The dashed line is calculated using the phase matching equation, and implies that the M equals negative one and N equals zero surface plasma polaritons are excited. This reflectivity map is from when the polarizer and analyser are at orthogonal positions.
The background is now zero, as indicated by the blue of the color map. The reflectivity profiles have peaks in green, instead of dips, as only the surface plasma polaritons remain after removing the background. This is the P-polarized reflectivity mapping and the orthogonal reflectivity mapping along the gamma X for cadmium, selenium, telluride, quantum dot doped PVA polymer, spin coated on the array.
In addition, here are data for the incident and detection angle photoluminescence measurements. The data can be used to determine the excitation and coupling rates between light emitters and resident cavities. Though this method can provide insight into fluorescence enhancement from periodic arrays, it can be also applied to other systems, such as metamaterials and photonic crystals.
Once mastered, this technique can be done in three hours, if it is performed properly. While attempting this procedure, it is important to remember to check the alignment of the array set-up, and the polarization and angles of the reflectivity spectrography. After its development, this technique paved the way for researchers in the field of photoluminescences to explore surface plasma-enhanced fluorescence in the field of resins enhancement.
After watching this video, you should have a good understanding of how to determine the excitation and the coupling rate between light emitters and the surface plasma polaritons arising from periodic arrays.
