This protocol uses a grooved parallel plate wave guide to measure the refractive index of a microfluidic sample at terahertz frequencies First design and fabricate the grooved wave guide to exhibit a resonance in the terahertz range. Then using a terahertz time domain spectroscopy system, measure the resonant frequency of the waveguide. Next, fill the waveguide with a carefully measured volume of the sample fluid.
The final step is to measure the resonant frequency of the filled waveguide. Ultimately, the difference between the resonant frequencies for the empty and filled wave guides can be used to determine the refractive index of the sample in the terahertz frequency range. Generally, individuals new to this method will struggle because of the very high accuracy required to obtain repeatable results.
Kim reel another graduate student from Daniel Middleman's Laboratory and I will now demonstrate the procedure. Design a parallel plate waveguide with one or more integrated cavities or grooves. Base the geometry on parameters detailed in the accompanying manuscript and also refer to our previous publications.
These are a few general guiding principles. Start with a plate size wide enough that it can be considered infinite compared to the input beam to allow for easy access to the groove. Make the bottom wave guide plate significantly wider than the top plate so that the groove extends almost the entire width of the plate.
Make the propagation length such that the wave guide as a whole is at least three times as long as the extent of the groove and the holes in the bottom plate are threaded while those in the top are not. The design for the groove will depend on the desired resonant frequency, the desired line width, and the chosen plate spacing among other factors. It's important to consider the limitations of your fabrication techniques for very narrow or very shallow grooves For use as a reference.
Also fabricate an identical design lacking a groove. Maintain the plate spacing using dielectric spacers such as shards from a shattered microscope. Slide Machine fabricate the wave guide.
Importantly, do not blunt the edges of the plates, particularly on the input face. Rounded edges are standard practice in many machine shops for safety reasons, but a rounded edge on the input face will distort the signal. Begin the assembly with a structure with two flat surfaces perpendicular to one another.
Place the bottom plate on the horizontal surface and press it. Flush against the vertical surface. Insert the dielectric spacers as close to the screw holes as possible.
Two per screw, one on each side. Check that the screws do not obstruct the groove or extend beyond the input face.Carefully. Place the top plate flush against the vertical surface and slide it down to sit on the bottom plate and spacers.
Now holding both plates flush against the vertical surface. Insert the screws, screw them down incrementally in an alternating pattern. Examine the final wave guide for a perfectly flat input face and uniform plate spacing.
Start by configuring the apparatus. If not already present, introduce four lenses into the rah hertz beam path in a confocal orientation. In order to provide a tight focus at the midpoint of the path, place an aperture of 12 millimeters at the focal point.
The aperture should be large enough to block all radiation from propagating except through the waveguide. The size of the aperture will determine the beam size propagating in the waveguide. Use a secure holder to ensure repeatable placement of the waveguide.
Next position the waveguide immediately behind the aperture with the input face in contact with the aperture and with the waveguide propagation axis aligned as closely as possible with the optical axis. The alignment here is critical reflections dispersion variations in the cutoff, in resonant frequencies, and other issues may arise due to improper alignment of the waveguide. Now, place the syringe holder so that the tip of the syringe is aligned with the groove.
For best results, use a different syringe for each material to prevent cross-contamination. Fill the syringe with the liquid to be tested and eliminate any bubbles. Also between runs.
Follow a cleaning procedure that requires first disassembling the wave guide. Then wash both plates thoroughly in an appropriate solvent to remove any residue from the experiment. Blow dry with compressed air, reassemble the waveguide as shown earlier.
Start with a reference wave form of the grod waveguide. A reference wave form is only necessary once every few hours during each experimental session, depending on the long-term stability of the time. Domain spectrometer signal.
Remove the ungrouped wave guide. Next place the clean grooved wave guide into the apparatus. Take a waveform for the empty grouped wave guide.
The process of removal and disassembly can lead to very small variations in the geometry of the waveguide. These variations will affect the absolute resonant frequency of the empty and filled grooves, but not the observed shift. Therefore, each full measurement requires its own empty reference.
To calculate the shift Without moving the waveguide, put the filled syringe in place in the holder. Slowly fill the groove keeping. Watch that the fill is good with no bubbles or overflow.
Take another waveform. If the system has more than one groove, continue filling grooves and taking as desired. Remove the wave guide and clean it prior to collecting the next data set.
In this example of a well fabricated waveguide, note that the groove does not extend the entire length or width of the waveguide. Once assembled the parallel plate, waveguide geometry is suitable for implementing a refractive index sensor for terahertz frequencies. A small volume of liquid is required for measurement of the refractive index.
These data show typical frequency spectra and are obtained from an analysis of tetra decane. The spectrum obtained from the reference un grooved wave guide is shown in black measurements of the grooved wave guide with no liquid fill is indicated in blue, and the grooved wave guide with a tetra decane sample is in red. The amplitude spectra for the empty and full grooved wave guides are then squared and divided by the spectrum from the reference wave guide to obtain power transmission spectra.
The difference in frequency between the resonant features of the empty and full wave guides is the resonant shift which relates to the refractive index. While attempting this procedure, it's important to remember to be as consistent as possible and to be careful to minimize cross-contamination in the waveguide. After watching this video, you should have a good understanding of how to obtain a measurement of the refractive index of a microfluidic sample by measuring the resonant frequency of a grooved parallel plate wave guide using Terahertz time domain spectroscopy.
