Engineering
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Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation
Chapters
Summary February 25th, 2017
We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.
Transcript
The overall goal of this procedure is to demonstrate the optical fabrication of a one-dimensional photonic crystal cavity on a tapered optical fiber with a sub-wavelength diameter waste. The key point of our method is to fabricate thousand-empiric nanocreators or a nanofiber but I think it's just a single laser part and the created nanostructure eventually acts as a one dimensional photonic crystal cavity which may open new possibilities for nanophotonics and quantum information science. One essential aspect of this work is that the nanofiber itself acts as a cylindrical lense and focuses the laser beam on its side surface.
Moreover, distinguish the fabrication makes it immune to any mechanical instabilities or any other fabrication imperfections. The voice reading is a procedure with Jameesh Keloth, a grad student from my laboratory. The nanofibers for the fabrication will be produced using a commercial device.
The fiber is heated with an oxohydrogen flame from this nozzle. The fiber is drawn by motorized stages to produce a tapered section. A computer monitors the transmission through the fiber using input from a probe laser and photodiode.
The nanofiber will be made from a length of single mode optimal fiber, about 210 millimeters long. Producing the nanofiber will require other equipment. To start, have a fiber coating stripper, a source of methanol, and cleanroom wipes.
Also, have a reservoir of acetone in which the single mode fiber can be immersed. To prevent dust from collecting on the nanofiber, be prepared to isolate it quickly. For this experiment, the nanofiber will be mounted in this nanofiber holder using UV curable epoxy.
The holder can be closed using the glass plated top cover. Begin with the length of single mode fiber and use the fiber coating stripper to remove five millimeters of the polymer jacket from each end. Dip a clean room wipe in methanol and use it to clean the ends.
Next, immerse the fiber between the two ends in the reservoir of acetone. Keep it there for 10 to 15 minutes until the fiber jacket falls off. When the fiber jacket has fallen off, remove the fiber from the acetone and clean the entire fiber with a clean room wipe dipped in methanol.
For the next steps, take the fiber to the commercial nanofiber device. This fiber is mounted on the motorized drives and ready for fabrication to begin. Close the device and start the probe laser to monitor the transmission.
Use software to ignite the flame, load the parameters, and start fabrication. After the fabrication is complete, take the nanofiber holder with epoxy to the device. Secure the fiber on either side of the taper using UV curable epoxy.
Once the fiber is in place, cover the nanofiber holder with the top cover. Place the sample in a clean box to transfer it to the experiment set up. This is the setup for femtosecond laser fabrication.
It is inside a clean booth with hepa filters. A laser beam enters from above a cylindrical lense. The nanofiber holder will sit on top of a stage for X Y Z translation and one for rotation.
This schematic provides a clearer idea of the apparatus. Laser light passes through a cylindrical lens. It then reaches a phase mask with a pitch of 700 nanometers.
The phase mask splits the beam into zero and plus and minus one orders. The zero order is blocked, but the plus minus one orders reflect from folding mirrors. The symmetrically placed mirrors lead to the creation of an interference pattern at the nanofiber in its holder.
A photodiode allows monitoring of the light in the fiber. A CCD Camera is used to monitor the nanofiber position. The laser fabrication setup must be aligned.
This requires use of a glass plate which can be ablated by the laser. Put the glass plate on the fabrication bench. With the translation stage, adjust the height of the bench to 15 millimeters then use the laser to eradiate the glass for five seconds at a pulse energy of one milijoules.
Use the CCD camera to observe the plate and identify the laser induced ablation. A damaged line can be seen on the glass with the ablation pattern. Change the horizontal position of the glass by a millimeter to allow for new ablation.
After that, change the height of the glass surface in order to test the strength of the ablation in a new position. Radiate the glass plate again for five seconds with a pulse energy of one milli joules. Then, assess the damage of the glass plate.
As has happened with this glass plate, adjust the glass hight and ablate a new region until the strongest ablation line is identified. With the stage at the height associated with the strongest ablation line, fine tune the angle of the mirrors and stage the further maximize ablation. After this optimization, go to the software for the CCD camera.
Use the software to mark the position of the ablation line in the field of view. Remove the glass plate to test the periodic structure of the ablation. To image the pattern, use a scanning electron microscope.
The pattern should show a periodic structure with a period of 350 nanometers. If not, repeat the alignment steps. Begin at the aligned fabrication bench.
Have ready a properly fabricated tapered fiber in its holder. Mount the fiber holder and couple the fiber to a probe laser. To be properly aligned, the fiber should be approximately parallel to the ablation line marked in the CCD software.
Continue by sending a probe laser through the tapered fiber and using the CCD camera to observe the scattering. Use the translation stage to move the fiber along its length and center it on the ablation line. Now, use the femtosecond laser with the minimum pulse energy.
Translate the fiber in the horizontal plane to overlap with the femtosecond laser beam. Then translate the fiber in the vertical plane to overlap its position with the ablation line. Again, translate in the horizontal plane to maximize the overlap with the femtosecond laser.
While translating the stage back and forth, observe the glass on the top cover of the fiber holder for the first two order reflections from the fiber. If the bright spots move along the line, the nanofiber is not parallel to the ablation line and the rotation stage must be rotated. If the spots appear in a flash, this indicates the nanofiber is parallel to the ablation line and the rotation stage does not need adjustment.
When the nanofiber is parallel to the ablation line, turn off the probe laser and measure the power through the fiber with the photodiode. Use the translation stage to adjust the fiber in the horizontal plane. The goal of the adjustments is to maximize the measured power scattered from the femtosecond laser.
When done, use the rotation stage to rotate the fiber to the angle of rotation. Next, take the power meter and use it to block the femtosecond laser beam. Adjust the pulse energy so the meter reads zero point two seven milli joules.
Change the femtosecond laser setting to single shot before removing the meter from the laser path. Complete the fabrication by firing a single femtosecond laser pulse. Begin fabrication with an aligned set up.
In addition, arrange for a wire to be supported above the cylindrical lens. This zero point five millimeter copper wire is supported by a post. The post is mounted on a translation stage to allow positioning the wire in the laser beam.
Make sure to set the height of the glass plate to where the strongest ablation line was found. Then insert the wire at the center of the laser beam and perpendicular to the ablation line. Observe the shadow of the wire and try to position it at the center of the ablation pattern.
Next, use a femtosecond laser pulse to produce an ablation pattern on the glass plate. Check the ablation pattern on the glass plate to see if the wire produces a gap at its center. If not, move the copper wire to center and ablate a new section of the glass plate.
Repeat until the gap is in the center of the ablation pattern. Before continuing, fix the wire in place by locking its translation stage. Then remove the glass plate from the fabrication platform.
Get the fiber holder with its mounted fiber and install it in the fabrication setup. Here, the holder is in place and the fiber is coupled to a probe laser. Send a probe laser pulse through the fiber.
It should be approximately parallel to the ablation line recorded in the CCD software. Translate the stage along the fiber length to center the nanfiber on the ablation line before switching off the probe. Turn on the femtosecond pulse and translate the fiber in the horizontal plane perpendicular to its length with the goal of maximizing the overlap of the fiber with the femtosecond laser pulse.
Check by measuring the power of the scattered light with the photodiode. After maximizing the overlap, set the angle of fabrication. Now, use the power meter to block the femtosecond laser.
Then adjust the pulse energy so that it is zero point two seven milli joules and change the femtosecond laser setting to be single shot. Remove the power meter from the laser path and fire a single femtosecond laser pulse to complete the fabrication. This scanning electron microscope image is of a typical segment of a fabricated nanofiber sample.
The nanocraters are formed on the shadow side of the fiber. The nanocraters are almost circular with the diameter of around 210 nanometers. In this sample, the periodicity is 350 nanometers.
This transmission spectrum from the apodized protonic crystal cavity is for light polarized perpendicular to the nanocrater faces. The spectrum shows a stopband region from about 794 to 799 nanometers in which the transmission is only a few percent. Compare this with the transmission spectrum of light polarized parallel to the nanocrater faces.
It also has a stop band but at longer wavelengths from approximately 796 to 803 nanometers. Both spectra have peaks that correspond to cavity modes. The transmission spectra from the same polarization modes in the defect induced photonic crystal cavities show similar behavior.
In these cases, cavity modes are at either side of the stop band. Note that the cavity mode spacing at shorter wavelengths is much larger than that at larger wavelengths. This single shot optical fabrication method is immune to mechanical instabilities ensuring height of the category and this fabrication technique may be implemented to make various nanophotonic devices from nanofibers and may be adapted to other nanofabrication processes.
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