Name: Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation (Video) | JoVE Journal
Uploaded: 2017-02-25T13:00:00.000+00:00
Duration: 13 min 2 s
Description: Learn femtosecond laser-induced ablation to fabricate one-dimensional photonic crystal cavities on tapered nanofibers. This robust optical method enables precise nanostructure creation for nanophotonics and quantum information science applications.

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5 minutes

This work was supported by the Japan Science and Technology Agency (JST) through the Strategic Innovation Program. KPN acknowledges support from a grant-in-aid for scientific research (Grant no. 15H05462) from the Japan Society for the Promotion of Science (JSPS).

CAUTION: Wear safety glasses and strictly avoid direct exposure to UV lamp and all lasers including the femtosecond laser. Wear a clean room suit and gloves to avoid contamination. Dispose any fiber trash properly in the designated trash box.
1. Nanofiber Preparation
<ol>
	<li>Use a fiber coating stripper to strip the polymer jacket of the single mode optical fiber for a length of 5 mm at two places separated by 200 mm. Clean the two mechanically stripped parts using cleanroom wipe dipped in methanol. Dip the fiber between these two stripped parts in acetone. Wait for 10 - 15 min till the jacket of the fiber fall apart. Take out the fiber from acetone and clean the entire stripped part using cleanroom wipe dipped in methanol.</li>
	<li>Set the stripped fiber on the two stages of the Optical Nanofiber Manufacture Equipment (ONME) to fabricate the nanofiber.
	<ol>
		<li>Launch the probe laser into the fiber and monitor the transmission using the photodiode and record the transmission data in the computer using the ADC card. Start the gas flow using the ONME software and ignite the flame. Load the pre-optimized parameter in the ONME software for the fabrication of tapered fiber with waist diameter of 500 nm and start the fabrication process. 
		NOTE: The ONME is a commercially available device, designed to fabricate tapered optical fibers using standard heat and pull technique. It uses oxyhydrogen flame to heat the fiber and two motorized stages to pull the fiber. The gas flow and the stage movements are controlled by computer program. The pre-optimized parameters may be obtained from the vendor, upon special request.</li>
	</ol>
	</li>
	<li>After the fabrication, catch the tapered fiber to the nanofiber holder using the UV curable epoxy. Cover the nanofiber holder using the top cover with glass plate (shown in the Figure 1a). Put the sample inside a clean box and transfer to the femtosecond laser fabrication unit.</li>
</ol>
2. Femtosecond Laser Fabrication
<ol>
	<li>Alignment of the fabrication setup
	<ol>
		<li>Put a glass plate on the fabrication bench at a height of 15 mm. Irradiate the femtosecond laser for 5 s at pulse energy of 1 mJ. Identify the femtosecond laser induced ablation from the white light generation, and the appearance of ablation pattern as a damage-line on the glass plate.</li>
		<li>Repeat the procedure by changing the height of the glass plate using the X-stage of the fabrication bench. For each fabrication, translate the Y-stage of the fabrication bench by 1 mm to make the fabrication in a new position.</li>
		<li>Find the height for the strongest ablation line. At this position, fine tune the tilt angle and position of one of the folding mirrors to maximize the ablation. Also, fine tune the tilt of the X-stage of the fabrication bench to maximize the ablation. 
		NOTE: The tilt angle of the folding mirror is tuned using the kinematic mirror holder tuning knobs and the position of the mirror is tuned by translating the Z-stage on which it is mounted.</li>
		<li>After the optimization, mark the position of the ablation line on the CCD camera software and remove the glass plate. 
		NOTE: The control software for the CCD camera enables image capture and drawing marks on the captured image. It also enables saving the data of the captured image and the markings. Since the X-stage of the fabrication bench does not have absolute position reference, the CCD image is used as the position reference in the X-axis. The resolution of the CCD image is 10 &#956;m per pixel.</li>
		<li>Using the Platinum (Pt)-coater, coat the glass plate for 60 s to deposit a 25 nm layer of Pt on the glass plate. Image the ablation pattern on the glass plate using a scanning electron microscope (SEM). If the ablation pattern shows periodic structure with a period of 350 nm (the expected interference fringe pattern) then the alignment is optimized. Else repeat the procedure (from Step 2.1.1 -&#160;2.1.4) for lower pulse energies (down to 300 &#956;J) until a periodic ablation pattern is seen.</li>
	</ol>
	</li>
	<li>Fabrication of apodized PhC cavity
	<ol>
		<li>Place the tapered fiber on the fabrication bench approximately parallel to the ablation line marked on the CCD camera.</li>
		<li>Send a probe laser (power = 1 mW) through the tapered fiber and observe the scattering from the tapered fiber on the CCD camera. The strongest scattering part corresponds to the nanofiber region due to its subwavelength diameter.</li>
		<li>Translate the Z-stage of the fabrication bench to center the nanofiber to the ablation line position marked on the CCD camera.</li>
		<li>Switch off the probe laser and irradiate the femtosecond laser with minimum pulse energy (&#60;10 &#956;J). Translate Y-stage to overlap the nanofiber with the femtosecond laser beam. The overlap is identified by the illumination of the nanofiber, observed on the CCD camera. 
		NOTE: The nanofiber is now aligned with respect to the femtosecond laser beam along Y and Z-axis.</li>
		<li>In order to align the nanofiber along the X-axis, translate the X-stage to overlap the nanofiber position to the ablation line position marked on the CCD camera.</li>
		<li>Translate the Y-stage to maximize the overlap of nanofiber with the femtosecond laser. Observe the reflection of the two first orders from the nanofiber (appears as two bright spots on the glass plate of the top cover). Observe the movement of these reflection spots while translating the Y-stage back and forth. 
		NOTE: If these spots move towards one side then nanofiber is not parallel to the ablation line. In this case, rotate the rotation stage to make the nanofiber parallel to the ablation line. When they are parallel, the reflection spots will appear as a flash.</li>
		<li>After making the nanofiber parallel to the ablation line, translate the Y-stage to maximize the overlap between the femtosecond laser beam and nanofiber, by measuring the power of the femtosecond laser scattered into the nanofiber guided modes using a photodiode at the end of the tapered fiber. After maximizing the overlap, rotate the rotation stage to the angle of fabrication &#952; = 0.5 deg. 
		NOTE: For maximum overlap between the femtosecond laser beam and nanofiber, one would expect the power of the femtosecond laser light scattered into the nanofiber guided modes to be maximized.</li>
		<li>Block the femtosecond laser with the power meter and set the pulse energy to 0.27 mJ. Change the femtosecond laser settings to the single-shot irradiation mode. 
		NOTE: In this mode, only a single pulse is generated when the fire-switch is pressed, otherwise there is no laser output.
		<ol>
			<li>Remove the power meter from the laser beam path and fire a single femtosecond laser pulse. This completes the fabrication process.</li>
		</ol>
		</li>
	</ol>
	</li>
	<li>Fabrication of defect-induced PhC cavity
	<ol>
		<li>Check the alignment of the setup by observing the ablation on a glass plate as described in the section 2.1. After finding the height for the strongest ablation line, insert a 0.5 mm copper wire at the center of the laser beam just before the phase mask. The copper wire should be along the Y-axis (perpendicular to the ablation line).</li>
		<li>Check the ablation pattern on the glass plate while changing the position of the copper wire along the Z-axis. Fix the position of the copper wire when the ablation pattern shows a single gap at the center of the ablation line.</li>
		<li>After the alignment, perform the femtosecond laser fabrication on the nanofiber following the procedure detailed in the section 2.2. For this fabrication, set the angle of fabrication to &#952; = 0 deg.</li>
	</ol>
	</li>
</ol>
3. Characterization of the Fabricated Samples
<ol>
	<li>Measurement of optical properties
	<ol>
		<li>Prepare the setup for the optical measurements as shown in Figure 1b. Launch the broadband light source into the tapered fiber and measure the transmission and reflection spectrum before and after the fabrication using the spectrum analyzer. After fabrication, the transmission spectrum will show a stopband corresponding to the Bragg resonance of the fabricated sample.</li>
		<li>Rotate the paddles of the fiber inline polarizer to select the polarization and take the spectra for two orthogonal polarizations X-pol and Y-pol. 
		NOTE: For the X-pol (polarization along the nano-craters) the stopband will be blue-shifted21 (towards the shorter wavelength) and the scattering from the nanofiber will be stronger. So, select the polarizations by looking at the spectrum and the CCD camera.</li>
		<li>For one of the polarizations, take the transmission spectra by stretching the tapered fiber using the PZT (shown in Figure 1b). Take the spectra by stretching the tapered fiber in steps of 2 &#956;m until the maximum stretching length of 20 &#956;m (limited by the PZT scanning range). Observe that the Bragg resonance will be red-shifted (towards the longer wavelength) by stretching the tapered fiber. From these spectra, calculate the shift of the Bragg resonance, per unit stretching length.</li>
		<li>For resolving the cavity modes and measuring the absolute cavity transmission, use the tunable CW laser source. Launch the laser into the tapered fiber and monitor the transmission using a photodiode.</li>
		<li>Set the laser wavelength to the red-side edge of the stopband for Y-pol and use the fiber inline polarizer to minimize the transmission. In this way, the X-pol component is suppressed and only the Y-pol is selected. Set the laser wavelength to further out of the red-side band-edge and record the transmission while stretching the tapered fiber from 0 - 20 &#956;m.
		<ol>
			<li>Repeat the measurement by changing the laser wavelength to blue-side in steps of 0.3 nm until the entire stopband is covered. From these data, reconstruct the entire spectrum using the data for the resonance shift per unit stretching length measured in step 3.1.3. 
			NOTE: For a typical sample, the stopband (Bragg resonance) along with the cavity modes shifts by 2 nm by stretching the tapered fiber by 20 &#956;m and the typical free spectral range for the cavity modes are between 0.05 - 0.5 nm. For a given wavelength of the input laser one can measure at least 3 - 4 cavity modes by stretching the tapered fiber. The frequency spacing between the modes is inferred from the data for the resonance shift per unit stretching length measured in step 3.1.3. Repeating the measurement by changing the laser wavelength in steps of 0.3 nm, at least 2 - 3 consecutive cavity modes are re-measured in the successive measurements. One can reconstruct the entire spectrum by overlaying the transmission data for the successive measurements while matching position of the re-measured cavity modes.</li>
		</ol>
		</li>
		<li>Now measure the spectrum for the other polarization using similar procedure as mentioned in steps 3.1.5 and 3.1.5.1.</li>
	</ol>
	</li>
	<li>Imaging the fabricated sample
	<ol>
		<li>Put the fabricated sample on a 2 cm long metal plate and fix the two ends of the tapered fiber to the metal plate using UV curable epoxy. Make sure the irradiation side of the sample faces the metal plate so that the shadow side can be imaged.</li>
		<li>Use the Pt-coater to coat the sample for 30 s and deposit a layer of Pt with a thickness of around 10 nm. Place the sample into the SEM. Take the SEM image of the sample at every 0.1 mm over the entire fabricated region.</li>
	</ol>
	</li>
</ol>

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</ol>

The authors have nothing to disclose.

The lensing effect of the nanofiber plays an important role in the fabrication technique, thereby creating nano-craters on the shadow surface of the nanofiber (shown in Figure 2). The lensing effect of the nanofiber also makes the fabrication process robust to any mechanical instabilities in the transverse direction (Y-axis). Moreover, due to single-shot irradiation, the instabilities along the other axes do not affect the fabrication as the irradiation time is only 120 fs (i.e. pulse width). As...

Strong confinement of light in nanophotonic devices has opened new frontiers in optical science. Modern nanofabrication technologies have enabled fabrication of 1-D and 2-D Photonic Crystal (PhC) cavities for new prospects in lasing1, sensing2 and optical switching applications3. Moreover, strong light-matter interaction in these PhC cavities has opened new avenues for quantum information science4. Apart from PhC cavities, plasmonic nanocavities have also shown promising prospects5,6,7. However, interfacing such cavities to fiber-based communication network remains a challenge.
In recent years, tapered single mode optical fiber with subwavelength diameter, known as optical nanofiber, has emerged as a promising nanophotonic device. Due to the strong transverse confinement of the nanofiber guided field and the ability to interact with the surrounding medium, the nanofiber is widely adapted and investigated for various nanophotonic applications8. Apart from that, it is also strongly investigated and implemented for quantum manipulation of light and matter9. Efficient coupling of emission from quantum emitters like, single/few laser-cooled atoms and single quantum dots, into the nanofiber guided modes has been studied and demonstrated10,11,12,13,14,15. The light-matter interaction on nanofiber can be significantly improved by implementing PhC cavity structure on the nanofiber16,17.
The key advantage for such a system is the fiber-in-line technology which can be readily integrated to communication network. Light transmission of 99.95% through the tapered nanofiber has been demonstrated18. However, the nanofiber transmission is extremely susceptible to dust and contamination. Therefore, fabrication of PhC structure on nanofiber using conventional nanofabrication technique is not very fruitful. Although cavity fabrication on nanofiber using Focused Ion Beam (FIB) milling has been demonstrated19,20, the optical quality and reproducibility is not as high.
In this video protocol, we present a recently demonstrated21,22 technique to fabricate PhC cavities on nanofiber using femtosecond laser ablation. The fabrications are performed by creating a two-beam interference pattern of the femtosecond laser on the nanofiber and irradiating a single femtosecond laser pulse. The lensing effect of the nanofiber plays an important role in the feasibility of such techniques, creating ablation craters on the shadow surface of the nanofiber. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. Such periodic nano-craters on nanofiber, act as a 1-D PhC. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber.
A key aspect of such nanofabrication is the all optical fabrication, so that high optical quality can be maintained. Moreover, the fabrication is done by the irradiation of just a single femtosecond laser pulse, making the technique immune to mechanical instabilities and other fabrication imperfections. Also this enables in-house production of PhC nanofiber cavity so that the probability of contamination can be minimized. This protocol is intended to help others implement and adapt this new type of nanofabrication technique.
Figure 1a shows the schematic diagram of the fabrication setup. The details of the fabrication setup and alignment procedures are discussed in21,22. A femtosecond laser with 400 nm center wavelength and 120 fs pulse width is incident on a phase mask. The phase mask splits the femtosecond laser beam in to 0 and &#177;1 orders. A beam block is used to block the 0-order beam. The folding mirrors symmetrically recombine the &#177;1-orders at the nanofiber position, to create an interference pattern. The pitch of the phase mask is 700 nm, so the interference pattern has a pitch (&#923;G) of 350 nm. The cylindrical lens focuses the femtosecond laser beam along the nanofiber. The beam size across (Y-axis) and along (Z-axis) the nanofiber is 60 &#181;m and 5.6 mm, respectively. The tapered fiber is mounted on a holder equipped with piezo actuator (PZT) for stretching the fiber. A top cover with glass plate is used to protect the nanofiber from dust. The holder with the tapered fiber is fixed on a fabrication bench equipped with translation (XYZ) and rotation (&#952;) stages. The &#952;-stage allows rotation of the nanofiber sample in the YZ-plane. The X-stage can also control the tilt angles along XY- and XZ-plane. A CCD camera is placed at a distance of 20 cm from the nanofiber and at an angle of 45&#176;&#160;in the XY-plane to monitor the nanofiber position. All the experiments are performed inside a clean booth equipped with HEPA (High-efficiency particulate arresting) filters to achieve dust-free conditions. Dust-free condition is essential to maintain the transmission of the nanofiber.
Figure 1b shows the schematic of the optical measurements. During fabrication, the optical properties are briefly monitored by launching a broadband (wavelength range: 700 - 900 nm) fiber-coupled light source into the tapered fiber and measuring the spectrum of the transmitted and reflected light using high resolution spectrum analyzer. A tunable CW laser source is used to properly resolve the cavity modes and to measure the absolute cavity transmission.
We present the protocol for the fabrication and characterization. The protocol section is divided in three subsections, nanofiber preparation, femtosecond laser fabrication and characterization of the fabricated samples.

<table><tbody><tr><td>Femtosecond Laser</td><td>Coherent Inc.</td><td>Libra HE</td><td></td></tr><tr><td>Phase Mask</td><td>Ibsen Photonics</td><td>Custom Made</td><td></td></tr><tr><td>Optial Nanofiber Manufacturing Equipment&amp;nbsp;&amp;nbsp;</td><td>Ishihara Sangyo</td><td>ONME</td><td></td></tr><tr><td>ADC Card</td><td>PicoTech</td><td>ADC-24</td><td></td></tr><tr><td>Single mode fiber</td><td>Fujikura</td><td>FutureGuide-SM</td><td></td></tr><tr><td>Broadband source</td><td>NKT Photonics</td><td>SuperK EXTREME</td><td></td></tr><tr><td>CW Tunable Laser</td><td>Coherent Inc.</td><td>MBR-110</td><td></td></tr><tr><td>Spectrum analyser (Transmission spectrum)</td><td>Thermo Fisher Scientific</td><td>Nicolet 8700</td><td></td></tr><tr><td>Spectrum analyser (Reflection spectrum)</td><td>Ocean Optics</td><td>QE65000</td><td></td></tr><tr><td>CCD Camera</td><td>Thorlabs</td><td>DCC1545M</td><td></td></tr><tr><td>Power Meter</td><td>Thorlabs</td><td>D3MM</td><td></td></tr><tr><td>Pt-Coater</td><td>Vacuum Device Inc.</td><td>MSP-1S</td><td></td></tr><tr><td>Scanning Electron Microscope</td><td>Keyence</td><td>VE-9800</td><td></td></tr><tr><td>UV Curable Epoxy</td><td>NTT-AT</td><td>AT8105</td><td></td></tr><tr><td>Photodiode</td><td>ThorLabs</td><td>PDA 36A-EC</td><td></td></tr><tr><td>Clean room wipe</td><td>TExWipe</td><td>TX-404</td><td></td></tr><tr><td>Fiber coating stripper</td><td>NTT-AT</td><td>Fiber nippers 250 &amp;mu;m&amp;nbsp;</td><td></td></tr><tr><td>Cover glass</td><td>Matsunami Glass IND,LTD</td><td>NEO micro cover glass 0.12-0.17 mm&amp;nbsp;</td><td></td></tr><tr><td>PZT</td><td>NOLIAC</td><td>NAC 2011-H20</td><td></td></tr><tr><td>Cylindrical lens stage</td><td>NewPort</td><td>M-481-A&amp;nbsp;</td><td></td></tr><tr><td>Y,Z stages</td><td>Chuo Precision Industrial Co., LTD.</td><td>LD-149-C7</td><td></td></tr><tr><td>Rotation stage</td><td>SIGMA KOKI</td><td>KSPB-1026MH</td><td></td></tr><tr><td>Z-stage(1), Z-stage(2)</td><td>NewPort</td><td>M-460P&amp;nbsp;</td><td></td></tr></tbody></table>

fabrication of 1-d photonic crystal cavity on a nanofiber using femtosecond laser-induced ablation

Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using femtosecond laser-induced ablation. We show that thousands of periodic nano-craters are fabricated on an optical nanofiber by irradiating with just a single femtosecond laser pulse. For a typical sample, periodic nano-craters with a period of 350 nm and with diameter gradually varying from 50 - 250 nm over a length of 1 mm are fabricated on a nanofiber with diameter around 450 - 550 nm. A key aspect of such a nanofabrication is that the nanofiber itself acts as a cylindrical lens and focuses the femtosecond laser beam on its shadow surface. Moreover, the single-shot fabrication makes it immune to mechanical instabilities and other fabrication imperfections. Such periodic nano-craters on nanofiber, act as a 1-D PhC and enable strong and broadband reflection while maintaining the high transmission out of the stopband. We also present a method to control the profile of the nano-crater array to fabricate apodized and defect-induced PhC cavities on the nanofiber. The strong confinement of the field, both transverse and longitudinal, in the nanofiber-based PhC cavities and the efficient integration to the fiber networks, may open new possibilities for nanophotonic applications and quantum information science.

Figure 2 shows the SEM image of a typical segment of the fabricated nanofiber sample. It shows that periodic nano-craters are formed on the shadow side of the nanofiber, with a periodicity of 350 nm corresponding well to the interference pattern. The inset shows the enlarged view of the sample. The shape of the nano-craters is almost circular and the diameter of a typical nano-crater is around 210 nm.
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We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

We present a protocol for fabricating 1-D Photonic Crystal (PhC) cavities on subwavelength-diameter tapered optical fibers, optical nanofibers, using ...

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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