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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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

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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Watch this Scientific Journal Video about Fabrication of 1-D Photonic Crystal Cavity on a Nanofiber Using Femtosecond Laser-induced Ablation at JoVE.com

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 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 ...

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9.7K Views.  University of Electro-Communications. We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

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

Fabrication of Silica Ultra High Quality Factor Microresonators

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the quality factor (Q) of the cavity, can be in excess of 500ns for cavities with Q factors above 100 million. As a result of their low material losses, silica microcavities have demonstrated some of the longest photon lifetimes to date1-2. Since a portion of the circulating light extends outside the resonator, these devices can also be used to probe the surroundings. This interaction has enabled numerous experiments in biology, such as single molecule biodetection and antibody-antigen kinetics, as well as discoveries in other fields, such as development of ultra-low-threshold microlasers, characterization of thin films, and cavity quantum electrodynamics studies.3-7
The two primary silica resonant cavity geometries are the microsphere and the microtoroid. Both devices rely on a carbon dioxide laser reflow step to achieve their ultra-high-Q factors (Q&gt;100 million).1-2,8-9 However, there are several notable differences between the two structures. Silica microspheres are free-standing, supported by a single optical fiber, whereas silica microtoroids can be fabricated on a silicon wafer in large arrays using a combination of lithography and etching steps. These differences influence which device is optimal for a given experiment.
Here, we present detailed fabrication protocols for both types of resonant cavities. While the fabrication of microsphere resonant cavities is fairly straightforward, the fabrication of microtoroid resonant cavities requires additional specialized equipment and facilities (cleanroom). Therefore, this additional requirement may also influence which device is selected for a given experiment.
Introduction
An optical resonator efficiently confines light at specific wavelengths, known as the resonant wavelengths of the device. 1-2 The common figure of merit for these optical resonators is the quality factor or Q. This term describes the photon lifetime (&tau;o) within the resonator, which is directly related to the resonator's optical losses. Therefore, an optical resonator with a high Q factor has low optical losses, long photon lifetimes, and very low photon decay rates (1/&tau;o). As a result of the long photon lifetimes, it is possible to build-up extremely large circulating optical field intensities in these devices. This very unique property has allowed these devices to be used as laser sources and integrated biosensors.10
A unique sub-class of resonators is the whispering gallery mode optical microcavity. In these devices, the light is confined in circular orbits at the periphery. Therefore, the field is not completely confined within the device, but evanesces into the environment. Whispering gallery mode optical cavities have demonstrated some of the highest quality factors of any optical resonant cavity to date.9,11 Therefore, these devices are used throughout science and engineering, including in fundamental physics studies and in telecommunications as well as in biodetection experiments. 3-7,12
Optical microcavities can be fabricated from a wide range of materials and in a wide variety of geometries. A few examples include silica and silicon microtoroids, silicon, silicon nitride, and silica microdisks, micropillars, and silica and polymer microrings.13-17 The range in quality factor (Q) varies as dramatically as the geometry. Although both geometry and high Q are important considerations in any field, in many applications, there is far greater leverage in boosting device performance through Q enhancement. Among the numerous options detailed previously, the silica microsphere and the silica microtoroid resonator have achieved some of the highest Q factors to date.1,9 Additionally, as a result of the extremely low optical loss of silica from the visible through the near-IR, both microspheres and microtoroids are able to maintain their Q factors over a wide range of testing wavelengths.18 Finally, because silica is inherently biocompatible, it is routinely used in biodetection experiments.
In addition to high material absorption, there are several other potential loss mechanisms, including surface roughness, radiation loss, and contamination loss.2 Through an optimization of the device size, it is possible to eliminate radiation losses, which arise from poor optical field confinement within the device. Similarly, by storing a device in an appropriately clean environment, contamination of the surface can be minimized. Therefore, in addition to material loss, surface scattering is the primary loss mechanism of concern.2,8
In silica devices, surface scattering is minimized by using a laser reflow technique, which melts the silica through surface tension induced reflow. While spherical optical resonators have been studied for many years, it is only with recent advances in fabrication technologies that researchers been able to fabricate high quality silica optical toroidal microresonators (Q&gt;100 million) on a silicon substrate, thus paving the way for integration with microfluidics.1
The present series of protocols details how to fabricate both silica microsphere and microtoroid resonant cavities. While silica microsphere resonant cavities are well-established, microtoroid resonant cavities were only recently invented.1 As many of the fundamental methods used to fabricate the microsphere are also used in the more complex microtoroid fabrication procedure, by including both in a single protocol it will enable researchers to more easily trouble-shoot their experiments.

We describe the use of a carbon dioxide laser reflow technique to fabricate silica resonant cavities, including free-standing microspheres and on-chip microtoroids. The reflow method removes surface imperfections, allowing long photon lifetimes within both devices. The resulting devices have ultra high quality factors, enabling applications ranging from telecommunications to biodetection.

Whispering gallery resonant cavities confine light in circular orbits at their periphery.1-2 The photon storage lifetime in the cavity, quantified by the ...

A Method to Fabricate Disconnected Silver Nanostructures in 3D

The standard nanofabrication toolkit includes techniques primarily aimed at creating 2D patterns in dielectric media. Creating metal patterns on a submicron scale requires a combination of nanofabrication tools and several material processing steps. For example, steps to create planar metal structures using ultraviolet photolithography and electron-beam lithography can include sample exposure, sample development, metal deposition, and metal liftoff. To create 3D metal structures, the sequence is repeated multiple times. The complexity and difficulty of stacking and aligning multiple layers limits practical implementations of 3D metal structuring using standard nanofabrication tools. Femtosecond-laser direct-writing has emerged as a pre-eminent technique for 3D nanofabrication.1,2 Femtosecond lasers are frequently used to create 3D patterns in polymers and glasses.3-7 However, 3D metal direct-writing remains a challenge. Here, we describe a method to fabricate silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm. The method enables the fabrication of patterns not feasible using other techniques, such as 3D arrays of disconnected silver voxels.8 Disconnected 3D metal patterns are useful for metamaterials where unit cells are not in contact with each other,9 such as coupled metal dot10,11or coupled metal rod12,13 resonators. Potential applications include negative index metamaterials, invisibility cloaks, and perfect lenses.
In femtosecond-laser direct-writing, the laser wavelength is chosen such that photons are not linearly absorbed in the target medium. When the laser pulse duration is compressed to the femtosecond time scale and the radiation is tightly focused inside the target, the extremely high intensity induces nonlinear absorption. Multiple photons are absorbed simultaneously to cause electronic transitions that lead to material modification within the focused region. Using this approach, one can form structures in the bulk of a material rather than on its surface.
Most work on 3D direct metal writing has focused on creating self-supported metal structures.14-16 The method described here yields sub-micrometer silver structures that do not need to be self-supported because they are embedded inside a matrix. A doped polymer matrix is prepared using a mixture of silver nitrate (AgNO3), polyvinylpyrrolidone (PVP) and water (H2O). Samples are then patterned by irradiation with an 11-MHz femtosecond laser producing 50-fs pulses. During irradiation, photoreduction of silver ions is induced through nonlinear absorption, creating an aggregate of silver nanoparticles in the focal region. Using this approach we create silver patterns embedded in a doped PVP matrix. Adding 3D translation of the sample extends the patterning to three dimensions.

Femtosecond-laser direct-writing is frequently used to create three-dimensional (3D) patterns in polymers and glasses. However, patterning metals in 3D remains a challenge. We describe a method for fabricating silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm.

The standard nanofabrication toolkit includes  techniques primarily aimed at creating 2D patterns in dielectric media. Creating  metal patterns on a ...

Fabrication And Characterization Of Photonic Crystal Slow Light Waveguides And Cavities

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view and for its considerable potential for practical applications. Slow light photonic crystal waveguides, in particular, have played a major part and have been successfully employed for delaying optical signals1-4 and the enhancement of both linear5-7 and nonlinear devices.8-11
Photonic crystal cavities achieve similar effects to that of slow light waveguides, but over a reduced band-width. These cavities offer high Q-factor/volume ratio, for the realization of optically12 and electrically13 pumped ultra-low threshold lasers and the enhancement of nonlinear effects.14-16 Furthermore, passive filters17 and modulators18-19 have been demonstrated, exhibiting ultra-narrow line-width, high free-spectral range and record values of low energy consumption.
To attain these exciting results, a robust repeatable fabrication protocol must be developed. In this paper we take an in-depth look at our fabrication protocol which employs electron-beam lithography for the definition of photonic crystal patterns and uses wet and dry etching techniques. Our optimised fabrication recipe results in photonic crystals that do not suffer from vertical asymmetry and exhibit very good edge-wall roughness. We discuss the results of varying the etching parameters and the detrimental effects that they can have on a device, leading to a diagnostic route that can be taken to identify and eliminate similar issues.
The key to evaluating slow light waveguides is the passive characterization of transmission and group index spectra. Various methods have been reported, most notably resolving the Fabry-Perot fringes of the transmission spectrum20-21 and interferometric techniques.22-25 Here, we describe a direct, broadband measurement technique combining spectral interferometry with Fourier transform analysis.26 Our method stands out for its simplicity and power, as we can characterise a bare photonic crystal with access waveguides, without need for on-chip interference components, and the setup only consists of a Mach-Zehnder interferometer, with no need for moving parts and delay scans.
When characterising photonic crystal cavities, techniques involving internal sources21 or external waveguides directly coupled to the cavity27 impact on the performance of the cavity itself, thereby distorting the measurement. Here, we describe a novel and non-intrusive technique that makes use of a cross-polarised probe beam and is known as resonant scattering (RS), where the probe is coupled out-of plane into the cavity through an objective. The technique was first demonstrated by McCutcheon et al.28 and further developed by Galli et al.29

Use of photonic crystal slow light waveguides and cavities has been widely adopted by the photonics community in many differing applications. Therefore fabrication and characterization of these devices are of great interest. This paper outlines our fabrication technique and two optical characterization methods, namely: interferometric (waveguides) and resonant scattering (cavities).

Slow light has been one of the hot topics in the photonics community in the past decade, generating great interest both from a fundamental point of view ...

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O2 pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (&gt;80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO2, Al2O3, WO3 and Ag4O4.

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Nanosecond Pulsed  Laser Deposition (PLD) in the presence of a background gas allows the deposition  of metal oxides with tunable morphology, structure, ...

Fabrication of Polymer Microspheres for Optical Resonator and Laser Applications

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, interface precipitation, and mini-emulsion. In all methods, well-defined, micrometer-sized spheres are obtained from a self-assembling process in solution. The vapor diffusion method can result in spheres with the highest sphericity and surface smoothness, yet the types of the polymers able to form these spheres are limited. On the other hand, in the mini-emulsion method, microspheres can be made from various types of polymers, even from highly crystalline polymers with coplanar, &#960;-conjugated backbones. The photoluminescent (PL) properties from single isolated microspheres are unusual: the PL is confined inside the spheres, propagates at the circumference of the spheres via the total internal reflection at the polymer/air interface, and self-interferes to show sharp and periodic resonant PL lines. These resonating modes are so-called "whispering gallery modes" (WGMs). This work demonstrates how to measure WGM PL from single isolated spheres using the micro-photoluminescence (&#181;-PL) technique. In this technique, a focused laser beam irradiates a single microsphere, and the luminescence is detected by a spectrometer. A micromanipulation technique is then used to connect the microspheres one by one and to demonstrate the intersphere PL propagation and color conversion from coupled microspheres upon excitation at the perimeter of one sphere and detection of PL from the other microsphere. These techniques, &#181;-PL and micromanipulation, are useful for experiments on micro-optic application using polymer materials.

Protocols for the synthesis of microspheres from polymers, the manipulation of microspheres, and micro-photoluminescence measurements are presented.

This paper describes three methods of preparing fluorescent microspheres comprising &#960;-conjugated or non-conjugated polymers: vapor diffusion, ...

Femtosecond Laser Filaments for Use in Sub-Diffraction-Limited Imaging and Remote Sensing

Probing remote matter with laser light is a ubiquitous technique used in circumstances as diverse as laser-induced breakdown spectroscopy and barcode scanners. In classical optics, the intensity that can be brought to bear on a remote target is limited by the spot size of the laser at the distance of the target. This spot size has a lower bound determined by the diffraction limit of classical optics. However, amplified femtosecond laser pulses generate intensity sufficient to modify the refractive index of the ambient air and undergo self-focusing. This self-focusing effect leads to the generation of highly intense laser filaments which maintain their intensity and small sub-millimeter diameter size at distances well beyond the classical Rayleigh length. Such intensity provides the capability of remote scanning, imaging, sensing, and spectroscopy with enhanced spatial resolution. We describe a technique for generating filaments with a femtosecond regenerative chirped-pulse amplifier, and for using the resulting filament to conduct imaging and spectroscopic measurements at remote distances of at least several meters.

High-intensity femtosecond pulses of laser light can undergo cycles of Kerr self-focusing and plasma defocusing, propagating an intense sub-millimeter-diameter beam over long distances. We describe a technique for generating and using these filaments to perform remote imaging and sensing beyond the classical diffraction limits of linear optics.

Probing remote matter with laser light is a ubiquitous technique used in circumstances as diverse as laser-induced breakdown spectroscopy and barcode ...

Low-cost Custom Fabrication and Mode-locked Operation of an All-normal-dispersion Femtosecond Fiber Laser for Multiphoton Microscopy

A protocol is presented to build a custom low-cost yet high-performance femtosecond (fs) fiber laser. This all-normal-dispersion (ANDi) ytterbium-doped fiber laser is built completely using commercially available parts, including $8,000 in fiber optic and pump laser components, plus $4,800 in standard optical components and extra-cavity accessories. Researchers new to fiber optic device fabrication may also consider investing in basic fiber splicing and laser pulse characterization equipment (~$63,000). Important for optimal laser operation, methods to verify true versus apparent (partial or noise-like) mode-locked performance are presented. This system achieves 70 fs pulse duration with a center wavelength of approximately 1,070 nm and a pulse repetition rate of 31 MHz. This fiber laser exhibits the peak performance that may be obtained for an easily assembled fiber laser system, which makes this design ideal for research laboratories aiming to develop compact and portable fs laser technologies that enable new implementations of clinical multiphoton microscopy and fs surgery.

A method is presented to build a custom low-cost, mode-locked femtosecond fiber laser for potential applications in multiphoton microscopy, endoscopy, and photomedicine. This laser is built using commercially available parts and basic splicing techniques.

A protocol is presented to build a custom low-cost yet high-performance femtosecond (fs) fiber laser. This all-normal-dispersion (ANDi) ytterbium-doped ...

Fabrication of Nanoheight Channels Incorporating Surface Acoustic Wave Actuation via Lithium Niobate for Acoustic Nanofluidics

Controlled nanoscale manipulation of fluids is known to be exceptionally difficult due to the dominance of surface and viscous forces. Megahertz-order surface acoustic wave (SAW) devices generate tremendous acceleration on their surface, up to 108 m/s2, in turn responsible for many of the observed effects that have come to define acoustofluidics: acoustic streaming and acoustic radiation forces. These effects have been used for particle, cell, and fluid manipulation at the microscale, although more recently SAW has been used to produce similar phenomena at the nanoscale through an entirely different set of mechanisms. Controllable nanoscale fluid manipulation offers a broad range of opportunities in ultrafast fluid pumping and biomacromolecule dynamics useful for physical and biological applications. Here, we demonstrate nanoscale-height channel fabrication via room-temperature lithium niobate (LN) bonding integrated with a SAW device. We describe the entire experimental process including nano-height channel fabrication via dry etching, plasma-activated bonding on lithium niobate, the appropriate optical setup for subsequent imaging, and SAW actuation. We show representative results for fluid capillary filling and fluid draining in a nanoscale channel induced by SAW. This procedure offers a practical protocol for nanoscale channel fabrication and integration with SAW devices useful to build upon for future nanofluidics applications.

We demonstrate fabrication of nanoheight channels with the integration of surface acoustic wave actuation devices upon lithium niobate for acoustic nanofluidics via liftoff photolithography, nano-depth reactive ion etching, and room-temperature plasma surface-activated multilayer bonding of single-crystal lithium niobate, a process similarly useful for bonding lithium niobate to oxides.

Controlled nanoscale manipulation of fluids is known to be exceptionally difficult due to the dominance of surface and viscous forces. Megahertz-order ...

Fabrication of a Fabry-P&#233;rot Etalon for Application in Trace-Gas Spectroscopy

Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-P&#233;rot Etalon

Fabry-P&#233;rot etalons (FPE) have found their way into many applications. In fields such as spectroscopy, telecommunications, and astronomy, FPEs are used for their high sensitivity as well as their exceptional filtering capability. However, air-spaced etalons with high finesse are usually built by specialized facilities. Their production requires a clean room, special glass handling, and coating machinery, meaning commercially available FPEs are sold for a high price. In this article, a new and cost-effective method to fabricate fiber-coupled FPEs with standard photonic laboratory equipment is presented. The protocol should serve as a step-by-step guide for the construction and characterization of these FPEs. We hope this&#160;will enable researchers to conduct fast and cost-effective prototyping of FPEs for various fields of application. The FPE, as presented here, is used for spectroscopic applications. As shown in the representative results section via proof of principle measurements of water vapor in ambient air, this FPE has a finesse of 15, which is sufficient for the photothermal detection of trace concentrations of gases.

Author Spotlight: Fabrication of a Low-Cost, Fiber-Coupled, and Air-Spaced Fabry-P&#233;rot Etalon  

This protocol describes the construction of a low-cost, discrete, fiber-coupled, and air-spaced Fabry-Perot etalon with various applications, such as in trace-gas spectroscopy. The fabrication is possible in any facility with standard optical laboratory equipment available.

Fabry-P&#233;rot etalons (FPE) have found their way into many applications. In fields such as spectroscopy, telecommunications, and astronomy, FPEs are ...

Synthesizing Nanoparticles and Nanostructures via Laser Ablation

Ultrafast Laser-Ablated Nanoparticles and Nanostructures for Surface-Enhanced Raman Scattering-Based Sensing Applications

The technique of ultrafast laser ablation in liquids has evolved and matured over the past decade, with several impending applications in various fields such as sensing, catalysis, and medicine. The exceptional feature of this technique is the formation of nanoparticles (colloids) and nanostructures (solids) in a single experiment with ultrashort laser pulses. We have been working on this technique for the past few years, investigating its potential using the surface-enhanced Raman scattering (SERS) technique in hazardous materials sensing applications. Ultrafast laser-ablated substrates (solids and colloids) could detect several analyte molecules at the trace levels/mixture form, including dyes, explosives, pesticides, and biomolecules. Here, we present some of the results achieved using the targets of Ag, Au, Ag-Au, and Si. We have optimized the nanostructures (NSs) and nanoparticles (NPs) obtained (in liquids and air) using different pulse durations, wavelengths, energies, pulse shapes, and writing geometries. Thus, various NSs and NPs were tested for their efficiency in sensing numerous analyte molecules using a simple, portable Raman spectrometer. This methodology, once optimized, paves the way for on-field sensing applications. We discuss the protocols in (a) synthesizing the NPs/NSs via laser ablation, (b) characterization of NPs/NSs, and (c) their utilization in the SERS-based sensing studies.

Author Spotlight: Advancements and Applications in Nanoparticle Synthesis Through Laser Ablation in Liquids 

Ultrafast laser ablation in liquid is a precise and versatile technique for synthesizing nanomaterials (nanoparticles [NPs] and nanostructures [NSs]) in liquid/air environments. The laser-ablated nanomaterials can be functionalized with Raman-active molecules to enhance the Raman signal of analytes placed on or near the NSs/NPs.

The technique of ultrafast laser ablation in liquids has evolved and matured over the past decade, with several impending applications in various fields ...