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

Summary

We present a protocol for fabricating 1-D photonic crystal cavities on subwavelength diameter silica fibers (optical nanofibers) using femtosecond laser-induced ablation.

Abstract

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.

Introduction

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 prospects^5,6,7. 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 demonstrated^{10,11,12,13,14,15}. The light-matter interaction on nanofiber can be significantly improved by implementing PhC cavity structure on the nanofiber^16,17.

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 demonstrated^19,20, the optical quality and reproducibility is not as high.

In this video protocol, we present a recently demonstrated^21,22 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 in^21,22. 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 ±1 orders. A beam block is used to block the 0-order beam. The folding mirrors symmetrically recombine the ±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 (Λ_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 µm 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 (θ) stages. The θ-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° 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.

Protocol

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

1. 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.
2. Set the stripped fiber on the two stages of the Optical Nanofiber Manufacture Equipment (ONME) to fabricate the nanofiber.
   1. 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.
3. 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.

2. Femtosecond Laser Fabrication

1. Alignment of the fabrication setup
   1. 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.
   2. 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.
   3. 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.
   4. 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 μm per pixel.
   5. 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 - 2.1.4) for lower pulse energies (down to 300 μJ) until a periodic ablation pattern is seen.
2. Fabrication of apodized PhC cavity
   1. Place the tapered fiber on the fabrication bench approximately parallel to the ablation line marked on the CCD camera.
   2. 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.
   3. Translate the Z-stage of the fabrication bench to center the nanofiber to the ablation line position marked on the CCD camera.
   4. Switch off the probe laser and irradiate the femtosecond laser with minimum pulse energy (<10 μJ). 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.
   5. 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.
   6. 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.
   7. 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 θ = 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.
   8. 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.
      1. Remove the power meter from the laser beam path and fire a single femtosecond laser pulse. This completes the fabrication process.
3. Fabrication of defect-induced PhC cavity
   1. 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).
   2. 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.
   3. 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 θ = 0 deg.

3. Characterization of the Fabricated Samples

1. Measurement of optical properties
   1. 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.
   2. 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.
   3. 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 μm until the maximum stretching length of 20 μm (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.
   4. 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.
   5. 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 μm.
      1. 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 μm 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.
   6. Now measure the spectrum for the other polarization using similar procedure as mentioned in steps 3.1.5 and 3.1.5.1.
2. Imaging the fabricated sample
   1. 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.
   2. 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.

Results

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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Discussion

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...

Materials

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