2.1.

Fiber Preparation

Single-mode fibers (SMF) with core/cladding diameters of $8/125\mu \mathrm{m}$ and multimode fibers (MMF) with core/cladding diameters of $62.5/125\mu \mathrm{m}$ were used. The protective coating was removed from a 1-mm long section after the fiber tip was cleaved. The cleaved angle of the fiber endface was controlled to an accuracy of 0.5 deg as measured by a fusion splicer (Fitel S177A). A gold layer with a thickness of 60 nm was sputter-coated onto the fiber endface. This gold layer served several purposes. First, it protected the fiber surface and prevented charging of the insulated fiber material. In effect, the quality of scanning electron microscope (SEM) imaging and FIB milling was increased. Second, after FIB patterning of the gold layer, the remaining gold sub-wavelength apertures or rods on the fiber endface were directly used as the functional material of the fiber-optic ultrasound generator. The objective gold nanopores on an optical fiber are shown in Fig. 1 with both top view and side view. The fiber tip was placed steadily upon the FIB stage using a conductive tape. The fiber pigtail was winded and pasted for further splicing and testing. The FIB stage was adjusted such that the FIB beam faced the fiber endface vertically.

Fig. 1

Schematic diagram of the objective gold nanopores on the fiber endface.

2.2.

FIB Conditions

A computer-controlled dual-beam FIB instrument (FIB-SEM, Auriga series, Carl Zeiss, Oberkochen, Germany) was employed to create different gold nanopores on the fiber. The instrument used a combination of SEM and FIB with two focused beams on the coincidence point. In addition to providing SEM imaging, this instrument can rapidly mill materials via ion sputtering. Before fabrication, most of the patterning conditions need to be optimized specifically to the fiber sample. Accurate pattern shaping is unachievable at high energies due to large protrusions and nanopores formations; however, it can be obtained with low energy ions. Therefore, in order to significantly decrease the amount of redeposition, the gallium ion energy was set at 30 keV or lower if necessary. If a superior resolution is required it would be preferable to use a protective layer with the sample. In this case, the fiber endface was initially coated with a gold layer with a thickness of 60 nm to improve conductivity. Ion scattering from walls is another critical factor to the large feature depth. In this case, most of the nanostructures were shallow, so the scattering could be neglected.

For operation of the FIB, factors including the focus, astigmatism, and proper magnification of the particular ion beam aperture must be controlled to obtain high-quality nanostructures. Other important parameters to be adjusted include dwell time, beam current, scanning strategy, and the number of passes. Dwell time refers to the time interval that the beam spends on a single milling spot, and is set according to the milling rate and the expected milling depth. Note that the beam spot diameter is a theoretically calculated value based on a Gaussian distribution and depends on the beam current. Therefore, it should be properly selected according to the nanopore feature size. Line scanning is typically used for rectangular shapes whereas spiral scanning is typically used for circular shapes. The number of passes for each scanned pattern is also critical to milling rate and sidewall slope. In this case, the beam diameter was controlled between 3 and 300 nm by varying beam current 1 pA and 4 nA at a consistent incidence angle of 0 deg. Although FIB image could also be used to measure the feature dimension, this imaging mode damages the completed pattern severely. The SEM mode was used to monitor the FIB created surface features in situ with a 54-deg angle, and to measure the feature size with angle compensation.

2.3.

Gold Nanopores

The key to the FIB milling technology is to operate FIB with the proper beam size, current, and energy used to remove a required amount of material from a pre-defined location and obtain high-precision structures in a controllable manner. In order to ensure the milling efficiency, the beam energy transferred from the ions to the target substrate should be sufficiently high. In order to ensure the milling accuracy, milling strategy such as a number of passes is important to prevent severe redeposition. For different samples and feature sizes, we need to select a proper beam current to balance the fabrication accuracy and the yield. For this case, to achieve a feature size of $\sim 100\mathrm{nm}$, beam currents less than 10 pA was used to ensure the fabrication accuracy. Since the theoretical beam probe size of 10 pA about 13 nm is much smaller than the objective feature size of $\sim 100\mathrm{nm}$, it is still potentially advantageous to further increase the beam current for higher throughput. The objective nanopores were fabricated by FIB milling nanopore array. After the removal of the gold material within the pore areas, the remaining gold materials form the gold nanopore array as shown in Fig. 2. Figure 2(a) shows gold nanostructure on the core area of a fiber endface. As shown in the embedded Fig. 2(b), FIB is flexible to align, mark, and fabricate various nanostructures on the fiber endface. In the zoom-in Fig. 2(c), the gold nanopores are 80 nm in diameter and 130 nm in spacing.

Fig. 2

(a) Gold nanostructure on the core area of the fiber endface; (b) zoom-in FIB alignment mark and nanopores on the fiber endface; and (c) gold nanopores with 80 nm in diameter.

Since only the fiber core area works for the photoacoustic ultrasound generation, only the core area was milled with the desired gold nanopores. Figure 3(a) and 3(b) shows the overview of the SMF before and after fabrication, respectively. Total milling time was about 30 min when a beam current of 10 pA was used.

Fig. 3

(a) Gold coated SMF before fabrication and (b) gold coated SMF after fabrication.

3.1.

Photoacoustic Principle

As shown in Fig. 4, the fiber-optic photoacoustic generator is an optical fiber endface coated with the FIB-milled gold nanopores. When laser pulses are coupled into the fiber and guided to the gold nanopores, a portion of the optical irradiation energy is converted into thermal energy in the gold nanopores due to the high absorption efficiency. Then, due to the thermal effect, mechanical vibrations result from thermal expansion. Finally, ultrasound is excited and propagates in the adjacent medium. The advantage of this method is that the ultrasound pulse profile can be tailored by the laser pulse profile. This means broad bandwidth ultrasound can be generated by ultrashort laser pulses.

Fig. 4

Schematic diagram of the photoacoustic principle.

3.2.

Experimental Setup

After FIB patterning of the gold layer on the fiber endface, the fiber sample was spliced with a fiber coupler (F810SMA-543, Thorlabs, Newton, New Jersey) to a nanosecond laser. Related experiments were performed in order to evaluate the photoacoustic generation efficiency through the gold nanopores. The schematic diagram of the experimental setup is shown in Fig. 5. A 532 nm $\mathrm{Nd}:\mathrm{YAG}$ nanosecond laser (Surelite series, Continuum, Santa Clara, California) with a pulse duration of 5 ns and a repetition rate of 10 Hz was used as the optical radiation source. The laser beam was split into two beams by a beam splitter. One of the beams was input into a photodetector (PDA10CS, Thorlabs) as a reference signal and another beam was coupled into the fiber pattern through the coupler. The gold nanopores were located directly on the fiber endface core area. The laser pulse was excited on the nanocomposite through the optical fiber. A hydrophone (HGL-0200, Onda, Sunnyvale, California) was placed about 1 to 2 mm away under water to collect the acoustic signals. The reference signals and the acoustic signals were transmitted to a data acquisition system (M2i.4032, Spectrum, Grosshansdorf, Germany) with a sampling rate of 50 MHz.

Fig. 5

Schematic diagram of the experimental setup.

Figure 6(a) shows a photograph of the overall experimental setup. Translation stages were used to control the distance between the fiber tip and the hydrophone. Figure 6(b) shows an enlarged photograph of the miniature fiber-optic ultrasound generator with gold nanopores on it. Compared to the piezoelectric hydrophone, the fiber-optic ultrasound generator is much smaller, with a diameter of about 125 μm.

Fig. 6

(a) Photograph of the overall experimental setup and (b) enlarged photograph of the miniature fiber-optic ultrasound generator with gold nanopores on it.