1. Introduction

Ultrasound, defined as sound at frequencies range of 20 kHz ~several GHz, has been applied to various applications depending on its frequency, and increases its applicable areas even further [1–4]. Especially, high-frequency ultrasound (HFUS) of tens-of-megahertz is used in biomedical ultrasound for diagnostic and therapeutic applications because of its unique merits; no danger of radiation exposure (X-ray) and interactive diagnosis by real-time imaging are the significant advantages of ultrasound over other biomedical imaging systems. As for therapeutic usage, high-intensity focused ultrasound (HIFU) can deliver local heating on the targeted and specific tissue through ultrasonic absorption in a non-invasive procedure [5, 6]. To date, the main approach to generate the ultrasound is a piezoelectric effect; by applying a high voltage to piezoelectric materials such as quartz and crystals, the materials vibrates and thus ultrasound generated through a vibrating medium. However, the mechanical weakness of piezo-materials, electrical issues such as increasing impedance with scale-down, difficulty with generating HFUS put limits on expanding its applications [7].

As an emerging approach, ultrasound transmitters based on optoacoustic effect have been actively investigated. Such transmitters typically consist of light-absorbing materials with specific nanostructures to increase optical absorption. When irradiated by laser pulses (~ns), the materials absorb part of the light energy which is converted to heat, and thus vibrate the surrounding medium through its thermal expansion and generate ultrasound with several tens MHz. To obtain high frequency ultrasound with high amplitude, various approaches have been reported: thin-metal layer (e.g. Chromium) [8], two-dimensional gold nanoparticle arrays (AuNPs) [9], carbon nanotube (CNT) and polydimethylsiloxane (PDMS) composites [10–14], etc.

Especially, optoacoustic ultrasound transmitter using CNT-PDMS composite was reported to produce the highest ultrasonic output up to ~80 MHz attributed to its high optical absorption coefficient at an incident pulse laser wavelength and high thermal conductivity, facilitating fast thermal energy conversion to the surrounding materials [10]. In other words, CNT-PDMS composites has shown the highest optoacoustic conversion efficiency. However, for the ultrasound transmitter based on the thermal-grown CNT composite, it is challenging to use flexible plastic substrates due to the high growth temperature (~770 °C) [15], and non-uniformity of CNT areal density results in a non-uniform intensity of the ultrasonic waves [16]. Furthermore, in order to synthesize a high density composite using as-grown CNT and a polymer, the process becomes complicated since it needs go through functionalizing the surface of CNT to prevent their agglomerate together due to van der Waals interaction and obtain uniform mixing as well as dispersion of CNT.

In this work, we demonstrate optoacoustic ultrasound transmitter based on nanostructured germanium (Ge) fabricated by simple metal-assisted chemical (MAC) etching in combination with spin-coated PDMS film. In order to compare and optimize optoacoustic conversion efficiency, we prepared Ge thin-films of three different thicknesses with various MAC etching times, and compared output ultrasonic waves with reference Cr film and CNT-PDMS composite. The nanostructured surface of Ge is intended to enhance the heat transfer of light-absorbed energy to the surrounding medium by increasing a contact area with PDMS, and therefore generate HFUS more efficiently. Frequency spectra also show that the proposed transmitter based on nanostructured Ge exhibits excellent optoacoustic conversion.

2. Experimental

Figure 1 shows a schematic flow chart of fabrication procedure of the nanostructured Ge ultrasound transmitter. First, Ge thin-films with three different thicknesses (300, 500, 700 nm) was deposited onto a quartz substrate by e-beam evaporator at room temperature [Fig. 1(a)]. Then a solution with well dispersed Au nanoparticles (AuNPs) were spin-coated and baked on a hotplate at 110 °C for 30 mins [Fig. 1(b)]. Metal-assisted chemical (MAC) etching of the Ge was performed under deionized water [17], and the etching time was controlled [Figs. 1(c) and (d)]; we varied the etching time from 10 to 50 hours with 10 hrs step. Chemical reactions of the MAC etching on the Ge surface can be expressed as below [17]:

 

Fig. 1 Schematic flow chart of the fabrication procedure of the nanostructured Ge-based high-frequency ultrasound transmitter using metal-assisted chemical etching. (a) Ge thin-film (300, 500, or 700 nm) deposition. (b) Dispersion of AuNPs on the Ge thin-film. (c) and (d) MAC etching of the Ge in DI water. (e) Spin-coating of PDMS.

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The experimental setup for characterizing acoustic performances of the fabricated transmitters is illustrated in Fig. 2. The nanostructured Ge transmitter is mounted on a water tank, and excited with ~7 ns pulsed laser (532 nm, 1.93 mJ/pulse, 3 mm in diameter) which is incident onto the device at an angle of 47.7°. The non-zero angle of laser incidence is not to interfere the alignment of a needle hydrophone (Precision Acoustics, UK) which is perpendicular to the transmitter film. The generated ultrasound was detected by the hydrophone 3.75 mm apart from the transmitter, and recorded by an oscilloscope. We compared the ultrasound output of nanostructured Ge transmitters with those of Cr (100 nm) and CNT-PDMS composite (~4-5 μm) films [14] used as references at the same condition. Especially, a thin Cr film is widely used as a control sample because of the simplest form with minimum process deviation and consistent optoacoustic properties [8, 10].

 

Fig. 2 Experimental setup for characterizing acoustic performances of the fabricated transmitters. A ~7 ns pulsed laser with 532 nm wavelength is incident onto the device at an angle of 47.7°, and the generated ultrasound was detected by a hydrophone and recorded by an oscilloscope.

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3. Results and discussion

Figure 3(a) shows X-ray diffraction (XRD) analysis of the Ge film deposited separately on a quartz substrate. No additional peak was observed other than the peaks of a quartz substrate, and so amorphous Ge was confirmed. Figure 3(b) shows the Ge surface after the deposition of Au nanoparticles, which are well dispersed and approximately 20 nm in diameter. Since the nanoparticle size is smaller than the diffraction limit of visible light, the surface is still smooth and reflective. The MAC etched Ge surface prior to PDMS spin-coating presented in Fig. 3(c) shows irregular nanostructures with various depths and diameters. While the MAC etching on a single crystalline Ge(100) generates inverted pyramidal shape of pits on the surface [17], the irregular nanostructures would be due to isotropic etching near the AuNPs in the amorphous Ge. This is very helpful to induce more scattering of light and, thus, more energy is absorbed by Ge.

 

Fig. 3 (a) X-ray diffraction (XRD) patterns of quartz substrate and 500 nm thick Ge film deposited on the quartz. (b) Scanning electron microscope (SEM) image of the deposited Ge layer with well-dispersed Au nanoparticles (approximately 20 nm in diameter). (c) SEM images of metal-assisted chemical etched Ge surface prior to PDMS spin-coating.

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Figure 4(a) shows the absorption spectra of Ge samples with three different Ge thicknesses of 300, 500, and 700 nm, which were all etched for 30 hours, in the 400-700 nm wavelength range. The absorption, reflection and transmission spectra of the etched Ge film of 500 nm thick, as an example, was also presented in the inset of Fig. 4(a). The absorption ratio increased with the decreasing Ge film thickness for the same etching time. Comparison of the absorption ratio at a laser wavelength of 532 nm for the 300, 500, 700 nm thick Ge samples in response to different MAC etching time (10, 20, 30, 40, 50 hrs) is presented in Fig. 4(b). As the etching time increased, the absorption ratio also increased due to reduced reflectance at the nanostructured Ge surface.

 

Fig. 4 (a) Absorption spectra of the nanostructured Ge samples (etching time: 30 hours) with three different Ge thicknesses of 300, 500, 700 nm in the 400-700 nm wavelength range. Inset: absorption, reflection and transmission spectra from the etched Ge film of 500 nm thick. (b) Comparison of the absorption ratio at a wavelength of 532 nm for 300, 500, 700 nm thick Ge samples in response to different MAC etching times (10, 20, 30, 40, 50 hrs).

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Figure 5(a) shows the output ultrasonic waveforms generated from the Ge samples with various thickness in comparison to the reference bare Cr layer and CNT-PDMS composite. Ge samples are all etched for 30 hrs. 300 nm thick Ge samples exhibited stronger pressure than other two Ge samples due to its high optical absorption ratio as shown in Fig. 4(a). Compared to the reference transmitters, the output amplitude was 7.5 times stronger than that of the Cr transmitter, and was around 42% of the CNT-PDMS composite. As for MAC etching time dependence of output ultrasonic waveforms, we representatively chose the 500 nm thick Ge samples with MAC etching times of 10~50 hrs, and measured the output waveforms under the same laser fluence (1.93 mJ/pulse). The signal amplitudes increased with longer etching time as shown in Fig. 5(b). The increased pressure can be explained with the enhanced optical absorption, consequent heating of Ge nanostructures and thus larger thermal expansion of the coated PDMS. Furthermore, Fig. 5(c) shows a linear relationship between the absorption ratio and generated ultrasound intensity with a correlation coefficient (Pearson’s r) of 0.89. Therefore, in order to enhance the optoacoustic conversion efficiency of the nanostructured Ge transmitter and thus to generate strong ultrasonic output, the nanostructures on the Ge surface formed by MAC etching need to be more dense as well as rough. Considering the pulsed laser energy, however, the generated ultrasound intensity was rather weak because of Ge’s indirect band gap and heat dissipation through quartz substrate; indirect band gap allows only part of the light energy to be absorbed and thus contribute to ultrasound generation. Furthermore, heat from the light absorbed energy was transferred to the PDMS layer as well as quartz substrate with high thermal conductivity (~1.4 W/m·K) [18]. To further enhance optoacoustic pressure intensity, the transmitter structure (e.g. nanostructured Ge in-between PDMS layers) should be optimized.

 

Fig. 5 (a) Output ultrasonic waveforms generated from the fabricated transmitters with various Ge thickness (MAC etching time: 30 hrs) in comparison to the reference bare Cr layer and CNT-PDMS composite-based ultrasound transmitters. (b) MAC etching time dependence of output ultrasonic waveforms for 500 nm thick Ge samples under a laser fluence of 1.93 mJ/pulse. (c) Relationship between the absorption ratio and generated ultrasound intensity with a correlation coefficient (Pearson’s r) of 0.89. (d) Frequency spectra for the measured time-domain waveforms in (a), which are normalized to each maximum intensity.

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Finally, we investigate the frequency-domain performance of the nanostructured Ge optoacoustic transmitter over broadband frequency. Figure 5(d) shows the corresponding frequency spectra obtained from the measured waveforms. Each spectrum was normalized to each maximum value. The spectral peaks of the transmitters are about ~15 MHz for nanostructured Ge with a thickness of 300 nm and ~14 MHz for CNT-PDMS composite. 6-dB frequency bandwidth are ~27 and ~25.5 MHz for nanostructured Ge and CNT-PDMS composite, respectively. The nanostructured Ge-based transmitters show comparably high frequency efficiency to that of the CNT-based transmitter. Unfortunately, the piezoelectric hydrophone used in our measurement has a 6-dB bandwidth of ~20 MHz and higher frequency responses were not exactly obtained and compared. Nonetheless, we can conclude that the nanostructured Ge-based transmitter generates high-frequency ultrasound as efficiently as the CNT-PDMS composite does. This is presumably due to the fast heat transfer from the absorption of nanostructured Ge to the surrounding PDMS.

4. Conclusions

In summary, using simple MAC etching process, we demonstrated a unanostructured Ge-PDMS based optoacoustic transmitter capable of generating strong and high frequency ultrasound; the optoacoustic signal amplitude was 7.5 times stronger than that of the Cr-based reference transmitter, and the frequency spectra analysis shows an efficient HFUS generation (primary: ~15 MHz, and 6dB roll-off at ~27 MHz). Further enhancement can be obtained from reducing heat dissipation to a quartz substrate and improving heat transfer to PDMS through optimization of the proposed transmitter structure. Comparing with other ultrasound transmitters such as a CNT-PDMS composite and an AuNPs array with a PDMS overlayer, moreover, the nanostructured Ge-based transmitter can be fabricated by a simple wet-etching process without substrate limitation. The demonstrated transmitter can be integrated with optical ultrasound detectors and therefore offer a promising approach for developing all-optical HFUS transducers.