Abstract
The scheme of a generation of ultrasound waves based on optically excited Tamm plasmon structures is proposed. It is shown that Tamm plasmon structures can provide total absorption of a laser pulse with arbitrary wavelength in a metallic layer providing the possibility of the use of an infrared semiconductor laser for the excitation of ultrasound waves. Laser pulse absorption, heat transfer and dynamical properties of the structure are modeled, and the optimal design of the structure is found. It is demonstrated that the Tamm plasmon-based photoacoustic generator can emit ultrasound waves in the frequency band up to 100 MHz with predefined frequency spectrum.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrasound waves are widely used for ultrasonic non-destructive evaluation, structure health monitoring, material characterization [1], orientation imaging microscopy [2] and acoustic microscopy [3,4] and optoacoustic tomography [5]. Existing conventional approach for generation of ultrasound waves utilize piezoelectric transducers, which bring some drawbacks associated with poor survivability in harsh environment, bulky size and weight, complexity of setups, high cost and susceptibility to electromagnetic interference [1]. Additionally, piezoelectric transducers exhibit a spectrum centered at the resonance frequency [6]. In contrary, photoacoustic transducers are a very attractive alternative for generation of ultrasound, because they are based on the effect of expansion of the optical absorbing layer heated by laser pulses [7,8]. In photoacoustic transducers, the absorber is heated and cooled, leading to mechanical deformations, which cause cycles of pressure, or, in the other words, acoustic waves in ambient surrounding [9]. The operation principle provides the number of advantages: reliability, compactness, full galvanic isolation [10], wideband operation [11], and possibility of realization of transducer at the edge of the optical fiber [12], that, in turn, leads to the immunity to harsh environment and electromagnetic interference. At the same time, further miniaturization, predefined spectrum of ultrasound as well as the increase of functional flexibility of the ultrasound generation is required. Such goals can be achieved by the application of plasmonic structures, especially the Tamm plasmon (TP) structures [13,14].
TP is the state of electromagnetic field localized at the interface between metal and the Bragg reflector (BR). TP structures provide a wide range of opportunities for electromagnetic mode engineering [15,16] and, in particular, for the control of the absorption of light [17]. In optoelectronic devices, absorption of light in metals is the enemy of high performance, but for photoacoustic generators, high performance of the device is based on controllable absorption of light.
In existing photoacoustic systems, the laser wavelength usually corresponds to green-blue visible range that limits the range of possible laser systems. The use of TP structures gives opportunity to provide a total absorption in the infrared band. In this case, GaAs-based semiconductor lasers emitting at a wavelength of 980 nm can be used. Such lasers combine affordability, high power, scalability, and possibility of a direct temporal modulation of the intensity for the frequencies up to few GHz [18,19]. Semiconductor lasers operating at the wavelength of 980 nm can produce an average power of 1 W focused on the 10 µm x 10 µm spot providing a flux density up to 106 W/cm2.
This paper aims to introduce the novel scheme of the photoacoustic generator based on the Tamm plasmon structure (PAG-TP) and the optimisation of its performance.
2. Design of the structure
In the proposed scheme the metallic layer of the TP structure is heated by a short laser pulse, expands and emits ultrasound waves, as shown in Fig. 1. Utilisation of the TP enhances the absorption of light in a predefined point of the structure and could improve the performance of the photoacoustic generator of ultrasound waves.
The structure consists of BR covered by a layer of metal and is illuminated from the BR side by the laser beam periodically modulated in time. The design of the TP structure ensures the total absorption of laser radiation at a desirable wavelength resulting in a periodic in time heating and cooling of the metallic layer. Therefore, its periodic expansion and shrinking will lead to emission of sound.
The design of the structure and the choice of the material should simultaneously provide: (i) total absorption of laser radiation at the desired wavelength; (ii) effective heating of the metallic layer by laser pulses; (iii) effective heat sink from the metallic layer in order to avoid overheating; (iv) maximal amplitude and desirable temporal shape of oscillation of the sample surface.
3. Results and discussion
When the layer of the material with thermal expansion coefficient $\varepsilon $ absorbs the laser pulse with energy density G, the thickness of the layer is increased by the value B defined by
where c and $\; \rho $ are the specific heat capacity and density of the material, respectively. In the case of a harmonic oscillation of the surface with amplitude of oscillation B and frequency f the intensity of sound J reads as [20,21,22]: where ${\rho _m}$ and$\; v$ are the density and speed of sound for the media, where ultrasound is emitted. Corresponding pressure, induced by of the acoustic wave P reads The efficiency of conversion of laser radiation to ultrasound waves $\eta $ can be estimated asTable 1 shows the properties of some metals and dielectric materials [23–27], which can be used to construct PAG-TP. Figure 2(a) shows the detailed scheme of the TP structure, which consists of a magnesium film deposited on top of the Bragg reflector made of 4 pairs of SiO2 and TiO2 layers with a thickness of 168 nm and 118 nm, respectively. The thickness of the layer of TiO2 adjacent to the magnesium layer is reduced to 100 nm in order to shift the TP to the center of the stopband of the Bragg reflector and provide better localization of light. The thickness of the magnesium layer is dM = 1000 nm.
Spectra of absorption $A(\lambda )$ and reflection$\; R(\lambda )$ coefficients at normal incidence of the TP structure are shown in Fig. 2(b). The reflection and absorption spectra, and the spatial distribution of electromagnetic field in the structure were obtained by solving Maxwell’s equations by means of transfer matrix method [28] (see Supplementary Supplement 1). Clearly, there is a sharp peak of absorption at the wavelength of 980 nm corresponding to the TP resonance, with the maximum value being close to unity. Note that when the light is incident on the interface of magnesium from the uniform material, the reflection coefficient at the wavelength of 980 nm is ∼0.8. The green line in Fig. 2(a) shows the spatial profile of ${|{E(x )} |^2}$, where E(x) is electric field established within the structure under illumination by the light with wavelength of 980 nm. It can be seen that the electric field is localized near the interface of the metal and the penetration depth is $\xi \approx $ 15 nm.
The total intensity absorbed by the structure is equal to the product of incident intensity $I({t,\lambda } )$ and absorption coefficient of the structure $A(\lambda )$. Penetration of laser radiation into metal and its absorption lead to the spatially distributed generation of the heat with a density F(x, t) described by
During the first pulse the temperature increases up to 80 °C, but subsequently the oscillation amplitude of $\mathrm{\Delta }T$ for this case is about 40 °C: the temperature does not reduce fast enough. The variation of the structure surface position induced by thermal expansion reads as:
Figure 5(a) shows that the shift of the surface by almost 1 nm at the first period of excitation followed by a sinusoidal oscillation with a magnitude of ∼0.5 nm accompanied by a constant shift of the surface about 0.8 nm due to the heating of the structure as a whole. The constant shift of the surface during few subsequent pulses slightly increases and after 100 ns its temporal dependence tends to fixed value.Note that for the parameters considered and the structure placed into a water-like medium the sound flux density is about 2 W/cm2 in CW mode and corresponding pressure of the acoustic wave, obtained by Eq. (3) is about 1 MPa.
It was reported that for other types of photoacoustic emitters, the laser fluxes of the order 10 MW/cm2 induce acoustic waves with pressure of the order of 10 MPa [30,31,32]. Thus, operational regime used for our structure does not approach conditions, leading to optical or mechanical destruction of the structures.
Since the power of sound emitted by the oscillating surface, defined by Eq.(2), is proportional to the magnitude of oscillation squared, it would be advantageous to provide the cool-down of the structure to the initial level after the end of the heating pulse, which can be achieved by the reduction of the repetition rate. In Fig. 4(b), the pattern of $\mathrm{\Delta }T({x,t} )$, i.e. the sequence of heating pulses with repetition rate of 50 MHz, is shown by the green line. In this case the structure cools down to the initial level between pulses and the magnitude of the surface oscillation increases up to 0.9 nm, as shown in Fig. 5(b).
4. Summary
The novel scheme of a photoacoustic generator based on the TP structure has been proposed. The TP structure provides a total absorption of the laser pulse at any desirable wavelength allowing the use of compact, powerful and feasible infrared semiconductor lasers with the wavelength about 1 µm for photoacoustic generators. Performance of various materials has been analyzed and it was shown that the structure utilizing magnesium for the active element possesses maximal efficiency. We have found that the power efficiency of light-to-sound conversion for the case considered reaches the value up to 10−5.
Different schemes of photoacoustic emitters demonstrate conversion efficiencies, varying by many orders of magnitude. Bare metallic film demonstrates conversion efficiency of the order of 1 from 10−8 [33]. The media, providing enhanced absorption light, like graphene, carbon nanotubes or metal nanoparticles demonstrates efficiency of the order of 10−5 [9,34,35]. The highest efficiency, of the order of 10−3, is demonstrated by composite made of polydimethylsiloxane (possessing extremely high thermal expansion coefficient) and gold nanoparticles (responsible for the absorption of light) [36]. The scheme proposed in our paper can combine the use of organic materials, possessing high thermal expiation coefficient, and advantages of thin film technologies, leading to even higher conversion efficiencies. The reported results provide a background for development of a new feasible and efficient type of photo-acoustic devices.
Funding
Belarusian Republican Foundation for Fundamental Research (F19RM-006); Energimyndigheten (# 46563-1); Vetenskapsrådet (#2019-05154); Russian Foundation for Basic Research (19-52-04005).
Disclosures
The authors declare no conflicts of interest.
See Supplement 1 for supporting content.
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