1. Introduction

The surface plasmon resonance (SPR) behavior of a metal thin film strongly depends on the refractive index near the metal surface. This behavior enables SPR sensors to be commonly used as biosensors and chemical sensors [1–4]. Au and Ag thin films have been widely utilized for SPR sensors, and their SPR energies are in the visible region. Visible-SPR sensors have received much attention and are practically used in fields such as biomedical analysis [5–7] and environmental monitoring [8–10].

SPR sensors, which can use light in the shorter wavelength region for surface plasmon excitation (in particular, in the deep-ultraviolet (DUV, 200-300 nm) and far-ultraviolet (FUV, 120-200 nm) regions), are expected to have remarkable properties. Although many target molecules for SPR sensors (such as DNA and proteins) have no absorption in the visible region, they have strong absorption and high refractive indices in the FUV and DUV regions [11,12]. In addition, even in the DUV region, a number of materials (such as water, alkanes, alcohols, some amino acids, and saccharides) do not exhibit any absorption, and they only absorb light with a wavelength below 200 nm [13]. The angle shift will therefore be greater in the FUV-DUV-SPR case (some materials only in the FUV-SPR case) than in the visible-SPR case.

Al is a suitable metal for FUV-DUV-SPR studies because its plasma frequency (2.4 × 10¹⁶ s⁻¹) [14] is higher than the frequency of light in the FUV-DUV region. The plasma frequencies of Au and Ag are 1.37 × 10¹⁶ s⁻¹ and 1.36 × 10¹⁶ s⁻¹, respectively [15,16]; hence, Au and Ag are not suitable metals for the FUV-DUV region. Recently, much attention has been paid to plasmonics in the DUV region [17–26], in applications such as fluorescent enhancement [18–21], photoelectron emission enhancement [22], surface-enhanced Raman scattering (SERS) [23,24], tip-enhanced Raman scattering (TERS) [25], and sensing [26].

Very few papers have been published to date about surface plasmons excited by light in the FUV region [27–29]. Callcott and Arakawa investigated the photo-induced SPR properties of Al thin films with light wavelengths from 120 nm to 300 nm using photo yield measurements [28]. Endriz and Spicer reported the effects of surface roughness of Al films on the SPR properties in the 100-200 nm wavelength region [29]. In the FUV region, both atmospheric H₂O and O₂ exhibit very intense absorptions. For this reason, the Al films were investigated under vacuum in these studies and photo-induced electrons were detected. The environment on the Al surface could not be controlled, and therefore, the effects of the surface atmosphere on the Al film have not been not reported.

In the present study, a new type of attenuated total reflectance (ATR) spectrometer was developed to investigate the FUV-DUV SPR properties (i.e. SPR angle and wavelength) of Al films with varying surface refractive indices. In this spectrometer, the optics and sample analysis sections are separated by an internal reflection element (IRE). One of the advantages of this system is that the sample analysis section is exposed to air and the environment around the sample can be readily controlled. The optics section is purged with dry N₂ gas, which does not absorb FUV light [13,30–35]. In addition, the instrument uses a deuterium lamp as a light source, not a laser. The ATR spectrometer allows the investigation of the dependence of the Al-SPR dispersion relationship on the surface refractive index at desired wavelengths in the FUV and DUV regions. The Al-SPR angle can be increased from 49° to 77° by increasing the refractive index of the environment near the Al film. This was achieved by changing the Al surface surrounding from air to 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). This shift is larger than that of a conventional Au-based visible-SPR sensor. The observed behavior agrees with simulation results based on Fresnel equations. These results indicate, for the first time, that an Al film can be used as an SPR sensor in the FUV and DUV regions. Compared with conventional visible-SPR sensors based on Au and Ag films, FUV-DUV-SPR sensors will have three practical advantages, viz. higher sensitivity, improved surface measurement accuracy, and better material selectivity. These benefits should enable FUV-DUV-SPR sensors to be utilized in applications requiring high biosensing sensitivity, such as monitoring antigen-antibody reactions.

2. Methods

The Kretschmann configuration was used to excite the SPR of an Al film deposited on a quartz prism by vapor deposition (5.0 × 10⁻⁴ Pa, deposition rate ≈10 nm s⁻¹). The film thickness was measured by a thickness meter (Alpha-step, KLA Tecor), and the dependence of reflectance on the incident angle was measured using the fourth harmonic of a Nd:YAG laser to characterize the prepared film. The output power of the laser was ~2 mW, and the intensity of the reflected excitation light was detected with Si photodiodes.

In this study, we used two ATR spectrometers with different measurable wavelength regions and variable incident angles. One instrument allowed us to measure reflectance spectra in the 180-450 nm region with 45°-70° incident angles. The other collected reflectance spectra in the 180-300 nm region with 60°-85° incident angles. The optics section was continuously purged with dry N₂ gas to exclude atmospheric O₂ and H₂O, which strongly absorb FUV light [5]. An outline schematic of the system is shown in Fig. 1.

 

Fig. 1 Outline schematic of the spectrometer system. The optics and sample sections are separated by a quartz prism, and the environment around the sample can be readily controlled.

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The system employed a 30 W deuterium (D₂) lamp as the light source. The light from a monochromator is split into a reference beam and a sample beam by a half mirror. The reference beam and the reflected sample beam pass through a synthetic quartz plate, coated with sodium salicylate, which fluoresces. Finally, the fluorescence of each beam is detected with a photomultiplier.

Prepared Al films on semi-cylindrical quartz prisms (15 mm diameter, ES grade, purchased from Opto-line, Tokyo) were placed in the ATR spectrometers, and reflection spectra were obtained, with the incident angle changing from 45° to 75° with a step increment of 1°. The reflection spectra of quartz prisms without Al films were also measured and were used as references. Reflectance is defined as I/I₀, where I and I₀ are the reflected light intensities from a prism without and with an Al film, respectively.

In order to increase the refractive index of the environment near the Al film, neat HFIP liquid (n_D = 1.275 at 25 °C) was deposited on the film. Reflectance spectra were collected by the second ATR spectrometer, with the incident angle varying from 60° to 85° with a step increment of 1°. HFIP was selected as it has little absorbance in the measurement wavelength region (180-450 nm), making it easier to investigate the refractive index dependence of SPR. Reflectance spectra with incident angles equal to or smaller than 63° could not be measured, due to the limitation arising from the critical angle between quartz and HFIP (~61° at 589.3 nm),.

In order to analyze the reflectance spectra and the dependence of reflectance on the incident angle, the relationship between the reflectance and the incident angle was investigated using Fresnel equations, based on a bilayer (aluminum and alumina) model with varying film thickness, incident light wavelength, and refractive index of the environment near the film.

3. Results and discussion

3.1 Characterization of the prepared Al film

The Al film on the quartz prism was characterized by laser illumination at the wavelength of 266 nm, (previously reported in [19]). Figure 2 shows the incident angle dependence of the film’s reflectance with p- and s- polarized excitations. For p-polarized excitation, the reflectance curve shows a dip at the incident angle of 47°, indicating that the SPR is excited at this angle. In contrast, the SPR is not excited by s-polarized light. The thickness of the Al film was about 23 nm.

 

Fig. 2 Incident angle dependence of reflectance with p-polarized (red line) and s-polarized (blue line) excitations at a laser wavelength of 266 nm.

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3.2 Dispersion relation of Al-SPR in air

Figure 3(a) shows the wavelength dependence of reflectance for the Al film on a quartz prism exposed to air, as measured by the system in Fig. 1. The spectra were obtained with an incident angle range between 46° and 68° with a step increment of 1°. Figure 3(a) shows the spectra with a step increment of 2°. The incident light from the deuterium lamp is unpolarized and includes both p-polarized and s-polarized light. As only p-polarized light can excite SPR, the minimum value of the reflectance is around 0.5. In the present system, space limitation makes it difficult to set a polarizer for the FUV-DUV regions.

 

Fig. 3 (a) Wavelength dependence of reflectance for the Al film on a quartz prism in air; (b) experimental and (c) simulated dependence of reflectance on incident angle and wavelength; and (d) (blue) experimental and (black) simulated dispersions in relation to air on the Al film. For (c), the simulation was modeled using Fresnel equations based on Al 19 nm/Al₂O₃ 4 nm on a quartz prism.

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Figure 3(b) shows the dependence of reflectance on the incident angle and wavelength of the excitation light. When the incident angle is set to 46°, the SPR wavelength is observed around 340 nm (~3.6 eV). Increasing the incident angle to 68°, shifts the SPR wavelength to a shorter wavelength (~190 nm (~6.5 eV)). From this data, the dependence of reflectance on the incident angle at desired wavelengths can be obtained. This is one of the advantages of the present spectrometer system, as the light source is a deuterium lamp that delivers a broad wavelength range of light. As shown in Fig. 3(b), at longer wavelengths, the SPR angle decreases and the bandwidth becomes narrower.

As stated above, the measured thickness of the Al₂O₃/Al sample film is ~23 nm. The thickness of the Al₂O₃ layer in the simulation model can therefore be changed from 0 to 6 nm, with an increment of 2 nm, on condition that the total thickness of the Al and Al₂O₃ layers is kept at 23 nm. To determine the most appropriate conditions, the experimental dependence of reflectance on the incident angle at four wavelengths (185, 225, 266, and 300 nm) was overlaid on the calculated results (Appendix A, Fig. 7). From these four cases, the calculated SPR angle matches best with the experimental data when the Al₂O₃ thickness is 4 nm. Optical constants used in the simulation are listed in Appendix B, Table 1. Figure 3(c) shows the simulated dependence of reflectance on the incident angle and wavelength, simulated by Fresnel equations based on the optimized bilayer (Al 19 nm/Al₂O₃ 4 nm) model. It should be mentioned that the calculation results show good correlation with the experimental results (Fig. 3(b)).

Table 1. Refractive index values of quartz, Al, Al₂O₃, and HFIP used in the simulations based on the Fresnel equations.

View Table

Doherty and Davis reported that the thickness of naturally formed aluminum oxide films on Al single crystals was about 3 nm [36], hence, a 4 nm Al₂O₃ layer seems too thick. These inconsistencies are probably due to the differences between modeled and measured films. The sample film may consist of an incomplete bilayer of Al and Al₂O₃, which is different from the simulation model; the oxidized Al may not form a complete Al₂O₃ layer on the Al film, and oxidation of Al may occur even in the Al film. Kirk Jr. and Huber Jr. reported that oxidation of Al occurs even in a low-pressure (~4 × 10⁻⁵ Pa) oxygen atmosphere [37]. These oxidation phenomena may change the effective refractive indices from the reference values [38] used in the simulations. The surface roughness of the metal film also has effects on the SPR [29]; the Al film has a surface roughness of ~4.815 × 10⁻¹ nm estimated by atomic force microscopy.

Figure 3(d) shows the experimental (blue) and simulated (black) dispersion relationships, based on the data in Fig. 3(b) and 3(c). For both experimental and simulated results, the angular frequency ω increases with the wavenumber k and eventually saturates, which corresponds with the characteristics of the SPR dispersion relationship. The experimentally obtained asymptotic value of ω is approximately 9.9 × 10¹⁵ s⁻¹ (~6.5 eV), while the calculated value is about 1.7 × 10¹⁶ s⁻¹, under the assumption that the plasma frequency ω_p of Al is 2.4 × 10¹⁶ s⁻¹ and the dielectric constant ε_m is 1.0 (air) near the Al film with no oxidation layer on top. This difference between the experimental and calculated values is also due to the presence of the oxidized Al layer in experiments.

3.3 Al-SPR changes in the presence of HFIP

As mentioned above, by adopting the ATR technique, the environment on the Al film can be arbitrarily controlled, which is a significant advantage of the present FUV-DUV spectrometer, compared with previously reported systems. Figure 4(a) displays the reflectance spectra of the Al film on which HFIP was deposited. Figures 4(b) and 4(c) show the experimental and simulated dependences of reflectance on the incident angle and wavelength of the excitation light, respectively. With the presence of HFIP on the Al film, the SPR wavelength in the reflectance spectra shifts to a longer wavelength region compared with that in the spectra measured in air. As shown in Fig. 4(d), the change of the dispersion relationship from that in Fig. 3(d) corresponds with the characteristics of the SPR properties that depend on the refractive index of surroundings.

 

Fig. 4 (a) Reflectance spectra of the Al film on which HFIP was deposited (b) experimental and (c) simulated dependence of reflectance on incident angle and wavelength, and (d) (red) experimental and (black) simulated dispersion relations with HFIP on the Al film. (The simulation model is the same as Fig. 3(c)).

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The SPR changes, arising from changes in the Al film environment, clearly show the potential for FUV-DUV SPR to be used for sensor applications. In previous studies [27–29], the Al film was investigated under vacuum, and therefore the refractive index on the Al film could not be changed. The present study is the first demonstration of refractive index dependence of Al-SPR.

In the presence of HFIP, the calculated curves for a 19 nm Al layer with a 4 nm Al₂O₃ layer also fit well with the experimental results. In the simulations, the real parts of the refractive indices of HFIP in the FUV and DUV regions were calculated by density functional theory (DFT) (functional: M062X; basis set: aug-cc-pVTZ) with Gaussian 09 (Rev C.01) [39]. The values used in the simulations are shown in Appendix B, Table 1. It is clear that the simulation reproduces well the experimentally observed shifts in the SPR angle and wavelength due to the presence of HFIP.

3.4 Discussion of advantages of FUV-DUV-SPR sensors

In order to discuss the sensor sensitivity of FUV-DUV-SPR sensors and conventional visible-SPR sensors, we compare their SPR wavelength shifts. Figure 5(a) shows the dependence of reflectance on the incident angle with air (red) or HFIP, on the Al film at 266 nm. The SPR angle is shifted from 49° to 77° by the deposition of HFIP on the Al film. According to the Fresnel equations, when a conventional visible-SPR sensor based on a 50 nm Au film is assumed, the SPR angle at 633 nm is shifted from 44° to 65°, due to the change of the surrounding n near the Au from 1.00 to 1.28 (Fig. 5(b)). The refractive index n of HFIP in the visible region is ~1.28; therefore, the shift of the SPR angle caused by the presence of HFIP for the FUV-DUV-SPR sensor (~29°) is larger than that for the visible-SPR sensor (~21°). Such a large shift would potentially contribute to the development of high sensitivity SPR sensors.

 

Fig. 5 (a) Experimentally obtained incident angle dependence of reflectance at 266 nm wavelength with air (red) and HFIP (blue) on the Al film. (b) Calculated incident angle dependence of reflectance at 633 nm wavelength with air (red) and HFIP(blue) on the Au film. (The simulation model is an Au film with a thickness of 50 nm on a quartz prism)

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In addition, FUV-DUV-SPR sensors may have two further advantages. One is surface-selective detection. In general, SPR sensors detect the average n in the measurement field, and the measurement field is in the evanescent wave range of the incident light. By using shorter wavelength light, the penetration depth of the evanescent wave becomes smaller, and thus FUV-DUV-SPR sensors can measure the change in n in a more closely defined space, unlike visible-SPR sensors. In the simulations shown in Fig. 6, the FUV-DUV-SPR sensors can detect changes in n in the 2-nm-thick region on the metal film, as a dip in the shift of the reflectance versus incident angle curve. The visible-SPR properties of the Au film show almost no shift. In particular, the shift is larger in the FUV region than in the DUV region. In Fig. 6, when n changes from 1.0 to 2.0, the SPR angle shifts 4.7° for FUV-SPR sensors, which is about 4 times and 16 times as large as the shift for FUV-DUV-SPR sensors (1.1°) and visible-SPR sensors (0.3°), respectively.

 

Fig. 6 Calculated incident angle dependence of (a) Au film at 633 nm wavelength, (b) Al film at 266 nm wavelength, and (c) Al film at 180 nm wavelength with varying surrounding refractive indices from 1.0 to 2.0. (d) Calculated shift of the SPR angle depending on the refractive index. (e) A model of the simulations.

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The other advantage of FUV-SPR sensors is the material-selective detection. Almost all materials exhibit unique spectral shapes in the FUV region, even if these materials have no absorption in the visible region. By using a certain wavelength of light, which can be absorbed by a particular material, material selective detection can be achieved.

These advantages mean that FUV-DUV-SPR sensors can be potentially be used as novel refractive index sensors with high sensitivity, surface selectivity, and material selectivity. In particular, the material selectivity is a powerful characteristic, which is not achieved by visible-SPR sensors. As mentioned above, some materials such as water and saccharides only have absorptions in the wavelength region below 200 nm. In addition, amino acids, for example, show much larger absorptions in the FUV region than in the DUV region. Therefore, FUV-SPR sensors can potentially be applied to more materials than the DUV-SPR sensors.

4. Conclusions

In summary, the FUV-DUV-SPR properties (i.e., SPR wavelength and angle) of the Al film with varying surrounding refractive index were investigated for the first time using ATR-FUV-DUV spectrometers. In the presence of HFIP, the SPR wavelength and angle became longer and larger, respectively, compared to those in the presence of air. These properties agreed well with the simulation results according to the Fresnel equations. Furthermore, it is expected that FUV-DUV-SPR sensors (in particular, FUV-SPR sensors) with tunable incident light wavelength will have three specific advantages compared with conventional visible-SPR sensors, i.e. higher sensitivity, surface-sensitive measurement, and better material selectivity. These advantages will potentially lead to the development of high-performance SPR sensors such as biosensors, monolayer sensors, and gas sensors.

Appendix A Simulation based on Fresnel equations

 

Fig. 7 Measured (color dots) and calculated (black lines) incident angle dependence of reflectance at 185, 225, 266, and 300 nm wavelengths with air (blue dots) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (red dots) on the Al film. Simulation models are (a) Al 23 nm/Al₂O₃ 0 nm, (b) Al 21 nm/Al₂O₃ 2 nm, (c) Al 19 nm/Al₂O₃ 4 nm, and (d) Al 17 nm/Al₂O₃ 6 nm on a quartz prism.

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Appendix B Optical constants used in the simulations based on the Fresnel equations

The values of quartz, Al, and Al₂O₃ are obtained from ref. 40, 38, and 41, respectively. The values of HFIP are calculated by DFT. Because the calculated n value at 589.3 nm (1.2810 at 298.15 K) was slightly larger (0.46%) than the experimental value (1.2752 at 298.15 K) [42], normalized n values in the FUV and DUV regions were used.

Funding

JSPS KAKENHI grant number JP16K14025; Izumi Science and Technology Foundation.

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