---
title: "Registration of long-range surface plasmon resonance by angle-scanning feedback and its implementation for optical hydrogen sensing"
authors: ["Valery N. Konopsky", "Dmitry V. Basmanov", "Elena V. Alieva", "Dmitry I. Dolgy", "Eugeny D. Olshansky", "Sergey K. Sekatskii", "Giovanni Dietler"]
affiliation: "Institute of Spectroscopy, Russian Academy of Sciences, Troitsk, Moscow region, 142190, Russia -."
journal: "New Journal of Physics"
year: 2009
volume: "11"
issue: ""
article_number: "063049"
pages: ""
doi: "10.1088/1367-2630/11/6/063049"
type: journal-article
site_group: "Surface plasmons and photonic bandgaps"
url_abstract: "https://valery.konopsky.com/paper/NJP2009/NJP2009.htm"
url_pdf: "https://valery.konopsky.com/kvnlocal/1367-2630_11_6_063049.pdf"
language: en
source_tex: "Z:\\ValeryData\\Valery_New\\my_articles\\NewJoPhysics2009\\NewJoPhysics\\Send\\Konopsky2NJP.tex"
source_pdf: "Z:\\ValeryData\\Valery_New\\my_articles\\NewJoPhysics2009\\NewJoPhysics\\Published\\1367-2630_11_6_063049.pdf"
---
## Abstract

...

# Introduction

Hydrogen-gas-leak detectors are needed in many hydrogen applications, where the rapid detection of hydrogen leaks below the lower explosive limit (LEL) of $\sim4\%$ in air is desirable. Optical detection methods have advantages due to the possibility for remote sensing and lower ignition risks near the sensing area in potentially explosive atmospheres. Unfortunately, the molecular structure of $H_2$ makes the selective detection of hydrogen by standard optical spectroscopy methods very difficult: $H_2$ is a homonuclear diatomic molecule and has no dipole vibrational transition in the IR range, while the electronic transitions of hydrogen lie in the vacuum ultraviolet range. At the same time, Raman and coherent Raman spectroscopy methods are costly, require relatively high laser power, and may be unsafe in explosive environments.

For these reasons the selective measurement of hydrogen is still a challenging technological problem in spite of the wide range of instruments (including commercial ones) that are currently available. In these instruments, a palladium film is commonly used as the selective layer, in conjunction with a range of transducers, such as thin film resistors [1], nanoparticle resistors [2, 3, 4], Schottky diodes [5, 6], MOS structures [7] and, certainly, optical sensors [8, 9, 10, 11, 12], including plasmonic [13] and nanoplasmonic ones [14].

Palladium is able to absorb up to 900 times its own volume in hydrogen in a reversible process, where the amount of hydrogen soluble in Pd is dependent on the temperature and partial pressure of the hydrogen. Hydrogen uptake by palladium strains the Pd lattice, leading to a volumetric expansion that reaches equilibrium for a given hydrogen concentration in the environment around the material [15]. This volumetric expansion leads to an increase in the thickness of the Pd film on the one hand and a decrease in the real and imaginary parts of the palladium refractive index (RI) on the other hand. In surface plasmon (SP) sensors, these changes lead to a shift in the surface plasmon resonance (SPR) position.

# Long-range surface plasmons at metal-gas interface

The SP is an excitation of a bound electromagnetic mode near a metal-dielectric interface [16]. Metals such as silver and gold are the most popular substrates for SP excitation due to their comparatively low losses in the visible spectral range. Palladium, on the other hand, is a lossy metal, with the imaginary part of its dielectric susceptibility being rather large and the SP propagation length on Pd in the visible spectral range being very short ($\le1~\mu$m).

It is possible to increase the SP propagation length and, consequently, the SPR sensitivity by using long-range SPs (LRSPs). LRSP can be excited in a thin metal film, imbedded between two dielectrics with identical refractive indexes (RIs) [17, 18, 19, 20, 21]. This requirement to match the RIs on both sides of the metal film poses serious complications for the practical employment of LRSPs for sensing in liquids and gaseous environments. For example, in gases ($n_e\simeq 1$), a freely suspended thin metal film is required to match the RIs on both sides and consequently ultrathin membranes are currently the only option as its support [22].

In the work [23] it was shown that this RIs matching requirement may be circumvented by using a one-dimensional (1D) photonic crystal (PC) instead of an RI matching layer to obtain a minimum electromagnetic (EM) field strength inside the metal film. The 1D PC is a simple periodic multilayer stack. Such 1D PC can exhibit photonic band gaps where the propagation of optical EM waves is forbidden. By changing the band gap parameters by modifying the thicknesses of alternating layers in the stack, the LRSP propagation can be excited in the terminal metal layer, bordering an external medium with any RI, including $n_e\simeq 1$. The use of the 1D PC instead of an RI matching layer to obtain a minimum electromagnetic (EM) field strength inside the metal film permits us to excite LRSPs along the metal nanofilm bordering with arbitrary environments (including gaseous environments). Moreover, this increase takes place even for lossy metals such as Pd, which recommends this structure as a promising system for hydrogen detection [24].

So far the long-range surface waves in lossy metals were obtained in a “quasisymmetric” scheme only, where a matching fluid was used to match the RIs on both sides of the metal film [25]. A resonance width about $0.15^0$ ($\sim2.6\!\times\!10^{-3}$ rad) was observed in thin vanadium and palladium films at $\lambda=3.391~\mu$m [19]. But, this “quasisymmetric” scheme will not work if $n_e\sim1$ near one interface and its implementation for sensing experiments seems to be difficult if not impossible. In contrast, in our structure, the unique wavelength tunable properties of our 1D photonic crystal permits us to achieve long-range propagation by simple tuning of the laser wavelength [23, 24].

The principal reason for the long-range plasmon propagation is the presence of the minimum electric field strength inside the thin metal film which is the result of destructive interference between SPPs from both film interfaces. It is because of this field minimum inside the film that LRSP propagation is possible even along lossy metals such as Pd. It may be shown that the thinner the final metal layer, the less the EM field is inside this layer (though the maximum of the EM field still occurs at the external interface of the layer). From the other hand, at a very small metal film thickness ($\sim10$ nm), the effective RI of the LRSP is approaching to RI of the external medium ($n_{\mathrm{LRSP}}\to n_e\simeq1$; see equation (1) from Ref. [23]) and, as a result, the scattering attenuation losses of LRSPs become prominent.

In this work we present a hydrogen detection system that uses this LRSP scattering as an input signal to pick up the SPR angle in the Pd nanofilm on 1D PC. The use of LRSP scattering as the input signal is possible because the scattering losses become a considerable attenuation channel of the LRSPs while the dissipation losses in the metal film are reduced due to the presence of the minimum electric field strength of the LRSP inside the thin metal film [23]. The detection of the LRSP scattering is possible by a simple photodiode, while a position-sensitive photodetector is needed for SPR dip registration at total internal reflection in standard SPR techniques.

# Experimental setup

The 1D PC structure used in the experiments is as follows: *substrate/$(HL)^{14}H'M$/air*, where $H$ is a $Ta_2O_5$ layer with a thickness $d_2=112.8$ nm, $L$ is a $SiO_2$ layer with $d_1=155.0$ nm, $H'$ is a $Ta_2O_5$ layer with $d_2'=103.4$ nm, and $M$ is the palladium layer with $d_3=d_M=8$ nm. The prism and the substrate are made from BK-7 glass. The $Ta_2O_5/SiO_2$ multilayer and palladium film are deposited by magnetron and cathode sputtering, respectively. The RIs of the substrate, $Ta_2O_5$, $SiO_2$, and $Pd$ are respectively: $n_0=1.513$, $n_2=2.076$, $n_1=1.455$, and $n_3=n_M=1.9+i4.8$.

![Sketch of the experimental setup. The dependence of the integrated scattering light vs incident angle is shown in the inset.](media/NJP2009/fig1.eps)

*Sketch of the experimental setup. The dependence of the integrated scattering light vs incident angle is shown in the inset.*

A sketch of the experimental setup is shown in figure 1: a parallel light beam from a fiber-coupled laser diode ($\lambda=737.7$ nm, $P=0.74$ mW) excites the LRSPs in the 1D PC structure at a resonance angle $\theta_0$ through an angle-scanning mirror. The angle scanning piezomirror modulates the incident angle around the resonance angle with its natural frequency ($\sim1.3$ kHz) of rotational vibrations of the piezomirror by the value of $\Delta\theta_0$. The integrated scattering light has its maximum precisely at the SPR angle $\theta^{\mathrm{SPR}}_0$ and diminishes while the angle-scanning mirror deflects the incident excitation angle from the resonance (see the inset outlined by dots in figure 1). As a consequence of this feature, the palladium surface flashes at the double modulation frequency ($\sim2.6$ kHz) when the system is precisely tuned to the SPR angle, while a signal of the first harmonic ($\sim1.3$ kHz) appears when the SPR angle and the excitation angle are untuned. The integrated scattering light from the LRSPs is collected on a photodiode by a lens. The lock-in amplifier picks out, detects, and amplifies the signal of the first harmonic and then this rectified signal (after an inverting) is fed back to the angle scanning piezomirror.

As a result, we have a closed negative feedback loop that picks up the SPR angle. If the SPR angle changes (for example, due to an increase in the Pd nanofilm thickness during hydrogen injection), the feedback loop changes the voltage at the piezomirror to tune the system to a new SPR angle. The voltage at the piezomirror is our useful signal, which is recorded by a computer via an analog-digital converter. The voltage-angle conversion ratio of the angle-scanning piezomirror is $\Delta\theta_0 = (\partial\theta_0/\partial V)\!\times\!\Delta V$, where $\partial\theta_0/\partial V\simeq 2\!\times\!10^{-5}$ rad/volts, except for the frequency range near $1.3$ kHz, where the natural resonance of rotational vibrations of the piezomirror occurs and where the coefficient $\partial\theta_0/\partial V$ is $Q=15$ times as great.

It should be noted that the distance between the piezomirror and the 1D PC is small in our system ($s\simeq 5$ cm) so the light beam displacement across the palladium surface during mirror scanning is negligible. For instance, in our experiments, the double amplitude of the natural rotational vibrations of the piezomirror excited by the $1.3$ kHz generator is about $\Delta\theta\simeq 6\!\times\!10^{-4}$ rad in air, which correlates with a laser beam’s natural divergence in air of $\sim 5\!\times\!10^{-4}$ rad (the angular width of the LRSP resonance in this system is also approximately the same [24]). Therefore, the displacement at a frequency $1.3$ kHz is $\Delta l\simeq s
\Delta\theta/(n_0\cos{\theta_0})\simeq 26~\mu$m, which is less then both the diameter of the parallel light beam ($\sim
1.5$ mm) and the LRSP propagation length across the palladium film in this system ($L_{1/e}\simeq 180~\mu$m [24]).

# Hydrogen detection

To verify the sensitivity of our system to hydrogen and to compare it with existing hydrogen sensors, we present experimental data showing changes in the feedback voltage at the piezomirror in response to exposure to 0.5% hydrogen. The data are shown in figure 2. The measurements were performed in a nitrogen environment at atmospheric pressure and room temperature (20$^0$C). The 0.5% (v/v) hydrogen was added to a gas chamber attached to the 1D PC structure with a terminal Pd layer. The time per point was about 0.5 second (including the time of light accumulation $\sim0.1$ second per point) and no posterior data averaging or smoothing was done. One can see that the response time following hydrogen injection was about 5 seconds and the recovery time was about 15 seconds. This was a rather fast response among palladium sensors and did not seem to be diffusion-limited, because the diffusion time of hydrogen in an 8 nm-thick Pd film should be less than $1~\mu$s (since the diffusion coefficients are $D_\alpha=10^{-7}$ cm$^2$/s and $D_\beta=10^{-6}$ cm$^2$/s for $\alpha$ and $\beta$ phase PdH). So this time response seems most likely is limited by the surface processes, as the surface is not atomically clean.

![Changes in the feedback voltage on the piezomirror in response to hydrogen injection.](media/NJP2009/fig2.eps)

*Changes in the feedback voltage on the piezomirror in response to hydrogen injection.*

The main physical reason for the LRSP resonance shift in response to hydrogen injection is the increase in Pd film thickness. The decrease in the real and imaginary parts of the palladium RI plays a minor role here, although it gives an important contribution in the response of ordinary SPR [13] or optical [8, 9, 11] hydrogen sensors. The reason is as follows: although the maximum of the EM field of LRSPs occurs at the external interface of the film as usual, there is a minimum of the EM field strength inside the Pd film, which makes LRSPs in this system less sensitive to changes in the film RI. This peculiarity may make this system less sensitive to temperature fluctuations in comparison with resistive [1] Pd sensors due to the next physical reason: in resistive Pd sensors the changes of the temperature produce a (false) signal through temperature coefficient of resistivity, which is of the order of $\sigma_\mathrm{Pd}\simeq4\!\times\!10^{-3}$ [1/ K] , while a (false) signal in the presented hydrogen sensor appears through temperature expansion coefficient, which is of the order of $\nu_\mathrm{Pd}\simeq1.7\!\times\!10^{-5}$ [1/ K] (we neglect of temperature stabilization methods in both examples). Experimental check of the temperature sensitivity of the present sensor will be done elsewhere.

From figure 2 one can see that the noise floor of the base line is about $\delta V=\mathrm{std}(V)\simeq
4\!\times\!10^{-2}~\mathrm{volts}$ (where std is the standard deviation) for our 0.1 second light accumulation time. Therefore, we detected 0.5% hydrogen with a signal/noise (S/N) ratio of about 100. For the 1 second light accumulation time, this would correspond to a noise floor equal to $\delta
V\simeq 1.3\times10^{-2}~\mathrm{volts}$, which is $\delta\theta_0\simeq 2.6\times10^{-7}~\mathrm{rad}$ in angle units or $\delta n\simeq 3\times10^{-7}$ in refractive index units.

# Conclusion

In conclusion, we excited long-range surface plasmons in a Pd nanofilm bordering a 1D PC at one side and a gas environment at another side. The ultrasharp plasmon resonance in this system let us measure changes in the Pd nanofilm thickness as a result of hydrogen injection with a fast time response. In contrast with several implementations of scanning mirrors in standard SPR systems [26, 27, 28], where light reflected from the prism is used for measurements, the long-range propagation of SP in our structure permits us to use LRSP scattering as the input signal for the feedback loop. The second important difference is the ultrasharp angular width of the long-range SP resonance in our system (which is comparable to natural laser beam divergence). As a result, a small value of angular scanning is needed for 100% modulation. These differences permit us to pick up the SPR angle with a precision of $2.6\times10^{-7}~\mathrm{rad}/\sqrt\mathrm{Hz}$. One eighth of the LEL hydrogen concentration was detected with a S/N ratio $\sim 100$. An additional benefit from the long-range propagation is the reduced need for the light beam to be spatially stationary at the probing surface, which simplified our setup.

# Acknowledgments

This work was partly supported by the Russian Foundation for Fundamental Researches and the Swiss Foundation for Fundamental Researches. One of us (Dmitry Basmanov) appreciates the Foundation for Assistance to Small Innovative Enterprises for financial support.

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## References

1. Hughes R C and Schubert W K 1992 em J. Appl. Phys./ bf 71 542--544.

2. Favier F, Walter E, Zach M, Benter T and Penner R 2001 em Science/ bf 293 2227--2231.

3. Walter E, Favier F and Penner R 2002 em Anal. Chem./ bf 74 1546--1553.

4. Xu T, Zach M, Xiao Z, Rosenmann D, Welp U, Kwok W and Crabtree G 2005 em Appl. Phys. Lett./ bf 86.

5. Domansky K, Baldwin D, Grate J, Hall T, Li J, Josowicz M and Janata J 1998 em Anal. Chem./ bf 70 473--481.

6. Potje-Kamloth K 2008 em Chem. Rev./ bf 108 367--399.

7. Petersson L G, Dannetun H M and Lundstr"om I 1984 em Phys. Rev. Lett./ bf 52 1806--1809.

8. Kalli K, Othonos A and Christofides C 2002 em J. Appl. Phys./ bf 91 3829--3840.

9. B'evenot X, Trouillet A, Veillas C, Gagnaire H and Cl'ement M 2002 em Meas. Sci. Technnol/ bf 13 118--124.

10. Minkovich V P, Monzon-Hernandez D, Villatoro J and Badenes G 2006 em Opt. Express/ bf 14 8413--8418.

11. Maier R R J, Jones B J S, Barton J S, McCulloch S, Allsop T, Jones J D C and Bennion I 2007 em J. Opt. A: Pure Appl. Opt./ bf 9 S45--S59.

12. Caucheteur C, Debliquy M, Lahem D and Megret P 2008 em Opt. Express/ bf 16 16854--16859.

13. Chadwick B and Gal M 1993 em Appl. Surf. Sci./ bf 68 135--138.

14. Langhammer C, Zoric I and Kasemo B 2007 em Nano Lett./ bf 7 3122--3127.

15. Lewis F A 1967 em The Palladium Hydrogen System/ (London: Academic).

16. Raether H 1988 em Surface Plasmons/ (Berlin: Springer).

17. Sarid D 1981 em Phys. Rev. Lett./ bf 47 1927--1930.

18. Craig A E, Olson G A and Sarid D 1983 em Opt. Lett./ bf 8 380--382.

19. Yang F, Sambles J R and Bradberry G W 1991 em Phys. Rev. B/ bf 44 5855--5872.

20. Dostalek J, Kasry A and Knoll W 2007 em Plasmonics/ bf 2 97--106.

21. Degiron A, Cho S Y, Tyler T, Jokerst N M and Smith D R 2009 em New J. Phys./ bf 11 015002.

22. Berini P, Charbonneau R and Lahoud N 2007 em Nano Lett./ bf 7 1376--1380.

23. Konopsky V N and Alieva E V 2006 em Phys. Rev. Lett./ bf 97 253904.

24. Konopsky V N and Alieva E V 2009 em Opt. Lett./ bf 34 479--481.

25. Yang F, Sambles J R and Bradberry G W 1990 em Phys. Rev. Lett./ bf 64 559--562.

26. Kooyman R, Lenferink A, Eenink R and Greve J 1991 em Anal. Chem./ bf 63 83--85.

27. Lenferink A, Kooyman R and Greve J 1991 em Sens. Actuator B-Chem./ bf 3 261--265.

28. Berger C and Greve J 2000 em Sens. Actuator B-Chem./ bf 63 103--108 thebibliography.

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