Hydrogen Detection Using Polarization Diversity via a ... - IEEE Xplore

5 downloads 0 Views 1MB Size Report
Sep 7, 2012 - National Science Foundation under Award ID 0901388 and was carried out in part in the Frederick Seitz. Materials Research Laboratory ...
Hydrogen Detection Using Polarization Diversity via a Subwavelength Fiber Aperture Volume 4, Number 5, October 2012 Steven J. McKeown Lynford L. Goddard

DOI: 10.1109/JPHOT.2012.2214475 1943-0655/$31.00 ©2012 IEEE

IEEE Photonics Journal

Hydrogen Detection Using Polarization Diversity

Hydrogen Detection Using Polarization Diversity via a Subwavelength Fiber Aperture Steven J. McKeown and Lynford L. Goddard Department of Electrical and Computer Engineering, Micro and Nanotechnology Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801 USA DOI: 10.1109/JPHOT.2012.2214475 1943-0655/$31.00 Ó2012 IEEE

Manuscript received June 30, 2012; revised August 12, 2012; accepted August 16, 2012. Date of publication August 23, 2012; date of current version September 7, 2012. This work was supported by the National Science Foundation under Award ID 0901388 and was carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois. Corresponding author: L. L. Goddard (e-mail: [email protected]).

Abstract: A photonic hydrogen gas sensor is fabricated by etching a subwavelength aperture into an optically thick palladium film deposited on the facet of an optical fiber. Upon adsorption of hydrogen onto the palladium surface, the complex refractive index of the film will change, altering the transmission through the aperture. Due to the plasmonic resonances and enhanced transmission of the C aperture, its response to hydrogen is several times larger than that of a plain film or a nonresonant aperture. Furthermore, the asymmetry of the aperture produces a different hydrogen response for the two polarizations. This leads to different sensitivities to hydrogen. By measuring the polarization-dependent loss (PDL), we can accurately quantify the hydrogen concentration since common-mode noise is eliminated. Index Terms: Gas detectors, nanophotonics, optical fiber sensors.

1. Introduction Hydrogen gas is one of the most promising candidates for portable or small-scale clean fuel applications, notably in situations where power grid access or size restrictions limit the use of other clean energy sources such as solar and wind [1]. Further, pure hydrogen gas is used in many industrial applications such as in the production of ammonia, reduction of metallic ores, rocket fuel, and methanol production. The primary concerns with using hydrogen as a fuel source or reactant are safety and control; hydrogen has a lower flammability limit of 4% in air, and the ability to accurately determine hydrogen concentration during processing or combustion is paramount. This necessitates the development of fast reliable sensors that are both cheap and versatile enough to be deployed in automobiles or industrial applications. Optical sensors are ideal because of their low risk of sparking and, in the specific case of fiber optic facet-based sensors, their small form factor allows them to be placed in structures with little to no design changes. Many current proposed sensor designs, both electrical and optical, rely on a change in the resistivity or optical constants of a catalyst film to detect hydrogen. Hydrogen is known to adsorb onto the surfaces of many of the transition metals and react with them [2]. Palladium (Pd) and platinum in particular are very good catalysts, with palladium being less expensive and the more popular choice. Composite films such as Pd/Au alloys, Pd/Ag, and Pd/WO3 have also been shown to be sensitive hydrogen [3], [4]. Usage of Pd as an optical sensor relies on the ability of the

Vol. 4, No. 5, October 2012

Page 1752

IEEE Photonics Journal

Hydrogen Detection Using Polarization Diversity

transition metal to catalyze the dissociation of hydrogen molecules at the surface where they subsequently diffuse into the metal lattice. The hydrogen atoms then form bonds at interstitial sites leading to the creation of palladium hydride (PdH). Depending on hydrogen content, this structural change is referred to as the  or  phase. As hydrogen content increases, the lattice expands and hydrogen begins to incorporate itself at lattice sites, resulting in a decrease in the resistivity of the metal and a change in the optical constants [5]. The phase transition occurs around 0.1%–2% hydrogen based on film properties, at which point the mechanical properties can degrade [6]. The change in conductivity of Pd films as a function of hydrogen concentration has led to very accurate and sensitive electrical sensors based on resistance measurements. In particular, thin granular films have been shown to be sensitive based on the lattice expansion producing conduction paths [7]. In addition to research into electrical sensors, there has also been work done on developing optical sensors. Some examples include devices such as lasers where palladium can affect the lasing conditions or wavelength [8], reflection and transmission systems where optical setups are used to monitor films [9], plasmonic sensors using nanoscale gratings and optical antennas [10], [11], and optical fiber systems where Pd is deposited onto the side or facet of an optical fiber [12]–[14]. The fiber optic sensors can be divided into several categories based on mechanism. The most fundamental is a simple mirror where reflection and transmission of a thin Pd film deposited on a facet can be monitored through an optical fiber [12]. Similarly, Pd can also be deposited on the sidewalls, typically with the cladding etched away, and the loss can be monitored as a function of hydrogen, so called evanescent or tapered fiber designs [13]. There are also sensors based on fiber Bragg gratings where the hydrogen-induced lattice expansion can introduce stress, altering the resonance of the grating [14]. In this paper, we present the design, fabrication, and testing of an optical hydrogen sensor based on the transmission characteristics of metallic nanoapertures. The functional metal layer used is palladium. In order to maximize transmission, and provide an opportunity for feature optimization, the aperture chosen is the so-called BC[ aperture. This aperture has been shown to have enhanced transmission and resonant characteristics due to a combination of guided modes and generated surface plasmons at the aperture interfaces [15]–[18]. As a control, both a circular aperture with equal aperture area and a plain film sensor are tested. The responses of the sensors at various hydrogen concentrations for both polarization states of the aperture are measured with a demonstrated sensitivity to hydrogen down to a test equipment limited value of 2500 ppm.

2. C Aperture and Device Parameters The C aperture was chosen to obtain a high throughput and sensitivity for measurement while minimizing the aperture area. Keeping the features subwavelength ensures that the propagating mode is confined to the metal–dielectric interface and increases interaction with the palladium layer. Based on prior work in the literature [15], the dimensions of the aperture were tuned to have high transmission at the test wavelength of 1550 nm using 3-D finite-difference time-domain (FDTD) simulations. Both the finite metal thickness and optical properties of palladium were taken into account. The C aperture is characterized by four dimensions that control the size of the two arms and the waist. A scanning electron microscope (SEM) image of the fabricated device and an inset showing the four aperture geometry parameters, namely, Wa , Wb , Hb , and Ht , are depicted in Fig. 1. A typical C aperture would have shorter arms, given by Wa , and a thinner ridge and arms, determined by Hb . It was found, however, that very small aperture sensors were not as reliable due to fabrication and measurement inconsistencies. Furthermore, exaggerating the aspect ratio of the aperture further distinguished the two polarization states. From simulations, it was found that there was a small broad transmission peak through the larger aperture that could be tuned by increasing the arm length, keeping the aperture subwavelength in the direction parallel to the guided electric field, i.e., the confining direction. The film thickness was chosen to be optically thick (90 nm) to increase interaction length and to isolate the effects of the aperture from simple thin-film reflection/ transmission, while still thin enough to provide a good signal-to-noise ratio for transmission.

Vol. 4, No. 5, October 2012

Page 1753

IEEE Photonics Journal

Hydrogen Detection Using Polarization Diversity

Fig. 1. SEM images of the fabricated C aperture (left) with design parameters and dimensions. On the right is a zoomed-out view with the aperture centered on the fiber facet as indicated by the arrow.

Fig. 2. Electric field profiles for the majority component of the two C aperture modes at  ¼ 1550 nm. The quasi-TE mode (left) is predominately Ex -polarized, and the quasi-TM mode (right) is predominately Ey -polarized.

Simulations were done using RSoft’s FullWAVE software package and COMSOL Multiphysics. Full 3-D FDTD simulations were carried out by simulating the aperture and a small portion of the surrounding metal and optical fiber. A ten-layer perfectly matched layer boundary was used along with a plane-wave excitation since the aperture is small compared with the fiber mode, and the exact positioning of the core center cannot be guaranteed due to variations in the fiber itself. Convergence studies and a small grid size of 2  2  2 nm were used to ensure accuracy. Transmission was measured with a monitor 10 nm from the exit port of the aperture. Fourier transforms were computed for time-domain pulse excitations to obtain spectra for aperture design. The field patterns for the quasi-TE and quasi-TM modes, computed through COMSOL, are shown in Fig. 2, where TE and TM are defined using the coordinate system in Fig. 2. The TM mode is guided primarily by the two metal–dielectric–metal (MDM) slot waveguides formed by the arms of the aperture and is mostly y -polarized. The TE mode is guided by the single MDM slot waveguide formed by the left edge of the aperture and interior ridge and is mostly x -polarized. In both cases, the field intensity is strongest along the metal interface due to propagating surface plasmon polaritons (SPPs). For comparison to experimental data, the theoretical transmission powers for the C and circular apertures were also computed and will be discussed in Section 5. This was done using a power confinement factor for the aperture area that was calculated by integrating the Poynting vector of the LP01 mode of our optical fiber over the simulated area. Using this factor, the transmission through the rest of the film could be taken into account through simple normal incidence transmission. Sensing with the aperture is based on hydrogen-induced changes in the palladium layer. Most straightforward is the change in optical constants as hydrogen content increases, which is due to an increase in the Fermi level. As hydrogen is incorporated into the lattice, the atoms act as electron donors, increasing the overall free-electron density and shifting the Fermi level upwards. This upward shift reduces the joint density of states for low energy transitions and results in a decrease

Vol. 4, No. 5, October 2012

Page 1754

IEEE Photonics Journal

Hydrogen Detection Using Polarization Diversity

Fig. 3. Schematic of the experimental test setup. The scrambler is exchanged for a manual polarization controller for fixed-state measurements. Laser wavelength is  ¼ 1550 nm.

in the absorption rate [19], [20]. This supports the measured decrease in reflectance (downward transitions) and optical constants as hydrogen content increases [5]. The complex index of refraction affects the transmission through the aperture both by altering the guidance conditions of the waveguide and by changing the loss in the cavity. Specifically, for palladium at 1550 nm, values of n ¼ 2:958  0:033 and  ¼ 8:356 þ 0:067 were found based on prior work [21], where  represents the percent hydrogen concentration, e.g.,  ¼ 1 for 1% hydrogen.

3. Fabrication and Testing First, small pieces of Corning SMF-28e optical fiber were stripped to bare fiber and cut and cleaved with both ends flat so that the aperture could be etched into one facet and the other facet could be spliced back to connectorized fiber. Batches of tips were then cleaned and rinsed to remove dust particles and debris from cleaving. They were dried and mounted at an angle in the evaporator where palladium was deposited through e-beam evaporation. The tips were placed at an angle to get sidewall coating, which provided a conduction path for SEM and focused ion beam (FIB) milling. Palladium was deposited at 1.5  A=s with a deposition thickness of 85 nm measured using a profilometer. The tips were then transferred to another mount to hold the samples during etching. Etching of the apertures was done with an FEI dual-beam DB-235 FIB/SEM, which was also used for SEM imaging. Alignment of the features to the sample was done manually through SEM imaging. The mask pattern for the aperture was placed at the center of the fiber and etched away. A low beam current of 10 pA was used to provide accuracy and minimize overetching. Overetching would alter the characteristics of the aperture, but most importantly, Gaþ implantation has been shown to increase absorption in SiO2 in the IR band [22]. The raster speed, dwell time, and overlap between pattern points were optimized to best match the aperture to the design dimensions. Images of the etched apertures were taken using SEM, which showed a good agreement between the design and the fabricated aperture dimensions (see Fig. 1). After the apertures were etched, excess palladium was removed from the tips, and they were spliced to connectorized fiber with a fusion splicer and mounted in a custom-built flow chamber for testing. The flow chamber is connected to mass flow controllers (MFCs) that control the hydrogen and nitrogen concentration through LabVIEW. To measure the transmission, a collimation lens was built into the chamber, which focuses the output from the fiber onto a broad area amplified germanium detector. The output from the detector is then recorded through LabVIEW. The input to the fiber was a 2-mW 1550-nm laser that passes through a polarization scrambler operating at 10 Hz for polarization-dependent measurements or a manual polarization controller for fixed-state measurements. Data were collected at 15 kHz and averaged over 15k samples per point using a PCI-6010 NI DAQ card.

4. Transmission Measurements To test the sensor, the transmitted power from the facet was recorded while cycling between a hydrogen/nitrogen mix and pure nitrogen. The experimental setup can be seen in Fig. 3; the

Vol. 4, No. 5, October 2012

Page 1755

IEEE Photonics Journal

Hydrogen Detection Using Polarization Diversity

Fig. 4. (a) A C aperture with different design dimensions exposed to hydrogen concentrations above the  phase change threshold. The ambient hydrogen concentration versus time is shown as a shaded bar chart. The arrows indicate the point at which the response diverges from that of lower concentrations. (b) A plain film exposed to a train of 5% hydrogen pulses for work hardening. The response direction at low concentrations changes as a result of plastic deformation of the film. Measured at  ¼ 1550 nm.

transmitted power is normalized to the input power to remove any laser fluctuations. For fixedpolarization measurements, a manual polarization controller was used to set the polarization to a state where the sensitivity to polarization changes was minimized. This turned out to be somewhere between the minimum and maximum transmitted power values. The hydrogen concentration was limited to 1% to avoid the  phase change. Evidence of the phase change was seen at concentrations of 1.5% hydrogen in other samples, which was denoted by a change in the shape of the response curve. As the sensors were exposed to the 1.5% hydrogen mixture, the power increased similarly to lower concentrations; however, instead of saturating at one value, there is a kink in the transmission, and it again starts to increase. The emergence of this double saturation is most likely due to a sharp change in the structural properties of the film due to increasing -phase content. The responses to 1%, 1.5%, and 2% are contrasted in Fig. 4(a) for a C aperture with different dimensions than those of Fig. 1. The behavior can be seen in both the saturation and relaxation phases. Worth noting is the almost doubling of the power change at 1.5% and the much longer response time. However, it has also been shown that the response of Pd-based sensors can be optimized by annealing and/or work hardening of the samples through hydrogen exposure [23]. Thus, to optimize the response of the sensors presented below, they were first exposed to a train of short 5% hydrogen pulses. This pushes the films through a phase change, which had a side effect of causing the response direction of some of the sensors to change, as seen in Fig. 4(b). This change is attributed to plastic deformation of the film in the  phase [24]. Since over work hardening can begin to mechanically degrade the films, this process was kept to several pulses. This unintended Bflipping[ behavior resulted in several sensors where the transmitted power decreased rather than increased at low hydrogen concentrations. At concentrations above the phase change threshold, the transmitted power always increased. After work hardening, the response at low concentrations was stable regardless of whether it was a transmission increase or decrease and did not further change direction without being passed through a phase change. The physics behind this behavior is currently unclear, but we believe it is most likely due to permanent plastic deformation,

Vol. 4, No. 5, October 2012

Page 1756

IEEE Photonics Journal

Hydrogen Detection Using Polarization Diversity

Fig. 5. Pulse test data for each of the three sensor types showing the change in transmitted power versus time for various hydrogen concentrations. Measured at  ¼ 1550 nm.

Fig. 6. Summary of power change versus hydrogen concentration for each sensor type averaged over multiple pulses and multiple sensors. Measured at  ¼ 1550 nm.

which changes either the optical properties of the palladium film or the way hydrogen incorporates itself into the lattice in the  phase. At low hydrogen concentrations, this may affect how the interstitial hydrogen atoms interact. At high concentrations, hydrogen bonds at lattice sites and again contributes to an overall increase in transmission. For testing, each sensor is exposed to the same hydrogen concentration at multiple times, separated by at least one cycle. Furthermore, multiple apertures of the same dimensions are tested. The length of exposure to each hydrogen concentration was chosen to be long enough so that the response saturated; for this, a cycle of 60 min was chosen, with 30-min hydrogen exposure and 30 min of nitrogen purge. The sensor was tested down to 0.25% hydrogen, or 2500 ppm, as limited by our MFCs. Since a consistent measurement of absolute power is hard to guarantee, the percent change in power is a better metric for sensing. The percent change in transmitted power for the C aperture, the circle aperture, and a plain film is shown in Fig. 5, along with the corresponding hydrogen concentration. Note that there is a small baseline drift in the circle aperture due to outgassing of the Pd film in the nitrogen chamber; a longer pre-exposure nitrogen purge would eliminate this behavior. The fractional power change, P ¼ ðPðtÞ  Pð0ÞÞ=Pð0Þ, versus hydrogen concentration for the three sensors is summarized in Fig. 6.

Vol. 4, No. 5, October 2012

Page 1757

IEEE Photonics Journal

Hydrogen Detection Using Polarization Diversity TABLE 1 Summary of response timesy

At the lowest concentration of 0.25% hydrogen, a transmitted power change of 8.5% was seen for the C aperture, whereas the circle and plain sensor only had changes of 3% and 0.5%, respectively. As hydrogen content increased to a maximum of 1%, the power change increased to around 13% for the C aperture, with values of 4.5% and 1.8% for the circle and plain film, respectively. Above 0.5% hydrogen, this is equivalent to a sensitivity P=H of 4, 2, and 1.7 for the C, circle, and plain film sensors, respectively. The 10%–90% response times of the sensors varied between 4–15 min for the three structures, as shown in Table 1. Due to the long linear ramp after the fast initial rise in transmission at 1% H2 , response times were only calculated up to 0.75% H2 . For the C aperture, the rise time decreased quickly with increasing hydrogen concentration, while the fall time was longer and did not change much. This is attributed to the outdiffusion of the hydrogen gas. During exposure, hydrogen quickly saturates the high-field-intensity region at the air/Pd interface, but during relaxation, hydrogen from deep in the palladium layer must pass out through the surface, keeping the hydrogen concentration at the interface higher for a longer period of time. However, due to the nonlinear dependence of P on H and strong confinement to the surface, the rise time for the C aperture decreases more rapidly at high concentrations when compared with a plain film. Typical response times for palladium-based sensors in literature are on the order of tens of seconds [13] or less; however, the combination of lower hydrogen concentrations and an order of magnitude thicker film produce longer response times. Furthermore, Gaþ contamination on the surface of the aperture may also increase the response time, as seen in the comparison to the plain film in Table 1. A thick film was chosen to highlight the role of the aperture, but a thinner film or modified layer structure may result in an ideal balance in response amplitude and response time.

5. Polarization Dependence Since the C aperture is asymmetric, the transmission should depend on the polarization of the incident light. Furthermore, since the confinement for the two modes is different, the response to hydrogen for the two modes will be different. The transmission for two orthogonal polarization states was measured concurrently by using a polarization scrambler to constantly cycle through the entire Poincare´ sphere and then recording the minimum and maximum values over several cycles through the sphere. This was done to ensure that the minimum and maximum were accurately represented for each measurement. For comparison to theoretical values, the measured versus theoretical transmission is shown in Table 2, where the values are normalized to the plain film transmission. Since the transmission is highly dependent on the alignment of the fiber tip, comparison between different apertures is difficult. This can be seen from the fact that the experimental transmission for the circle aperture is much higher than expected. However, comparing two different modes from a single device is possible, and the TE-to-TM ratio for the C aperture matches well with theory. The slight deviation is due to error in the film thickness, aperture imperfections, and potential scattering from etching into the fiber. The same hydrogen pulse test was then done with the scrambler. These results, as shown in Fig. 7, show that the two modes respond differently to hydrogen for the C aperture and that, as expected, there is no polarization dependence for the symmetric circle aperture or the plain film. The difference in response for the C aperture is apparent from the divergence of the two curves at higher concentrations, which can be seen in Fig. 8. Furthermore, the flipping effect of work

Vol. 4, No. 5, October 2012

Page 1758

IEEE Photonics Journal

Hydrogen Detection Using Polarization Diversity TABLE 2 Simulated and experimental transmission

Fig. 7. Polarization-dependent pulse test data for each of the three sensors types. The light-blue curve represents the maximum power measured over several cycles of the Poincare´ sphere. The black curve is the minimum measured power over the same interval. The two curves overlap for the circle aperture and plain film sensors. Measured at  ¼ 1550 nm.

hardening can be seen in the circle and plain film plots for these specific devices as the transmitted power is now decreasing with hydrogen, compared with the devices shown in Fig. 5, which were increasing with hydrogen. To further analyze the differences between the two modes, the polarization-dependent loss (PDL) of the C aperture was calculated. To calculate the PDL, the recorded minimum and maximum were smoothed over 5-s intervals using a median filter to remove spikes associated with nonuniformities in the sampling of the polarization states. Given the heavy one-sidedness to the noise, this provided better results than simple averaging. These results are shown in Fig. 9, where there is a clear and repeatable increase in the PDL with hydrogen. At 0.25% hydrogen, this was around a 0.07-dB increase in PDL, and at 1% hydrogen, this was around a 0.14-dB increase in PDL. Using the PDL as a sensing mechanism, rather than transmitted power, would eliminate common-mode noise such as small changes in the fiber-to-detector alignment and, thus, would provide for a more reliable sensor. A summary of the sensor’s response is shown in Table 3. For concentrations above 0.5% hydrogen, the sensor’s response to hydrogen can be roughly approximated as linear. For the minimum polarization, this gives a sensitivity of 3.84% power change per 1% hydrogen. For the

Vol. 4, No. 5, October 2012

Page 1759

IEEE Photonics Journal

Hydrogen Detection Using Polarization Diversity

Fig. 8. Summary of the hydrogen response at  ¼ 1550 nm for the two polarization states of the C aperture.

Fig. 9. PDL for the two states of the C aperture at  ¼ 1550 nm as a function of time for various hydrogen concentrations.

TABLE 3 Summary of device performance

maximum polarization, however, this value is a 5.56% power change per 1% hydrogen. By comparing the response at our minimum measured concentration of 2500 ppm with the noise in the data (see Figs. 7 and 9), we believe the minimum detection limit (MDL) of the plain film sensor is about 2500 ppm, the circle aperture sensor’s MDL is below 2500 ppm, but the C aperture sensor’s MDL is very far below 2500 ppm using the minimum polarization, maximum polarization, or PDL.

6. Conclusion The results presented offer new insight into the use of metal apertures as optical sensors using subwavelength transmission. The C aperture demonstrates the advantage of using resonant

Vol. 4, No. 5, October 2012

Page 1760

IEEE Photonics Journal

Hydrogen Detection Using Polarization Diversity

structures with extraordinary transmission and polarization diversity. It showed up to 15% change in transmitted power at 1% hydrogen, 3 improvement over the circular aperture, and 8 improvement over the plain film sensor. By using two different polarizations, the C aperture sensor can reduce common-mode noise. The effects of work hardening are also contrasted, showing how exposure to high concentrations can result in a reversal in the direction of the power change for a given aperture. The nanoaperture sensor would be particularly useful for in situ monitoring of processes with limited space due to the small form factor of the sensor and the ease with which an optical fiber can be routed.

Acknowledgment The authors thank Amir Arbabi and Benjamin Griffin for their assistance in setting up the simulations and measurements, respectively.

References [1] T. N. Vezirolu and S. S´ahin, B21st century’s energy: Hydrogen energy system,[ Energy Convers. Manag., vol. 49, no. 7, pp. 1820–1831, Jul. 2008. [2] C. Christofides and A. Mandelis, BSolid-state sensors for trace hydrogen gas detection,[ J. Appl. Phys., vol. 68, no. 6, pp. R1–R30, Sep. 1990. [3] J. Dai, M. Yang, Y. Chen, K. Cao, H. Liao, and P. Zhang, BSide-polished fiber Bragg grating hydrogen sensor with WO3 Pd composite film as sensing materials,[ Opt. Exp., vol. 19, no. 7, pp. 6141–6148, Mar. 2011. [4] Z. Zhao, Y. Sevryugina, M. A. Carpenter, D. Welch, and H. Xia, BAll-optical hydrogen-sensing materials based on tailored palladium alloy thin films,[ Anal. Chem., vol. 76, no. 21, pp. 6321–6326, Nov. 2004. [5] T. B. Flanagan and W. A. Oates, BThe palladium–hydrogen system,[ Annu. Rev. Mater. Sci., vol. 21, no. 1, pp. 269– 304, Aug. 1991. [6] M. Wang and Y. Feng, BPalladium–silver thin film for hydrogen sensing,[ Sens. Actuators B, Chem., vol. 123, no. 1, pp. 101–106, Apr. 2007. [7] J. van Lith, A. Lassesson, S. A. Brown, M. Schulze, J. G. Partridge, and A. Ayesh, BA hydrogen sensor based on tunneling between palladium clusters,[ Appl. Phys. Lett., vol. 91, no. 18, pp. 1819101-1–1819101–3, Oct. 2007. [8] B. Griffin, A. Arbabi, A. Kasten, K. Choquette, and L. Goddard, BHydrogen detection using a functionalized photonic crystal vertical cavity laser,[ IEEE J. Quantum Electron., vol. 48, no. 2, pp. 160–168, Feb. 2012. [9] M. Raval, S. McKeown, A. Arbabi, and L. L. Goddard, BPalladium based Fabry–Pe´rot etalons for hydrogen sensing,[ presented at the Optical Sensors, Monterey, CA, Jun. 2012, STh2B.5. [10] P. Tobisˇka, O. Hugon, A. Trouillet, and H. Gagnaire, BAn integrated optic hydrogen sensor based on SPR on palladium,[ Sens. Actuators B, Chem., vol. 74, no. 1–3, pp. 168–172, Apr. 2001. [11] N. Liu, M. L. Tang, M. Hentschel, H. Giessen, and A. P. Alivisatos, BNanoantenna-enhanced gas sensing in a single tailored nanofocus,[ Nat. Mater., vol. 10, no. 8, pp. 631–636, May 2011. [12] M. A. Butler, BOptical fiber hydrogen sensor,[ Appl. Phys. Lett., vol. 45, no. 10, pp. 1007–1009, Nov. 1984. [13] J. Villatoro, D. Luna-Moreno, and D. Monzo´n-Herna´ndez, BOptical fiber hydrogen sensor for concentrations below the lower explosive limit,[ Sens. Actuators B, Chem., vol. 110, no. 1, pp. 23–27, Sep. 2005. [14] C. Caucheteur, M. Debliquy, D. Lahem, and P. Megret, BHybrid fiber gratings coated with a catalytic sensitive layer for hydrogen sensing in air,[ Opt. Exp., vol. 16, no. 21, pp. 16 854–16 859, Oct. 2008. [15] X. Shi and L. Hesselink, BDesign of a C aperture to achieve =10 resolution and resonant transmission,[ J. Opt. Soc. Amer. B, vol. 21, no. 7, pp. 1305–1317, Jul. 2004. [16] H. Gai, J. Wang, Q. Tian, W. Xia, and X. Xu, BMatching the emitting wavelength from a very-small-aperture laser to the resonant property of a nanometric C-aperture,[ Appl. Opt., vol. 46, no. 31, pp. 7746–7750, Nov. 2007. [17] Z. Rao, BHigh-intensity nano-aperture lasers for near-field optics,[ Ph.D. dissertation, Stanford Univ., Stanford, CA, Nov. 2007. [18] S. McKeown, B. Griffin, and L. Goddard, BFiber optic hydrogen sensor utilizing facet-etched metal nano-apertures,[ in Proc. 23rd Annu. Meeting IEEE Photon. Soc., Nov. 2010, pp. 730–731. [19] A. Mandelis and J. A. Garcia, BPd/PVDF thin film hydrogen sensor based on laser-amplitude-modulated opticaltransmittance: dependence onH2 concentration and device physics,[ Sens. Actuators B, Chem., vol. 49, no. 3, pp. 258– 267, Jul. 1998. [20] E. Wicke and H. Brodowsky, Hydrogen in Metals II, G. Alefeld and J. Vo¨lkl, Eds. Berlin, Germany: Springer-Verlag, 1978. [21] L. Goddard, K. Y. Wong, A. Garg, E. Behymer, G. Cole, and T. Bond, BMeasurements of the complex refractive index of Pd and Pt films in air and upon adsorption ofH2 gas,[ in Proc. 21st Annu. Meeting IEEE LEOS, Nov. 2008, pp. 569–570. [22] Y. Fu and N. Bryan, BInvestigation of physical properties of quartz after focused ion beam bombardment,[ Appl. Phys. B, Lasers Opt., vol. 80, no. 4, pp. 581–585, Feb. 2005. [23] R. Smith and D. Otterson, BThe effect of hydrogen on the tensile properties of palladium,[ J. Less Common Metals, vol. 24, no. 4, pp. 419–426, Aug. 1971. [24] F. A. Lewis, The Palladium Hydrogen System. New York: Academic, 1967.

Vol. 4, No. 5, October 2012

Page 1761