An experimental investigation of long-ranging surface plasmon-polariton waves guided along Bragg gratings fabricated from a thin finite width Au film embedded in a homogeneous background dielectric is reported. The operation of four grating families is demonstrated near the free space optical wavelength of 1550 nm. The influence of the length of the grating and the depth of effective index modulation on the performance of these elements is presented and discussed.
©2005 Optical Society of America
A metal film of finite width in an optically infinite homogeneous dielectric supporting the mode1 can be used as the foundation waveguide for a new integrated optics technology. This paper reports experimental work conducted on a Bragg grating concept implemented with this waveguide structure2, building on demonstrations of other key passive elements, including for instance straight waveguides3,4, bends, y-junctions and couplers5,6
The grating concept consists in patterning the width of a metal film of constant thickness in a periodic manner over a prescribed length Lg as illustrated in Fig. 1. Part (a) shows a grating consisting of a metal-gap periodic pattern where the gap corresponds to an absence of metal filled by the background dielectric. Part (b) shows a grating consisting of a step-in-width periodic pattern. Part (c) shows four generic unit cells with w 1, d 1 and w 2, d 2 corresponding to the dimensions of the first and second metal segments, respectively, within the first unit cell. A gap corresponds to w 2=0. The pitch or period of the grating is denoted Λ and the duty cycle is d 1/Λ. This is a waveguide grating so the reflected and transmitted waves propagate longitudinally along the structure and are collected via the straight input and output waveguide segments shown on either side of the grating in Parts (a) and (b).
Changing the width of the film changes the effective index of the mode, with the lowest value of effective index occurring for a vanishing width and the highest value occurring for the widest segment. In the case where a metal segment is absent (w 2=0), the mode couples into plane waves supported by the background dielectric, which then propagate paraxially over the short gap length d 2 to couple back into the mode supported by the next metal segment.
A periodic pattern in the film thus engenders a corresponding periodic modulation in the effective index of the mode along the length of the structure thereby inducing reflections at and near the Bragg wavelength λB given by:
In the above, N is an integer corresponding to the order of the grating and nave is the average effective index of the mode in the grating. The effective index modulation depth can be larger in the metal-gap architecture than in the step-in-width architecture, leading to stronger gratings for the former for identical grating lengths Lg .
The grating concept described herein differs substantially from other gratings described in the literature involving a metal(s) or surface plasmons7–13, in that the long-ranging mode supported by a finite width metal film is utilized here and the structure corresponds to a waveguide grating. Tredicucci et al 7 used a periodic arrangement of Ti strips covered with an Au film to form a bi-metallic grating supporting surface plasmons at the top metal-semiconductor interface of a dielectric (semiconductor) waveguide. Osowski et al 8 used a dielectric waveguide loaded periodically with Ti strips to form a grating structure, the propagating optical mode not being a surface plasmon wave. Weeber et al 9, characterized Bragg micro gratings similar to those proposed here, consisting of a periodic arrangement of narrow slits engraved (using focused ion beam milling) into Au strips evaporated onto an indium tin oxide doped glass substrate, and propagating surface plasmons over short distances (non long-ranging). Work on surface plasmon polariton photonic crystals10 bears some relationship to the Bragg structures described herein in that they too can also be viewed as 1-D photonic crystals. Finally, corrugated metal gratings supporting and diffracting surface plasmon polaritons11 represent perhaps the most mature application of periodicity for the manipulation of surface plasmons and their coupling to free radiation fields. Within this context, Hooper at al12 have recently reported a theoretical study on the excitation and response of unsupported corrugated Ag slabs where both surfaces of the film are corrugated either conformally or non-conformally. Their study emphasized the role played by the coupled short and long ranging surface plasmon polaritons in the reflection and transmission response of the structure under broadside excitation. Gómez Rivas et al 13, recently demonstrated the operation of corrugated gratings fabricated on n and p doped Si at THz frequencies using a time domain spectrometer. Corrugated gratings11,12 are generally based on slabs and are typically excited and used at out-of-plane angles (including broadside), in contrast to the waveguide gratings of interest here which are based on narrow metal films of constant thickness and modulated width, and which are excited and used in an end-fire arrangement.
Within the broader context of integrated optics, the gratings proposed herein are substantially different from gratings in dielectric waveguides due to a fundamental distinction: the former operate via a surface plasmon polariton mode (ss 0 b) while the latter operate via a dielectric waveguide mode ( , ). The well-known attributes of these modes lead to the principal distinguishing features: the former are TM polarised only, have higher loss, but are surface waves, while the latter can be TE or TM polarised, have lower loss, but are essentially “bulk” waves. The implications of these distinctions must be carefully considered in any intended application of the gratings proposed herein.
2. Fabrication and experimental procedures
Bragg grating designs having λB near 1550 nm were implemented using 20 nm thick Au (εr ,Au=-131.9475-j12.6514) on SiO2 and an index-matched polymer upper cladding. All designs are third order with a pitch of Λ=1.6 µm and a duty cycle of 0.5, leading to a minimum feature dimension of 0.8 µm. Such dimensions can be patterned using contact lithography. Four grating families were designed where the only difference between the members of a family is the length of the grating Lg . Access waveguides, each comprising an 8 µm wide Au film, were added to both ends (input and output) of each grating. The grating families designed and implemented include: (i) metal-gap with w 1=8 µm denoted C8g; (ii) step-in-width with w 1=8 µm and w 2=2 µm denoted C82; (iii) step-in-width with w 1=8 µm and w 2=3 µm denoted C83; (iv) step-in-width with w 1=8 µm and w 2=4 µm denoted C84.
The gratings were fabricated on 15 µm thick native thermal oxide (SiO2) on Silicon <100> wafers, the SiO2 layer forming the lower cladding of the structures. A wafer was spin-coated with a bi-layer of resist, exposed using broadband light in a contact aligner and then developed in a chemical bath using standard microfabrication techniques. A thin Au film of the desired thickness was then e-beam evaporated under vacuum directly onto the cleaned SiO2 surface. The deposition rate and thickness of the metal film were monitored during deposition using a quartz crystal microbalance. The metal in the field areas was subsequently lifted off to reveal the metal features. An adhesion layer between the metal and the lower cladding was not used in the fabrication of the waveguides. The upper cladding is an infinitely thick (optically) optical polymer (OCK-433, Nye Optical), which is thermally tuned during the measurements to precisely match the index of the SiO2 substrate over the operating wavelength range. Using this material as the upper cladding instead of SiO2 reduces the fabrication effort (cost and time) and is suitable for demonstration prototypes.
2.2 Experimental procedures
An end fire coupling technique (fibres butt-coupled to the waveguides on the die) is used to excite the grating under test and to collect the emerging reflected and transmitted optical signals as shown in Fig. 2(a). A polarization maintaining (PM) fibre is used at the input of the grating and a single mode fibre (SMF) is used at the output. The polarization of the light emerging from the input PM fibre is polarization aligned to match the polarization of the mode supported by the grating (TM). Optical index matched gel is placed between the input and output fibres and the die.
Before proceeding with the measurement of wavelength responses, and in fact before introducing the output fibre into the set-up, the output mode emerging from the grating at a wavelength off-resonance but near λB is viewed using a lens system and an IR camera. The temperature of the thermo-electric cooler (TEC) is then adjusted in order to achieve a nicely symmetrical mode output and thus precisely index-matched claddings. The temperature of the die is then maintained constant using a temperature monitor in a feedback control loop to the TEC. The lens system and the IR camera are then removed and the output fibre is introduced into the set-up and aligned.
The set-up used to measure the wavelength response is shown schematically in Fig. 2(b). The low power tuneable laser source has a PM output fibre and is connected directly to port y of a 3 dB PM fused silica fibre coupler having a centre wavelength of 1550 nm. The PM coupler allows the measurement of the reflectance and transmittance spectra simultaneously. The coupled port of the PM coupler (port b) is terminated in index-matched gel to minimise back reflections through the coupler that can interfere with the measurements. The through port of the coupler (port a) delivers the incident light from the tuneable laser to the grating under test (DUT). The transmitted power is measured on channel C1 of the optical power meter. The input PM fibre collects the reflected light from the grating under test, which then emerges from port x of the PM coupler and is detected on channel C2. Calibration sets the measurement reference planes to the end facet of the input and output fibres. Instrument control and data acquisition are achieved via computer.
3. Experimental results
3.1 Metal-gap designs
Measurements related to a family of C8g gratings located on the same die are shown in Fig. 3. The die characterised is 6.3 mm long and the gratings vary in length from Lg =1 to 5 mm. The difference between the die length and grating length is taken up by the input and output access waveguides. The spectral response of these access waveguides (insertion loss) has been removed from the grating responses but the coupling losses at the fibre/die interfaces remain (about 0.4 dB per facet).
Several features are readily apparent from the transmittance curves plotted in Fig. 3(a). Firstly, the uniformity of the Bragg wavelength λB among the gratings is quite good as the transmission dip coincides very well for all gratings. Secondly, both the off- and on-resonance transmittances decrease with increasing length Lg as expected. The off-resonance transmittance decreases with Lg since the gratings exhibit increasing loss. The on-resonance transmittance decreases even more with Lg since the gratings become stronger, reflecting more of the incident wave back toward the input. Thirdly, the off-resonance transmittance is lower on the short wavelength side of λB than on its long-wavelength side. This might be caused by coupling to radiation modes of the structure at shorter wavelengths or to diffraction (or both). Reduced transmittance on the shorter wavelength side in semiconductor gratings was ascribed by Gómes Rivas et al13 to radiative losses, which supports the former.
Figure 3(b) gives the reflectance curves in dB obtained for the same gratings, showing that the reflectance of the gratings increases with increasing length, as expected, and in agreement with the transmittance trend. Some asymmetry is noted in the sidelobe levels on either side of the main reflectance peak, primarily in the levels of the first sidelobe and first null. The full width at half maximum (FWHM) bandwidth of the gratings is about 0.5 nm, except for the 1 mm long grating which exhibits a FWHM bandwidth of about 1.5 nm. The bandwidth narrows with increasing grating length as expected.
Figure 4 gives a series of infrared mode outputs captured for the 5 mm long C8g grating as the excitation wavelength was swept through the transmission dip. A nicely symmetric output mode is observed over the entire measurement wavelength range. There is slight radiation into the upper cladding as indicated by the lower intensity ring surrounding the mode. The intensity on-resonance is about 10 dB lower than off-resonance.
3.2 Step-in-width designs
Measurements related to a family of C83 gratings located on the same die are shown in Fig. 5(a) (transmittance) and in Fig. 5(b) (reflectance). The die characterised is 6.3 mm long, the gratings have lengths of Lg =2.5 to 5 mm, the difference between the die length and grating length is taken up by the input and output access waveguides and their spectral response (insertion loss) has been removed from the grating responses but the coupling losses at the fibre/die interfaces remain (about 0.4 dB per facet).
The same general trends are noted in these spectral responses as for the C8g gratings, the principal differences between the families being the depth of the transmission dip (or strength of the reflection peak) and the improved symmetry of the spectral response with respect to λB . The FWHM bandwidth of these gratings also decreases with length and is less than 0.5 nm. It is noted that the off-resonance transmittance on the short wavelength side of λB is essentially the same as that on the long wavelength side (Fig. 5(a)), whereas this was not the case for the C8g designs (Fig. 3(a)). In fact, improved symmetry was observed as the gap was filled with an increasingly wider second metal segment in going from the C8g to C84 families.
Figure 6 gives a series of infrared mode outputs captured for the 5 mm long C83 grating as the excitation wavelength was swept through the transmission dip. A well confined and nicely symmetric output mode displaying very little background radiation is observed over the entire measurement wavelength range. There is less background radiation in these mode outputs compared to the C8g design given in Fig. 4, due to the increased confinement provided by the w 2=3 µm wide segments. The intensity on-resonance is about 3 dB lower than off-resonance.
3.3 Summary of reflectance strengths
Figure 7 plots the reflectance strength versus grating length Lg for the four grating families characterized in this study (the case Lg =3 mm was not used in the polynomial fit of the C8g family). The spectral responses of the C82 and C84 families resemble those of the C83 family given in Fig. 5. The peak reflectance at a given grating length decreases with effective index modulation depth (from C8g down to C84) as expected. For each family, the peak reflectance increases with Lg up to a maximum value beyond which it remains approximately constant or shows a small decrease. The reason for the existence of a maximum in the peak reflectance is clear: the gratings have loss so adding length produces diminishing reflectance looking into the input of the grating. The largest reflectance observed among these families is about -1.8 dB, corresponding to 66% power reflection, and occurring for the C8g grating of length Lg =4.5 mm. Higher reflectances are expected for first order designs.
The operation of Bragg gratings designed using thin Au films of finite width in an SiO2/polymer background and supporting a single long-ranging mode has been demonstrated. The impact of varying a few important design parameters, such as the length of the grating and the effective index modulation depth, was explored. A simple fabrication approach was used to construct the devices and end-fire couplings were achieved using a polarization maintaining fibre butt-coupled to the input of the device and a single mode fibre butt-coupled to its output. Detailed quantitative comparisons with results generated by a theoretical model of the gratings are currently being made and will be reported in a subsequent paper.
The authors would like to acknowledge the fabrication work performed by A. Burns and J. Hempinstall, formerly with Spectalis Corp, and would like to thank L. Berndt and Professor G. Tarr of Carleton University for their support.
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