The suitability of a terahertz plasmonic sensor for sensing applications is successfully demonstrated using a hybrid planar waveguide composed of a subwavelength plastic ribbon waveguide and a diffraction metal grating. The subwavelength-confined terahertz plasmons on the hybrid waveguide resonantly reflect from the periodic metal structure under phase-matched conditions and perform resonant transmission dips. The resonant plasmonic frequencies are found to be strongly dependent on the refractive indices and thicknesses of analytes laid on the hybrid planar waveguide. Both plastic films with varying thicknesses and granular analytes in different quantities are successfully identified according to the spectral shifts of resonant dips. An optimal refractive index sensitivity of 261 GHz per refractive index unit is achieved. Within localized and enhanced terahertz plasmonic fields, the minimum detectable optical path difference can be reduced to 2.7 μm corresponding to λ/289, and the minimum detectable amount of analytes in powdered form reaches 17.3 nano-mole/mm2. The sensing technique can be used to detect particles in a chemical reaction or monitor pollutants.
© 2013 OSA
Various terahertz (THz) sensors are widely used for sensitive and label-free detection of materials with different THz spectral fingerprints or dielectric constants [1–3]. Different sensors generally exhibit spectral features such as resonant dips/peaks or band edges in transmission power or dispersion. Analytes can thus be identified by the measurement of spectral variation. These sensors include THz waveguides [4–6], photonic crystals , filters , and resonators [9, 10]. Waveguide sensing modality has drawn considerable research interest because waveguide configuration allows THz radiation to effectively interact with analytes in a small region. This modality also improves detection sensitivity and integrates easily with different kinds of platforms and resonator/filter structures for various applications [7, 11–13]. One method for sensing minute amounts of materials such as ultrathin overlayers or small amounts of molecules for use in biochips or lab-on-a-chip applications is to markedly increase the electromagnetic field overlap with analytes by the subwavelength-scaled confinement of the waveguide modes [6, 14, 15].
The use of optical surface plasmon resonance (SPR) for label-free sensing subwavelength-thick analytes has received wide acceptance in near-field microscopy and sensing applications [16, 17]. The sensitivity and functionality of SPR sensing in an integrated waveguide can be engineered by optimizing SPR generation and ensuring highly efficient interaction between electromagnetic fields and analytes. Generally, THz waves cannot be confined to a perfect conductor-like metal surface to form localized surface plasmons because THz frequencies are considerably lower than the intrinsic plasmon frequencies of metals. This difference prevents field penetration inside the metals to directly excite SPPs. Metal surfaces exhibiting different textures, such as 2D hole arrays , periodic slits , and various patterns of metamaterials [20, 21], have been demonstrated to generate THz spoof surface plasmons (THz-SSPPs) and thus enhance lateral confinement. In addition, THz-SPR has recently been demonstrated to exhibit extraordinary optical transmission (also referred to as resonant transmission) after passing through a metal hole array (MHA) [2, 22–26] and a planar metamaterial device . This transmission results from the interference by the incident waves and the re-emitted radiation from the metal holes or the resonator elements. The resonant frequency of THz-SPPs from the MHA and planar metamaterial devices depend on the dielectric constants in the vicinity of the metal structure, and the sensing scheme has been approved for the successful identification of different concentrations  and thicknesses of molecular overlayers . Based on the spectral resonance shift, the detectable molecular amounts and thickness of the analyte’s overlayer on the MHA and the planar metamaterials are, respectively, demonstrated down to 0.01 mmole/mm2  and 100nm . Combining the enhanced lateral confinement and the refractive index dependence of THz-SSPPs allows molecular sensing in the configuration of metal structured waveguide. However, current THz-SSPPs propagated on plasmonic waveguides do not exhibit obvious spectral features, and the transmission power of the well-confined waveguide mode is too weak to interact with analytes [27, 28].
To generate THz-SSPPs with sufficiently high lateral confinement, high transmission power, and distinct spectral features related to the refractive index of the analyte [29, 30], we propose and experimentally demonstrate a THz hybrid plasmonic waveguide for sensing minute materials. This waveguide consists of a plastic ribbon waveguide integrated with a diffraction metal grating. A THz surface plasmonic wave (SPW) propagated along the hybrid waveguide is generated by transferring the plastic ribbon waveguide modes into the THz-SSPPs by means of the integrated diffraction metal grating structure [31, 32]. The generated THz-SPWs are not only confined within subwavelength scales; they are also reflected resonantly from the periodic metal structure under phase matching conditions . The phase-matched resonance of THz-SPWs resembles Bragg resonance in the periodic structure. The spectral positions of the resonant dips in the transmission spectrum depend on the ambient refractive index of the grating. When the thickness of an overlayer covered on the plasmonic waveguide changes, the resonant dips of THz-SPW shift based on the variance in the optical path difference (OPD). Therefore, various polyethylene (PE) films with thicknesses of 20, 50, and 90 μm are used as test analytes and to calibrate the spectral dip shift sensitivity of the THz-SPWs.
In thin film sensing, the hybrid plasmonic waveguide exhibits high sensitivity because the near-field-confined THz-SPW provides high fractional overlaps with the thin film analytes, contributing to large waveguide-index variation per analyte thickness unit. The sensitivity of thin-film detection depends on the thickness because there are frequency-dependent evanescent mode sizes of THz-SPWs. For a 20 μm thick PE film substrate, the minimum detectable OPD can be reduced to 2.7 μm based on spectral resolution of 0.004THz, corresponding to λ/289 according to the sensitivity of 261 GHz/refractive index unit (RIU); however, the particle detection sensitivity of the sensing scheme is less than that of the thin film for the same PE substrate. Based on the sensitivity of a 60 μm thick PE film substrate, 163 GHz/RIU, and the spectral resolution of 0.004 THz, the minimum detectable molecular quantity is about 13 μmol, which corresponds to the molecular density of about 17.3 nano-mole/mm2. This amount can potentially detect high-dispersion particles when monitoring various pollutants and chemical reactions.
2. Sensing configuration
Figure 1(a) presents a schematic of the side view of a hybrid plasmonic waveguide for THz sensing; the waveguide consists of a plastic ribbon waveguide integrated with a metallic diffraction grating. The 1D diffraction grating is made of a 200 μm thick brass. The grating period of the rectangular grooves is 1.5 mm, including air and brass sections with lengths of 1.0 and 0.5 mm, respectively. The plastic ribbon waveguide consists of a PE film that is 20 μm thick, 220 mm long, and 15 mm wide. The ribbon thickness is much smaller than the wavelengths of the THz waves, ranging from 0.05 THz to 1 THz. Thus, the delivered transverse magnetic (TM) waves are almost evanescent in the air cladding as the dominant waveguide mode . Figure 1(a) shows a 50 mm long metallic diffraction grating attached to one side of the PE ribbon at the output end to couple the low-loss TM waves from the plastic ribbon waveguide transferring into the subwavelength-confined THz-SPWs to propagate on the grating surface . As shown in Fig. 1(a), the evanescent fields of the THz-SPWs can be sensitively modified by the attached analytes, and the THz-SPWs interact with the analytes on the grating for a sufficiently long distance to generate obvious spectral loss for sensing applications. Despite resonant THz-SSPPs generated by a metal mesh for sensing air hole-loaded thin films, the reflection power in a transmission spectrum is indistinct because of the short interaction, obtaining a poorly distinguishable spectral feature that cannot be used for sensitive detection .
The resonant reflection of THz-SPWs on the grating waveguide follows the momentum conservation principle. The wave vector geometry of the phase-matching condition shown in Fig. 1(b) follows the vector equation The magnitude equation derived from the vector calculation is expressed as where Kin, Kref and KΛ are the propagation constants of the incident, resonant, and grating wave vectors, respectively, and ϕ is the angle between Kin and KΛ. The propagation constants of the incident wave Kin and the resonant wave Kref are equal to where neff, ν, and C denote the effective refractive index, resonant frequency of the THz-SPWs, and light speed in a vacuum, respectively. The grating wave vector is defined as where Λ and m represent the grating period and the Bragg diffraction order of the metal grating, respectively. Thus, the resonant frequencies of the THz-SPWs shown in Eq. (1) can be derived from the propagation constants in the magnitude equation.Fig. 2(b), the resonant wave propagates forward and reflects among the periodical air-metal interface in the highest coupling efficiency. THz-SPW resonance thereby shows serious losses as spectral dips in a transmission spectrum, providing a straightforward spectral feature. This feature can recognize various analytes when the partial cladding space of the hybrid waveguide is replaced by a different overlayer, contributing to the different refractive indices of the waveguide.
3. Resonance spectrum of terahertz surface plasmonic waves
The transmission spectra, as shown in Fig. 2, of the PE ribbon waveguide with and without attaching the metal structure, as sketched in Fig. 1(a), is measured by means of a waveguide based terahertz time-domain spectroscopy (THz-TDS) . A THz-pulse is coupled to the input end of the ribbon using an off-axis parabolic mirror with effective focal length of 75mm, and collimated out of the output end of the hybrid waveguide via a plastic lens with effective focal length of 50mm. In Fig. 2, the spectrum of the 220mm-long blank ribbon is so broad ranging from 0.1 to 1 THz with the best signal-to-noise ratio up to 104, where the sharp dips at 0.550 and 0.750THz result from the environment water vapor. The power spectrum can be used to observe various resonant frequencies of the 50mm long grating. Based on the integration scheme, THz waves are directly end-coupled into the hybridplasmonic waveguide . The grating period Λ equals 1.5 mm, and the effective index neff is assumed to be the air index equal to 1.0. There is about 0.100THz spectral range between resonant dips in the measurement; however, only two obvious spectral dips of the 3rd and 4th order resonant waves are found in the transmission spectrum of Fig. 2. The 1st and 2nd resonant dips are not observed because of the poor mode match, i.e. the ribbon mode size in Y-axis is too large to couple into the metal grating . The higher orders beyond the 4th resonant dip are unobvious in the transmission spectrum due to the low signal-to-noise ratio, ~10, which is not reliable for the sensing application.
The incident angle ϕ is modified in the discussion to determine its correlation to the Bragg resonance frequencies defined in Eq. (1). The grating is oriented with respect to Y-axis for various incident angles when THz waves are delivered from the ribbon waveguide. As shown in Fig. 3(a), the two spectral dips at 0.380 and 0.296 THz are simultaneously shifted to the high-frequency range when the input angle of the TM- polarized THz-wave is increased from 0° (black line) to 30° (golden line). Figure 3(b) compares the calculated resonance frequencies according to Eq. (1) with the measured spectral dips. Both the calculated and the measured Bragg resonant dips of the THz-SPWs exhibit a blue-shift trend as the incident angle increases. However, obvious deviation occurs in the high-order resonant frequency, whereas deviation is minimized in the low-order Bragg resonance. Given that the evanescent waves of the low THz frequencies are extended in the air to obtain effective waveguide indexes approximate to the air index, the measured 3rd-order Bragg frequency is consistent with the calculated frequency, as shown in Fig. 3(b). This result indicates that the modal fields of THz-SPWs in the low-frequency range of about 0.300 THz are mainly contributed by the TM waves of the ribbon waveguide. These waves directly and freely pass through the hybrid grating waveguide with considerably low coupling efficiency to excite the THz-SSPPs. At approximately 0.400 THz, the field distribution is significantly affected by the excited THz-SSPPs. This effect yields an effective waveguide index larger than the air index, causing large deviation between the measured and the calculated results, as shown in Fig. 3(b).
To test the sensing capability of the hybrid plasmonic waveguide according to the THz-SPW resonance, PE films with thicknesses of 20, 50, and 90 μm are attached to the grating surface to form the sensing configuration at an incident angle of 0°, as shown in Fig. 1(a). Figure 4(a) shows the transmitted spectrum of the hybrid plasmonic waveguide attached to PE films exhibiting varying thicknesses. The original spectral dip at 0.380 THz for the blank hybrid plasmonic waveguide obviously shifts to the low-frequency range as the PE film thickness is increased from 20 μm to 90 μm. However, the spectral shift of the 3rd-order SPW-Bragg resonance at 0.296 THz does not occur because the 0.296 THz modal field is not efficiently coupled to the metal grating to form the THz-SSPPs. In the low-order Bragg resonant mode shown in Fig. 3(b), the modal field is dominated by the TM modes of the ribbon waveguide to directly pass through the outer region of the metal grooves without being modulated by the metal grating. Thus, the surface wave of 0.296 THz directly propagates along the attached PE film with weak resonance reflection by the metal grating. By contrast, Fig. 3(b) shows that large portions of the modal field for the 4th-order Bragg resonance at 0.380 THz is confined as THz-SSPPs inside the metal grating, and a small portion of the modal field evanesces in the air cladding with a subwavelength modal tail strongly interacting with the covered thin PE films. The Bragg resonance reflections of THz-SSPPs are greatly influenced by the effective refractive index of the covered thin film because of the small modal distribution. Therefore, the resonant frequency of the 4th-order SPW-Bragg resonance, 0.380 +/− 0.020 THz, can be completely modified when the effective refractive index of the waveguide is changed by the attached PE film according to Eq. (1) with an inversely proportional relationship.
Figure 4(b) summarizes the spectral shift of the THz-SPW resonance from 0.380 THz to frequencies of 0.318, 0.281, and 0.263 THz, which are modified by the 20, 50, and 90 μm thick PE films, respectively, on the grating surface with a system deviation of +/−0.004 THz. On the basis of the relation between the resonant frequency of the THz-SPW and the effective refractive index shown in Eq. (1), the corresponding effective refractive indices are increased from the blank condition of 1.05 to 1.25, 1.42, and 1.51 for 20, 50, and 90 μm thick PE films, respectively, as shown in Fig. 4(c). The power distribution along the X-axis of the hybrid waveguide is measured as 2 mm whether the PE film is attached or not. The effective waveguide index resulted from the variation in the PE film thickness is determined from the power-occupied ratio of the modal field in each PE film along the Y-axis. The effective index can thus be estimated by the formula neff = (1-σ).nair + σ.nPE, where σ, nair, and nPE represent the volume ratio of the waveguide field occupied in a PE film, refractive indices of air, and PE film, respectively. The occupied ratios of the resonant THz-SPWs in the 20, 50, and 90 μm thick PE films are estimated as 0.5, 0.8, and 1.0, respectively, as shown in Fig. 4(d). The overall power distribution along the Y-axis outside the grating can be clearly covered by the 90 μm thick PE film, which leads the effective waveguide index to approach the refractive index of a PE material in THz frequency range .
Therefore, to identify various thin films according to the spectral shift of the Bragg resonant dip in a transmission spectrum, the evanescent field of the propagated THz-SPWs that interact with thin film analytes should be significantly confined [13, 14]. The decay lengths of the 4th-order Bragg resonance for various PE films attached to the metal grating can be calculated from the standard thickness d of the PE films, and the occupied field ratio σ inside the PE films as shown in Fig. 4(d). The power decay length ΔY is illustrated in the inset of Fig. 5(a) and can be calculated from the relation ΔY = d/σ. As shown in Fig. 5(a), the calculated decay lengths of the resonant THz-SPWs located at 0.318, 0.281, and 0.263 THz are approximately 40 μm (~λ/21), 60 μm (~λ/16), and 90 μm (~λ/12) apart from the metal-grating surface in the Y direction, respectively. In addition, the evanescent decay lengths of THz-SPWs can also be obtained from the waveguide transmittance T according to the formula T = (σ.e-αL) + (1-σ), where α, L denote the absorption coefficient (~1 cm−1)  and the PE film length (50 mm), respectively. σ of the guided THz-SPWs is expressed as σ = (1- T)/(1-e-αL), where the measured transmittance T of a PE film ranging from 0.160 THz to 0.280 THz is obtained using the transmission power shown in Fig. 4(a) and the decay lengths are then estimated, as shown in Fig. 5(b). The estimated minimal decay lengths of the delivered THz-SPWs for PE thicknesses of 20, 50, and 90 μm are approximately 40, 60, and 90 μm, respectively, which are approximately the same as the measured results at THz-SPW resonant frequencies shown in Fig. 5(a). The spectral ranges of the best confined THz-SPWs with the minimal decay length at the 4th-order Bragg resonance are estimated of around 70 GHz, located between 0.250 and 0.318 THz, 0.210 and 0.281 THz, as well as 0.190 and 0.263 THz for attaching 20, 50, and 90 μm thick PE films, respectively. Within these spectrum ranges, THz-SSPPs are excited with the best field confinement and the obviously index-dependent spectrum-shift that is not influenced by analyte absorption or scattering loss.
4. Near-field sensing results
The use of hybrid THz plasmonic waveguides for the sensitive detection of thin film analytes is based on the strong coupling of subwavelength-confined evanescent field with the analytes, which changes the index-dependent resonance-frequency. The modified waveguide index represents a certain fraction of the evanescent field replaced by the analytes. The waveguide-sensing mechanism can be used to detect different thicknesses or quantities of overlayer- or powder-form analytes. For conceptual demonstration of identifying different concentrations of analytes using the hybrid waveguide sensor, a chemical compound of Bi2CuISe3 with irregular grain shapes and non-uniform particle sizes, about less than 300 μm, was used as a sample to slightly change the effective index of the resonant THz-SPW and test the sensing capability of minute particle distributions in the waveguide sensing configuration, as illustrated in Fig. 1(a). A 60 μm thick PE film was treated as a sample substrate attached to the grating surface to adhere to chemical compounds in different quantities (e.g., 16.5, 29.3, and 55.3 μmole). The adsorbed particles with amounts of 16.5, 29.3, and 55.3 μmole are respectively and randomly adhered on the PE ribbon substrate, which is 50mm-long and 15mm-wide. The sensing result in Fig. 6(a) shows that the 4th-order resonance dip obviously shifts from 0.276 THz to 0.266, 0.259, and 0.237 THz, respectively for the blank substrate and with adsorbed molecular quantities of 16.5, 29.3, and 55.3 μmole, respectively. The apparent spectral shifts of the resonant dips from granular particles are attributed to the high material dispersion in THz frequency, which is verified by the THz-TDS measurement of the tablet sample, as shown in the inset of Fig. 6(a).
Figure 6(b) summarizes the resonant dip-shifts of THz-SPWs that correspond to waveguide-index variations from different thicknesses of PE films and different quantities of chemical compounds. For thin-film sensing, the waveguide index variation attributed to different thicknesses of PE films ranges from 0.2 to 0.467. A fitting curve (black line) is presented as a quadratic function in Fig. 6(b), and the tangential slope of each data point represents the detection sensitivity of the hybrid plasmonic waveguide. The detection sensitivity is defined as the frequency-shift range per refractive index unit of the waveguide, i.e. Δν/Δn. Thus, the sensitivities derived from the differential of the fitting curve according to substrates of 20, 50, and 90 μm thick PE films are 261, 197, and 159 GHz/RIU, respectively.
Sensitivity increases as the thickness of the PE film used as a substrate for powder or molecular overlayer sensing decreases because such a PE film has a larger fraction of evanescent field interacting with the added overlayer compared with a thick PE substrate. In addition to thin-film detection using the waveguide sensor, powder sensing is also demonstrated based on a 60 μm thick PE film substrate, in which the waveguide index variation is almost linearly proportional to the spectral shift in the waveguide index variation range of 0.395 to 0.626, as indicated by the red fitting line in Fig. 6(b). By using the slope of the fitting curve (red line) for varying quantities of powders adhered on the substrate, the sensitivity is estimated at 163 GHz/RIU and less than that of the thin film, which is approximately 187 GHz/RIU from the slope of the quadratic fitting curve (black line). This result indicates that a uniform overlayer can be more easily detected than a powder analyte by using the hybrid waveguide sensing scheme despite the markedly higher THz-wave dispersion of a granular material compared with that of a uniform thin film material.
To summarize the sensing capability of the hybrid THz plasmonic waveguide from the polynomial fits in Fig. 6(b), we use the 20 μm thick PE film substrate with the detection sensitivity of 261 GHz/RIU as an example. The minimum detectable thickness variation of this PE film is estimated to be 1.8 μm based on a 0.004 THz spectral resolution, corresponding to an OPD of 2.7 μm. The thickness resolution approaches λ/289 for the 4th-order resonance wave at 0.380 THz. For the powder sensing ability, the detection sensitivity of a 60 μm thick PE substrate is 163 GHz/RIU; this sensitivity results in a minimum detectable molecular quantity increment of the chemical compound of approximately 13 μmole covered on the 15mm-wide and 50mm-long substrate area, corresponding to the molecular density of about 17.3 nano-mole/mm2. Moreover, the detectable PE-film thickness is possible to be reduced to 454 nm, corresponding to OPD of 681nm, when the spectral resolution of THz-spectroscopy system is further decreased to 1 GHz. The waveguide sensing scheme is certainly compatible with the planar terahertz metamaterials  as sensitive thin film sensors in THz frequency for nano-thin film detection. MHA and planar terahertz metamaterials are difficult to sense non-uniform and random located particles, sprayed on the resonator array, even though the highest sensitivity of MHA approaches 2 THz/RIU . This is because the particles layer can be considered as an effective waveguide-cladding of the hybrid plasmonic waveguide sensor to induce the waveguide index change from various particle quantities.
The suitability of a THz plasmonic waveguide composed of a metal grating and a plastic ribbon waveguide is experimentally demonstrated in near-field sensing. The generated THz-SSPPs in THz-SPWs are not only subwavelength confined to the metal surface but also delivered over a long distance with resonant reflection by the periodic metal structure. THz-SPW resonance follows the Bragg principle, and the transmission spectrum depends on the ambient refractive index of the grating. PE films with varying thicknesses are used to confirm spectral dip-shifts correlated to the analyte indices without being influenced by analyte absorption. The detectable OPD on a 20 μm thick PE film substrate can be reduced to 2.7 μm, which corresponds to a thickness resolution of λ/289 at the 4th-order Bragg resonance. In addition, the granular analytes exhibiting high dispersions with different quantities are successfully identified by the plasmonic waveguide sensing scheme, and the minimum detectable molecular quantity variation per unit area is reduced to approximately 17.3 nano-mole/mm2. This THz plasmonic waveguide sensor could potentially be used to sense particles in monitoring various pollutants or chemical reactions.
This work was supported by the Advanced Optoelectronic Technology Center, National Cheng Kung University, under projects from the Ministry of Education and the National Science Council (NSC 100-2221-E-006 −174 -MY3) of Taiwan. The authors are grateful for the preparation of chemical compounds by the associated professor, Kuei-Fang Hsu, in Department of Chemistry, National Cheng Kung University.
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