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A terahertz artificial material composed of metal rod array is experimentally investigated on its transmission spectral property and successfully incorporated into microfluidics as a miniaturized terahertz waveguide with an extended optical-path-length for label-free fluidic sensing. Theoretical and experimental characterizations of terahertz transmission spectra show that the wave guidance along the metal rod array originates from the resonance of transverse-electric-polarized waves within the metal rod slits. The extended optical path length along three layers of metal-rod-array enables terahertz waves sufficiently overlapping the fluid molecules embedded among the rods, leading to strongly enhanced phase change by approximately one order of magnitude compared with the blank metal-parallel-plate waveguide. Based on the enhanced phase sensitivity, three kinds of colorless liquid analytes, namely, acetone, methanol, and ethanol, with different dipole moments are identified in situ using the metal-rod-array-based microfluidic sensor. The detection limit in molecular amounts of a liquid analyte is experimentally demonstrated to be less than 0.1 mmol, corresponding to 2.7 μmol/mm2. The phase sensitive terahertz metal-rod-array-based sensor potentially has good adaptability in lab-chip technology for various practical applications, such as industrial toxic fluid detection and medical breath inspection.
© 2017 Optical Society of America
1. Introduction
Artificial optical materials, or namely metamaterials, are extensively developed as various functional devices to freely manipulate photons via engineering their geometric parameters of periodic structures [1]. Optical sensing based on such new artificial materials has been realized in biomedical inspection, spectroscopic sensing, and label-free detection [2]. The sensing principle is mainly based on the optical signal change, which is sensitive to the refractive index change or the absorption variation coming from the target analyte near a periodical structure. Compared with the optical lights and microwaves, terahertz electromagnetic (THz EM) waves fairly match the low-frequency molecular resonance of intermolecular forces, thereby possessing unique absorption and dielectric constant features in the THz spectrum [3, 4]. Metamaterials in THz regime are thoroughly investigated in their spectral properties [5, 6], and various THz metasurfaces with different pattern designs are applied to sense thin films, biomolecular membranes [7], and liquid analytes [8, 9] under the planar metamaterial sensing platforms. In the adopted THz sensing platforms, coupling schemes based on prisms or metal blades are necessarily added onto the metamaterial surfaces for spoof surface plasmon (SSP) excitation. The generated THz SSPs are precisely controlled within a near field distance in order to optimally overlap the confined fields with the target analytes for sensitivity enhancement. However, the presented coupling configurations are difficult to planarly integrate with microfluidics for in situ sensing liquid analytes because either the analyte thickness or waveguide mode excitation is strictly related to the bulky prism- or metal-based scatters [8,9]. Fortunately, the waveguide sensing modality can solve the difficulty to directly combine the sensing device with microfluidics on one chip and facilitate the delivered EM fields to efficiently interact with the minute analytes [10, 11]. However, the traditional THz waveguides are too bulky to compatibly combine with the microfluidics on one chip [12–15]. An artificial waveguide based on the metamaterial concept is possible to be tailored for miniaturization and highly sensitive performances for one microfluidic integration [16].
A periodic metal rod (or pillar, wire) structure is one of metamaterials with a negative permittivity below the plasma (cutoff) frequency [17]. Usually, a periodic metal structure can be scaled to match the operated EM wavelength and engineered to freely tailor its spectral feature in a broad spectral range. Therefore, the cutoff frequency of a plasmonic material can be shifted from the UV–visible frequency range down to GHz–THz through properly scaling the structural period in a metal rod array (MRA) [18]. According to the EM wave theory [19], metal-guided waves with real dielectric constants are allowed to propagate when the frequency of an incident wave is higher than the plasma frequency. Hence, the one- or multiple-layered MRA structure was demonstrated as a high pass or bandpass filter, enabling EM wave transmission higher than the artificial plasma frequency via the transverse-electric (TE)-polarized wave excitation (i.e., // rod axis) [20, 21]. Besides, we have previously demonstrated a THz plasmonic waveguide composed of multiple arrays of metal rods to transport the THz-SSPs deeply confined on the MRA surface within a subwavelength range [22]. The THz-SSPs of the MRA structures are excited by the transverse magnetic (TM)-polarized THz waves (i.e., ⊥ rod axis) and have transmission spectra with multiple forbidden bands resembling those of photonic crystals [22]. The wave propagation characteristics of the MRA waveguides for the TE- or TM-wave excitation have been experimentally investigated from visible to THz frequency ranges, but the structure dependent spectral properties of MRAs have not been explored yet. Sensing applications based on MRA structures are few as well. Recently, the possibility of nanofilm sensing using a TM-polarized-wave excited MRA was successfully proven based on the phase sensitive feature of the THz-SSPs [22]. However, the array quantity of the MRA to guide THz-SSPs should be increased up to 30 layers and fully covered by sufficient amounts of sample to produce distinguishable phase variations from different analytes [22, 23]. The over long interaction lengths between the THz-SSPs and analytes would strongly decay THz waves and be detrimental for high THz absorption of liquid sensing.
In this presentation, a rectangular MRA is incorporated into microfluidics composed of two metal parallel plates to form a miniaturized THz waveguide for TE-polarized wave transmission. The MRA waveguides with different layer thicknesses along propagation direction are fabricated for studying the structure dependent spectral properties. From the spectral characterization, the MRA-based THz waveguide performs as a bandpass filter with transmission peaks above the cutoff frequency and its spectral performance is varied with the MRA-waveguide length. The origin of the transmission peaks at pass bands is experimentally investigated and well explained by the TE-wave resonance among the metal rod interspace in theory [24]. The spectral redshift of the transmission peaks is also observed while the waveguide length of a MRA is varied from single layer to three layers, revealing that the extraordinarily extended optical path length (OPL) can be produced by merely few layers of a MRA waveguide. The extended OPL in a short length of a MRA waveguide benefits phase sensitive detection of highly absorbed analytes due to less power decay. From the time-domain electric field oscillation and power spectrum of THz waves transmitted through an analyte-loaded MRA structure, it is found that the MRA-guided TE wave is quite sensitive to the embedded fluidic molecules in the phase instead of the amplitude of THz wave. Three types of colorless liquid analytes, namely, acetone, methanol, and ethanol, are successfully distinguished using an integrated MRA waveguide with the optimal phase sensitivity, where the THz phase response of the liquid analytes is in situ and dynamically detected during evaporation. This work demonstrates the feasibility to incorporate a MRA artificial material into various functional lab chips or biochips as one sensing unit for different molecular sensing purposes.
2. Configuration and fabrication of MRAs
Figure 1(a) shows the configuration of a MRA and its microscopic photograph at the inset. The structural period (Λ) is approximately 0.62 mm along the x- and z-axes. The diameter and interspace of metal rods are approximately 0.16 and 0.46 mm, respectively. The length of each rod is almost consistent (approximately 1 mm), and the structural total width along the x-axis is 12Λ. Such uniform MRA structure is originally prepared from the rectangular array of polymer rods via microstereolithography [25], and then the polymer rods are deposited with a 100 nm-thick aluminum metal film via a sputter coating process. Microstereolithography is a method of 3D printing, and the process is divided into two steps, namely, photolithography and development. During the photolithography process, a spot-array pattern is projected from a mask onto a liquid photopolymer (NOA81, Norland Optical Adhesive 81) using a UV lamp light for the photocuring. A 0.6 mm-thick glass plate is used as a substrate of the rod array and first sunken under the liquid polymer surface (approximately 0.2 mm) to start photolithography. The NOA81 monomers in the photo-defined region are cross-linked after the first UV exposure for several seconds, and the substrate is continuously moved down to the next 0.2 mm distance via a manual elevator for the second UV light exposure. The photo polymerized rods are formed layer by layer by repeating the downward translation and UV irradiation, stacking up to 1 mm high after five times of operation. After the development process of photolithography, the 3D rod structure is then soft baked in an oven under 50 °C for 12 h to further harden the polymerized rods. Then, the uncross-linked monomers on the rod array structure are washed out by acetone solutions. Finally, a 100 nm-thick aluminum layer is coated on the surface of polymer rod array via a sputter coating method to increase its surface conductivity [26]. The sputter coating process deposits a uniform metal film on the 3D rod structure at low temperature (<70 °C) to prevent melting the polymer material. Based on the high-quality fabrication in 3D printing process, four devices with the same rods but different thicknesses of one-, two-, three- and four-layer MRAs are successfully constructed to observe THz wave transmission along various lengths of the waveguides in z-axis [Fig. 1(a)]. The incident THz wave is oriented to TE polarization that is parallel to the rod axis as described in Fig. 1(a) to operate the MRA as a metamaterial waveguide with a plasma (cutoff) frequency. Figure 1(b) illustrates the input and transmitted THz waveforms through the MRAs with different layer thicknesses measured by the waveguide-based THz-TDS [14]. When THz waves pass though the four-layered MRA, the electric field amplitude is severely decayed by the metal rods so that it cannot provide sufficient signal change for liquid fluid sensing.
Fig. 1 (a) Schematic MRA configuration to pass TE-polarized THz waves. (Inset) Microscopic photograph of a MRA. (b) Input and output THz waveforms for the MRAs with different transmission thicknesses.
3. Characterization of THz wave guidance along a MRA
Figures 2(a) and 2(b) show the measured and calculated spectral transmittances within 0.1–1 THz for one, two, and three layers of MRA along the z-axis. The input THz wave spectrum, denoted as a blue curve in Fig. 2(a), is illustrated as reference, where the absorption lines at 0.550 and 0.750 THz come from the ambient water vapor. The one-layered MRA obviously transmits THz waves higher than ~0.350 THz, resembling a metal wire grid structure [21]. The measured cutoff frequency of 0.350 THz [Fig. 2(a)] reasonably agrees with the calculated artificial plasma frequency of the one-layered MRA [Fig. 2(b)] based on the finite-difference time-domain (FDTD) calculation. Beyond the cutoff frequency of the one-layered MRA, there are two distinct transmission bands with spectral peaks at 0.440 and 0.910 THz. The spectral performance of the one-layered MRA resembles a THz bandpass filter [20]. Both of the spectral peaks obviously redshift while the propagation length of THz waves in the MRA increases from one to three layers. The cutoff frequency for different layers of MRAs with the same MRA period approximately retains at around 0.350 THz. The high-frequency peaks of the two- and three-layered MRAs individually shift to 0.650 and 0.460 THz, which are denoted by arrows in Fig. 2(a) and compared with the high frequency peak of one-layer MRA at 0.910 THz. The low-frequency peak also shifts from 0.440 THz to 0.420 and 0.380 THz when the transmitted MRA is increased from one layer to the two and three layers, respectively. Obviously, the spectral shift range of the high frequency peak is larger than that of the low-frequency one. The transmission bandwidth of the one-layered MRA is further compressed by adding two additional MRA layers. For the MRA with a 620μm-rod-period, the THz transmission spectrum of TM-polarized-wave excitation [22] is very different from that of TE-polarized-wave excitation [Fig. 2(a)]. For example, the spectral properties of MRA at 0.100–0.250 THz and 0.400–0.600 THz for the TM-polarized-wave excitation respectively belong to high transmission and forbidden bands [22], revealing the polarization dependent spectral property. The theoretical TE-polarized transmission results of the one-, two-, and three-layered MRAs, simulated by the FDTD method, also show the effects of redshift and spectral compression as sketched in Fig. 2(b), where the high-frequency peaks are also indicated by arrows. Such the MRA thickness dependent spectral feature is reasonably agreed between measurement and FDTD simulation.
The measured transmission peaks of THz waves at pass band of the MRAs can be explained by the TE guiding waves in one dielectric (air) slit, which is located between two adjacent metal rods. Here, we consider the aluminum metal in the THz frequency region as a perfect conductor without surface plasma excited by the EM wave on the metal surface. Thus, the TE wave existing in one dielectric slit (along the x-axis) can be assumed as a sine wave function as expressed in Eq. (1), where the x-axial position of one rod surface is assumed as 0.
where Ey, h, and β are the y-polarized electric field, and the propagation constants along the x- and z-axes, respectively. We substitute Ey into the wave equation and consider the boundary condition, ∂Ey /∂y = 0, to obtain the relation of the propagation constants [27],where K0 and n are the wavenumber in vacuum (i.e., 2π/λ) and the refractive index in the slit, respectively. The propagation of TE-polarized waves in the MRA is different from the waveguide modes of TM waves but the TE waves outside the metal surface can pass through the dielectric slit based on the wave propagation criterion, β 2 >0 [27]. The propagation wave criterion is directly imposed with a threshold value in the wavelength/frequency of the TE waves, namely cutoff wavelength/frequency, which is observed in the measured results [Fig. 2(a)] and the theoretical calculation based on FDTD [Fig. 2(b)]. From the tangential field component Ey at the metal boundary, i.e., Sin(hd) = 0 [24], the transverse wave vector h can thus be derived as mπ/d, where d is the slit width along the x-axis of a MRA and m is the mode number, equaling to 1, 3, 5.... Furthermore, when β approaches zero to achieve the wave resonance condition (i.e., β ~0), we can get the approximation from Eq. (2), (K02n2-h2) ~0, where the correlating wavelength, λc, can thus be expressed asThe measured result in Fig. 2(a) shows that the TE-polarized propagating waves, satisfying β2>0 and the resonance conditions (β ~0), have relatively high transmittance. The measured transmission spectrum of the one-layer MRA waveguide shows the resonant guiding waves with high transmittance at 0.440 and 0.910 THz. Based on Eq. (3), the 1st (m = 1) and 2nd (m = 3) resonant waves of a 0.46 mm-wide slit (i.e., the MRA interspace in a period [Fig. 1(a)]) are individually estimated at 0.330 and 0.980 THz, which are very close to those in measurement [Fig. 2(a)] and FDTD calculation [Fig. 2(b)] results [24]. Therefore, the THz wave transmission and guiding principle of a MRA-based artificial material can be explained by the TE wave transmission of each rod slit, and the spectral positions of transmission peaks are correlated to the transverse wave vector, h, determined by the resonance condition in the slit. In our observation, only the 1st and 2nd resonant modes appear within the measurable spectrum, and their spectral positions are shifted to low frequency range for transmission of multiple-layer MRAs. Actually, the spectral redshifts of resonant modes can be explained by the TE guiding wave propagated in different OPLs of the MRA waveguide. In general, the OPL is not only defined by a physical length of ray trace (L) but also by the dispersion property along the trace (n), i.e., OPL = n·L [19]. According to Eq. (3), the redshift of 1st and 2nd resonance modes [Figs. 2(a) and 2(b)] is possibly resulted from two factors when the TE waves propagate along the multiple layers of MRAs, including that the effective waveguide index is larger than 1.00 or the effective slit width along the x-axis is larger than 0.46 mm. Both effects make the MRA OPL longer than the physical propagation length (z-axis) for THz wave guidance. From different redshift ranges of the resonant modes, the high-frequency guiding waves (i.e., the 2nd resonant mode) obviously have larger OPLs compared with those of low-frequency waves at the 1st spectral peak, which is very similar to the spectral properties of a general dielectric slab waveguide [27].Figures 3(a)–3(c) show the FDTD calculation results of one-layer MRA regarding the amplitude distribution of the electric field, Ey, in the x-z plane at 0.250, 0.320, and 0.440 THz frequencies, respectively. The result shows that THz waves at 0.250 and 0.320 THz cannot pass the MRA structure and the incident Ey fields accumulate at the input face, which is also consistent to the low transmittance in the measured spectral results [Fig. 2(a)]. The 0.320 THz wave with a shorter wavelength invades the slits deeper in the z direction compared with the 0.250 THz wave. When input wave frequency increases to 0.440 THz, the Ey field accumulates at the output end [Fig. 3(c)], opposite to those of low-frequency ones [Figs. 3(a) and 3(b)], which indicates that the guided THz wave possesses a real propagation constant (β) and the electric field can thus pass through the slits along the z-axis. Figures 3(d) and 3(e) show the calculation results of the electric fields and Poynting vectors at different THz frequencies for the three-layered MRA. In Fig. 3(d), the x-axial field distribution at 0.250 THz is apparently different from that of Fig. 3(a) because of the multiple-layered structure. The time-average Poynting vector, denoted as the arrows, shows the input Ey field almost reflects from the first layer of the MRA. For the guiding wave at 0.440 THz [Fig. 3(e)], the spatial mode pattern in the x-z plane is also changed compared with that in Fig. 3(c) when the MRA structure is changed from one layer to three layers. The simulation results of Figs. 3(d) and 3(e) also illustrate that the accumulating field at the MRA input end is still able to invade the three-layer MRA while the input THz wave frequency is increased from 0.250 to 0.440 THz. Figure 3(f) shows one example about the higher interspace refractive index of 1.1. The field concentration becomes uniformly denser near the input and output ends of the three-layer MRA in contrast to that of 1.00 interspace refractive index in Fig. 3(e). The modal pattern response induced by the slight index variation implies that the interspace of MRA plays a crucial role to make different waveguide refractive indices when different analytes are loaded in the MRA interspace for THz-wave sensing applications.
Fig. 3 Electric field distributions in the x-z plane at (a) 0.250 THz, (b) 0.320 THz, and (c) 0.440 THz for the one-layered MRA structure. Time-average Poynting vectors in the x-z plane of the three-layered MRA under different interspace refractive indices and input THz wave frequencies. (d) 1.00 refractive index unit (RIU), 0.250 THz. (e) 1.00 RIU, 0.440 THz. (f) 1.1 RIU, 0.440 THz. The black solid circles in each figure represent the metal rods.
4. Sensing principle of a MRA
Based on the analysis of Figs. 1–3, a MRA medium, composed by 0.16 mm-diameter metal rods and the 0.46 mm-wide interspace, can be considered as one artificial dielectric waveguide to guide the THz waves within 0.300–0.500 THz. On the basis of the aforementioned redshift phenomenon of spectral peaks, the OPL of MRA-guided waves is found with distinct extension in different layers of the MRA structure. A large OPL possessed by a waveguide sensor is certainly sensitive to distinguish similar analytes because the interaction length, cross section, or probability between the photon and target analyte can be enhanced to reveal more distinctions in optical signals that are induced by different analytes [10, 23]. For the three layers of MRA with the most extended OPL in this experiment [Fig. 2], its waveguide dimension is only 1.4 mm long (i.e., three layers of MRA), 1.0 mm thick and 7.4 mm wide, which is not only compact to conveniently integrate with microfluidics on one chip but also with sufficient transmission power for fluid sensing [Fig. 1]. Comparing to the TM-polarized-waves excited MRA waveguide with a 18 mm-length (i.e., 30 layers of MRA), a 1.0 mm-thickness and a 7.4 mm-width [22], the MRA dimension operated for TE guiding waves is more miniaturized to load fewer liquid for minute analyte sensing. A semiconductor rod array proposed for THz liquid sensing also requires a large number of rods for THz beam spot coverage to achieve the sensing purpose [28], which is also limited to integrate microfluidic channels for minute material detection.
Using the MRA as a waveguide sensor, the THz response in the electric field oscillations of waveguide mode to the loaded analytes involves the amplitude decay and phase retardation. Before liquid sensing, a calibration experiment is required to clarify the sensitive feature of the guided THz electric field, responding to the standard analyte. A vaporized analyte can uniformly distribute inside the rod interspace and has relatively low loss to sustain the guiding wave power, which is suitable to be a standard sample medium in the calibration. In this presentation, the acetone vapor with a high dipole moment of 2.91 Debye (D) is used as the standard analyte to modulate the propagation properties of THz waves in amplitudes and refractive indices [29, 30]. To conveniently manipulate the vapor interacting with the MRA, the three-layer MRA is incorporated into a microfluidic sensing unit as displayed in Figs. 4(a)–4(c). The MRA-based microfluidic sensing unit is mechanically assembled by two aluminum metal plates, equipped with a parallel plate waveguide (PPWG), a MRA structure, microfluidic channels, and a reservoir; the first two items are the waveguiding and sensing parts of the unit, respectively, whereas the others belong to the sample treatment part.
Fig. 4 (a) Photograph of a mechanical assembly, including a MRA structure (I) and aluminum metal substrates, 42(x) × 10(y) × 52(z) mm3 (II). (b) Photograph of an aluminum substrate, 42(x) × 10(y) × 52(z) mm3, equipped with (III) a rectangular PPWG channel waveguide, 10(x) × 1(y) × 52(z) mm3, (IV) a liquid reservoir, (V) liquid inlet, (VI) five air-microfluidic channels, 8.5(x) × 0.5(y) × 0.8(z) mm3, and (VII) PE tubing. (c) Mechanical drawing and photograph (inset) of a MRA-based microfluidic sensing unit for vapor sensing. The blue arrow represents the PE tubing for liquid analyte injection. (d) Transmittance and (e) phase change of THz-field oscillation induced by acetone vapors inside the PPWG hollow core with and without the three-layer MRA structure.
The function of PPWG resembles a THz optofluidic channel to transport both transverse electromagnetic (TEM) THz waves [31] and vapor analytes in the same channel. The overall THz microfluidic sensing composite [Fig. 4(c)] is formed by mechanically assembling the aluminum plate with a MRA structure [Fig. 4(a)] as a cap to cover the other aluminum plate [Fig. 4(b)], containing the channel waveguide and the sample treatment part. The reservoir in the sample treatment part acts as a liquid container to load liquid analytes via an injection port connected by a polyethylene (PE) tubing [Fig. 4(b)]. The vaporized analyte naturally evaporates from the reservoir and uniformly disperses inside the PPWG via the transport of five microfluidic channels. THz radiation is initially edged-coupled from free space into the hollow core of PPWG and then passed through the MRA, allowing TE resonant guiding waves to interact with fluid analytes embedded among metal rods. Finally, the MRA-guided waves transmit through the 40 mm-long PPWG channel to the output end for the THz waveform measurement. The microfluidic sensing unit in vapor sensing is sealed to measure the output THz electric field oscillations with and without the MRA structure.
Figure 4(d) shows the spectral transmittance of the acetone vapor for the PPWG with and without the three-layer MRA, which covers the THz-TE guiding waves of the 1st and 2nd resonance modes, 0.300–0.500 THz. We found that the THz transmittance of acetone vapor in the microfluidic sensing platform is almost unchanged whether the PPWG is integrated with the MRA structure or not. It represents the extended OPL guidance via the three-layered MRA in the PPWG channel does not significantly enhance THz absorption from the target analyte. The corresponding phase variation (also called phase retardation, ΔΦ) induced by the acetone vapor in the PPWG optofluidic channel with and without the three-layer MRA is analyzed in Fig. 4(e). The phase variation obtained from the target analyte molecules is obviously enhanced via the OPL-extended wave guidance along the three-layer MRA. The enhancement ratio is also estimated and sketched as a green curve in Fig. 4(e), where the strong enhancement range covers the spectrum of 0.400–0.500 THz, corresponding to the spectral position of the 2nd resonance mode in the three-layer MRA [Fig. 2(a)]. The maximum enhancement factor is approximately up to 11 at 0.475 THz, and the measurement deviation in ΔΦ is only about 1% of 2π-unit, which is expressed in the red curve of Fig. 4(e). As calibrated in this experiment, the acetone vapor-inducing THz-wave phase variation is obviously enhanced in the 2nd resonance mode with the maximum value over one order at 0.475 THz [Fig. 4(e)] when the three-layer MRA is inserted in a PPWG-based THz optofluidics.
5. Liquid sensing method and results
Based on the phase-sensitive response of the MRA-guided waves to the fluid analytes, the liquid sensing experiment is then conducted using the same microfluidic sensing composite as shown in Figs. 4(a)–4(c). The inset of Fig. 5(a) shows the photograph of the sensing unit for liquid detection, connected with an outer PE tube for liquid input. The liquid sensor assembly is flipped upside down, contrary to the vapor-sensing configuration [Fig. 4(c)]. In the liquid sensing experiment, the aluminum substrate with channel waveguide and microfluidic channels acts as a cap to cover the MRA substrate, and the injection inlet of liquid analytes, formed by mechanically piercing through the cap and indicated as V in Fig. 4(b), is located on top of the MRA structure to transport the liquid analyte into the MRA interspace. Figure 5(a) shows the mechanical drawing of liquid detection using the MRA-based microfluidic-sensing unit, illustrating the injected liquid analyte to interact the guided THz TEM waves in the PPWG. In the PPWG, a MRA is a 3D structure with a large surface area, resembling a porous medium, to facilitate the liquid analytes to efficiently interact with the MRA guided THz waves in the microfluidic sensor. Thus, the liquid molecules can directly modulate the propagation properties of the MRA guided THz waves to display in the time-domain electric field oscillations.
Fig. 5 (a) Mechanical drawing and photograph (inset) of a MRA-based microfluidic sensing unit for liquid sensing. (b) Electric field distribution of 0.475THz wave in the MRA across the section of the x-y plane.
In this work, three types of colorless liquid analytes, namely, acetone (Anaqua Chemicals Supplys, Absolute Acetone, 99.5%), methanol (Merck Inc., Methanol for spectroscopy, 99.9%), and ethanol (Anaqua Chemicals Supplys, Ethanol, 99.9%), are used to demonstrate the liquid sensing performance. The input and output ends of PPWG are opened to make the injected liquid drop naturally evaporate at an ambient pressure of 1 atm and a temperature of 25 °C. The amounts of liquid analyte inside the MRA therefore change during the natural evaporation. Although the input volume for all liquid analytes is only 0.05 C. C., at least one hour is required to completely exhaust the residual liquids via natural evaporation under normal temperature and pressure. The MRA-based microfluidic sensor can therefore be repetitively used to detect the other liquid analytes. In this experiment, we recorded the remaining analyte weights and THz-electric-field oscillations in situ at 300, 600, and 1100 s after liquid injection. To precisely measure the weight of the remaining analyte, an electronic weighing-scale is equipped with the MRA-based microfluidic sensing unit for the simultaneous monitor of analyte weight during the THz waveform scan in a liquid sensing experiment. The recorded residual liquid amounts of acetone, methanol, and ethanol in the time series, 300/600/1100 s, are 0.532/0.456/0.389 mmol, 1.226/1.070/0.858 mmol, and 0.866/0.771/0.66 mmol, respectively. The gravity draws the liquid to fill the rod interspace from the bottom substrate up to near the top of the rod while the liquid analyte is injected into the MRA. Inversely, the evaporation process starts to exhaust the liquid drop at the top surface, and the remaining liquid on the bottom substrate lastly evaporates. In contrast to vapor sensing, the loaded liquid analytes among the metal rods is not uniform along the y-axis, and eventually affects the phase response of THz waveforms that interact with the liquid analyte at different evaporation times. In Fig. 5(b), we take 0.475 THz wave as an example to simulate its electric field distribution in the x-y plane at the output end of the three-layer MRA. The electric field distribution along the rod axis is indeed not uniform, and the field near the MRA substrate is weaker than that at the upper section of the rods. Such weak field is attributed from the THz wave boundaries at the rod surface and substrate plane. For other MRA-guided waves, the electric field distributions in the x-y plane and at the output end are similar in the FDTD simulation.
Figures 6(a)–6(c) show the sensing results of THz phase response induced by different liquid amounts measured at different evaporation times. We consider the phase variations, ΔΦ, between the conditions of blank MRA and the MRA with the remaining liquids at evaporation times of 300, 600, and 1100 s to cancel the waveform phase contributed from the blank substrate for only sensing the MRA-loaded analyte. ΔΦ is then divided by the remaining molecular amounts to be normalized as the phase difference per 1 mmol to manifest the intrinsic molecular properties. The phase retardation measured at 300 s, obtained from the largest analyte amount, shows the highest distinction among the three liquids using the 1st and 2nd resonance modes within 0.35–0.5 THz. The sequence order of the phase variation for 1 mmol liquid analyte is acetone > methanol > ethanol, which is consistent with the measured result using the traditional THz spectroscopy [30]. This result can be related to the distinct molecular dipole moments of the three liquid analytes [30, 32], where the acetone molecule has the largest dipole moment (2.91 D) and thus to strongly retard the THz electric field oscillation along the three-layer MRA. In other words, the phase variation produced by methanol (1.70 D) and ethanol (1.69 D) within the same THz frequency range are apparently lower than that of acetone because of their smaller dipole moments. As indicated in Figs. 6(a) and 6(b), the phase variation per 1 mmol liquid obviously decays when the loaded liquid amounts gradually reduce during an evaporation time of 300–600 s. The phase changes measured at 600 s [Fig. 6(b)] and 1100 s [Fig. 6(c)] show that acetone and alcoholic liquids are still distinguishable but the smaller analyte amounts of ethanol and methanol are not recognized. Therefore, the experimental results in Figs. 6(a)–6(c) represent that the available distinguishability of different liquids only occurs in sufficiently thick overlayers to cover the waveguide fields in the rod axis (y-axis). The required analyte amount is at least 1 mmol.
Fig. 6 Schematic description about liquid sensing (a)–(c) without and (d)–(f) with considering the interface layer between the metal substrate and the liquid analytes.
The non-uniform THz field distribution along the y-axis of MRA-guided waves can affect the detection sensitivity because of different overlapped integrals between the field and analyte. To investigate this effect, we in situ monitor the dynamic evaporation of the liquid analyte using the MRA microfluidic sensing scheme [Fig. 5(a)] to analyze the overlayer’s phase retardation of certain thickness. Figures 6(d)–6(f) show the experimental results of the THz phase changes produced by distinct molecular overlayers during the dynamic evaporation of each liquid analyte. The phase change of a certain overlayer is obtained by comparing the THz phases of the analyte at distinct evaporation times, t1 and t2, and the residual liquid layer at t2 is labeled as the interface layer, adjacent to the metal material and sketched in Figs. 6(d)–6(f). The phase variation spectrum obtained by comparing the phase responses at evaporation times of 300 s and 600 s (Fig. 6(d)) reveals that the three liquid analytes can be easily recognized based on the 2nd resonant guiding mode within 0.400–0.500 THz. The large distinction in phase change among the three analytes is caused by the strong field–analyte interaction at the upper overlayer, leading to high detection sensitivity. The spectral range of the high sensitivity region is consistent to that of the enhanced phase variations with significant enhanced ratio caused by the 2nd resonance mode because of the extended OPL [Fig. 4(e)]. The analyzed molecular amounts of acetone, methanol, and ethanol in the Fig. 6(d) are 0.076, 0.156, and 0.095 mmol, respectively, corresponding to the liquid volumes of 5.57, 6.31, and 5.54 μL. The detectable amounts (i.e., detection limit) are further decreased to 0.1 mmol, equivalent to 2.7 μmol/mm2 for the MRA surface area of approximately 36 mm2. The sensitivity is improved by approximately one order of magnitude because of the optimal overlapping between the analyte and the electric THz field in the y-axis compared with the sensing result in Fig. 6(a). Further analysis of the phase variation between 300 s and 1100 s for the three liquids [Fig. 6(e)] shows that the variations are attributed to more amounts of the liquid analyte and thinner interface layers, compared with those of 300–600 s [Fig. 6(d)]. The phase-difference response at 300–1100 s, obtained from acetone and methanol, are both smaller than those in the Fig. 6(d) [at 300–600 s] because the increased thickness of target liquid analytes is not fully covered by the effective MRA guiding field in the y-axis. In other words, the additional liquid thicknesses not only cannot offer effective THz wave phase variation due to the weak THz-field nearby the substrate but also decrease the phase contribution per 1 mmol analyte. The overlayer thickness of Fig. 6(f) is similar to that of Fig. 6(d) but closer to the MRA substrate. However, the THz phase variations at 600–1100 s [Fig. 6(f)] perform the indistinguishable phase variations among the three liquid analytes, different from those of 300–600 s [Fig. 6(d)]. This evaluation also indicates the THz cross field distribution near the substrate is too weak to perform the detectable signals of THz phase variation per 1 mmol liquid.
6. Conclusion
We have experimentally demonstrated a MRA-based THz artificial material to transport the TE waves resulted from the resonance among rod slits. The miniaturized MRA-based waveguide, fabricated by the high accuracy of the 3D printing technique, is also successfully incorporated with the microfluidics to transport THz waves and fluidic analytes in the same channel for THz optofluidic sensing applications. Analysis of the THz transmission spectra for different layers of MRAs shows the OPL of the MRA-guiding wave is longer than the physical waveguide length. The three-layered MRA is thus suggested as the optimal waveguide sensing length because of the sufficiently long OPL, adequate transmission power and high overlapping field to enhance the wave-analyte interaction. The acetone vapor sensing result shows that the phase variation, rather than the amplitude decay, of the THz field can be strongly enhanced using the 2nd resonance waveguide mode, compared with the sensing result of blank PPWG. Finally, the composite MRA-based sensing unity is utilized for liquid sensing based on the phase-sensitive property of the TE guiding waves. With the optimal overlapping between the modal field and liquid overlayer along the rod axis, three types of colorless liquid analytes are successfully recognized at microliter volumes based on their distinct molecular dipole moments. This work demonstrates the realization of a miniaturized THz waveguide using a MRA-based artificial material, and the feasibility of label-free microfluidic detection based on the waveguide sensing platform to distinguish minute amounts of liquid analytes with similar dipole moments. Using the phase-sensitive method for minute liquid detection is not restricted to the broadband THz radiation and the uniform sample cell thickness, which are usually adopted for sensing the absorption lines of materials. The MRA waveguide can be potentially integrated into the micro-total-analysis system as one unit or conjugated with lab chip technology for various sensing applications.
Funding
Ministry of Science and Technology of Taiwan (MOST 104-2221-E-006-163-MY3).
Acknowledgment
This work was supported by the Advanced Optoelectronic Technology Center of National Cheng Kung University and the Ministry of Science and Technology in Taiwan.
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