Just like ocean waves, electromagnetic waves travel in a defined direction. While the speed of ocean waves can vary, however, the speed of electromagnetic waves — commonly referred to as the speed of light — is essentially a constant, approximately million meters per second.
This is true whether we are talking about gamma radiation or visible light. Obviously, there is a big difference between these two types of waves — we are surrounded by the latter for more than half of our time on earth, whereas we hopefully never become exposed to the former to any significant degree. The different properties of the various types of electromagnetic radiation are due to differences in their wavelengths, and the corresponding differences in their energies: shorter wavelengths correspond to higher energy.
High-energy radiation such as gamma- and x-rays is composed of very short waves — as short as 10 meter from crest to crest. Longer waves are far less energetic, and thus are less dangerous to living things.
Visible light waves are in the range of — nm nanometers, or 10 -9 m , while radio waves can be several hundred meters in length. The notion that electromagnetic radiation contains a quantifiable amount of energy can perhaps be better understood if we talk about light as a stream of particles , called photons , rather than as a wave. If we describe light as a stream of photons, the energy of a particular wavelength can be expressed as:.
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Because electromagnetic radiation travels at a constant speed, each wavelength corresponds to a given frequency, which is the number of times per second that a crest passes a given point. For mid-infrared spectroscopy, this property is significant because various biological moieties are characterized by multiple vibrational fingerprints.
The mid-infrared response characteristics of the surface-enhanced perfect absorption enable the implementation of precision identification of individual molecular species and can even enable the detection and characterization of complex biological molecular regions, thereby improving the specificity of biodetection In particular, the plasmonic resonances of PAMs can be tuned to certain spectral regions of interest with tolerant spatial reproducibility.
The advantages of multiple-band absorption and intuitive tunability make PAMs particularly attractive for the development of a new generation of label-free mid-infrared molecular absorption spectroscopies 32 and make it possible to approach the detection limit for single-molecule binding events Particularly in mid-infrared spectroscopy, dual- or multi-band perfect absorbers have been designed to explore the structural diversity of two molecular stretches of polymer films 35 , 36 ; these absorbers have potential for various applications, such as the simultaneous monitoring of multiple fingerprint characteristics during structural transformation and the dissimilarity between different molecular regions 21 , 23 , Although these achievements only cover part of the mid-infrared range 19 , they have the potential to be readily achieved for arbitrary frequency segments for implementing molecular absorption spectroscopy in the mid-infrared region with multi-band responses and surface enhancement, with the goals of sensitivity, specificity and multiplex detection 17 , 38 , To achieve surface-enhanced multi-band infrared spectroscopy, a perfect-absorber metamaterials array was developed; in this array, cross-shaped perfect absorbers were arranged in diagonal patterns.
We first tailored the geometry of individual PAMs array to form resonant structures that match the molecular vibrational modes of interest. The array was specifically designed to attain multiple-wavelength spectral resonances in the mid-infrared region.
The amplitudes and resonant frequency shifts at two frequency bands were tuned to match the molecular vibrational modes of the analyte and to maximize the enhancement and accuracy for the analyte. The surface enhancement and multiplex spectral response in the mid-infrared region make this platform a powerful and versatile technology for ultra-sensitive molecular vibrational spectroscopy A concept schematic of the embedded PAMs on the surface of the substrate chip is shown in figure 1 a.
The real device is one chip measured using an Fourier-transform infrared spectrometer FTIR equipped with an infrared microscope. The device consists of a few square detection regions with elaborately designed PAM arrays. One of cross-shaped PAMs array is shown in figure 1 b , in which the inset gives the arrays of cross-shaped nanoresonators arranged in diagonal pattern. A molecular film of Parylene C, tens of nanometers in thickness, was chosen as a sample analyte to coat the PAM. Then the FTIR is used to measure the absorption response.
The absorption responses before and after coating are used to identify the existing of molecule. A few PAMs arrays were designed and fabricated in the substrate chip. A Parylene C molecular film was chosen as an example. Beams of incident and reflect light are used to measure the absorption response by FTIR. PAMs array is shown in background.
A magnified image of the device is shown in the insert. The PAM arrays were deliberately designed to induce distinct resonances through the surface-enhanced perfect absorption. The resonance wavelength was strongly dependent on the occurrence and the concentration of the analyte in the adjacent medium around the metallic nanostructure, as shown in figure 1 c.
A single unit of an infrared PAM, which is a typical metal—insulator—metal absorber structure, as shown in figure 2 a , consists of a metallic cross-shaped nanoresonator on top of a barrier film above a ground plane deposited on a silicon substrate. The array of metallic nanoresonators and the ground plane film are separated by a thin Al 2 O 3 film.
The array of cross-shaped nanoresonators plays a role similar to that of an electric ring resonator ERR 40 , which strongly manipulates the incident electromagnetic wave. Compared to the arrays of cross-shaped PAMs investigated in the previous literature 28 , 41 , 42 , the diagonal patterns provide two bands and sharper spectral responses in the mid-infrared region. The electric response can be tuned by altering the sizes of the cross-shaped resonators, and the magnetic response can be tuned by the thickness of the dielectric layer between the resonator arrays and the ground plane 43 , 44 , At specific frequencies, impedance-matching conditions are met for the nanoresonator array and the surrounding medium, and these conditions result in a maximized narrow-band absorbance.
The cross-sectional view displays the layers, including the barrier layer of gold, the dielectric spacer layer of Al 2 O 3 and the topmost gold antenna. Two thin layers of titanium material are included to improve adhesion between the layers. The parameters of the designed PAMs are characterized by the arm length L , the arm width W and the length of the periodic region P. The thickness of the topmost gold layer of the nanoantenna is labeled H.
To identify an infrared molecular fingerprint, one must be able to easily position the resonance response throughout the mid-infrared. This capability is demonstrated in figure 2 b , which shows the resonance tunability of the PAM structure for different values of the arm length L.
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As the arm length L increases, the associated resonant mode becomes red-shifted. The absorption spectra of the PAM structure measured are in close agreement with numerical simulations. The resonance response at short wavelengths M2 is attributed to the propagation surface plasmons SPPs excited at the interface through a coupling mechanism The mode M1 is red-shifted as the distance between the two arms increases and exhibits characteristics typical of surface plasmon resonance. The linear sensitivity relationship has been observed previously and can be explained by circuit theory The resonant absorption peaks are primarily sensitive to the arm lengths of the PAM structure, although other geometric factors contribute slightly to the resonance.
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The displayed resonance tunability is highly advantageous for surface-enhanced measurements in the infrared region 48 , The lithographically fabricated structures are then coated with a monolayer of the polymer, and the vibrational dipole moments of the molecules act as a load for the PAM resonators. Nanoresonator arrays, driven by an infrared resonance source, efficiently focalize the incident wave to the load through the collectively enhanced plasmonic excitations.
Under resonant absorption conditions, the nanostructure arrays efficiently tailor the incident electromagnetic wave through the enhanced plasmonic excitations For an individual cross-shaped perfect absorber, the EM wave response is controlled by the incident field that excites the localized surface plasmons LSPs serving as the electric dipoles. However, for an array, the resonance responses are attributed to not only the single excitations induced by the incident electromagnetic field but also to the induced scattered fields from the dipoles of the other nanoresonators.
The local electromagnetic fields around the nanoparticle are created by the incident electromagnetic fields and the other induced dipole fields. The values represent the dipolar interaction between the nanoparticles without the phase term If the induced dipolar field interactions within the array patterns are almost in phase, the local fields satisfy the resonance oscillation and become extremely large.
Therefore, in an array of perfect absorbers in phase, the extremely localized field redistribution results in strongly enhanced near-field excitation. For normal incident light at the nanoresonator array, previous simulations showed that the near-field intensities were enhanced by a factor of 10 2 —10 3 Although these enhancement factors are quite impressive, higher near-field intensities are required for monolayer molecular measurements.
In the array structure, hybrid excitations in well-engineered perfect-absorber ensembles can be exploited to achieve dramatically enhanced near-field intensities with much narrower far-field spectral responses. As shown in figure 3 b , the near-field enhancement of the Poynting field P 2 in the cross-sectional plane of the PAM was computed by three-dimensional FDTD simulations at excitation wavelengths of 6.
The simulations indicated that the electromagnetic energy was efficiently confined in the Al 2 O 3 spacer layer and that a locally enhanced electromagnetic field was established at the gaps and tips between adjacent nanoantenna resonators.
The largest Poynting intensity enhancement of 10 4 —10 5 was obtained at the tips of the cross-shaped nanoantennas. The large spectrally selective enhancement holds promise for sensing applications, as demonstrated below. The sectional monitor P3 grey plane transects one cross-shaped resonator grey plane.
The color bar shows the enhancement factor. As a proof of concept, Parylene C film was chosen as a model system to prove the mechanisms of the infrared surface enhancement spectroscopy. Parylene has exceptional mechanical and optical properties for nanostructures 43 , Additionally, it has several well-known characteristic absorption bands in the infrared range.
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First, the absorption spectral properties of a Parylene C film evaporated on a bare substrate were measured as displayed in figure 4 a. There are three distinct absorption peaks in the infrared range at 3. The optical constants in the mid-infrared were also measured as shown in figure 4 b , which agrees well with the absorption spectral positions.
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The optical constants below give reasonable results. The strong peaks at approximately The side lobes of the peaks at 6. The peak at approximately 9. The small peak at 3.
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The data agree well with the vibrational reflection dips reported in the mid-infrared 43 , although the spectra of the vibrational bonds are slightly displaced from their typical positions because of the illumination duration. The repeating unit of the Parylene C polymer is shown in the inset. Three measurements were repeated for Parylene C films with slightly different thicknesses. The obvious strong absorption bands correspond to the vibrational stretches of C-H at approximately 3.
The permittivity of Parylene C contains a nondispersive component in the refractive index. Based on the optical constant results shown in figure 4 , the infrared absorption spectra of the PAMs before and after coating with Parylene C film were studied. The presence of the thin polymer film changes the dielectric environment of the PAMs and results in frequency shifts. The complex value of the frequency shifts affects both the spectral position index-induced shift and the surface-enhanced absorption The frequency shifts scale with the thickness of the film; this result was verified by numerical simulations and experiments.
The spectral difference before and after the coating process was calculated, as shown in figure 5 b. The inset chart indicates the linear relation between the frequency shift of the peak and the thickness of the polymer for films less than tens of nanometers thick; this relation allows the polymer thickness to be estimated Analogous to the figure of merit FOM for the refractive index sensitivity measurements 52 , the function for assessing the sensing potential is defined by where represents the thickness and m is the linear regression slope in figure 5 b for the thickness dependence.
This definition allows nanostructures to be judged against one another as sensing platforms, independent of the shapes or sizes of the nanostructures.