Density functional theory (DFT) at the B3LYP/6-31++G(d,p) level was employed to obtain the optimi... more Density functional theory (DFT) at the B3LYP/6-31++G(d,p) level was employed to obtain the optimized geometries and vibrational spectra of several pyridine(Py)-water(W) complexes with stoichiometric ratios ranging from 2:1 (Py 2 W) to 1:3 (PyW 3 ). The harmonic vibrational wavenumbers of pyridine ring modes and the fundamental modes of water were calculated in order to examine the influence of hydrogen bonding on the normal modes of both pyridine and water upon complexation. The Raman spectra in the wavenumber region 960-1060 cm -1 covering the ring modes ν 1 and ν 12 of pyridine (in Wilson's notation) as a function of pyridine mole fraction were recorded. The integrated Raman intensities in the isotropic components of the spectra were used to determine the relative concentration of "free" pyridine molecules in close neighborhood with other Py-W complexes. The combination of both experimental wavenumbers yielding the overall shift induced by the entirety of hydrogen-bonded complexes in the mixture and the DFT-derived vibrational wavenumbers of the isolated species provides the possibility to probe concentration profiles as a function of pyridine mole fraction. The examination of the concentration dependence of line widths reveals that the counter competing influences of different dynamic processes are simultaneously present in this binary mixture.
We present a systematic study of a close-ring pair based freestanding metamaterial fabricated by ... more We present a systematic study of a close-ring pair based freestanding metamaterial fabricated by double-layer, self-aligned photolithography. Terahertz time-domain spectroscopy transmission measurements and numerical simulations have revealed negative index of refraction in the frequency range of 0.66-0.90 THz under normal wave incidence. The observed resonance behaviors can be well explained by a theoretical circuit model. The electromagnetic properties and the figure of merit of such close-ring metamaterials are also explored in terms of geometrical parameters of the unit cell with a goal of providing optimized design for three-dimensional metamaterials and potential device applications.
We investigate the effect of metal permittivity on resonant transmission of metamaterials by tera... more We investigate the effect of metal permittivity on resonant transmission of metamaterials by terahertz timedomain spectroscopy. Our experimental results on double split-ring resonators made from different metals confirm the recent numerical simulations [Phys. Rev. E 65, 036622 ] that metamaterials exhibit permittivity-dependent resonant properties. In the terahertz regime, the measured inductive-capacitive resonance is found to strengthen with a higher ratio of the real to the imaginary parts of metal permittivity, and this remains consistent at various metal thicknesses. Furthermore, we found that metamaterials made even from a generally poor metal become highly resonant owing to a drastic increase in the value of the permittivity at terahertz frequencies.
We study ways to enhance the sensitivity and dynamic tuning range of the fundamental inductor-cap... more We study ways to enhance the sensitivity and dynamic tuning range of the fundamental inductor-capacitor (LC) resonance in split ring resonators (SRRs) by controlling the aspect ratio of the SRRs and their substrate thickness. We conclude that both factors can significantly affect the LC resonance. We show that metafilms consisting of low height SRRs on a thin substrate are most sensitive to changes in their dielectric environment and thus show excellent potential for sensing applications.
Using terahertz time domain spectroscopy we investigate the normal incidence transmission through... more Using terahertz time domain spectroscopy we investigate the normal incidence transmission through periodically and randomly arranged planar split ring resonators (SRRs). Introduction of positional disorder in metamaterials has no effect on the quality factor of the fundamental Inductive-Capacitive (LC) resonance. The dipole resonances undergo broadening and shift in their resonance frequencies. The experiment reveals that the randomly distributed SRR structures interact incoherently at LC resonance but couple coherently at the higher frequency dipole resonance.
Terahertz time domain spectroscopy and rigorous simulations are used to probe the coupling betwee... more Terahertz time domain spectroscopy and rigorous simulations are used to probe the coupling between a dark and a bright plasmonic eigenmode in a metamaterial with broken symmetry. The metamaterial consists of two closely spaced split ring resonators that have their gaps in non-identical positions within the ring. For normal incidence and a fixed polarization both lowest order eigenmodes of the split ring resonators can be excited; although one of them has to be regarded as dark since coupling is prohibited because of symmetry constraints. Emphasis in this work is put on a systematic evaluation of the coupling effects depending on a spectral tuning of both resonances.
We report on the experimental and theoretical study of the resonant eigenmodes of spiral-type ter... more We report on the experimental and theoretical study of the resonant eigenmodes of spiral-type terahertz antennas. The analysis is carried out for a varying number of spiral windings. For larger numbers the structure possesses a self-complementary property which allows the application of the Mushiake principle predicting that the impedance of such structures is half the impedance of free space. This permits to observe an equal and frequency independent reflection and transmission coefficient. This property makes the spiral-type terahertz antenna not only a fascinating example of a medium supporting strong resonances in the long wavelength limit but also a medium which can be easily and reasonably homogenized at higher frequencies. This is in stark contrast to most of the existing metamaterials.
We report an experimental observation of three distinct resonances in split ring resonators (SRRs... more We report an experimental observation of three distinct resonances in split ring resonators (SRRs) for both vertical and horizontal electric field polarizations at normal incidence by use of terahertz time domain spectroscopy. Breaking the symmetry in SRRs by gradually displacing the capacitive gap from the center towards the corner of the ring allows for an 85% modulation of the fundamental inductive-capacitive resonance. Increasing asymmetry leads to the evolution of an otherwise inaccessible high quality factor electric quadrupole resonance that can be exploited for bio-sensing applications in the terahertz region.
Resonant properties of optically thin metamaterials are studied by terahertz time-domain spectros... more Resonant properties of optically thin metamaterials are studied by terahertz time-domain spectroscopy. Both the lower energy inductorcapacitor (LC) and the higher energy dipole resonances of the planar double split-ring resonators (SRRs) exhibit characteristic evolution with various sub-skin-depth thicknesses of the constituent Pb film. The signature of the LC resonance begins to emerge at a critical thickness near 0.15 skin depth. The resonances reveal a characteristic enhancement; they are strengthened remarkably with increasing SRR thicknesses at sub-skin-depth level and then gradually saturate beyond the skin depth.
The limiting effects of varying the thickness of a dielectric overlayer on planar double split-ri... more The limiting effects of varying the thickness of a dielectric overlayer on planar double split-ring resonator (SRR) arrays are studied by terahertz time-domain spectroscopy. Uniform dielectric overlayers from 100 nm to 16 μm thick are deposited onto fixed SRR arrays in order to shift the resonance frequency of the electric response. We discuss the bounds of resonance shifting and emphasize the resulting limitations for SRR-based sensing. These results are presented in the context of typical biosensing situations and are compared to previous work and other existing sensing platforms.
We experimentally demonstrate at terahertz frequencies that a planar metamaterial exhibits a spec... more We experimentally demonstrate at terahertz frequencies that a planar metamaterial exhibits a spectral response resembling electromagnetically induced transparency. The metamaterial unit cell consists of a split ring surrounded by another closed ring where their dimensions are such that their excitable lowest order modes have identical resonance frequencies but very different lifetimes. Terahertz time-domain spectroscopy verifies that the interference of these two resonances results in a narrow transparency window located within a broad opaque region. In contrast to previous studies this enhanced transmission is achieved by independently exciting two resonances in which their coupling to the radiation field, and thus their linewidth, differs strongly. Rigorous numerical simulations prove that the transparency window is associated with a large group index and low losses, making the design potentially useful for slow light applications. This experiment opens an avenue to explore quantum-mechanical phenomena using localized resonances in metallic structures.
Optical properties of planar thin film metamaterials were measured at room and liquid nitrogen te... more Optical properties of planar thin film metamaterials were measured at room and liquid nitrogen temperatures using terahertz time-domain spectroscopy. The operation of metamaterials at cryogenic temperatures is anticipated to be a promising path toward low-loss metamaterials since nonradiative losses are strongly suppressed due to higher charge mobility. A 14% increase in the quality factor of the resonances was experimentally observed. It was limited by the high electron scattering rate due to defects in thin films. Supplementary simulations assuming metamaterials made of thick films reveal a temperature controlled behavior and a 40% increase in the quality factor at 10 K.
Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique... more Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique can be used to attain extremely low temperatures, as well as the ranges used in common refrigerators.
Density functional theory (DFT) at the B3LYP/6-31++G(d,p) level was employed to obtain the optimi... more Density functional theory (DFT) at the B3LYP/6-31++G(d,p) level was employed to obtain the optimized geometries and vibrational spectra of several pyridine(Py)-water(W) complexes with stoichiometric ratios ranging from 2:1 (Py 2 W) to 1:3 (PyW 3 ). The harmonic vibrational wavenumbers of pyridine ring modes and the fundamental modes of water were calculated in order to examine the influence of hydrogen bonding on the normal modes of both pyridine and water upon complexation. The Raman spectra in the wavenumber region 960-1060 cm -1 covering the ring modes ν 1 and ν 12 of pyridine (in Wilson's notation) as a function of pyridine mole fraction were recorded. The integrated Raman intensities in the isotropic components of the spectra were used to determine the relative concentration of "free" pyridine molecules in close neighborhood with other Py-W complexes. The combination of both experimental wavenumbers yielding the overall shift induced by the entirety of hydrogen-bonded complexes in the mixture and the DFT-derived vibrational wavenumbers of the isolated species provides the possibility to probe concentration profiles as a function of pyridine mole fraction. The examination of the concentration dependence of line widths reveals that the counter competing influences of different dynamic processes are simultaneously present in this binary mixture.
We present a systematic study of a close-ring pair based freestanding metamaterial fabricated by ... more We present a systematic study of a close-ring pair based freestanding metamaterial fabricated by double-layer, self-aligned photolithography. Terahertz time-domain spectroscopy transmission measurements and numerical simulations have revealed negative index of refraction in the frequency range of 0.66-0.90 THz under normal wave incidence. The observed resonance behaviors can be well explained by a theoretical circuit model. The electromagnetic properties and the figure of merit of such close-ring metamaterials are also explored in terms of geometrical parameters of the unit cell with a goal of providing optimized design for three-dimensional metamaterials and potential device applications.
We investigate the effect of metal permittivity on resonant transmission of metamaterials by tera... more We investigate the effect of metal permittivity on resonant transmission of metamaterials by terahertz timedomain spectroscopy. Our experimental results on double split-ring resonators made from different metals confirm the recent numerical simulations [Phys. Rev. E 65, 036622 ] that metamaterials exhibit permittivity-dependent resonant properties. In the terahertz regime, the measured inductive-capacitive resonance is found to strengthen with a higher ratio of the real to the imaginary parts of metal permittivity, and this remains consistent at various metal thicknesses. Furthermore, we found that metamaterials made even from a generally poor metal become highly resonant owing to a drastic increase in the value of the permittivity at terahertz frequencies.
We study ways to enhance the sensitivity and dynamic tuning range of the fundamental inductor-cap... more We study ways to enhance the sensitivity and dynamic tuning range of the fundamental inductor-capacitor (LC) resonance in split ring resonators (SRRs) by controlling the aspect ratio of the SRRs and their substrate thickness. We conclude that both factors can significantly affect the LC resonance. We show that metafilms consisting of low height SRRs on a thin substrate are most sensitive to changes in their dielectric environment and thus show excellent potential for sensing applications.
Using terahertz time domain spectroscopy we investigate the normal incidence transmission through... more Using terahertz time domain spectroscopy we investigate the normal incidence transmission through periodically and randomly arranged planar split ring resonators (SRRs). Introduction of positional disorder in metamaterials has no effect on the quality factor of the fundamental Inductive-Capacitive (LC) resonance. The dipole resonances undergo broadening and shift in their resonance frequencies. The experiment reveals that the randomly distributed SRR structures interact incoherently at LC resonance but couple coherently at the higher frequency dipole resonance.
Terahertz time domain spectroscopy and rigorous simulations are used to probe the coupling betwee... more Terahertz time domain spectroscopy and rigorous simulations are used to probe the coupling between a dark and a bright plasmonic eigenmode in a metamaterial with broken symmetry. The metamaterial consists of two closely spaced split ring resonators that have their gaps in non-identical positions within the ring. For normal incidence and a fixed polarization both lowest order eigenmodes of the split ring resonators can be excited; although one of them has to be regarded as dark since coupling is prohibited because of symmetry constraints. Emphasis in this work is put on a systematic evaluation of the coupling effects depending on a spectral tuning of both resonances.
We report on the experimental and theoretical study of the resonant eigenmodes of spiral-type ter... more We report on the experimental and theoretical study of the resonant eigenmodes of spiral-type terahertz antennas. The analysis is carried out for a varying number of spiral windings. For larger numbers the structure possesses a self-complementary property which allows the application of the Mushiake principle predicting that the impedance of such structures is half the impedance of free space. This permits to observe an equal and frequency independent reflection and transmission coefficient. This property makes the spiral-type terahertz antenna not only a fascinating example of a medium supporting strong resonances in the long wavelength limit but also a medium which can be easily and reasonably homogenized at higher frequencies. This is in stark contrast to most of the existing metamaterials.
We report an experimental observation of three distinct resonances in split ring resonators (SRRs... more We report an experimental observation of three distinct resonances in split ring resonators (SRRs) for both vertical and horizontal electric field polarizations at normal incidence by use of terahertz time domain spectroscopy. Breaking the symmetry in SRRs by gradually displacing the capacitive gap from the center towards the corner of the ring allows for an 85% modulation of the fundamental inductive-capacitive resonance. Increasing asymmetry leads to the evolution of an otherwise inaccessible high quality factor electric quadrupole resonance that can be exploited for bio-sensing applications in the terahertz region.
Resonant properties of optically thin metamaterials are studied by terahertz time-domain spectros... more Resonant properties of optically thin metamaterials are studied by terahertz time-domain spectroscopy. Both the lower energy inductorcapacitor (LC) and the higher energy dipole resonances of the planar double split-ring resonators (SRRs) exhibit characteristic evolution with various sub-skin-depth thicknesses of the constituent Pb film. The signature of the LC resonance begins to emerge at a critical thickness near 0.15 skin depth. The resonances reveal a characteristic enhancement; they are strengthened remarkably with increasing SRR thicknesses at sub-skin-depth level and then gradually saturate beyond the skin depth.
The limiting effects of varying the thickness of a dielectric overlayer on planar double split-ri... more The limiting effects of varying the thickness of a dielectric overlayer on planar double split-ring resonator (SRR) arrays are studied by terahertz time-domain spectroscopy. Uniform dielectric overlayers from 100 nm to 16 μm thick are deposited onto fixed SRR arrays in order to shift the resonance frequency of the electric response. We discuss the bounds of resonance shifting and emphasize the resulting limitations for SRR-based sensing. These results are presented in the context of typical biosensing situations and are compared to previous work and other existing sensing platforms.
We experimentally demonstrate at terahertz frequencies that a planar metamaterial exhibits a spec... more We experimentally demonstrate at terahertz frequencies that a planar metamaterial exhibits a spectral response resembling electromagnetically induced transparency. The metamaterial unit cell consists of a split ring surrounded by another closed ring where their dimensions are such that their excitable lowest order modes have identical resonance frequencies but very different lifetimes. Terahertz time-domain spectroscopy verifies that the interference of these two resonances results in a narrow transparency window located within a broad opaque region. In contrast to previous studies this enhanced transmission is achieved by independently exciting two resonances in which their coupling to the radiation field, and thus their linewidth, differs strongly. Rigorous numerical simulations prove that the transparency window is associated with a large group index and low losses, making the design potentially useful for slow light applications. This experiment opens an avenue to explore quantum-mechanical phenomena using localized resonances in metallic structures.
Optical properties of planar thin film metamaterials were measured at room and liquid nitrogen te... more Optical properties of planar thin film metamaterials were measured at room and liquid nitrogen temperatures using terahertz time-domain spectroscopy. The operation of metamaterials at cryogenic temperatures is anticipated to be a promising path toward low-loss metamaterials since nonradiative losses are strongly suppressed due to higher charge mobility. A 14% increase in the quality factor of the resonances was experimentally observed. It was limited by the high electron scattering rate due to defects in thin films. Supplementary simulations assuming metamaterials made of thick films reveal a temperature controlled behavior and a 40% increase in the quality factor at 10 K.
Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique... more Magnetic refrigeration is a cooling technology based on the magnetocaloric effect. This technique can be used to attain extremely low temperatures, as well as the ranges used in common refrigerators.
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