Absorption-induced Transparency Research Articles

Highly Sensitive and Tunable Absorption-Induced Transparency for Terahertz Fingerprint Sensing With Spoof Surface Plasmon Polaritons

Highly Sensitive and Tunable Absorption-Induced Transparency for Terahertz Fingerprint Sensing With Spoof Surface Plasmon Polaritons

Frequency selective fingerprint sensor: the Terahertz unity platform for broadband chiral enantiomers multiplexed signals and narrowband molecular AIT enhancement

Chiral enantiomers have different pharmacological and pharmacokinetic characteristics. It is important to strictly detect chiral component for avoiding being harmful to the human body due to side effects. Terahertz (THz) trace fingerprint detection is essential because the molecular vibrations of various biological substances such as chiral enantiomers are located in THz range. Recent reported enhanced trace fingerprint technologies have some drawbacks. For instance, multiplexing technology suffered from narrow operation range and limitation by frequency resolution of commercial THz time domain spectroscopy; Absorption induced transparency (AIT) identification for narrowband molecular oscillations suffered from random resonance frequency drift due to fabrication error. In this paper, we proposed frequency-selective fingerprint sensor (FSFS), which can experimentally achieve enhanced trace fingerprint detection by both broadband multiplexing technology and robust AIT identification. Such FSFS is based on polarization independent reconfiguration metasurfaces array. Broadband absorption lines of trace-amount chiral carnitine were boosted with absorption enhancement factors of about 7.3 times based on frequency-selective multiplexing at 0.95–2.0 THz. Enhanced trace narrowband α-lactose fingerprint sensing can be observed at several array structures with absorption enhancement factors of about 7 times based on AIT, exhibiting good robustness. The flexibility and versatility of proposed FSFS has potential applications for boosting trace chiral enantiomer detection as well as diversity of molecular fingerprints identification by both multiplexing and AIT.

Amplification of stimulated light emission in arrays of nanoholes by plasmonic absorption-induced transparency.

Absorption induced transparency is an optical phenomenon that occurs in plasmonic nanostructures when materials featuring narrow lines in their absorption spectra are deposited on top of it. First reported in the visible range for metallic arrays of nanoholes, using dye lasers as covering, it has been described as transmission peaks unexpectedly close to the absorption energies of the dye. In this work, amplification of stimulated light emission is numerically demonstrated in the active regime of absorption induced transparency. Amplification can be achieved in the regime where the dye laser behaves as a gain material. Intense illumination can modify the dielectric constant of the gain material in a short span of time and thus the propagation properties of the plasmonic modes excited in the hole arrays, providing both less damping to light and further optical feedback that enhances the stimulated emission process.

Tunable Multimode Plasmon–Exciton Coupling for Absorption-Induced Transparency and Strong Coupling

The interaction between plasmons and molecular excitons, which can usher in a wide range of novel results under both an electronic and a vibrational strongly coupled state, has attracted great inte...

Nonlinear absorption-induced transparency and extinction of boron nanosheets

Abstract Two-dimensional (2D) materials exhibit interesting nonlinear optical properties, and they have been used in bioimaging, ultrafast photon and all-optical switch devices. Boron nanosheets is a recently emerging 2D material that has attracted the attention of a large number of researchers. In this work, the saturation absorption (SA) effect of boron nanosheets has been investigated through a typical z-scan method, it is demonstrated that the nonlinear absorption coefficient and the quality factor are greatly improved compared with the other 2D materials such as black phosphorus, MoS2 and Ti3C2Tx. By combining the all-optical modulation and z-scan technology, the competition between the laser intensity-dependent SA and nonlinear scattering in boron nanosheets are revealed, and hence the optically-induced transparency and optically-induced extinction in nanosecond regime have been confirmed. Based on this competitive effect, an effective modulation of continuous light (λ = 671 nm) by nanosecond pulses (λ = 532 nm) has been realized, the maximum modulation depth of 5.5 dB is obtained.

Molecular methylation detection based on terahertz metamaterial technology.

Terahertz wave has a good ability to identify biomolecules due to its fingerprint spectrum characteristics. However, the minimum detectable limit of terahertz technology by the conventional tablet pressing method is on the order of milligrams, which cannot meet the application requirements of low concentration detection in the biomedical field-near or below micrograms. Here, we proposed a method to enhance the detection sensitivity by designing a metamaterial chip with the absorption-induced transparency (AIT) effect, which can enhance the interaction between terahertz waves and biomolecules and lower the detectable limit. Taking 7-methylguanine (7-MG) as an example, based on its terahertz characteristic absorption peak, we designed a split-ring resonator (SRR) metamaterial chip, which has the advantages of high sensitivity, unlabeled detection, fast response and simple measurement. Its quantitative detection limit can reach 6.30 μg, which is about 500 times smaller than that of the traditional tablet pressing method (2.95 mg). In addition, for methylated and unmethylated substances, the chip exhibits different frequency shifts, which also realizes the qualitative identification effectively. These results provide a reference for the rapid and accurate diagnosis of diseases associated with molecular methylation in clinical medicine.

Waveguide and Plasmonic Absorption-Induced Transparency

Absorption-induced transparency (AIT) is one of the family of induced transparencies that has emerged in recent decades in the fields of plasmonics and metamaterials. It is a seemingly paradoxical phenomenon in which transmission through nanoholes in gold and silver is dramatically enhanced at wavelengths where a physisorbed dye layer absorbs strongly. The origin of AIT remains controversial, with both experimental and theoretical work pointing to either surface (plasmonic) or in-hole (waveguide) mechanisms. Here, we resolve this controversy by carefully filling nanoholes in a silver film with dielectric material before depositing dye on the surface. Our experiments and modeling show that not only do plasmonic and waveguide contributions to AIT both exist, but they are spectrally identical, operating in concert when the dye is both in the holes and on the surface.

Thermally Tunable Absorption‐Induced Transparency by a Quasi 3D Bow‐Tie Nanostructure for Nonplasmonic and Volumetric Refractive Index Sensing at Mid‐IR

A nonresonant transmission process namely absorption‐induced transparency (AIT) is demonstrated in a quasi 3D bow‐tie nanostructure at mid‐IR spectrum for the first time. The quasi 3D structure is formed by self‐aligning two metasurfaces obtained by the Babinet's principle which are linked to each other through a patterned layer of PMMA. It is shown that the vibrational absorption of poly(methyl methacrylate) (PMMA) at 5.79 μm can strongly influence the propagation characteristics of the quasi 3D structure and generate the absorption‐induced transmission peak. A detailed parametric study is conducted on the process and its nonplasmonic characteristics are confirmed. Later, the usefulness of the quasi 3D nanochannels of the geometry is addressed and volumetric refractive index sensing (refractive index unit (RIU) shift of 123.33 nm) by the AIT effect is experimentally demonstrated for the first time. Lastly, the thermal responsiveness of the metal–dielectric hybrid platform is exploited and a thermal tuning up to 25 nm for a moderate range of temperature increase (100°) is obtained. It is believed that the hybrid system with 3D nanochannels assisted transparency can be a new avenue for volumetric plasmonic sensing and stimuli responsive device operating solely on the principle of changing molecular conformation of material.

Absorption-induced transparency metamaterials in the terahertz regime.

Contrary to what might be expected, when an organic dye is sputtered onto an opaque holey metal film, transmission bands can be observed at the absorption energies of the molecules. This phenomenon, known as absorption-induced transparency, is aided by a strong modification of the propagation properties of light inside the holes when filled by the molecules. Despite having been initially observed in metallic structures in the optical regime, new routes for investigation and applications at different spectral regimes can be devised. Here, to illustrate the potential use of absorption-induced transparency at terahertz, a method for molecular detection is presented and supported by a theoretical analysis.

Terahertz gas sensor based on absorption-induced transparency

A system for the detection of spectral signatures of gases at the Terahertz regime is presented. The system consists in an initially opaque holey metal film whereby the introduction of a gas provokes the appearance of spectral features in transmission and reflection, due to the phenomenom of absorption-induced transparency (AIT). The peaks in transmission and dips in reflection observed in AIT occur close to the absorption energies of the molecules, hence its name. The presence of the gas would be thus revealed as a strong drop in reflectivity measurements at one (or several) of the gas absorption resonances. As a proof of principle, we theoretically demonstrate how the AIT-based sensor would serve to detect tiny amounts of hydrocyanic acid.

Plasmonic electrodes for bulk-heterojunction organic photovoltaics: a review

Here, we review recent progress on the integration of plasmonic electrodes into bulk-heterojunction organic photovoltaic devices. Plasmonic electrodes, consisting of thin films of metallic nanostructures, can exhibit a number of optical, electrical, and morphological effects that can be exploited to improve performance parameters of ultrathin photovoltaic active layers. We review the various types of plasmonic electrodes that have been incorporated into organic photovoltaics such as nanohole, nanowire, and nanoparticle arrays and grating electrodes and their impact on various device performance parameters. The use of plasmonic back electrodes can impact device performance in a number of ways because the mechanisms of performance improvements are often a complex combination of optical, electrical, and structural effects. Inverted bulk heterojunction device architectures have been shown to benefit from the multifunctionality of plasmonic back electrodes as they can minimize space-charge effects and reduce hole carrier collection lengths in addition to providing improved light localization in the active layer. The use of semi-transparent plasmonic electrodes can also be beneficial for organic photovoltaics as they can exhibit a variety of optical properties such as light scattering, light localization, extraordinary transmission of light, and absorption-induced transparency, in addition to providing an alternative to metal oxide–based transparent electrodes.

Theory of absorption-induced transparency

Recent experiments (Angew. Chem. Int. Ed. 50, 2085 (2011)) have demonstrated that the optical transmission through an array of subwavelength holes in a metal film can be enhanced by the intentional presence of dyes in the system. As the transmission maxima occurs spectrally close to the absorption resonances of the dyes, this phenomenon was christened Absorption Induced Transparency. Here, a theoretical study on Absorption Induced Transparency is presented. The results show that the appearance of transmission maxima requires that the absorbent fills the holes and that it occurs also for single holes. Furthermore, it is shown that the transmission process is non-resonant, being composed by a sequential passage of the EM field through the hole. Finally, the physical origin of the phenomenon is demonstrated to be non-plasmonic, which implies that Absorption Induced Transparency should also occur at the infrared or Terahertz frequency regimes.

Absorption‐Induced Transparency

Optisch dicke Metallfilme, die mit subwellenlängengroßen Löchern perforiert sind, werden transparent, wenn sie eine dünne Schicht von Molekülen auf ihrer Oberfläche adsorbieren (siehe Bild). Transmissionsspektren von Filmen mit (schwarze Kurve) und ohne Löcher (rote Kurve) belegen, dass die Transmission unerwartet bei Wellenlängen stattfindet, bei denen die Molekülschicht stark absorbiert.

Absorption‐Induced Transparency

In the past decade, the field of optics has been stimulated by new concepts such as plasmonics and extraordinary optical transmission, which are paving the way for next-generation photonic components. In this context, hybrid materials that combine the properties of structured metals with semiconductor or molecular materials to create novel functionalities offer much potential. While studying molecule– metal interactions, we have found a new phenomenon whereby molecules can induce transparency in optically thick metal films perforated with subwavelength holes. Nonintuitively, transparent windows are opened up at wavelengths at which the molecules absorb strongly, that is, where one would normally expect no transmission. Here we report a detailed study of this phenomenon showing, among other things, that the molecular material must be within the dipole coupling distance (less than ca. 20 nm) from the metal surface, and that the mechanism involves surface plasmons but is independent of the arrangement of the holes. The phenomenon thus provides new flexibility for tailoring extraordinary optical transmission through subwavelength holes and points to new directions for preparing plasmonic hybrid materials for photonics and energy-conversion applications. Absorption-induced transparency (AIT) is best illustrated by the schematic and spectra in Figure 1. Figure 1a shows the transmission spectrum of a square array of 100 nm-diameter holes milled by focused ion beam (FIB) in a 200 nm-thick Ag film with a period of 250 nm (black curve). Only the transmission peak associated with the (1,0) surface plasmon (SP) resonance on the glass/metal interface of the hole array is visible at 518 nm. When an approximately 30 nm layer of a J-aggregate of a cyanine compound (2,2’-dimethyl-8-phenyl5,6,5’,6’-dibenzothiacarbocyanine chloride) is adsorbed on the hole array, its transmission spectrum shows an intense new transmission with a sharp onset at 685 nm (red curve). Figure 1b shows the absorption spectrum of the cyanine Jaggregate layer (measured as 1 reflection) taken on a