Optical Terahertz Research Articles

Optical Design Study with Uniform Field of View Regardless of Sensor Size for Terahertz System Applications

The focal length in a typical optical system changes with the angle of view, according to the size of the sensor. This study proposed an optical terahertz (THz) system application where the focal length changed while the angle of view was fixed; thus, the image height was variable and responded to various sensor sizes. Therefore, it is possible to respond to various sensors with one optical system when the inspection distance is fixed. The fundamental optical system was designed by arranging the refractive power, which was determined according to the sensor size using the Gaussian bracketing method. A zoom optical system that changed the image height by fixing the angle of view and changed the focal length by moving the internal lens group was designed. THz waves exhibit minimal change in the refractive index depending on the wavelength. Moreover, their long-wavelength characteristics facilitate the development of millimeter-level pixel sizes. Therefore, the root mean square size of the maximum spot was 0.329 mm, which corrected the aberration to less than 1 mm (smaller than the pixel size). Further, a lighting analysis at 3 and 6 m locations confirmed the expansion of the lighting area by the magnification of the sensor size. After turning off certain light sources, we checked the contrast ratio via lighting analysis and confirmed that the size of one pixel was clearly distinguishable. Consequently, this newly designed optical system performed appropriately as an optical inspection system for THz system applications.

Multimodal Non-Destructive In Situ Observation of Crystallinity Changes in High-Density Polyethylene Samples with Relation to Optical Parameters during Tensile Deformation

Abstract: Many non-destructive optical testing methods are currently used for material research, providing various information about material parameters. At RECENDT, a multimodal experimental setup has been designed that combines terahertz (THz) spectroscopy, optical coherence tomography (OCT), infrared (IR), and Raman spectroscopy with a tensile test stage. This setup aims to gather material information such as crystallinity and optical parameters of high-density polyethylene (HDPE) during a tensile test. The setup compares common IR and Raman spectroscopy and the less common optical methods THz and OCT. Complementarity is achieved through different frequency ranges and measurement approaches, resulting in different measured optical material parameters and depths. During tensile testing, HDPE samples with varying crystallinity were analysed, and the determined optical parameters such as refractive index, birefringence, scattering coefficient of decay, and penetration depth can be correlated with the change in crystallinity. These findings demonstrate that the optical methods and their outcomes can be interconnected. With further optimization of the experimental setup, it would be possible to observe the alignment of fibres in fibre composite panels and the stress distribution of polymers effectively. This opens interesting possibilities for polymer characterization in the future, including quality control during moulding processes and material testing.

Unveiling enamel demineralization mechanisms by sensitive dielectric differentiation based on terahertz nanospectroscopy.

The early stage of dental caries, i.e. demineralization, has always been a topic of concern to dentists. Understanding the essential mechanism of its occurrence is of great significance for the prevention and treatment of dental caries. However, owing to limitations in resolution and the detection capabilities of diagnostic tools, the study of enamel demineralization has always been a challenge. Terahertz (THz) technology, especially the combination of scanning near-field optical microscopy (s-SNOM) and THz time-domain spectroscopy (TDS), due to its nanoscale resolution, has shown great advantages in the field of biological imaging. Here, a THz s-SNOM system is used to perform near-field imaging of enamel before and after demineralization at the nanoscale. It can be found that near-field signals decrease significantly after demineralization. This is due to the changes of the crystal lattice and the transfer of mineral ions during demineralization, which leads to a decrease in the permittivity of the enamel. The novel approach in this study reveals the essence of demineralization and lays the groundwork for additional research and potential interventions.

Generation of terahertz beam with longitudinally varied polarization state via coherent superposition based on metasurface

Polarization is an important dimension in the research and applications of light waves. However, traditional polarization optics often only focus on the polarization characteristics in the transverse plane. Here, we demonstrate a new scheme for the generation of longitudinally varied polarization state in terahertz beam using all-silicon metasurface. We employ wavefront transformation designs with long-focal-depth for orthogonal circularly polarized terahertz waves, achieving varied amplitude and phase along the propagation direction in opposite spin states. Based on the principle of coherent superposition of polarized waves, different linear and elliptical polarization states are obtained in transverse planes along the propagation path, with variable ellipticity and azimuth angle. Simulation results show that a large-scale evolution of the elliptical polarization azimuth angle from 45° to -60° and ellipticity from 20° to -74° can be observed within a focal depth range of 0.45-0.8 mm. We also intuitively display the helical trajectory of the polarization state from left-hand elliptical ones to right-hand elliptical ones within the focal depth range, using the Poincaré sphere. This work expands the application of metasurface devices for multifunctional polarization devices and can be applied to polarization generation and transformation for optical imaging or terahertz communications.

Optical terahertz metamaterial switch controlled via high-stability CsPbBr3 microcrystals

Dynamic control of terahertz metamaterials using thin organic perovskite active layers has been extensively researched. However, the preparation of organic perovskite devices requires strict environmental conditions, and the devices are prone to hydrolysis in air, which reduces performance. Herein, we report an optical terahertz metamaterial switch controlled via hybridization with high-stability CsPbBr3 microcrystals prepared through precipitation from a water–dimethylformamide (DMF) mixed-solution. Under light excitation, a modulation factor of 24% was achieved based on the photoelectric and photothermal effects of the CsPbBr3 microcrystals. After exposure to air for four months, the modulation factor remained essentially unchanged, demonstrating the exceptional stability of the system generated. Following the integration of CsPbBr3 microcrystals with the metamaterial, a frequency-shift of its dipole resonance and switching of Fano resonance was achieved, providing a novel approach to dynamic control of terahertz waves.

Solitons, bifurcation and chaotics in a bidirectional mamyshev oscillator

Bidirectional mode-locked fiber lasers are able to deliver two outputs of pulses in the clockwise (CW) and counterclockwise (CCW) directions, having important applications in dual-comb spectroscopy, asynchronous optical sampling and terahertz time-domain spectroscopy. In this paper, we have experimentally demonstrated an all-fiber Er/Yb-doped bidirectional passively mode-locked Mamyshev oscillator (MO) without an external seed. Two stable mode-locked states with remarkable different characteristics are observed by adjusting the pump power. The transition process between the two mode-locked states shows chaotic behaviors, which are further investigated with numerical simulations based on the Ginzburg-Landau equation. The bidirectional MO proposed in this work is not only a good laser source, but also a good platform to study soliton dynamics, system bifurcation, and chaos in ultrafast fiber laser systems.

Optical Tweezing Terahertz Probing for a Single Metal Nanoparticle.

Recently, extensive research has been reported on the detection of metal nanoparticles using terahertz waves, due to their potential for efficient and nondestructive detection of chemical and biological samples without labeling. Resonant terahertz nanoantennas can be used to detect a small amount of molecules whose vibrational modes are in the terahertz frequency range with high sensitivity. However, the positioning of target molecules is critical to obtaining a reasonable signal because the field distribution is inhomogeneous over the antenna structure. Here, we combine an optical tweezing technique and terahertz spectroscopy based on nanoplasmonics, resulting in extensive controllable tweezing and sensitive detection at the same time. We observed optical tweezing of a gold nanoparticle and detected it with terahertz waves by using a single bowtie nanoantenna. Furthermore, the calculations confirm that molecular fingerprinting is possible by using our technique. This study will be a prestep of biomolecular detection using gold nanoparticles in terahertz spectroscopy.

High photoelectric conversion efficiency and fast relaxation time of FA0.4MA0.6PbI3 applied in ultrafast modulation of terahertz waves

Active control of terahertz (THz) waves is attracting tremendous attentions in terahertz communications and active photonic devices. Perovskite, due to its excellent photoelectric conversion performance and simple manufacturing process, has emerged as a promising candidate for optoelectronic applications. However, the exploration of perovskites in optically controlled THz modulators is still limited. In this work, the photoelectric properties and carrier dynamics of FA0.4MA0.6PbI3 perovskite films were investigated by optical pumped terahertz probe (OPTP) system. The ultrafast carrier dynamics reveal that FA0.4MA0.6PbI3 thin film exhibits rapid switching and relaxation time within picosecond level, suggesting that FA0.4MA0.6PbI3 is an ideal candidate for active THz devices with ultrafast response. Furthermore, as a proof of concept, a FA0.4MA0.6PbI3-based metadevice with integrating plasma-induced transparency (PIT) effect was fabricated to achieve ultrafast modulation of THz wave. The experimental results demonstrated that the switching time of FA0.4MA0.6PbI3-based THz modulator is near to 3.5 ps, and the threshold of optical pump is as low as 12.7 μJ cm−2. The simulation results attribute the mechanism of ultrafast THz modulation to photo-induced free carriers in the FA0.4MA0.6PbI3 layer, which progressively shorten the capacitive gap of PIT resonator. This study not only illuminates the potential of FA0.4MA0.6PbI3 in THz modulation, but also contributes to the field of ultrafast photonic devices.

Double plasmon-induced transparency 3 bit graphene encoder

In this paper, a 3 bit encoder was proposed which was based on double plasmon-induced transparency (PIT) effect. This structure consists of the bright-bright mode and realizes a double PIT window in the 0.1–3 THz range. Electric field distributions reveal that the double PIT effect is originated from the strong destructive interference between three bright modes, the Lorentz oscillation coupling model is utilized to confirm the finite-integration time-domain (FITD) simulation regardless of the x-polarization direction or y-polarization direction. When the incident light is x-polarized, encoding frequencies were at 0.976 THz, 1.525 THz and 2.069 THz, respectively; while in y-polarization, encoding frequencies were at 1.579 THz, 1.935 THz and 2.589 THz, respectively. Results depict that the Modulation Depth (MD) could reach 98 % regardless of the x-polarization direction or the y-polarization direction. As an electro-optical switch, the maximum MD could reach ≥98 %, the minimum IL could reach 0.35 dB, the maximum ER could reach ≥14 dB. Besides the encoder, the proposed structure is of great importance for the design of optical switches, terahertz modulators and slow light devices.

PERFORMANCE ANALYSIS OF OPTICAL PARALLEL FULL ADDER USING ARTIFICIAL NEURAL NETWORK

A verbal exchange today wishes for quick operational progress. This can be accomplished by replacing devices that are primarily concerned with commutation and logic with photon-based systems instead of the usual data service, the electron. The basic building blocks of superior frames are called gates. With the aid of these gates, various logical and mathematical operations can be performed. All-optical arithmetical and logical processes are eagerly expected in high-speed dialogue frameworks. In this chapter, we’ve introduced parallel models for adding two binary digits that are based on Sagnac gates with help from semiconductor optical amplifiers (SOA) and terahertz optical asymmetric demultiplexers (TOAD). We created a Full adder that works in parallel using only two TOADs as total switches. Using artificial neural networks (ANN), we have created a model of this circuit that is equivalent. Utilizing ANN, this circuit design has been validated. This optical circuit is now capable of synthesizing light as an input and successfully structuring the aspiration output in addition to speeding up calculation. This parallel circuit’s biggest advantage is that it doesn’t need synchronization for distinct inputs. An ANN model was used to analyze this circuit’s performance in detail.

A Novel Optical Fiber Terahertz Biosensor Based on Anti-Resonance for The Rapid and Nondestructive Detection of Tumor Cells.

The sensitive and accurate detection of tumor cells is essential for successful cancer therapy and improving cancer survival rates. However, current tumor cell detection technologies have some limitations for clinical applications due to their complexity, low specificity, and high cost. Herein, we describe the design of a terahertz anti-resonance hollow core fiber (THz AR-HCF) biosensor that can be used for tumor cell detection. Through simulation and experimental comparisons, the low-loss property of the THz AR-HCF was verified, and the most suitable fiber out of multiple THz AR-HCFs was selected for biosensing applications. By measuring different cell numbers and different types of tumor cells, a good linear relationship between THz transmittance and the numbers of cells between 10 and 106 was found. Meanwhile, different types of tumor cells can be distinguished by comparing THz transmission spectra, indicating that the biosensor has high sensitivity and specificity for tumor cell detection. The biosensor only required a small amount of sample (as low as 100 μL), and it enables label-free and nondestructive quantitative detection. Our flow cytometry results showed that the cell viability was as high as 98.5 ± 0.26% after the whole assay process, and there was no statistically significant difference compared with the negative control. This study demonstrates that the proposed THz AR-HCF biosensor has great potential for the highly sensitive, label-free, and nondestructive detection of circulating tumor cells in clinical samples.

Diabetes noninvasive diagnostics and monitoring through volatile biomarkers analysis in the exhaled breath using optical absorption spectroscopy.

The review is aimed on the analysis the abilities of noninvasive diagnostics and monitoring of diabetes mellitus (DM) and DM-associated complications through volatile molecular biomarkers detection in the exhaled breath. The specific biochemical reactions in the body of DM patients and their associations with volatile molecular biomarkers in the breath are considered. The applications of optical spectroscopy methods, including UV, IR, and terahertz spectroscopy for DM-associated volatile molecular biomarkers measurements, are described. The applications of similar technique combined with machine learning methods in DM diagnostics using the profile of DM-associated volatile molecular biomarkers in exhaled air or "pattern-recognition" approach are discussed.

New Soliton Regime of Generation of Broadband Terahertz Radiation by Laser Pulses with Tilted Wave Fronts

A new soliton-like regime of generation of terahertz radiation by optical pulses with tilted wave fronts is analyzed. It has been shown that the diffraction of an optical pulse is of fundamental importance for the formation of optical–terahertz soliton. A nonsoliton broadband terahertz component is generated synchronously with the soliton component of radiation. Two matching conditions called “super-Cherenkov” and “anti-Cherenkov” have been revealed under which generation is the most efficient. In the former and latter cases, the optical terahertz soliton propagates ahead and behind the nonsoliton terahertz component, respectively.

Large angle electromagnetic induced reflection – Like of phase coupled eccentric ring in metasurface

Electromagnetic induced reflection (EIR)-like is a phenomenon of electromagnetic induced transparency(EIT) -like has potential applications in optical delay lines, slow light devices and high sensitivity sensors. In this paper, a metasurface structure composed of four eccentric rings was designed. And the reflection and magnetic field distributions are investigated by finite integral method in time domain method. The results show that the EIR-like effect was obtained by designed structure through tuned the phase of eccentric rings. The equivalent impedance of the structure was calculated by inversion method. And the multipole scattering spectrum was calculated by the theory of multipole scattering to further explain the EIR-like effect. The influence of the structural parameters on the reflection illustrates the EIR-like effect was studied. To further explore the potential application of the phenomenon, the effect of the large angle on the EIR-like was also studied. The results show that the structure has a large angle of polarization insensitive EIR-like effect. The structure has potential applications in terahertz optical communication, terahertz sensing and reflector.

Enhanced crystallinity, optical conductivity and terahertz cutoff frequency of stacked layers of FeSe2 by Al nanosheets

Herein a 250 nm thick two stacked layers of FeSe2 (abbreviated as FF) thin films and FF stacks comprising aluminum nanosheets of thicknesses of 50 nm (FAF) are studied. Amorphous FeSe2 thin films are deposited using thermal evaporation technique under a vacuum pressure of 10−5 mbar. It is observed that insertion of Al nanosheets induced the evolution of orthorhombic phase of FeSe2. nanowire and elongated rectangular grains are also formed. Optically Al nanosheets enhanced the light absorption by 4.1 times and redshifted the indirect/direct energy band gaps from 2.80/2.49 eV to 2.42/1.85 eV, respectively. It also improved the optical conductivity and terahertz cutoff frequency as well. In addition the optical conductivity parameters of FF and FAF terahertz resonators are determined by the Drude-Lorentz approach. It is observed that in the infrared range of light Al nanosheets improved the drift mobility of FF from 2.06 cm2/Vs to 12.55 cm2/Vs. The hole density is reduced and the scattering time constant at femtosecond level is increased. FF and FAF terahertz resonators performed as optical filters suitable for terahertz technology applications with terahertz cutoff frequency varying in the range of 17.2–300 THz.

Regulating Terahertz Photoconductivity in Two-Dimensional Materials

Two-dimensional materials represented by graphene have attracted extensive interest owing to the unique layer-dependent physical properties that are tunable with various external fields. In addition, by stacking two or more 2D materials together, a new material with the desired properties can be tailored and designed. Fully understanding the dynamical photoconductive response in 2D materials is uttermost important to design and develop the advanced optoelectronic devices. Terahertz (THz) time-domain spectroscopy (TDS) and time-resolved THz spectroscopy are powerful spectroscopic tools with the advantages of being contact-free and noninvasive, which have been widely used to study the photoconductivity (PC) of 2D materials. In this review, firstly, we provide a short introduction of the 2D materials and THz spectroscopy, and then a brief introduction of the experimental setup and experimental data analysis based on time-resolved THz spectroscopy are presented. After that, we overview the latest progress on the regulation of the THz PC that includes: (1) regulating the THz PC of graphene (Gr) and transition metal dichalcogenide (TMD) thin films with oxygen adsorption; (2) regulating the THz PC of Gr and Gr/TMDs heterostructures by electric gating and a built-in field introduced by a substrate; (3) regulating the THz PC of Gr/TMD heterostructures via optical gating; and (4) we overview the latest progress on the observation of elementary excitations in 2D materials with THz PC spectra following optical excitation and THz PC regulation via the photoexcitation of quasi-particles. Finally, we conclude the review and present a short overview of future research directions.

Engineering Band Gap and Photoconduction in Semiconducting Metal Organic Frameworks: Metal Node Effect.

We report a systematic study on the correlation of the metal nodes in M-THQ conducting MOFs (M = Fe, Ni, Cu, and Zn; THQ = tetra-hydroxybenzoquinone) with their structure, photophysical property, and photoconductivity. We found that the structural preference in these MOFs is controlled by metal node identity where Cu prefers a square planar coordination which leads to a 2D Kagome-type structure. Fe, Ni, and Zn prefer an octahedral sphere which leads to a 3D structure. Fe-THQ has the smallest band gap and highest photoconduction as well as a long-lived ligand-to-metal charge transfer state due to the mixed valence state revealed by time-resolved optical and X-ray absorption and terahertz spectroscopy. These results demonstrate the importance of the metal node in tuning the photophysical and photocatalytic properties of MOFs.

Lead-tungsten oxide interfaces designed as gigahertz/terahertz filters

Herein amorphous tungsten oxide thin films of thicknesses of 300 nm are coated onto semitransparent Pb films (100 nm) by the thermal evaporation technique under a vacuum pressure of 10−5 mbar. Optical investigations in these films have shown that Pb nanosheets enhances the light absorbability in the visible and infrared ranges of light without significant change in the energy band gap value. In addition deep analyses of the optical conductivity and terahertz cutoff frequency spectra of the Pb/WO3 optical filters revealed that the cutoff frequency value in the visible range of light is invariant with light signal energy indicating the possibly of filtering none-monochromatic light signals. On the other hand imposing an ac signal between the terminals of Pb/WO3/Au devices has proofed the ability of the device to perform as low pass and as band stop filters workable in the microwave frequency domain. The microwave cutoff frequency for this device reached ∼9 GHz nominating it for use in 5 G mobile technology. The current study showed that coating of tungsten oxide onto semitransparent Pb substrate and coverage small area of the top contact with Au can allow functioning the device as gigahertz/terahertz band filters suitable for communication technology.

Polarization-resolved ultrafast all-optical terahertz micro-grating array modulator based on Weyl semimetallic microfilm towards 6G technology

The 6th generation (6G) of communication technology, defines 0.1–1 THz as the communication band. As a result, ultrafast modulation and multi-dimensional resolution coding technologies in the terahertz band pose a big challenge.In this work, a polarization-resolved ultrafast optical controlled terahertz modulator has been realized by combining tungsten ditelluride (WTe2) thin film with subwavelength metallic gratings.In the frequency range of 0.1–1.2 THz, which covers the 6G frequency band, the modulation depth of the device reaches up to 36.8% with a centimeter-scale effective area. An ultrafast response time of 0.95 ps is obtained due to the limitations of the grating for carrier degrees of freedom. In addition, the polarization sensitivity of terahertz waves polarized in the x- and y-direction is up to 0.77 due to the sub-wavelength grating structure. The results give a chance to develop the large-area, high-performance polarization-resolved ultrafast THz modulators based on Weyl semimetal, which could provide a reliable alternative for 6G communications equipment.

Optically Controlling Broadband Terahertz Modulator Based on Layer-Dependent PtSe2 Nanofilms

In this paper, we propose an optically controlling broadband terahertz modulator of a layer-dependent PtSe2 nanofilm based on a high-resistance silicon substrate. Through optical pump and terahertz probe system, the results show that compared with 6-, 10-, and 20-layer films, a 3-layer PtSe2 nanofilm has better surface photoconductivity in the terahertz band and has a higher plasma frequency ωp of 0.23 THz and a lower scattering time τs of 70 fs by Drude-Smith fitting. By the terahertz time-domain spectroscopy system, the broadband amplitude modulation of a 3-layer PtSe2 film in the range of 0.1-1.6 THz was obtained, and the modulation depth reached 50.9% at a pump density of 2.5 W/cm2. This work proves that PtSe2 nanofilm devices are suitable for terahertz modulators.