Glucose refractometer based on terahertz resonant waveguide grating structure

The transmission spectra of sub-wavelength cross metallic fractal slits in terahertz (THz) frequency region are presented by means of finite-difference-time-domain (FDTD) simulation. The transmission spectra with multiple pass bands and stop bands are observed. To understand the physical mechanism of the enhanced transmissions, we simulated the electric field distribution of THz radiation within the metallic slits at the resonance frequencies by the electromagnetic design software named CONCERTO. Further analysis reveals that the two transmission peaks in the low frequency is the local resonance of electric field of the two cross slit, respectively. The third transmission peak is the co-effect of the two level cross slits. Our simulation is helpful for the understanding of THz wave propagation and THz transmission through the cross metallic fractal structures. It is also useful for the development of THz photonic devices.

SummaryLogic gates are important components in integrated photonic circuitry. Here, a series of logic gates to achieve fundamental logic operations based on linear interference in spoof surface plasmon polariton waveguides are demonstrated at terahertz frequencies. A metasurface-based plasmonic source is adopted to couple free-space terahertz radiation into surface waves, followed by a funnel-shaped metasurface to efficiently couple the surface waves to the waveguides built on a domino structure. A single Mach-Zehnder waveguide interferometer can work as logic gates for four logic functions: AND, NOT, OR, and XOR. By cascading two such interferometers, NAND and NOR operations can also be achieved. Experimental investigations are supported by numerical simulations, and good agreement is obtained. The logic gates have compact sizes and high intensity contrasts for the output “1” and “0” states. More complicated functions can be envisioned and will be of great value for future terahertz integrated computing.

The characteristics of spatiotemporal shaping and filtering of terahertz （THz） pulses passing through metal slits with finite thickness have been investigated by employing the finite-difference time-domain （FDTD） method. It is shown that this diffraction-based quasi-optical technique has a good influence on shaping and filtering of transverse magnetic（TM） polarized THz pulse, while has no effect on transverse electric（TE） polarized THz pulse. Furthermore, the numerical simulation results have been interpreted qualitatively by the planar waveguide theory and can be employed in the relevant experiments.

Dispersion-free characterization of terahertz slab–waveguide modal confinement based on a metal Bragg grating structure

We present experimental and theoretical studies on terahertz (THz) notch filter having a wide THz frequency range and low-pass filter (LPF) having a 0.78 THz cutoff frequency. The notch filter has a dispersion-free and low-loss transverse magnetic (TM) mode. On the other hand, the cut off frequency of the LPF was determined using a Bragg stop band, which depends on slit period.

The characteristics of spatiotemporal shaping and filtering of terahertz (THz) pulses passing through metal slits with finite thickness have been investigated by employing the finite-difference time-domain (FDTD) method. It is shown that this diffraction-based quasi-optical technique has a good influence on shaping and filtering of TEM-polarized THz pulses for thick-slits, while no effect for thin-slits. Furthermore, the numerical simulation results can be employed in the relevant experiments.

Extreme terahertz (THz) science and technologies, the next disruptive frontier in nonlinear optics, provide multifaceted capabilities for exploring strong light‐matter interactions in a variety of physical systems. However, current techniques involve the need for an extremely high‐field free space THz source that is difficult to generate and has limited investigations to a rather weak and linear regime of light‐matter interactions. Therefore, new approaches are being sought for the tight confinement of THz waves that can induce nonlinear effects. Here, a nonlinear “tera‐nano” metasurface is demonstrated exhibiting extremely large THz nonlinearity and sensitive self‐modulation of resonances at moderate incident THz field strengths. A record deep‐subwavelength (≈λ/33 000) confinement of strongly enhanced (≈3200) THz field in a nano‐gap (15 nm) exhibits remarkable THz field‐tailored nonlinearity. Further, ultrafast injection of photocarriers reveals a competition between nonlinear THz field‐induced intervalley scattering and optically driven interband excitations. The results on “tera‐nano” metasurfaces enable a novel platform to realize enhanced nonlinear nano/micro composites for field‐sensitive extreme THz nonlinear applications without the need for intense THz light sources.

Extreme terahertz (THz) science and technologies, the next disruptive frontier in nonlinear optics, provide multifaceted capabilities for exploring strong light-matter interactions in a variety of physical systems. However, current techniques involve the need for an extremely high-field free space THz source that is difficult to generate and has limited investigations to a rather weak and linear regime of light-matter interactions. Therefore, new approaches are being sought for the tight confinement of THz waves that can induce nonlinear effects. Here, a nonlinear “tera-nano” metasurface is demonstrated exhibiting extremely large THz nonlinearity and sensitive self-modulation of resonances at moderate incident THz field strengths. A record deep-subwavelength (≈λ/33 000) confinement of strongly enhanced (≈3200) THz field in a nano-gap (15 nm) exhibits remarkable THz field-tailored nonlinearity. Further, ultrafast injection of photocarriers reveals a competition between nonlinear THz field-induced intervalley scattering and optically driven interband excitations. The results on “tera-nano” metasurfaces enable a novel platform to realize enhanced nonlinear nano/micro composites for field-sensitive extreme THz nonlinear applications without the need for intense THz light sources.

In this study, we introduce and experimentally validate, to our knowledge, a new type of terahertz (THz) fiber waveguide. The waveguide features a core made from petroleum jelly (commonly known as Vaseline) and a cladding made of holey polytetrafluoroethylene (PTFE), also known as Teflon. Since the core is biocompatible and the cladding is safe for human use, this design has promising applications for biocompatible probes in the THz range. We rigorously analyzed the transmission properties of the waveguide using the finite element method (FEM) and followed up with experimental validation using a THz time-domain spectroscopy (THz-TDS) system. The fiber supports single-mode operation for frequencies below 0.9 THz and demonstrates low-loss transmission of THz waves, even when tightly bent. For instance, with a bending radius as small as 1.61 cm, the fiber exhibited minimal losses of 0.23 dB/cm at 0.2 THz and 0.27 dB/cm at 0.5 THz, surpassing previous technical limitations. Another key advantage is the strong confinement of the THz waves within the petroleum jelly core, which helps maintain low dispersion and ensures stable pulse transmission, even under tight bends. The exceptional stability and flexibility of this biocompatible THz fiber make it highly suitable for sensing and imaging applications in confined, flexible environments, including potential uses within the human body.

Terahertz (THz) waves laterally confined in a 1 mm-thick microstructured planar waveguide are demonstrated on a free-standing metal rod array (MRA), and one apparent rejection band of a transmission spectrum, resembling the bandgap of a photonic crystal, is found in 0.1-0.6 THz. The visibility of the photonic bandgap in the spectral width and power distinction can be manipulated by changing the MRA geometry parameters, including the rod diameter, the interspace between adjacent rods, and the propagation length based on the interactive MRA-layer number. THz transmission ratio enhanced by a large interactive length is verified in 30 MRA layers due to the longitudinally resonant guidance of transverse-magnetic-polarized waveguide modes along the MRA length, which is critical to the interspace width of adjacent rods and the metal coating of the rod surface. For an MRA with respective rod diameter and interspace dimensions of about 0.16 and 0.26 mm, the highest transmission of the guided resonant THz waves are performed at 0.505-0.512 THz frequency with strong confinement on the metal rod tips and a low scattering loss of 0.003 cm-1.

The broadband and high rate advantages of Terahertz (THz) wave communication attract tremendous attention of many researchers, and the study of the THz sources and detectors are more popular in recent years than previous. The study on the propagation characteristics of THz wave in troposphere is relatively rare, and it is necessary due to the rapid development of the THz devices. The content of water vapor and the oxygen in the tropospheric atmosphere are the main factors that influence the absorption of THz wave because of the resonance of THz wave with the water and oxygen molecules, and as a result, the propagation characteristics of THz wave in different climate zones can be very different. THz wave propagation characteristics in Kashgar, Xinjiang, China are investigated and compared to that of Qingdao herein, and the results show that Kashgar is more suitable for THz earth-space communication owing to its drier atmosphere. The propagation characteristics are analyzed along a path of 14km through troposphere on earth-space links in zenith in Kashgar. The attenuation of 0.14THz wave is as small as 0.48 dB in winter and 1.5 dB in summer, both of the attenuation are much smaller than that of surface attenuation, and the similar results are obtained for 0.22 THz and 0.34THz wave, while all of frequencies used here are window frequencies in atmosphere. The analysis above offers theory basis for the high efficiency communication system. The effect of elevation angle on the tropospheric propagation of THz wave is also calculated and analyzed, while the results show that the bigger the elevation angle, the more suitable for the communication applications.

Integrated sensing platforms, which possess active layers of local electric-field oscillation in the terahertz (THz) frequency, can sensitively detect analytes via electromagnetic (EM) wave guidance, resonance, and spatial confinement in the system of THz time-domain spectroscopy. A Bragg grating structure of periodically perforated metal slits (PPMSs) is presented here as an active layer to generate the local electric-field oscillation of metal-cavity EM wave resonance that is coupled from 0.1–1 THz metal surface waves along the wavelength-scaled segments of metal slabs. The cavity modal confinement property, which is characterized from device transmittance or loss spectra, leads to the distribution of the metal-cavity wave resonance over a 35-pitch length and a 0.1–0.2 mm metal thickness of PPMSs. These two distribution dimensions of metal-cavity wave resonance represent longitudinal and transverse confinement performances, respectively. The PPMS-THz waves specified by 2D spatial confinement can realize the functional integration of a dielectric superstrate component and a sample adsorbing layer to adjust the dispersion responsivity and concentration of analytes, respectively. A refractive-index sensing application of glucose is presented by this integrated scheme for a molecular density range of 0–24 μg mm−2 and a resolution density of 1.6 μg mm−2. The PPMS-based THz integrated sensing platform facilitates the developments of specific test papers and relevant molecular surface modification to target molecules.

We present a tunable notch filter having a wide terahertz (THz) frequency range and a low-pass filter (LPF) having a 0.78 THz cutoff frequency. Single slit and multiple slits are positioned at the center of air gaps in tapered parallel-plate waveguides (TPPWG) to obtain the notch filter and LPF, respectively. The notch filter has a dispersion-free and low-loss transverse magnetic (TM) mode. The Q factor was proved to be 138, and the resonant frequency is easily tunable by adjusting the air gaps between TPPWG. On the other hand, the cut off frequency of the LPF was determined using a Bragg stop band, which depends on slit period. The LPF has a transition width of 68 GHz at the cutoff frequency and a dynamic range of 35 dB at stop bands. In addition, the characteristics of such filters were analyzed using finite-difference time-domain (FDTD) simulations.

This paper studies the interaction effect between the micro-structure photoconductive antenna (PCA) and femtosecond laser, the control of radiated terahertz (THz) wave, and the radiation regulation mechanism of the THz wave for the photoconductive antenna (PCA). The Drude-Lorentz theory model is used to solve the photocurrent density, which is then iterated to the excitation grid by FDTD method. And then the time-varying electromagnetic fields is solved by Maxwell’s equations. The radiated THz wave from the near field to far field in the multi-layer medium is got through the Green's Function of transmission line, and the relationship between the photocurrent and impedance of the radiation is established. At the same time, the relationship between the photocurrent and the magnetic resonance model is also established. The control mechanism of THz wave radiation from micro-structure S-shaped PCA is analyzed by simulation. The results show that the radiation impedance of the equivalent model is changed after split ring resonator (SRR) is introduced into the H-shaped PCA. Meanwhile, it is known that the coupling effect exists when the coupling coefficient is not zero. With the increasing of the coupling coefficient, and the radiation intensity of the resonance frequency peak increases and shifts. The adjusting range of the center frequency between 0.50−0.80 THz, the frequency modulation degree is 75%, and the peak radiation efficiency increases by 70% after simulating the S-shaped PCA. This work lays an important foundation for the design of THz wave resonance center frequency range and structure of high-power PCA.

THz biosensing applications for clinical laboratories: Bottlenecks and strategies