The paper proposes an ultra-narrow band graphene refractive index sensor, consisting of a patterned graphene layer on the top, a dielectric layer of SiO₂ in the middle, and a bottom Au layer. The absorption sensor achieves the absorption efficiency of 99.41% and 99.22% at 5.664 THz and 8.062 THz, with the absorption bandwidths 0.0171 THz and 0.0152 THz, respectively. Compared with noble metal absorbers, our graphene absorber can achieve tunability by adjusting the Fermi level and relaxation time of the graphene layer with the geometry of the absorber unchanged, which greatly saves the manufacturing cost. The results show that the sensor has the properties of polarization-independence and large-angle insensitivity due to the symmetric structure. In addition, the practical application of testing the content of hemoglobin biomolecules was conducted, the frequency of first resonance mode shows a shift of 0.017 THz, and the second resonance mode has a shift of 0.016 THz, demonstrating the good frequency sensitivity of our sensor. The S (sensitivities) of the sensor were calculated at 875 GHz/RIU and 775 GHz/RIU, and quality factors FOM (Figure of Merit) are 26.51 and 18.90, respectively; and the minimum limit of detection is 0.04. By comparing with previous similar sensors, our sensor has better sensing performance, which can be applied to photon detection in the terahertz band, biochemical sensing, and other fields.

We study the low-intensity light pulse propagation through an asymmetric double quantum well via Fano-type interference based on intersubband transitions. The propagation of the pulse across the quantum well is studied analytically and numerically with the coupled Maxwell-Schrödinger equations. We show the generation of ultraslow bright and dark optical solitons in this system. Whether the solitons are dark and bright can be controlled by the ratio of dipole moments of the intersubband transitions. Such investigation of ultraslow optical solitons in the present work may lead to important applications such as high-fidelity optical delay lines and optical buffers in semiconductor quantum wells structure.

Perfect metamaterial absorbers have attracted researchers’ attention in solar energy harvesting and utilization. An ideal solar absorber should provide high absorption, be ultra-wideband, and be insensitive to polarization and incident angles, which brings challenges to research. In this paper, we proposed and optimized an ultra-wideband solar absorber based on Ti-Al₂O₃ cross elliptical disk arrays to obtain the ultra-wideband absorption of solar energy. The addition of a cavity greatly improves the energy-absorbing effect in the operating band, which has research value. The absorption spectrum and field distribution were analyzed by the finite difference time domain method. For the physical mechanism, the electric and magnetic field distribution indicates that ultra-wideband absorption is caused by propagation surface plasmon resonance (SPR), localized SPR and Fabry–Perot (F-P) resonance excited between Ti and Al₂O₃ disks. The results demonstrate that the absorption bandwidth with the absorption rate beyond 90% reaches 1380 nm (385–1765 nm), and the average absorption reaches an astonishing 98.78%. The absorption bandwidth matches the main radiation bandwidth of the solar energy, which is approximately 295–2500 nm according to the data from the literature, and the total thickness of the structure is only 445 nm. Moreover, the ultra-wideband solar absorber is insensitive to the polarization angle and oblique incidence angle. The proposed ultra-wideband solar absorber has research and application value in solar energy harvesting, photothermal conversion and utilization.

We theoretically investigate the simultaneous formation and stable propagation of slow optical soliton pairs in semiconductor quantum dots with a four-level biexciton-exciton cascade configuration. Owing to the destructive interference set up by two continuous wave control fields that couple to a biexciton state, the linear as well as nonlinear dispersion can be dramatically enhanced simultaneously with the absorptions of two weak probe fields being almost suppressed. These results reveal that the detrimental distortions of the two weak-pulsed probe fields due to dispersion effects can be well balanced by the self-phase modulation effect under very low input light intensity, which leads to the slow temporal optical soliton pairs with matched group velocity and amplitude. We also show that the propagation of slow optical solitons can be strongly modified by the biexciton coherence.

Here, the broadband absorber can achieve electromagnetic wave absorption at 18–28 THz and realize multi-layer Fermi energy tunable. In addition, it shows high irrelevance between incident angle and polarization direction.

We show the possibility to generate bright and dark optical solitons based on bound-to-bound intersubband transitions in an asymmetric three-coupled quantum well structure. By presenting the concept of detuning management, we show that the bright optical soliton can evolve into a dark one by adiabatically changing the corresponding one- and two-photon detunings. With adjustable carrier frequencies within the terahertz regime, we also demonstrate numerically shape-recovered collisions of two solitons in our proposed quantum wells. The present investigation may provide research opportunities in nonlinear optical experiments and may have impact on the technology for the design of semiconductor devices.