Omnidirectional and near-unity nonreciprocal thermal radiation with trilayer cavities-enhanced approach.

From the standpoint of thermal radiation, omnidirectional nonreciprocal thermal radiation (NTR) is strongly desired for thermal energy harvesting. Here, we propose theoretically lithographic free thermal emitter made in a dielectric-Weyl semimetal (WSM)-dielectric fashion and terminated by a metallic substrate. By engineering the structural parameters, a surprising result of spectrally selective as well as omnidirectional (along both polar and azimuthal angles) NTR is realized. It is shown that the magnitude and sign of the contrast between emission (e) and absorption (α) can be managed simultaneously. The suggested structure shows good nonreciprocity stability in a wide range of polar and azimuthal angles for transverse magnetic (TM) polarized incident wave. The ability to fine tune nonreciprocal radiative properties of our design suggests a relatively simple way to manifest the NTR with high performance, which could lead to the development of power scavenging and conversion devices.

Weyl semimetal/dielectric/Weyl semimetal stack for highly circularly polarized thermal radiation.

Highly circularly polarized (CP) infrared thermal radiation is greatly in demand because of its significant potential in mid-infrared (mid-IR) applications. To exploit the magnitude and quality factor of circular dichroism (CD) simultaneously, a lithography-free platform consisting of a Weyl semimetal (WSM)/dielectric (Ge)/WSM stack sitting on a metallic substrate (Mo) is proposed. A chiral response and varying CD values from -1 to 0.957 have been demonstrated. The numerical results from a generalized 4 × 4 transfer matrix algorithm verify that the chiral structure manifests a remarkably high quality factor (Q-factor) of 605. The effect of the thickness of each layer in the stack on the CD value is investigated. Moreover, it is identified that the design results in an angle-independent performance. A dual-channel chiral absorber operating in the 18-21 µm wavelength range has also been achieved. Our simple yet powerful paradigm could offer a new way of manipulating the Q-factor and resonance wavelength of a chiral absorber while maintaining near-unity CD, offering a new approach for the advancement of more efficient and tunable chiral optical devices. The approach is generally applicable to other planar configurations with different WSMs and can be extended to other wavelengths.

Weyl semimetal mediated epsilon-near-zero hybrid polaritons and the induced nonreciprocal radiation.

Polaritonic excitation and management in ultra-thin polar crystals has recently received significant attention and holds new promise for epsilon-near-zero (ENZ) modes. However, manipulation of the ENZ mode via anisotropic magneto-optic (MO) material remains elusive. Herein, we provide an effective strategy for constructing an ENZ polar thin film with dependence on Weyl semimetals (WSM). The thermal radiation of the proposed device is explored with electromagnetic (EM) simulations that utilize the anisotropic rigorous coupled-wave analysis (aRCWA) method. Strong coupling of the ENZ mode to WSM polaritons has been demonstrated, and the structural parameters hold tolerance on the order of hundreds of nanometers, which is highly favorable for low-cost fabrication and high-performance application. By changing both the azimuthal angle (ϕ) and angle of incidence (θ), the nonreciprocity (η) can be effectively influenced. The distribution of η is symmetrical with ϕ = 180°, η = 0 when ϕ = 90° and ϕ = 270°. The mechanism of this proposal is owing to the hybrid polaritons supported by the polar thin film and nonreciprocal radiation of WSM, which is validated by examining the amplitude distribution of the magnetic field. The nonreciprocal emitter described herein allows simultaneous control of spectral distribution and polarization of radiation, which will facilitate the active design and application of mid-infrared (MIR) thermal emitters.

Heterodyne Doppler velocity measurement of moving targets by mode-locked pulse laser.

In this study, heterodyne detection is adopted to measure the velocity of a target simulated by a rapidly rotating plate by using a mode-locked pulse laser as the resource. The coherent beat frequency of the signal light reflected by target and local oscillation light occurred on the surface of the detector. Then the waveform of beat frequency was processed by filtering to obtain the Doppler frequency shift of the signal light induced by target. With this frequency shift, the velocity of target could be obtained by calculation. Results indicate that the measurement has a high precision. The error on average is within 0.4 m/s.

Passive frequency stabilization in Nd:YAG pulse laser using reflective volume Bragg grating.

We demonstrate a stable Q-switched single-longitudinal-mode (SLM) Nd:YAG laser using a volume Bragg grating as the output coupler. The reflective volume Bragg grating, serving as a longitudinal selector and passive frequency stabilizer, effectively eliminates the mode hopping effect of the laser. The maximum output energy of the SLM obtained from the current experimental setup is 18.5 mJ. The maximum separation of frequencies is significantly less than the longitudinal mode separation, indicating that a stable SLM laser is achieved.

Research on heterodyne detection of a mode-locked pulse laser based on an acousto-optic frequency shift.

Heterodyne detection research is carried out on a mode-locked pulse laser using an acousto-optic frequency shifter (AOFS). Theoretical calculation and numerical simulation of the beat frequency of the mode-locked pulse were carried out, and the results show that the frequency offset can be calculated according to the beat period of the pulse waveform, and the offset is the frequency difference between the two adjacent longitudinal modes. Experiments were implemented with a self-built mode-locked laser using an AOFS to simulate the frequency shift of the detection laser induced by the object, the coherent beat frequency of the signal light and local oscillation light that were observed on the surface of the detector. The beat frequency wave was processed by filtering to obtain the frequency shift of the signal light. The experimental results are in good agreement with the numerical simulation.