Optical generation,detection and non-destructive testing applications of terahertz waves

Optoelectronic terahertz generation and detection play a key role in the applications of non-destructive testing,which involves different areas such as physics,biological,material science,imaging,explosions detection,astronomy applications,semiconductor technology and superconductiong electronics. In this article,we present a reviewof the principle and performance of typical terahertz sources,detectors and non-destructive testing applications. On this basis,the newdevelopment and trends of terahertz radiation detectors are also discussed.

Hybrid bound states in the continuum in terahertz metasurfaces

Bound states in the continuum(BICs)have exhibited extraordinary properties in photonics for enhanced light-matter interactions that enable appealing applications in nonlinear optics,biosensors,and ultrafast optical switches.The most common strategy to apply BICs in a metasurface is by breaking symmetry of resonators in the uniform array that leaks the otherwise uncoupled mode to free space and exhibits an inverse quadratic relationship between quality factor(Q)and asymmetry.Here,we propose a scheme to further reduce scattering losses and improve the robustness of symmetry-protected BICs by decreasing the radiation density with a hybrid BIC lattice.We observe a significant increase of radiative Q in the hybrid lattice compared to the uniform lattice with a factor larger than 14.6.In the hybrid BIC lattice,modes are transferred toГpoint inherited from high symmetric X,Y,and M points in the Brillouin zone that reveal as multiple Fano resonances in the far field and would find applications in hyperspectral sensing.This work initiates a novel and generalized path toward reducing scattering losses and improving the robustness of BICs in terms of lattice engineering that would release the rigid requirements of fabrication accuracy and benefit applications of photonics and optoelectronic devices.

Terahertz spectral properties of melamine and its deuterated isotope,melamine-d_6

The far-infrared optical properties of melamine and its deuterated isotope,melamine-d_6 were experimentally and theoretically investigated in the frequency range from 0.2 to 3.0 THz.Under the room temperature and dry air nitrogen conditions,three absorption bands were observed at 2.0,2.3 and 2.6 THz in the melamine sample by use of terahertz time-domain spectroscopy.Whereas,in the melamine-d_6 sample,the observed absorption bands shift towards lower frequencies and the relative intensity of the absorption bands reduces.Numerical simulation results based on the Parameterized Model number 3(PM3) were compared with the experimental data and the observed vibration spectra were assigned according to the PM3 calculations.The absorption bands of the measured melamine samples at terahertz frequencies are highly correlated with the intermolecular hydrogen bond stretching and π-π stacking vibration.Also,the red shift of the absorption bands is due to hydrogen/deuterium substitution.

Effect of terahertz pulse on gene expression in human eye cells

In recent years, the advances in terahertz applications have stimulated interest in the biological effects associated with this frequency range. We study the gene expression profile in three types of cells exposed to terahertz radiation,i.e., human ARPE-19 retinal pigment epithelial cells, simian virus 40-transformed human corneal epithelial cells, and human MIO-M1 Müller cells. We find that the gene expression in response to heat shock is unaffected, indicating that the minimum temperature increases under controlled environment. The transcriptome sequencing survey demonstrates that 6-hour irradiation with a broadband terahertz source results in specific change in gene expression and also the biological functions that are closely related to these genes. Our results imply that the effect of terahertz radiation on gene expression can last over 15 hours and depends on the type of cell.

Non-thermal effects of 0.1 THz radiation on intestinal alkaline phosphatase activity and conformation

Alkaline phosphatase(ALP) plays an integral role in the metabolism of liver and development of the skeleton in humans. To date, the interactions between different-duration terahertz(THz) radiation and ALP activities, as well as the influence mechanism are still unclear. In this study, using the para-nitro-phenyl-phosphate(p NPP) method, we detect changes in ALP activities during 40-minute THz radiation(0.1 THz, 13 m W/cm^2). It is found that the activity of ALP decreases in the first 25 min, and subsequently increases in the later 15 min. Compared with the activity of ALP being heated, the results suggest that short-term terahertz radiation induces a decrease in enzyme activity through the non-thermal mechanism. In order to explore the non-thermal effects of THz radiation on ALP, we focus on the impacts of 0.1 THz radiation for 20 min on the activity of ALP in different concentrations. The results reveal that the activity of ALP decreases significantly after exposure to THz radiation. In addition, it could be deduced from fluorescence, ultraviolet-visible(UV-vis), and THz spectra results that THz radiation has induced changes in ALP structures. Our study unlocks non-thermal interactions between THz radiation and ALP, as well as suggests that THz spectroscopy is a promising technique to distinguish ALP structures.

Active-Thermal-Tunable Terahertz Absorber with Temperature-Sensitive Material Thin Film

It is shown that active-tunable terahertz absorbers can be realized in a sandwich-structured system comprising an ultrathin dielectric film(polyimide) on a temperature-sensitive substrate(InSb) with a metal film on the back by utilizing the intrinsic carrier density(N) variation in InSb. When increasing the temperature from 250 to 320 K, N in InSb varied from ～5.50×1015 to ～2.98×1016 cm–3. Fixing the thickness of dielectric film with the value of 1.37 μm, the absorption peak shifted from 1.41 to 3.29 THz while keeping absorption higher than 99%. This active tunability can respond to even a slight temperature perturbation, and shows polarization insensitivity as well as high tolerance of incidence-angle(absorption peak can still exceed 90% even the incidence angle reaches 60°). Besides, the refractive index of polyimide(PI) has thermal stability at the terahertz range and the merit of good workability. These characteristics guarantee the stability of activetunable performance. The peculiarities and innovations of this proposal promise a wide range of high efficiency terahertz devices, such as thermal sensors, spatial light modulators(SLMs) and so on.

Rabi Oscillations and Coherence Dynamics in Terahertz Streaking-Assisted Photoelectron Spectrum

We present an approach,a Terahertz streaking-assisted photoelectron spectrum(THz SAPS),to achieve direct observations of ultrafast coherence dynamics with timescales beyond the pulse duration.Using a 24 fs probe pulse,the THz SAPS enables us to well visualize Rabi oscillations of 11.76 fs and quantum beats of 2.62 fs between the 5S_(1/2) and 5P_(3/2) in rubidium atoms.The numerical results show that the THz SAPS can simultaneously achieve high resolution in both frequency and time domains without the limitation of Heisenberg uncertainty of the probe pulse.The long probe pulse promises sufficiently high frequency resolution in photoelectron spectroscopy allowing to observe Autler-Townes splittings,whereas the streaking THz field enhances temporal resolution for not only Rabi oscillations but also quantum beats between the ground and excited states.The THz SAPS demonstrates a potential applicability for observation and manipulation of ultrafast coherence processes in frequency and time domains.

Terahertz Metamaterial Sensor Based on Electromagnetically Induced Transparency Effect

A terahertz metamaterial sensor adopting the metamaterial-based electromagnetically induced transparency（EIT） effect is presented for determining the 1,4-dioxane concentration in its aqueous solution. The metamaterial sensor, which consists of an EIT element unit with a cut-wire metallic resonator and two split-ring metallic resonators fabricated on a 490-μm thick silicon substrate, operates in a transmission geometry. The EIT peak was red-shifted and decreased with the increase of the water volume. A maximum redshift about 54 GHz of the EIT peak was detected between the 1,4-dioxane and water. The presented linear behavior and high sensitivity of the EIT peak depending on the water concentration pave a novel avenue for sensor applications.

Electrically tunable resonant terahertz transmission in subwavelength hole arrays

An actively enhanced resonant transmission in a plasmonic array of subwavelength holes is demonstrated by use of terahertz time-domain spectroscopy. By connecting this two-dimensional element into an electrical circuit, tunable resonance enhancement is observed in arrays made from good and relatively poor metals. The tunable feature is attributed to the nonlinear electric response of the periodic hole array film, which is confirmed by its voltage-current behavior. This finding could lead to a unique route to active plasmonic devices, such as tunable filters, spatial modulators, and integrated terahertz optoelectronic components.

Polarization-independent terahertz wave modulator based on graphene-silicon hybrid structure

In this study, we propose and demonstrate a broadband polarization-independent terahertz modulator based on graphene/silicon hybrid structure through a combination of continuous wave optical illumination and electrical gating.Under a pump power of 400 mW and the voltages ranging from-1.8 V to 1.4 V, modulation depths in a range of-23%–62% are achieved in a frequency range from 0.25 THz to 0.65 THz. The modulator is also found to have a transition from unidirectional modulation to bidirectional modulation with the increase of pump power. Combining the Raman spectra and Schottky current–voltage characteristics of the device, it is found that the large amplitude modulation is ascribed to the electric-field controlled carrier concentration in silicon with assistance of the graphene electrode and Schottky junction.

Recent progress in graphene terahertz modulators

Graphene has been recognized as a promising candidate in developing tunable terahertz(THz)functional devices due to its excellent optical and electronic properties,such as high carrier mobility and tunable conductivity.Here,we review graphene-based THz modulators we have recently developed.First,the optical properties of graphene are discussed.Then,graphene THz modulators realized by different methods,such as gate voltage,optical pump,and nonlinear response of graphene are presented.Finally,challenges and prospective of graphene THz modulators are also discussed.

New assembly route for three-dimensional metamaterials obtained through effective medium theory

In this study, we illustrate the effective medium theories in the designs of three-dimensional composite metama- terials of both negative permittivity and negative permeability. The proposed metamaterial consists of random coated spheres with sizes smaller compared to the wavelength embedded in a dielectric host. Simple design rules and formulas following the effective medium models are numerically and analytically presented. We demonstrate that the revised Maxwell-Garnett effective medium theory enables us to design three-dimensional composite metamaterials through the assembly of coated spheres which are random and much smaller than the wavelength of the light. The proposed ap- proach allows for the precise control of the permittivity and the permeability and guides a facile, flexible, and versatile way for the fabrication of composite metamaterials.

Momentum Spectroscopy for Multiple Ionization of Cold Rubidium in the Elliptically Polarized Laser Field

Employing recently developed magneto-optical trap recoil ion momentum spectroscopy(MOTRIMS)combined with cold atoms,strong laser pulse,and ultrafast technologies,we study momentum distributions of the multiply ionized cold rubidium(Rb)induced by the elliptically polarized laser pulses(35 fs,1.3×10^15 W/cm^2).The complete vector momenta of Rb^n+ions up to charge state n=4 are recorded with extremely high resolution(0.12 a.u.for Rb^+).Variations of characteristic multi-bands are displayed in momentum distributions because the ellipticity varies from the linear to circular polarization,are interpreted qualitatively with the classical overbarrier ionization model.Present momentum spectroscopy of cold heavy alkali atoms presents novel strong-field phenomena beyond the noble gases.

Active terahertz beam steering based on mechanical deformation of liquid crystal elastomer metasurface

Active metasurfaces are emerging as the core of next-generation optical devices with their tunable optical responses and flat-compact topography.Especially for the terahertz band,active metasurfaces have been developed as fascinating devices for optical chopping and compressive sensing imaging.However,performance regulation by changing the dielectric parameters of the integrated functional materials exhibits severe limitations and parasitic losses.Here,we introduce a C-shape-split-ring-based phase discontinuity metasurface with liquid crystal elastomer as the substrate for infrared modulation of terahertz wavefront.Line-focused infrared light is applied to manipulate the deflection of the liquid crystal elastomer substrate,enabling controllable and broadband wavefront steering with a maximum output angle change of 22°at 0.68THz.Heating as another control method is also investigated and compared with infrared control.We further demonstrate the performance of liquid crystal elastomer metasurface as a beam steerer,frequency modulator,and tunable beam splitter,which are highly desired in terahertz wireless communication and imaging systems.The proposed scheme demonstrates the promising prospects of mechanically deformable metasurfaces,thereby paving the path for the development of reconfigurable metasurfaces.

Tailoring spatiotemporal dynamics of plasmonic vortices

Plasmonic vortices confining orbital angular momentums to surface have aroused wide research interest in the last decade.Recent advances of near-field microscopes have enabled the study on the spatiotemporal dynamics of plasmonic vortices,providing a better understanding of optical orbital angular momentums in the evanescent wave regime.However,these works only focused on the objective characterization of plasmonic vortex and have not achieved subjectively tailoring of its spatiotemporal dynamics for specific applications.Herein,it is demonstrated that the plasmonic vortices with the same topological charge can be endowed with distinct spatiotemporal dynamics by simply changing the coupler design.Based on a near-field scanning terahertz microscopy,the surface plasmon fields are directly obtained with ultrahigh spatiotemporal resolution,experimentally exhibiting the generation and evolution divergences during the whole lifetime of plasmonic vortices.The proposed strategy is straightforward and universal,which can be readily applied into visible or infrared frequencies,facilitating the development of plasmonic vortex related researches and applications.

Interaction of colliding laser pulses with gas plasma for broadband coherent terahertz wave generation

Colliding of two counter-propagating laser pulses is a widely used approach to create a laser field or intensity surge.We experimentally demonstrate broadband coherent terahertz(THz)radiation generation through the interaction of colliding laser pulses with gas plasma.The THz radiation has a dipole-like emission pattern perpendicular to the laser propagation direction with a detected peak electric field 1 order of magnitude higher than that by single pulse excitation.As a proof-of-concept demonstration,it provides a deep insight into the physical picture of laser–plasma interaction,exploits an important option to the promising plasma-based THz source,and may find more applications in THz nonlinear near-field imaging and spectroscopy.

Anisotropic coding metamaterials and their powerful manipulation of differently polarized terahertz waves

Metamaterials based on effective media can be used to produce a number of unusual physical properties(for example,negative refraction and invisibility cloaking)because they can be tailored with effective medium parameters that do not occur in nature.Recently,the use of coding metamaterials has been suggested for the control of electromagnetic waves through the design of coding sequences using digital elements‘0’and‘1,'which possess opposite phase responses.Here we propose the concept of an anisotropic coding metamaterial in which the coding behaviors in different directions are dependent on the polarization status of the electromagnetic waves.We experimentally demonstrate an ultrathin and flexible polarization-controlled anisotropic coding metasurface that functions in the terahertz regime using specially designed coding elements.By encoding the elements with elaborately designed coding sequences(both 1-bit and 2-bit sequences),the x-and y-polarized waves can be anomalously reflected or independently diffused in three dimensions.The simulated far-field scattering patterns and near-field distributions are presented to illustrate the dual-functional performance of the encoded metasurface,and the results are consistent with the measured results.We further demonstrate the ability of the anisotropic coding metasurfaces to generate a beam splitter and realize simultaneous anomalous reflections and polarization conversions,thus providing powerful control of differently polarized electromagnetic waves.The proposed method enables versatile beam behaviors under orthogonal polarizations using a single metasurface and has the potential for use in the development of interesting terahertz devices.

Reflective chiral meta-holography: multiplexing holograms for circularly polarized waves

By allowing almost arbitrary distributions of amplitude and phase of electromagnetic waves to be generated by a layer of sub-wavelength-size unit cells,metasurfaces have given rise to the field of meta-holography.However,holography with circularly polarized waves remains complicated as the achiral building blocks of existing meta-holograms inevitably contribute to holographic images generated by both left-handed and right-handed waves.Here we demonstrate how planar chirality enables the fully independent realization of high-efficiency meta-holograms for one circular polarization or the other.Such circular-polarization-selective meta-holograms are based on chiral building blocks that reflect either left-handed or right-handed circularly polarized waves with an orientation-dependent phase.Using terahertz waves,we experimentally demonstrate that this allows the straightforward design of reflective phase meta-holograms,where the use of alternating structures of opposite handedness yields independent holographic images for circularly polarized waves of opposite handedness with negligible polarization cross-talk.

Terahertz surface plasmonic waves: a review

Terahertz science and technology promise many cutting-edge applications.Terahertz surface plasmonic waves that propagate at metal–dielectric interfaces deliver a potentially effective way to realize integrated terahertz devices and systems.Previous concerns regarding terahertz surface plasmonic waves have been based on their highly delocalized feature.However,recent advances in plasmonics indicate that the confinement of terahertz surface plasmonic waves,as well as their propagating behaviors,can be engineered by designing the surface environments,shapes,structures,materials,etc.,enabling a unique and fascinating regime of plasmonic waves.Together with the essential spectral property of terahertz radiation,as well as the increasingly developed materials,microfabrication,and time-domain spectroscopy technologies,devices and systems based on terahertz surface plasmonic waves may pave the way toward highly integrated platforms for multifunctional operation,implementation,and processing of terahertz waves in both fundamental science and practical applications.We present a review on terahertz surface plasmonic waves on various types of supports in a sequence of properties,excitation and detection,and applications.The current research trend and outlook of possible research directions for terahertz surface plasmonic waves are also outlined.

Mechanically reprogrammable Pancharatnam-Berry metasurface for microwaves

Metasurfaces have enabled the realization of several optical functionalities over an ultrathin platform,fostering the exciting field of flat optics.Traditional metasurfaces are achieved by arranging a layout of static meta-atoms to imprint a desired operation on the impinging wavefront,but their functionality cannot be altered.Reconfigurability and programmability of metasurfaces are the next important step to broaden their impact,adding customized on-demand functionality in which each meta-atom can be individually reprogrammed.We demonstrate a mechanical metasurface platform with controllable rotation at the meta-atom level,which can implement continuous Pancharatnam–Berry phase control of circularly polarized microwaves.As the proof-of-concept experiments,we demonstrate metalensing,focused vortex beam generation,and holographic imaging in the same metasurface template,exhibiting versatility and superior performance.Such dynamic control of electromagnetic waves using a single,low-cost metasurface paves an avenue towards practical applications,driving the field of reprogrammable intelligent metasurfaces for a variety of applications.