Ultrafast terahertz emission from emerging symmetry-broken materials

Nonlinear optical spectroscopies are powerful tools for investigating both static material properties and light-induced dynamics.Terahertz(THz)emission spectroscopy has emerged in the past several decades as a versatile method for directly tracking the ultrafast evolution of physical properties,quasiparticle distributions,and order parameters within bulk materials and nanoscale interfaces.Ultrafast optically-induced THz radiation is often analyzed mechanistically in terms of relative contributions from nonlinear polarization,magnetization,and various transient free charge currents.While this offers material-specific insights,more fundamental symmetry considerations enable the generalization of measured nonlinear tensors to much broader classes of systems.We thus frame the present discussion in terms of underlying broken symmetries,which enable THz emission by defining a system directionality in space and/or time,as well as more detailed point group symmetries that determine the nonlinear response tensors.Within this framework,we survey a selection of recent studies that utilize THz emission spectroscopy to uncover basic properties and complex behaviors of emerging materials,including strongly correlated,magnetic,multiferroic,and topological systems.We then turn to low-dimensional systems to explore the role of designer nanoscale structuring and corresponding symmetries that enable or enhance THz emission.This serves as a promising route for probing nanoscale physics and ultrafast light-matter interactions,as well as facilitating advances in integrated THz systems.Furthermore,the interplay between intrinsic and extrinsic material symmetries,in addition to hybrid structuring,may stimulate the discovery of exotic properties and phenomena beyond existing material paradigms.

Making and breaking terahertz waves with fluid plasmas

Vigorous research efforts during the past several decades have successfully closed the"terahertz gap"between microwaves and infrared lig ht,offering new and increasingly efficient ways to produce,detect,and manipulate radiation fields in the terahertz(THz)frequency range.In our laboratory,THz time he itspy IDS)and optical-pumрTHz-its have been routinely utilized as ulltrafast spectrosopy tools to investigate a variety of emerg ing quantum materinls pulses are produced by femtosecond laser excitation of photo-and metamaterials!.The ultrafast THz conductive antennas,semiconductor surfaces(InAs),and non-linear crystals(ZnTe,GaSe.and LiNbO,).Photoconductive an tennas and nonlinear crystals also al low for the coherent de-tection of these pulses in the time domain,with amplitude and phase spectra obtained in the frequency domain via fast Fourier transform.Although such solid-state schemes are highly desirable in many aspects and thus also commonly utilized in many research laboratories and industrial applications,they typically sufer from limited bandwidths due to the absorption and frequency dispersion induced primarily by pho-non resonances,limiting THz applications such as spectroscopрy of emerging materinl,l,imaging ansd detection4.5 and biomedi-cal chuacterization.

Hybrid graphene metasurfaces for highspeed mid-infrared light modulation and single-pixel imaging

During the past decades,major advances have been made in both the generation and detection of infrared light;however,its efficient wavefront manipulation and information processing still encounter great challenges.Efficient and fast optoelectronic modulators and spatial light modulators are required for mid-infrared imaging,sensing,security screening,communication and navigation,to name a few.However,their development remains elusive,and prevailing methods reported so far have suffered from drawbacks that significantly limit their practical applications.In this study,by leveraging graphene and metasurfaces,we demonstrate a high-performance free-space mid-infrared modulator operating at gigahertz speeds,low gate voltage and room temperature.We further pixelate the hybrid graphene metasurface to form a prototype spatial light modulator for high frame rate single-pixel imaging,suggesting orders of magnitude improvement over conventional liquid crystal or micromirror-based spatial light modulators.This work opens up the possibility of exploring wavefront engineering for infrared technologies for which fast temporal and spatial modulations are indispensable.

Semiconductor activated terahertz metamaterials

Metamaterials have been developed as a new class of artificial effective media realizing many exotic phenomena and unique properties not normally found in nature. Metamaterials enable functionality through structure design, facilitating applications by addressing the severe material issues in the terahertz frequency range. Consequently, prototype functional terahertz devices have been demonstrated, including filters, antireflection coatings, perfect absorbers, polarization converters, and arbitrary wavefront shaping devices. Further integration of functional materials into metamaterial structures have enabled actively and dynamically switchable and frequency tunable terahertz metamaterials through the application of external stimuli. The enhanced light-matter interactions in active terahertz metamaterials may result in unprecedented control and manipulation of terahertz radiation, forming the foundation of many terahertz applications. In this paper, we review the progress during the past few years in this rapidly growing research field. We particularly focus on the design principles and realization of functionalities using single-layer and fewlayer terahertz planar metamaterials, and active terahertz metamaterials through the integration of semiconductors to achieve switchable and frequency-tunable response.