Xiaoguang Zhao

Xiaoguang Zhao
Tsinghua University | TH · Department of Precision Instruments and Mechanical Engineering

About

78
Publications
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1,172
Citations

Publications

Publications (78)
Article
Full-text available
Radiofrequency identification (RFID), particularly passive RFID, is extensively employed in industrial applications to track and trace products, assets, and material flows. The ongoing trend toward increasingly miniaturized RFID sensor tags is likely to continue as technology advances, although miniaturization presents a challenge with regard to th...
Article
Full-text available
Auxetics refers to structures or materials with a negative Poisson's ratio, thereby capable of exhibiting counter-intuitive behaviors. Herein, auxetic structures are exploited to design mechanically tunable metamaterials in both planar and hemispherical configurations operating at megahertz (MHz) frequencies, optimized for their application to magn...
Article
Full-text available
This cover image shows the reconfigurable terahertz metamaterials where artificial toroidal metamolecules and traditional MEMS bi-material cantilever structures are integrated within the same unit cell. This enables enhancement of the toroidal dipole response through reorientation of the metamolecules by thermal actuation. These results, presented...
Article
Compared with the traditional electric and magnetic multipoles, the existence of a dynamic toroidal moment has received increasing interest in recent years. This is due to its novel electromagnetic response, including dynamic non-radiating charge-current configurations and non-reciprocal interactions. Reconfigurable terahertz metamaterials where ar...
Preprint
Auxetics refers to structures or materials with a negative Poisson's ratio, thereby capable of exhibiting counter-intuitive behaviors. Herein, auxetic structures are exploited to design mechanically tunable metamaterials in both planar and hemispherical configurations operating at megahertz (MHz) frequencies, optimized for their application to magn...
Article
The resonant modes and their coupling effect of metamaterials is a rapidly growing research topic with potential for applications in optoelectronics. In this work, strong absorption coupling between the spoof surface plasmon polariton (SSPP) modes and the intrinsic modes have been investigated in a novel terahertz metamaterials. The anti-crossing e...
Article
In this study, we used two-photon polymerization 3D printing technology to successfully print the first true pore-scale rock proxy of Berea sandstone with a submicrometer resolution. Scanning electron microscope (SEM) and computed tomography (CT) images of the 3D-printed sample were compared with the digital file used for printing to verify the roc...
Conference Paper
We utilize a periodic lattice of elliptical holes in a thin silicon membrane to demonstrate an optically tunable broadband terahertz metasurface absorber with a bandwidth of ~500 GHz that achieves >90% absorption.
Article
The concept of "bound states in the continuum"(BIC) describes an idealized physical system exhibiting zero radiative loss composed, for example, of an infinitely extended array of resonators. In principle, vanishing of radiative losses enables an infinitely high-quality factor and corresponding infinite lifetime of the resonance. As such, BIC inspi...
Article
Full-text available
We present a polarization-insensitive air-spaced triple-band metamaterial perfect absorber (MMPA), consisting of a metamaterial layer and metallic ground plane operating at terahertz frequencies. Three near-unity absorption peaks can be individually determined by the geometry of the ring resonators within one unit cell, since the inter-unit-cell co...
Article
Full-text available
Breaking Lorentz reciprocity is fundamental to an array of functional radiofrequency (RF) and optical devices, such as isolators and circulators. The application of external excitation, such as magnetic fields and spatial–temporal modulation, has been employed to achieve nonreciprocal responses. Alternatively, nonlinear effects may also be employed...
Cover Page
Full-text available
In article number 2001443, Stephan W. Anderson, Xin Zhang, and co‐workers demonstrate a nonreciprocal system based on the linear and nonlinear meta‐atoms. Theoretical studies and experiments reveal the bounds of the nonreciprocity based on the passive and nonlinear resonators. The paradigm may be employed to construct magnetless radiofrequency isol...
Article
Full-text available
Detecting low energy photons, such as photons in the long-wave infrared range, is a technically challenging proposition using naturally occurring materials. In order to address this challenge, we herein demonstrate a micro-bolometer featuring an integrated metamaterial absorber (MA), which takes advantage of the resonant absorption and frequency se...
Article
Full-text available
Devices designed to dynamically control the transmission, reflection, and absorption of terahertz (THz) radiation are essential for the development of a broad range of THz technologies. A viable approach utilizes materials which undergo an insulator‐to‐metal transition (IMT), switching from transmissive to reflective upon becoming metallic. However...
Article
Full-text available
Metamaterials provide a powerful platform to probe and enhance nonlinear responses in physical systems toward myriad applications. Herein, the development of a coupled nonlinear metamaterial (NLMM) featuring a self‐adaptive response that selectively amplifies the magnetic field is reported. The resonance of the NLMM is suppressed in response to hig...
Cover Page
Full-text available
In article number 1905461, Stephan W. Anderson, Xin Zhang, and co‐workers present a coupled nonlinear metamaterial featuring a self‐adaptive response that selectively amplifies the magnetic field depending on the radio‐frequency excitation energy. The nonlinearity in metamaterials is harnessed to achieve in excess of a 10‐fold enhancement in the si...
Article
The effects of large external fields on semiconductor nanostructures could reveal much about field-induced shifting of electronic states and their dynamical responses, and could enable electro-optic device applications that require large and rapid changes in optical properties. Studies of quasi-DC electric field modulation of quantum dot (QD) prope...
Conference Paper
Full-text available
We present a single-layer H-shaped all-silicon array exhibiting tunable ultra-broadband terahertz wave absorption. Experiment and simulation reveal near unity absorption at ~1 THz, with a bandwidth of ~913 GHz for ≥90% absorbance. The absorption is optically tunable, exhibiting a resonance frequency blueshift by 420 GHz, while the peak absorbance r...
Article
Terahertz spectroscopy of the c‐axis Josephson plasma resonance (JPR) in high‐temperature cuprates is a powerful probe of superconductivity, providing a route to couple to and interact with the condensate. Electromagnetic coupling between metasurface arrays of split ring resonators (SRRs) and the JPR of a La2−xSrxCuO4 single crystal (Tc = 32 K) is...
Article
Manipulating the phase of electromagnetic radiation is of importance for applications ranging from communication to imaging. Here, real-time reconfigurable phase response and group delay of a tunable terahertz metamaterial consisting of dual-layer broadside coupled split-ring resonators is demonstrated. Utilizing electrostatic comb-drive actuators,...
Conference Paper
Metamaterials, artificially engineered materials with extraordinary and controllable effective properties, have expedited the development of photonic and optical devices. The electric permittivity and magnetic permeability of metamaterials can be tailored efficiently by changing the structural design of metamaterials. Recent research has mainly foc...
Conference Paper
Metamaterials and metamaterial absorbers have been widely reported to be used in sensing and detection applications. Among them, metamaterial absorber is superior by its high-quality factor, and its high sensitivity to the change of the surrounding environment. However, the conventional metamaterials and metamaterial absorbers are configured with r...
Conference Paper
Full-text available
In this paper, we present a polarization insensitive air-spacer triple band absorber. With the fabricated absorber, the three near unity-absorption peaks can be fine controlled with different structure designs, which have a great advantage in variety applications. Also, with air instead of the traditional dielectric layer, the absorber will exhibit...
Conference Paper
Metamaterials, artificially engineered materials with extraordinary and controllable effective properties, have expedited the development of photonic and optical devices. The electric permittivity and magnetic permeability of metamaterials can be tailored efficiently by changing the structural design of metamaterials. Recent research has mainly foc...
Article
Full-text available
Diatoms are photosynthetic algae that exist ubiquitously throughout the planet in water environments. Over the preceding decades, the diatom exoskeletons, termed frustules, featuring abundant micro‐ and nanopores, have served as the source material and inspiration for myriad research efforts. In this work, it is demonstrated that frustule‐inspired...
Cover Page
Full-text available
In article number 1809029, Xin Zhang and co‐workers use a hierarchical hexagonal design of nanoresonators to realize a near‐infrared metamaterial absorber, inspired by the hierarchical design of diatom frustule pores. With this method, an additional degree of freedom (hierarchy) has been introduced into the design of electromagnetic metamaterials....
Article
In this letter, we report optical pump terahertz (THz) near-field probe (n-OPTP) and optical pump THz near-field emission (n-OPTE) experiments of graphene/InAs heterostructures. Near-field imaging contrasts between graphene and InAs using these newly developed techniques as well as spectrally integrated THz nano-imaging (THz s-SNOM) are systematica...
Article
Full-text available
Magnetic resonance imaging (MRI) represents a mainstay among the diagnostic imaging tools in modern healthcare. Signal-to-noise ratio (SNR) represents a fundamental performance metric of MRI, the improvement of which may be translated into increased image resolution or decreased scan time. Recently, efforts towards the application of metamaterials...
Article
In the past few decades, electromagnetic metamaterial absorbers have attracted tremendous attention due to near unity absorption of incident electromagnetic waves over a desired frequency range determined by the metamaterial inclusions as opposed to the constituent material properties. Importantly, metamaterial absorbers enable numerous potential a...
Article
Full-text available
Electromagnetic metamaterials, which are a major type of artificially engineered materials, have boosted the development of optical and photonic devices due to their unprecedented and controllable effective properties, including electric permittivity and magnetic permeability. Metamaterials consist of arrays of subwavelength unit cells, which are a...
Article
Terahertz perfect absorbers represent an essential photonic component for detecting, modulating and manipulating terahertz radiation. We utilize single-layer H-shaped all-silicon arrays to demonstrate tunable ultra-broadband terahertz wave absorption. Experiment and simulation reveal near unity absorption at 1 THz, with a bandwidth of ~913 GHz for...
Article
Full-text available
A typical metamaterial perfect absorber (MPA) is comprised of a metamaterial layer, a dielectric spacer, and a ground plane. The conventional spacer material is usually a lossy dielectric with little-dispersion for the purpose of easing the design and optimization procedure of the MPA. In this paper, we present the design, fabrication, and characte...
Article
Full-text available
Dynamic fracture of borosilicate glass through focusing of high-amplitude nanosecond surface acoustic waves (SAWs) at the micron scale is investigated in an all-optical experiment. SAWs are generated by a picosecond laser excitation pulse focused into a ring-shaped spot on the sample surface. Interferometric images capture the SAW as it converges t...
Cover Page
In article number 1801822, Xin Zhang and co‐workers employ a vapor‐liquid‐solid method to incorporate silicon dioxide nanowires on frustules, the exoskeleton structures from diatoms. This process features preferential nanowire growth on frustules, which is thermodynamically analyzed. Compared to the original frustule structures, the frustule‐nanowi...
Article
Diatom frustules are a type of porous silicon dioxide microparticle that has long been used in applications ranging from biomedical sensors to dye‐sensitized solar cells. The favorable material properties, enormous surface area, and enhanced light scattering capacity support the promise of diatom frustules as candidates for next generation biomedic...
Article
Metamaterial absorbers are typically comprised of a layer of split-ring resonators and a ground plane with a dielectric spacer layer that provides structural support and in which absorbed energy is deposited. We address the question “What happens to the absorption if the spacer layer is removed?” through the design, fabrication, and characterizatio...
Article
We present a comprehensive investigation of a continuously tunable metamaterial perfect absorber operating at terahertz frequencies. In particular, we investigate a three-layer absorber structure consisting of a layer of split ring resonators and a metallic ground plane, with a central layer consisting of a mechanically tunable air-spaced layer. Th...
Article
Dynamic polarization control of light is essential for numerous applications ranging from enhanced imaging to materials characterization and identification. We present a reconfigurable terahertz metasurface quarter-waveplate consisting of electromechanically actuated micro-cantilever arrays. Our anisotropic metasurface enables tunable polarization...
Article
Full-text available
Metamaterial absorbers typically consist of a metamaterial layer, a dielectric spacer layer, and a metallic ground plane. We have investigated the dependence of the metamaterial absorption maxima on the spacer layer thickness and the reflection coefficient of the metamaterial layer obtained in the absence of the ground plane layer. Specifically, we...
Article
Full-text available
The terahertz (THz) dielectric properties of super-aligned multi-walled carbon nanotube (MWCNT) films were characterized in the frequency range from 0.1 to 2.5 THz with terahertz time-domain spectroscopy. The refractive index, effective permittivity, and conductivity were retrieved from the measured transmission spectra with THz incident wave polar...
Article
We present a detailed analysis of the conditions that result in unity absorption in metamaterial absorbers to guide the design and optimization of this important class of functional electromagnetic composites. Multilayer absorbers consisting of a metamaterial layer, dielectric spacer, and ground plane are specifically considered. Using interference...
Conference Paper
A miniaturized double layered antenna (0.74 by 0.74 mm) was fabricated and integrated with a commercialized passive RFID chip [1] to form a microsensor to work with a highly efficient magnetically coupled reader antenna for wireless charging and communication. Two techniques were developed to boost the magnetic field strength more than 100 times pa...
Article
Full-text available
Unique optical properties of colloidal semiconductor quantum dots (QDs), arising from quantum mechanical confinement of charge within these structures, present a versatile testbed for the study of how high electric fields affect the electronic structure of nanostructured solids. Earlier studies of quasi-DC electric field modulation of QD properties...
Article
We present a three-dimensional terahertz metamaterial perfect absorber (MPA) that exhibits a high quality factor and is polarization insensitive. The unit cell is composed of two orthogonally oriented copper stand-up split ring resonators deposited on a copper ground plane with capacitive gaps in free space away from the substrate. Near unity (99.6...
Article
A double layer spiral antenna with side length of 740 μm was fabricated by a multilayer electroplating process and bonded with an radio frequency identification chip by silver epoxy to form a microsensor chip. A theoretical power transfer model was built to optimize the power transfer efficiency. The resonant frequency of the microsensor was charac...
Conference Paper
In this paper, we present a three-dimensional (3D) terahertz metamaterial perfect absorber (MPA) with a high quality factor. The absorption response of the proposed structure is analyzed and optimized using coupled mode theory and numerical simulations. Subsequently, we fabricate the 3D MPA by employing the multi-layer electroplating process and ch...
Conference Paper
We designed, fabricated, and characterized a metamaterial perfect absorber at terahertz (THz) frequency range utilizing air as the dielectric material. Due to the avoidance of the loss usually introduced by the dielectric material, there was a three times improvement of the quality factor. Also, with metamaterials fabricated on a free-standing sili...
Conference Paper
This paper reports a mechanically tunable terahertz metamaterial based on microscale electrostatic actuators. The device consists of an array of suspended cantilevers, forming LC resonators with underlying capacitive pads. The applied voltage can pull the cantilever downwards to increase the capacitance value in the resonator, and thus, redshift th...
Article
We present a superconducting metamaterial saturable absorber at terahertz frequencies. The absorber consists of an array of split ring resonators (SRRs) etched from a 100nm YBaCu3O7 (YBCO) film. A polyimide spacer layer and gold ground plane are deposited above the SRRs, creating a reflecting perfect absorber. Increasing either the temperature or i...
Article
A double layer spiral antenna with side length of 380 μm was fabricated by a multi-step electroplating process, and integrated with a commercialized passive RFID chip to realize the RF power harvesting and communication functions of a microsensor. To power up and communicate with the microchips, a single layer spiral reader antenna was fabricated o...
Article
We experimentally demonstrate the extraordinary transmission of THz waves through super-aligned multi-walled carbon nanotube (MWCNT) films with one-dimensional arrays of sub-wavelength rectangular gratings in the broad frequency range from 0.2 to 2.5 THz. To achieve this, two kinds of MWCNT films (1 μm and 3 μm in thickness) were fabricated by draw...
Article
Full-text available
This paper presents the design, fabrication, and characterization of a real-time voltage-tunable terahertz metamaterial based on microelectromechanical systems and broadside-coupled split-ring resonators. In our metamaterial, the magnetic and electric interactions between the coupled resonators are modulated by a comb-drive actuator, which provides...
Article
Full-text available
We investigate the nonlinear response of terahertz (THz) metamaterial perfect absorbers consisting of electric split ring resonators on GaAs integrated with a polyimide spacer and gold ground plane. These perfect absorbers on bulk semi-insulating GaAs are characterized using high-field THz time-domain spectroscopy. The resonance frequency redshifts...
Conference Paper
This paper reports our recent progress on a dynamically tunable terahertz metamaterial based on broadside-coupled split ring resonators (BC-SRRs) and a comb-drive actuator. In the structure, the coupling between two arrays of SRR is modulated with the lateral displacement exerted by the comb-drive actuator. In the spectrum of BC-SRRs, two resonant...
Article
Full-text available
The development of responsive metamaterials has enabled the realization of compact tunable photonic devices capable of manipulating the amplitude, polarization, wave vector, and frequency of light. Integration of semiconductors into the active regions of metallic resonators is a proven approach for creating nonlinear metamaterials through optoelect...
Article
Full-text available
We use intense terahertz pulses to excite the resonant mode (0.6 THz) of a micro-fabricated dipole antenna with a vacuum gap. The dipole antenna structure enhances the peak amplitude of the in-gap THz electric field by a factor of ~170. Above an in-gap E-field threshold amplitude of ~10 MVcm-1, THz-induced field electron emission is observed (TIFEE...
Conference Paper
Full-text available
A miniaturized antenna, 380μm by 380μm in size, was fabricated and integrated with a commercialized passive RFID chip to form a micro-tracer, whose size was 2mm by 1mm in total. The micro-tracer was wirelessly powered and interrogated by a single layer spiral reader antenna through near field coupling. To maximize the working distance, the resonant...
Conference Paper
The functionality of terahertz metamaterials can be dramatically increased through judicious materials integration. In addition to semiconductors, materials ranging from graphene to superconductors can enhance or enable new functionality. Following a brief review, we present recent results creating nonlinear metamaterials using InAs and YBa 2 Cu 3...
Article
We present our recent progress on a highly flexible tunable perfect absorber at terahertz frequencies. Metamaterial unit cells were patterned on thin GaAs patches, which were fashioned in an array on a 10 μm polyimide substrate via semiconductor transfer technique, and the backside of the substrate was coated with gold film as a ground plane. Optic...
Conference Paper
This paper reports a real-time tunable metamaterial based on broadside-coupled split ring resonators (BC-SRRs). The device is composed of two layers of SRRs stacked together with an air gap spacer, forming the BC-SRR configuration. One of the layers is fixed, while the other can be driven by an electrostatic comb-drive actuator. The lateral displac...