Two-dimensional metamaterials - metasurfaces - offer tremendous opportunities in realizing exotic optical phenomena and functionalities to address the technological challenges encountered in the terahertz frequency regime. By tailoring the resonant response of basic building blocks as well as their mutual interactions, we are able to effectively control of amplitude, phase, and polarization state of terahertz waves. Here we report the realization of highly efficient polarization conversions including: (1) Reflective linear polarization rotation using an array of anisotropic resonators backed with a ground plane; (2) Transmissive linear polarization rotation using an array of anisotropic resonator array sandwiched by two orthogonal gratings; and (3) Reflective linear-to-circular polarization conversion using two cascading arrays of complementary resonators. They operate over a broad bandwidth more than one octave and approaching two octaves in some cases. We further show that the linear polarization rotation is accompanied by a tunable phase discontinuity, which allows us to demonstrate an ultrathin terahertz flat lens enabling diffraction-limited focusing. The broadband linear-to-circular polarization may also find applications including terahertz circular dichroism spectroscopy and excitation of valley polarization in 2D materials.
In this work we experimentally demonstrate high-performance narrowband terahertz (THz) bandpass filters through cascading multiple bilayer metasurface antireflection structures. Each bilayer metasurface, consisting of a square array of silicon pillars with a self-aligned top gold resonator-array and a complementary bottom gold slot-array, enables near-zero reflection and simultaneously close-to-unity single-band transmission at designed operation frequencies in the THz spectral region. The THz bandpass filters based on stacked bilayer metasurfaces allow a fairly narrow, high-transmission passband, and a fast roll-off to an extremely clean background outside the passband, thereby providing superior bandpass performance. The demonstrated scheme of narrowband THz bandpass filtering is of great importance for a variety of applications where spectrally clean, high THz transmission over a narrow bandwidth is desired, such as THz spectroscopy and imaging, molecular detection and monitoring, security screening, and THz wireless communications.
Conventional optical lenses focus electromagnetic waves by imparting position-dependent phase delay through shaping their geometry. This poses difficulties in eliminating the geometric aberrations in high numerical aperture lenses, in addition to the fabrication challenges when operating at short wavelengths (e.g. visible light), and bulky devices operating at long wavelengths (e.g. microwaves). In contrast, metasurfaces realize full control of phase through tailoring the subwavelength resonant structures, allowing for the demonstration ultrathin flat lens, although the efficiency is still rather low using single-layer metasurfaces. Here we report the demonstration of high-performance flat lens in the terahertz frequency range using few-layer metasurfaces. The three-layer metasurface structure is capable of rotating the incident linear polarization by 90° with a very high efficiency over a bandwidth of two octaves. More importantly, the phase of the output light can be tuned over the entire 2π range with subwavelength resolution through simply tailoring the structure geometry of the basic building blocks. Based on this success, we design, fabricate, and characterize a metasurface lens operating at 0.4 THz. With a lens diameter and focal length both 5 cm, we realize a high numerical aperture of 0.5 and diffraction-limited terahertz beam focusing. Terahertz time-domain spectroscopy measurements show that the metasurface lens is capable of achieving the same signal intensity as compared to a bulk TPX lens of the same size and focal length.
Tailoring the geometry and arrangement of metasurface structures yields a complete control of the reflection/transmission amplitude, phase, and polarization states. The planar nature of the structures can be readily fabricated using existing technologies, which makes them more accessible particularly in the optical regime and potentially revolutionizes the design of integrated photonics. Although the ultrathin thickness in the wave propagation direction can greatly suppress the absorption, the impedance mismatching in many metasurfaces results in undesirable high insertion losses, and the performance of single-layer metasurface devices is yet unsatisfying in real world applications. In contrast, few-layer meta-surfaces can circumvent this impedance mismatching issue. Recently, we have developed a three-layer metasurface structure that is capable of rotating the incident linear polarization by 90° with a very high efficiency over a bandwidth of nearly two octaves. More importantly, the phase of the output light can be tuned over the entire 2π range with sub-wavelength resolution through simply tailoring the structure geometry of the basic building blocks. This creates an important opportunity in designing highly efficient optical devices for wavefront engineering, such as at lenses working in the microwave, terahertz, and infrared frequency ranges. Here we will present the design, fabrication, and characterization of high-performance terahertz metasurface lenses based on this concept.
The promise of metamaterials lies in the realization of desirable electromagnetic functionalities simply through tailoring the geometric structure and deliberate arrangement of metal/dielectric building blocks (meta-atoms) to yield envisaged material properties that may be difficult or impossible to accomplish using natural materials. Integration of functional materials into metamaterial structures further extends switchable and frequency tunable functionalities through applying an external stimulus such as temperature change, photoexcitation, and voltage bias. Among them electrically switchable metamaterials are of particular interest for a host of applications. In this work we present our recent progress in this direction. More specifically, hybrid terahertz metamaterials can be formed through integrating semiconducting Schottky junctions into the metallic resonators, enabling highly efficient, electrically switchable resonant response. Such hybrid terahertz metamaterials can be applied in creating terahertz spatial light modulators and active diffraction gratings. Furthermore, graphene can be used to extend the active metamaterials to the mid-infrared frequency range.
Electromagnetic metamaterials (MMs) consisting of highly conducting sub-wavelength metallic resonators enable many unusual electromagnetic properties at designed frequencies which are not permissible with the naturally occurring materials. The electromagnetic properties of metamaterial are typically controlled by the clever design of the MM unit cell, often termed as meta-molecule, consisting of metallic split ring resonators (SRRs) or meta-atoms. The near field coupling between meta-atoms plays a vital role in tuning the natural resonances of individual SRR and, therefore, has the ability to modify the far-field radiation properties significantly. It is shown that near field coupling between the meta-atoms could lead to resonance tuning, mode splitting, and ultrafast switching in passive and active resonators. In this article, we present a brief review on tuning the metamaterial properties by active and passive manipulation of near field coupling between neighboring split ring resonators.
We demonstrate thermal and ultrafast optical tuning in planar terahertz (THz) superconducting metamaterials. The
fundamental resonance of an array of split-ring resonators (SRRs) fabricated from a 50-nm-thick high-temperature
superconducting (HTS) YBa2Cu3O7-δ (YBCO) film is characterized as a function of temperature and near-infrared
photoexcitation fluence. The HTS metamaterial exhibits a very strong resonant response at temperatures much lower
than the transition temperature Tc. Increasing the temperature reduces the density of Cooper pairs, which results in a
dramatically decreasing imaginary part of the complex conductivity, and thereby tunes the metamaterial resonance. We
observe switched resonance strength and large red shift of resonance frequency when the temperature increases from 20
K to Tc. Similar resonance switching and frequency tuning is also demonstrated in an ultrafast time scale through near-infrared
femtosecond laser excitation. We further compare the thermal tuning behaviour of the 50-nm-thick HTS
metamaterial with a metamaterial sample comprised of gold SRRs with identical geometry and dimensions, which has
Extraordinary optical transmission through subwavelength metallic hole-arrays has been an active research area
since its first demonstration. The frequency selective resonance properties of subwavelength metallic hole arrays,
generally known as surface plasmon polaritons, have potential use in functional plasmonic devices such as filters,
modulators, switches, etc. Such plasmonic devices are also very promising for future terahertz applications. Ultrafast
switching or modulation of the resonant behavior of the 2-D metallic arrays in terahertz frequencies is of particular
interest for high speed communication and sensing applications. In this paper, we demonstrate ultrafast optical control of
surface plasmon enhanced resonant terahertz transmission in two-dimensional subwavelength metallic hole arrays
fabricated on gallium arsenide based substrates. Optically pumping the arrays creates a thin conductive layer in the
substrate reducing the terahertz transmission amplitude of both the resonant mode and the direct transmission. Under low
optical fluence, the terahertz transmission is more greatly affected by resonance damping than by propagation loss in the
substrate. An ErAs:GaAs nanoisland superlattice substrate is shown to allow ultrafast control with a switching recovery
time of ~10 ps. We also present resonant terahertz transmission in a hybrid plasmonic film comprised of an integrated
array of subwavelength metallic islands and semiconductor hole arrays. Optically pumping the semiconductor hole
arrays favors excitation of surface plasmon resonance. A large dynamic transition between a dipolar localized surface
plasmon mode and a surface plasmon resonance near 0.8 THz is observed under near infrared optical excitation. The
reversal in transmission amplitude from a stop-band to a pass-band and up to π/ 2 phase shift achieved in the hybrid
plasmonic film make it promising in large dynamic phase modulation, optical changeover switching, and active terahertz
The terahertz (THz) region has been shown to have considerable application potential for spectroscopic imaging,
nondestructive imaging through nonpolar, nonmetallic materials and imaging of biological materials. These applications
have all been possible due to the recent progress in THz sources, detectors and measurement techniques. However, only
moderate progress has been made in developing passive and active devices to control and manipulate THz radiation,
which can enhance current imaging capabilities. One promising approach for implementing passive and active devices at
THz frequencies are metamaterials - composite materials designed to have specific electromagnetic properties not found
in naturally occurring materials. The most common implementation utilizes a metallic resonant particle periodically
distributed in an insulator matrix where the periodicity is significantly smaller than the wavelength of operation. We
have designed and implemented three metamaterial based devices with potential applications to THz imaging. We
present an electrically-driven active metamaterial which operates as an external modulator for a ~2.8 THz CW quantum
cascade laser. We obtained a modulation depth of ~60%. We also demonstrate a polarization sensitive metamaterial
which can be used as a continuously variable attenuator or as a wave plate. The latter may be useful for the development
of THz phase contrast imaging.
The fascinating properties of plasmonic structures have had significant impact on the development of next
generation ultracompact photonic and optoelectronic components. We study two-dimensional plasmonic structures
functioning at terahertz frequencies. Resonant terahertz response due to surface plasmons and dipole localized surface
plasmons were investigated by the state-of-the-art terahertz time domain spectroscopy (THz-TDS) using both
transmission and reflection configurations. Extraordinary terahertz transmission was demonstrated through the
subwavelength metallic hole arrays made from good conducting metals as well as poor metals. Metallic arrays made
from Pb, generally a poor metal, and having optically thin thicknesses less than one-third of a skin depth also contributed
in enhanced THz transmission. A direct transition of a surface plasmon resonance from a photonic crystal minimum was
observed in a photo-doped semiconductor array. Electrical controls of the surface plasmon resonances by hybridization
of the Schottky diode between the metallic grating and the semiconductor substrate are investigated as a function of the
applied reverse bias. In addition, we have demonstrated photo-induced creation and annihilation of surface plasmons
with appropriate semiconductors at room temperature. According to the Fano model, the transmission properties are
characterized by two essential contributions: resonant excitation of surface plasmons and nonresonant direct
transmission. Such plasmonic structures may find fascinating applications in terahertz imaging, biomedical sensing,
subwavelength terahertz spectroscopy, tunable filters, and integrated terahertz devices.
In this paper we present our recent developments in terahertz (THz) metamaterials and devices. Planar THz metamaterials and their complementary structures fabricated on suitable substrates have shown electric resonant response, which causes the band-pass or band-stop property in THz transmission and reflection. The operational frequency can be further tuned up to 20% upon photoexcitation of an integrated semiconductor region in the split-ring resonators as the metamaterial elements. On the other hand, the use of semiconductors as metamaterial substrates enables dynamical control of metamaterial resonances through photoexcitation, and reducing the substrate carrier lifetime further enables an ultrafast switching recovery. The metamaterial resonances can also be actively controlled by application of a voltage bias when they are fabricated on semiconductor substrates with appropriate doping concentration and thickness. Using this electrically driven approach, THz modulation depth up to 80% and modulation speed of 2 MHz at room temperature have been demonstrated, which suggests practical THz applications.
We demonstrate external control of metamaterials operating at terahertz frequencies. Through photodoping of
semiconducting substrates, used to support metamaterial arrays, we show ultrafast switching times. New metamaterial
"grids" are presented, which may be formed by the union of electric metamaterials arrays. Metamaterial
grids are then utilized to form a Schottky contact are used to demonstrate voltage switching of the metamaterials
resonance. Both devices presented may be utilized to form novel devices at terahertz frequencies and also scaled
to other energy regimes of interest.
Compared to the neighboring infrared and microwave regions, the terahertz regime is still in need of fundamental
technological advances. This derives, in part, from a paucity of naturally occurring materials with useful electronic or
photonic properties at terahertz frequencies. This results in formidable challenges for creating the components needed
for generating, detecting, and manipulating THz waves. Considering the promising applications of THz radiation, it is
important overcome such material limitations by searching for new materials, or by constructing artificial materials with
a desired electromagnetic response. Metamaterials are a new type of artificial composite with electromagnetic properties
that derive from their sub-wavelength structure. The potential of metamaterials for THz radiation originates from a
resonant electromagnetic response which can be tailored for specific applications. Metamaterials thus offer a route
towards helping to fill the so-called "THz gap". In this work we discuss novel planar THz metamaterials. Importantly,
the dependence of the resonant response on the supporting substrate enables the creation of active THz metamaterials.
We show that the resonant response can be efficiently controlled using optical or electrical approaches. This has resulted
in the creation of efficient THz switches and modulators of potential importance for advancing numerous real world THz
Tunable electromagnetic metamaterials can be designed through the incorporation of semiconducting materials.
We present theory, simulation, and experimental results of metamaterials operating at terahertz frequencies.
Specific emphasis is placed on the demonstration of external control of planar arrays of metamaterials patterned
on semiconducting substrates with terahertz time domain spectroscopy used to characterize device performance.
Dynamical control is achieved via photoexcitation of free carriers in the substrate. Active control is achieved by
creating a Schottkey diode, which enables modulation of THz Transmission by 50 percent, an order of magnitude
improvement over existing devices. Because of the universality of metamaterial response over many decades of
frequency, these results have implications for other regions of the electromagnetic spectrum and will undoubtedly
play a key role in future demonstrations of novel high-performance devices.
We report on the development of an apertureless scanning near-field optical microscope for characterization of dielectric properties of nano-structures at terahertz frequencies. A spatial resolution of ≈ 150 nm is achieved, which corresponds to a sub-wavelength factor of ≈1/1000. The imaging mechanism is due to a resonant coupling between light field and the tip-surface system. This allows for image contrasts which exceed those can be expected from Mie scattering by orders of magnitude. Terahertz images of organic and inorganic structures show that the apertureless terahertz microscopy gives insight into the dielectric properties on submicron scale.
Do-nut Mode Optical Trap is the kind of Optical Trap that the laser distribution in the center part of the beam is approximately zero. Because the trapping effect of an Optical Trap correlates closely with the distribution of optical field, the scattering force of Do-nut Mode Optical Trap is markedly reduced. We realized Do-nut Mode Optical Trap by rebuilding wave front. Then we studied the trapping effect and trap stiffness of Solid Mode Optical Trap and Do-nut Mode Optical Trap in the cases of upright microscope and inverted microscope. We measured the stiffness of the Optical Trap near the focus with Boltzmann statistics method.
The distribution of the optical field of Do-nut Optical Trap is like aura. And the trapping effect changes with the diameter ratio of the aura.
Based on the micron precision manipulation and measurement in the optical tweezers, we built an optical-trap system that can make quantitative measurement of displacement with nanometer resolution in millisecond time scale and then we can measure the piconewton force involved in the process. It is an eminent tool for manipulating biological specimens at the macromolecular level. In this paper we describe its technical properties, design, and make some discussion.