Based on the complementary V-shaped antenna structure, ultrathin vortex phase plates are designed to achieve the terahertz (THz) optical vortices with different topological charges. Utilizing a THz holographic imaging system, the two dimensional complex field information of the generated THz vortex beam with the topological number l=1 is directly obtained. Its far field propagation properties are analyzed in detail, including the rotation, the twist direction, and the Gouy phase shift of the vortex phase. An analytic Laguerre-Gaussian mode is used to simulate and explain the measured phenomena. The experimental and simulation results overlap each other very well.

two authors contributed equally to this work.Abstract: Conventional optical components shape the wavefront of propagating light by adjusting the optical path length, which requires the use of rather thick lenses, especially for the adjustment of terahertz (THz) radiation due to its long wavelength. Two ultrathin THz planar lenses were designed and fabricated based on interface phase modulation of antenna resonance. The lens thicknesses were extremely reduced to 100 nm, which is only 1/4000 th of the illuminating light wavelength. The focusing and imaging functions of the lenses were experimentally demonstrated. The ultrathin optical components described herein are a significant step toward the development of a micro-integrated THz system. One-Sentence Summary: Ultrathin planar lenses with a thickness of 100 nm were designed and fabricated to implement THz beam focusing and imaging.Terahertz (THz) radiation lies in the frequency range between infrared and microwaves, typically having wavelengths ranging from 10 μm to 3 mm. THz technology is developing rapidly in many independent fields and has many potential applications (1). However, due to the relatively long wavelengths of THz radiation, most THz components, such as lenses and prisms, are on a large scale and are not suitable for system integration.Conventional optical components shape the wavefront of propagating light via gradual phase changes that accumulate along the optical path, usually via alterations to the spatial distribution using the thickness or refraction index of the components. Early optical components possessed continuous curved surfaces to achieve phase modulation, as indicated in Fig. 1A. This continuity determines the bulkiness of the components. Further technological developments utilized the 2π phase jump to reduce component thickness to the wavelength scale, as shown in Fig. 1B. Subsequently, metamaterials with extremely large effective refractive indices have been used to further reduce the thickness of the optical components (2, 3). However, the basic theory for wavefront shaping is still based on phase accumulation along the optical path, and the thickness of the corresponding components is still quite large. The question remains as to whether it is possible to further reduce the thickness of the optical components.Alternatively, phase changes can also be introduced by an optical resonator. Electromagnetic cavities (4-6), nanoparticles clusters (7,8), and plasmonic antennas (9,10) have previously been employed for tailoring phase changes. Recently, a novel method was proposed to introduce a phase discontinuity at the interface between two media (11)(12)(13). In this method, the geometry of planar V-shaped antennas was spatially selected, and the phase shift between the emitted and illuminating lights could be controlled arbitrarily; the generalized laws of reflection and refraction using this method were described. If the antennas are spatially arranged according to a customized phase distribution, an ultrathin planar optical componen...

A waveguide-plasmonic scheme is constructed by coating the matrix of randomly distributed gold nanoisland structures with a layer of dye-doped polymer, which provides strong feedback or gain channels for the emission from the dye molecules and enables successful running of a random laser. Excellent overlap of the plasmonic resonance of the gold nanoislands with the photoluminescence spectrum of the dye molecules and the strong confinement mechanism provided by the active waveguide layer are the key essentials for the narrow-band and low-threshold operation of this random laser. This kind of feedback configuration potentially enables directional output from such random lasers. The flexible solution-processable fabrication of the plasmonic gold nanostructures not only enables easy realization of such a random laser but also provides mechanisms for the tuning and multicolor operation of the laser emission.

Terahertz (THz) technology is a developing and promising candidate for biological imaging, security inspection and communications, due to the low photon energy, the high transparency and the broad band properties of the THz radiation 1-3 . However, a major encountered bottleneck is lack of efficient devices to manipulate the THz wave, especially to modulate the THz wave front. A wave front modulator should allow the optical or electrical control of the spatial transmission (or reflection) of an input THz wave and hence the ability to encode the information in a wave front 4 . Here we propose a spatial THz modulator (STM) to dynamically control the THz wave front with photo-generated carriers. A computer generated THz hologram is projected onto a silicon wafer by a conventional spatial light modulator (SLM). The corresponding photo-generated carrier spatial distribution will be induced, which forms an amplitude hologram to modulate the wave front of the input THz beam. Some special intensity patterns and vortex beams are generated by using this method. This all-optical controllable STM is structure free, high resolution and broadband. It is expected to be widely used in future THz imaging and communication systems.S andwiched between the microwave and infrared, THz radiation has been notoriously difficult to produce, modulate and detect [1][2][3] . Recent progresses such as quantum-cascade lasers 5,6 , terahertz wave generation through a nonlinear crystal 7 and THz time-domain spectroscopy 8 are promoting this subject into one of the most rapidly growing fields. High performance devices to control and manipulate the THz radiation are in urgent demand to develop sophisticated imaging and communication system. The filters, absorbers and polarizers based on graphene, frequency selective surface, metamaterials and photonic crystals have been reported [9][10][11][12][13][14][15][16][17][18] . However, the wave front modulation devices are still lacking. Recently, a novel technology based on the metasurface has been demonstrated to generate the desired wave front distribution 19,20 . Unfortunately, the specific function of this kind of devices has been determined at the moment of their design and could not be flexibly changed any more. The SLM which has been widely used in the visible light band can optically or electrically control the spatial transmission (or reflection) of an input light beam and encode information in the wave front 4 . The SLM always plays an important role in optical information processing, three dimensional image display, optical interconnections and real-time beam shaping. Usually, the SLM is realized through liquid crystals, magneto-optic materials or deformable mirrors. However, such mechanisms cannot work well in the THz regime due to the lack of suitable materials and the size mismatching between the micro-machined components and the THz wavelength 21 . In order to obtain the STM, a novel technology needs to be explored.The STM requires an array of small building blocks that can independentl...

The plasmonic resonance effect on metasurfaces generates an abrupt phase change. We employ this phase modulation mechanism to design the longitudinal field distribution of an ultrathin terahertz (THz) lens for achieving the axial long-focal-depth (LFD) property. Phase distributions of the designed lens are obtained by the Yang-Gu iterative amplitude-phase retrieval algorithm. By depositing a 100 nm gold film on a 500 μm silicon substrate and etching arrayed V-shaped air holes through the gold film, the designed ultrathin THz lens is fabricated by the micro photolithography technology. Experimental measurements have demonstrated its LFD property, which basically agree with the theoretical simulations. In addition, the designed THz lens possesses a good LFD property with a bandwidth of 200 GHz. It is expected that the designed ultrathin LFD THz lens should have wide potential applications in broadband THz imaging and THz communication systems.

A polarization-independent optical sensor is created by fabricating a concentric gold ring grating with a period of 900 nm on the end facet of an optical fiber. The sensing function of this miniaturized device is realized by sending white light as a probe to the gold rings and collecting the response signal in the back-reflection through the optical fiber. A pronounced peak due to the Rayleigh anomaly of the gold ring grating is observed in the reflection spectrum, the center wavelength of which is sensitive to the change in the environmental refractive index of the fiber end facet. Theoretical analysis not only shows excellent agreement with the experimental results, but also gives insights into the mechanisms of this kind of sensor. Using the center position of the Rayleigh peak as the response signal, a high sensitivity dλ/dn of 900 nm per unity refractive index is realized for this sensor and a resolution of Δn/n ≈ 1% is demonstrated in preliminary experiments. The sensitivity is solely determined by the period of the grating.

Spin of light provides a route to control photons. Spin-based optical devices which can manipulate photons with different spin states are imperative. Here we experimentally demonstrated a spin-selected metasurface lens based on the spin-orbit interaction originated from the Pancharatnam-Berry (PB) phase. The optimized PB phase enables the light with different spin states to be focused on two separated points in the preset plane. Furthermore, the metasurface lens can perform the spin-selected imaging according to the polarization of the illuminating light. Such a spin-based device capacitates a lot of advanced applications for spin-controlled photonics in quantum information processing and communication based on the spin and orbit angular momentum.

of applications. Spatial light modulators, which can impose programmable modification of the spatial intensity and/or phase distribution of an optical beam, are commonly used in applications, such as optical information processing, imaging, and communications. However, in the THz frequency range, the development of spatial light modulators remains a great challenge due to the lack of electrooptical materials, although they are urgently requested with the ongoing advancement of THz technology and the identification of a variety of potential applications. [5] While a few spatial THz intensity modulators have already been proposed, [6][7][8][9] a pure phase spatial modulation can greatly increase the immunity to noise, therefore is more robust and applicable in THz systems where the intensity of THz radiation is typically low. Steinbusch et al. used the photogenerated graded index grating to actively steer the THz beam; the phase modulation was considered. [10] However, the phase modulation is caused by the change of reflection index of the thin semiconductor film pumped by the visible light, which is quite small. Other attempts used semiconductor quantum well structures at cryogenic temperatures and liquid crystals with low modulation speed. [11,12] More recently, metamaterials/ metasurfaces integrated with semiconductor and graphene have allowed more efficient modulation of THz phase. [13,14] However, the range of THz phase modulation needs further improvement, and one should also address the drawback of accompanied intensity modulation associated with the resonant response.Despite these limitations to overcome, the emergence of metasurfaces has provided an excellent opportunity to accomplish a variety of functional THz devices. [15][16][17] Metasurfaces are constructed from carefully designed discrete metallic or dielectric elements and have spatially varying characteristics across its surface, [18] enabling effective manipulation of the amplitude, phase, and polarization state of electromagnetic waves with continuously broadband achromatism for ultrathin planar lenses, [19,20] complex light field generation, [21] spin orbital angular momentum coupling, [22,23] and metasurface holograms. [24,25] Activating these functionalities will no doubt expand the application scope and create a greater societal impact of metasurfaces, which necessitates the integration of functional materials within the metasurface structures, including most widely used semiconductors, phase transition materials, and Terahertz (THz) radiation has many potential applications. However, comparing with the rapid development of THz sources and detectors, functional devices for THz modulation, especially the spatial modulation devices, are still insufficient. Here, a novel approach for generating arbitrary wavefronts of a THz beam is presented. By dynamically creating metasurface structures through illuminating a thin silicon wafer with femtosecond laser, which is spatially modulated, an array of reconfigurable subwavelength resonators is generate...