Broadband and tunable terahertz absorption via photogenerated carriers in undoped silicon

Terahertz absorbers based on doped silicon have achieved broadband and high-efficiency absorption due to their high concentration of carriers.However,their tunable performance is obviously insufficient.Here,we propose a new scheme for active terahertz absorption based on undoped silicon with a metamaterial antireflection layer,which realizes both strong absorption and ultrahigh modulation depth.Benefiting from the weak absorption and high transmission of undoped silicon for 1064-nm continuous wave,uniformly distributed carriers across the entire thickness of the absorber are excited,and efficient free carrier absorption of the terahertz wave is obtained.We use only a 500-μm thick absorber and achieve absorption greater than 90%in the range of 0.58 to 1.92 THz,with a peak value of 99%.More important,the absorber can be switched between two working states of nonabsorption and high-efficiency absorption by changing the pump power,which means the modulation depth reaches 100%.This simple and high-performance implementation scheme provides a new idea for the design of terahertz tunable absorbers.

Time-Modulated Transmissive Programmable Metasurface for Low Sidelobe Beam Scanning

Programmable metasurfaces have great potential for the implementation of low-complexity and low-cost phased arrays.Due to the difficulty of multiple-bit phase control,conventional programmable metasurfaces suffer a relatively high sidelobe level(SLL).In this manuscript,a time modulation strategy is introduced in the 1-bit transmissive programmable metasurface for reducing the SLLs of the generated patterns.After the periodic time modulation,harmonics are generated in each reconfigurable unit and the phase of the first-order harmonic can be dynamically controlled by applying different modulation sequences onto the corresponding unit.Through the high-speed modulation of the real-time periodic coding sequences on the metasurface by the programmable bias circuit,the equivalent phase shift accuracy to each metasurface unit can be improved to 6-bit and thus the SLLs of the metasurface could be reduced remarkably.The proposed time-modulated strategy is verified both numerically and experimentally with a transmissive programmable metasurface,which obtains an aperture efficiency over 34%and reduced SLLs of about-2o dB.The proposed design could offer a novel approach of a programmable metasurface framework for radar detection and secure communication applications.

Microfluidic integrated metamaterials for active terahertz photonics

A depletion layer played by aqueous organic liquids flowing in a platform of microfluidic integrated metamaterials is experimentally used to actively modulate terahertz(THz)waves.The polar configuration of water molecules in a depletion layer gives rise to a damping of THz waves.The parallel coupling of the damping effect induced by a depletion layer with the resonant response by metamaterials leads to an excellent modulation depth approaching 90%in intensity and a great difference over 210°in phase shift.Also,a tunability of slow-light effect is displayed.Joint time-frequency analysis performed by the continuous wavelet transforms reveals the consumed energy with varying water content,indicating a smaller moment of inertia related to a shortened relaxation time of the depletion layer.This work,as part of THz aqueous photonics,diametrically highlights the availability of water in THz devices,paving an alternative way of studying THz wave–liquid interactions and developing active THz photonics.