Metamaterial-based octave-wide terahertz bandpass filters

We present octave-wide bandpass filters in the terahertz(THz)region based on bilayer-metamaterial(BLMM)structures.The passband region has a super-Gaussian shape with a maximum transmittance approaching 70%and a typical stopband rejection of 20 dB.The design is based on a metasurface consisting of a metallic squarehole array deposited on a transparent polymer,which is stacked on top of an identical metasurface with a subwavelength separation.The superimposed metasurface structures were designed using finite-difference time-domain(FDTD)simulations and fabricated using a photolithography process.Experimental characterization of these structures between 0.3 and 5.8 THz is performed with a time-domain THz spectroscopy system.Good agreement between experiment and simulation results is observed.We also demonstrated that two superimposed BLMM(2BLMM)devices increase the steepness of the roll-offs to more than 85 dB/octave and enable a superior stopband rejection approaching 40 dB while the maximum transmittance remains above 65%.This work paves the way toward new THz applications,including the detection of THz pulses centered at specific frequencies,and an enhanced time-resolved detection sensitivity toward molecular vibrations that are noise dominated by a strong,off-resonant,driving field.

Spatial sampling of terahertz fields with sub-wavelength accuracy via probe-beam encoding

Recently,computational sampling methods have been implemented to spatially characterize terahertz(THz)fields.Previous methods usually rely on either specialized THz devices such as THz spatial light modulators or complicated systems requiring assistance from photon-excited free carriers with high-speed synchronization among multiple optical beams.Here,by spatially encoding an 800-nm near-infrared(NIR)probe beam through the use of an optical SLM,we demonstrate a simple sampling approach that can probe THz fields with a single-pixel camera.This design does not require any dedicated THz devices,semiconductors or nanofilms to modulate THz fields.Using computational algorithms,we successfully measure 128×128 field distributions with a 62-μm transverse spatial resolution,which is 15 times smaller than the central wavelength of the THz signal(940μm).Benefitting from the noninvasive nature of THz radiation and sub-wavelength resolution of our system,this simple approach can be used in applications such as biomedical sensing,inspection of flaws in industrial products,and so on.

Single-End Adaptive Optics Compensation for Emulated Turbulence in a Bi-Directional 10-Mbit/s per Channel Free-Space Quantum Communication Link Using Orbital-Angular-Momentum Encoding

A single-end adaptive-optics(AO)module is experimentally demonstrated to mitigate the emulated atmospheric turbulence efects in a bi-directional quantum communication link,which employs orbital angular momentum(OAM)for data encoding.A classical Gaussian beam is used as a probe to detect the turbulence-induced wavefront distortion in the forward direction of the link.Based on the detected wavefront distortion,an AO system located on one end of the link is used to simultaneously compensate for the forward and backward channels.Specifcally,with emulated turbulence and when the probe is turned on,the mode purity of photons carrying OAMℓ=1 is improved by∼21%with AO mitigation.We also measured the performance when encoding data using OAM{ℓ=−1,+2}and{ℓ=−2,+1}in the forward and backward channels,respectively,at 10 Mbit/s per channel with one photon per pulse on average.For this case,we found that the AO system could reduce the turbulence efects increased quantum-symbolerror-rate(QSER)by∼76%and∼74%,for both channels in the uni-directional and bi-directional cases,respectively.Similar QSER improvement is observed for the opposite direction channels in the bi-directional case.