Labao Zhang

Tuning of superconducting niobium nitride terahertz metamaterials

Superconducting planar terahertz (THz) metamaterials (MMs), with unit cells of different sizes, are fabricated on 200 nm-thick niobium nitride (NbN) films deposited on MgO substrates. They are characterized using THz time domain spectroscopy over a temperature range from 8.1 K to 300 K, crossing the critical temperature of NbN films. As the gap frequency (f(g) = 2Δ0/h, where Δ0 is the energy gap at 0 K and h is the Plank constant) of NbN is 1.18 THz, the experimentally observed THz spectra span a frequency range from below f(g) to above it. We have found that, as the resonance frequency approaches f(g), the relative tuning range of MMs is quite wide (30%). We attribute this observation to the large change of kinetic inductance of superconducting film.

Noncollinear parametric fluorescence by chirped quasi-phase matching for monocycle temporal entanglement

Quantum entanglement of two photons created by spontaneous parametric downconversion (SPDC) can be used to probe quantum optical phenomena during a single cycle of light. Harris [Opt. Express 98, 063602 (2007)] suggested using ultrabroad parametric fluorescenc generated from a quasi-phase-matched (QPM) device whose poling period is chirped. In the Harriss original proposal, it is assumed that the photons are collinearly generated and then spatially separated by frequency filtering Here, we alternatively propose using noncollinearly generated SPDC. In our numerical calculation, to achieve 1.2 cycle temporal correlation for a 532 nm pump laser, only 10% -chirped device is sufficien when noncollinear condition is applied, while a largely chirped (50%) device is required in collinear condition. We also experimentally demonstrate an octave-spanning (790-1610 nm) noncollinear parametric fluorescenc from a 10% chirped MgSLT crystal using both a superconducting nanowire single-photon detector and photomultiplier tube as photon detectors. The observed SPDC bandwidth is 194 THz, which is the largest width achieved to date for a chirped QPM device. From this experimental result, our numerical analysis predicts that the bi-photon can be compressed to 1.2 cycles with appropriate phase compensation.

Strategy for realizing magnetic field enhancement based on diffraction coupling of magnetic plasmon resonances in embedded metamaterials

We have demonstrated a straightforward strategy to realize magnetic field enhancement through diffraction coupling of magnetic plasmon (MP) resonances by embedding the metamaterials consisting of a planar rectangular array of U-shaped metallic split-ring resonators (SRRs) into the substrate. Our method provides a more homogeneous dielectric background allowing stronger diffraction coupling of MP resonances among SRRs leading to strong suppression of the radiative damping. We observe that compared to the on-substrate metamaterials, the embedded ones lead to a narrow-band hybridized MP mode, which results from the interference between MP resonances in individual SRRs and an in-plane propagating collective surface mode arising from light diffraction. Associated with the excitation of this hybridized MP mode, a twenty-seven times enhancement of magnetic fields within the inner area of the SRRs is achieved as compared with the pure MP resonance. Moreover, we also found that besides the above requirement of homogeneous dielectric background, only a collective surface mode with its magnetic field of the same direction as the induced magnetic moment in the SRRs could mediate the excitation of such a hybridized MP mode.

Photon-Counting Optical Time-Domain Reflectometry Using a Superconducting Nanowire Single-Photon Detector

A novel photon-counting optical time-domain reflectometry (ν-OTDR) based on superconducting nanowire single-photon detector (SNSPD) is proposed and demonstrated experimentally. Benefiting from the low noise equivalent power (NEP), high repetition rate and low timing jitter of the SNSPD, our ν-OTDR system achieves a dynamic range of 22 dB after measurement time of 15 minutes. This obtainable dynamic range corresponds to a sensing length of 110 km. The system exhibits 6.0 cm spatial resolution at the end of 2 km and 1.1 m spatial resolution at the end of 26 km standard single-mode fiber. Considering the performance we obtained now and the increasing improvement of the fabrication technology, the SNSPD is promising in the field of fiber sensors.

Counting rate enhancements in superconducting nanowire single-photon detectors with improved readout circuits

Counting rates of superconducting nanowire single-photon detectors are usually estimated at hundreds of MHz by their kinetic-inductive reset time. This maximum is also limited by capacitor coupling effects in conventional readout circuits. In this Letter, we design and demonstrate an improved readout circuit that reduces the reset time and removes circuit limits. The counting rate at the 3 dB compression point is increased by four times for a large active area detector. We also discuss nonlinear dependences of the counting rate on the incident continuous-wave optical power and give a numerical model to explain our observations.

Long-haul and high-resolution optical time domain reflectometry using superconducting nanowire single-photon detectors.

In classical optical time domain reflectometries (OTDRs), for sensing an 200-km-long fiber, the optical pulses launched are as wide as tens of microseconds to get enough signal-to-noise ratio, while it results in a two-point resolution of kilometers. To both reach long sensing distance and sub-kilometer resolution, we demonstrated a long-haul photon-counting OTDR using a superconducting nanowire single-photon detector. In a 40-minute-long measurement, we obtained a dynamic range of 46.9 dB, corresponding to a maximum sensing distance of 246.8 km, at a two-point resolution of 0.1 km. The time for measuring fiber after 100 km was reduced to one minute, while the fiber end at 217 km was still distinguished well from noise. After reducing the pulse width to 100 ns, the experimental two-point resolution was improved to 20 m while the maximum sensing distance was 209.47 km.

A method to study the crack healing process of glassformers

A mechanical spectroscopy method of quantitatively monitoring the healing of stress-induced microcracks in small glass samples is described. Whereas the cracks are generated catastrophically at some unpredictable interval below the glass temperature Tg the healing process proves to be highly reproducible and a characteristic temperature for the crack healing process, the temperature of maximum healing rate Tch coincides with Tg.

Multimode Fiber Coupled Superconductor Nanowire Single-Photon Detector

High-performance superconductor nanowire single-photon detectors (SNSPDs) coupled by 50-μm multimode fiber were designed and fabricated for the purpose of space applications. Microlens coupling between multimode fiber and detector chip was proposed, resulting in coupling efficiency of 86% at the chips with detection area of 15 x 15 μm2. An optical structure for enhancing the photons absorption and superconductor nanowires with low critical current of 3 μA were fabricated on the chips. A cryogenic amplifier was adopted to increase the signal-to-noise ratio and time jitter of output pulse. The SNSPD exhibited system efficiency (1550 nm) of 56% at a dark count rate of 100 cps, 50% at a dark count rate of 5 cps, and 40% at a dark count rate of 1 cps. The time jitter was 46 ps at full-width at half-maximum. The multimode fiber coupling system with high performance will promote its applications in free space.

Surface-Plasmon-Polaritons-Assisted Enhanced Magnetic Response at Optical Frequencies in Metamaterials

We theoretically study the coupling of magnetic plasmon polaritons (MPPs) with propagating surface plasmon polaritons (SPPs) in a system composed of an array of metal nanowires close to a metal film with a dielectric spacer. Strong coupling between MPPs and SPPs is observed, manifested by the anticrossing behavior of the resonant positions in the reflection spectra. It creates narrow-band hybridized MPPs with Rabi-type splitting as large as 250 meV. Moreover, we also found that the coupling between the MPPs and the SPPs can be tailored by the period of the metal nanowire array to affect the magnetic response of the plasmonic structure. Above the resonant wavelength of the MPPs, coupling between two kinds of resonance modes can lead to a 20-fold enhancement of the magnetic fields in the dielectric spacer, as compared with the pure magnetic resonance upon the excitation of the hybridized MPPs, whereas below it, coupling cannot lead to a magnetic field enhancement. We suggest that this feature could offer a feasible way to achieve huge magnetic field enhancement at optical frequencies and hold promising potential applications in magnetic nonlinearity and sensors.

Investigation of the Performance of an Ultralow-Dark-Count Superconducting Nanowire Single-Photon Detector

The realization of an ultralow-dark-count rate (DCR) along with the conservation of high detection efficiency (DE) is critical for many applications using single photon detectors in quantum information technologies, material sciences, and biological sensing. For this purpose, a fiber-coupled superconducting nanowire single-photon detector (SNSPD) with a meander-type niobium nitride nanowire (width: 50 nm) is studied. Precise measurements of the bias current dependence of DE are carried out for a wide spectral range (from 500 to 1650 nm in steps of 50 nm) using a white light source and a laser line Bragg tunable band-pass filter. An ultralow DCR (0.0015 cps) and high DE (32%) are simultaneously achieved by the SNSPD at a wavelength of 500 nm.