Yundong Zhang

Experimental evidence of enhanced rotation sensing in a slow-light structure

We report the experimental observation of enhanced rotation sensing in a slow-light structure. We demonstrate that the differential of the phase difference, introduced by rotation, between the counterpropagating waves is proportional to the group index, and thus we can obtain large rotation sensitivity in the slow-light structure with high dispersion (large group index). The results also predict the realization of highly sensitive and compact integrated rotation sensors and gyroscopes by using microwaveguides.

Nonlinear optical properties of phenoxy-phthalocyanines at 800nm with femtosecond pulse excitation

We investigated nonlinear optical properties of Phenoxy-phthalocyanine (Pc1) and Phenoxy-phthalocyanine-Zinc(II) (Pc2) at a wavelength of 800 nm with 100 fs pulses. The nonlinear absorption coefficient (alpha) and nonlinear refractive index (n(2)) are measured using standard Z-scan technique. Open aperture Z-scan indicates strong three-photon absorption in both phthalocyanines. With good solubility and excellent nonlinear optical coefficient,the samples are expected to be a potential candidate for optical applications.

Observation of superluminal propagation at negative group velocity in C60 solution

The authors observed the superluminal propagation at negative group velocity in C60 toluene solution firstly. The theoretical analysis was made based on density matrix model of superluminal propagation. The simulation result was well consistent with experimental measurement. The largest time advancement of 3.58ms was got at 5.6×10−4mol∕l of sample concentration; the corresponding group velocity is −0.28m∕s.

Reversible Fano resonance by transition from fast light to slow light in a coupled-resonator-induced transparency structure

We theoretically propose and experimentally perform a novel dispersion tuning scheme to realize a tunable Fano resonance in a coupled-resonator-induced transparency (CRIT) structure coupled Mach-Zehnder interferometer. We reveal that the profile of the Fano resonance in the resonator coupled Mach-Zehnder interferometers (RCMZI) is determined not only by the phase shift difference between the two arms of the RCMZI but also by the dispersion (group delay) of the CRIT structure. Furthermore, it is theoretically predicted and experimentally demonstrated that the slope and the asymmetry parameter (q) describing the Fano resonance spectral line shape of the RCMZI experience a sign reversal when the dispersion of the CRIT structure is tuned from abnormal dispersion (fast light) to normal dispersion (slow light). These theoretical and experimental results indicate that the reversible Fano resonance which holds significant implications for some attractive device applications such as highly sensitive biochemical sensors, ultrafast optical switches and routers can be realized by the dispersion tuning scheme in the RCMZI.

NONLINEAR-OPTICAL AND OPTICAL LIMITING PROPERTIES OF PHENOXY-PHTHALOCYANINES STUDIED USING THE Z-SCAN TECHNIQUE

The nonlinear-optical and optical limiting properties of 2(3), 9(10), 16(17), 23(24) phenoxy-phthalocyanines have been investigated using a 10-ns-pulse laser at 532 nm. The nonlinear absorption coefficient (β) is measured by the single beam Z-scan technique. We have observed low power optical limiting, with low limiting thresholds, based on nonlinear absorption in the sample. These studies indicate that the phthalocyanine material is a potential candidate for low power optical limiting applications.

Enhancing the sensitivity of fiber Mach–Zehnder interferometers using slow and fast light

We demonstrate theoretically and experimentally that the sensitivity of a fiber Mach-Zehnder (M-Z) interferometer can be enhanced by coupling fiber ring resonators with it. The experimental results agree well with theoretical predications that combining slow light with fast light will further increase the sensitivity of a fiber M-Z interferometer.

Rotation sensing based on a side-coupled spaced sequence of resonators

A side-coupled spaced sequence of resonators (SCSSOR) displays strong dispersion with a magnitude much larger than that of conventional waveguides. We investigate the Sagnac effect in a SCSSOR structure. An explicit expression of the Sagnac phase difference of the structure is derived and discussed. The results show the sensitivity is proportional to the number and dispersion intensity of resonators. Compared with other resonator structures, one advantage is that it is not necessary to preserve the same circumference of each resonator, which is difficult to realize in practice. The results also predict a SCSSOR structure can be used for the realization of highly sensitive and compact integrated rotation sensors and gyroscopes.

Observation of microsphere movement driven by optical pulse

A laser beam was coupled into a tapered optical fiber tip with a diameter of about 2.6 μm, and a 46 μm diameter microsphere was propelled by the outgoing optical pulse. With the change of pulse energy from 1.35 to 7.22 μJ, the calculated average velocity of the driven microsphere varied from 0.38 to 10.68 cm/s. The scanning electron microscope images of the fiber tip show that there is no thermal damage during the experiment.

Luminescence probe for temperature sensor based on fluorescence intensity ratio

The β-NaLuF4: Yb3+/Ho3+ nanocrystals have been synthesized via a solvothermal method, which were protected in a SiO2 capillary tube. Upon 980 nm continuous laser excitation, the micro-tube waveguide characteristic was studied. Additionally, the upconversion emissions from the 5F4, 5S2→5I8 (~545 nm), 5F5→5I8 (~650 nm) and 5F4, 5S2→5I7 (~750 nm) transitions of Ho3+ ions in the micro-tube were obtained and studied as a function of temperature in the range of 300~500 K. The maximum sensitivities derived from the fluorescence intensity ratio technique are 1.53% K−1 (I750/I650) and 0.09% K−1 (I545/I650) at 300 K. The result demonstrated that the near infrared-red absolute sensitivity is more sensitive as a thermometer than the green-red at the whole temperature range. The device is a promising candidate for high resolution luminescent thermometers operating under temperature of 300~500 K.

Observation of whispering gallery modes in microtube-microspheres system

We proposed that a fluorescent microsphere with diameter of 6 μm was manipulated into a microtube with inner diameter of 6.2 μm. The whispering gallery modes (WGMs) of fluorescence resonance were observed by 532 nm laser pumping the microspheres-mircotube system. Another microsphere with the same diameter was manipulated into the microtube and mode splitting in the system of two spheres in contact in the mircotube was demonstrated. We also discussed relationship between WGMs peak intensity and the excitation power. The scheme will bring more insight into the applications of WGMs for biomedical diagnostics and microfluidics.