Shuangqiang Liu

Tapered optical fiber waveguide coupling to whispering gallery modes of liquid crystal microdroplet for thermal sensing application

We demonstrate efficient coupling to the optical whispering gallery modes (WGMs) of nematic liquid crystal (NLC) microdroplets immersed in an immiscible aqueous environment. An individual NLC microdroplet, confined at the tip of a microcapillary, was coupled via a tapered optical fiber waveguide positioned correctly within its vicinity. Critical coupling of the taper-microdroplet system was facilitated by adjusting the gap between the taper and the microdroplet to change the overlap of the evanescent electromagnetic fields; efficient and controlled power transfer from the taper waveguide to the NLC microdroplet is indeed possible via the proposed technique. We also found that NLC microdroplets can function as highly sensitive thermal sensors: A maximum temperature sensitivity of 267.6 pm/°C and resolution of 7.5 × 10-2 °C were achieved in a 78-μm-diameter NLC microdroplet.

Tunable whispering gallery modes lasing in dye-doped cholesteric liquid crystal microdroplets

This paper reports the observation of optically pumped whispering gallery mode (WGM) lasing emission from dye-doped emulsion microdroplets of cholesteric liquid crystals (CLCs) suspended in an immiscible aqueous environment. The higher index contrast between the immersion liquid and CLC microdroplet contributes to the generation of WGM resonance so that lasing emission can be realized in the CLC microdroplet via total internal reflection. The WGM lasing nature is verified by numerical simulations as well as experiment of size-dependent lasing action. The lasing wavelength depends on the refractive index of the CLC microdroplet and can be tuned by varying the temperature. A tuning range of 9.1 nm within 6 °C temperature interval is realized in a 20-μm-diameter microdroplet. Such a temperature tunable microlaser is promising for applications of flexible photothermic devices.

Humidity-insensitive temperature sensor based on a quartz capillary anti-resonant reflection optical waveguide

A compact, humidity-insensitive, fiber optic temperature sensor based on a quartz capillary, antiresonant reflecting optical waveguide was proposed and experimentally demonstrated. The transmission spectral responses of the proposed sensor were experimentally investigated regarding temperature variation and environmental humidity. Resonant dips exhibited temperature sensitivity as large as 201 pm/°C from -30 to 45 °C in a humid environment. By coating a sufficiently thick gold film onto the sensor surface, the humidity cross-sensitivity issue was effectively resolved. This proposed sensor was anticipated to find potential applications in humid environments, and moreover, immunity to humidity-sensitivity ensures its applicability in marine environments.

Thermal sensing based on whispering gallery modes in tapered-fiber-coupled liquid crystal microdroplets

We report the efficient coupling of optical whispering gallery modes (WGMs) in liquid crystal microdroplets suspended in immiscible aqueous environment. Individual nematic liquid crystal (NLC) microdroplet is confined at the tip of a microcapillary used to generate the microdroplets and coupled through a tapered optical fiber waveguide positioned in the vicinity of the microdroplets. Efficient coupling of WGMs is observed in the NLC microdroplets with a diameter of 50–150 μm. In addition, the wavelengths of the WGMs can be tuned by temperature, making such NLC microdroplets suitable for thermal sensors. A temperature sensitivity of 0.244 nm/°C is achieved in a 75-μm-diameter microdroplet. The estimated thermal resolution of the microdroplet sensor is 8.2 × 10−2 °C.

Micro-gun based on laser pulse propulsion

This paper proposes a novel “micro-gun” structure for laser pulse propulsion. The “micro-bullets” (glass microspheres) are irradiated by a laser pulse with a 10 ns duration in a dynamic process. Experimental parameters such as the microsphere diameter and the laser pulse energy are varied to investigate their influence on laser pulse propulsion. The energy field and spatial intensity distribution in the capillary tube were simulated using a three-dimensional finite-difference time-domain method. The experimental results demonstrate that the propulsion efficiency is dependent on the laser pulse energy and the microsphere size. The propulsion modes and sources of the propelling force were confirmed through direct observation and theoretical calculation. Waves also generated by light-pressure and thermal expansions assisted the propulsion.

Whispering gallery modes in a liquid-filled hollow glass microsphere

We develop a hydrofluoric (HF) etching process to open a microhole on the hollow glass microsphere (HGM). The typical whispering gallery mode (WGM) resonance was observed by coupling the HGM with a tapered fiber. Dioctyl phthalate was filled into the HGM, and the resonance wavelength decreased at elevated temperatures. We analyzed the WGM resonance properties inside the liquid-filled HGM with a higher or lower refractive index in comparison to the HGM wall. Four different liquids were also injected into the HGM to investigate the influence of the thermo-optic coefficient on the temperature sensitivity. Size-dependent experiments further showed that HGMs with varying sizes have varying temperature sensitivity. The maximum temperature sensitivity observed was 334.3  pm/°C.

Observation of thermally induced tuning of lasing emission from melamine–formaldehyde resin microspheres

We report on the observation of the thermally induced tuning of the whispering gallery mode lasing emission from active microcavities. Melamine–formaldehyde resin microspheres were doped with a fluorescent dye and pumped by a 532 nm pulse laser. We show that microspheres with different sizes display lasing emissions in different parts of the spectrum, with a varying number of lasing modes present in the spectrum. The dominant thermo-optic effect leads to a blue shift of the whispering gallery mode lasing. The thermal sensing with a sensitivity of 0.33 nm/°C is higher than that of conventional polymer microcavities.