聚合物微球 Whispering-Gallery 模式激光器用于湿度传感

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" Whispering-gallery 模式激光器在聚合物微球中的应用，用于湿度传感" 本文报道了一种基于高质量（Q 值）回音壁模式（WGM）共振的微激光器，这种微激光器在生物传感和成像应用中具有巨大的潜力。研究者通过控制乳液溶剂蒸发法成功制备了染料掺杂的聚合物微球。在单个微球中，利用飞秒激光泵浦实现了低阈值和高Q因子的WGM激光。这种微激光器的一个关键特性是其对环境相对湿度（RH）的敏感性。当水分子暴露于微球附近时，由于湿度的轻微变化，可以监测到激光模式的位移，从而显示出高达6 pm/RH%的高灵敏度。微球的制造工艺是实现WGM激光的关键步骤。控制乳液溶剂蒸发方法使得聚合物微球具有均匀的结构和理想的光学特性，这对于维持稳定的WGM非常重要。WGM激光器的工作原理是，光在微球的边界上连续反射，形成类似于教堂回音壁的效果，即光在微球内部绕行多次，导致非常高Q因子的谐振模式。高Q因子意味着微激光器可以在较低的激发功率下工作，这对于节能和提高灵敏度至关重要。湿度传感应用中，聚合物微球激光器的响应源于微球对环境湿度变化的敏感性。当湿度增加时，微球表面的折射率会发生变化，进而影响WGM的共振频率。通过精确测量激光模式的频率位移，可以实时监测环境湿度的变化，这在环境监控、工业过程控制以及医疗诊断等领域具有潜在的应用价值。此外，染料掺杂是增强微球激光性能的一种手段，染料能够吸收泵浦光并将其转化为激光辐射，使得微激光器能够在特定波长范围内高效工作。实验中使用的飞秒激光泵浦技术是一种非线性光学过程，它能够快速激发染料分子并启动激光振荡，从而实现低阈值激光发射。总结来说，这篇论文展示了基于聚合物微球的WGM激光器在湿度传感方面的创新应用，这种激光器具有低阈值、高灵敏度的特性，有望推动微纳光学传感器的发展，并为生物传感和成像领域带来新的解决方案。同时，该研究也为设计和优化新型微激光器提供了理论依据和技术参考。

Whispering-gallery mode lasing from polymer

microsphere for humidity sensing

Wei Xu (徐巍)

, Chunxiang Xu (徐春祥)

*, Feifei Qin (秦飞飞)

, Yaqi Shan (单雅琦)

Zhu Zhu (朱珠)

, and Ye Zhu (朱烨)

State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering,

Southeast University, Nanjing 210096, China

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China

*Corresponding author: xcxseu@seu.edu.cn

Received April 9, 2018; accepted June 6, 2018; posted online July 30, 2018

Microlasers based on high quality (Q) whispering-gallery mode (WGM) resonance are promising low threshold

laser sources for bio-sensing and imaging applications. In this Letter, dye-doped polymer microspheres were

fabricated by a controlling emulsion solvent evaporation method. WGM lasing with low threshold and high

Q factors was realized in an individual microsphere under femtosecond laser pumping. The slight change of

environmental relative humidity (RH) can be monitored by measuring the shift of the lasing modes at the ex-

posure of water molecules, which demonstrates the sensitivity is as high as 6 pm/RH%. The results would offer

an insight into employing microlasers as sensors.

OCIS codes: 140.2020, 160.2540, 280.3420.

doi: 10.3788/COL201816.081401.

The contro l of relative humidity (RH) is extremely funda-

mental in many industrial processes and human life,

including meteorology, agriculture, and medical opera-

tions. A large amount of humidity detection devices that

monitor RH have been explored mainly based on the prin-

ciples of optics and electronics. It is well-known that

optical approaches for humidity sensing have many out-

standing advantages, including immunity to the electrical

and vibrational sources of noise, explosion proof, remote

sensing, small size, and the possibility of multimode detec-

tion integrated in an optical device compared to conven-

tional electronic detection approaches

[1]

. One main optical

approach is based on whispering-gallery mode (WGM) mi-

crocavities, which have high quality (Q) factors and small

mode volume

[2,3]

. Because a portion of the optical field (the

so-called evanescent tails) slightly extends into the envi-

ronment, WGM cavities and their resonant wavelengths

are inherently sensitive to the refractive index changes

of the surroundings or the binding of biological/chemical

molecules to the resonator surface. For instance, Zhang

et al. have demonstrated the use of non-resonating hydro-

gel spheres coupl ed to an optical fiber core as an RH op-

tical sensor. Due to the fact that the refractive index of the

hydrogel changes with the ambient humidity, the light is

scattered out from the core of the fiber in a different

amount, and, consequently, the tapered fiber transmit-

tance changes

[4]

However, t he study of the WGM sensor is mainly con-

centrated in the field of a passive cavity. From the appli-

cation point of view, the detection schemes of most solid

WGM sensors usually adopt tapered fibers or prism cou-

pling configurations, which are bulky and difficult to ap-

ply in the micro- and nanoscale sample environments.

This problem can be solved in the high Q microlasers that

are achieved by incorporating optical gain media, which

are desired to have a better sensitivity

[5]

. Dye-doped poly-

mer hemispheres with a diameter 53 μm were applied in

refractive index sensing, and the sensitivity of 103 nm per

refractive index unit (RIU) was achieved

[6]

. According to

the requirements of practical applications, there is some

urgent need of lasing with miniaturized dimensions, whi ch

is more important in sensing in the micro scale, such as

sensing in cells. However, the Q factor of WGM microcav-

ities decreased with the diameter because the bending ra-

diation loss of circular resonators shows an exponential

decline with their diameters, which is not beneficial for

sensing purposes, as it amplifies the interferences of ampli-

tude noise and spectral resolution. Thus, the design of high

Q and compact size WGM cavities should be proposed in

sensing areas.

In this Letter, high Q and compact size organic WGM

resonators were obtained by controllably evaporating the

solvent of the emulsion solution. Optically pumped lasing

action is observed at room temperature from the dye-

doped microspheres by using a micro-photoluminescence

(μ-PL) system. Lasing performances were explored by

experiment and finite-differen ce time-domain (FDTD)

simulation. By measuring the shift of their lasing modes,

we successfully monitored the slight change of environ-

mental RH. The sensitivity of polymer microlasers is up

to 6 pm/RH%.

The polymer microspheres were prepared with an emul-

sion solvent evaporation method, as reported before

[5]

The experiment process for fabricat ion of lasing cavities

was shown in Fig.

1(a). Specifically, 50 mg polystyrene

(PS) particles were dissolved in 500 μLCH

. Then,

2 mg orga nic dyes 4- (dicyanomethylene)-2-methyl-6-

(4-dimethylaminostyryl)-4H-pyran (DCM) were mixed

COL 16(8), 081401(2018) CHINESE OPTICS LETTERS August 10, 2018

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聚合物微球 Whispering-Gallery 模式激光器用于湿度传感

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