Effect of physisorption and chemisorption of water on resonant modes of rolled-up tubular microcavities
© Zhong et al.; licensee Springer. 2013
Received: 13 November 2013
Accepted: 9 December 2013
Published: 18 December 2013
Both blue- and redshifts of resonant modes are observed in the rolled-up Y2O3/ZrO2 tubular microcavity during a conformal oxide coating process. Our investigation based on spectral analyses suggests that there are two competitive processes during coating: desorption of both chemically and physically absorbed water molecules and increase of the tube wall thickness. The redshift is due to the increase of the wall thickness and corresponding light confinement enhancement. On the other hand, desorption of water molecules by heating leads to a blueshift. The balance of these two factors produces the observed bi-directional shift of the modes while they both contribute to promoted quality factor after coating.
KeywordsMicrocavity Coating Mode shift Desorption Detection
Optical microcavities with tubular geometry exhibit several advantages compared to other types of optical microcavities[1–4]. They naturally assume a hollow structure and are fully integrative into lab-on-chip systems. In the past years, rolled-up tubular microcavities have been used as cell culture devices[6, 7], microlasers[8, 9], sensors, and so on. Especially, rolled-up microcavities with (ultra)thin wall thickness are sensitive to tiny alterations and modifications in the vicinity of the inner and outer tube wall surfaces. Thus, the microcavities exhibit excellent potential applications as sensors in the fields of optoelectronics, biosensing[6, 12], and integrated optofluidics[10, 13]. Very recently, preliminary results concerning detection of dynamic molecular processes were demonstrated on a self-rolled-up SiO/SiO2 optical microcavity with sub-wavelength wall thickness. In fact, the molecule absorption/desorption are quite complex processes, and their interaction with the evanescent field is even intricate, especially in the nanoscale. Before this sensing technique can be put into practical applications like other label-free methods, more work must be done to disclose the mechanism and to exhibit the general and diverse capability of the approach.
In this letter, we focus on the detection of physically and/or chemically absorbed water molecules by using a rolled-up tubular microcavity as a core component. The microcavities used in this work were prepared by releasing prestressed 33.5-nm-thick Y2O3/ZrO2 circular nanomembranes on photoresist sacrificial layers. The influence of surface composition (e.g., coating formed by atomic layer deposition (ALD) and water molecules absorbed from atmosphere) on the mode (including the sub-modes) positions as well as the Q-factors will be discussed on the basis of detailed spectral analyses. Different from a previous work, we conduct a much more meticulous ALD coating process and observe an unusual blueshift of the resonant mode in the present case. We find that the observation originates from the effects of both chemisorption and physisorption water molecules, suggesting a rather complicated nature of the interaction between the evanescent field and the surrounding environment.
The bare Y2O3/ZrO2 tubular optical microcavities are prepared via self-rolled nanotechnology as described elsewhere. These Y2O3/ZrO2 microtubes are uniformly coated with up to 150 monolayers (MLs) of HfO2 by ALD to tune the optical resonant modes. Tetrakis(dimethylamino)hafnium (Hf[N(CH3)2]4) and H2O are used as precursor sources; pulse times for hafnium source and water source are both 15 ms per circle. The abovementioned two precursors react completely in each circle at 150°C and 30 Pa (N2 as the carrying gas) to obtain HfO2 coating layer on the wall of the microtube. The thickness of the HfO2 layer is approximately 2 Å/ML, which is calibrated using an atomic force microscope (AFM). After coating of every 10 HfO2 MLs, the sample is taken out and the microphotoluminescence (micro-PL) spectra (excitation wavelength 514 nm) are collected from the center spot of the microtube. All the optical measurements were carried out in the air at room temperature. Light emission from defect-related luminescent centers can circulate and interfere constructively in the circular cross section of the tubular microcavity forming stable resonance at certain wavelengths, noticed as an optical resonance mode[16, 17].
Results and discussion
According to the literature, the mode positions show a strong relationship with the evanescent field and the surrounding medium[5, 10], and the interaction of evanescent field with the absorption molecules on the wall of tubular microcavity leads to a detectable shift in the resonant frequency (i.e., mode position)[10, 18] The previous experimental and theoretical results indicated that the resonant wavelength monotonically redshifts with increasing thickness of the high-refractive-index oxide (Al2O3 or HfO2) coating. In the present case, the modes show an obvious redshift with the HfO2 coating increasing from 20 to 150 MLs (Figures 1c and2a), which fits well with the previous experimental results and theoretical prediction. However, when the HfO2 coating is less than 20 MLs, the mode positions show an unusual blueshift, which in any case suggests a change in the light's circulation and interference in the microcavity. To check the light confinement therein, we calculated the Q-factor using the formula Q = λ/∆λ, where λ and ∆λ denote the mode position and the full width at half maximum (FWHM) of the mode, respectively, and the results are plotted in Figure 2b. It is not surprising that as a consequence of the improved light confinement, the Q-factor appears to have a pronounced enhancement with increasing coating layers. However, the blueshift of modes in the case of a few coating layers ought to be related to other effects different from the increasing wall thickness.
Finally, we would like to discuss more about the influence of surface condition on the Q-factor. It is already well known that an oxide coating layer with high refractive index promotes an effective refractive index and light confinement which leads to low light loss and higher Q-factor[3, 16, 21]. For the tubular microcavity in our work, the most important loss terms are bulk adsorption (Qmat-1) and loss introduced by surfaced contaminants (Qcont-1): Q-1 = Qmat-1 + Qcont-1[5, 18]. The adsorption of water molecules on the surface will increase the roughness of the tube wall as one kind of contaminant which magnifies Qcont-1 and consequently deteriorates the entire Q-factor. The desorption of water molecules, on the contrary, will enhance the Q-factor. Both the water molecule desorption and the increase of the tube wall thickness during ALD contribute to the enhancement of the Q-factor, as shown in Figure 2b.
In summary, we have demonstrated that physisorption and chemisorption of water can influence the optical resonance in rolled-up Y2O3/ZrO2 tubular microcavity. Desorption of these two kinds of water molecules from the surface of the tube wall at high temperature can cause a blueshift of optical modes while additional coating of oxide layers with high refractive index leads to a redshift of the modes. Although both effects promote the Q-factor of the microcavity, the competition among them produces a bi-directional shift of the modes during the ALD process. Our current work demonstrates the feasibility of precisely modulating the modes of the rolled-up microcavity with a fine structure and high Q-factor. These discoveries may find potential applications in environmental monitoring. For instance, a humidity sensor using a tubular microcavity as a core component can be fabricated to detect the humidity variation of the environment.
Atomic force microscope
Atomic layer deposition
Full width at half maximum
This work is supported by the Natural Science Foundation of China (nos. 51322201 and 51102049), 'Shu Guang’ project by Shanghai Municipal Education Commission and Shanghai Education Development Foundation, Project Based Personnel Exchange Program with CSC and DAAD, Specialized Research Fund for the Doctoral Program of Higher Education (no. 20120071110025), and Science and Technology Commission of Shanghai Municipality (nos. 12520706300 and 12PJ1400500). JW thanks the support from China Postdoctoral Science Foundation (no. 2011 M500731). We thank Dr. Zhenghua An from Fudan Nano-fabrication and Devices Laboratory for the assistance in sample fabrications.
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