Temperature dependence of whispering gallery modes of quantum dot-doped microbottle resonators
Introduction
Dielectric microresonators have applications as micro-lasers [[1], [2], [3]], micro-sensors [4,5] and in frequency stabilization [6]. The symmetric, dielectric resonators give rise to whispering gallery modes (WGMs) in the electromagnetic spectrum. Geometries such as circular [7], rectangular [8], spherical [9] and microtoroids [10] have been extensively studied. However, microbottle resonators (MBRs) are relatively new in literature [11]. In contrast to the spheres or toroids, the modes in MBRs are extended along the axis of the resonator. The free spectral range (FSR) of MBRs is an order of magnitude smaller than that of the microspheres of same dimension [12]. This is due to the fact that the light ray in MBRs traverses a longer path - an added advantage for improved light-interrogation with measurands for sensing applications. Among a few methods of preparing MBRs [11,13], the self-assembly process is simpler and easier to fabricate quickly in large numbers. Recently, the self-assembled MBRs supporting Q values of the order 106 have been reported [14] at 1550 nm.
Quantum dots (QDs) are semiconductor particles with sizes less than the exciton Bohr radius. Their optical properties lie in between that of the bulk and a single molecule [15]. The optical, photoluminescence (PL) and electronic properties of QDs can be tuned by varying their sizes. One of the popularly studied semiconductor QDs is cadmium selenide (CdSe). The CdSe QDs have band gap in the range of 1.7–2.1 eV. These have been used in optoelectronic devices, solar cells [16] and light emitting diodes applications [17].
Since WGMs are highly sensitive to the size and refractive index of the resonator, any change in these parameters causes a shift in the position of a mode. Temperature dependent studies of bare microresonators and doped with dyes have been reported in literature. It is reported that the Q values for passive microresonators are ~106 [18,20] at 1550 nm. For active microresonators [19,21] the Q values fall in the range 103-104 due to material absorption by dopants and Rayleigh scattering at shorter wavelengths. The photobleaching of dyes is known to induce errors in measurement [21]. To avoid photobleaching, inorganic fluorophores such as QDs of CdSe with high photobleaching threshold can be used to prepare self-assembled MBRs (sMBRs). Poly (methyl methacrylate) (PMMA) can be used due to its good optical transparency in the visible and near-infrared wavelengths [22]. In this paper, finite element modelling has been used to mimic the experimental system to understand the amount of shift in WGMs for the temperature range of −50 °C to 75 °C. The advantage of self-assembled MBR lies in its fabrication process and handling and fixing it with its tails on the substrates for temperature controlled experiments. The experiments are performed on the temperature dependence of CdSe QD doped PMMA sMBRs with free-space pumping. It is observed that the simulated and experimental data are in good agreement and the system has high photo- and temporal stability for probable applications in sensing.
Section snippets
Experimental details
CdSe/CdS core shell QDs dispersed in hexane were obtained from CANdots. PMMA (average molecular wt. 120,000) and toluene (99.8%) were obtained from Sigma-Aldrich. The tapered fibers were made by heat and pull method using a standard single mode optical fiber [11]. The sMBRs were prepared by a method described elsewhere [14]. Briefly, a tapered fiber was dipped in the PMMA solution of toluene and withdrawn. During this process, a thin layer of PMMA gets coated on the surface of the cylindrical
Modelling of the PMMA bottle resonator
The MBR has been modelled to look into the behavior of the resonance frequency of a particular bottle mode as a function of the temperature. Fig. 2A shows the schematic of a sMBR. The blue lines indicate the trajectory of the bottle modes. The resonant modes are identified by three quantum numbers (m, p, q) where m is the azimuthal quantum number or mode number, p is the radial quantum number and q is the axial quantum number.
The finite element modelling (Comsol Multiphysics 5.2) was used to
Steady state absorption and PL spectra
The absorption spectrum of the colloidal solution of CdSe QD in hexane is shown in Fig. 3A. The first excitonic peak of the QD is located at 600 nm. The PL spectrum shows a maximum at 610 nm. These values are in agreement to those reported in literature. A Tauc plot [26] has been used for estimating the energy band gap of CdSe/CdS core shell QDs suspended in hexane as follows. For a direct band gap semiconductor is plotted as a function of energy where is the absorption
Conclusions
QD doped self-assembled microbottle resonators exhibit WGMs in the luminescence spectra. The effect of temperature has been studied from −50 °C to 75 °C. The WGMs show a linear blue-shift with increase in the temperature. It has been observed that the smaller sMBR shows higher sensitivity with temperature compared to the larger one. The shift in the positions of the WGMs in sMBRs with change in temperature along with the high photo-stability of the system can be used in fluorescence based
CRediT authorship contribution statement
Shubhayan Bhattacharya: Conceptualization, Investigation, Methodology, Data curation, Writing - original draft. Aneesh V. Veluthandath: Writing - review & editing. Ganapathy Senthil Murugan: Supervision, Writing - review & editing. Prem B. Bisht: Conceptualization, Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
The authors thank Department of Science and Technology (DST), New Delhi, for financial assistance. PBB thanks financial support by IITM grant no. TTR1819015IITMPREM.
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