Open Access
Add to BibSonomy
Abstract
The potential of whispering-gallery-modes (WGMs) microcavities in sensing applications has been being released continuously with improvements from various aspects. Introducing smart materials and structures into the WGMs microcavities based sensing systems are an effective approach to promote their applications in real world. Here, we propose a smart grating as the coupling setup to a WGMs microcavity of polystyrene microsphere to enhance the responses to chemical and thermal stimulations. The changes of the coupling distance due to the deformation of the smart grating induce additional increments to the intrinsic wavelength shifts of the WGMs of the microcavity, which is proved to be the mechanism of the response enhancements. We use two-photon lithography based “lab on fiber” technology to realize the device and the demonstration of the response enhancements. Our results may be of great significance to the design of the WGMs microcavity based chemical and temperature sensors.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Enhancing the response and robustness of the WGMs microcavity sensing systems to external stimulations are fundamental approaches to improve the performance and practicability of them [1,2]. Generally, the response enhancements are realized by optimizing the microcavity, while the robustness improvements are achieved by modifying the auxiliary part in the system. Improving the quality factor of the WGMs microcavities can enhance the light field intensities of the WGMs inside them, so as to enhance the response of the sensing systems to external stimulations. Therefore, researchers have been making strenuous efforts to improve the quality factor of the WGMs microcavity by optimizing the specific configuration, surface smoothness and the corresponding fabrication process [3–6] or using active materials [7]. In addition, combining sensitive material to microcavity is another effective way to improve their response to the external stimulations, such as gold nanoparticles [8,9], magnetic materials [10,11], biological materials [12,13], smart materials [14–16] and so on. The way to enhance the robustness of the WGMs microcavity sensing systems to the external stimulations is mainly achieved by optimizing and improving the classical coupling setup of tapered optical fiber in the systems or designing all-new coupling setup [17]. Considering that the fragility of the tapered optical fiber is originated from the long and thin region for the evanescent wave, a long period grating-based fiber coupler with thick diameter [18] and a fiber-attached coupler with thin region fixed were proposed [19]. Various all-new coupling setup such as metallic grating [20,21], metamaterial engineered silicon photonic coupler [22], optical nanoantenna coupler [23], and perpendicular coupler [24,25] which can coupled light into WGMs microcavity in the way of end-fire injection were invented and created.
On the other hand, besides the desired performance of sensitive response to external stimulations and robustness of the microcavity sensing systems, intellectualization and miniaturization are also desirable merits to the systems. Although, the microcavity can be fabricated with smart materials [15,16,26], thereby, smart functions can be introduced into the microcavity, it is hard for the traditional couplers of bulky prism and tapered optical fiber to satisfy the requirement of miniaturization at the same time. While the abovementioned couplers provided more options to satisfy the requirement of miniaturization and integration, there is no intelligence has been introduced so far.
In this work, we demonstrated a smart grating coupled WGMs microcavity system with enhanced responses to both chemical and thermal stimulations. The smart grating is a suspended two-dimensional grid made of smart materials [27,28] of metal-polymer double layer film. The smart grating will bend when it is exposed to external stimulations, and the coupling distance between the smart grating and the WGMs microcavity will be changed accordingly, therefore, the resonant wavelengths of the WGMs are shifted additionally. If such wavelength shift induced by the bend of the smart grating are additive to that caused by the same stimulations to the microcavity, the system will show enhanced response to the stimulations.
2. Working principle: response enhancement mechanism of the smart grating coupled WGMs microcavity
Figure 1 shows the mechanism of how the responses of the smart grating coupled WGMs microcavity to the external stimulations can be enhanced. A microsphere is used as the WGMs microcavity, and a suspended metal-polymer double layer film with two-dimensional periodic holes on it plays the role of the smart grating (Fig. 1(a) and (b)). light being incident from underside is diffracted by the grating with suitable period and then coupled into the microsphere to excite both the clockwise and counterclockwise WGMs. Resonant dips of WGMs can be observed in the reflected spectra. When the smart grating coupled microsphere system is exposed to some simulations, which cause changes of both the microsphere and the grating, wavelength shift (Δλ) of the WGMs will happen. Usually, the external stimulations cause the changes of the radius (Δr) and refractive index (Δn) of the microsphere, then, induce the wavelength shift of the WGMs of ΔλMicrosphere(Δr, Δn). While the external stimulations cause the swelling and bend of the smart grating which induces the change of the coupling distance between the grating and the microsphere, therefore, results in another wavelength shift ΔλGrating (Δd, ngold), where, d is the coupling distance between the smart grating and the microsphere, Δd is the change of the distance, ngold is the refractive index of the metal layer (gold is used in the experiment). Considering that the metal layer is located at the side of the microsphere and it causes more significant wavelength shift compared with dielectric materials, the refractive index of metal is a main factor for ΔλGrating. Supposing that the external stimulations cause red shift of ΔλMicrosphere, the ΔλGrating can be red or blue shift, therefore, the overall wavelength shift can be expressed as (Fig. 1(c))
Fig. 1. Schematic of the mechanism of the response enhancement of the smart grating coupled WGMs microcavity to the external stimulations. (a) The side view of the smart grating coupled spherical microcavity system and light propagation in it. (b) The smart grating is a suspended metal-polymer double layer film with two-dimensional periodic square holes on it. (c) The response of the smart grating coupled microcavity system to the external stimulations interrogated with wavelength shift of the WGMs. (d), (e) and (d) the corresponding situations of stiff grating coupled microcavity system. The thiknesses of the polymer layer, the size and period of the square holes both for smart and stiff gratings are the same h1 = 1.0 µm, 0.675 µm × 0.675 µm, and P = 1.35 µm, in experiment.
There are also two origins for the Δd which can be denoted as ΔdSwelling and ΔdGrating bending with the former one describes the distance change caused by the swelling of the polymer layer of the grating, and the latter one caused by the bend. The swelling always cause the decrease of the coupling distance, while the bend can cause increase or decrease of the distance, therefore, the overall changes Δd = ΔdSwelling ± ΔdGrating bending. When the bend of the grating results in the decrease of the coupling distance, “+” works in this formula; otherwise, “-” works.
As a contrast, the situation of a stiff polymer (which is a kind of photoresist in the experiment) grating coupled WGMs microcavity is also shown (Fig. 1(d) and 1(e)), where the word of “stiff” means that the grating is fixed on the substrate. The wavelength shift can be expressed as (Fig. 1(f))
The significant difference of the two situations is that the wavelength shift caused by the bend of the smart grating coupler are much larger than that caused by the swelling of the stiff grating, which is the physical mechanism of the response enhancement of the smart grating coupled WGMs microcavity.
3. Design of the smart grating coupled WGMs microcavity on the tip of multicore optical fiber
3.1 Spectra and modes analysis of smart grating coupled WGMs microcavity
In order to quantitatively show the wavelength shift ΔλGrating (Δd, ngold) caused by the change of the coupling distance between the smart grating and the WGMs microcavity, the system is modeled in two dimension and simulated by the FDTD (Finite Difference Time Domain) method, where a cylinder is employed to mimic the spherical microcavity. The properties of the WGMs in the cylinder is very similar to those of WGMs in a sphere when the polar and azimuth angular numbers have the same value. Therefore, in this condition, the cylinder is a good approximation to sphere. The radius and the refractive index of the cylinder are 10 µm and 1.59. The refractive index of the polymer layer of the smart grating is 1.52, and the period is 1350 nm, the duty cycle is 0.5, the thickness of the gold layer is 200 nm. Figure 2(a) shows that the simulated reflection spectra with coupling distance varying from 0 to 600 nm with steps of 100 nm. The thickness of the polymer layer is 1000 nm. The normal incident light is set as transverse electric (TE) polarized. As can be seen in the spectra, six WGMs are excited in the wavelength range of 1520 ∼ 1570 nm, which are $T E_{59}^{1}, T E_{49}^{3}, T E_{53}^{2}, T E_{58}^{1}, T E_{48}^{3} \,\textrm{and}\, T E_{52}^{2}$, respectively (Fig. 2(d)). All the modes shift to short wavelength and the speed of the shift slows down with the coupling distance increasing. Large wavelength shift is observed for small coupling distance (d < 100 nm). As a contrast, the reflection spectra with varying thickness of h from 400 to 1700 nm with steps of 300 nm while the coupling distance is kept unchanged are shown in Fig. 2(b). Even for such large thickness change of the polymer layer in the smart grating, the caused wavelength shift is still almost negligible. This enabled us believe that the observed obvious differences of the wavelength shifts of the WGMs for smart and stiff grating coupled WGMs microcavities in the experiments were indeed caused by the bend of the smart grating.
Fig. 2. The modes excited in the smart grating coupled WGMs microcavity and their tuning properties with the parameters varying. (a) Reflection spectra of the smart grating coupled WGMs microcavity with varying coupling distance d = 0, 100, 200, 300, 400, 500, 600 nm. The metal layer in the grating is fixed as 200 nm. The period of the grating is 1350 nm. (b) Reflection spectra of the smart grating coupled WGMs microcavity with varying thickness of the smart grating h = 400, 700, 1000, 1300, and 1700nm, while keep the distance fixed d = 300 nm. (c) The typical experimental reflection spectra of a sample and the corresponding simulation spectrum. (d) The electrical field distribution of the WGMs modes in the observed reflection spectrum.
Figure 2(c) shows the typical spectrum obtained in experiment with the above optimal parameters (h = h1 + h2 = 200 nm +1000 nm, d = 300 nm) and the corresponding simulated one. The six WGMs in the simulated spectrum mach well with the corresponding experimental ones.
The aforementioned design strategy is universally applicable for various gratings and WGMs microcavites made of different materials, as long as the gratings and the microcavities response to the external stimulations at the same time. In the following two sections, we demonstrate the effectiveness of the design strategy by using the system of a WGMs microcavity of polystyrene microsphere coupled with gold-photoresist double layer grating.
3.2 Design of smart grating coupled WGMs microcavity on fiber tip
We used the “lab on fiber” technology developed in our lab [29–32] to realize the smart grating coupled WGMs microcavity experimentally as shown in Fig. 3.
For the design of the system, four requirements must be satisfied. Firstly, the smart grating must be supported by a pedestal to ensure it to be suspended. Secondly, the coupling distance between the smart grating and the WGMs microcavity must be controlled precisely. Thirdly, the WGMs microcavity of polystyrene microsphere must be assembled onto the design position with some fabrication strategy. Fourthly, it must be convenient to achieve the optical measurements. Figure 3 shows a design which satisfies the above four requirements, where the system consists five parts. The smart grating and the microsphere are supported by pedestals, respectively. The coupling distance can be controlled by setting the height and diameters of the pedestal with optimized values. A funnel is designed for guiding the microsphere to be assembled onto the pedestal. Finally, the end facet of a seven-core optical fiber is employed as the platform for the realization of seven systems with each of them is located on one core of the fiber. Light can be incident into the systems and reflected back to the cores for conveniently detection.
4. Realization and characterizations of the device
4.1 Fabrication strategy for the smart grating coupled WGMs microcavity on the tip of multicore optical fiber
A template-assisted assembly fabrication strategy based on two-photon lithography direct writing technology on the tip of seven-core optical fibers (Fibercore Ltd., UK) is developed to fabricated the system.
Figure 4(a) shows the process of the fabrication: (I) direct laser writing of seven units with two-photon lithography on the tip of the seven-core optical fiber (photoresist of IP-L 780 from Nanocribe Ltd was used); (II) developing with propylene glycol monomethyl ether acetate (PGMEA); (III) 200 nm gold film evaporation on the structure; (IV) polystyrene microsphere colloid with radii of 10 µm (from Thermo Fisher Ltd., US) with concentration of 4% dropping to the tip of the optical fiber, then, the microspheres are self-assembled onto the pedestals under the force of the surface tension of water; (V) seven units on the tip of the seven-core optical fiber are obtained after dry. Scanning electron microscopy (SEM) and microscopy images of the obtained device are shown in Fig. 4(b)–4(g). Figure 4(b) and 4(c) are the SEM images of the template and the smart grating after gold evaporation, where the grating is a periodical square hole array with period of 1350 nm. The gold deposited directly on the facet of the optical fiber can be see through the square holes, which indicates that the smart grating is suspended on the fiber. Figure 4(d) and 4(e) are the SEM images system with polystyrene microspheres on the pedestals. Figure 4(f) and 4(g) are the SEM and microscopy images of one sample with only one microsphere at the center while the other six ones are empty. The holes of the two-dimension grating at the center are imaged by the microsphere. The colorful cross-shaped diffractive patterns imply that the good quality of the smart gratings.
4.2 Response of the metal-polymer double layer film to chemical stimulation
We designed and fabricated a gold coated micro-propeller to demonstrate that the gold-photoresist double layer film has smart response to the isopropanol stimulation. The micro-propeller is direct writing with two-photon lithography with the length, width and thickness of the fan blade of 10 µm, 10 µm and 1 µm, and then 5 nm of chromium and 200 nm gold were evaporated onto it. Figure 5 shows the optical microscopy images of the deformation process of the micro-propeller with stimulation of isopropanol. As can be seen, after being soaked for 20 minutes, the free ends of the blades deviated from original position 1 µm. The deformations were recovered completely when the micro-propeller was put into the air environment to dry for 720 minutes.
4.3 Response enhancement of the smart grating coupled WGMs microcavity to chemical stimulations
A experimental setup was established for the spectral characterization of the grating coupled WGMs microcavity microsphere structures on the tip of the seven-core optical fiber (as shown in Fig. 6). A tunable diode laser (TLB-6728-P, Newport Ltd., USA) is used as the light source. The polarization of the light is manipulated by an optical polarization controller, and the light is then transmitted into a circulator and coupled into a fan-out (MCFFO-S-07/37-1550-SM01-FC/APC-C, Fibercore Ltd., UK). The fan-out enables connections to the seven cores of the optical fiber to obtain reflected light from any one of the grating coupled microsphere structures. The reflected light passes back through the fan-out and circulator and is converted into an electrical signal using the photodetector (DETO8CFC/M, Thorlabs Ltd., USA). The signal is finally displayed and recorded using an oscilloscope (DSO7052B, Tektronix Ltd., USA).
Fig. 3. Schematic of the design of the smart grating coupled WGMs microcavity on the tip of multicore optical fibers.
Fig. 4. Schematic of the template-assisted assembly fabrication strategy for the the smart grating coupled WGMs microcavity on the tip of multicore optical fiber and the scanning electron microscopy (SEM) and microscopy images of the obtained device.
Fig. 5. The demonstration of the smart response of gold-photoresist double layer film to the isopropanol stimulation. A gold coated micro-propeller were created and then soaked into isopropanol solution, deformations of the fan blades were observed. The deformations ware recovered when the micro-propeller was put into air to dry.
Fig. 6. Experimental setup for the spectral characterization of the grating coupled WGMs microcavity microsphere structures on the tip of the seven-core optical fiber. A tunable diode laser was used as the light source. The fan-out is a device with seven single mode optical fibers on one side that is able to connect the circulator to one seven-core optical fiber on the other side, where microspheres are fabricated on the tip. The reflected light that passes back through the fan out and circulator is converted into an electrical signal via a photodetector, and the signal is displayed and recorded using an oscilloscope. The tip of the fiber is placed in a cabinet with temperature and vapor environment being changeable.
The photoresist layer in the smart grating and the polystyrene show swelling when they are located in the vapor environment of some volatile organic compounds, therefore, response enhancement was expected to be observed if the wavelength shifts induced by the grating bend is additive to the ones induced by the microsphere swelling. Here, we use the vapor of isopropanol (IPA) as the stimuli which is produced by its water solution with concentration of CIPA. Considering that the time need for the fully swelling of the 1000 nm photoresist layer is less than 100 s, we recorded the spectra after the system being put into the vapor for 100 s, and then, the system was put into air for 600 s for dry as shown in Fig. 7.
Fig. 7. Reflection spectra of (a) the smart grating and (b) the stiff grating coupled WGMs microcavity when they are located in isopropanol vapor produced by its aqueous solution with concentration (CIPA) varying from 0% to 17.6% in step of 0.8%.
In order to show the relationship between the wavelength shifts of the six WGMs ($T E_{59}^{1}, T E_{49}^{3},$ $T E_{53}^{2}$, $T E_{58}^{1}$, $T E_{48}^{3}$, and $T E_{52}^{2}$) and the concentration of the IPA aqueous solution clearly, the exact values of the wavelength shifts should be extracted from the spectra in Fig. 7 to draw the Δλ ∼ CIPA curves. However, as can be seen (Fig. 7(a)), the line shape of WGMs changes dramatically from dips to peaks when the concentration of the IPA solution is near 8.0% for the case of smart grating coupled WGMs microcavity. The line shape change is originated from that the Fano resonance of the system. Indeed, the grating coupled WGMs microcavity is exactly a coupled-resonators system. Therefore, we use the following strategy to define the wavelength shifts: for concentration lower than 8.0%, we use dips to define the wavelength shift, while for concentration higher than 8.0%, we use peaks to define the wavelength shift. For the concentration equal to 8.0%, we use a middle value for concentrations of 7.2% and 8.8%. Furthermore, the wavelength shift (2.5 nm) due to IPA is much larger than the linewidth of the dips or peaks (about 0.3 nm, the corresponding quality factor is 5200 for the case of smart grating), which implies that the shape changes of the Fano-like line have limited influence to the Δλ ∼ CIPA curves.
As the contrast, the wavelength shifts of the corresponding WGMs in the system of a stiff grating coupled WGMs microcavity of polystyrene microsphere are also shown (the $T E_{59}^{1}$ and $T E_{58}^{1}$, were not observed for the case of stiff grating).
As shown in Fig. 8(a)–8(c), there are clearly differences of the Δλ ∼ CIPA curves of the two systems. The smart grating system shows less wavelength shifts in the range 0% < CIPA < 8% than that of the stiff grating system, while the situation was reversed in the range 8% < CIPA < 17.6%, a turning point at CIPA = 8% was observed for the smart grating system. The smart grating system shows suppressed responses for vapor with lower concentration, while it shows enhanced responses for vapor with higher concentration.
Fig. 8. (a)-(c) The wavelength shifts of the six WGMs of $T E_{59}^{1}, T E_{49}^{3}, T E_{53}^{2}, T E_{58}^{1}, T E_{48}^{3} \,\textrm{and}\, T E_{52}^{2}$ when the smart and stiff grating coupled WGMs microcavities sensing systems are located in the isopropanol vapor environments. (d) The bend behavior of the smart grating with isopropanol vapor stimulation.
The observed results in Fig. 8(a)–8(c) imply that “-” works in the Eq. (1) in the concentration range of 0% ≤ CIPA ≤ 8%, while “+” works in the range of 8% ≤ CIPA ≤ 17.6%. The “-” and “+” are corresponding to the increase and decrease of the coupling distance, therefore, the smart grating must bend down in the range of 0% ≤ CIPA ≤ 8%, while the bend must recover in the range of 8% ≤ CIPA ≤ 17.6% as shown in Fig. 8(d).
Although it is easy to understand that the gold-photoresist double layer smart grating will bend down due to the swelling of the photoresist layer, the reason for the bend recover is not straightforward. The bend and recover of the smart grating are the results of the competition of the internal stresses. The intensities of the internal stresses of polymer materials are interrelated with their Young's modulus. The Young's modulus of polymer materials can be decreased dramatically due to chemical stimulation. For lower concentration of the vapor stimuli, the Young's modulus of the photoresist does not decrease much, the internal stresses induced by swelling are large and dominate the bend of the smart grating. When the concentration of the vapor stimuli reaches a value, the Young's modulus of the photoresist decreases much, the photoresist turns into rubber state, the internal stress induced by the swelling is released. The compressed gold layer begin to stretch out, therefore, the smart grating recovers to flat. The final coupling distance (d4) is less than the original one (d1) because that the thickness increase of the photoresist induced by the swelling does not recover.
The other character of the Δλ ∼ CIPA curves is that the red shift of Δλ is not always increasing with the increasing of the concentration of the isopropanol vapor, but a little decrease in the range of 12% ≤ CIPA ≤ 17.6% both for the two cases of smart and stiff gratings. The reason for this decrease is that ΔλMicrosphere (Δr, Δn) is not monotonically increasing with the increasing of the concentration of the IPA vapor. The sensing system were put into and taken out from the environment of IPA vapor, there is always residual of some IPA molecular in the microsphere. The penetration depth (δr) of the IPA molecular in the microsphere was increased with the times increasing of the system being put into and taken out from the vapor (as shown in Fig. 8(d)). the ΔλMicrosphere (Δr, Δn) will show a drop when the penetration depth exceeds the first peak of the electric field distribution of the WGMs inside the microsphere.
In order to show such drops caused by the the increase of the penetration depth more clearly. The dynamic evolutions of the wavelength shifts of the $ T E_{52}^{2}$ mode with time both for systems of smart and stiff gratings in isopropanol vapor (CIPA = 10%) are shown in Fig. 9. It clearly shows that there are two drops both for systems of smart and stiff gratings, which is corresponding to the two peaks of the electric field distribution of the $ T E_{52}^{2}$ mode. And, the system of the smart grating almost always show larger wavelength shift than that of the case of stiff grating.
Fig. 9. The dynamic evolutions of the wavelength shifts of the $ T E_{52}^{2}$ mode with time for both systems of smart and stiff gratings in isopropanol vapor (CIPA = 10%).
The response speed of the smart grating shown in Fig. 9 seems very fast than the results shown in section 4.2, where we show the response of gold-photoresist bilayer film to IPA solution with a micro-propeller. The response speed is slow with shape deformation of 1 µm in 20 minutes. There are two reasons for this difference. Firstly, the smart grating is double-layer thin film with two-dimensional array of holes on it which effectively increase the contact area between vapor and the photoresist, so as to speed up the vapor adsorption speed and improve the response speed of vapor sensing of the device. Secondly, in the smart grating coupled WGMs microcavity system, the actual changes of the coupling distance are very small (only hundred nanometers), but significant wavelength changes in the spectrum can be induced, so a relatively fast response is achieved in the experiment.
4.4 Response enhancement of the smart grating coupled WGMs microcavity to thermal stimulation
The polystyrene and the gold-photoresist duble layer grating also response to the thermal stimulation at the same time, therefore, response enhancement was observed as well. Figure 10(a)–10(c) show the wavelength shifts of three WGMs of TE1 58, $ T E_{48}^{3}$ and $ T E_{52}^{2}$ for both cases of smart and stiff grating when the temperature of the sensing system is increased from 293 to 393 K with steps of 5 K. In contrast to the red shifts of the WGMs induced by the vapor stimulation, blue shifts induced by the thermal stimulation were observed. This is because that the refractive index of the polystyrene decrease with the temperature increasing, which causes blue shifts of the modes. Although the polystyrene microsphere expands with the temperature increasing, which cause red shift of the modes, the decrease of the refractive index dominate the wavelength shifts.
Fig. 10. Response of the WGMs of $ T E_{58}^{1}$, $ T E_{52}^{2}$ and $ T E_{48}^{3}$ of the smart and stiff grating coupled WGMs microcavity to thermal stimulation.
The wavelength shifts of the WGMs can be divided into three stages: stage I from 293 to 363 K, stage II form 363 to 393 K, stage III beyond 393 K. The stage I is the normal thermal expansion, the refractive index of the polystyrene decrease slow with the temperature increasing. The stage II is the glass transition of the polystyrene, the refractive index of it decrease dramatically with the temperature increasing. In the stage III the polystyrene microsphere maybe melt and no clear modes can be observed.
The smart grating will bend down when the temperature is increased due to the larger thermal expansion of photoresist than that of gold. In fact, such double layer film of different materials with significant difference of thermal expansion is a common design for thermal actuator. Similarly to the case of vapor stimulation, such bend down cause blue shifts of the WGMs. Thereby, response enhancements were observed (Fig. 10(b) and 10(c)) in the stage I.
There are also turning points observed on the curves in Fig. 10(b) and 10(c), where the wavelength shifts of the system of smart grating are exceeded by that of the system of stiff grating. The reason is also the recover of the bend of the smart grating. Similar to the situation of vapor stimulation, the Young's modulus of polymer materials can be also decreased dramatically due to thermal stimulation when the temperature is high enough. Thereby, response suppress were observed in the stage II.
5. Conclusion and discussion
In conclusion, we show that the introduce of the smart grating coupling setup to WGMs microcavities can effectively enhance the response of the whole system to external stimulations. We realized the smart grating coupling WGMs microcavities sensing system with “lab on fiber” technology, and demonstrated response enhancements to two kinds of stimulations i.e., vapor of volatile organic compounds and temperature.
For the isopropanol vapor stimulation, the response enhancements of the smart grating coupled WGMs microcavity happened when the concentration of the isopropanol solution is larger than 8%. Above this value, the coupling distance between the smart grating and the, WGMs microcavities decreased, therefore cause additional wavelength shift of the WGMs.
The response enhancement of the smart grating coupled WGMs microcavity to temperature stimulation is happened in the temperature range from 293 to 363 K. This enhancement can go further, if the edge of the smart grating is not fixed by the pedestal and large deformation happens. A temperature sensor with higher sensitivity maybe achieved with this design strategy.
In fact, both the ranges of the concentration of the vapor and the temperature can be tuned to the values one is interesting. There are two ways to tune the system for vapor sensing with lower concentration. The microcavity maybe created with function materials which will contract under vapor stimulation, while the smart grating is still created with the swelling materials. Then, the wavelength shifts induced by the bending of the smart grating will be alike to the response of the microspheres (both are blue shifts), and the response can be enhanced in the low concentration range. The other way is that using the materials to create the smart grating, while the micosphere is remained the swelling materials so that the smart grating bends upward under the stimulation of vapor. Then, the wavelength shifts induced by the bending of the smart grating will be also alike to the response of the microspheres (both are red shifts), and the response can be enhanced as well.
We believe that our research is of great significance to the design of WGMs microcavity based chemical and temperature sensors.
Funding
National Natural Science Foundation of China (11374216, 11404224, 11474206, 11774243, 11774246, 61735002); Beijing Nova Program (Z161100004916100); the Scientific Research Base Development Program of the Beijing Municipal Commission of Education; the Youth Innovative Research Team of Capital Normal University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. F. Vollmer and L. Yang, “Label-free detection with high-Q microcavities: a review of biosensing mechanisms for integrated devices,” Nanophotonics 1(3-4), 267–291 (2012). [CrossRef]
2. M. R. Foreman, J. D. Swaim, and F. Vollmer, “Whispering gallery mode sensors,” Adv. Opt. Photonics 7(2), 168–240 (2015). [CrossRef]
3. D. K. Armani, T. J. Kippenberg, S. M. Spillane, and K. J. Vahala, “Ultra-high-Q toroid microcavity on a chip,” Nature 421(6926), 925–928 (2003). [CrossRef]
4. C. H. Dong, L. He, Y. F. Xiao, V. R. Gaddam, S. K. Ozdemir, Z. F. Han, G. C. Guo, and L. Yang, “Fabrication of high-Q polydimethylsiloxane optical microspheres for thermal sensing,” Appl. Phys. Lett. 94(23), 231119 (2009). [CrossRef]
5. H. Lee, T. Chen, J. Li, K. Y. Yang, S. Jeon, O. Painter, and K. J. Vahala, “Chemically etched ultrahigh-Q wedge-resonator on a silicon chip,” Nat. Photonics. 6(6), 369–373 (2012). [CrossRef]
6. M. W. Puckett, K. K. Liu, N. Chauhan, Q. C. Zhao, N. J. Jin, H. T. Cheng, J. F. Wu, R. O. Behunin, P. T. Rakich, K. D. Nelson, and D. J. Blumenthal, “422 Million intrinsic quality factor planar integrated all-waveguide resonator with sub-MHz linewidth,” Nat. Commun. 12(1), 934 (2021). [CrossRef]
7. L. N. He, S. K. Ozdemir, J. G. Zhu, and L. Yang, “Ultrasensitive detection of mode splitting in active optical microcavities,” Phys. Rev. A. 82(5), 053810 (2010). [CrossRef]
8. M. D. Baaske, M. R. Foreman, and F. Vollmer, “Single-molecule nucleic acid interactions monitored on a label-free microcavity biosensor platform,” Nat. Nanotechnol. 9(11), 933–939 (2014). [CrossRef]
9. F. X. Gu, L. Zhang, Y. B. Zhu, and H. P. Zeng, “Free-space coupling of nanoantennas and whispering-gallery microcavities with narrowed linewidth and enhanced sensitivity,” Laser Photonics Rev. 9(6), 682–688 (2015). [CrossRef]
10. S. Forstner, E. Sheridan, J. Knittel, C. L. Humphreys, G. A. Brawley, H. Rubinsztein-Dunlop, and W. P. Bowen, “Ultrasensitive Optomechanical Magnetometry,” Adv. Mater. 26(36), 6348–6353 (2014). [CrossRef]
11. B. B. Li, J. Bilek, U. B. Hoff, L. S. Madsen, S. Forstner, V. Prakash, C. Schafermeier, T. Gehring, W. P. Bowen, and U. L. Andersen, “Quantum enhanced optomechanical magnetometry,” Optica 5(7), 850–856 (2018). [CrossRef]
12. X. L. Feng, G. Zhang, L. K. Chin, A. Q. Liu, and B. Liedberg, “Highly Sensitive, Label-Free Detection of 2,4-Dichlorophenoxyacetic Acid Using an Optofluidic Chip,” ACS Sens. 2(7), 955–960 (2017). [CrossRef]
13. Y. Y. Wang, S. W. Zeng, G. Humbert, and H. P. Ho, “Microfluidic Whispering Gallery Mode Optical Sensors for Biological Applications,” Laser Photonics Rev. 14(12), 2000135 (2020). [CrossRef]
14. S. Yang, Y. Q. Wang, Y. Kong, G. S. Huang, Z. Zhao, Y. Wang, B. R. Xu, J. Z. Cui, and Y. F. Mei, “Enhanced Evanescent Field Coupling of Smart Particles in Tubular Optical Microcavity for Sensing Application,” Adv. Optical Mater. 10(4), 2102158 (2022). [CrossRef]
15. Y. L. Sun, Z. S. Hou, S. M. Sun, B. Y. Zheng, J. F. Ku, W. F. Dong, Q. D. Chen, and H. B. Sun, “Protein-Based Three-Dimensional Whispering-Gallery-Mode Micro-Lasers with Stimulus-Responsiveness,” Sci. Rep. 5(1), 12852 (2015). [CrossRef]
16. Y. Zhang, C. H. Zhang, Y. Q. Fan, Z. Liu, F. Q. Hu, and Y. S. Zhao, “Smart Protein-Based Biolasers: An Alternative Way to Protein Conformation Detection,” ACS Appl. Mater. Interfaces 13(16), 19187–19192 (2021). [CrossRef]
17. L. Cai, J. Y. Pan, and S. Hu, “Overview of the coupling methods used in whispering gallery mode resonator systems for sensing,” Opt. Laser Eng. 127, 105968 (2020). [CrossRef]
18. D. Farnesi, F. Chiavaioli, G. C. Righini, S. Soria, C. Trono, P. Jorge, and G. Nunzi Conti, “Long period grating-based fiber coupler to whispering gallery mode resonators,” Opt. Lett. 39(22), 6525–6528 (2014). [CrossRef]
19. L. L. Shi, Q. Q. Wang, and T. Zhu, “A Fiber-Attached Coupler for Transmission Bandpass Whispering Gallery Mode Resonator,” J. Lightwave Technol. 39(8), 2454–2459 (2021). [CrossRef]
20. Y. Y. Zhou, D. Zhu, X. Yu, W. Ding, and F. Luan, “Fano resonances in metallic grating coupled whispering gallery mode resonator,” Appl. Phys. Lett. 103(15), 151108 (2013). [CrossRef]
21. Y. Y. Zhou, D. Zhu, X. Yu, H. X. Zhang, and F. Luan, “Metallic diffraction grating enhanced coupling in whispering gallery resonator,” Opt. Express 21(7), 8939–8944 (2013). [CrossRef]
22. D. Farnesi, S. Pelli, S. Soria, G. N. Conti, X. Le Roux, M. O. Ballester, L. Vivien, P. Cheben, and C. Alonso-Ramos, “Metamaterial engineered silicon photonic coupler for whispering gallery mode microsphere and disk resonators,” Optica 8(12), 1511–1514 (2021). [CrossRef]
23. A. Z. Li, K. Tian, J. B. Yu, R. A. Minz, J. M. Ward, S. Mondal, P. F. Wang, and S. N. Chormaic, “Packaged whispering gallery resonator device based on an optical nanoantenna coupler,” Opt. Express 29(11), 16879–16886 (2021). [CrossRef]
24. F. J. Shu, C. L. Zou, and F. W. Sun, “Perpendicular coupler for whispering-gallery resonators,” Opt. Lett. 37(15), 3123–3125 (2012). [CrossRef]
25. S. Liu, W. Z. Sun, Y. J. Wang, X. Y. Yu, K. Xu, Y. Z. Huang, S. M. Xiao, and Q. H. Song, “End-fire injection of light into high-Q silicon microdisks,” Optica 5(5), 612–616 (2018). [CrossRef]
26. A. V. Saetchnikov, E. A. Tcherniavskaia, V. A. Saetchnikov, and A. Ostendorf, “A Laser Written 4D Optical Microcavity for Advanced Biochemical Sensing in Aqueous Environment,” J. Lightwave Technol. 38(8), 2530–2538 (2020). [CrossRef]
27. X. Li, X. B. Cai, Y. F. Gao, and M. J. Serpe, “Reversible bidirectional bending of hydrogel-based bilayer actuators,” J. Mater. Chem. B 5(15), 2804–2812 (2017). [CrossRef]
28. L. Y. Tan, A. C. Davis, and D. J. Cappelleri, “Smart Polymers for Microscale Machines,” Adv. Funct. Mater. 31(9), 2007125 (2021). [CrossRef]
29. Z. W. Xie, S. F. Feng, P. J. Wang, L. S. Zhang, X. Ren, L. Cui, T. R. Zhai, J. Chen, Y. L. Wang, X. K. Wang, W. F. Sun, J. S. Ye, P. Han, P. J. Klar, and Y. Zhang, “Demonstration of a 3D Radar-Like SERS Sensor Micro- and Nanofabricated on an Optical Fiber,” Adv. Optical Mater. 3(9), 1232–1239 (2015). [CrossRef]
30. S. Y. Zhang, S. J. Tang, S. F. Feng, Y. F. Xiao, W. Y. Cui, X. K. Wang, W. F. Sun, J. S. Ye, P. Han, X. P. Zhang, and Y. Zhang, “High-Q Polymer Microcavities Integrated on a Multicore Fiber Facet for Vapor Sensing,” Adv. Optical Mater. 7(20), 1900602 (2019). [CrossRef]
31. Y. X. Zhan, Q. Q. Liu, S. F. Feng, J. S. Ye, X. K. Wang, W. F. Sun, and Y. Zhang, “Photonic molecules stacked on multicore optical fiber for vapor sensing,” Appl. Phys. Lett. 117(17), 171107 (2020). [CrossRef]
32. Q. Q. Liu, Y. X. Zhan, S. Y. Zhang, S. F. Feng, X. K. Wang, W. F. Sun, J. S. Ye, and Y. Zhang, ““Optical tentacle” of suspended polymer micro-rings on a multicore fiber facet for vapor sensing,” Opt. Express 28(8), 11730–11741 (2020). [CrossRef]
