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Abstract
As a promising infrared optical material, the physical characteristics of patterned chalcogenide glass (ChG) membranes are of great importance for the improvement of device performances. In this work, based on the suspended membrane configuration, we have demonstrated the mechanical and thermal characterizations of the Ge11.5As24Se64.5 ChG optical microdisk resonator. By approximation of ChG cantilever configuration, the out-of-plane minimum mechanical strength of the microdisk membrane was measured to be 150 MPa by exploiting atom force microscope (AFM). This value is two orders of magnitude smaller than that of the bulk material, which is beneficial to achieve better mechanical compliance in terms of the ChG membrane sensors. To illustrate the effect of environmental temperature variation on the optical response of the ChG microdisk membrane with quality factor (Q-factor) of 2.87 × 104, the thermal drift was characterized to be 90.2 pm/°C by changing the substrate temperature from 30 °C to 44 °C. The characterization of multi-parameters in combination with the ChG free-standing microdisk prototype is conducive to further expand the potentials of ChG membrane in the ultrasound and other cavity optomechanical sensing.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In order to further excavate the potentials of the whispering-gallery modes (WGMs) microcavity resonators, a large number of various optical materials, such as the silicon dioxide (SiO2) [1,2], SU-8 [3], lithium niobate [4], polystrene [5], have been chosen as resonator matrixes in combination with their unique mechanical, thermal, electrical, and acoustical characterization to extend their multi-disciplinary applications in the optomechanics, optothermal physics, electro-optics, and optoacoustics. Meanwhile, the specific microcavity structures were designed and fabricated to enhance these physical effects. As a new type of infrared optical material, the bulk materials of ChGs have some intrinsic optical, mechanical and thermal characteristics. In optics, being composed of covalently bonded heavy elements leads to a window of transparency ranging from the near-infrared to the mid-infrared (up to 25 μm) [6,7]. By controlling the deposition of one or more of the chalcogen elements (sulphur, selenium or tellurium), the infrared transmittance ranges could be appropriately adjusted. Combined with their high refractive indices (RIs) [8,9] and strong optical nonlinearities [9,10], the ChGs have been the promising candidates in infrared optical microcavity resonators fabrication for cavity-enhanced chemical and biological sensors due to the molecular fingerprint regions covered in the infrared windows of the devices [11].
To better take advantage of mechanical characteristics of ChGs for cavity optomechanical sensing, the suspended ChGs microdisks are always expected to maximize mechanical compliance. Compared with the case of silica (Young’s modulus of silica is 73 GPa) [12,13], the ChGs have smaller mechanical strengths (e.g. Young’s modulus of Ge5As38Se57 is 18 GPa) [14,15], thus they were also called soft glasses. It means that there exists a trade-off between the mechanical strength and geometry sizes allowing the configuration of free-standing ChG microdisk membrane. In general, the out-of-plane mechanical strength of optical thin film is dependent on the thickness of film [16]. However, there is less work to study the uniaxial mechanical characteristics of ChGs films. The effect from the patterned geometry structure is yet to be demonstrated. With the aid of cantilever structure [17,18], the Young’s modulus of suspended ChG membrane would be estimated to clarify its ability in optomechanical application.
The generation of mechanical vibration can introduce the changes of optical modes in cavity optomechanical sensor [19,20]. Thermal accumulation in optical WGM resonator-based sensor can also give rise to the shifts of optical modes [21,22]. Monitoring of optical response caused by mechanical action necessitates one to remove the noise from the thermal effect. It means that the thermal drift characterization of device is directly related to the estimation of sensitivity and reliability of a sensor [23]. Using a suspended silica wedge resonator as the bottom cladding, Ge28Sb12Se60 membrane as the core layer of a hybrid optical microdisk resonator was deposited onto it and the thermal drift of 60.5 pm/K was reported by Kang et al [24]. While the high Q-factor could be obtained, the mechanical compliance of the resonator as cavity optomechanical sensor will still limited by the properties of silica. For this reason, the optical, mechanical and thermal characteristics of the suspended Ge11.5As24Se64.5 glass microdisk membrane, as only resonator matrix and an unknown constituent in the family of ChGs, would be tested to demonstrate its potentials as optomechanical sensor in the future.
In this paper, the Ge11.5As24Se64.5 ChG with the mean coordination number (MCN) of about 2.45 was chosen due to its advantages of the excellent thermal stability, near-optimally constrained glass network, indistinguishable RI and bandgap from the bulk glass material, which have been reported in previous works [25–27]. The suspended Ge11.5As24Se64.5 ChG microdisk membrane was fabricated by the photolithography and wet-etching techniques. Through by taper fiber coupling, the optical transmission characterization of the ChG microdisk membrane was measured at the C-band. To estimate the mechanical performance of suspended ChG microdisk membrane, the ChG cantilever configuration with comparable thickness was fabricated approximately as the prototype of the out-of-plane Young’s modulus assessment of suspended microdisk membrane. At last, the thermal characterization of microdisk resonator was carried out to reflect the thermal drift of the device. The analysis of multi-parameters is conducive to understand the abilities of device as cavity optomechanical sensor in the following work.
2. Fabrication and measurement
2.1 Device fabrication
Prior to the experiments, we have examined as-prepared film with the energy dispersive spectrometer (EDS) after deposition. The test results show that the composition of the membrane sample is Ge10.4As26.8Se62.8, which is close to that of the equivalent bulk glass (Ge11.5As24Se64.5). Referring to the schematic depicted in Fig. 1(a), we have fabricated the suspended microdisk resonator from an approximately 650 nm-thick Ge11.5As24Se64.5 film deposited by the thermal evaporation onto an oxidized silicon wafer, using the maskless lithography and reactive ion etching (RIE). After the photoresist (PR) was removed, the isotropically wet-etching with buffered hydrofluoric acid (BHF) (NH4F:HF = 6:1) was performed to remove the silica underlayer controllably to yield a ~2 μm-high pedestal, which was used to support the free-standing ChG microdisk. It should be noted that the isopropyl alcohol (IPA) rinsing in a hot water and baking were conducted to release the surface tension after the BHF etching [28]. A scanning electron microscope (SEM) image of a suspended ChG microdisk resonator with a radius of 40 μm and a thickness of 650 nm was shown in Fig. 1(b).
Fig. 1 (a) The fabrication schematic of the suspended ChG microdisk resonator. (b) SEM image of a suspended ChG microdisk membrane with a radius R = 40 μm and a thickness d = 650 nm.
2.2 Measurement setup
To couple light into ChG microdisk resonator, a silica taper fiber (TF) was produced using oxyhydrogen flame and pulling with motorized stages. The final waist diameter was around 1 μm with an insertion loss of about 0.22 dB. As shown in Fig. 2, to characterize the ChG microdisk resonator, a tunable laser (TL) with 100 kHz linewidth (Keysight 8164B) was launched into a tapered silica fiber prepared in advance. The fiber was placed in contact with the rim of a microdisk due to the electrostatic forces as the taper fiber was brought close to the edge of the microdisk. The transmission was monitored as a function of frequency using a digital signal analyzer (DSA) (Tektronix CSA7404). Before the signal got into the DSA, a photo detector (PD) (Agilent 11982A) was used to convert the optical signals into electrical signals. The input light with the transverse magnetic (TM) polarization, which is defined as the normal direction with respect to the plane of the disk, was controlled using a polarization controller (PC). The suspended ChG microdisk sample located over the temperature controller (TEC) was mounted onto a 3-axis stage, which could be monitored from the above using a CCD.
Fig. 2 The schematic of the experimental measurement setup. Inset was the optical image of a suspended ChG microdisk resonator evanescently coupling to a silica taper fiber under the test.
3. Results and discussion
3.1 Optical characterization
Figure 3 shows the transmission spectrum of the fabricated ChG microdisk resonator. Multiple lossy modes simultaneously existed in our resonator due to the large RI difference between the silica taper fiber and ChG microdisk resonator (RI: 2.6), which is the main reason in terms of the noisy transmission curve. The full width at half maximum (FWHM), denoted as Δλ, was measured to be 54.3 pm by the Lorentz fitting. Based on the formula Q = λ/Δλ, the loaded Q-factor was calculated to be 2.87 × 104 at 1556.7 nm. To understand the origin of this resonance mode, the Oxborrow’s model with perfect matched layers (PMLs) modified by Cheema et al was adopted to calculate the electric field distributions of the WGMs in the axisymmetric ChG microdisk membrane using the finite element method (FEM) in the COMSOL Multiphysics software [29–31]. By matching the resonance wavelength, the radial order and azimuthal quantum number of the resonance mode were simulated to be 12 and 300, respectively. The inset in Fig. 3(b) shows the electric field distribution of the TM mode corresponding to the resonance dip around 1556.7 nm.
Fig. 3 (a) Transmission spectrum of a suspended ChG microdisk resonator with the radius of 40 μm. (b) Zoomed-in transmission spectrum corresponding to a high-order mode given a loaded Q-factor ~2.87 × 104. Inset was the electric field distribution of TM resonance mode around 1556.7 nm.
We set γt as the minimum power transmission in the through-port at the resonance wavelength λ0 = 1556.7 nm. In this case, the value of γt was measured to be 0.2031. According to the following formula [32], the intrinsic Q-factor of the ChG microdisk resonator, denoted as Qi, was calculated to be 6.38 × 104.
Herein, is the fraction of intrinsic power losses (such as bending loss, absorption loss and surface scattering loss due to the roughness) per round-trip in the microdisk. FSR is the free spectral range (in wavelength span). According to the simulation, the FSR was calculated to be 3.2 nm. In order to estimate the optical confinement of the suspended ChG microdisk membrane, the intrinsic loss of 0.2 dB/round-trip, denoted as L, can be obtained by the following Eq. (2).3.2 Mechanical characterization
The mechanical characterization of the suspended ChG microdisk membrane is of great importance for the applications of cavity optomechanical sensing. In our experiments, the cantilever configuration with the comparable thickness (∼1300 nm) was chosen to simplify the suspended ChG microdisk membrane and was used for approximately estimating the out-of-plane Young’s modulus of the membrane by employing AFM system (Bruker Dimension Fastscan bio). Based on this solution, the ChG cantilevers were fabricated with the fixed width (W) of 20 μm using similar process techniques mentioned above. The lengths (L) of cantilever from 5 μm to 40 μm with interval of 5 μm was designed in sequence to reveal the variation of modulus. Figure 4(a) shows the SEM image of a ChG cantilever with length of 30 μm.
Fig. 4 (a) SEM image of the ChG cantilever with length of 30 μm and width of 20 μm. (b) Measured Young's moduli of the ChG cantilevers with different aspect ratios.
Herein, the Hertz model [33] was chosen to analyze the Young's modulus of the ChG cantilever. From this model, the loading force is defined as
where E is the Young's modulus, σ is the indentation depth, r is the tip radius and ν is the Poisson ratio. The aspect ratio is defined as the ratio of the length to width. Young's moduli of the ChG cantilevers with different aspect ratios were shown in Fig. 4(b). It can be found that the Young’s modulus decreases with the increase of aspect ratio. When the aspect ratio is larger than 1, the Young’s moduli of the ChG cantilevers nearly have no change. It indicates that the generation of mechanical displacements of the ChG cantilevers under the AFM tip is dominated by the intrinsic material damping and is independent on the geometries of cantilevers when the lengths of cantilevers are beyond a specific value. Table 1 shows the Young’s moduli of different materials under the bulk and membrane states. For the SiO2, the Young’s modulus of membrane state nearly has comparable order of magnitude with that of bulk material. However, it can be seen that the value of the Ge11.5As24Se64.5 glass membrane is approximately two orders of magnitude smaller than that of bulk material with close composition in comparison with the case of the polymethyl methacrylate (PMMA), which demonstrated the soft property of ChG microdisk membrane. The reduction of Young’s modulus of the ChG membrane was mainly attributed to the molecular arrangement of material. As a promising candidate used for cavity optomechanical sensor fabrication, the suspended ChG membrane configurations are beneficial to further enhance the mechanical compliance and improve the sensitivity of device in the future.
Table 1. Young's moduli of different materials.
3.3 Thermal characterization
To analysis the thermal drift of device, the transmission spectra of the suspended ChG microdisk membrane were monitored under the different temperatures. In the experiments, a temperature controller (TED200C) including a thermoelectric cooler (TEC2FS) and temperature probe was used to enable the stable temperature adjustment. A thermoelectric cooler attached underneath the ChG wafer was used to control the temperature of the substrate and the temperature probe was used to monitor the surface temperature of the sample. Figure 5 shows the temperature dependence of resonant wavelength of the ChG microdisk membrane. It could be seen that the resonant wavelength tended to red-shift with the increase of temperature. A plot of the corresponding shifts in resonant wavelength as a function of temperature reveals a temperature sensitivity of 90.2 pm/°C, as shown in Fig. 5(b). In general, both the thermal-optic and thermal expansion effects could give rise to a shift in resonant wavelength. The temperature-dependent wavelength shift is then expressed by Eq. (4) [3],
where δ, α, n, R are the thermal-optic coefficient, thermal expansion coefficient, effective refractive index, and radius of the Ge11.5As24Se64.5 glass microdisk resonator, respectively. Referring to the data involved in the literature [34], the thermal-optic coefficient and thermal expansion coefficient of the bulk Ge14As23.2Se62.8 glass used as an approximation of the target composition glass were calculated to be 50.8 ppm/k and 55.5 ppm/k, respectively. Based on the thermal expansion coefficient, the thermal-optic coefficient of our membrane was estimated to be 6.4 ppm/k, which is smaller than 50.8 ppm/k. The difference between them was mainly attributed to the faster heat dissipation caused by the large specific surface area of the membrane sample in comparison with the case of the bulk material, which reduces the influence of heat on the RI. Once the geometry of the disk changes, the conclusion obtained from the thermal characterization for a particular ChG microdisk will not be useful. It is because that the overlaps between the optical modes and thermal fields in the different device geometries are different, which mainly determines the effect of heat on the optical resonance property.Fig. 5 (a) Variations of transmission spectra of the suspended ChG microdisk resonator at the different temperatures (b) A linear fitting to the red-shift of the resonant wavelength as a function of temperature.
4. Conclusion
Suspended Ge11.5As24Se64.5 ChG microdisk membrane have been systematically studied including optical, mechanical, and thermal characterization. Based on this membrane, an optical loaded Q-factor of 2.87 × 104 at 1556 nm was obtained by using a silica taper fiber coupling. The uniaxial Young’s modulus of 150 MPa of the microdisk membrane was calculated by exploiting the ChG cantilever configuration as model approximation. To estimate the thermal drift of the suspended ChG microdisk resonator, the thermal instability of 90.2 pm/°C was observed by changing substrate temperature from 30 °C to 44 °C. The analysis of multi-parameters of the free-standing ChG microdisk resonator will further facilitate the applications of ChG membranes in cavity optomechanical sensing.
Funding
National Natural Science Foundation of China (NSFC) (61805104, 61435006, 61525502); The Science and Technology Planning Project of Guangdong Province (2017B010123005); Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01X121); Natural Science Foundation of Xuzhou under Grant (KC16SG267).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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