1. Introduction

Integration of high-performance electronic/photonic elements on a mechanically stretchable substrate exhibits particularly attractive characteristics and enables new applications in soft robotic as well as body-attachable, conformable devices [1–12]. Various integration strategies of inorganic and organic materials in micro- and nano-structured forms on deformable substrates have been demonstrated for stretchable strain gauges or touch sensors [13–16], which are supposed to be used to monitor mechanical deformations in submicron or nanometer scale. However, micro local strain characterization remains challenging for many flexible electronic sensors due to the large device footprint and high-power consumption. One approach to overcome these shortcomings is to use microscale nanolasers. For example, 1D photonic crystal nanorods laser working at band-edge mode with very small device footprint (~11 μm) and low threshold (~280 μW) wrapped in a deformable and transparent polydimethylsiloxane (PDMS) substrate was introduced and nearly 7.7 nm wavelength tuning was obtained under 1% compression [17]. Besides, a high-resolution strain-gauge consisted of a band-edge laser with footprint ~10 μm and threshold ~600 μW was demonstrated by embedded InGaAsP iron-nail-shaped rode array into deformable PDMS, and the applied strain range was from −10 to 12% [18]. However, the band-edge photonic crystal laser needs precise fabrication methods with low tolerance, prerequisite photonic crystal band-gap calculation and small allowable stretching/compression range, which hinder the large-scale application of such kind of strain gauges. Besides, these reported strategies all integrated microlasers emitting at conventional band communication ambit, strain gauges based on visible microlasers has not been demonstrated.

Whispering gallery modes (WGMs) microdisk lasers can be a promising candidate to address these limitations. WGMs microdisk lasers with extremely highly confined wavelength-scale mode volume and high quality factors (Q factors) are widely used for on-chip photonic and optoelectronic devices, such as Raman sources [19], nanoparticle detection [20,21], temperature or micromechanical vibration sensors [22,23]. Submicron or wavelength-scale micordisk lasers exhibit large mode-spacing and single-mode lasing with the sub-nanometer spectral resolution [24]. In addition, the lasing spectra and mode behaviors of WGMs are highly sensitive to the environmental changes such as structural deformation [25] and changes in the temperature or refractive index of the surrounding medium [26,27].

Here, we present visible microdisk lasers operating at low threshold and being embedded in a PDMS polymer substrate, of which the spectra are sensitive to the flexible substrate deformation and can be functioned as microscale local strain gauges with low-power consumption. The lasing spectra of microdisk lasers were modulated mainly by strains due to the change in surrounding refractive index of PDMS and the deformation of microcavity. The measured relative wavelength tuning under strains of the novel strain sensors indicates a linear behavior with the slope indicating the gauge factor (defined as Δλ/ε, Δλ and ε indicates wavelength shift and strain percentage, respectively) being ~4.0 nm and ~6.7 nm per stretching unit for microdisk lasers with the diameter of 1.2 μm and 1.5 μm, which corresponds to a smooth wavelength tuning of 1.5 nm and 2.6 nm under 36% strain, respectively. In addition to be used as microscale strain gauges, the visible lasers on deformable substrate can also function as tunable light source for the photonic integrated circuits and flexible laser projection displays.

2. Experiments and discussions

Figures. 1(a)-1(c) schematically illustrate the key fabrication processes for strain gauges consisted of microscale microdisk lasers. The microdisk patterns were defined by using electron beam lithography with Zep520 acting as electron beam resist, and the lithographic pattern from that of electron beam resist was transfered into silicon dioxide hard mask by using reactive ion etching (RIE). After removing the electron beam resist, the hard mask pattern was transferred through the InGaP/InGaAlP multi-quantum wells (MQWs) with inductively coupled plasma RIE [24]. The supporting posts were formed by selectively wet-etching and the dioxide silicon layer was removed by dilute buffered HF as shown in Fig. 1(a). Then the resonators were embedded in the optically transparent flexible PDMS polymer as shown in Fig. 1(b) and peeled off from the hard GaAs substrate with supporting posts removed by using selectively wet-etching as shown in Fig. 1(c). Figure. 1(d) illustrates photography and optical microscopic images of compact microdisk resonators embedded in PDMS substrate, which indicates microdisk resonators are peeled off successfully form the rigid GaAs substrate. And the tilted scanning electron microscope (SEM) image of a microdisk resonator with diameter 1.2 μm is shown in Fig. 1(e), the thickness of microdisk is about 180 nm. After embedding the microdisk resonators in the PDMS substrate and removing of supporting posts, the devices were mounted on a 2-axis piezo-stage with a resolution of 30 nm. To understand lasing characteristics, the microdisks were optically pumped at room temperature by a 405-nm laser diode with 10 ns pulses at a repetition rate of 100 kHz. The pumping laser with spot size around 3 μm was focused normal to the microdisk surface and the emitted light was collected from the top using a microscope objective with x50 magnification (N.A. = 0.42). About 16% of incident pumping energy was absorbed and used for carrier generation considering the effective pumping area and surface reflection.

 

Fig. 1 Schematic diagrams of key fabrication processes for the proposed microscale strain gauges. Visible microdisk resonators with mushroom shape were fabricated (a) and embedded in the PDMS polymer (b). The resonators were peeled off form the hard substrate and embedded in the flexible substrate with the supporting posts being selectively removed by wet-etching (c). Photography and optical microscopic images of compact microdisk resonators embedded in a PDMS substrate (d). (e) indicates a tilted SEM image of microdisk resonator with diameter 1.2 μm before being embedded in PDMS.

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As mentioned above, the lasing wavelength of nanolasers are sensitive to environmental refractive index and microcavity deformation. In addition, the refractive index of PDMS can be manually controlled by changing the volume of PDMS through stretching or compression. As elongating the flexible PDMS substrate, the environmental refractive index decreases due to the material`s change in volume [28]. Before the experimental characteristics, 3D finite-difference time-domain (FDTD) solutions were used to calculate the wavelength tuning of microdisk resonators embedded in PDMS modulated by the index of surrounding materials. The refractive index and diameter of microdisk resonator were set to be 3.4 and 1.2 μm, respectively. Figure. 2(a) demonstrates the calculated normalized intensity spectra map with refractive index of surrounding medium changing from 1.2 to 1.4, which indicates blue-shift of resonant wavelength with decreasing index of deformable substrate as verified from the experimental results shown below. The inset in Fig. 2(a) illustrates a magnified normalized intensity map of spectra for TE_1,13 mode, which indicates around 4 nm wavelength tuning with index of polymer changing from 1.2 to 1.4. 1-D slice of the normalized spectra map with refractive index n = 1.26 of polymer is shown in Fig. 2(b) with transverse-electric (TE) modes labeled in the figure. The Q-factors of three first-order WGMs have been calculated under various refractive indexes as shown in Fig. 2(c). The Q-factor increases with decreasing index which can be explained by enhanced light confinement in high-index gain region. Figures 2(d)–2(h) indicate the calculated magnetic field profiles of various first- and second-order resonant WGMs with green dashed line indicating the microdisk resonator boundary.

 

Fig. 2 (a) Calculated normalized intensity spectra map for a micodisk resonator embedded in polymer with refractive index changing from 1.2 to 1.4. The index and diameter of microdisk is 3.4 and 1.2 μm, respectively. The inset illustrates a magnified normalized intensity map of spectra for TE_1,13 mode. (b) 1-D slice of the normalized spectra map with refractive index n = 1.26 of polymer. (c) Calculated Q factors of three first-order WGMs: TE_1,14, TE_1,13 and TE_1,12, respectively. (d) - (h) Calculated magnetic field profiles of various WGMs. Resonant modes from (d) to (h) are identified as TE_1,14, TE_2,10, TE_1,13 TE_2,9 and TE_1,12, respectively. The green dashed line indicates the boundary of microdisk.

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As indicated from simulated results, lasing wavelength can be manipulated by modifying the index of surrounding material and resonant peaks are blue-shift with decreasing index of surrounding polymer, which indicates a strain gauge can be made based on monitoring variances in lasing spectra due to index changing. Besides, microcavity deformation can be induced during stretching the flexible substrate, which also functions as a wavelength tuning mechanism. Microscale strain gauges with two sizes had been characterized. The single-mode lasing behaviors for a microdisk embedded in the PDMS without stretching are shown in Fig. 3(a), the diameter of the microdisk is 1.2 μm. The lasing wavelengths under different pumping power as shown in the inset of Fig. 3(a) indicate the wavelength shift can be around 0.3 nm in these optical-pumping conditions, which also helps to confirm the lasing threshold. The blue-shift of wavelength below and above threshold is mainly caused by the increasing carrier density as increasing the pumping power, and the increasing rate of carrier density slows down when the pumping power is above the threshold. The measured output power and linewidth as a function of peak pumping power (L-L curve) are illustrated in Fig. 3(b), which indicates the lasing action and shows a threshold of 580 μW. The WGMs of the first-order with azimuthal modes being 13 has been identified for the microdisk resonator embedded in the PDMS by using 3D-FDTD solutions. Figures. 3(c) and 3(d) show the top-view and side-view for calculated magnetic field distribution profiles of microdisk laser embedded in the PDMS, respectively.

 

Fig. 3 The single-mode lasing spectra of a microdisk laser with diameter 1.2 μm embedded in the flexible PDMS substrate without stretching (a) and the inset indicates the lasing wavelengths under different pumping power. Collected output power and measured linewidth of the lasing wavelength as a function of peak pumping power are plotted in (b), and the lasing threshold is ~580 μW. Calculated magnetic field top-view (c) and side-view (d) profiles of WGM mode of microdisk with diameter 1.2 μm embedded in PDMS.

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Based on embedded in flexible and deformable PDMS substrate, the lasing wavelength of microdisk lasers can be manipulated by stretching the PDMS substrate. The refractive index of PDMS is decreasing during stretching process [28], which indicates the blue-shift of lasing wavelength. The blue-shift of lasing wavelength is verified by measuring the peak positions of photoluminescent lasing spectra with gaussian fitting at several stretching conditions, as described in Fig. 4. The strain or stretch percentage ε is defined by the ratio between the walking distance of piezo-stage and the original length of PDMS substrate. To avoid the wavelength tuning induced by carrier density, the optical-pumping power is fixed during various deformation conditions and the pumping spot is precisely aligned to the same positions. The characterizations of lasing spectra for several stretching conditions of a microdisk laser with diameter 1.2 μm are demonstrated in Fig. 4(a), the wavelength of lasing can be nearly linear manipulated and shows a blue-shift action during stretching the PDMS substrate, the inset in Fig. 4(a) indicates the lasing spectra without stretching. The wavelength shifts of four microdisks lasers (labelled as laser A to D) fabricated on the same chip with diameter 1.2 μm under stretching from 0 to 36% are shown in Fig. 4(b), the inset diagram of Fig. 4(b) indicates stretching diagram of the strain gauge. The difference of wavelength shifts for four microdisk lasers (laser A to laser D) may be caused by the fabrication fluctuations such as surface roughness and imperfections, material inhomogeneity or tiny air bubbles nearby. Around 1.5 nm wavelength tuning is obtained for a microdisk laser with diameter 1.2 μm, which is nearly three times of its lasing linewidth. The gauge factor of laser A is ~4.0 nm per stretching unit by linear curve fitting of measured lasing positions.

 

Fig. 4 Measured lasing spectra from a microdisk laser with diameter 1.2 μm under various stretching conditions (a). The inset in (a) indicates the lasing spectra without stretching (b). Measured lasing wavelength shift under various stretching conditions for four microdisk lasers with diameter 1.2 μm and the inset illustrates the stretching diagram of microdisk lasers embedded in PDMS.

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Another microdisk laser with diameter of 1.5 μm for strain gauge had been characterized. The measured wavelength shifts and Q factors (defined by λ/Δλ) under several stretching conditions are described in Fig. 5(a). Figure. 5(b) illustrates the magnified normalized lasing spectra under several stretching conditions, which indicates nearly linear wavelength tuning and blue-shift of lasing wavelength under strains. And the linear behaviors of wavelength tuning under strains were linear fitted as shown by the red line in Fig. 5(a). The wavelength tuning under strains could be caused by both of index change of surrounding PDMS and defomation of microdisk laser during stretching the PDMS [25]. Further research will be taken on the explanations the mechanism of wavelength tuning behaviors. The strain gauge factor obtained from the fitting line is ~6.7 nm per stretching unit, which is higher than that of microdisk laser strain gauge with diameter 1.2 μm as described above. Figures 5(c) and 5(d) demonstrate the L-L curve under strains of 0% and 36%, respectively. The lasing threshold under strains of 0% and 36% are 1.1 mW and 930 μW, respectively. The decreasing of threshold may be owing to enhanced light confinement in gain region with decreasing refractive index of PDMS under strains.

 

Fig. 5 Spectra characteristics of a microdisk laser with diameter of 1.5 μm embedded in flexible PDMS. Measured lasing wavelength shifts and Q factors under various stretching conditions (a). (b) The normalized lasing spectra with 0%, 8%, 16%, 24% 36% stretching percentage, respectively. (c) and (d) showing the L-L curves of lasing at 0% and 36% stretching, respectively.

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In conclusion, low-threshold microdisk lasers were embedded in a deformable and transparent PDMS polymer substrate, of which the spectra were sensitive to flexible substrate deformation and can be functioned as microscale local strain gauges with low-power consumption and large strain range. Microdisk laser strain gauges with size 1.2 μm and 1.5 μm and gauge factor being ~4.0 nm and ~6.7 nm per stretching unit had been characterized, which corresponded to a smooth wavelength tuning of 1.5 nm and 2.6 nm under 36% strain, respectively. In addition to being used as microscale local strain gauges, the visible lasers on deformable substrate can also function as tunable light source for the photonic integrated circuits and flexible laser projection displays.