A high-resolution strain-gauge nanolaser

Abstract

Interest in mechanical compliance has been motivated by the development of flexible electronics and mechanosensors. In particular, studies and characterization of structural deformation at the fundamental scale can offer opportunities to improve the device sensitivity and spatiotemporal response; however, the development of precise measurement tools with the appropriate resolution remains a challenge. Here we report a flexible and stretchable photonic crystal nanolaser whose spectral and modal behaviours are sensitive to nanoscale structural alterations. Reversible spectral tuning of ∼26 nm in lasing wavelength, with a sub-nanometre resolution of less than ∼0.6 nm, is demonstrated in response to applied strain ranging from -10 to 12%. Instantaneous visualization of the sign of the strain is also characterized by exploring the structural and corresponding modal symmetry. Furthermore, our high-resolution strain-gauge nanolaser functions as a stable and deterministic strain-based pH sensor in an opto-fluidic system, which may be useful for further analysis of chemical/biological systems.

Figure 1. Flexible and stretchable nanolaser.

(a–d) Schematic illustrations of fabrication process. A high-index semiconductor iron-nail-shaped rod array is fabricated (a) and embedded in an optically transparent low-index flexible polymer (b). The rod array is peeled off from the substrate (c), and the supporting posts are selectively removed by wet-etching (d). (e) Tilted SEM image of a square lattice InGaAsP iron-nail-shaped rod-type PhC structure before embedding it in PDMS. The lattice constant, radii of individual nail heads and supporting posts, and thicknesses of individual nail heads and supporting posts are 650, 200, 120, 250 and 800 nm, respectively. Scale bar, 1 μm. (f) Optical microscopy image of a PhC structure embedded deeply in PDMS. Scale bar, 5 μm. (g) Optical microscopy image of the array of PhC structures embedded in PDMS. Scale bar, 20 μm. Inset: a photograph showing the flexibility and transparency of the fabricated laser sample. (h) Schematic illustration of optically pumped lasing from a PhC structure embedded in a flexible and transparent substrate. Inset: measured single-mode lasing peak at a wavelength of 1,364.9 nm. (i) Schematic illustrations exhibiting the tuning mechanism of the lasing wavelength in stretched or compressed PhC laser devices. (j) Calculated transverse-electric-like photonic band diagrams of the PhC structures embedded in PDMS. The lattice constant is varied from 550 to 700 nm in one direction (the x direction), whereas the lattice constant in the other direction (the y direction) is fixed at 650 nm. The other structural parameters are the same as those in e. The y axis is the normalized frequency in units of a/λ and the x axis is the wavevector, where a is a fixed lattice constant of 650 nm and λ is the wavelength in free space. Inset: magnified diagram around the first Γ-point band-edge mode, where the x direction lattice constant is 550 (green), 600 (red), 650 (black) and 700 nm (blue).

Figure 2. Spectroscopic characterization of the strain-gauge laser sensor.

(a) Optical microscopy images of a fabricated PhC structure embedded in PDMS. Strains of −9.9% (left of panel), 0.0% (middle of panel) and 11.8% (right of panel) are applied along the x direction. Strain is defined as the percentage change of one side length (L) of the PhC structure. Scale bars, 5 μm. (b) Strains along the x (solid line) and y directions (dashed line) as a function of displacement of the sample stage in the x direction. (c) Measured lasing spectra of the strain-gauge PhC laser in a under different strains along the x direction. The applied strain varies from −10 to 12%. The PhC laser is optically pumped using a pulsed diode laser with a peak pump power of 700–1,200 μW. The linewidths of the lasing peaks are less than ∼0.6 nm for strains of ≤6.9%, and ∼0.8 nm and ∼0.9 nm for strains of 8.9% and 11.8%, respectively. (d) Measured output intensity as a function of the incident peak pump power under the strains in (i)–(v) in c. The lasing threshold powers are ∼600 (i), ∼590 (ii), ∼580 (iii), ∼640 (iv), and ∼1,050 μW (v). (e) The lasing wavelength shift in c with respect to that of the unstrained laser device is plotted as a function of the applied strain. The dotted line indicates a linear fit to the data. (f) The estimated optical strain-resolving factor is plotted as a function of the applied strain. The black and red dotted lines are the mean strain-resolving factor based on the strain sensitivity obtained in e and the experimental singularity line, respectively.

Figure 3. Visualization of applied strains and comparison with numerical simulations.

(a) Measured visual responses of the lasing mode under different strains of −7.3% (left of panel), 0.0% (middle of panel) and 6.9% (right of panel) along the x direction. Scale bars, 5 μm. (b) Calculated z-components of the time-averaged Poynting vectors at a position 15 μm above the PhC structure. Lattice constants of 600 (left of panel), 650 (middle of panel) and 700 nm (right of panel) are used in the simulation, which give strains similar to those in a: lattice constants of 600, 650 and 700 nm correspond to strains of −7.7%, 0.0% and 7.7%, respectively. Scale bars, 5 μm. (c) Measured (red dots) and calculated (black line) resonant wavelengths plotted as a function of the x direction lattice constant of PhC structure. The measured wavelengths are from Fig. 2c. (d) Calculated Q-factors as a function of the x direction lattice constant. The y direction lattice constant is fixed at 650 nm.

Figure 4. Mechanical strain-based tunable laser pH sensor.

(a) Schematic illustrations showing the working principle of pH sensing using the laser device. An aqueous solution with a given pH level is injected into the fluidic channel and meets the pH-sensitive hydrogel in the chamber. Different mechanical strains are applied to the strain-sensitive flexible laser device on top of the chamber, depending on the volume change of the pH-sensitive hydrogel. (b) A photograph of the opto-fluidic pH sensor that includes a strain-sensitive flexible PhC laser device, the pH-sensitive hydrogel, and the transparent plastic fluidic channels. Scale bar, 1 cm. (c) Measured lasing spectra of an optically pumped strain-sensitive flexible PhC laser in dry conditions (no solution; top of panel), acetic acid (pH 2.5; middle of panel) and a neutral solution (pH 7.0; bottom of panel). The incident pump power is 700–800 μW, and the lasing wavelengths are 1,257.6 (top of panel), 1,264.6 (middle of panel) and 1,267.4 nm (bottom of panel). (d) Measurement of the reversible repeatability and temporal stability of the pH sensor of c. The spectral behaviour of lasing wavelength was monitored over time while the pH level in the aqueous solution was reversibly and repeatedly switched between acetic acid (red) and neutral (blue) solutions. The measured lasing wavelengths are 1,264.7±0.2 nm (4–15 min; red), 1,267.2±0.2 nm (19–30 min; blue), 1,264.1±0.1 nm (36–45 min; red) and 1,267.4±0.1 nm (53–62 min; blue). Inset: multi-cycle test over longer time.