Pylon

A 980-nm pulsed laser diode (10-ns pulses with 1% duty cycle) was used to optically pump the coupled PhC cavities at room temperature. The light emitted from the cavities was collected by a × 40 microscope objective lens with a numerical aperture of 0.55 and focused onto either an IR 1D array detector (PyLoN, Princeton Instruments) or an InGaAs IR camera (C10633, Hamamatsu). To control the pump position in the scanning PL measurement, the motorized single-axis translation stage was used to move the PhC samples with a scanning step of 0.2 μm, while a fixed pump laser with a spot size of ∼3.0 μm illuminated the samples. The pump positions were accurately determined using a reference position near the coupled PhC cavities. In Fig. 5, a gate voltage was applied with ion gel using a DC voltage source (R6142, Advantest).

All PL spectra are acquired at room temperature in a micro-PL set-up with samples placed in vacuum. A 685-nm diode laser coupled to a single mode fibre is used as the excitation source. After passing through a 700-nm short-pass filter, the laser is focussed onto the sample through an infinity corrected objective (Mitutoyo × 100 NA = 0.7). For the data reported in Fig. 2, the average power entering the objective is 30  $\mathrm{\mu }\mathrm{W}$ for monolayer PL and 120  $\mathrm{\mu }\mathrm{W}$ for bilayer PL. The resulting laser spot has a radius of $\approx 3.5\mathrm{\mu }$ m, which is large enough to illuminate entire individual dimers, and kept constant for a uniform excitation of all the different nano-antennas sizes when determining the reported enhancement factor in Eq. (1). The dimers are separated by 10 μm, which allows optical measurement of an individual dimer. The emitted light is collected by the same objective and filtered with a 700-nm long-pass filter before coupling into a spectrometer (Princeton Instruments SP2750) and detection with a high-sensitivity liquid nitrogen cooled charge-coupled device (CCD; Princeton Instruments PyLoN). For polarisation measurements, a Glan–Thompson linear polarizer, followed by a half-wave plate mounted onto a motorised rotation stage, are inserted in the excitation path in order to control the linear polarisation angle of the laser source. No polarisation optics is used in the collection path.

The flexible PDMS samples with PhC laser devices were mounted and fixed to a customized digital controller that consisted of a micrometre and a single-axis translation stage. The controller was used to stretch or compress the flexible samples (Supplementary Fig. 1). In Figs 2 and 3, a 980-nm pulsed laser diode (10-ns pulses with 1% duty cycle) was used to optically pump the PhC laser devices at room temperature. The light emitted from the laser devices was collected by a × 40 microscope objective lens with a numerical aperture of 0.55 and focused onto either an infrared one-dimensional array detector (PyLoN, Princeton Instruments) or an InGaAs infrared camera (C10633, Hamamatsu).