We investigate the lasing spectra, threshold gain values, and emission directionalities for a two-dimensional microcavity laser with a "kite" contour. The cavity modes are considered accurately using the linear electromagnetic formalism of the lasing eigenvalue problem with exact boundary and radiation conditions. We develop a numerical algorithm based on the Muller boundary integral equations discretized using the Nystrom technique, which has theoretically justified and fast convergence. The influence of the deviation from the circular shape on the modal characteristics is studied numerically for the modes polarized in the cavity plane, demonstrating opportunities of directionality improvement together with preservation of a low threshold. These advantageous features are shown for the perturbed whispering-gallery modes of high-enough azimuth orders. Other modes can display improved directivities while suffering from drastically higher threshold levels. Experiments based on planar organic microcavity lasers confirm the coexistence of Fabry-Perot-like and whispering-gallery-like modes in kite-shaped cavities and show good agreement with the predicted far-field angular diagrams.

Nonlinear chiroptical effects of precisely designed chiral plasmonic nanomaterials can be much stronger than such effects observed in the linear regime. We take advantage of this property to demonstrate the use of circularly polarized second-harmonic generation microscopy towards the efficient read-out of a microscopic pattern encoded by an array of triangular gold nanoprisms forming arrangements of adequate chiral symmetry. Strong chiroptical effects are observed in the backscattered second-harmonic generation intensity, enabling clear distinction of the laterally arranged enantiomers, down to nearly 1 µm resolution with an overall intensity contrast of about 40 % (second-harmonic generation circular dichroism of 20 %). Numerical simulations show a noticeable change in the spatial distribution of plasmonic hot spots within the individual nanostructures under excitation by circularly polarized light of different handedness. This leads to rather weak chiroptical effect in the linear backscattering (theoretical circular dichroism not exceeding 3 %), in contrast with the much more significant change of the second-harmonic generation in the far-field (secondharmonic generation circular dichroism from 16 up to 37%). These results open the possibility of designing deeply subwavelength chiral nanostructures for encoding microscopic "watermarks", which cannot be easily accessed by linear optical methods, moreover requiring a nonlinear microscopy set-up for reading out the encoded pattern. Graphic Over the past two decades, research devoted to the second-harmonic generation (SHG) from plasmonic nanostructures evolved from the investigation of hyper-Rayleigh scattering in colloidal suspensions of metal nanoparticles [1-3] towards the measurements of SHG from individual plasmonic nano-objects [4][5][6]. Far-field SHG radiation pattern has been demonstrated as a highly sensitive probe of the nanoscale optical field distribution [7], allowing for the investigation of multipolar plasmon modes [8][9][10], the analysis of surface plasmon polariton (SPP)-mediated interactions [11,12], refractive index sensing [13] and precise characterization of the nanostructure geometry [14][15][16]. In parallel, advanced theoretical tools for SHG modelling were developed, based on numerical methods such as finite elements to solve Maxwell equations and yield the scattered SHG radiation [17][18][19]. Among the geometrical properties that can be addressed by SHG microscopy, chirality stands-out as one of the most interesting probes of light-matter interactions, especially in the context of chiral plasmonic meta-materials which have been reported to exhibit giant optical activity in the linear regime [20,21]. Valev et al. have thus reported extensive work on chirality in SHG from specially patterned superchiral meta-surfaces [7,[22][23][24]. The location of plasmonic hot spots and their influence on the chiroptical response have been thoroughly addressed [22,23], leading to reports of nonlinear chiroptical effects reaching magnitudes as high as 5...