Remote Nanoscopy with Infrared Elastic Hyperspectral Lidar

Abstract

Monitoring insects of different species to understand the factors affecting their diversity and decline is a major challenge. Laser remote sensing and spectroscopy offer promising novel solutions to this. Coherent scattering from thin wing membranes also known as wing interference patterns (WIPs) have recently been demonstrated to be species specific. The colors of WIPs arise due to unique fringy spectra, which can be retrieved over long distances. To demonstrate this, a new concept of infrared (950-1650 nm) hyperspectral lidar with 64 spectral bands based on a supercontinuum light source using ray-tracing and 3D printing is developed. A lidar with an unprecedented number of spectral channels, high signal-to-noise ratio, and spatio-temporal resolution enabling detection of free-flying insects and their wingbeats. As proof of principle, coherent scatter from a damselfly wing at 87 m distance without averaging (4 ms recording) is retrieved. The fringed signal properties are used to determine an effective wing membrane thickness of 1412 nm with ±4 nm precision matching laboratory recordings of the same wing. Similar signals from free flying insects (2 ms recording) are later recorded. The accuracy and the method's potential are discussed to discriminate species by capturing coherent features from free-flying insects.

Discrimination of Hover Fly Species and Sexes by Wing Interference Signals

“…Thus, specular flashes can be observed by insects in flight. [34] Spectrally, this wing flash is dominated by fringy thin-film interference resonances [32,34] referred to as a wing interference signal (WIS) in this study. The flash instances represent the rare occasions when the wing orientation is entirely known and the reflectance spectra [34,43] may provide the quantitative information to determine the species and sex.…”

“…The left-right discrepancy of the effective thickness is just 10 nm, values are shown in Figure 1D for the examined individual, similar to the precisions achieved by lidar estimates on free-flying insects. [32] The spectral modulation depth varies across the wing surface (Figure 1E), implying that all parts of the wing surfaces does not contribute equally to the effective fringe. In particular, the wing veins do not produce spectral fringes and display low modulation, as illustrated by the histogram of wing thicknesses in individual pixels (Figure 1G).…”

“…To improve species identification of free-flying insects, the frequency-, polarimetric-and spectral domains have been explored, [26] and real-time field sensors that discriminate freeflying insect species based on wing beat frequencies (WBFs) and harmonic overtones developed. [27][28][29][30] WBF and harmonic spectrum depend on both the wing dynamics, [31] wing surface roughness, [11] and wing membrane thickness [32][33][34] in relation to the wavelength. The WBFs of hover flies are in the 150-300 Hz range, [35] which is between the WBFs of bees [36] and mosquitoes.…”

“…The polarimetric domain provides sensitivity to microstructural features, [34,40] whereas different spectral bands [41] may yield molecular or nanoscopic information on the wing membrane thickness. [32,33] Importantly, most of the light backscattered by insects is oscillatory and contributed by insect wings, [39,42] and most of that light is scattered by specular reflections in insect wings. Thus, specular flashes can be observed by insects in flight.…”

“…[44] As the specular WIS is highly directional, the resonant reflectances exceed the white Lambertian reference (100% diffuse reflectance). This implies that WIS could be detectable over extended distances with a considerable contrast against background, [32,34] with consequences both for visual ecology and the prospects for detection and monitoring. The spectra obtained from an individual pixel exhibit significant fluctuations, reaching up to 88% modulation (Figure 1F, orange solid line, see also Section 4.5).…”