Optical Anisotropy of Black Phosphorus in the Visible Regime

Abstract

The striking in-plane anisotropy remains one of the most intriguing properties for the newly rediscovered black phosphorus (BP) 2D crystals. However, because of its rather low-energy band gap, the optical anisotropy of few-layer BP has been primarily investigated in the near-infrared (NIR) regime. Moreover, the essential physics that determine the intrinsic anisotropic optical property of few-layer BP, which is of great importance for practical applications in optical and optoelectronic devices, are still in the fancy of theory. Herein, we report the direct observation of the optical anisotropy of few-layer BP in the visible regime simply by using polarized optical microscopy. On the basis of the Fresnel equation, the intrinsic anisotropic complex refractive indices (n-iκ) in the visible regime (480-650 nm) were experimentally obtained for the first time using the anisotropic optical contrast spectra. Our findings not only provide a convenient approach to measure the optical constants of 2D layered materials but also suggest a possibility to design novel BP-based photonic devices such as atom-thick light modulators, including linear polarizer, phase plate, and optical compensator in a broad spectral range extending to the visible window.

Size-dependent saturable absorption and mode-locking of dispersed black phosphorus nanosheets

“…where c is the velocity of light, λ is the wavelength of the incident light and n is the linear refractive index, about 3.5 for BP [29]. The Im χ at 1030 nm decrease almost linearly with the linear transmittance when the size of nanosheets becomes smaller, as shown in Fig.…”

Visualizing Optical Phase Anisotropy in Black Phosphorus

“…Previous studies have revealed a range of intriguing anisotropic behaviors of BP in terms of its optical spectrum, Raman scattering, light absorption, photo-detection, and electrical conductivity. [27][28][29][30][31][32][33][34] While these properties reported in the literature have formed a valuable basis, many applications in the optical domain demand more information regarding the wave characteristics of the interacting light with the BP medium, specifically its phase. Moreover, the manipulation of the phase retardance of light in conjunction with interferometric techniques allows us to achieve optical contrast much stronger than an intensity measurement alone could 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 offer, as has been demonstrated extensively in optical metrology.…”

Layer Identification of Colorful Black Phosphorus

“…However, this method is hard to give the BP thickness information directly without other characterization methods. In addition, BP samples with different layer numbers are found to present different colors under the illumination of halogen lamp in optical microscope . This thickness‐dependent color variation rule can be used to easily and quickly identify BP layer numbers but it has not been clarified yet.…”

“…The red value reaches the maximum 255 from 59L to 138L, where the ranges of green and blue values are 128–184 and 13–76, respectively, which indicate that the colors of BP from 59L to 138L are between yellow and red. Refractive index of BP used here is from ZZ crystalline direction of few‐layer BP calculation results . According to the first‐principles simulation result, few‐layer black phosphorous strongly absorbs armchair‐polarized light (AC direction) and is transparent to zigzag‐polarized light (ZZ direction).…”