Optical properties of bismuth nanostructures towards the ultrathin film regime

Abstract

Bulk bismuth presents outstanding optical properties, such as a giant infrared refractive index (n ~ 10) and a negative ultraviolet -visible permittivity induced by giant interband electronic transitions. Although such properties are very appealing for applications in nanophotonics, the dielectric function of bismuth nanostructures has been scarcely studied. Here, we determine by spectroscopic ellipsometry the far infraredto ultraviolet dielectric function of pulsed laser deposited bismuth thin films with nominal thickness tBi varied from near 10 nm to several tens of nm. For tBi > 15 nm, the films display a continuous structure and their dielectric function is comparable with that of bulk bismuth. For tBi < 15 nm, the film structure is discontinuous, and the dielectric function differs markedly from that of bulk bismuth. It is proposed from FDTD simulations that this marked difference arises mainly from effective medium effects induced by the discontinuous film structure, where quantum electronic confinement does not play a dominant role. This suggests that ultrathin and continuous bismuth films should present the same outstanding optical properties as bulk bismuth for high performance nanophotonic devices.Bulk bismuth (Bi) presents outstanding optical properties, related with its giant interband electronic transitions, such as a giant infrared refractive index (n ~ 10) and a negative ultravioletvisible permittivity [1]. These properties are thought to enable a strong visible and infrared absorption [1][2][3][4] and an ultravioletvisible plasmonic response [1-3, 5, 6] in deeply subwavelength Bi nanostructures. These effects are of utmost importance for a growing number of applications based on Bi nanostructures, including photocatalysis [7-10], photodetection [16,12], or optical modulation [13,14]. For exploiting the full potential of such applications by rational nanostructure design, knowing the dielectric function of Bi nanostructures in a broad spectral range, from the far infrared to the ultraviolet, is needed. However, such data are not available, despite of several claims of quantum electronic confinement effects implying a size-dependence for the electronic structure of bismuth nanostructures [15][16][17][18]. In fact, very few attempts to determine it were reported in the past years, in the case of bismuth thin films [19] and nanowires [15], and they were limited to narrow spectral windows in the infrared.In here, we determine the far infraredtoultraviolet dielectric function of Bi thin films with nominal thicknesses tBi varied from near 10 nm to several tens of nm. All the studied films display a continuous structure and a dielectric function comparable to that of bulk Bi, except the thinnest one (tBi = 11 nm). In this case, our analysis suggests that the deviation from bulk values originates from the discontinuous film structure, the incident light interacting with an effective medium consisting of Bi and air. This leads us to propose that the observed deviation does not primarily originate from quan...

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Spectrally Tailored Light‐Matter Interaction in Lithography‐Free Functional Nanomaterials

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“…[65] This suitable wavelength dependence for the dielectric function might be the result of a discontinuous film structure. [66]…”