Natural Hyperbolic Dispersion with Anisotropic Epsilon‐Near‐Zero and Epsilon‐Near‐Pole in Squaraine Molecular Film

Abstract

and a deep direct writing lithography, [7] to name a few. Furthermore, the nonlinear optical processes, both quadratic and cubic, have been found to be enhanced in ENZ spectral range. [8][9][10][11][12][13][14] When isotropic bulk metal is modeled as free electron gas, Drude permittivity exhibits isotropic ENZ corresponding to the frequency of longitudinal plasma oscillation. [15] Note that ENZ frequency is the same as plasma frequency ω p in the absence of collision, while introduction of collision causes ENZ to appear below ω p . [16] Moreover, ENZ frequency of silver (3.7 eV, 327 nm) is significantly red-shifted from ω p (9.1 eV, 136 nm), which is attributed to bound electrons in atomic layers adjacent to the metal surface. [17] In deeply subwavelength-thick metal slab, the signature of ENZ permittivity is manifested as plasma resonance absorption at ENZ wavelength for non-normal incident p-polarized light, called Ferrell-Berreman mode. [18][19][20] ENZ occurring in inorganic films of transparent conducting oxide [21] such as indium tin oxide [12] and Al-doped ZnO [13] and organic film of HTS-Squaraine [14] is also isotropic, where enhanced cubic nonlinearity is observed at ENZ wavelength.Epsilon-near-zero (ENZ) optical material has been employed in a number of novel linear and nonlinear optical applications, owing to the vanishing polarization upon an incident optical wave. In a uniaxial medium possessing hyperbolic energy-momentum dispersion of optical wave, ENZ can take place at ordinary and extraordinary permittivities. Organic thin films presenting a lamellar structure have been reported to exhibit a transverse negative hyperbolic dispersion with ENZ at ordinary permittivity. Here, organic thin film with ENZ at extraordinary permittivity is demonstrated. Newly synthesized polymethine dye (i.e., squaraine indolenine triethyleneglycol molecule) self-organizes to form a layered structure in a pristine film, and both transverse negative and positive hyperbolic dispersions are observed at visible wavelengths. Analysis of tens-nanometer-thick pristine film shows that both ENZ and epsilon-near-pole (ENP) occur at longitudinal as well as transverse component of dielectric permittivity. Optical characterization of squaraine pristine film is presented, and the importance of transverse positive hyperbolic dispersion in such monolithic thin film is discussed.

Effect of the electron donating group on the excited-state electronic nature and epsilon-near-zero properties of curcuminoid-borondifluoride dyes

“…Notably, for isotropic films of 4 and 5 a double zero-crossing of Re{ ε } takes place, similar to that observed in dielectric spectrum of squaraine indolenine triethyleneglycol molecular film. 22 That is, in films of 4 and 5, as wavelength increases from blue to red spectral region, Re{ ε } changes sign from positive to negative (ENZ) at short wavelength, while from negative to positive (epsilon-near-pole, ENP) at long wavelength. 27,28 The presence of ENP is associated with a high value of refractive index n , and in fact the maximum value of n is over 3 in films of 4 and 5.…”