Nanometer-thick flat lens with adjustable focus

Abstract

We report laser beam focusing by a flat, homogeneous film with a thickness of less than 100 nm. The effect relies on refractive index changes occurring in vanadium dioxide as it undergoes a phase transition from insulator to metal. Phase front curvature is achieved by means of temperature gradients, and adjustable focal lengths from infinity to 30 cm are attained. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4903887] Thin flat lenses have found applications in areas such as integrated optics and satellite optical instruments, where lightweight and small-volume optical components are desired, as well as x-ray optics, where standard lens materials are ineffective. While the traditional approach to creating thin lenses is based on the Fresnel zone plate design, there has been a resurgence of activity exploiting effects of nanoplasmonics and metamaterials. For example, a quasiperiodic arrangement of sub-wavelength holes on a thin gold film was found to focus light to sub-wavelength spatial resolution. 1-3 Similarly, light focusing by ultrathin lenses was demonstrated by exploiting the properties of nano-antennas etched on a surface, 4,5 thus forming a type of optically functionalized metasurface. 6,7 In this paper, we report yet an alternative method for focusing light with a thin layer of material. There are three key differences that distinguish this work from previous studies:(1) it exploits a different mechanism, namely, the large refractive index change occurring during a phase transition from insulator-to-metal, (2) the focusing capability is dynamic, and can be adjusted in real time, and (3) the material used is uniform and does not require prior physical patterning. In the latter aspect, it is analogous to gradient-index (GRIN) and other such thin lenses, [8][9][10][11][12] with the exception that its index of refraction is not spatially profiled beforehand.When light interacts with a layer of vanadium dioxide (VO 2 ), it acquires a phase shift which depends on the state of the material. In the temperature range where VO 2 undergoes a transition from insulator to metal (roughly 50-80 C), the optical phase shift is approximately linear with temperature, as was shown in a previous study. 13 Figure 1 illustrates the concept. Figure 2 presents ellipsometry measurements showing the change in refractive index as VO 2 undergoes a phase transition. Given the complex refractive indices, Fresnel reflection coefficients at the air/VO 2 and VO 2 /substrate interfaces introduce phase shifts of 0.51 rad and 0.75 rad, respectively, during the phase transition of the material. Meanwhile, the round-trip propagation phase in the layer varies by À1.0 rad. From this we conclude that both propagation and reflection phases are responsible for the observed phase shift.From Figure 1(b), we see that between a temperature T A (the onset of the phase transition) and a temperature T B (the end of the phase transition), the optical phase shift from VO 2 changes by some amount D/. As a result, by mean of a temperature gradien...

VO2 Thermochromic Metamaterial-Based Smart Optical Solar Reflector

“…Several studies have reported the effects of a global, thermally, or optically driven phase transition in VO 2 on the plasmonic response of nanoparticles and metamaterials over a broad spectral range from terahertz, − mid-infrared, , and near-infrared − to visible. − In the visible and near-infrared, studies of plasmonic effects on the optical excitation of the IMT have focused on hot-electron injection-driven ultrafast processes , and on nonresonant, continuous-wavemediated plasmonic heating, , including our demonstration of picosecond control of switching …”

Controlling light polarization with VO2 films

“…This phase shift, along with the field amplitudes, determine the polarization state of reflected and transmitted light beams. For example, with Δ = ± π or Δ = ± π/2, linearly polarized light may rotate by up to 90 o , or be switched between linear and circular states [1][2][3].…”

Polarization modulation by vanadium dioxide on metallic substrates

“…Changes in optical properties have been useful mostly for applications of spectral filtering and fieldamplitude modulation of infrared light, but recently, applications have also been found for phase and polarization modulation of visible and infrared light [6][7][8][9][10]. Phase and polarization modulations rely on the principle that in a material with complex-valued refractive indices like VO 2 , optical phase shifts are generally unequal in s-and p-polarizations.…”