page_title: Graphene lens
section_title: Graphene lenses as ultra-broad photodetectors
hierarchical_section_title: Graphene lens / Graphene lenses as ultra-broad photodetectors
caption_attribution_description: English: Figure 2: Optical tunability of graphene under strong electric gating
context_page_description: Graphene applications as optical lenses. The unique honeycomb 2-D structure of graphene contributes to its unique optical properties. The honeycomb structure allows electrons to exist as massless quasiparticles known as Dirac fermions. Graphene's optical conductivity properties are thus unobstructed by any material parameters represented by equation 1, where e is the electron charge, h is Planck's constant and "e^2"/"h" represents the universal conductance.
  Equation 1
  This simple behavior is the result of an undoped graphene material at zero temperature. In contrast to traditional semiconductors or metals; graphene's band gap are near nonexistent as the conducting and valence bands make contact. However, the band gap is tunable via doping and electrical gating, resulting in changes in graphene's optical properties. As a result of its tunable conductivity, graphene is employed in various optical applications.
context_section_description: Electrical gating and doping allows for adjustment of graphene's optical absorptivity. The application of electric fields transverse to staggered graphene bilayers generates a shift in Fermi energy and an artificial, non-zero band gap (equation 2 figure 1).
  Equation 2
     δD=Dt - Db    where   Dt = top electrical displacement field   
                            Db = bottom electrical displacement field
  Varying δD above or below zero(δD=0 denotes non-gated, neutral bilayers) allows electrons to pass through the bilayer without altering the gating-induced band gap. As shown in Figure 2, varying the average displacement field, ▁D, alters the bilayer's absorption spectra. The optical tunability resulting from gating and electrostatic doping (also known as charge plasma doping) lends to the application of graphene as ultra-broadband photodetectors in lenses.
  Chang-Hua et al. implemented graphene in an infrared photodetector by sandwiching an insulating barrier of Ta2O5 between two graphene sheets. The graphene layers became electrically isolated and exhibited an average Fermi difference of 0.12eV when a current was passed through the bottom layer (figure 3). When the photodetector is exposed to light, excited hot electrons transitioned from the top graphene layer to the bottom, a process promoted by the structural asymmetry of the insulating Ta2O5 barrier. A consequence of the hot electron transition, the top layer accumulates positive charges and induces a photogating  effect on the lower graphene layer, which is measured as a change in current correlating with photon detection. Utilizing graphene both as a channel for charge transport and light absorption, the graphene ultra-broadband photodetectors ably detects the visible to mid-infrared spectrum. Nanometers thin and functional at room temperature, graphene ultra-broadband photodetectors show promise in lens application.
caption: Optical tunability of graphene under strong electric gating
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