Optically transparent microwave polarizer based on quasi-metallic graphene

Abstract

An optically transparent graphene-based wire-grid polarizer for operating at microwave frequencies (X band) has a glass substrate having multiple strips or layers of SOCl.sub.2 doped graphene. The strips are separated by portions of the glass substrate such that the strips are arranged in parallel. The SOCl.sub.2 doped graphene strips have a quasi-metallic quality allowing for the transmission of an electric field with horizontal polarization in the horizontal direction while reflecting the vertical portion of the electric field.

Claims

1. A microwave polarizer, comprising: a glass substrate; a plurality of SOCl.sub.2 doped graphene layers, and wherein: said plurality of SOCl.sub.2 doped graphene layers is arranged on the top of said glass substrate so as to form a sheet and are separated by portions of said glass substrate such that said plurality of SOCl.sub.2 doped graphene layers are in parallel with one another; and wherein each layer of said plurality of SOCl.sub.2 doped graphene layers is about 1 nm thick, said plurality of SoCl.sub.2 doped graphene layers comprises at least four SOCl.sub.2 doped graphene layers, and each layer of said plurality of SOCl.sub.2 doped graphene layers has a hole concentration greater than 10.sup.12 cm.sup.2.

2. A microwave polarizer according to claim 1, wherein: the sheet formed by said plurality of SoCl.sub.2 doped graphene layers and said glass substrate has a sheet resistance R.sub.s range between 10 /sq and 2 k/sq.

3. A microwave polarizer according to claim 1, wherein: said plurality of SoCl.sub.2 doped graphene layers have a quasi-metallic quality allowing for the transmission of an electric field with horizontal polarization in the horizontal direction while reflecting a vertical electric field.

Description

DESCRIPTION OF THE DRAWINGS

(1) A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained by reference to the following detailed description when considered in connection with the accompanying drawings.

(2) FIG. 1 is a top view of the graphene-based grid-polarizer of the present invention.

(3) FIG. 2 is a perspective view of the graphene-based grid-polarizer of the present invention.

(4) FIG. 3 is a graphical illustration which compares normalized transmittances of the graphene-based grid polarizer of the present invention with those of a copper-based polarizer.

(5) FIG. 4 is a graphical illustration showing the Raman spectrum in pristine CVD graphene.

(6) FIG. 5 is a graphical illustration showing the Raman spectrum in SOCl.sub.2 doped graphene in accordance with the present invention.

(7) FIG. 6 is a graphical illustration demonstrating how the optical transmittance decreases as layers of SOCl.sub.2 doped graphene are added (from 1 to 5 layers) in accordance with the present invention.

(8) FIG. 7 is a graphical illustration showing sheet resistance as layers of SOCl.sub.2 doped graphene are added in accordance with the present invention.

(9) FIG. 8 is a graphical illustration showing analytical model lines for reflectance, transmittance and absorbance for the polarizer of the present invention with the analytical models being compared with actual experimental values when sheet resistance R.sub.s is varied in the range 10 /sq-2 k/sq.

DETAILED DESCRIPTION

(10) In FIG. 1 graphene-based grid-polarizer 10 has graphene strips 12A, 12B, 12C, 12D, and 12E positioned on a clear glass substrate 16. The widths of respective portions 16A, 16B, 16C, 16D of the glass substrate 16 separate the respective graphene strips 12A, 12B, 12C, 12D and 12E. The graphene strips are doped with SOCl.sub.2. Reference numerals 14A, 14B, 14C, 14D and 14E indicate that graphene strips 12A, 12B, 12C, 12D and 12E contain SOCl.sub.2 doping. The hole concentration for each graphene strip is greater than 10.sup.12 cm.sup.2.

(11) FIG. 2 gives further perspective of graphene strips 12A, 12B, 12C, 12D, and 12E being positioned on the clear glass substrate 16. The graphene-based grid-polarizer 10 transmits only the horizontal component 22 of the electric field with the vertical component of the electric field being filtered out.

(12) FIG. 3 shows the normalized transmittance (T.sub.90T.sub.pol)/T.sub.90 of the polarizer T.sub.pol with respect to transmission at 90 (T.sub.90). This normalization emphasizes the difference at 0 (reflected signal). The graphene-based wire-grid polarizer 10 of the present invention shows a near-unity transmission at 90.

(13) FIG. 3 gives a graphical comparison of the graphene-based grid-polarizer 10 of the present invention with a prior art copper based polarizer. Normalized transmittance (T.sub.90T.sub.pol)/T.sub.90 for the graphene-based polarizer (depicted with solid lines and squares) is compared with the normalized transmittance of the copper-based polarizer (depicted by dashed lines and circles) where T.sub.90 and T.sub.pol are the transmittance at 90 and the polarizer transmittance, respectively.

(14) In testing the present invention, the Raman spectrum of CVD graphene was compared before and after SOCl.sub.2 treatment. In FIG. 4 the Raman spectrum of pristine CVD graphene is shown. FIG. 5 shows the Raman spectrum of SOCl.sub.2 doped graphene in accordance with the present invention. Attention is made to the peaks 30 and 32 of the pristine CVD graphene and peaks 34 and 36 of the SOCl.sub.2 doped graphene.

(15) The hole-doping effect is clearly demonstrated by the red-shift of both the G and 2D peaks and by the increase of the ratio between their intensities. The absence of the D peak in the spectrum of the chemically-doped graphene attests to the absence of any breaking in the C-sp.sup.2 lattice conjugation induced by the doping treatment. The interaction between SOCl.sub.2 and graphitic materials occurs by removing hydroxyl and carboxyl functionalities, and by introducing both covalently and ionically bonded chlorine atoms. Moreover, the interaction with other SOCl.sub.2/graphene reaction products, including SO.sub.2, cannot be excluded.

(16) With reference to FIG. 6, the optical transmittance of chemically-doped graphene in accordance with the present invention is demonstrated as the chemically-doped graphene layers are progressively increased from one to five layers. No differences were found between the transmittance values of pristine and chemically-doped multilayer graphene. The higher absorbance found for three, four and five graphene layers are related mainly to residual impurities due to the transferal processes, and not to impurities introduced by SOCl.sub.2 treatment.

(17) In FIG. 7, in accordance with the present invention, the effect of the combined use of SOCl.sub.2 chemical doping and multilayer graphene in terms of sheet resistance is shown. In going from a monolayer or single strip to five layers or strips the sheet resistance R.sub.s of pristine CVD graphene monotonically decreases and plateaus to values larger than 100 /sq (black solid squares). In contrast, chemically-doped graphene can overcome the low threshold limit with only two layers. The minimum sheet resistance value measured for chemically-doped graphene is 27 /sq, and was achieved with five-layer graphene samples. If additional graphene layers are added, the effect of series resistances between adjacent graphene layers becomes predominant, resulting in higher R.sub.s.

(18) The very low sheet resistance of the present invention (quasi-metallic graphene) led to testing of the electromagnetic response of hole-doped graphene at microwave frequencies. In particular, reflectance, transmittance, and absorbance of samples with different sheet-resistances were measured. The measurements were carried out by means of a microwave setup consisting of a klystron connected to a WR90 rectangular waveguide (the waveguide supported only the TE.sub.10 mode). A slotted-line acquired the electromagnetic power (square law) with a spatial resolution of 1 mm in the rectangular waveguide at 9 GHz (the guide wavelength is .sub.g=48 mm, while the free-space wavelength is .sub.0=33 mm).

(19) That analysis is based on the measurement of the standing waves that originate in the rectangular waveguide from the discontinuity at one end of the waveguide. In order to validate the experimental protocol, the reflection coefficient when the waveguide is either shorted with a metallic plate or open-ended was measured.

(20) First the response of glass samples covered by graphene sheets with different sheet resistances was measured. The area covered by the doped graphene was about 3 cm2 cm, allowing total coverage of the WR90 waveguide cross-section. It is noted that the thickness of the hole-doped graphene as used in the present invention is of the order of 1 nm, corresponding to /10.sup.6, thus confirming the two-dimensional nature of graphene at microwave frequencies.

(21) FIG. 8 shows analytical model (lines) and experimental findings (symbols) for the reflectance R.sub.GR (dot-dashed, square), transmittance T.sub.GR (dashed, circle) and absorbance A.sub.GR (solid, asterisk) when sheet resistance R.sub.s is varied in the range 10 /sq-2 k/sq. (The x-axis of FIG. 8 is in logarithm scale). The maximum absorbance (obtained considering R.sub.s=n.sub.0/2 where n.sub.0 is the vacuum impedance) separates the quasi-metallic region (R.sub.s<n.sub.0/2) from the lossy-dielectric region (R.sub.s>n.sub.0/2). The reflectance and transmittance were measured by means of a microwave setup operating at 9 GHz.

(22) As FIG. 8 demonstrates, the comparison between the analytical models and the experimental results shows very good agreement.

(23) The analytical models can be formulated by considering the boundary conditions at the interface z=0, between two (semi-infinite) media (impinging from medium 1 to medium 2), that introduces a finite sheet conductivity .sub.2D this can be expressed as:
n(.sub.1.sub.2)|.sub.z=0=0
n(H.sub.1H.sub.2)|.sub.z=0=J.sub.s=s.sub.2D(Eq. 1)
Where n.sub.1 and n.sub.2 are the wave impedances in the two media, respectively, and J.sub.s=s.sub.2D corresponds to Ohm's law.

(24) When graphene is considered in the microwave regime, the graphene sheet conductivity can be approximated by the DC sheet conductivity, i.e. s.sub.2Ds.sub.DC=1/R.sub.s. Moreover, the sheet conductivity is independent over a wide frequency range. At the same time, the graphene sheet may be considered as an interface (current sheet) between two media, since its thickness is /10.sup.6 at microwave frequencies. Applying the boundary conditions for the electric and magnetic fields at the interface z=0 gives:

(25) $\begin{array}{cc}1+=T\frac{1}{n_1}-\frac{}{n_1}-\frac{T}{n_2}=s_{2D}T& \left(\mathrm{Eq}.2\right)\end{array}$
where and T are the reflection and transmission coefficients, respectively.

(26) Solving the Equation 2 yields to:

(27) $\begin{array}{cc}=\frac{1+s}{1-s}T=\frac{2}{1+s}& \left(\mathrm{Eq}.3\right]\end{array}$
where

(28) $s=\frac{n_1}{n_2}\left(1+s_{2D}{n}_2\right).$

(29) Reflection and transmission coefficients allow calculation of reflectance R.sub.GR and transmittance

(30) $T_{\mathrm{GR}},{.Math..Math.}^2\mathrm{and}\frac{n_1}{n_2}{.Math.T.Math.}^2,$
respectively, while absorbance A.sub.GR may be evaluated as A.sub.GR=1R.sub.GRT.sub.GR. Assuming n.sub.1=n.sub.2=n.sub.0 (where .sub.0 corresponds to the impedance of free space) it is possible to define two different regions for a single graphene sheet: the lossy-dielectric region

(31) $\left(R_s>\frac{n_0}{2}\right)$
and the quasi-metallic region

(32) $\left(R_s<\frac{n_0}{2}\right),$
where

(33) $R_s=\frac{n_0}{2}$
defines the sheet resistance that corresponds to the absorbance maximum.

(34) For the present invention, the maximum achievable absorption with the doped graphene sheet was found equal to 50%, while the graphene in the quasi-metallic region acts as a metal by efficiently reflecting the impinging electromagnetic field. For example, when R.sub.s=27 /sq the reflectance is larger than 80%. At the same time, the doped graphene sheet conductivity is independent over a wide frequency range which makes the results reported in FIG. 8 also relatively frequency-independent, and may consequently be applied to other regions of the spectrum.

(35) The graphene used in the prototype of the present invention was grown by chemical vapor deposition methodology using copper foils (thickness ranging from 15 to 50 m) as substrates and CH.sub.4 as gaseous carbon precursors. Copper substrates were annealed at 1000 C. in a CH.sub.4 and H.sub.2 atmosphere (with a CH.sub.4/H.sub.2 ratio going from 99/1 to 1/99) and a total gas pressure of 1-100 mbar. The annealing of copper substrates was carried out for a time ranging from 10 to 200 minutes in order to achieve the complete coverage of the copper substrate by graphene.

(36) Then, the fabricated copper/graphene samples were cooled, exposed to air, and covered by thermal tape. These copper/graphene/thermal tape samples were dipped in a solution of ammonium persulfate until the complete etching of the copper substrates.

(37) Finally, the transfer of graphene on corning glass substrates was performed by pressing the graphene/tape samples to the corning substrates (treated by O.sub.2 plasma to improve graphene adhesion). Then, these corning/graphene/thermal tape samples were heated for detaching the thermal tape that loses its adhesive properties at temperatures above 100 C. Additional transfer steps were performed on graphene/corning samples for fabricating multilayer graphene samples.

(38) SOCl.sub.2 treatments were performed in a dry chamber by placing graphene/glass substrate and 1 mL of liquid SOCl.sub.2 (avoiding direct contact) at 105 C. for 60 min. Doping of multilayer samples was performed by repeating SOCl.sub.2 treatment after transferring and stacking each graphene layer.

(39) Sheet resistance measurements were carried out using four-point contacts geometry in the Van der Pauw configuration on a sampled area of 44 mm.sup.2 in air and at room temperature.

(40) The chemically-optimized graphene was exploited to realize a graphene-based wire-grid polarizer, by superimposing four graphene strips. Using this procedure achieved a minimum sheet resistance of 50 /sq (quasi-metallic region) in the strips over an area of about 5055 mm.sup.2 (FIG. 2). The presence of quasi-metallic strips allows the transmission of the electric field (E.sub.H) with horizontal polarization 22 in the horizontal direction, while the electric field (E.sub.V) with vertical polarization 20 is reflected (FIG. 2).

(41) In order to verify its behavior in terms of polarization, the graphene-based wire-grid polarizer was placed in between two horn pyramidal antennas with a mutual distance larger than that the Fraunhofer distance, thus allowing operation in the far-field. The device was placed on a plastic goniometer ( step was set at 15) and the transmitted signal was normalized with respect to a reference structure (glass substrate without graphene).

(42) A comparison was made of the graphene-based wire-grid polarizer 10 of the present invention with a wire-grid polarizer made with copper stripes of identical dimensions (5 mm width) and identical foot-print with respect to the graphene-based polarizer.

(43) The comparison reveals that the two polarizers only show a constant 3 dB difference over the entire angular range while their angular behavior is identical, (see, FIG. 3). This result is quite remarkable, considering that the copper layer is 35 m thick, while graphene thickness is about 1 nm.

(44) The present invention in combining the use of SOCl.sub.2 chemical doping with multiple layers of graphene can reduce material sheet resistance down to 30 /sq while optical transparency remains above 85%.

(45) Also, it should be emphasized that dimensions of the sheet of the present invention (i.e., substrate with parallel strips of doped graphene) can range in size from a few cm.sup.2 to several m.sup.2 with the number of strips ranging from four to multiple, multiple quantities of strips.

(46) The present invention has many uses. Various modifications and embodiments of the invention may be made by those skilled in the art without departing from the scope and spirit of the foregoing disclosure. Accordingly, the scope of the invention is limited only by the following claims.