LUMPED PLASMONIC "DIODE" AND LUMPED PLASMONIC "RECTIFIER" FOR OPTICAL FIELD RECTIFICATION AND LUMPED OPTICAL SECOND HARMONIC GENERATION

Abstract

A lumped nanocircuit element design at IR and optical frequencies is provided that can effectively act as a lumped “diode” and a lumped “rectifier” for rectifying optical field displacement currents or optical electric field. The lumped nanocircuit element design can also act as a lumped second harmonic generator. The element is formed by juxtaposing an epsilon-negative nanoparticle with a nonlinear optical nanostructure.

Description

FIELD OF THE INVENTION

The invention relates generally to a plasmonic lumped “diode” and lumped “rectifier” that, when exposed to an optical electric field, can provide optical field displacement current predominantly rectified in a particular polarity.

BACKGROUND OF THE INVENTION

In an article by N. Engheta, et al. entitled “Circuit elements at optical frequencies: nano-inductors, nano-capacitors and nano-resistors,” Physical Review Letters 95, 095504 (2005), the inventors of that application explored and studied the concept of lumped nanocircuit elements such as nanocapacitors, nanoinductors, and nanoresistors at optical frequencies using the plasmonic and non-plasmonic nanoparticles. The concept described therein can provide the possibility of utilizing and extending the concept and mathematical tools of circuit theory, which has traditionally belonged to the radio frequency (RF) and microwave domains, into the THz, IR and optical frequencies, and can open doors to various innovations in future photonic circuits and systems. In another recent article (N. Engheta, “Nanocircuits with Light at Nanoscales: Optical Nanocircuits Inspired by Metamaterials”, Science, Vol. 317, pages 1698-1702, Sep. 21, 2007), N. Engheta overviews and envisions nanocircuits at optical frequencies at nanoscale, which can be formed by collections of nanoparticles with various different materials. Using these concepts, the present inventor has also developed the notion of 1-D, 2-D, and 3-D nanotransmission lines in optics that can provide the possibility for photonic metamaterials with negative refraction. The case of 1-D nanotransmission line is discussed by A. Alu and N. Engheta in “Theory of Linear Chains of Metamaterial/Plasmonic Particles as Subdiffraction Optical Nanotransmission Lines” Physical Review B, Vol. 74, 205436 (2006). The case of the 2-D nanotransmission line is discussed by A. Alù and N. Engheta in “Optical nano-transmission lines: synthesis of planar left-handed metamaterials in the infrared and visible regimes,” Journal of the Optical Society of America B 23, 571-583 (2006), and the case of 3-D nanotransmission line is discussed by A. Alu and N. Engheta in “Three-Dimensional Nanotransmission Lines at Optical Frequencies: A Recipe for Broadband Negative-Refraction Optical Metamaterials,” Physical Review B, Vol 75, 024304 (2007).

Furthermore, several other related concepts have been developed recently. These include the concept of Optical ‘Shorting’ wire by A. Alu and N. Engheta in “Optical ‘Shorting Wire’” Optics Express, Vol. 15, Issue 21, pp. 13773-13782, Oct. 5, 2007; the concept of coupling among lumped nanocircuits, by A. Alu, A. Salanrino, and N. Engheta in “Coupling of Optical Lumped Nanocircuit Elements and Effects of Substrates,” Optics Express, Vol. 15, Issue 21, pp. 13865-13876, Oct. 5, 2007; the concept of nanoinsulators and nanoconnectors by M. G. Silveirinha, A. Alu, J. Li, and N. Engheta in “Nanoinsulators and Nanoconnectors for Optical Nanocircuits” posted in http://www.arxiv.org/ftp/condmat/papers/0703/0703600.pdf, 2007; the concept of parallel, series, and intermediate interconnections of optical nanocircuit elements, by A. Salandrino, A. Alu, and N. Engheta in “Parallel, Series, and Intermediate Interconnections of Optical Nanocircuit Elements: Part I: Analytical Solutions” posted in http://www.arxiv.ori/abs/0707.1002; and Part II of this concept by A. Alu, A. Salandrino, and N. Engheta in “Parallel, Series, and Intermediate Interconnections of Optical Nanocircuit Elements: Part II: Nanocircuit and Physical Interpretations” posted in http://www.arxiv.orttabs/0707.1003; and the concept of lumped nanofilters by A. Alu, M. Young, and N. Engheta in “Nanofilters for Optical Nanocircuits” posted in http://www.arxiv.org/ftp/arxiv/papers/0710/0710.0616.pdf, 2007

It is desired to extend such concepts to provide other circuit elements, such as plasmonic lumped “diodes” and lumped “rectifiers” at optical frequencies that function in the optical domain the same way that their counterparts function in the microwave domain.

SUMMARY OF THE INVENTION

The inventor discloses a plasmonic lumped “diode” and lumped “rectifier” that, when exposed to an optical electric field, can provide optical field displacement current predominantly rectified in a particular polarity. Specifically, when the electric field of the optical signal is pointed to one direction in a half cycle, the displacement current can be relatively large, whereas when the electric field is reversed in the next half cycle, the displacement current is relatively small in the opposite direction. In these designs, a plasmonic nanostructure with negative permittivity is paired and juxtaposed with another nanostructure that is formed by a nonlinear optical material. By properly choosing the shape, size and the material parameters for both segments of this element, the inventor has found that the induced dipole moment (and therefore the displacement current) on this lumped element would be nonsymmetric as the optical electric field varies sinusoidally with optical frequencies. This provides a mechanism for rectification of the induced dipole (or displacement current), providing a diode-like functionality for this lumped element at IR and optical frequencies. This can also lead to a direct second-harmonic generation at this element at the local level without needs for filtering out the fundamental harmonic and/or without need for phase matching concern.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the general geometry of a nanoelement as a lumped plasmonic diode/rectifier at IR and optical frequencies in accordance with the invention.

FIG. 2 illustrates the series configuration of a lumped nanoelement in accordance with the invention.

FIG. 3 is a sketch of the function of this lumped plasmonic “rectifier” at optical frequencies, generating a direct second harmonic.

FIG. 4 illustrates the parallel configuration of such lumped nanoelements at IR and optical frequencies.

FIG. 5 is a sketch of the function of this nanoelement as a lumped rectifier at IR and optical frequencies.

FIG. 6 illustrates the diode behavior of the lumped nanoelement shown in FIG. 4.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The invention will be described in detail below with reference to FIGS. 1 through 6. Those skilled in the art will appreciate that the description given herein with respect to those figures is for exemplary purposes only and is not intended in any way to limit the scope of the invention. All questions regarding the scope of the invention may be resolved by referring to the appended claims.

Consider a nanostructure (which is smaller than the operating wavelength) formed by mixing two parts: an epsilon-negative plasmonic material (for example, silver or gold or any other materials with negative values for real part of the permittivity), and a nonlinear optical material (NLO) (with quadratic nonlinearity such as KDP). FIG. 1 shows the sketch of such a two-part nanoelement. One of the goals of the present inventor is to choose the material properties and the geometric dimensions of this nanoelement such that when the electric field of the optical signal is pointed to one direction in a half cycle, the displacement current can be relatively large (see the left panel of FIG. 1), whereas when the electric field is reversed in the next half cycle, the displacement current is relatively small in the opposite direction (see the right panel of FIG. 1). In this sense, the resulting element will be a lumped plasmonic “diode” at IR and optical frequencies.

In order to achieve this goal, the following “series” configuration for this nanoelement, is considered as shown in FIG. 2. When the external (i.e., input) electric field and the electric displacement vectors excite this nanoelement (and for simplicity it is assumed that this vector is normal to the interface of this nanoelement), the normal component of the displacement vector D_input should stay continuous across the boundaries of this nanoelement. Inside the nonlinear section of this nanoelement, one can write D=ε_oE+P=εE where the polarization vector P can be expressed using the nonlinear expression. Therefore, one can write D=ε_oε_rlE+ε_o(χ_n: EE), which can be expressed as D=ε_o(ε_rl+χ_nE) E=ε_oε_NLE. The quadratic nonlinearity term χ_n is usually very small, i.e.,

|χ_nE_o|<<ε_rl.

Due to the continuity of the normal component of the displacement vector, one can write D_input=ε_oε_NLE_NL=ε_oε_plE_pl where ε_pl represents the relative permittivity of plasmonic material, and E_NL and E_pl denote the electric fields inside the NLO and the plasmonic materials, respectively. The “effective” electric field across the entire nanoelement can be defined as

$E_{\mathrm{eff}}\equiv \frac{E_{\mathrm{NL}}d_1+E_{\mathrm{pl}}d_2}{\left(d_1+d_2\right)},$

where d₁ and d₂ are the thicknesses of the two segments, respectively. This effective electric field can thus be expressed in terms of the input electric and displacement vectors as follows:

$\begin{array}{c}{E}_{\mathrm{eff}}=\frac{1}{d_1+d_2}\left\{\left[\frac{d_1}{{\varepsilon }_{\mathrm{rl}}}+\frac{d_2}{{\varepsilon }_{\mathrm{pl}}}\right]E_{\mathrm{input}}-\frac{2{\chi }_nd_1E_{\mathrm{input}}^2}{{\varepsilon }_{\mathrm{rl}}^3}\right\}\\ =\frac{1}{d_1+d_2}\left\{\left[\frac{d_1}{{\varepsilon }_{\mathrm{rl}}}+\frac{d_2}{{\varepsilon }_{\mathrm{pl}}}\right]\frac{D_{\mathrm{input}}}{{\varepsilon }_o}-\frac{2{\chi }_nd_1D_{\mathrm{input}}^2}{{\varepsilon }_{\mathrm{rl}}^3{\varepsilon }_o^2}\right\}\end{array}$

In this expression, there are two specific terms: one is the “linear term” and the other is the nonlinear term. If instead of the plasmonic material a regular conventional dielectric with positive permittivity were used, the linear term would be the dominant term when compared with the nonlinear term. However, by using the plasmonic material, one can choose the material parameters such that the linear term vanishes, since ε_pl can be negative. Therefore, if the parameters are chosen such that:

$\frac{d_1}{{\varepsilon }_{\mathrm{rl}}}+\frac{d_2}{{\varepsilon }_{\mathrm{pl}}}=0,i.e.,\mathrm{if}$ $\frac{d_1}{{\varepsilon }_{\mathrm{rl}}}+\frac{d_2}{{\varepsilon }_{\mathrm{pl}}},\mathrm{then}\mathrm{one}\mathrm{obtains}$ $\begin{array}{c}{E}_{\mathrm{eff}}=-\frac{2{\chi }_nd_1E_{\mathrm{input}}^2}{{\varepsilon }_{\mathrm{rl}}^3\left(d_1+d_2\right)}\\ =-\frac{2{\chi }_nd_1D_{\mathrm{input}}^2}{{\varepsilon }_{\mathrm{rl}}^3{\varepsilon }_o^2\left(d_1+d_2\right)}.\end{array}$

This implies that the effective field across the entire nanoelement varies quadratically with input D_input. In other words, if one has a sinusoidal variation of D_input=ε_oE_input=A sin(ω_ot), the effective field across the nanoelement varies as E_eff(t)∝ sin²(ω_ot) as illustrated in FIG. 3. In other words, there is a “direct” second harmonic generation due to the “rectification” of the optical field. In this way, this lumped element will be able to “rectify” the optical electric field at the IR and optical frequencies.

One can also consider the “parallel” configuration as shown in FIG. 4, in which the two elements (one plasmonic and the other NLO) have been placed in parallel with each other, and very thin layers of materials with relatively high value of permittivity (either positive or negative) are used as the top and bottom cover layers. In this case, the input electric field E_input can lead to an effective displacement current exiting the nanoelement, which can be described as:

$I_{\mathrm{eff}}^{\mathrm{dis}}=-\omega \frac{{\varepsilon }_o}{A_1+A_2}\left\{\left[{\varepsilon }_{\mathrm{rl}}A_1+{\varepsilon }_{\mathrm{pl}}A_2\right]+{\chi }_nA_1E_{\mathrm{input}}\right\}E_{\mathrm{input}},$

where A₁ and A₂ are the cross-sectional areas of the two segments of this nanoelement, and I_eff^dis≡−iωD_eff^dis. Here if the material parameters and geometrical dimensions are chosen such that ε_rlA₁+ε_plA₂=0, then one will have:

$I_{\mathrm{eff}}^{\mathrm{dis}}=-\omega \frac{{\varepsilon }_o{\chi }_nA_1E_{\mathrm{input}}^2}{A_1+A_2},$

which shows that the effective displacement current coming out of this nanoelement varies quadratically with the input electric field as illustrated in FIG. 5. This results in a “direct” second harmonic generation at the IR and optical frequencies without any need for filtering out the fundamental harmonic and without need for the phase matching, and also suggests that one can have a lumped “rectifier” at optical frequencies.

If instead of the above condition ε_rlA₁+ε_plA₂=0, the material parameters are selected such that (ε_rl−χ_n|E_input^peak|)A₁+ε_plA₂=0 where E_input^peak is the peak of the input sinusoidal electric field, then one will have

$I_{\mathrm{eff}}^{\mathrm{dis}}=-\omega \frac{{\varepsilon }_o{\chi }_nA_1}{A_1+A_2}\left(E_{\mathrm{input}}+E_{\mathrm{input}}^{\mathrm{peak}}\right)E_{\mathrm{input}},$

which shows that the effective displacement current coming out of this nanoelement is not symmetric with respect to the sinusoidal variation of the input electric field. In other words, when the input electric field of the optical signal is pointed to one direction in a half cycle, the effective displacement current can be different as compared to that when the input electric field is reversed in the next half cycle, as shown in FIG. 6. This means that one has a lumped “diode” at optical frequency that provides the “diode” behavior for the displacement current.

In addition to parallel and series configurations individually, one can also consider the other combinations of plasmonic and nonlinear layers, e.g., concentric shells and coaxial shells of these materials in which the materials and the dimensions are chosen such that these shells act as lumped “diode” at optical frequency that provide “diode” behavior for the displacement current or for electric field.

Those skilled in the art will also appreciate that numerous other modifications to the invention are possible within the scope of the invention. Accordingly, the scope of the invention is not intended to be limited to the preferred embodiments described above, but only by the appended claims.

Claims

1. A plasmonic circuit element that provides field rectification and second harmonic generation at infrared and optical frequencies, comprising: an epsilon-negative plasmonic material; anda nonlinear optical material juxtaposed with said epsilon-negative plasmonic material such that when an electric field of an applied signal at an infrared or optical frequency points to one direction in a half-cycle of the electric field a displacement current in the epsilon-negative plasmonic material is large relative to the displacement current in the epsilon-negative plasmonic material that occurs in an opposite direction when the electric field of the applied signal is reversed in a next half cycle of the electric field.

2. A circuit element as in claim 1, wherein the electric field and the displacement current are normal to an interface between the epsilon-negative plasmonic material and the nonlinear optical material so as to rectify said applied electric field.

3. A circuit element as in claim 1, wherein the electric field is applied in a direction parallel to an interface between the epsilon-negative plasmonic material and the nonlinear optical material so as to rectify said displacement current.