Slot-waveguide

A slot-waveguide is an optical waveguide that guides strongly confined light in a subwavelength-scale low refractive index region by total internal reflection.

A slot-waveguide consists of two strips or slabs of high-refractive-index (n_H) materials separated by a subwavelength-scale low-refractive-index (n_S) slot region and surrounded by low-refractive-index (n_C) cladding materials.

Principle of operation

The principle of operation of a slot-waveguide is based on the discontinuity of the electric field (E-field) at high-refractive-index-contrast interfaces. Maxwell’s equations state that, to satisfy the continuity of the normal component of the electric displacement field D at an interface, the corresponding E-field must undergo a discontinuity with higher amplitude in the low-refractive-index side. That is, at an interface between two regions of dielectric constants ε_S and ε_H, respectively:

:"D_S^N=D_H^N"

:"ε_SE_S^N=ε_HE_H^N"

:"n_S²E_S^N=n_H²E_H^N"

where the superscript N indicates the normal components of D and E vector fields. Thus, if n_S<H, then E_S^N>>E_H^N.

Given that the slot critical dimension (distance between the high-index slabs or strips) is comparable to the exponential decay length of the fundamental eigenmode of the guided-wave structure, the resulting E-field normal to the high-index-contrast interfaces is enhanced in the slot and remains high across it. The power density in the slot is much higher than that in the high-index regions. Since wave propagation is due to total internal reflection, there is no interference effect involved and the slot-structure exhibits very low wavelength sensitivity V.R. Almeida, Q. Xu, C.A. Barrios, and M. Lipson, “Guiding and confining Light in void nanostructure,” Optics Letters, vol. 29, no. 11, pp. 1209-1211, 2004.] .

Invention

The slot-waveguide was born in 2003 as an unexpected outcome of theoretical studies on metal-oxide-semiconductor (MOS) electro-optic modulation in high-confinement silicon waveguides by Vilson Rosa de Almeida and Carlos Angulo Barrios, then a Ph.D. student and a Postdoctoral Associate, respectively, at Cornell University. Theoretical analysis V.R. Almeida, Q. Xu, C.A. Barrios, and M. Lipson, “Guiding and confining Light in void nanostructure,” Optics Letters, vol. 29, no. 11, pp. 1209-1211, 2004.] and experimental demonstration Q. Xu, V.R. Almeida, R.R. Panepucci, and M. Lipson, “Experimental demonstration of guiding and confining light in nanometer-size low-refractive-index material,” Optics Letters, vol. 29, no. 14, pp. 1626-1628, 2004.] of the first slot-waveguide implemented in the Si/SiO₂ material system at 1.55 μm operation wavelength were reported by Cornell researchers in 2004.

Since these pioneering works, several guided-wave configurations based on the slot-waveguide concept have been proposed and demonstrated. Relevant examples are the following:

In 2005, researchers at the Massachusetts Institute of Technology proposed to use multiple slot regions in the same guided-wave structure (multi-slot waveguide) in order to increase the optical field in the low-refractive-index regions [N.-N. Feng, J. Michel, and L.C. Kimerling, “Optical field concentration in low-index waveguides,” IEEE J. Quantum Electron. 42 (9), p. 885, 2006.] .

In 2006, the slot-waveguide approach was extended to the terahertz frequency band by researchers at RWTH Aachen University [M. Nagel, A. Marchewka, and H. Kurz, “Low-index discontinuity terahertz waveguides,” Optics Express, 14(21), p. 9944, 2006.] .

In 2007, a non-planar implementation of the slot-waveguide principle of operation was demonstrated by researchers at the University of Bath. They showed concentration of optical energy within a subwavelength-scale air hole running down the length of a photonic-crystal fiber G.S. Wiederhecker, C.M.B. Cordeiro, F. County, F. Benabid, S.A. Maier, J.C. Knight, C.H.B. Cruz and H.L. Fragnito, “Field enhancement within an optical fibre with a subwavelength air core,” Nature Photonics, 1, 115-118 (2007).] .

Fabrication

Planar slot-waveguides have been fabricated in different material systems such as Si/SiO₂Q. Xu, V.R. Almeida, R.R. Panepucci, and M. Lipson, “Experimental demonstration of guiding and confining light in nanometer-size low-refractive-index material,” Optics Letters, vol. 29, no. 14, pp. 1626-1628, 2004.] [T. Baehr-Jones, M. Hochberg, C. Walker, A Scherer, “High-Q optical resonators in silicon-on-insulator based slot waveguides,” Appl. Phys. Lett. 86, 081101 (2005).] and Si₃N₄/SiO₂ [C.A. Barrios, B. Sanchez, K.B. Gylfason, A. Griol, H. Sohlström, M. Holgado and R. Casquel “Demonstration of slot-waveguides structures on silicon nitride/silicon oxide platform,” Optics Express, 15(11), pp.6846-6856, 2007.] . Both, vertical (slot plane is normal to the substrate plane) and horizontal (slot plane is parallel to the substrate plane) configurations have been implemented by using conventional micro- and nano-fabrication techniques. These processing tools include electron beam lithography, photolithography, chemical vapour deposition [usually low-pressure chemical vapour deposition (LPCVD) or plasma enhanced chemical vapour deposition (PECVD)] , thermal oxidation, reactive-ion etching and focused ion beam.

In vertical slot-waveguides, the slot and strips widths are defined by electron- or photo-lithography and dry etching techniques whereas in horizontal slot-waveguides the slot and strips thicknesses are defined by a thin-film deposition technique or thermal oxidation. Thin film deposition or oxidation provides better control of the layers dimensions and smoother interfaces between the high-index-contrast materials than lithography and dry etching techniques. This makes horizontal slot-waveguides less sensitive to scattering optical losses due to interface roughness than vertical configurations.

Fabrication of a non-planar (fiber-based) slot-waveguide configuration has also been demonstrated by means of conventional microstructured optical fiber technology G.S. Wiederhecker, C.M.B. Cordeiro, F. County, F. Benabid, S.A. Maier, J.C. Knight, C.H.B. Cruz and H.L. Fragnito, “Field enhancement within an optical fibre with a subwavelength air core,” Nature Photonics, 1, 115-118 (2007).] .

Applications

A slot-waveguide produces high E-field amplitude, optical power, and optical intensity in low-index materials at levels that cannot be achieved with conventional waveguides. This property allows highly efficient interaction between fields and active materials, which may lead to all-optical switching [C.A. Barrios, “High-performance all-optical silicon microswitch,” Electronics Letters, 40 (14), 862-863 (2004).] , optical amplification [C.A. Barrios and M. Lipson, “Electrically driven silicon resonant light emitting device based on slot-waveguide,” Optics. Express 13, 10092-10101, (2005).] and optical detection [T. Baehr-Jones, M. Hochberg, G. Wang, R. Lawson, Y. Liao, P. A. Sullivan, L. Dalton, A. K.-Y. Jen, and A. Scherer, “Optical modulation and detection in slotted silicon waveguides,” Opt. Express 13, 5216-5226 (2005).] on integrated photonics. Strong E-field confinement can be localized in a nanometer-scale low-index region; therefore the slot waveguide can be used to greatly increase the sensitivity of compact optical sensing devices [C.A. Barrios, “Ultrasensitive nanomechanical photonic sensor based on horizontal slot-waveguide resonator,” IEEE Photon. Technol. Lett. 18, 2419-2421 (2006).] [C.A. Barrios, K.B. Gylfason, B. Sanchez, A. Griol, H. Sohlström, M. Holgado and R. Casquel, “Slot-waveguide biochemical sensor,” Optics Letters, 32(21), pp. 3080-3082, 2007.] [C.A. Barrios, M.J. Bañuls, V. Gonzalez-Pedro, K.B. Gylfason, B. Sánchez, A. Griol, A. Maquieira, H. Sohlström, M. Holgado, and R. Casquel. “Label-free optical sensing with slot-waveguides,” Optics Letters, 33(7), pp. 708-710, 2008.] [J.T. Robinson, L. Chen, and M. Lipson, “On-chip gas detection in silicon optical microcavities,” Optics Express, 16 (6), pp. 4296-4301, 2008] or to enhance the efficiency of near-field optics probes.

References