Table 1: Comparison of various nanocoupling approaches.

ApproachCoupling efficiency: low ( 10%), medium (10–50%), high ( 50%)Size: compact ( 10  m) or large (>10  m)Subwavelength coupling or focusingLateral or vertical couplingReferences

4.1. Tapered dielectricHighBothNoLateral[1825]
4.2. Tapered metalMedium and largeCompactYesLateral[2639]
4.3. Tapered metamaterialLow and mediumCompactYesLateral[4042]
5.1. Next to anotherHighBothYesLateral[4353]
5.2. End-fireHighCompactNoLateral[5460]
5.3. Resonant stubHighCompactYesLateral[11, 6164]
6.1. Dielectric lensHighLargeNoLateral[66, 67, 69, 70]
6.2. Plasmonic lensMediumCompactYesLateral[7178]
6.3. Negative index lensN/aN/aYesLateral[79, 80, 84, 100]
6.4. Photonic crystal lensN/aCompactYesLateral[101105]
6.5. HyperlensLow or mediumCompactYesLateral[107122]
7.1. AntennaMediumCompactYesBoth[125131]
7.2. GratingHighLargeNoVertical[132140]
7.3. Random scatterersLowN/aYesBoth[141]