Near-field characterization of low-loss photonic crystal waveguides

A scanning near-field optical microscope is used to directly map the propagation of light in the wavelength range of 1500–1630 nm along straight photonic crystal waveguides (PCWs) fabricated on silicon-on-insulator wafers. The PCWs were formed by removing a single row of holes in the triangular 428-nm-period lattices with different filling factors (0.76 and 0.82) and connected to access ridge waveguides. Using the near-field optical images we investigate the light propagation along PCWs for TM and TE polarizations (the electric field is perpendicular/parallel to the sample surface). Efficient guiding (for both samples) of the TM-polarized radiation is observed in the whole range of laser tunability. For TE polarization, the efficient guiding is limited to the wavelengths shorter than 1552 or 1570 nm for the PCW with the filling factor of 0.76 or 0.82, respectively. For longer wavelengths, we observe drastic and rapid deterioration of the waveguiding and PCW mode confinement, ending up with the complete disappearance of the PCW mode once the wavelength exceeds the cutoff value by merely 2 nm. Using averaged cross sections of the intensity distributions along the PCW axis, the propagation loss is evaluated and found to be in good agreement with the corresponding transmission spectra. Considering spatial frequency spectra of the intensity variations along the PCW axis, we determine (for both polarizations) the dispersion of the PCW mode propagation constant and, thereby, the mode group and phase velocities.

Figure 1

(a) Schematic layout of the experimental setup. (b) Optical microscope image showing a top view of the central part of the sample, which contains straight PCWs.

Figure 2

Grey-scale (a) topographical and (b) near-field optical images $(66\times 66\mu {\mathrm{m}}^2)$ obtained with a $40\text{−}\mu \mathrm{m}$-long PCW of sample N1 at the wavelength $\lambda \cong 1520\mathrm{nm}$ for TM polarization.

Figure 3

Grey-scale (a) topographical and (b)–(f) near-field optical images $(5\times 25\mu {\mathrm{m}}^2)$ obtained with a $40\text{−}\mu \mathrm{m}$-long PCW (sample N1) at the wavelengths $\lambda \cong$ (b) 1500, (c) 1520, (d) 1550, (e) 1590, and (f) 1630 nm for TM polarization.

Figure 4

Grey-scale (a) topographical and (b)–(f) near-field optical images $(5\times 25\mu {\mathrm{m}}^2)$ obtained with a $40\text{−}\mu \mathrm{m}$-long PCW (sample N1) at the wavelength $\lambda \cong$ (b) 1500, (c) 1522, (d) 1524, (e) 1560, and (f) 1600 nm for TE polarization.

Figure 5

Grey-scale (a) topographical and (b)–(f) near-field optical images $(5\times 25\mu {\mathrm{m}}^2)$ obtained with a $40\text{−}\mu \mathrm{m}$-long PCW (sample N2) at the wavelength $\lambda \cong$ (b) 1500, (c) 1530, (d) 1570, (e) 1600, and (f) 1630 nm for TM polarization.

Figure 6

Grey-scale (a) topographical and (b)–(f) near-field optical images $(5\times 25\mu {\mathrm{m}}^2)$ obtained with a $40\text{−}\mu \mathrm{m}$-long PCW (sample N2) at the wavelength $\lambda \cong$ (b) 1500, (c) 1570, (d) 1572, (e) 1590, and (f) 1630 nm for TE polarization.

Figure 7

Averaged cross sections of the near-field optical images shown in Figs. 3b, 4b along the PCW axis. Solid lines indicate linear fits to the experimental data.

Figure 8

Transmission spectra measured with a $10\text{−}\mu \mathrm{m}$-long PCW of sample N1 (black line corresponds to TM polarization, light gray line—to TE polarization) and $40\text{−}\mu \mathrm{m}$-long PCW of sample N2 (gray line) plotted together with the results obtained from the averaged cross sections of SNOM images.

Figure 9

(a) Spatial frequency spectrum of the optical image obtained with the PCW of sample N1 for TE polarization at the wavelength $\lambda \cong 1510\mathrm{nm}$. Positions of the main peaks fitted to the experimental data are indicated with solid lines. Dotted lines indicate combination frequencies that may appear due to the propagating backward PCW modes and/or nonlinear detection. (b) Wavelength dependence of the propagation constant of the main PCW mode determined from the corresponding near-field optical images obtained for TM and TE polarizations.

Figure 10

Group and phase velocities normalized to the speed of light in vacuum for the (a) TM-polarized and (b) the TE-polarized PCW mode of sample N1 as functions of the light wavelength. Solid lines (used to guide the eye) represent quadratic approximations by the least-square method.

Figure 11

3D FTDT calculated (a) TE and (b) TM band diagrams with the light line indicated. Black lines represent (a) the PBG guided mode and (b) the refractive-index guided mode. Note that the group velocity being proportional to the slope of a dispersion curve is expected to become very small at the edge of the Brillouin zone.