Observation of amplitude and phase in ridge and photonic crystal waveguides operating at 155??m by use of heterodyne scanning near-field optical microscopy

Abstract

We apply heterodyne scanning near-field optical microscopy (SNOM) to observe with subwavelength resolution the amplitude and phase of optical fields propagating in several microfabricated waveguide devices operating around the 1.55 m wavelength. Good agreement between the SNOM measurements and predicted optical mode propagation characteristics in standard ridge waveguides demonstrates the validity of the method. In situ observation of the subwavelength-scale distribution and propagation of optical fields in straight and 90°bend photonic crystal waveguides facilitates a more detailed understanding of the optical performance characteristics of these devices and illustrates the usefulness of the technique for investigating nanostructured photonic devices. © 2005 Optical Society of America OCIS codes: 040.2840, 120.5050, 180.5810, 230.7370. Photonic nanostructures, and in particular photonic crystals (PCs), provide a number of possibilities for developing novel devices based on near-field optical phenomena, as well as for monolithic integration of large-scale photonic systems. 1-3 However, as many of these devices depend on the subwavelength-scale localization of optical fields, traditional far-field characterization techniques are incapable of fully capturing the propagation characteristics of light in such structures. Scanning near-field optical microscopy 4,5 (SNOM) provides the ability to observe directly the optical fields propagating in a photonic structure with subwavelength resolution and has been applied to investigate PC resonators and various other devices. 6,7 In heterodyne SNOM, light collected by the scanning probe is combined with a frequencyshifted reference beam to permit simultaneous measurement of the local amplitude and phase of the probed field. [8][9][10] This technique has been used to characterize several devices, including periodically structured waveguides 11 and structured optical fibers, 12 demonstrating the usefulness of SNOM for the detailed characterization of optical field propagation inside microstructured waveguides. In this Letter, we apply this technique to investigate microfabricated ridge and PC waveguides and demonstrate the measurement of the phase and amplitude of the propagating fields in silicon-based photonic devices at the 1.55 m wavelength. 13 The obtained results are in good agreement with predicted values, illustrating the viability of the technique as a method for investigating the complex amplitude of optical fields as they propagate inside the waveguides, and could have a significant effect on the understanding and development of nanostructured optical devices.In the present setup, the source is a semiconductor cw laser operating in a wavelength band around 1.55 m. The cantilevered tapered fiber probe has an aperture of 200 nm and is scanned in tapping mode. This scanning technique reduces lateral interaction forces and is particularly advantageous for the investigation of silicon membranes or other fragile samples having abrupt topographical changes.First, we c...

Mapping of complex optical field patterns in multimode photonic crystal waveguides by near-field scanning optical microscopy

“…Similar near-field phenomena have been reported by Tortara and coworkers. 10,13 The minimum and maximum FWHM are about 0.50 m and 1.00 m. At 1588 nm ͓Fig. 4͑d͔͒, the propagating light exhibits a dominant peak at the left side of the waveguide, whose FWHM is only about 0.42 m. There exists another weak peak at the right side of the waveguide.…”

“…The field distributions in this coexisting region of two guided modes are quite similar to those discussed by Tortora and coworkers, except for different lengths of the "snake-like" structure caused by superposition of two modes with different propagation wavevectors. 10,13 In the fourth region of Table I, there exist three guided modes, and things become much more complex than in the other situations involving only one or two modes. The calculated eigenmode field distributions of the first even guided mode, the second odd guided mode, and the third even guided mode at a wavelength of 1588 nm are shown in Figs.…”

“…Many previous NSOM studies have also shown the importance of connecting experimental observations to an appropriate physical model in order for good understanding of the observed complex nearfield phenomena. [6][7][8][9][10][11][12][13][14][15][16] On the other hand, our NSOM can only offer the intensity ͑or amplitude͒ information of the optical near field. It would be very interesting to look at the phase information involved in the complex near-field patterns for this multimode W3 PC waveguide.…”

“…Fortunately, nearfield scanning optical microscopy ͑NSOM͒ can circumvent the optical diffraction limit and map the topography and optical intensity distribution with subwavelength resolution simultaneously. [6][7][8][9][10][11][12][13][14][15][16] In the past few years, the optical intensity distributions in PC single-mode or dual-mode straight and bent waveguides, [10][11][12][13][14] micro-cavities and lasers [6][7][8][9] have been studied elaborately with NSOM. In particular, the field distribution of two coexisting modes in a dual-mode waveguide have been explored systematically by means of a deliberate heterodyne NSOM technique, which allows the probe of both the amplitude and phase information of optical near fields.…”

“…In particular, the field distribution of two coexisting modes in a dual-mode waveguide have been explored systematically by means of a deliberate heterodyne NSOM technique, which allows the probe of both the amplitude and phase information of optical near fields. 10,13 On the other hand, the dispersion relation and light propagation in a W3 PC multimode waveguide ͑with three lines of air holes unperforated along the ⌫K direction in a triangular lattice PC membrane͒ have been measured with a timeresolved NSOM. 15,16 In the time-resolved NSOM, a pulsed laser with picosecond or femotosecond duration is used as the light source.…”