Photonic near-field imaging in multiphoton photoemission electron microscopy

We report the observation of optical near fields in a photonic waveguide of conductive indium tin oxide (ITO) using multiphoton photoemission electron microscopy (PEEM). Nonlinear two-photon photoelectron emission is enhanced at field maxima created by interference between incident 410-nm and coherently excited guided photonic waves, providing strong phase contrast. Guided modes are observed under both transverse magnetic field (TM) and transverse electric field (TE) polarized illuminations and are consistent with classical electromagnetic theory. Implications on the role of multiphoton PEEM in optical near-field imaging are discussed.

Figure 1

Dispersion relationship for SPPs propagating at the interface of indium-tin oxide with vacuum. Waves observed in PEEM are excited with 410-nm light at 3 eV, well above the surface plasma frequency in the infrared (0.8–1.0 eV). Calculated from optical constants of Refs. 9 (red line) and 10 (blue dotted line).

Figure 2

Photonic structure imaged by PEEM. A $6.0\times 0.4$-$\mu$m${}^2$ slit and a shallow, $2.0\times 5.0$-$\mu$m${}^2$ trench milled into ITO thin film on glass. Structure is illuminated by light at 60${}^{\circ }$ to surface normal, and emitted electrons are imaged using a photoemission electron microscope (PEEM).

Figure 3

PEEM images. Laser is obliquely incident from the bottom edge at 60${}^{\circ }$ to surface normal. Regions of interest are indicated by A, B, and C. (a) Single-photon, 244-nm, continuous-wave (cw), TM polarized illumination. (b),(c) Two-photon, 410-nm, 100-fs pulsed, (b) TM and (c) TE polarized illumination; shown in false color (online).

Figure 4

Two-photon PEEM interference pattern profiles, averaged from the regions marked in insets. (a),(b) TM polarized, (c),(d) TE polarized.

Figure 5

Image formation model for near-field enhancement in multiphoton PEEM. Incident wave fronts 1 and 4 excite guided wave fronts 1 and 4. These propagate away from the slit and interfere constructively with the next incident wave fronts 2 and 5, enhancing the photoelectron emission rate there.

Figure 6

Fourier transform spectrum of Fig. 4, where $k=2\pi /410\text{nm}$. Six interference peaks are highlighted in each spectrum, with signals 4–6 unique to multiphoton interference. The six spectra are (a)–(c) TM polarized, (d)–(f) TE polarized; from the regions marked in insets.

Figure 7

Allowed modes of the vacuum/ITO/glass waveguide model for ITO films of 200–320 nm. Experimental results determined from FFT indicated with error bars.