Near-Field Probing Surface Plasmon Enhancement Effect on Two-Photon Emission Yuzhen Shen, Jacek Swiatkiewicz, Tzu-Chau Lin, Przemyslaw Markowicz, and Paras N. Prasad* Photonics Research Laboratory, Institute for Lasers, Photonics and Biophotonics, Departments of Chemistry, Physics, Electrical Engineering, and Medicine, UniVersity at Buffalo, The State UniVersity of New York, Buffalo, New York 14260 ReceiVed: December 20, 2001; In Final Form: March 1, 2002 A photon scanning tunneling microscope is employed to probe the surface-plasmon-induced local field enhancement effect on two-photon fluorescence from organic nanoparticles adsorbed on silver surface. A size dependence of fluorescence enhancement and photodecomposition is observed, which results from the competition between the surface-plasmon-enhanced two-photon fluorescence and the nonradiative energy transfer from the excited dye molecules to the silver surface. A surface-plasmon (SP) is a localized electromagnetic field at metal-dielectric interfaces. 1,2 The field associated with SP is sensitive to the variations in the environment adjacent to the interfaces and therefore has potential as a sensing probe. 3 The field enhancement due to SP resonance is predicted to be ∼10 2 times larger than the incident field, 2 and therefore can be used to increase nonlinear optical effects at interfaces. 4-9 The SP- induced electromagnetic field is evanescent, and is not accessible by far-field SP optics. Furthermore, the resolution in far-field SP optics is limited to SP decay length, and the information about spatial structures obtained by far-field SP optics is the average response over a macroscopic region. The advances in near-field scanning optical microscopy (NSOM) 10-13 and photon scanning tunneling microscopy (PSTM) 14-18 makes it possible to perform direct measurements on SP in the near field and probe SP related effects on a nanoscopic region. The principle of NSOM is to scan a nanoscopic light source in the near field above a sample with the resulting field intensity being detected. With PSTM, a nanoscopic probe senses the evanescent field above the sample that is illuminated under attenuated total reflection (ATR). Because SP can be excited by evanescent wave arising from ATR, ATR-based PSTM has potential benefit in near-field SP studies. Recently, PSTM has been used to detect SP resonance in randomly rough surface, fractal colloid clusters, continuous metal film, and single metal particles. 19-24 However, there is no attempt to apply SP resonance to near-field nonlinear fluorescence study. In this letter, we employ PSTM to probe SP-induced local field enhancement and apply the enhancement effect to near-field two-photon fluorescence imaging and spectroscopic study on organic nanoparticles. The schematic of the experimental setup is shown in Figure 1. A self-mode-locked Ti:Sapphire laser (Coherent) is used as an excitation source at 800 nm with an average power of 2 W. The pulse width is 80 fs at a repetition rate of 80 MHz. The laser output is passed through a polarizer, focused by a lens, then incident on the sample that is mounted with an index matching oil on a BK7 prism under total internal reflection. Position lens L and align mirror M so that the sample surface lies in the focal plane and the incident beam that is parallel to the optical axis of the lens L is refracted through the focus at a different angle when mirror M is scanned up and down. As a result, the angle of incidence can be adjusted by scanning mirror M in increments of 0.085 mm without changing focus point on the sample, which corresponds to 0.05° each step. The dynamic range of angular scan is ∼10°. The reflected light intensity is detected with a photodiode to measure SP resonance curve. The transmission or fluorescence signal is collected by an aluminum- coated fiber probe (Topometrix) with an aperture of 50 nm in the near-field above the sample, passed through band-pass filter, and detected by a cooled photomultiplier (Hamamatsu) con- nected to photon-counting electronics (Standford Research System) and a computer to process data or generate optical image. Shear-force feedback that uses tuning-fork detection monitors the oscillation of the probe, and keeps the probe-sample separation constant. A Shear-force image can be produced simultaneously with the optical image, as the probe is rastered across the sample surface. A silver film of 50 nm is evaporated on half of a BK7 microscope cover glass at 10 -6 Torr and then used as a sample. The excitation light is transmitted through the silver film as an exponentially decaying wave, which is localized at silver surface and can be detected only in the near-field. To measure the SP- Figure 1. Schematic of the experimental setup. 4040 J. Phys. Chem. B 2002, 106, 4040-4042 10.1021/jp014639g CCC: $22.00 © 2002 American Chemical Society Published on Web 04/02/2002