Yuzhen Shen

Two-photon fluorescence imaging and spectroscopy of nanostructured organic materials using a photon scanning tunneling microscope

Photon scanning tunneling microscopy and spectroscopy using femtosecond two-photon excitation are demonstrated. The measurement of both intensity dependence and spectral dependence is performed on a two-photon chromophore. A subdiffraction-limited resolution is obtained, and the domain-size dependence of spatial and spectral features is observed, which indicates the high degree of molecular order in the isolated nanoparticle. It is shown that the light confinement due to a quadratic dependence of the fluorescence intensity leads to an optical contrast enhancement with a coated probe.

Second-harmonic and sum-frequency imaging of organic nanocrystals with photon scanning tunneling microscope

Second-harmonic generation and sum-frequency generation with photon scanning tunneling microscopy and shear-force detection are used to map the nonlinear optical response and the surface topograph of N-(4-nitrophenyl)-(L)-prolinol crystals with a subdiffraction-limited resolution. The domain-size dependence of the spatial feature is obtained, which shows the local orientational distribution of the optical near field radiated by nonlinear nanocrystals and reveals the difference between nanoscopic and macroscopic second-order optical nonlinearities of molecular crystals.

Enhancement of two-photon emission in photonic crystals

We report the influence of photonic stopgaps on two-photon excited emission from highly efficient nonlinear chromophores infiltrated into high-quality photonic crystals. We have observed a sharp decrease (filter effect) in emission within the frequency range and direction of the stopgap as well as sharp enhancement of the two-photon excited emission associated with the stopgaps edge. This effect may be important for the development of low-threshold upconversion lasers.

High-density optical data storage with one-photon and two-photon near-field fluorescence microscopy.

A spatially localized photochemical reaction induced by near-field femtosecond laser pulses is demonstrated on a nanometer scale and used for high-density optical data storage. Recorded domains down to 120 and 70 nm are obtained with one-photon and two-photon excitation, respectively. It is shown that the local-field confinement that is due to the quadratic dependence of two-photon excitation on light intensity has the potential to increase the near-field optical storage density.

Near-field probing of nanoscale nonlinear optical processes

We report a study of two nonlinear optical processes at the nanoscale level, using a near-field probe: (i) two-photon-pumped upconversion from ZnS:Mn nanoparticles encapsulated with 2-[{(?) -2-[4 -(ethylsulfonyl) phenyl]- 1-ethenyl} (methyl)anilino]- 1-ethanethiol and (ii) second-harmonic generation (SHG) from N -(4-nitrophenyl)- (L) -prolinol crystallites. The use of highly efficient nonlinear organic chromophores together with special processing on the nanometer scale made it possible to observe and characterize what are believed to be the smallest topographically distinguishable objects reported so far, using nonlinear optical techniques. Issues pertaining to the study of two-photon-excited emission and SHG by use of a near-field probe are discussed.

Nanoscopic study of second-harmonic generation in organic crystals with collection-mode near-field scanning optical microscopy

Collection-mode near-field scanning optical microscopy (NSOM) is used to map nanoscopic second-harmonic generation (SHG) in N -(4-nitrophenyl)- (L) -prolinol crystals. A spatial resolution of 98 nm is achieved. Near-field polarization-dependent SHG measurement is performed, and a local effective SHG susceptibility of 224+/-18 pm/V is obtained.

Nanoscale nonlinear optical process: theoretical modeling of second-harmonic generation for both forbidden and allowed light.

Based on a combination of the multiple-multipole method and nonlinear coupled-wave equations, a rigorous three-dimensional numerical simulation of nonlinear optical interactions between an optical near field and a nonlinear medium is performed, allowing us to study the dependence of second-harmonic (SH) near-field intensity on tip-sample distance and the polarization state of the incident fundamental wave. It is demonstrated that allowed and forbidden light make different contributions to the SH near-field intensity.

Nanophotonics: Nanoscale Optical Science and Technology

Nanophotonics defined as nanoscale optical science and technology is a new frontier, which includes nanoscale confinement of radiation, nanoscale confinement of matter, and nanoscale photophysical or photochemical transformation. Selected examples of our research work in each of these areas are presented here. Nonlinear optical interactions involving nanoscale confinement of radiation is both theoretically and experimentally studied using a near-field geometry. The effort in nanoscale confinement of optical domains is focused to control excitation dynamics and energy transfer as well as to produce photon localization using nanostructured rare-earth doped glasses and novel inorganic-organic photorefractive nanocomposites. Spatially localized photochemistry using a near-field two-photon excitation is being developed for nanofabrication and nanoscale memory.