Nien Hua Lu

Implementation of a short-tip tapping-mode tuning fork near-field scanning optical microscope

We present the implementation of a short‐tip tapping‐mode tuning fork near‐field scanning optical microscope. Tapping frequency dependences of the piezoelectric signal amplitudes for a bare tuning fork fixed on the ceramic plate, a short‐tip tapping‐mode tuning fork scheme and an ordinary tapping‐mode tuning fork configuration with an 80‐cm optical fibre attached are demonstrated and compared. Our experimental results show that this new short‐tip tapping‐mode tuning fork scheme provides a stable and high Q factor at the tapping frequency of the tuning fork and will be very helpful when long optical fibre probes have to be used in an experiment. Both collection and excitation modes of short‐tip tapping‐mode tuning fork near‐field scanning optical microscope are applied to study the near‐field optical properties of a single‐mode telecommunication optical fibre and a green InGaN/GaN multiquantum well light‐emitting diode.

Tapping‐mode tuning‐fork near‐field scanning optical microscopy of low power semiconductor lasers

The newly developed inverted tapping‐mode tuning‐fork near‐field scanning optical microscopy (TMTF‐NSOM) is used to study the local near‐field optical properties of strained AlGaInP/Ga0.4In0.6P low power visible multiquantum‐well laser diodes. In contrast to shear‐force mode NSOM, TMTF‐NSOM provides the function to acquire the evanescent wave intensity ratio |I(2ω)|/|I(ω)| image, from which the evanescent wave decay coefficient q can be evaluated for a known tapping amplitude. Moreover, we probe the near‐field stimulated emission spectrum, which gives the free‐space laser light wavelength λo and the index of refraction nr of the laser diode resonant cavity. Once q, λo, and nr are all measured, we can determine the angle of incidence θo of the dominant totally internally reflected waves incident on the front mirror facet of the resonator. Determination of such an angle is very important in modelling the stability of the laser diode resonator.

Demonstrating Applications of Non-optically Regulated Tapping-Mode Near-Field Scanning Optical Microscopy to Nano-optical Metrology and Optical Characterization of Semiconductors

We demonstrate the applications of a near-field scanning optical microscopy (NSOM) system based on a short-probe tapping-mode tuning fork (TMTF) configuration to nano-optical metrology and the optical characterization of semiconductors. The short-probe TMTF–NSOM system is constructed to operate in both collection and excitation modes, in which a cleaved short fiber probe attached to one tine of the tuning fork is used as a light collector/emitter as well as a force-sensing element. Interference fringes due to standing evanescent waves generated by total internal reflection are imaged in the collection mode. Excitation-mode short-probe TMTF–NSOM is applied to near-field surface photovoltage measurement on distributed-Bragg-reflector-enhanced absorbing substrate AlGaInP light-emitting diode structures.

Near-field optical characterization of visible multiple quantum well semiconductor lasers

Both collection and excitation modes of scanning near-field optical microscopy (SNOM) were used to study a low power visible multiquantum-well laser diode (LD). Collection mode SNOM provides the near-field optical propagating intensity distribution at the facet of LD. Excitation mode SNOM gives local photoconductivity information of the structure of LD facet. Results show highly localized spatial correlation of LD structure and its optical performance at the facet. Different sizes of apertures were used in both modes, and results of near-field interactions can be quite different. Results show obvious difference of photocurrent distribution caused by the different sizes of apertures in excitation mode. Two wavelengths of 543.5 nm and 632.8 nm were used in excitation mode SNOM. It can be deduced from the two pump photon energies that there exists defect level in the energy range of 60 - 380 meV below the conduction band edge in the n-(Al0.7Ga0.3)0.5In0.5P cladding layer. In addition to the highly localized images of topography, optical output, and optical beam induced current at the facet of LD, local near- field optical spectroscopy was performed as well. Spatially resolved near-field optical spectra of both stimulated and spontaneous emissions were obtained at the facet of LD. Longitudinal modes of stimulated emission of LD were observed locally.

Optimum instrumentation of a tapping mode, non-optically regulated near-field scanning optical microscope and its applications

We describe the optimum design of the near-field scanning optical microscope (NSOM) based on a short probe tapping mode tuning-fork (TMTF) configuration and its applications in optoelectronic characterization and optical measurements. The short probe TMTF-NSOM is constructed to operate both in collection and excitation modes, in which a cleaved short fiber probe attached to one tine of the tuning fork is used as the light collector/emitter as well as the force sensing element. Interference fringes due to standing evanescent waves generated by total internal reflection are imaged by collection mode. On the other hand, excitation mode of short probe TMTF-NSOM is applied to perform near-field surface photovoltage measurements on AlGaInP light emitting diode structures.

Optical enhancements on gold thin films through surface nanostructure modulation

Surface plasmons (SPs) on gold thin film coupling with the grating on the surface were studied. For films with periodic nanostructures, experimental results and simulations showed that the incident light was reflected as well as excited. Because the propagating wave is particular about the surface plasmons excitation, we used the FDTD simulations to predict the grating conditions such as the thickness, period, width, and depth. Optical transmission intensity enhancements due to the nanostructures on the gold thin films were observed in our results.

Near-field simulations and measurements of surface plasmons on perforated metallic thin films

Surface plasmon excitations with different nanostructure modulations on the interface between metal and dielectric were interesting to investigate. The study was performed by measuring optical transmission through perforated metallic thin film. Experimental observations on dependences of periods, depths, and widths of nanostructures on the transmission of gold film were reported. Furthermore, simulations by finite difference time domain (FDTD) method were used to predict the variations of transmission and reflection with periods.

Short fiber probe scheme for tapping-mode tuning fork near-field scanning optical microscopy

Construction of a tapping-mode tuning fork with a short fiber probe as the force sensing element for near-field scanning optical microscopy is reported. This type of near-field scanning optical microscopy provides stable and high Q factor at the tapping frequency of the tuning fork, and thus gives high quality NSOM and AFM images of samples. We present results obtained by using the short tip tapping-mode tuning fork near-field scanning optical microscopy measurements performed on a single mode telecommunication optical fiber and a silica based buried channel waveguide.

Study of the mechanism of the light-scattering-mode superresolution near-field structure

The near-field recording mechanism of the super resolution near-field structure, glass/ZnS-SiO2/AgOx/ZnS-SiO2, has been studied experimentally. Near-field optical effects of the glass/ZnS-SiO2/AgOx/ZnS-SiO2 have been observed by a tapping mode tuning-fork near-field scanning optical microscope (TMTF-NSOM) on the transmitting light spot. Laser-excited surface plasmon at the interfaces of AgOx/ZnS-SiO2 thin film was detected by this technique. Results showed that the transmitting focused light through the AgOx type super resolution near-field structure consists of a propagating term and an evanescent one resulted from the localized surface plasmon of the AgOx thin film. A strong enhancement of the near-field intensity and the dynamic localized enhancement of the transmitting focused light were observed as well.

Imaging the near-field intensity gradients of a low-power semiconductor laser

A newly developed inverted tapping-mode tuning-fork near- field scanning optical microscope is used to study the local near-field radiation properties of a strained AlGaInP/Ga0.4In0.6P low power visible multiquantum-well laser diode. With this novel technique, we can easily image the local near-field optical intensity gradients. In the intensity ratio image there are remarkable contrasts among the various regions on the laser diode facet. The anomalous phenomenon manifests the different origins of the near-field optical waves from various regions on the laser diode facet. We believe that this method should be very important for further understanding the optical radiation properties in the near-field region.