James W. M. Chon

Calibration of rectangular atomic force microscope cantilevers

A method to determine the spring constant of a rectangular atomic force microscope cantilever is proposed that relies solely on the measurement of the resonant frequency and quality factor of the cantilever in fluid (typically air), and knowledge of its plan view dimensions. This method gives very good accuracy and improves upon the previous formulation by Sader et al. [Rev. Sci. Instrum. 66, 3789 (1995)] which, unlike the present method, requires knowledge of both the cantilever density and thickness.

Five-dimensional optical recording mediated by surface plasmons in gold nanorods.

Multiplexed optical recording provides an unparalleled approach to increasing the information density beyond 1012 bits per cm3 (1 Tbit cm-3) by storing multiple, individually addressable patterns within the same recording volume. Although wavelength, polarization and spatial dimensions have all been exploited for multiplexing, these approaches have never been integrated into a single technique that could ultimately increase the information capacity by orders of magnitude. The major hurdle is the lack of a suitable recording medium that is extremely selective in the domains of wavelength and polarization and in the three spatial domains, so as to provide orthogonality in all five dimensions. Here we show true five-dimensional optical recording by exploiting the unique properties of the longitudinal surface plasmon resonance (SPR) of gold nanorods. The longitudinal SPR exhibits an excellent wavelength and polarization sensitivity, whereas the distinct energy threshold required for the photothermal recording mechanism provides the axial selectivity. The recordings were detected using longitudinal SPR-mediated two-photon luminescence, which we demonstrate to possess an enhanced wavelength and angular selectivity compared to conventional linear detection mechanisms. Combined with the high cross-section of two-photon luminescence, this enabled non-destructive, crosstalk-free readout. This technique can be immediately applied to optical patterning, encryption and data storage, where higher data densities are pursued.

Experimental validation of theoretical models for the frequency response of atomic force microscope cantilever beams immersed in fluids

Detailed measurements of the frequency responses of a series of rectangular atomic force microscope (AFM) cantilever beams, immersed in a range of fluids, have been performed to test the validity and accuracy of the recent theoretical model of Sader [J. Appl. Phys. 84, 64 (1998)]. This theoretical model gives the frequency response of a cantilever beam, that is immersed in a viscous fluid and excited by an arbitrary driving force. Very good agreement between experimental measurements and theoretical calculations is found for all fluids considered. Furthermore, a critical assessment of the well-known inviscid model is presented, which demonstrates that this model is not applicable to AFM cantilever beams in general.

Rheological measurements using microcantilevers

The use of microcantilevers in rheological measurements of gases and liquids is demonstrated. Densities and viscosities of both gases and liquids, which can range over several orders of magnitude, are measured simultaneously using a single microcantilever. The microcantilever technique probes only minute volumes of fluid (<1 nL), and enables in situ and rapid rheological measurements. This is in direct contrast to established methods, such as “cone and plate” and Couette rheometry, which are restricted to measurements of liquid viscosity, require large sample volumes, and are incapable of in situ measurements. The proposed technique also overcomes the restrictions of previous measurements that use microcantilevers, which are limited to liquid viscosity only, and require independent measurement of the liquid density. The technique presented here only requires knowledge of the cantilever geometry, its resonant frequency in vacuum, and its linear mass density. A simple yet robust calibration procedure is des...

Acoustic Oscillations and Elastic Moduli of Single Gold Nanorods

We present the first acoustic vibration measurements of single gold nanorods with well-characterized dimensions and crystal structure. The nanorods have an average size of 90 nm x 30 nm and display two vibration modes, the breathing mode and the extensional mode. Correlation between the dimensions obtained from electron microscope images and the vibrational frequencies of the same particle allows us to determine the elastic moduli for each individual nanorod. Contrary to previous reports on ensembles of gold nanorods, we find that the single particle elastic moduli agree well with bulk values.

Laser trapping and manipulation under focused evanescent wave illumination

Laser trapping is based on the radiation pressure on a small particle in the focal region of a high numerical-aperture objective. Currently, the focal spot of a trapping beam is elongated along the longitudinal direction and thus the axial size of the trapping volume is approximately three times larger than that in the transverse direction. We report on a laser trapping technique under focused evanescent wave illumination. Therefore laser trapping of micro/nano-objects can be achieved in the near-field region with an axial trapping size of approximately 60 nm, which is reduced by approximately one order of magnitude. Hence, this technique is of significant importance in nanometry including single molecule detection and manipulation.

Splitting of the focal spot of a high numerical-aperture objective in free space

Reported in this letter is a phenomenon that the focal spot of a high numerical-aperture objective in free space can split into two spots if a ring beam is used for illumination. Diffraction by a high numerical-aperture objective results in a depolarization such that the diffracted field in the focal region includes not only a component with the same polarization as the incident beam, but also orthogonal and longitudinal components. The use of a ring beam enhances the relative contribution from the longitudinal component. As a result, a single focal spot splits into two spots along the incident polarization direction. It is revealed theoretically that for an objective of given numerical aperture there is a threshold of the central obstruction size of a ring beam for the appearance of a two-peak focus.

Experimental investigation of single voxels for laser nanofabrication via two-photon photopolymerization

We report critical roles that are played by laser system parameters in two-photon laser nanofabrication, which are not significant in rapid prototyping at larger scale, including: (i) polarization-induced lateral deshaping of volume elements, voxels, the primitive building block of micronanostructures, and (ii) lateral size reduction of voxels at low numerical aperture lens focusing due to thresholding effect. Also interesting is (iii) simultaneous recording of zeroth- and higher-order diffraction patterns, which was not hindered by the two-order light intensity difference by taking the advantage of the large dynamic exposure time range of general resins, a concept that is proposed in contrast to dynamic power range.

Below Melting Point Photothermal Reshaping of Single Gold Nanorods Driven by Surface Diffusion

Plasmonic gold nanorod instability and reshaping behavior below melting points are important for many future applications but are yet to be fully understood, with existing nanoparticle melting theories unable to explain the observations. Here, we have systematically studied the photothermal reshaping behavior of gold nanorods irradiated with femtosecond laser pulses to report that the instability is driven by curvature-induced surface diffusion rather than a threshold melting process, and that the stability dramatically decreases with increasing aspect ratio. We successfully utilized the surface diffusion model to explain the observations and found that the activation energy for surface diffusion was dependent on the aspect ratio of the rods, from 0.6 eV for aspect ratio of 5 to 1.5 eV for aspect ratio less than 3. This result indicates that the surface atoms are much easier to diffuse around in larger aspect ratio rods than in shorter rods and can induce reshaping at any given temperature. Current plasmonics and nanorod applications with the sharp geometric features used for greater field enhancement will therefore need to consider surface diffusion driven shape change even at low temperatures.

Three-photon excited band edge and trap emission of CdS semiconductor nanocrystals

We report on three-photon excited band edge and trap emission of CdS semiconductor nanocrystals. While the band edge emission intensity clearly shows a cubic dependence on excitation intensity, demonstrating three-photon absorption process, the trap emission does not exhibit such a cubic dependence. A simple theoretical model based on the assumption that there exist a limited number of trap states in nanocrystals shows good agreement with the experiment, suggesting that the number of trap states play an important role in their emission intensity dependence of multiphoton excitation. The three-photon absorption cross section of CdS nanocrystals is measured to be ∼10−79 cm6 s2 photon−2, which is three to four orders of magnitude higher than those of the previously reported common UV fluorescent dyes.