Position- and orientation-controlled polarized light interaction of individual indium tin oxide nanorods

Spatially distinct Raman scattering characteristics of individual ZnO nanorods under controlled polarization: intense end scattering from forbidden modes

In this study, we characterize incident/scattered polarization-specific and NR position-resolved Raman scattering behaviors of individual zinc oxide nanorods (ZnO NRs). We quantify Raman signals from the five key ZnO phonon modes of E_2L, E_2H-2L, A_1T, E_1T, and E_2H, and reveal the NR position-dependent Raman scattering characteristics of the phonon modes per given light-matter interaction geometry. We then present Raman intensity maps and elucidate Raman behaviors consistent and incongruous with Raman selection rules. In particular, we identify an intriguing Raman scattering phenomenon from the forbidden modes, distinctively occurring at the two NR ends. Their unexpectedly strong and localized scattering signals at the NR termini are contrasted by the scattering behaviors from the rest of the NR positions agreeing with the selection predictions. By carrying out control measurements on isotropic ZnO microparticles (MPs), we ascertain that the unique NR position-specific Raman responses observed on ZnO NRs originate from their high shape anisotropy. Owing to the superior optical property coupled with reduced dimensionality and high geometric anisotropy, ZnO NRs have gained much attention recently for use in optoelectronic, photonic, and biosensor technologies. Raman scattering has been increasingly exploited as a noninvasive and sensitive analytical tool to investigate NR properties pertinent to these applications. Hence, our endeavors, explicitly providing the spatially distinct, polarized Raman scattering behaviors from individual ZnO NRs, will be central to the correct interpretation of Raman data of both the individual and ensemble NRs as well as to the accurate correlation of the measurement outcomes to their chemical/physical/optical properties. Our efforts may also promote novel applications for polarized Raman scattering whose optical outputs on the various positions along the ZnO NRs can be selectively modulated.

Polarization-resolved mechanistic investigation of fluorescence signal intensification on zinc oxide nanorod ends

The superior optical properties of zinc oxide nanorods (ZnO NRs) have continued to promote their broad use in photonic, photoelectric, light detecting, and biosensing applications. One particularly important property pertinent to biodetection is fluorescence intensification on nanorod ends (FINE), a phenomenon in which a highly spatially localized and strongly intensified fluorescence signal with its extended photostability at the NR ends is seen from the emission profiles of fluorophore-coupled biomolecules on ZnO NRs. Therefore, understanding key parameters affecting the FINE phenomenon and the degree of FINE (DoF) is critical for their applications in biosensors. In this study, we describe in detail the outcomes of polarization-resolved measurements by systematically considering the polarization effects on FINE and DoF as a function of NR tilt angle and position along the NR. Specifically, we elucidate the exact roles of the different states of light polarization in FINE and quantitatively determine the explicit contributions arising from distinctive polarization states to the DoF. We confirm that the presence of the FINE phenomenon is ubiquitous from the fluorophore-coupled ZnO NR systems, regardless of the polarization setting. We subsequently show that DoF is significantly affected by the light-matter interaction geometry. We reveal the specific polarization conditions that contribute dominantly to the FINE effect. The highest DoF from a NR and the greatest NR end intensity can be achieved when both the excitation and collection polarization states are perpendicular to the NR main axis. Insights from this study provide valuable design principles for selecting the polarization state and light-matter interaction geometry to attain maximum FINE as well as DoF on ZnO NRs. The precise understanding of polarization-derived consequences on FINE and DoF manifested differently as a function of the position on individual NRs can also be important for warranting accurate interpretation and quantification of the position-dependent, fluorophore-emitted signals on single ZnO NRs. Hence, our findings from this study can be extremely beneficial in fluorescence-based sensing and detection settings utilizing polarization.

Evaluation of polarization rotation in the scattering responses from individual semiconducting oxide nanorods

We investigate the interaction of visible light with the solid matters of semiconducting oxide nanorods (NRs) of zinc oxide (ZnO), indium tin oxide (ITO), and zinc tin oxide (ZTO) at the single nanomaterial level. We subsequently identify an intriguing, material-dependent phenomenon of optical rotation in the electric field oscillation direction of the scattered light by systematically controlling the wavelength and polarization direction of the incident light, the NR tilt angle, and the analyzer angle. This polarization rotation effect in the scattered light is repeatedly observed from the chemically pure and highly crystalline ZnO NRs, but absent on the chemically doped NR variants of ITO and ZTO under all measurement circumstances. We further elucidate that the phenomenon of polarization rotation detected from single ZnO NRs is affected by the NR tilt angle, while the phenomenon itself occurs irrespective of the wavelength and incident polarization direction of the visible light. Combined with the widespread optical and optoelectronic use of the semiconducting oxide nanomaterials, these efforts may provide much warranted fundamental bases to tailor material-specific, single nanomaterial-driven, optically modulating functionalities which, in turn, can be beneficial for the realization of high-performance integrated photonic circuits and miniaturized bio-optical sensing devices.

Scattering Intensity and Directionality Probed Along Individual Zinc Oxide Nanorods with Precisely Controlled Light Polarization and Nanorod Orientation

We elucidated the light-matter interaction of individual ZnO NRs with a monochromatic beam of linearly polarized light that scatters elastically from the ZnO NRs by performing forward scattering and back-aperture imaging in a dark-field setting. We precisely controlled the electric field vector of the incident light and the NR orientation within the plane of light interaction during both modes of measurement, and spatially resolved the scattering response from different interaction points along the NR long axis. We then discerned, for the first time, the effects of light polarization, analyzer angle, and NR orientation on the intensity and directionality of the optical responses both qualitatively and quantitatively along the length of the single ZnO NRs. We identified distinctive scattering profiles from individual ZnO NRs subject to incident light polarization with controlled NR orientation from the forward dark-field scattering and back-aperture imaging modes. The fundamental light interaction behavior of ZnO NRs is likely to govern their functional outcomes in photonics, optoelectronics, and sensor devices. Hence, our efforts provided much needed insight into unique optical responses from individual 1D ZnO nanomaterials, which could be highly beneficial in developing next-generation optoelectronic systems and optical biodetectors with improved device efficiency and sensitivity.

Scattering attributes of one-dimensional semiconducting oxide nanomaterials individually probed for varying light-matter interaction angles

We report the characteristic optical responses of one-dimensional semiconducting oxide nanomaterials by examining the individual nanorods (NRs) of ZnO, SnO₂, indium tin oxide, and zinc tin oxide under precisely controlled, light-matter interaction geometry. Scattering signals from a large set of NRs of the different types are evaluated spatially along the NR length while varying the NR tilt angle, incident light polarization, and analyzer rotation. Subsequently, we identify material-indiscriminate, NR tilt angle- and incident polarization-dependent scattering behaviors exhibiting continuous, intermittent, and discrete responses. The insight gained from this study can advance our fundamental understanding of the optical behaviors of the technologically useful nanomaterials and, at the same time, promote the development of highly miniaturized, photonic and bio-optical devices utilizing the spatially controllable, optical responses of the individual semiconducting oxide NRs.