Nanoscale characterization in functional perspectives
The waveproperties in nanoscale revolutionized the optoelectronics in general. Accordingly, the charaterization methods should fit well with the scale and also the required functionalities of the devices. In the materials' viewpoint, two-dimensional materials and nanomaterials, all in nanoscale, that replace Si and other conventional 3-dimensional semiconductors have emerged recently. In this regard, techniques for analyzing nanomaterials became pramount both in the terms of better operation and also envisioning new functionalities. In our lab., we developed nano-scale characterization methods beyond the convetional AFM and optical microscopy. For example, we can analyze the luminescence characteristics for temporally and spatial carrier dynamics, occuring in nanoscale. The experimental schemes have specialized goals such as locating defects, visualizing lattice deformation, estimating thermally conduting/insulating parameters, and mechanical properties such as nanoscale strains.
Two-photon laser scanning microscopy (TPLSM)
Two-photon microscopy (TPM) provides a means of depth-resolved PL study. Two-photon absorption (TPA) is a third-order nonlinear optical process in which two incident low-energy photons are simultaneously absorbed and excite excitons. The cross section of TPA is usually several orders of magnitude smaller than that of linear absorption at low light intensities and is proportional to the square of the incident photon density. Therefore, significantly improved TPA can be achieved by using an extremely temporally and spatially bound excitation volume produced by femtosecond laser pulses and a highly focused beam. Such small excitation volume and high penetration incident light facilitate depth resolution by stage movement relative to the confocal plane. Figure 1 describes the setting of the TPM in conjuction with confocal system. In conjuction with laser scanning system, Two-photon laser scanning micrscopy (TPLSM) with nanoscale resolution are valuable characterization techniques for temporal and spectral carrier dynamics analysis in a nanostruture.
Figure 1. TPM scheme for TPA-induced PL. The laser beam was focused and PL was excited at tip of the nanorod.
Figure 2. (a) Top-view scanning electron microscopy (SEM) image of ZnO nanorods. The dashed white square of the SEM image is corresponding to (b). (b) Two-dimensional PL peak intensity map of the ZnO NRs by laser scanning system.
Time-Correlated Single Photon Counting (TCSPC)
In nano-photonics, understanding and controlling dynamic behavior of carriers (electron, hole, and exciton) due to light-matter interaction is very important issue for the advanced optical application as well as the fabrication of novel photonic device. In particular, for the temporal characterization of carriers, various time-resolved techniques to measure ultrafast carrier dynamics have been utilized. Among them, TCSPC is a powerful analysis tool recording the time dependent intensity profile of the emitted photon (fluorescence) from nano-materials. In TCSPC, single photons are repetitively registered upon the reference timing of periodic excitation pulse like a stopwatch (start-stop) and then single photon histrogram is formed over multiple cycles as shown in Fig3. Currently, we are operating TCSPC system for the study of exciton-phonon interaction as well as the fluorescence decay (lifetime) measurement in nanomaterials.
Figure 3. Schematics of TCSPC system for single photon detection.
In general, most fluorescence decay event is ended within nanosecond time scale. To measure this ultrafast event, of course, ultrashort laser pulse determining temporal resolution is essential. However, the time resolution (IRF) of TCSPC is limited due to by the timing uncertainty of detector introducing conversion from a photon to an electrical pulse. Although femtosecond source such as Ti:Sapphire mode-locked pulse laser is used, it is difficult to acquire IRF of several tens picoseconds. Nevertheless, our TCSPC system provides the excellent temporal resolution by using the highly sensitive detector such as single-photon avalanche diode (SPAD). This ultrashort timing resolution enables us to measure the event related to the exciton-phonon interaction as well as the fluorescence decay dynamics in nanomaterials.