We measured the reflected THz signal as the height over an extended Au surface decreases and Fig. 6 illustrates the THz waveform and spectrum measured at different tip height. The changes of waveform and spectrum near 0.5 THz are due to the near-field THz wave. Not only Au thin film, but also Au nanoparticles were prepared for the investigation of the signal enhancement by the surface plasmon. 2D assembly and 3D multilayer Au structures were tested in Z-scan measurement system separately developed to investigate the nonlinear optical properties. The high repetition rate of our laser system, however, causes the thermal excitation of highly conductive Au nanostructures so that the correct information of material properties could not be obtained. This problem can be
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solved only by reducing the repetition rate of the laser system using, so-called, a pulse picker to at least a few tens of kHz, which we do not have currently. The performance of newly established THz reflection spectroscopy system was tested by measuring the previously identified materials. We took Si substrate and a c-plane InN film as the reference materials. Figure 7(a) shows the
Drude-like electrical conductivity of the c-plane InN film and the real part of refractive index of
~3.41 is consistent with the well-known value of Si wafer These results are in the excellent agreement with those measured by the separately installed transmission-based THz-TDS system in our laboratory. Further measurements on other materials and the comparison with the results of transmission-based THz-TDS confirm the performance of our reflection-based THz-TDS system and we used this system to measure the electrical properties of Ag nanowires.
Fig. 7. (a) Frequency-dependent electrical conductivity of the c-plane InN film measured by THz reflection TDS system. The frequency dependence of conductivity could be described by the simple Drude model and the fitting parameters used in the simulation are consistent with those obtained by THz transmission TDS system. (b) The complex refractive index of Si substrate measured by THz reflection TDS system. The real part of refractive index of ~3.41 over the frequency range between 0.1 THz to 2.0 THz is consistent with that of THz transmission TDS.
We have also performed several researches on III-V nitrides using THz spectroscopy and ultrafast carrier dynamics. As an extension of the research on photoluminescence measurement of semiconductors, we studied the PL of InGaN/GaN nanorods on metal oxide and succeeded in building the smallest nanolaser based on surface plasmonics. The results of these research are summarize in the following.
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
FIG. 6. Reflected THz waveforms and the THz spectrum as the distance between the Au film and the metal tip is adjusted
Oxidation of Ag nanowires studied by THz-reflection-TDS
Indium oxide (ITO) has been widely used as plastic substrates with high transparency and low sheet resistance for printed electronic devices. However, ITO is quite brittle so that there has been a demand on a new material to replace ITO. Conducting polymers, metal inks, nanoparticulate metal oxides, carbon nanotubes, and graphene have been investigated as potential alternatives, but none can yet compete in terms of transparency and sheet resistance. Thin meshes of silver nanowires have recently emerged as promising electrodes due to their ability to provide good transmittances (>85%) at sheet resistances less than 20 Ω sq-1. Their application to printed electronics, however, is challenging due to a highly non-uniform topography (as it can be seen in Fig. 5), which can cause shorting through other layers. We used the THz reflection spectroscopy to measure the conductivity and oxidation problem of Ag NWs. Through the collaboration with Prof. P. Yu in Department of Photonics in National Chiao Tung University, we prepared the Ag NW film by spin coating method. Figure 8(a) shows the frequency-dependent electrical conductivity of Ag NWs with the NW density of 1/36 right after the NW film was prepared. Despite the individual NW has nanostructures, nanowires can be overlapped in the junction and form a wide mesh of silver. Therefore, the frequency-dependent electrical conductivity in Fig. 8(a) still shows the simple Drude-like behavior, indicating very conductive metal mesh. We measured THz spectroscopy of the same Ag NW film four weeks later and the frequency-dependent conductivity in Fig. 8(b) is nearly identical to the one in Fig. 8(a).
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Fig. 8. Frequency-dependent electrical conductivity of AgNWs on Si with the NW density of 1/36 measured (a) right after the growth and (b) one month later. Both show the Drude-like metal behavior.
We performed the same measurement on the Ag NW film with the NW density of 1/72.
The conductivity measured right after the film was prepared [Fig. 9(a)] shows the Drude-like behavior, but the one measured about four weeks later shows clearly non-Drude dependence on the frequency. This frequency-dependence can be described by, so-called, Drude-Smith model which is well-accepted to explain the frequency-dependent conductivity of nanostructures. We attribute this change of conductivity to oxidation of Ag NWs. Right after the growth of Ag NWs, the junctions
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between wires are electronically conductive and build a large conductor mesh over the whole film and behave like a two-dimensional conductive film. As the time passes, the surface of each NW oxidizes and forms thin Ag-oxide layer surrounding the NW. Then at the junction of NWs, electron transfer can be prevented by the thick-enough Ag-oxide and electrons would be confined in each NW. Then the whole film is like a stacked nanowires which are electrically independent.
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Fig. 9. Frequency-dependent electrical conductivity of AgNWs on Si with the NW density of 1/72 measured (a) right after the growth and (b) one month later. While conductivity signals in Fig. (a) can be explained by the Drude model, the one in Fig. (b) follows the Drude-Smith model which is due to the confined electrons in nanostrucutres.
We measured the same Ag NW samples in THz transmission spectroscopy system. But the absorption of THz waves in Ag NWs is very high so that we could not get the reasonable data. This proves the benefit of performing reflection-based THz spectroscopy and by coulpling with metal tips, we can measure the electrical properties of nanostructures and even can perform the fine-resolution mapping.
Mg-induced terahertz transparency of indium nitride films[13]
Terahertz time-domain spectroscopy (THz-TDS) has been used to investigate electrical properties of Mg-doped InN. Mg-doping in InN was found to significantly increase terahertz transmittance. THz-TDS analysis based on the Drude model shows that this high transmittance from Mg-doped InN is mainly due to the reduction in mobility associated with ionized dopants. The Hall-effect-measured mobility is typically lower than the THz-TDS-measured mobility for the same samples. However, the results of both measurements have the same slope in the linear relation between mobility and density. By introducing a compensation ratio of ~0.2, an excellent agreement in mobilities of two methods is obtained. This result can explain the discrepancy in the values of mobilites measured by THz spectroscopy and the Hall effect measurement and suggest the method to calibrated the result of Hall effect measurement.
Fig. 10. (a) Terahertz transmittance of an undoped InN film and four Mg doped InN films with different carrier densities. (b)Electron-density-dependent electron mobility measured by the THz-TDS and the Hall-effect method. Blue circles are THz-TDS-measured mobility corrected by including a compensation ratio = 0.2.
Background and photoexcited carrier dependence of terahertz radiation from Mg-doped nonpolar indium nitride films [3]
Terahertz generation from Mg-doped nonpolar (a-plane) InN (a-InN:Mg) was systematically studied and compared with those from undoped a- and c-InN. While the amplitude and polarity of the THz field from Mg-doped polar (c-plane) InN depend on the background carrier density, the p-polarized THz field from a-InN:Mg has background carrier-insensitive intensity and polarity, which can be attributed to carrier transport in a polarization-induced in-plane electric field. A small but apparent azimuthal angle dependence of the THz field from a-InN:Mg shows the additional contribution of the second-order nonlinear optical effect. Meanwhile, in this study, we did not observe the contribution of the intrinsic in-plane electric field which is significant for high stacking fault density nonpolar InN.
Fig. 11. Peak amplitudes of the THz radiation from Mg-doped c- (squares) and a-InN (circles) films as a function of background carrier concentration. The open circle indicates the peak amplitude obtained from an undoped a-InN and the solid triangle corresponds to an n-type InAs (100) with a carrier density of ~2×1017 cm-3. The two insets in the figure illustrate the waveforms of THz radiation with positive and negative polarities, respectively.
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Carrier dynamics in InN nanorod arrays [14]
We investigated ultrafast carrier dynamics of vertically aligned indium nitride (InN) nanorod (NR) arrays grown by molecular-beam epitaxy on Si(111) substrates. Dominant band filling effects were observed and were attributed to a partial bleaching of absorption at the probe wavelengths near the absorption edge. Carrier relaxation in nanorod samples was strongly dependent on the rod size and length. In particular, a fast initial decay was observed for carriers in NRs with a small diameter (~30 nm), the lifetime of which is much shorter than the carrier cooling time, demonstrating the substantial surface-associated influence on carrier relaxation in semiconductor nanostructures.
Fig. 12. Differential reflectivity transient of (a) InN epilayer and (b)-(d) NR samples A, B, and C measured at various probing wavelengths. Sample A with large rod diameter and long height has the sign flipping in R/R as the probe wavelength increases beyond c whereas sample B and C consisted of small diameter NRs have the positive change in R/R for all probe wavelengths.
Curve fits to the measured data indicate that the wavelength-dependent cooling time 2
for InN film is of the order of 2 ps, which is similar to those for samples A and C. Figure 4(a) shows that the cooling time of sample B is comparable, but is relatively longer than those of other samples, the reason for which remains unclear. Here, it is worthy to note that in addition to 2, samples B and C have an initial rapid decay time 1 which is in the range of 500 fs to 1 ps. Because samples B and C are consisted of the 30-nm NRs, 1 is expected to be strongly correlated with the carrier confinement in the small NRs. For InN, surface electron accumulation layer extends to approximately 10 nm below the surface.
For samples B and C, the electron accumulation layer is comparable to the radius of NRs (~15 nm) and subsequently, the photoexcited carriers can be easily trapped by the surface-related defects with a short lifetime.
Fig. 13. (a) Carrier lifetimes of NR samples with different heights and diameters compared to that of epilayer film. Slow recombination time constants are not shown here. (b) Pump fluence dependence of differential reflectivity transient of sample B. Inset shows the carrier lifetimes 1
and 2 at the corresponding pump fluences.
Terahertz emission mechanism of magnesium doped indium nitride [6]
The carrier concentration-dependence of terahertz emission from magnesium doped indium nitride (InN:Mg) films was investigated. It has been a main issue among the nitride research society to realize p-type InN to achieve p-n junction. In addition, the demonstration of embedded p-type layer below the narrow surface layer is another challenge. We investigated the terahertz emission mechanism of Mg-doped InN since the emission mechanism is based on the electron dynamics in the photoexcited area, holding the information of conduction type. Near the critical concentration (nc~1×1018 cm−3), the competition between two emission mechanisms determines the polarity of terahertz emission. InN:Mg with n > nc exhibits enhanced positive-polarity terahertz emission compare to the undoped InN, which is due to the reduced screening of the photo-Dember field. For InN:Mg with n < nc, the polarity of terahertz signal changes to negative, indicating the dominant contribution of the surface electric field due to the large downward surface band-bending within the sub-surface layer extending over the optical absorption depth.
Fig. 14. (a) The time-domain terahertz waveforms of the as-grown (undoped) InN and Mg-doped InN films. InN:Mg films with the electron concentration below ~1×1018 cm−3 have a negative
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polarity, while others including undoped InN film have a positive polarity. The terahertz waveforms of n- and p-type GaAs are shown together for comparison. (b) The amplitude of terahertz emission field vs. pump fluence for InN:Mg films with n=4.3×1017 cm−3 and 1.6×1018 cm−3. The sign of data of InN:Mg film with n=4.3×1017 cm−3 is inverted to be positive. Inset shows a typical azimuthal angle-dependent terahertz radiation from InN:Mg film.
Carrier dynamics of Mg-doped indium nitride [4]
In our previous results, we have reported a significant enhancement (>500 times in intensity) in terahertz emission from Mg-doped indium nitride (InN:Mg) films compared to undoped InN. It was found that the intensity of terahertz radiation strongly depends on the background electron density. The carrier dynamics of InN:Mg is studied by employing ultrafast time-resolved reflectivity measurement. We find that the decay time constant of InN:Mg also depends on background electron density in the same way as terahertz radiation does as it shown in Fig. 16. The spatial redistribution of carriers in diffusion and drift is found to be responsible for the recombination behavior as well as terahertz radiation.
Fig. 15. Peak amplitudes of the THz radiation from Mg-doped c- (squares) and a-InN (circles) films as a function of background carrier concentration. The open circle indicates the peak amplitude obtained from an undoped a-InN and the solid triangle corresponds to an n-type InAs (100) with a carrier density of ~2×1017 cm-3. The two insets in the figure illustrate the waveforms of THz radiation with positive and negative polarities, respectively.
Plasmonic green nanolaser based on a metal-oxide-semiconductor structure[15]
Realization of smaller and faster coherent light sources is critically important for the emerging applications in nanophotonics and information technology. Semiconductor lasers are arguably the most suitable candidate for such purposes. However, the minimum size of conventional semiconductor lasers utilizing dielectric optical cavities for sustaining laser oscillation is ultimately governed by the diffraction limit (∼(/2n)3 for
three-dimensional (3D) cavities, where is the free-space wavelength and n is the refractive index). Here, we demonstrate the 3D subdiffraction-limited laser operation in the green spectral region based on a metal_oxide_semiconductor (MOS) structure, comprising a bundle of green-emitting InGaN/GaN nanorods strongly coupled to a gold plate through a SiO2 dielectric nanogap layer. In this plasmonic nanocavity structure, the analogue of MOS-type “nanocapacitor” in nanoelectronics leads to the confinement of the plasmonic field into a 3D mode volume of 8.0 × 10-4 m3 (∼0.14(/2n)3). This laser is the smallest nanolaser ever demonstrated based on surface plasmonic coupling of lasing enhancement.
Figure 16. Plasmonic green nanolaser. (a) Schematic representation of the lasing MOS structure consisting of a bundle of green-emitting InGaN/GaN semiconductor nanorods, which is coupled to an underlying colloidal gold triangular plate through an SOG dielectric gap layer.
The supporting substrate is silicon and the thickness of the SOG layer is about 5 nm. The average nanorod diameter is 30 nm, while the lengths of InGaN and GaN sections are 300 and 380 nm, respectively. (b) FE-SEM image of the hybrid system. The magnified image shows the detailed view of the measured InGaN/GaN nanorod bundle on top of the gold plate (c) The green laser emission from the InGaN/SOG/gold hybrid system in a cyrostat (at 7 K) and under the excitation of a frequency-doubled Ti:sapphire laser system
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Figure 17. Lasing characteristics. (a) Power-dependent laser emission spectra of the InGaN/GaN nanorod bundle supported on the SOG covered gold plate. These spectra were recorded at 7 K with varying excitation intensities, showing the transition from spontaneous emission to lasing at 533 nm. (b) In comparison, the power-dependent photoluminescence spectra of the InGaN/GaN nanorod bundle directly positioned on an SOG/Si substrate (without the gold plate) show no signs of lasing. (c) For the lasing MOS structure, the superlinear response of the peak intensity becomes obvious when the excitation intensity is above the threshold intensity (∼300 kW/cm2). The inset shows the simultaneous line width narrowing of the emission peak above the lasing threshold. (d) The complete lasing characteristics are shown as a log_log plot together with the corresponding slopes (S) for different regions. The inset shows the defocused lasing mode image. The appearance of the high-contrast fringes indicates spatial coherence due to lasing.