H.-M. Lee

Direct evidence of 8:9 commensurate heterojunction formed between InN and AlN on c plane

We show that, despite a large difference in lattice constants, high-quality InN∕AlN heterostructures can be formed on Si(111) due to the existence of “magic” ratios between the lattice constants of comprising material pairs: 2:1 (Si∕Si3N4), 5:4 (AlN∕Si), and 8:9 (InN∕AlN). For InN growth on AlN with nitrogen polarity, by using reflection high-energy electron diffraction and cross-sectional transmission electron microscopy, we have found that the pseudomorphic to commensurate lattice transition occurs within the first monolayer of growth, resulting in an abrupt heterojunction at the atomic scale. This new route of lattice match allows the formation of commensurate and nearly strain-free interface with a common two-dimensional superlattice.

Study of optical phase transduction on localized surface plasmon resonance for ultrasensitive detection

Localized surface plasmon resonance (LSPR) has shown its remarkable applications in biosensing, bioimaging, and nanophotonics. Unlike surface plasmon polariton (SPP), the current studies regarding LSPR as biosensor were restricted in probing the extinction spectra, and thus limit the performance in biosensing and bioimaging. Here, we reveal that optical phase of LSPR provides an acute change at resonance beyond extinction spectra, which permits an ultra-high sensitivity in phase interrogation. We found that optical phases of LSPR show two orders of magnitude higher sensing resolution than extinction spectra among the same nanostructures. For the first time, we demonstrated the feasibility of probing optical phase transduction in LSPR for biosensing, and the sensitivity is superior to not only the extinction spectra among the same metallic nanostructures, but also the LSPR sensors among the current literatures. In summary, the exploitation of LSPR by phase interrogation essentially complements the sensitivity insufficiency of LSPR, and provides new access to understanding and using the rich physics of LSPR.

Experimental and theoretical studies of lattice dynamics of Mg-doped InN

Raman studies of Mg-doped InN films with a Mg content from NMg=3.3×1019to5.5×1021cm−3 are reported. Raman and secondary ion mass spectroscopy data on the Mg content have been found to correlate well. Lattice dynamics of hexagonal InN with substitutional impurities and vacancies has been investigated in the framework of the cluster approach. Energy positions of local vibrational modes in InN have been calculated and compared with experimental findings. It is concluded that Raman spectroscopy is a good tool for quantitative characterization of Mg-doped InN.

Demonstration of an ultrasensitive refractive-index plasmonic sensor by enabling its quadrupole resonance in phase interrogation.

We present an ultrasensitive plasmonic sensing system by introducing a nanostructured X-shaped plasmonic sensor (XPS) and measuring its localized optical properties in phase interrogation. Our tailored XPS exhibits two major resonant modes of a low-order dipole and a high-order quadrupole, between which the quadrupole resonance allows an ultrahigh sensitivity, due to its higher quality factor. Furthermore, we design an in-house common-path phase-interrogation system, in contrast to conventional wavelength-interrogation methods, to achieve greater sensing capability. The experimental measurement shows that the sensing resolution of the XPS reaches 1.15×10(-6) RIU, not only two orders of magnitude greater than the result of the controlled extinction measurement (i.e., 9.90×10(-5) RIU), but also superior than current reported plasmonic sensors.

An innovative cloak enables arbitrary multi-objects hidden with visions and movements

We propose an innovative cloak to enable different sizes, shapes and also constituent parameters of arbitrary hidden objects not only to observe the fields with 360-degree eyesight from the environment outside the cloak but also to need neither to be ‘custom-made’ nor to be confined by the position of the corresponding anti-objects. We design the spatially varying constituent parameters of an innovative cloak by the aid of transformation optics with two steps of coordinate transformations and testify the performances of an innovative cloak by COMSOL simulation software. Herein, we demonstrate the corresponding Ez field distribution as a testimony of invisible hidden objects and claim that the simulation results demonstrate a good agreement with analytical study of an innovative cloak.

Influence of structural anisotropy to anisotropic electron mobility in a-plane InN

This study reports on the anisotropic electron transport properties and a correlation between the electron mobility (μ) and the stacking faults (SFs) in the a-plane InN film. Electron mobilities measured by terahertz time-domain spectroscopy and Hall effect measurement along the in-plane [1¯100] (c⊥) orientation were much higher than those of the in-plane [0001] (c∥) orientation. This result shows a sharp contrast to higher defect density for the c⊥ orientation as measured by x-ray diffraction. The electrons transporting through the planar SFs aligned along the c⊥ direction are expected to experience more scattering by defects, resulting in lower μ for the c∥ orientation.

Magnetic Surface Polariton in a Planar Biaxial Metamaterial with Dual Negative Magnetic Permeabilities

The magnetic surface polaritons (MSPs) mode rarely exists in natural materials, mainly due to their limited magnetic response. In order to understand the relationship between the MSPs mode and the limited magnetic response, we theoretically derive a general dispersion equation of the MSPs mode for two kinds of biaxial anisotropic media. The dispersion equation suggests the requirements of μx < 0 < εz, μxμy > 1, and momentum conservation need to be satisfied, which guides us to design a planar biaxial magnetic metamaterials (PBMM) with two orthogonal negative permeabilities (i.e., μx < 0 and μy < 0). In addition, the results of the retrieval magnetic permeability and electric permittivity indeed point out that our PBMM satisfies the aforementioned requirements. Meanwhile, we investigate the mechanism of the magnetic field by a consideration of an induced current loop, and subsequently we demonstrate the artificial MSPs mode under transverse electric excitation by numerical simulation. After a numerical fit of simulation results, we find that the field of the MSPs mode is in good agreement with the analytical calculation of Maxwell’s equations. Moreover, we also simulate three unsatisfied aforementioned requirements as control conditions to verify the general dispersion equation of the MSPs mode.

Resonant raman scattering and dispersion of polar optical and acoustic phonons in hexagonal inn

It is shown that a study of the dependence of impurity-related resonant first-order Raman scattering on the frequency of excitation light makes it possible to observe the dispersion of polar optical and acoustic branches of vibrational spectrum in hexagonal InN within a wide range of wave vectors. It is established that the wave vectors of excited phonons are uniquely related to the energy of excitation photon. Frequencies of longitudinal optical phonons E1(LO) and A1(LO) in hexagonal InN were measured in the range of excitation-photon energies from 2.81 to 1.17 eV and the frequencies of longitudinal acoustic phonons were measured in the range 2.81–1.83 eV of excitation-photon energies. The obtained dependences made it possible to extrapolate the dispersion of phonons A1(LO) and E1(LO) to as far as the point Γ in the Brillouin zone and estimate the center-band energies of these phonons (these energies have not been uniquely determined so far).

The electrostatic coupling of longitudinal optical phonon and plasmon in wurtzite InN thin films

We utilize coherent longitudinal optical phonon as an optical nanoprobe to investigate the plasmonic behavior of wurtzite c-plane InN thin films. The transition from the three-dimensional electron gas in InN bulk toward the two-dimensional electron gas in InN thin film is revealed via measuring the coupling strength of coherent A1(LO) phonon and plasmon coupling mode. The coupling strength diminishes as the film thickness reduces and finally vanishes at 3±1 nm. This phenomenon is ascribed to the detuning of the intersubband plasmon frequency above the A1(LO) frequency, which is originated from the electronic quantum confinement in InN thin films.

Mg-induced terahertz transparency of indium nitride films

Terahertz time-domain spectroscopy (THz-TDS) has been used to investigate electrical properties of Mg-doped indium nitride (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.