K S Chan

Application of spectral surface plasmon resonance to gas pressure sensing

An optical sensor to be used for gas pressure detection is described. The device is based on the spectral response associated with the surface plasmon resonance (SPR) occurring on a 50-nm gold surface. The operation of the device relies on the fact that the refractive index of a gas changes with pressure, which leads to a shift in the resonance wavelength dip SPR in the SPR curve. Experiments performed on nitrogen gas at room temperature demonstrated the sensor resolution of 4.4×10–5 RUI, which corresponds to a pressure resolution of 16 kPa. The new device offers the benefits of wide dynamic range, no moving parts, and possibly 2-D imaging of pressure distribution. Further development of the device may lead to a new type of optical pressure sensors.

Charge transport in polyguanine–polycytosine DNA molecules

A double chain tight-binding model is proposed to interpret the experimental I–V curves for polyguanine–polycytosine DNA molecules reported in Porath et al (2000 Nature 493 635). The proposed model includes the salient features of existing transport models of DNA molecules. The proposed double chain model fits excellently with the experimental I–V curves and provides a theoretical interpretation of features found in the I–V curves, which so far do not have a satisfactory explanation. Steps in the I–V curves are explained as the result of transmission gaps caused by hybridization with reservoirs and inter-chain coupling. Variations in I–V curves are due to the variation of inter-chain and intra-chain hopping parameters caused by structural changes in the DNA molecules.

Equilibrium spin current in ferromagnetic graphene junction

We report a theoretical study of the equilibrium spin current (ESC) flowing in a ferromagnet/ferromagnet (FM/FM) graphene junction with noncollinear magnetizations, in which the FM correlation in graphene is induced by depositing an FM insulator on it. Using the scattering matrix method, we derived an analytical expression of the ESC which is formally the same as that of a normal FM/FM junction. The ESC comes from the exchange coupling between the two FM magnetizations. Owing to the linear dispersion of graphene, the ESC disappears in the one-dimensional case, while it exists in the two-dimensional case. It exhibits damped oscillations with change in the Fermi energy, the layer length between the two FM regions, and the FM exchange splitting. It is also found that ESC in FM/FM graphene junctions has electron-hole inversion symmetry whereas it has electron-hole inversion antisymmetry in normal FM/FM junctions.

Detection of spin bias by quantum interference effect in a Rashba ring

We propose to use the quantum interference effect in a Rashba ring to detect the pure spin current driven by a spin bias. By means of the Keldysh Greens function method, we demonstrated that a tunneling charge current could be induced flowing through the ring due to the combined quantum interference effect of the electron spin precession phase from Rashba spin-orbit coupling (RSOC) and the electron traveling phase difference in the two arms of the ring. The nonzero charge current can be used to deduce not only the magnitude of the spin bias but also its spin polarization. This charge current depends on some system parameters such as RSOC strength and the asymmetry of two arms. Our proposal may provide a practical and all-electrical way to indirectly detect the pure spin current (spin bias) by measuring the induced charge current/bias in a Rashba ring.

A scattering method for the equilibrium spin current in a ferromagnet junction

We extended McMillans Greens function method to study the equilibrium spin current (ESC) in a ferromagnet/ferromagnet (FM/FM) tunnelling junction, in which the magnetic moments in both FM electrodes are not collinear. The single-electron Greens function of the junction system is directly constructed from the elements of the scattering matrix, which can be obtained by matching wavefunctions at boundaries. The ESC is found to be determined only by the Andreev-type reflection amplitudes as in the Josephson effect. The obtained expression of ESC is an exact result and at the strong barrier limit gives the same explanation for the origin of ESC as the linear response theory, that is, ESC comes from the exchange coupling between the magnetic moments of the two FM electrodes, J ∼ h 1 x h r . In the weak barrier region, ESC cannot form spontaneously in a noncollinear FM/FM junction when there is no tunnelling barrier between the two FM electrodes.

Theoretical analysis of wavelength tuning in distributed-feedback lasers by quantum-well intermixing

Wavelength tuning in distributed-feedback (DFB) lasers using quantum-well intermixing is analyzed. A 0.42-nm tuning range is obtained when the bandgap is blue-shifted by 5.9 nm, and this value agrees well with the experimental value of 0.36 nm. The limitation of the tuning range is also discussed, and is because of the increase in carrier density that raises the gain above the threshold after intermixing. The dependence of wavelength shift on bandgap blueshift is not affected by the details of the intermixing process. A maximum tuning range of 3.5 nm is predicted, demonstrating that after device fabrication intermixing process can be used to adjust DFB lasers operating wavelengths to match the predefined wavelength channels of wavelength-division-multiplexing system.

Spin current pumped by a rotating magnetic field in zigzag graphene nanoribbons.

We study electron spin resonance in zigzag graphene nanoribbons by applying a rotating magnetic field on the system without any bias. By using the nonequilibrium Greens function technique, the spin-resolved pumped current is explicitly derived in a rotating reference frame. The pumped spin current density increases with the system size and the intensity of the transverse rotating magnetic field. For graphene nanoribbons with an even number of zigzag chains, there is a nonzero pumped charge current in addition to the pumped spin current owing to the broken spatial inversion symmetry of the system, but its magnitude is much smaller than the spin current. The short-ranged static disorder from either impurities or defects in the ribbon can depress the spin current greatly due to the localization effect, whereas the long-ranged disorder from charge impurities can avoid inter-valley scattering so that the spin current can survive in the strong disorder for the single-energy mode.

Single-parameter charge pump in a zigzag graphene nanoribbon

We report a theoretical study of a single-parameter quantum charge pump in the clean zigzag graphene nanoribbon (ZGNR) system. By Keldysh Greens function method, we show that a pumped current in the ZGNR with an even number of zigzag chains can sharply increase from zero as the frequency matches the Fermi energy, whereas the pumped charge current is always absent in the ZGNR with an odd number of zigzag chains as well as the GNR with armchair edges, it is attributed to the peculiar zero-energy edge state in the ZGNR and the symmetry breaking of the topologically inequivalent carbon atoms due to the zigzag edges. The pumped current in the even-ZGNR decreases with the Fermi energy and its direction is determined by Fermi energy below or above the Dirac point as well as the type of carbon atoms at one edge of the ZGNR. The two-parameter charge pump in the ZGNR is also discussed and the magnitude of the pumped current is comparable to the single-parameter pump when the pumping frequency matches the Fermi energy.

Spin and charge transport in hybrid triplet Josephson junctions

We report a theoretical study on the spin and charge transport in hybrid triplet Josephson junctions, of which the triplet pair potentials could have both different orbital symmetries and spin states. Based on a lattice model and a Hamiltonian method, we find that the spin/charge supercurrent is proportional to sin2 with being the macroscopic superconducting phase, coming from the second-order Josephson effect, when the orbital symmetries of pair potentials in the two triplet superconductors are orthogonal to each other. A dissipationless transverse spin current is also found flowing at the interface of the junction and its polarization points along the cross-product of two d vectors, which arises from the combined effect of the orthogonal orbital symmetries and misalignment of d vectors. In a special hybrid junction, where the zero-energy states are absent at the interface of the junction, there is no net spin supercurrent flowing through the junction although two d vectors can be perpendicular to each other, whereas a mode-resolved spin supercurrent is flowing in the system instead.

Two-dimensional surface plasmon resonance biosensor array based on phase-sensitive measurement

We report a 2-dimensional surface plasmon resonance (SPR) imaging array sensor based on differential phase measurement between p- and s-polarization. This parallel detection provides the advantage of high-throughput sensing, which is essential in recent biosensing technology. In the differential measurement approach, the signal (p) and reference (s) beams go through exactly identical optical path. This greatly improves the phase detection stability. In the present setup we use a low-cost imaging device and a simple data analysis program to perform the required arrayed sensing operation. The system demonstrates a refractive index resolution of 1x10-4 RIU per degree phase change.