H. C. Guo

The Physics of ultrafast saturable absorption in graphene

The ultrafast saturable absorption in graphene is experimentally and theoretically investigated in the femtosecond (fs) time regime. This phenomenon is well-modeled with valence band depletion, conduction band filling and ultrafast intraband carrier thermalization. The latter is dominated by intraband carrier-carrier scattering with a scattering time of 8 ( +/- 3) fs, which is far beyond the time resolution of other ultrafast techniques with hundred fs laser pulses. Our results strongly suggest that graphene is an excellent atomic layer saturable absorber.

In vitro cancer cell imaging and therapy using transferrin-conjugated gold nanoparticles

Gold nanoparticles were conjugated with transferrin molecules for targeting, imaging and therapy of breast cancer cells (Hs578T, ATCC). Results show that, the transferrin-transferrin receptor-mediated cellular uptake of gold nanoparticles is six times of that in the absence of this interaction. As a consequence, the laser power effective for photothermal therapy of the cancer cells was reduced to values of two orders of magnitude lower. To demonstrate the efficiency of the conjugated gold nanoparticles in selectively targeting cancer cells, the cellular uptake of the gold nanoparticles by noncancerous cells (3T3, ATCC) was also investigated. The cellular uptake by the normal cells is only one fourth of that by the cancerous cells indicating that the transferrin-transferrin receptor interaction plays an important role in controlling the cellular uptake of the gold nanoparticles.

Recent progress of atomic layer deposition on polymeric materials

As a very promising surface coating technology, atomic layer deposition (ALD) can be used to modify the surfaces of polymeric materials for improving their functions and expanding their application areas. Polymeric materials vary in surface functional groups (number and type), surface morphology and internal structure, and thus ALD deposition conditions that typically work on a normal solid surface, usually do not work on a polymeric material surface. To date, a large variety of research has been carried out to investigate ALD deposition on various polymeric materials. This paper aims to provide an in-depth review of ALD deposition on polymeric materials and its applications. Through this review, we will provide a better understanding of surface chemistry and reaction mechanism for controlled surface modification of polymeric materials by ALD. The integrated knowledge can aid in devising an improved way in the reaction between reactant precursors and polymer functional groups/polymer backbones, which will in turn open new opportunities in processing ALD materials for better inorganic/organic film integration and potential applications.

Polarization dependent state to polarization independent state change in THz metamaterials

We experimentally demonstrated a polarization dependent state to polarization independent state change in terahertz (THz) metamaterials. This is accomplished by reconfiguring the lattice structure of metamaterials from 2-fold to 4-fold rotational symmetry by using micromachined actuators. In experiment, it measures resonance frequency shift of 25.8% and 12.1% for TE and TM polarized incidence, respectively. Furthermore, single-band to dual-band switching is also demonstrated. Compared with the previous reported tunable metamaterials, lattice reconfiguration promises not only large tuning range but also changing of polarization dependent states, which can be used in photonic devices such as sensors, optical switches, and filters.

Near-infrared upconversion nanoparticles for bio-applications

Upconversion nanoparticles (UCNs) attract intensive attentions in biomedical applications. They have shown great potential in bioimaging, biomolecule detection, drug delivery, photodynamic therapy and cellular molecules interactions. Due to the anti-Stokes optical property and NIR excitation, UCNs overcome the drawbacks encountered in conventional luminescent biomarkers. High signal to noise ratio, low cytotoxicity and stable high throughput results are obtained using UCNs as luminescent labels or light triggers in biomedical applications. In this review article, the reason for choosing UCNs as biomedical agents, the progress of the UCNs development and case studies of their biomedical applications will be discussed.

Terahertz carrier dynamics and dielectric properties of GaN epilayers with different carrier concentrations

Using terahertz time-domain spectroscopy, we measured the complex conductivity and dielectric function of n-type GaN with various carrier concentrations on sapphire substrate. The measured complex conductivity, which is due to the free carriers, is well fitted by simple Drude model. The contribution from the lattice vibration to the complex dielectric function increases with the decrease in free carrier concentration. A better fitting of the frequency-dependent complex dielectric response was obtained by considering both of the Drude and the classical damped oscillator model.

Terahertz dielectric response and optical conductivity of n-type single-crystal ZnO epilayers grown by metalorganic chemical vapor deposition

Using terahertz time-domain spectroscopy, we measured the frequency dependent complex dielectric response and conductivity of n-type single-crystal ZnO epilayers with different carrier concentrations over the frequency range from 0.1 to 3.0 THz. The measured complex dielectric response and conductivity are analyzed using Drude model.

Rapid Copper Metallization of Textile Materials: a Controlled Two-Step Route to Achieve User-Defined Patterns under Ambient Conditions

Throughout history earth-abundant copper has been incorporated into textiles and it still caters to various needs in modern society. In this paper, we present a two-step copper metallization strategy to realize sequentially nondiffusive copper(II) patterning and rapid copper deposition on various textile materials, including cotton, polyester, nylon, and their mixtures. A new, cost-effective formulation is designed to minimize the copper pattern migration on textiles and to achieve user-defined copper patterns. The metallized copper is found to be very adhesive and stable against washing and oxidation. Furthermore, the copper-metallized textile exhibits excellent electrical conductivity that is ~3 times better than that of stainless steel and also inhibits the growth of bacteria effectively. This new copper metallization approach holds great promise as a commercially viable method to metallize an insulating textile, opening up research avenues for wearable electronics and functional garments.

Mid-infrared radiation in an aperiodically poled LiNbO3 superlattice induced by cascaded parametric processes

The cascaded parametric processes, i.e. optical parametric oscillation (OPO) and difference frequency generation (DFG), can be used to induce efficient CW mid-infrared radiation at 4–5 µm in a single optical superlattice obtained by aperiodically poling a ferroelectric crystal. The superlattice consists of basic building blocks modulated by a spatial function. We numerically solve the coupling equations considering pump depletion and mid-IR absorption and realize the optimal condition for the cascaded frequency-down-conversion processes. Simultaneously, it is seen that the output performance is enhanced by the cascaded parametric design compared to the standard OPO configuration.

Parametric downconversion via cascaded optical nonlinearities in an aperiodically poled MgO:LiNbO3 superlattice

Quasi-phase-matched parametric downconversion via cascaded optical nonlinearities in an aperiodically poled MgO:LiNbO3 superlattice was studied in theory and experiment. Due to the cascading effect and the abundant reciprocal structure in an aperiodic optical superlattice, multiple-wavelength parametric downconversion in a wide spectrum range can be obtained. Enhancement of the conversion efficiency and output stability through coupling of two nonlinear processes is demonstrated. The result also reveals that the cascaded parametric downconversion process can be used to efficiently downconvert the fundamental wavelength to a longer wavelength of the infrared region. The process can be additionally served as an efficient mechanism to enhance THz wave propagation in a nonlinear optical medium.