Ran Jia

Polarization Enhanced Charge Transfer: Dual-Band GaN-Based Plasmonic Photodetector

Here, we report a dual-band plasmonic photodetector based on Ga-polar gallium nitride (GaN) for highly sensitive detection of UV and green light. We discover that decoration of Au nanoparticles (NPs) drastically increases the photoelectric responsivities by more than 50 times in comparition to the blank GaN photodetector. The observed behaviors are attributed to polarization enhanced charge transfer of optically excited hot electrons from Au NPs to GaN driven by the strong spontaneous polarization field of Ga-polar GaN. Moreover, defect ionization promoted by localized surface plasmon resonances (LSPRs) is also discussed. This novel type of photodetector may shed light on the design and fabrication of photoelectric devices based on polar semiconductors and microstructural defects.

Microcantilever Actuation by Laser Induced Photoacoustic Waves

We present here a combined theoretical and experimental investigation on effective excitation of microcantilever by using photoacoustic waves. The photoacoustic waves arose from a vibrating Al foil induced by an intensity-modulated laser. We demonstrate that, superior to photothermal excitation, this new configuration avoids direct heating of the microcantilever, thus minimizing undesired thermal effects on the vibration of microcantilever, while still keeps the advantage of being a remote, non-contact excitation method. We also measured the vibration amplitude of the microcantilever as a function of distance between the microcantilever and the Al foil and found that the amplitudes decay gradually according to the inverse distance law. This method is universal and can be adopted in bio-microelectromechanical systems (BioMEMs) for the detection of small signals where detrimental thermal effects must be avoided.

Enhanced photoluminescence of gallium phosphide by surface plasmon resonances of metallic nanoparticles

We report here enhanced photoluminescence (PL) from GaP, an indirect band gap semiconductor, by metallic Au and Ag nanoparticles. The metallic Au and Ag nanoparticles were sputtering-coated onto GaP followed by heat treatment. After optimization of the surface coverage ratios, we find 5.6-fold and 14.5-fold PL enhancement for the Au and Ag coated samples, respectively. Time resolved photoluminescence spectroscopy indicates that Ag nanoparticles can effectively accelerate the PL decay time. Our results indicate that the PL enhancement comes from enhanced photon scattering and localized field enhancement of metallic Au and Ag nanoparticles.

Plasmonic GaInP solar cells with improved energy conversion efficiency

We report here a new method to improve the performance of GaInP solar cells by fabricating Ag nanoparticles (Ag NPs) atop the devices through a simple wet-chemical method. The size and density of Ag NPs can be easily optimized by changing the concentration of AgNO3 solution and/or the reaction time. The Ag NPs support surface plasmonic resonances (SPRs) and thus greatly increase light absorbance and photocurrent responses of the solar cells by the scattering and re-reflection of incident light. We find that, for a Ag NPs density of ∼3.37 × 1010 cm−2, the short-circuit current density (Jsc) increases by 19.5% from 14.9 to 17.8 mA cm−2, and the power conversion efficiency (PCE) increases by 20.4% from 15.2% to 18.3%.

Periodic indentation patterns fabricated on AlGaInP light emitting diodes and their effects on light extraction

We report a novel method for fabricating periodic indentation patterns on AlGaInP light emitting diodes (LEDs) that can effectively improve the light extraction efficiency. The patterns consist of hemispherical pits created by directly imprinting the top GaP window layer of AlGaInP LEDs with patterned sapphire (PS). The angle resolved electroluminescence tests reveal that the patterned chips show an average light emission enhancement of 155% over the original planar chips.

Optically induced reversible wettability transition on single crystal lithium niobate surfaces

Solid surfaces with controllable and reversible wettability are scientifically and technologically important. Here, we report on the reversible wettability transitions of single crystal (0001) lithium niobate (LiNbO3) surfaces by alternate ultraviolet (UV) and infrared (IR) light irradiation. The UV irradiation (170 mW/cm2) could markedly reduce the contact angle of LiNbO3 over 30 min from 55.3° to 10.7°. IR irradiation (200 mW/cm2) recovered the water contact angle from 10.7° to 55.1° over 1 h. First-principles calculations showed that under both O-poor and O-rich conditions, oxygen vacancies preferred to form at the Li-terminal (0001) surface rather than at the Nb-terminal surface and the O-terminal surface or in the bulk. We further show that this light induced wettability transition has a dependence on the light wavelength. The influences of relative humidity and oxygen concentration were also investigated.

Plasmonic Microcantilever with Remarkably Enhanced Photothermal Responses

Plasmonic nanostructures exhibit abundant optoelectronic properties. We explore here the technological potentials of plasmonic nanostructures as active component to actuate microcantilever sensors. We find that the photothermal excitation of microcantilevers can be greatly enhanced by Au nanoparticle (NPs). A detailed investigation reveals that the enhancement is wavelength dependent and can be attributed to selective excitation of localized surface plasmon resonance (LSPR). The associated effects are discussed based on a thorough examination of the geometric aspects of Au NPs, microcantilever lengths, and incident optical power. Some technological advantages offered by this method are also discussed.

Photoresistless fabrication of periodic patterns on GaAs by laser interference photochemical lithography

Patterns on semiconductors are crucial for optoelectronic microdevices in a wide variety of applications. We report here a facile photochemical method based on laser interference etching for fabrication of two-dimensional (2D) gratings and quasi-hexagonal photonic patterns on GaAs wafers. The periods of the patterns can be adjusted by tuning the incident angles without any significant change of the optical alignment. We find that the optical reflectivity of the etched samples is greatly reduced, especially for samples after three-beam interference etching with the minimum average reflectance of less than 2%, which can be easily adopted for fabricating anti-reflection layer of solar cells. Photoluminescence (PL) study shows that slightly blue shifted PL peaks can be observed due to quantum confinement effect.