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Modern scattering-type scanning near-field optical microscopy (s-SNOM) has become an indispensable tool in material research. However, as the s-SNOM technique marches into the far-infrared (IR) and terahertz (THz) regimes, emerging experiments sometimes produce puzzling results. For example, "anomalies" in the near-field optical contrast have been widely reported. In this Letter, we systematically investigate a series of extreme subwavelength metallic nanostructures via s-SNOM near-field imaging in the GHz to THz frequency range. We find that the near-field material contrast is greatly impacted by the lateral size of the nanostructure, while the spatial resolution is practically independent of it. The contrast is also strongly affected by the connectivity of the metallic structures to a larger metallic "ground plane". The observed effect can be largely explained by a quasi-electrostatic analysis. We also compare the THz s-SNOM results to those of the mid-IR regime, where the size-dependence becomes significant only for smaller structures. Our results reveal that the quantitative analysis of the near-field optical material contrasts in the long-wavelength regime requires a careful assessment of the size and configuration of metallic (optically conductive) structures.

We report in this paper the design and fabrication of single-chip alternating current-LEDs (AC-LEDs) by monolithically integrating, for the first time, high-breakdown Schottky barrier diodes (SBDs) and micro-LED arrays using III-nitride LED epi-wafers of standard specs for volume production. A technique of cyclic mixed-etching has been introduced to restore the surface of inductively coupled-plasma (ICP) etching processed gallium nitride to device quality for fabricating high-breakdown Schottky junctions. Proof-of-concept single-chip AC-LED devices and a prototype driver-free white AC-LED lamp were demonstrated, showing high-efficiency LED emission, high chip area utilization efficiency, low power loss of the on-chip SBD bridge rectifier, and good luminous efficacy of the prototype AC-LED lamp. This paper paves the way toward mass-producing reliable and low-cost driver-free AC-LED lamps for solid-state lighting with existing LED manufacturing infrastructures.

Crystalline 4-N, N-dimethylamino-4ʹ-Nʹ-methyl-stilbazolium tosylate (DAST) is known as an excellent organic nonlinear optical (NLO) material. However, it is vulnerable to damage caused by high-intensity laser irradiation, which severely limits its NLO applications. In this study, DAST molecules were included with β-cyclodextrin (β-CD), and DAST@β-CD supramolecular nanowires (NWs), microplates (MPs), and chaotic microcrystals (CMCs) were successfully prepared. All the crystals not only possess excellent NLO properties, but also exhibit greatly enhanced optical damage tolerances (ODTs) and photoluminescence. Under the pumping of a 532-nm picosecond (ps) pulsed laser, we achieved single-mode lasing from MPs and random lasing from CMCs, with thresholds of 55.22 and 88.38 nJ/cm2, respectively. When the laser irradiation had a peak intensity of ~17.5 GW/cm2, which is 8-9 orders of magnitude higher than the DAST bulk-crystal photobleaching intensity, the CMC laser emission intensity retained ~95% of its initial value after 4 h of operation in an ambient atmosphere.

By using terahertz time-domain spectroscopy (THz-TDS), the transmission spectral images of a series of aliphatic monohydric liquid alcohols were investigated in this work. The image gray value indicated the non-uniform distribution of the liquid. After noise reduction, the depth distribution was computed from the image content, which depended on the wettability of the container surface by the liquid. Then the volume of the liquid crept up to the container surface was compared quantitatively. Our result showed that the surface wettability of the PS container surface by the liquids increased with the alcohol alkyl chain increasing, which was verified by the contact angle of the liquid and the surface. The study indicated that the THz image might offer a potential technique for detecting the wettability of liquid directly without an additional contact angle experiment.

All-optical encoders with high response speed are required to achieve efficient information conversion in the field of ultra-fast information encoding, which can break the bottleneck of photoelectric conversion delay in traditional optical encoders and achieve a tunable bandwidth up to THz. The modifiable advantages of light path about all-optical encoder can widely satisfy the functional requirements of the modern high-precision device. This review introduces two principles of all-optical encoders, based on light propagation design and radiation source parameter design. We introduced the principle and miniaturization development of all-optical encoders based on the fabrication of two-dimensional (2D) photonic crystal defects to control optical path propagation, which can effectively use the photonic band gap effect to construct waveguide and the ring resonator to improve the coupling efficiency. In addition, we compared the previous method of the all-optical encoder with the design of radiation source parameters, and offer a practicable idea for the design of high-order encoder.

Directly predicting the line positions of samples in the terahertz (THz) band is of significant importance for their THz identification. However, it is really a challenge to gain accurately the line positions by means of theoretical calculation, because the calculation typically involves various parameters, such as level energy and transition moment, which usually we hardly get directly. Based on the classical forced vibration model of dipoles, we propose a quantitative expression, i.e. the forced radiation intensity of molecular electric dipoles, which intend to predict the line positions of absorption peaks in the THz fingerprint spectra of a sample. We verified our expression by 9 recognized frequencies selected from the fingerprint spectra of water vapor in the THz band. Both the line positions and intensities of the absorption peaks of water vapor we calculated by the expression are well consistent with the experimental measurements. The line positions we calculated are also more accurate and comprehensive than that of water clusters simulated from Density Functional Theory (DFT). Our findings further support the theory of coherent superposition to advance a new method to exactly analyze the generation mechanism of molecular THz-fingerprint spectroscopy of a sample.

In a photoconductive antenna (PCA), femtosecond-laser-excited carriers will form a broadband terahertz photon-carrier (PC) comb, and the terahertz PC comb can be used as a multi-frequency local oscillator to carry out heterodyne detection of continuous terahertz sources with high frequency accuracy. In this paper, the heterodyne efficiency and the bias effects of a PCA terahertz PC comb are investigated. The results show that the pair beat signals (with the beat frequencies lower than the repetition frequency of femtosecond laser) of a continuous terahertz source and the two adjacent comb teeth do not decrease with the increase of beat frequency. Applying a bias voltage to the PCA can effectively enhance the terahertz emission efficiency. However, such a bias voltage has no positive effects on the heterodyne detection responsivity because the heterodyne detection is intrinsically based on the terahertz rectification effect that is proportional to the photo-excited electrons. In addition, by using a reference terahertz source with high frequency stability, it is possible to measure the fluctuation and the drift of the repetition frequency of femtosecond lasers with higher accuracy. The results are helpful for improving the performance of terahertz frequency measurement system based on PCA PC combs.

Infrared and optical spectroscopy represents one of the most informative methods in advanced materials research. As an important branch of modern optical techniques that has blossomed in the past decade, scattering-type scanning near-field optical microscopy (s-SNOM) promises deterministic characterization of optical properties over a broad spectral range at the nanoscale. It allows ultrabroadband optical (0.5-3000 µm) nanoimaging, and nanospectroscopy with fine spatial (

The bias-enhanced performance of visible-blind ultraviolet (UV) detection of GaN Schottky barrier diodes has been studied. The UV response of the Schottky diodes was found to be vastly amplified at elevated reverse bias, leading to the observation of the voltage-dependent gain arising from the defect-induced Schottky barrier lowering effect. In contrast, the visible light response of the GaN Schottky diodes shows insignificant voltage dependence because of the dominance of the internal photoemission absorption. Thus, the visible rejection ratio, defined as the responsivity of UV over visible range, can be greatly enhanced at high operating bias for the UV detectors based on the GaN Schottky barrier diodes. The observation has been supported by both experimental results and simulation data, and has been utilized to minimize the interference between the monolithically integrated GaN photodetectors and power light-emitting diodes (LEDs) in the present study of LED communication.

This letter reports the design and simulation of novel AlGaN/GaN double-gate high electron mobility transistors (DG HEMTs) featuring enhanced back gate-control of the two dimensional electron gas in AlGaN/GaN heterostructures. A comparison study was carried out to reveal the difference between the performance of the AlGaN/GaN DG HEMTs and single-gate devices. The results show that the DG GaN-HEMTs can potentially offer a higher transconductance gain and better immunity of the short channel effects of drain induced barrier lowering and subthreshold swing than traditional single-gate HEMTs.

Nonradiative energy transfer between colloidal quantum-dot (QD) phosphors and the active region of an indirect-bandgap semiconductor diode was investigated. A silicon carbide (SiC) p–n junction was fabricated and surface-patterned with arrays of holes, facilitating the sidewall coupling between the QDs and the SiC p–n junction. Nonradiative energy transfer was observed from the SiC diode to the colloidal QD phosphors, which was characterized with a color conversion efficiency of 3.1%. Time-resolved photoluminescence measurements were conducted to verify and characterize the energy transfer process between the diode and QDs. The carrier recombination lifetime of SiC was found to decrease upon the presence of QD phosphors, which provides further evidence for the presence of the nonradiative energy transfer path between the QDs and the SiC diode.

Au/Ag core–shell nanorods with variable shell thickness were prepared by using a simple method, and their optical properties were characterized by using steady state extinction spectra and ultrafast transient absorption spectroscopy. The extinction spectra and discrete dipole approximation calculations showed that the longitudinal surface plasmon resonance (SPR) peak of Au/Ag core–shell nanorods shifted to blue as the Ag shell thickness increased. Although these Au/Ag core–shell nanorods are expected to display two longitudinal SPR bands, only one longitudinal SPR band was observed in the steady state extinction spectra and simulation results. We resolved this conflict by using ultrafast transient absorption spectroscopy under different excitation wavelengths: 400 and 800 nm. We have experimentally demonstrated for the first time the existence of two closely spaced longitudinal SPR bands in Au/Ag core–shell nanorods, corresponding to the contributions from the Ag shell and Au core.

We report a general description of quasi-phase-matched parametric process in nonlinear photonic crystals (NLPC) by extending the conventional X-ray diffraction theory in solids. Under the virtual wave approximation, phase-matching resonance is equivalent to the diffraction of the scattered virtual wave. Hence a modified NLPC Ewald construction can be built up, which illustrates the nature of the accident for the diffraction of the virtual wave in NLPC and further reveals the complete set of diffractions of the virtual wave for both of the air-dielectric and dielectric-dielectric contacts. We show the two basic linear sequences, the anti-stacking and para-stacking linear sequences, in one-dimension (1D) NLPC and present a general rule for multiple phase-matching resonances in 1D NLPC. The parameters affecting the NLPC structure factor are investigated, which indicate that not only the Ewald construction but also the relative NLPC atom size together determine whether a diffraction of the virtual wave can occur in 2D NLPC. The results also show that 1D NLPC is a better choice than 2D NLPC for a single parametric process.

Copper nanoparticles embedded in soda-lime glass were fabricated by ion exchange followed by thermal treatment in hydrogen. The ultrafast dynamics of the embedded Cu nanoparticles formed under different fabrication conditions were investigated by applying femtosecond pump-probe technique. Non-Fermi electrons were suggested to be dominant in the transient behavior of the nanocomposites far from surface plasmon resonance of Cu. The long ion-exchange processing time was found to benefit and improve the ultrafast response of the fabricated nanocomposites.

Ag nanoparticles embedded in soda-lime silicate glasses were fabricated by the ion-exchange technique. Effects of thermal treatment on the optical nonlinearity and ultrafast dynamics of Ag nanoparticles were investigated by applying time-resolved optical Kerr effect and pump-probe techniques. The results indicate that thermal treatment is an efficient method to improve the nonlinear optical performance of this kind of material.

Single-crystalline α-Fe2O3 hollow nanocrystals from nanotube to nanoring have been successfully synthesized by a facile hydrothermal method. Size and shape control of the hollow nanocrystals is ach...

Ultrafast third-order nonlinear optical responses of the as-deposited and annealed Cu:Bi 2 O 3 nanocomposite films were investigated by using femtosecond Z-scan and time-resolved optical Kerr effect (OKE) techniques at the wavelength of 800 nm. The Z-scan measurement showed that both the real and imaginary parts of the third-order nonlinear optical susceptibility were enhanced after thermal annealing treatment. The OKE response of the as-deposited film was instantaneous, and the enhanced OKE signal of the annealed film presented additional slow decay processes which imply the important contribution from photoinduced hot electrons in Cu nanoparticles.

The solution-processed PbSe colloidal quantum-dot (CQD) infrared photodetector with tandem architecture is proposed to address the high dark current issue. The electrical transport mechanism in tandem has been fundamentally changed in which the recombination of carriers at an intermediate layer becomes dominant rather than carriers hopping between nearest neighbors in CQD materials. As a result, the tandem photodetector exhibits ultra-high detectivities of 4.7 × 10(13) Jones and 8.1 × 10(13) Jones under 34 μW cm(-2) illumination at 1100 nm, at 275 K and 100 K, respectively.

Ag : Si3N4 nanocermets were fabricated by the magnetron cosputtering method and were thermally treated at different temperatures. The off-resonant third-order nonlinear susceptibilities were determined to be of the order of 10−10 esu at 800 nm by the optical Kerr effect experiment. Further time-resolved pump–probe researches prove that heat treatment is a feasible way to improve the performance of ultrafast response for this material. The research results demonstrate that in addition to the common use in microelectronics, the Si3N4 nanocermets are promising in future applications in optical communication and photoelectric fields.

Micrometer-sized tubular ZnO crystal was fabricated by a hydrothermal method, and was characterized by scanning electron microscopy (SEM). Back-scattering micro-Raman spectrum was applied to investigate the vibrational modes of the microtubes with the comparison to that of ZnO powder. Raman scattering in a single micrometer-sized tubular ZnO was also recorded with different polarization configurations consequently. Most of the observed vibrational modes were assigned on the basis of group theoretical analysis. Besides the typical Raman-active modes assigned to ZnO, a new mode with Raman shift of 860 cm −1 was observed which has not been reported in previous relative researches yet. In addition to this, the Raman signal of E 1T mode at 410 cm −1 increased dramatically and become asymmetric in some cases.

The ultrafast nonlinear optical properties of co-sputtered silver/bismuth oxide (Ag:Bi2O3) nanocomposite films with different Ag concentration (13.2–59.3 at%) were investigated by femtosecond (fs) pump–probe and fs optical Kerr effect (OKE) techniques. The pump–probe measurements revealed the Ag concentration-related energy relaxation dynamics: for films with high concentration of Ag (>50%), the relaxation dynamics consist of a fast excitation power-dependent component (1–2 ps) and a long-lived component (>100 ps). While for films with relative low Ag concentration ( χ ( 3 ) ) of Ag:Bi2O3 films have a maximum of 4.1×10−10 esu at Ag concentration of 35.7%.

The energy relaxation dynamics in Ag:Bi2O3 nanocomposite films with high density of Ag nanoparticles were investigated by femtosecond pump–probe technique. For the films with relatively low Ag concentration (13.2–35.7 at.%), the energy relaxation is nearly completed within 1 picosecond (ps) at moderate excitation regime. For the film with 52.0 at.% Ag concentration, the relaxation process consists of a fast power-dependent component (1–2 ps) and a long-lived one. The different relaxation characteristics are suggested to be resulted from the differences in the structural and thermal properties of the surrounding Bi2O3 matrix coupling with Ag nanoparticles. � 2005 Elsevier B.V. All rights reserved.

By using femtosecond optical Kerr effect (OKE) and pump–probe methods, we investigated the ultrafast nonlinear optical response of TiO 2 films doped with gold nanoparticles. These films fabricated in a sol–gel way were heat treated at different temperatures, and the red shift of peak of the surface plasmon resonance with increase of heat treatment temperature was observed. The results of the experiment indicated that the third-order nonlinear susceptibility, about 10 −10 esu, depended on the heat treatment temperature.

Pure ZnO films and PbS/ZnO nanocomposite films of optical quality were prepared with a sol–gel method. The particle size, optical absorption, and linear refractive index of the PbS/ZnO films were measured by TEM (transmission electron microscopy), UV-Vis absorption, and ellipsometry techniques, respectively. For high Pb/Zn ratios, PbS particles of size 5–8 nm are observed in TEM, and a large refractive nonlinearity of 10−7 esu with a saturable absorption was measured in picosecond Z-scan experiments.

Au-core CdS-shell composite nanoparticles were synthesized by a direct self-assembly process and integrated into BaTiO3 thin films. Characterization by transmission electron microscopy showed that the average diameter of these composite nanoparticles was about 8 nm. Using the femtosecond time-resolved optical Kerr effect method, we investigated the third-order nonlinear optical response of the Au@CdS nanoparticles embedded in the BaTiO3 thin films at a wavelength of 800 nm. An ultrafast nonlinear response and a large effective third-order nonlinear susceptibility of χ(3)=7.7×10-11 esu were observed. We attributed the enhancement of the third-order optical nonlinearity to a localized electric field effect originating from the core-shell structure under off-surface-plasmon resonance conditions.

Ultrafast excitation relaxation dynamics of magnesium phthalocyanine (MgPc) thin film were investigated by femtosecond pump-probe spectroscopy. The excited state dynamics of MgPc's Q-band were found to be strongly dependent on the probe wavelength. Ultrafast exciton–exciton annihilation with a t−1/2 time dependence of the excited-state population was observed.

Design and fabrication of monolithically integrated III-nitride visible light-emitting-diodes (LEDs) and ultraviolet Schottky barrier-photodetectors (SB-PDs) have been proposed and demonstrated. Responsivity up to 0.2 AW(-1) at 365 nm for GaN SB-PDs has been achieved. It is shown that those UV SB-PDs were capable of sensitive UV light detection down to 7.16×10(-4) W/cm2 at 365 nm, whereas simultaneous operation of on-chip blue LEDs has produced negligible crosstalk at practical illumination brightness. Monolithically integrated LEDs and SB-PDs can function as transmitters to emit visible light signals, and as receivers to analyze incoming UV signals, respectively; this offers the potential of using such devices for bi-directional optical wireless communication applications.

In this paper, Monte Carlo simulations were performed to determine the potential efficiencies of luminescent solar concentrator (LSC) systems using PbSe quantum dots (QDs) as the active fluorescent material. The simulation results suggest that PbSe QD LSCs display good absorption characteristics, but yield limited LSC power conversion efficiency due to self-absorption and down-conversion loss. It is proposed that the self-absorption loss can be reduced by utilizing Forster resonance energy transfer between two different sizes of PbSe QDs, yielding pronounced improvement in the optical efficiency of LSCs.

Silver/bismuth oxide composite thin films with Ag concentration varying from 13.2% to 59.3% were prepared by cosputtering in a multitarget magnetron sputtering system. The absorption spectra of these thin films were measured, and the absorption peaks due to surface plasmon resonance of Ag particles showed a redshift behavior with increasing Ag concentration. By using the femtosecond time-resolved optical Kerr effect technique under 800 nm wavelength, the nonlinear optical properties of these films were investigated. The Kerr signals showed an ultrafast rise, suggesting that the main physical mechanism involved in the optical nonlinearity of Ag:Bi2O3 thin films on the femtosecond scale was the pure electronic contributions. The maximum value of the third-order nonlinear optical susceptibility of Ag:Bi2O3 thin films was 4.1×10−10 esu, and occurred at a Ag concentration of about 35.7%.

PbSe quantum dot light emitting diodes (QD-LEDs) of a multi-layer architecture are reported in the present work to exhibit high external quantum efficiencies. In these devices, a ligand replacement technique was employed to activate PbSe QDs, and ZnO nanoparticles were used for the electron transport layer. The emission wavelength of this solution processed device is QD size tunable over a broad spectral range, and an LED efficiency of 0.73% was measured at 1412 nm. Higher efficiencies at longer wavelengths are also inferred from spectral characterization.

The degradation mechanism of CdSe/ZnS quantum dots (QDs) light-emitting diodes (LEDs) was investigated with steady-state and time-resolved photoluminescence measurements. Our study reveals that the degradation is associated with the decreasing quantum efficiency of the CdSe/ZnS QDs in the devices. Two mechanisms that cause the efficiency reduction were verified in the experiments: i.e., thermal instability and luminescence quenching.

Multi-photon absorption pumped ultraviolet ZnO nanolasers were demonstrated with femtosecond pulses at room temperature. Time resolved photoluminescence revealed that the exciton collision effect played an important role in the nonlinear lasing mechanisms.

In an attempt to suppress the dark current, the barrier layer engineer for solution-processed PbSe colloidal quantum-dot (CQD) photodetectors has been investigated in the present study. It was found that the dark current can be significantly suppressed by implementing two types of carrier blocking layers, namely, hole blocking layer and electron blocking layer, sandwiched in between two active PbSe CQD layers. Meanwhile no adverse impact has been observed for the photo current. Our study suggests that this improvement resides on the transport pathway created via carrier recombination at intermediate layer, which provides wide implications for the suppression of dark current for infrared photodetectors.

With intense femtosecond laser excitation, multiphoton absorption-induced stimulated emission and laser emission in ZnO bulk crystal and nanowires have been demonstrated at room temperature. UV-stimulated emission peaks appeared in both bulk crystal and nanowires when the excitation exceeded certain thresholds, and a sharp lasing peak with a linewidth of ∼0.5 nm was observed from ZnO nanowires. The emission properties were attributed to the band-edge emission of the recombination of carriers excited by two- and three-photon absorption processes in the wide-bandgap semiconductor.

Ultraviolet lasing emission was observed from the wide band semiconductor ZnO excited by the fs pulses with photon energy less than the half of the bandgap via the Rabi flopping assisted Two-photon process.

We report a high-performance colloidal quantum dot (CQD)-based near-infrared tandem photodetector fabricated on flexible substrates via solution-processed method. The tandem photodetector on poly(ethylene terephthalate) substrates exhibited low dark current and high detectivities over ∼8.8 × 1011 Jones at near infrared range at −0.5 V bias and over ∼1013 Jones near 0 bias. The critical bend radii of ∼8 mm and ∼3 mm have been demonstrated for tensile and compressive bending, respectively. The performance of photodetectors remains stable under mechanical stress, making PbSe CQD material a promise candidate for flexible infrared sensing applications.