Low Threshold Power Density Research Articles

Efficient Near-Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface.

Solid state photon upconversion by triplet-triplet annihilation (TTA), particularly near-infrared (NIR)-to-blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR-to-blue upconversion has remained particularly challenging and elusive due to inherent multiple energy-downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid-state NIR-to-blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR-to-blue upconversion with high quantum yield (ΦUC = 1.2%, out of 50%), low threshold power density (Ith = 110 mW/cm2) and a record-high apparent anti-Stokes shift of 1.12 eV. Further spin- and time-resolved spectroscopy reveals an ultrafast (< 500 fs) electron spin flip to triplet-like excitons in semiconductor sensitizer and subsequent picosecond (~ 6×1010 s-1) interfacial dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR-to-blue photon upconversion and implementation.

Efficient Near‐Infrared to Blue Photon Upconversion by Ultrafast Spin Flip and Triplet Energy Transfer at Organic/2D Semiconductor Interface

Solid state photon upconversion by triplet‐triplet annihilation (TTA), particularly near‐infrared (NIR)‐to‐blue upconversion, holds instant promise for enhancing optoelectronic and photochemical applications. Despite extensive studies, NIR‐to‐blue upconversion has remained particularly challenging and elusive due to inherent multiple energy‐downhill processes in TTA upconversion. In this study, using atomically thin two dimensional (2D) monolayer semiconductor as a triplet sensitizer, we demonstrate an efficient and robust solid‐state NIR‐to‐blue photon upconversion system. The ultrathin and flexible organic/2D bilayer heterostructure exhibits a NIR‐to‐blue upconversion with high quantum yield (ΦUC = 1.2%, out of 50%), low threshold power density (Ith = 110 mW/cm2) and a record‐high apparent anti‐Stokes shift of 1.12 eV. Further spin‐ and time‐resolved spectroscopy reveals an ultrafast (&lt; 500 fs) electron spin flip to triplet‐like excitons in semiconductor sensitizer and subsequent picosecond (~ 6×1010 s‐1) interfacial dexter energy transfer to annihilator molecules. The triplet energy transfer rate and efficiency depend strongly on driving force, exhibiting Marcus normal region behavior. This work demonstrates 2D monolayer semiconductor as a superior ultrathin light harvesting and triplet sensitization layer and reveals the key knob to overcome the compromise between upconversion efficiency and energy loss, offering a viable pathway to efficient solid state NIR‐to‐blue photon upconversion and implementation.

Continuous-wave terahertz quantum cascade laser based on a hybrid bound to bound quantum design

We report a low threshold power density and high power output terahertz quantum cascade laser emitting at ∼3.9 THz operating in continuous-wave mode. The high output power and wall-plug efficiency are achieved based on a hybrid bound-to-bound quantum active design. A record output power of 312 mW and a low threshold power density of 0.8 kW/mm3 (threshold current density of 109 A/cm2) in continuous-wave mode at 20 K is demonstrated for a 300-μm-wide and 2-mm-long single-ridge device. The highest wall-plug efficiency is 1.38% and the slope efficiency is 684 mW/A with an internal quantum efficiency of ∼120 photons per injected electron. The demonstration of this low-threshold and high-power THz laser will promote THz-based remote sensing and standoff detection for pharmaceutical and health industry applications.

Infrared stimulated emission with an ultralow threshold from low-dislocation-density InN films grown on a vicinal GaN substrate

Near-infrared stimulated emission from a high-quality InN layer under optical pumping was observed with a threshold excitation power density of 0.3 and 4 kW cm−2 at T = 8 and 77 K, respectively. To achieve such a low threshold power density, vicinal GaN substrates were used to reduce the edge-component threading dislocation (ETD) density of the InN film. Cross-sectional transmission electron microscopy images reveal that the annihilation of ETDs can be divided into two steps, and the ETD density can be reduced to approximately 5 × 108 cm−2 near the surface of the 5-µm-thick film. The well-resolved phonon replica of the band-to-band emission in the photoluminescence spectra at 9 K confirm the high quality of the InN film. As a result, the feasibility of InN-based photonic structures and the underlying physics of their growth and emission properties are demonstrated.

Effects of Mg and Si doping in the guide layers of AlGaN-based ultraviolet-B band lasers

We investigated the effects of Si and Mg doping concentrations in the guide layers on the threshold power density of an AlGaN-based optically pumped ultraviolet-B band (UV-B) laser. For Si, a decrease in the threshold power density required for laser oscillation was confirmed at doping concentrations up to an order of 1017 cm−3. However, an increase was confirmed at high concentrations of approximately 1018 cm−3. For Mg doping, no decrease in the threshold power density of the optically pumped laser was confirmed even at low Mg concentrations, and it increased rapidly at 1018 cm−3. These results were characterized by weakly excited photoluminescence and variable stripe length, indicating that they were due to changes in optical gain with increasing or decreasing internal quantum efficiency and changes in internal loss. This suggests that Mg doping of the guide layer in UV-B lasers should be lower than 1018 cm−3 to realize low-threshold power density lasers. On the other hand, doping on an order of up to 1017 cm−3 for Si was effective at reducing the threshold power density. This indicates that the threshold power density increases if it is too high. These results could provide guidance for designing UV-B lasers in the future.

Direct Coupling of Coherent Emission from Site-Selectively Grown III–V Nanowire Lasers into Proximal Silicon Waveguides

Semiconductor nanowire (NW) lasers are nanoscale coherent light sources that exhibit a small footprint, low-threshold lasing characteristics, and properties suitable for monolithic and site-selective integration onto Si photonic circuits. An important milestone on the way toward novel on-chip photonic functionalities, such as injection locking of laser emission and all-optical switching mediated by coherent optical coupling and feedback, is the integration of individual, deterministically addressable NW lasers on Si waveguides with efficient coupling and mode propagation in the underlying photonic circuit. Here, we demonstrate the monolithic integration of single GaAs-based NW lasers directly onto lithographically defined Si ridge waveguides (WG) with low threshold power densities of 19.8 μJ/cm2 when optically excited. The lasing mode of individual NW lasers is shown to couple efficiently into propagating modes of the underlying orthogonal Si WG, preserving the lasing characteristics during mode propagati...

The sensitization of laser-induced heat of Er3+ doped Y2O3 nanocrystals by codoped with Mn2+

Low threshold power density cw laser-induced heat has been observed in [Formula: see text] and [Formula: see text] codoped [Formula: see text] nanocrystals under excitation by a 980 nm IR laser. Codoped [Formula: see text] remarkably reduces the power density threshold of laser-induced heat compared with [Formula: see text] doped [Formula: see text] nanocrystals. When the excitation power density exceed [Formula: see text], [Formula: see text] codoped [Formula: see text] nanocrystals emit strong blackbody radiation. The thermal emission of [Formula: see text] should originate from the multiphonon relaxation between neighboring energy levels. One additional UC-PL enhancement is observed. The UC-PL intensity can be enhanced by an order of magnitude through high temperature calcination caused by light into heat.

Optically pumped GaN vertical cavity surface emitting laser with high index-contrast nanoporous distributed Bragg reflector.

Laser operation of a GaN vertical cavity surface emitting laser (VCSEL) is demonstrated under optical pumping with a nanoporous distributed Bragg reflector (DBR). High reflectivity, approaching 100%, is obtained due to the high index-contrast of the nanoporous DBR. The VCSEL system exhibits low threshold power density due to the formation of high Q-factor cavity, which shows the potential of nanoporous medium for optical devices.

Room temperature ultraviolet GaN metal-coated nanorod laser

This study demonstrates a room-temperature ultraviolet GaN/Al nanorod (NR) metal laser with an optimized sidewall. A wet-chemical etching process with potassium hydroxide was used to control the GaN NR sidewall angle and polish the NR surface. The lasing action was observed near a wavelength of 365 nm with a low threshold power density of 5.2 mJ/cm2. The high-quality factor (Q) surface plasmon lasing modes were characterized with experiments and three-dimensional finite-element method simulations. We also studied the optical modes in GaN metal-coated NR with and without an Al layer and verified the metal layer is necessary for high-Q resonant modes.

Large-area ultraviolet GaN-based photonic quasicrystal laser with high-efficiency green color emission of semipolar {10-11} In0.3Ga0.7N/GaN multiple quantum wells

In this study, a multi-color emission was observed from the large-area GaN-based photonic quasicrystal (PQC) nanopillar laser. The GaN PQC nanostructure was fabricated on an n-GaN layer by using nanoimprint lithographic technology. The regrown InGaN/GaN multiple quantum wells (MQWs) formed a nanopyramid structure on top of the PQC nanopillars. A lasing action was observed at ultraviolet wavelengths with a low threshold power density of 24 mJ/cm2, and a green color emission from InGaN/GaN MQWs was also achieved simultaneously.

Tunable two-dimensional photonic crystal cavity all-optical switching infiltrated with liquid-crystal

A novel photonic crystal (PC) all-optical switch based on the shift of defect mode is demonstrated. Two photonic crystal waveguides are connected by a photonic crystal cavity. The point defect of the photonic crystal cavity is filled with phenylacetylene liquid cyrstals intermingled with azobenzene. The output of the optical switch is controlled by adjusting the rotation angle of the liquid crystals' orientation vector. And the optical switch is numerically investigated by using the finite difference time domain (FDTD) method and plane wave expansion method. Numerical simulation shows that the optical switch based on the PC cavity can be changed by adjusting the applied linearly polarized light. Results show that the switch has low threshold power density, great extinction ratio, and small size.

Large Area of Ultraviolet GaN-Based Photonic Quasicrystal Laser

In this study, large-area GaN-based photonic quasicrystal (PQC) nanopillars were fabricated on an n-GaN substrate using the nanoimprint lithography (NIL) technique. Under optical pumping condition, a high lasing action from the GaN photonic quasicrystals was observed. The lasing wavelength is at 366 nm with a low threshold power density of 0.009 kW/cm2. To confirm the band-edge lasing mode, the finite-element method (FEM) was used to perform the simulation for the 12-fold symmetry photonic quasicrystal lattices.

Low threshold power density for the generation of frequency up-converted pulses in bismuth glass by two crossing chirped femtosecond pulses

We investigated the generation of frequency up-converted femtosecond laser pulses by nondegenerate cascaded four-wave mixing (CFWM) in a bismuth-oxide glass (BI glass). Broad-bandwidth light pulses with different propagation directions were simultaneously obtained by using two small-angle crossing femtosecond laser pulses in BI glass. Experimental results show that the threshold power density for the generation of broad-bandwidth femtosecond pulses in BI glass is one order of magnitude lower than that in fused silica.

Lasing in metal-coated GaN nanostripe at room temperature

This study demonstrated a metal-coated GaN nanostripe laser operable at room temperature. The ultraviolet lasing mode was observed at a wavelength of approximately 370 nm with a low threshold power density of 0.042 kW/cm2. The lasing mode of the metal-coated nanostripe was characterized using finite-element method simulation. The results showed the significance of metal coatings in this nanocavity structure for lasing at room temperature.

Soft-solution route to ZnO nanowall array with low threshold power density

ZnO nanowall array (ZNWA) has been directionally grown on the buffer layer of ZnO nanoparticles dip-coated on Si-wafer under a soft solution process. Nanowalls on substrate are in most suitable shape and orientation not only as an optical trap but also as an optical waveguide due to their unique growth habit, V[011¯0]⪢V[0001]≈V[0001¯]. Consequently, the stimulated emission at 384 nm through nanowalls is generated by the threshold power density of only 25 kW/cm2. Such UV lasing properties are superior to those of previously reported ZnO nanorod arrays. Moreover, there is no green (defect) emission due to the mild procedure to synthesize ZNWA.

Ultraviolet GaN-based microdisk laser with AlN/AlGaN distributed Bragg reflector

We demonstrated a 4.7 μm GaN-based microdisk laser with 25-pair AlN/AlGaN distributed Bragg reflector in ultraviolet range without undercut or deeply-etching procedures. The distributed Bragg reflector provides a high reflectivity of 85%, and selects lasing mode around 375 nm wavelength. Under optical pumping conditions, the lasing action was observed with a low threshold power density of 0.03 kW/cm2. We also characterized the whispering gallery mode profiles of the microdisk with finite-different time-domain simulation.

Interband Cascade Lasers with Wavelengths Spanning 3.2–4.2 μm

We report a detailed experimental investigation of five interband cascade lasers with five active stages each and emitting at wavelengths between 3.2 μm and 4.2 μm at room temperature. Pulsed threshold current densities as low as 394 A/cm2 and voltage efficiencies as high as 76% are obtained at 300 K. The low pulsed threshold power densities (0.9–1.6 kW/cm2 at 300 K) imply that ambient-temperature cw operation should be possible over the entire spectral band once optimized narrow ridges can be fabricated.

CW Argon-ion laser initiated aluminum induced crystallization of amorphous silicon thin films

ABSTRACTCW Argon-ion laser initiated aluminum induced crystallization (AIC) of RF magnetron sputtered amorphous silicon (a-Si) thin films has been investigated. It was found that lasers could be effectively used to initiate AIC process at very low threshold power densities. An argon-ion laser (λ=514.5 nm) was used to anneal Al/a-Si/glass structures with varying power densities ranging between 55 and 125 W/cm2 and exposure times ranging from 10 to 120 s. X-ray diffraction analysis showed the resulting films to be polycrystalline. The crystallization rate increased both with power density and exposure time. Environmental scanning electron microscopy (ESEM) analysis showed that the surface features change with increasing power density and irradiation time. A dendritic growth pattern was observed in the initial stages of interaction between the films. A strong crystalline Raman peak at around 520 cm-1 was observed in the Raman spectra of the crystallized samples.

2.5–3.5 μm optically pumped GaInSb/AlGaInSb multiple quantum well lasers grown on AlInSb metamorphic buffer layers

Room-temperature emission is observed as long as 3.26 μm in optically pumped type-I quantum well lasers on relaxed epitaxial layers grown by molecular-beam epitaxy. A superlattice is used to filter dislocations in the metamorphic buffer to reduce Shockley–Read–Hall losses. The longest wavelength emission of 3.45 μm from these structures is observed at 170 K, and the brightest room-temperature laser emits 0.5 W/facet peak power at 2.81 μm. It has a low threshold power density of 169 W/cm2 and a differential quantum efficiency of 28%. The characteristic temperatures, T0 and T1, are 119 K and 171 K, respectively. Stimulated emission is observed in this sample at a maximum operating temperature of 370 K.

ZnSe-based blue-green lasers with a short-period superlattice waveguide

We report the successful application of alternatively strained short-period superlattices for the waveguide region of optically pumped and injection room-temperature ZnSe-based lasers operating within the 470–523 nm spectral range. The design of optically pumped ZnMgSSe/ZnSSe/ZnCdSe lasers provides extremely low threshold power densities due to the enhanced electronic and optical confinement. Room-temperature BeMgZnSe/ZnCdSe injection lasers with threshold current density of about 750 A/cm2 and characteristic temperature as high as 366 K are demonstrated. The peculiarities of carrier transport across the short-period superlattices are explained by a thermally activated mechanism.