Unraveling the efficiency losses and improving methods in quantum dot-based infrared up-conversion photodetectors

Quantum dot-based up-conversion photodetector,in which an infrared photodiode(PD)and a quantum dot light-emitting diode(QLED)are back-to-back connected,is a promising candidate for low-cost infrared imaging.However,the huge efficiency losses caused by integrating the PD and QLED together hasn’t been studied sufficiently.This work revealed at least three origins for the efficiency losses.First,the PD unit and QLED unit usually didn’t work under optimal conditions at the same time.Second,the potential barriers and traps at the interconnection between PD and QLED units induced unfavorable carrier recombination.Third,much emitted visible light was lost due to the strong visible absorption in the PD unit.Based on the understandings on the loss mechanisms,the infrared up-conversion photodetectors were optimized and achieved a breakthrough photon-to-photon conversion efficiency of 6.9%.This study provided valuable guidance on how to optimize the way of integration for up-conversion photodetectors.

Surface modification of an up-conversion luminescence material by overcoating with SiO_2 and its characterization

The surface of an up-conversion luminescence material was modified by overcoating with SiOa, which was synthesized from a hydrolysis progress of telraethoxysilane （TEOS） in alkalescent condition. By analyzing the hydrolyzed mechanism of TEOS, it was found that there was not only physical adsorption but also chemical bonding between the up-conversion material and SiO2. At the same time, some adsorption bands at 1100, 475, 950, and 3500 cm^-1 were found by FI-IR, which were the characteristic bands of Si-OH and Si-O-Si. By analyzing the surface elements of the coated material by XPS, it was found that its surface only included Si, O, and C elements, and not F and Y. In the picture of XRD, there was no additional peak after surface modification, suggesting that the silica shell was amorphous. The small peak at 20 = 23° in the X-ray diffraction pattern of the coated material was caused by the amorphous SiO2 shell, and the TEM image also proved that the surface of the material was successfully modified by overcoating with SiO2. The amount of hydroxyls was then increased on the surface of the material, which made it easy to connect with other active groups.

CMOS Quadrature Modulator and Up-Conversion Mixer for 802.11a Wireless LAN Systems

A quadrature modulator and an up-conversion mixer for an 802. lla wireless LAN system are designed and fabricated in 0.18μm gate length standard CMOS technology. A current feedback loop with a transconductor is used to improve the linearity of the quadrature modulator;An LC resonant tank is used as the load of the upconversion mixer to improve its gain and increase the voltage swing. The measurement results show that the input P1dB achieves -3.6dBm, the transducer power gain of the circuit is -3.6dB,and the current consumes about 45.8mA with a 1.8V power supply.

Nanocrystals precipitation and up-conversion luminescence in Yb^(3+)-Tm^(3+) co-doped oxyfluoride glasses

Rare earth ions doped oxyfluoride glass with composition of 28SiO2·22AlO1.5·40PbF2·10PbO·（4.8-x） GdFy0.1NdF3.xYbF3·0.1TmF3 （x=-0, 0.1, 0.2, 0.5, 1, 2, 3, 4 and 4.8） in molar ratio was developed. When the oxyfluoride glasses were heat-treated at the first crystallization temperature, the glasses gave transparent glass-ceramics in which rare earth containing fluorite-type nanocrystals of about 17.2 nm in diameter uniformly precipitated in the glass matrix. Compared with the glasses before heat treatment, the glass-ceramics exhibited very strong blue up-conversion luminescence under 800 nm light excitation. Rare earth containing nanocrystals were also space selectively precipitated upon laser irradiation in an oxyfluoride glass, the size of precipitated nanocrystals could be controlled by laser power and scan speed. The intensity of the blue up-conversion luminescence was strongly dependent on the precipitation of β-PbF2 nanocrystal and the YbF3 concentration. The reasons for the highly efficient Tm^3＋ up-conversion luminescence after laser irradiation were discussed.

Research on Up-Conversion Mechanism in Er^(3+)/Yb^(3+)-Codoped Oxyfluoride Glass

Er3 +/Yb3 +-codoped oxyfluoride crystallite glass was prepared with melting technique. The compositions and the melting temperature and the annealing temperature of the rare earth-doped crystallite glass were studied in detail. The emission spectra of samples were measured with the Hitachi F-4500 fluorescent photometer pumped by 980 nm wavelength laser. The up-conversion luminescence mechanism was illuminated on the view of the photophysics. By measuring the relationship between luminescent intensity and pump power, it is confirmed that the emission peaks at 550 nm belong to two-photon process, while that at 665 nm belongs to three-photon process. Moreover, the distributions of crystalline were determined by SEM.

Optical temperature sensor based on up-conversion fluorescence emission in Yb^(3+):Er^(3+) co-doped ceramics glass

yb^3＋：Er^3＋ co-doped oxy-fluoride ceramics glass has been prepared. The mechanism of up-conversion emissions about Er^3＋ was discussed, and the temperature properties of green up-conversion fluorescence between 303 and 823 K were investigated. The results show that the sensitivity of this sample reaches its maximum value, about 0.0047 K^-1, when the temperature is 383 K, indicating that this kind of sample can be used as high temperature and high sensitivity optical temperature sensor.

Investigation of ultrafast dynamics of CdTe quantum dots by femtosecond fluorescence up-conversion spectroscopy

The ultrafast carrier relaxation processes in CdTe quantum dots are investigated by femtosecond fluorescence upconversion spectroscopy.Photo-excited hole relaxing to the edge of the forbidden gap takes a maximal time of ～ 1.6 ps with exciting at 400 nm,depending on the state of the photo-excited hole.The shallow trapped states and deep trap states in the forbidden gap are confirmed for CdTe quantum dots.In addition,Auger relaxation of trapped carriers is observed to occur with a time constant of ～ 5 ps.A schematic model of photodynamics is established based on the results of the spectroscopy studies.Our work demonstrates that femtosecond fluorescence up-conversion spectroscopy is a suitable and effective tool in studying the transportation and conversion dynamics of photon energy in a nanosystem.

Up-conversion detection of mid-infrared light carrying orbital angular momentum

Frequency up-conversion is an effective method of mid-infrared(MIR) detection by converting long-wavelength photons to the visible domain, where efficient detectors are readily available. Here, we generate MIR light carrying orbital angular momentum(OAM) from a difference frequency generation process and perform up-conversion on it via sum frequency conversion in a bulk quasi-phase-matching crystal. The maximum quantum conversion efficiencies from MIR to visible are 34.0%, 10.4%, and 3.5% for light with topological charges of 0, 1, and 2, respectively, achieved by utilizing an optimized strong pump light. We also verify the OAM conservation with a specially designed interferometer, and the results agree well with the numerical simulations. Our study opens up the possibilities for generating, manipulating, and detecting MIR light that carries OAM, and will have great potential for optical communications and remote sensing in the MIR regime.

Up-Conversion Properties of Oxy-Fluoride Glasses Co-Doped with Ho^(3+) and Yb^(3+)

Oxy-fluoride glasses with composition of 25SiO2-65PbF2-9.4AlF3-0.1HoF3-0.5YbF3 were prepared. Their up-conversion fluorescence characteristics were investigated by 980 nm laser. Two emission peaks were observed at 540 and 650 nm. The up-conversion mechanism and processes were analyzed. The relationship between pumping power and relative intensity of emissions was discussed. From the dependence, it is known that the emissions centered at 540 and 650 nm are both attributed to two-photon process.

Green and red up-conversion emissions and thermometric application of Er^(3+) -doped silicate glass

The green and red up-conversion emissions centred at about 534, 549 and 663 nm of wavelength, corresponding respectively to the ^2H11/2 → ^4I15/2, ^4S3/2 → ^4I15/2 and ^4F9/2 → ^4I15/2 transitions of Er^3＋ ions, have been observed for the Er^3＋-doped silicate glass excited by a 978 nm semiconductor laser beam. Excitation power dependent behaviour of the up-conversion emission intensity indicates that a two-photon absorption up-conversion process is responsible for the green and red up-conversion emissions. The temperature dependence of the green up-conversion emissions is also studied in a temperature range of 296-673 K, which shows that Er^3＋-doped silicate glass can be used as a sensor in high-temperature measurement.

Up-conversion luminescence from Nd^(3+) ions doped in NaBi(WO_4)_2 single crystal

The luminescence of Nd3+-doped in NaBi(WO4)2 single crystal was investigated from 10 K to room temperature.The excitation source was a pulsed dye laser in resonance into the(4G5/2+2G(1)7/2) levels of Nd3+ ions.Several blue emission bands in the up-conversion luminescence spectra corresponded to transitions from 2P1/2 to 4I9/2.Some violet bands corresponding to transitions of 4D3/2→(4I9/2,4I11/2,4I13/2) were also observed.For comparison,the luminescence spectra and decay curves excited by the pulsed 355 nm l...

Up-conversion Properties of Er^(3+)/Yb^(3+) Co-doped Li_3Ba_2Gd_3(MoO_4)_8 Phosphors

Er3＋/Yb3＋ co-doped Li3Ba2Gd3（MoO4）8 phosphors were synthesized by conven- tional solid state reaction method, and their structure and spectral properties were investigated. The diffuse reflectance spectra showed that the 4I15/2→4I11/2 transition of Er3＋ and the 2F7/2→2F5/2 transition of Yb3＋ ions were highly overlapped. Under the excitation of 980 nm, three up-conver- sion （UC） luminescence bands around 530, 555 and 660 nm were observed, corresponding to the 2H11/2→ 4I15/2, 4S3/2 → 4I15/2 and 4F9/2-→4I15/2 transitions of Er3＋ ions, respectively. The effects of the concentration and pumping power on the UC intensities of Li3Ba2Gd3（MoO4）8：Er3＋/yb3＋ phosphors were investigated, and the possible UC mechanism was proposed based on the results.

Intense up-conversion emissions of yb3＋/Dy3＋ co-doped A12O3 nanopowders prepared by non-aqueous sol-gel method＊

yb3＋/Dy3＋ co-doped A1203 nanopowders have been prepared by the non-aqueous sol-gel method and their up- conversion photoluminescence spectra are measured under excitation by a 980-nm semiconductor laser. The results show that there are comparatively abundant spectra of up-conversion emissions centered at 378, 408, 527 and 543, and 663 nm, corresponding to 4C9/2→ 6H13/2, 4C9/2→ 6Hll/2, 4115/2 → 6H13/2, and 4F9/2 →6Hll/2 transitions of Dy3＋, respectively. Two-photon and three-photon processes are involved in ultraviolet, violet, green, and red up-conversion emissions. The energy transition between Yb3＋ and Dy3＋ is discussed.

Synthesis and Characterization of IR Up-Conversion Material SrS∶Eu, Sm

Europium and samarium co-doped strontium sulfide (SrS∶Eu, Sm) infrared up-conversion phosphor was synthesized through calcining the precursor, which prepared by wet-method with strontium carbonate (SrCO_3), sulphur (S), europium oxide (Eu_2O_3) and samarium oxide (Sm_2O_3) as the starting materials, lithium fluoride (LiF) as the fluxing agent, at 750～1200 ℃ in carbon-reducing atmosphere. XRD analysis shows that SrS crystal structure is formed at 750 ℃, most completely at 1100 ℃. The IR up-conversion luminescence properties were characterized by excitation, emission, up-conversion excitation, up-conversion emission and thermoluminescence spectra. The spectral analysis associated with the physical model of up-conversion luminescence shows that the IR up-conversion luminescence is resulted from electron trapping process of Sm 3+ energy level. The thermoluminescence peak does not appear below 500 ℃ indicating the trapping energy level is appropriately deep, 800～1400 nm near infrared light can be the release light to realize up-conversion luminescence.

Hydrothermal synthesis and up-conversion luminescence of Ho^(3+)/Yb^(3+)co-doped CaF_2

CaF2：Ho3＋/Yb3＋ nano-particles with intense green up-conversion （UC） luminescence are successfully synthesized via a facile hydrothermal approach by using NH4F as the fluoride source and Na2EDTA as a chelating reagent. Powder X- ray diffraction （XRD）, transmission electron microscopy （TEM）, field emission scanning electron microscopy （FE-SEM）, and UC emission spectra are used to characterize the structures, shapes, and luminescent properties of the samples. The effects from fluoride sources and chelating reagents on the formations of CaF2 nano-particles are investigated, and the for- mation process is also deduced. Under the excitation of a 980-nm laser diode, the samples each show a green up-conversion emission centered at 540 nm corresponding to the 5S2/5F4-＋518 transitions of Ho3＋. Moreover, the UC mechanisms of Ho3＋/Yb3＋ co-doped CaF2 nano-particles are also discussed.

Up-conversion luminescence tuning in Er^3+ -doped ceramic glass by femtosecond laser pulse at different laser powers

The up-conversion luminescence tuning of rare-earth ions is an important research topic for understanding luminescence mechanisms and promoting related applications. In this paper, we experimentally study the up-conversion luminescence tuning of Er3+-doped ceramic glass excited by the unshaped, V-shaped and cosine-shaped femtosecond laser field with different laser powers. The results show that green and red up-conversion luminescence can be effectively tuned by varying the power or spectral phase of the femtosecond laser field. We further analyze the up-conversion luminescence tuning mechanism by considering different excitation processes, including single-photon absorption(SPA), two-photon absorption(TPA), excited state absorption(ESA), and energy transfer up-conversion(ETU). The relative weight of TPA in the whole excitation process can increase with the increase of the laser power, thereby enhancing the intensity ratio between green and red luminescence(I547/I656). However, the second ETU(ETU2) process can generate red luminescence and reduce the green and red luminescence intensity ratio I547/I656, while the third ESA(ESA3) process can produce green luminescence and enhance its control efficiency. Moreover, the up-conversion luminescence tuning mechanism is further validated by observing the up-conversion luminescence intensity, depending on the laser power and the down-conversion luminescence spectrum under the excitation of 400-nm femtosecond laser pulse. These studies can present a clear physical picture that enables us to understand the up-conversion luminescence tuning mechanism in rare-earth ions, and can also provide an opportunity to tune up-conversion luminescence to promote its related applications.

High-sensitive terahertz detection by parametric up-conversion using nanosecond pulsed laser

A high-sensitive terahertz detector operating at room temperature was demonstrated based on parametric upconversion.A nanosecond 1064-nm Nd:YAG laser was used to pump the parametric up-conversion detector and the upconversion from terahertz wave to NIR laser was realized in a lithium niobate crystal.The minimum detectable terahertz energy of 9 p J was realized with the detection dynamic range of 54 d B,which was three orders of magnitude higher than that of commercial Golay cell.The detectable terahertz frequency range of the detection system was 0.90 Thz–1.83 THz.Besides,the effects of pump energy and effective gain length on the detection sensitivity were studied in experiment.The results showed that higher pump energy and longer effective gain length are helpful for improving the detection sensitivity of parametric up-conversion detector.

A New Organic Up-conversion Laser Dye HEASPS

A new organic dye, trans-4-[p-(N-hydroxyethyl-N-ethylamino)styryl]-N-methyl- pyridinium p-toluene sulfonate (HEASPS) has been synthesized and its structure has been determined. Pumped with a 1064 nm, 50 ps laser pulses, 0.05 mol/L HEASPS/DMF solution showed the two-photon pumped (TPP) output/input efficiency of 9.1% which is higher than that of known TPP laser dyes trans-4-[p-(N-hydroxyethyl-N-methylamino) styryl]-N-methylpyridinium iodide (ASPI) and trans-4-[p-(N-hydroxyethyl-N-methylamino) styryl]-N-methylpyridinium p-toluene sulfonate (HMASPS) at the same experimental conditions.

Up-conversion luminescence properties and energy transfer of Er^(3+)/Yb^(3+)co-doped oxyfluoride glass ceramic containing CaF_2 nano-crystals

The Er^3＋/yb^3＋ co-doped transparent oxyfluoride glass-ceramics containing CaF2 nano-crystals were successfully prepared. After heat treatments, transmission electron microscopy （TEM） images showed that CaF2 nano-crystals of 20-30 nm in diameter precipitated uniformly in the glass matrix. luminescence of Er^3＋ at 540 nm and 658 nm was observed in Comparing with the host glass, high efficiency upconversion the glass ceramics under the excitation of 980 nm. Moreover, the size of the precipitated nano-crystals can be controlled by heat-treatment temperature and time. With the increase of the nano-crystal size, the intensity of the red emission increased more rapidly than that of the green emission. The energy transfer process of Er^3＋ and Yb^3＋ was convinced and the possible mechanism of Er^3＋ up-conversion was discussed.

Sensitized Up-Conversion Luminescence of Ho^(3+) in Nd^(3+)-Yb^(3+)-Ho^(3+) Co-doped ZrF_4-Based Glass

Up-conversion processes for the blue, green and red emissions were foundtwo-photon phenomenon, known as the cooperative phenomenon. This phenomenon was assisted by Nd^(3+)→ Yb^(3+) → Ho^(3+) energy transfer. The strong green emission due to the Ho^(3+) : (~5F_4, ~5S_2)→ ~5I_8 transitions was observed in Nd^(3+) - Ho^(3+) co-doped ZrF_4-based fluoride glasses under800 nm excitation. As an attempt to enhance Ho^(3+) up-conversion luminescences in the Nd^(3+) -Ho^(3+) co-doped ZrF_4-based glasses, Yb^(3+) ions were added to the glasses. As a result it wasfound that, in 800 nm excitation of 60ZrF_4. 30BaF2. (8-x)LaF_3. 1NdF_3. xYbF_3. 1HoF_3 glasses (x =0 to 7), sensitized up-conversion luminescences are observed at around 490 nm (blue), 545 nm(green), and 650 nm (red), which correspond to the Ho^(3+) : ~5F_3 → ~5I_8, ( ~5F_4, ~5S_2) →~5I_8 and ~5F_5 → ~5I_8 transitions respectively. The intensities of the green and red emissions ina 3 mol% YbF_3-containing glass were about 50 times stronger than those glasses without YbF_3. Thisis based on sensitization due to Yb^(3+) ions. In particular, the green emission was extremelystrong and the Nd^(3+) - Yb^(3+) - Ho^(3+) co-doped ZrF_4-based glasses have a high possibility ofrealizing a green up-conversion laser glass. In this paper the up-conversion mechanism in theglasses is discussed in detail.