Efficient Down-conversion Research Articles

Experimental simulation of spin-1 states by second-order type-II parametric down-conversion

In this paper, by using the second-order parametric down-conversion of the nonlinear crystal, the spin-1 state is simulated by the two-photon polarization entangled modes. Through adjusting the laser pulse power density, the efficiency of second-order parametric down-conversion is enhanced. The intensity of the spin-1 state is 0.5/s. The fidelity of the state is up to F=0.891±0.002, and the contrast is C=17.3. The results provide a new method for Stern-Gerlach measurement on the spin-1 system.

Origin and suppression of back conversion in a phase-matched nonlinear frequency down-conversion process

Back conversion is an intrinsic phenomenon in nonlinear frequency down-conversion processes. However, the physical reason for its occurrence is not well understood. Here, we theoretically reveal that back conversion is the result of a π-phase jump associated with the depletion of one interacting wave. By suppressing the idler phase jump through a deliberate crystal absorption, the back conversion can be inhabited, thus enhancing the conversion efficiency from the pump to the signal. The results presented in this Letter will further the understanding of nonlinear parametric processes and pave the way toward the design of highly efficient down-conversion systems.

Efficient near-infrared downconversion and energy transfer mechanism in Tb^4+-Yb^3+ co-doped NaYF_4 nanoparticles

Tb4+-Yb3+ co-doped NaYF4 nanoparticles (NPs) are prepared by sintering the as-synthesized NaYF4:Tb3+, Yb3+ NPs at 380°C under air atmosphere. The oxidization of Tb3+ ions to Tb4+ ions in NaYF4 NPs after sintering is demonstrated through X-ray photoelectron spectroscopy (XPS). The near-infrared (NIR) downconversion (DC) luminescence of Tb4+-Yb3+ couple is measured and investigated for the first time. The results show that DC luminescence of Tb4+-Yb3+ couple enhance obviously compared with Tb3+-Yb3+ couple in as-synthesized sample. The enhancement factor is about 14 and 19 excited at 379nm and 487nm, respectively. On analyzing the exponential dependence of NIR fluorescence intensity on the pumping power, we reveal that the energy transfer (ET) mechanism from Tb4+ to Yb3+ in NaYF4 NPs occurs by the single-step ET process. Our study may provide a promising DC layer on the top of silicon-based solar cells to improve the photovoltaic conversion efficiency.

Multi-photon quantum cutting in Gd2O2S:Tm3+ to enhance the photo-response of solar cells

Conventional photoluminescence (PL) yields at most one emitted photon for each absorption event. Downconversion (or quantum cutting) materials can yield more than one photon by virtue of energy transfer processes between luminescent centers. In this work, we introduce Gd2O2S:Tm3+ as a multi-photon quantum cutter. It can convert near-infrared, visible, or ultraviolet photons into two, three, or four infrared photons of ∼1800 nm, respectively. The cross-relaxation steps between Tm3+ ions that lead to quantum cutting are identified from (time-resolved) PL as a function of the Tm3+ concentration in the crystal. A model is presented that reproduces the way in which the Tm3+ concentration affects both the relative intensities of the various emission lines and the excited state dynamics and providing insight in the quantum cutting efficiency. Finally, we discuss the potential application of Gd2O2S:Tm3+ for spectral conversion to improve the efficiency of next-generation photovoltaics. Tm3+:Gd2O2S downconverts high-energy photons to infrared photons, making it promising for extending the spectral responses of solar cells. Conventional photoluminescence produces one photon per absorption event. Now, researchers at South China University of Technology and Utrecht University in the Netherlands have experimentally shown that the downconversion material Gd2O2S:Tm3+ respectively converts near-infrared, visible, and ultraviolet photons to two, three, and four infrared photons with a wavelength of about 1800 nanometres. By evaluating emission spectra and dynamics for various Tm3+ concentrations, they determined the rates and efficiencies of the cross-relaxation processes involved in four-photon quantum cutting. Based on this analysis, the researchers discuss the practical application of this material for enhancing the spectral response of solar cells and hence boost their efficiency. They also note that the efficient downconversion of Tm3+ may be useful for bio-imaging applications.

Dual Mode Emission In NaYF4:Yb Er Nanorods For Photovoltaics Application

In the present study, synthesis, characterization, of dual mode emission (Up and downconversion) in lanthanide-doped NaYF4 nanorods is presented. The unique and rich energy-level structure of these lanthanide ions allows for efficient spectral conversion, including up and downconversion processes mediated by resonant energy transfer between neighboring lanthanide ions. The careful examination of nanorods indicates the perfect uniformity, monodispersity and well defined crystallographic facets. Er 3 + , Yb 3 + doped NCs exhibit green/red dual mode emission with low power density. As solar cells have majority of losses due to spectral mismatch, so with the help of these dual mode emission of nanorods can be utilized as up/down converter to increase the efficiency solar cell.

Enhancement of Luminescence of YPO<sub>4</sub>:Eu<sup>3+</sup>,Yb<sup>3+</sup> Nanophosphor Synthesized by Hydrothermal Method

YPO4 nanowires (NWs) and nanoparticles (NPs) have been synthesized by hydrothermal methods respectively in the absence and presence of EDTA. The Eu3+ and Yb3+ codoped YPO4 NWs and NPs prepared directly from hydrothermal systems show efficient red down-conversion luminescence (DL) and upconversion luminescence (UL) at room temperature without further annealing process. EDTA plays a critical role in controlling the morphologies of YPO4 nanocrystals in hydrothermal systems and enhances both the DL and UL intensities of Eu3+ by the factor of 100%. Monodispersed Eu3+ and Yb3+ codoped YPO4 NPs will be a new candidate for application in infrared displays and biological labels.

Efficient near-infrared downconversion and energy transfer mechanism of ce(3+)/yb(3+) codoped calcium scandate phosphor.

An efficient near-infrared (NIR) downconversion has been demonstrated in CaSc2O4: Ce(3+)/Yb(3+) phosphor. Doping concentration optimized CaSc2O4: 1%Ce(3+)/5%Yb(3+) shows stronger NIR emission than doping concentration also optimized typical YAG: 1%Ce(3+)/5%Yb(3+) under 470 nm excitation. The NIR emission from 900 to 1100 nm is enhanced by a factor of 2.4. In addition, the main emission peak of Yb(3+) in the CaSc2O4 around 976 nm matches better with the optimal spectral response of the c-Si solar cell. The visible and NIR spectra and the decay curves of Ce(3+): 5d → 4f emission were used to demonstrate the energy transfer from Ce(3+) ions to Yb(3+) ions. The downconversion phenomenon has been observed under the direct excitation of Ce(3+) ions. On analyzing the dependence of energy transfer rate on Yb(3+) ion concentration, we reveal that the energy transfer (ET) from Ce(3+) ions to Yb(3+) ions in CaSc2O4 occurs mainly by the single-step ET process. Considering that the luminescence efficiency of CaSc2O4: Ce(3+) is comparable to that of commercial phosphor YAG: Ce(3+), the estimated maximum energy transfer efficiency reaches 58% in the CaSc2O4: 1%Ce(3+)/15%Yb(3+) sample, indicating that CaSc2O4: Ce(3+)/Yb(3+) sample has the potential in improving the conversion efficiency of c-Si solar cells.

Antireflective downconversion ZnO:Er3+,Yb3+ thin film for Si solar cell applications

Hexagonal wurtzite phased ZnO:Er3+/Yb3+ thin films with various Yb concentrations were deposited on Si(111) substrate by Aerosol Assisted Chemical Vapor Deposition process. Post-annealed films at 1000 °C in air atmosphere showed a crystallinity enhancement. Yb3+ (4F7/2 → 4F5/2) 1000 nm emission increased with the increase of Yb3+ concentration emanating from an Er-Yb energy transfer. The reflectance percentage of 12% was achieved in the [250–1000 nm] range, and the refractive index of 1.97 was obtained for 632 nm wavelength. These results suggest that the (3 mol. % Er, 9 mol. % Yb) codoped film is a highly efficient antireflective downconversion layer for enhancing Si solar cell efficiency.

Continuous-wave 1.57 m/3.84 m intra-cavity multiple optical parametric oscillator based on MgO:APLN

Continuous-wave (CW) coherent sources emitting two wavelengths of 1.57 m and 3.84 m have aroused much interest of scientists due to their many applications such as military multiband composite guidance, remote monitoring of the special environment, etc. Quasi-phase matching (QPM) optical parametric oscillator (OPO) device with periodically inverted structure of nonlinear coefficient can implement an efficient and wavelength conversion at arbitrary wavelength in the transparent range of the QPM material. Nowadays, using MgO:PPLN for QPM, various MgO:PPLN-OPOs pumped by conventional 1.06 m laser source can produce 1.57 m and 3.84 m laser and also achieve good results. But as a result of the limitation of momentum conservation condition and periodically poled structure, 1.57 m and 3.84 m laser can only meet a single band. To obtain the two-wavelength laser output at the same time, the MgO:PPLN-OPO could not be applied. In this paper, a CW 1.57 m and 3.84 m intra-cavity multiple optical parametric oscillator based on MgO:APLN is reported. The cross period parameter light is obtained by using a folded type doubly cavity which consists of 1064 nm resonator and multiple optical parametric oscillator. Considering both its thermal stability under high power pump and the light spot mode matching of multiple optical parametric oscillation process, through numerical simulation and theoretical analysis of two sub cavities, the optimum parameters of the cavity structure are determined. On this basis, the influences of output coupler transmittance on oscillation threshold, the down-conversion efficiency, output power stability are investigated in experiment. With T=10% at 1.47 m and 3.3 m output coupler used, the maximum output powers of 3.13 W at 1.57 m and 0.85 W at 3.84 m are obtained, corresponding to slope efficiencies of 6.8% and 1.9%, respectively. The power stabilities are better than 1.8% and 3% at the maximum output power in half an hour. The experimental results show that the intra-cavity multiple optical parametric oscillator based on a single poled crystal MgO:APLN is an effective method of obtaining a 1.57 m and 3.84 m CW laser.

Mirrorless optical parametric oscillators with stitching faults: backward downconversion efficiency and coherence gain versus stochastic pump bandwidth

Parametric downconversion in submicronic periodically poled (PP) quadratic materials allows for the generation of signal and idler waves in opposite directions. The distributed feedback mechanism enables a backward mirrorless optical parametric oscillator (BMOPO), which has been experimentally realized using a periodically poled KTiOPO (PPKTP) crystal with 800 nm periodicity. A remarkable spectral property of the BMOPO is that the bandwidth of the forward co-propagating wave is comparable to the pump bandwidth, whereas that of the backward wave is typically several orders of magnitude narrower. In the backward idler configuration realized in PPKTP the backward idler exhibits a coherence gain of two orders of magnitude. In this paper we will consider the backward signal configuration, where the effect of coherence enhancement of the backward signal wave is especially pronounced for exact group velocity matching of the co-propagating pump and idler waves. This quasi-phase-matched (QPM) scheme requires a 335 nm scale poling periodicity, which may be achieved in GaN waveguides, combining e-beam lithography and epitaxy. However, during the lithography step necessary to obtain the PP material, stitching faults may occur. The BMOPO device is then formed by a sequence of 200 micron long PP elements stuck together. We assume that the stitching faults do not exceed half of the poling period in order to prevent the parametric reverse process. The original three-wave QPM model with the explicit periodicity of the nonlinear parametric coupling is necessary for taking into account the periodicity faults in the junctions. Thanks to the high performance computing facility available at Nice University, we analyze the parametric downconversion efficiency and the coherence gain of the backward signal versus the stochastic pump bandwidth in a large pump-bandwidth range. We show that if the stitching faults do not exceed a quarter of the poling period, i.e., less than 80 nm, which is technically feasible, the efficiency is comparable to the BMOPO in the perfect PP material. In any case, for an incoherent pump of broad spectral bandwidth we obtain a coherence gain of almost three orders of magnitude.

Microwave down-conversion with an impedance-matched Λ system in driven circuit QED.

By driving a dispersively coupled qubit-resonator system, we realize an "impedance-matched" Λ system that has two identical radiative decay rates from the top level and interacts with a semi-infinite waveguide. It has been predicted that a photon input from the waveguide deterministically induces a Raman transition in the system and switches its electronic state. We confirm this through microwave response to a continuous probe field, observing near-perfect (99.7%) extinction of the reflection and highly efficient (74%) frequency down-conversion. These proof-of-principle results lead to deterministic quantum gates between material qubits and microwave photons and open the possibility for scalable quantum networks interconnected with waveguide photons.

Efficient near-infrared down-conversion in KCaGd(PO_4)_2:Ce^3+,Yb^3+

Abstract: A novel near-infrared (NIR) quantum cutting KCaGd(PO4)2:Ce3+,Yb3+ phosphor was successfully developed. Due to the cooperative energy transfer from one Ce3+ to two Yb3+, an intense NIR emission around 1021 nm of Yb3+:2F5/2-2F7/2 transition was obtained under 324 nm excitation. Yb3+ concentration dependent quantum efficiency has been calculated and the theoretical maximum efficiency approaches up to 158.2%. Because the emission of Yb3+ around 1021 nm is matched with the band gap of crystalline Si, the phosphors could be a potential candidate for silicon-based solar cells.

Near-infrared down-conversion in Pr3+–Yb3+ co-doped transparent glass ceramic containing Ca5(PO4)3F nanocrystals

Pr3+–Yb3+ co-doped transparent oxyfluoride glass ceramics containing Ca5(PO4)3F nanocrystals were synthesized by a melting-quenching method. Spectral analyses demonstrate that Pr3+ and Yb3+ ions are both incorporated into the Ca5(PO4)3F nanocrystals. Near-infrared down-conversion around 1000nm was observed when the glass ceramics were excited at 435nm. The maximum actual down-conversion efficiency for 0.3mol% Pr3+–0.8mol% Yb3+ co-doped glass ceramic is about 125%.

Octave-spanning coherent mid-IR generation via adiabatic difference frequency conversion

We demonstrate efficient downconversion of a near-IR broadband optical parametric chirped pulse amplifier (OPCPA) pulse to a 1.1-octave-spanning mid-IR pulse (measured at -10 dB of peak) via a single nonlinearly and adiabatically chirped quasi-phase-matching grating in magnesium oxide doped lithium niobate. We report a spectrum spanning from 2 to 5 μm and obtained by near full photon number conversion of μJ-energy OPCPA pulses spanning 680-870 nm mixed with a narrowband 1047-nm pulse. The conversion process is shown to be robust for various input broadband OPA pulses and suitable for post-amplification conversion for many near-IR systems.

Broadband down-conversion in Bi3+–Yb3+-codoped yttrium and yttrium–aluminum oxides

Comparative study of the broadband down-conversion processes in Bi3+–Yb3+-codoped yttrium oxide (Y2O3) and various yttrium–aluminum oxides (Y3Al5O12, YAlO3 and Y4Al2O9) has been performed from the point of view of search for materials suitable for enhancement of efficiency of silicon solar cells. The studied materials in the form of nanopowders have been synthesized by sol–gel method and characterized by X-ray powder diffraction, scanning electron microscopy and luminescence techniques. Relative down-conversion efficiency for studied materials has been estimated. It was shown that optimal concentration of Yb3+ ions should be 2–4 at.% simultaneously with Bi3+ ions in the amount of about 1 at.%. Such dopants content provides the highest emission intensity of Yb3+ ions in near-infrared when excited into Bi3+ ions absorption in ultraviolet. Perspectives of the studied materials for enhancement of silicon solar cells are discussed.

Near-infrared luminescence and energy transfer in CaMoO4: Ho3+, Yb3+ phosphor

Down-conversion (DC) is a promising avenue to break Shockley–Queisser detailed balance limit, and efficient DC from one ultraviolet visible photon to two near-infrared (NIR) photons is realized in the Ho3+–Yb3+ co-doped CaMoO4 system. The scheelite samples have been synthesized via solid-state reaction method. Upon blue light excitation (447 nm), Ho3+ transfers its energy to Yb3+ via resonant cross-relaxation process, bringing about NIR luminescence. The visible emissions decrease as a function of Yb3+ contents, while NIR emission at 996 nm first increases and then decreases due to CQ at Yb3+ contents beyond 15 mol %. The lifetime of Ho3+: 5F4, 5S2 → 5I8 with various Yb3+ contents is calculated as 114, 85, 73, 70, 64, 59 and 55 μs, respectively. The maximum energy transfer efficiency and the corresponding DC quantum efficiency are estimated to be 48 and 148 %. The phosphor may be a potential candidate for application in enhancing the performance of crystalline silicon solar cells.

Single-pass parametric generator made of DFB diodes, specialty-fiber amplifier, and periodically-poled lithium niobate

A compact pulsed mid-IR source with 10-ns pulsewidth, few hundred-microjoule energy, and repetition rate adjustable from 10 through 500 kHz is reported. In order to reach up to 30-% efficiency of down-conversion from 1064 nm to 1500/3500 nm, a 1064-nm narrow-line master oscillator is followed by a two-stage fiber amplifier. In turn, this amplifier is spliced with a large-mode-area photonic crystal fiber to ensure high quality of the beam that pumps a 5-mm thick periodically poled MgO-doped lithium niobate. The spectrum of the device replicates that of a cw 1500-nm seed laser.

Fabrication of warm, high CRI white LED using non-cadmium quantum dots

We reported on the synthesis of two bright non-cadmium quantum dots (QDs) of green (512 nm)-emitting InP/ZnS and orange (583 nm)-emitting CuInS2 (CIS)/ZnS core/shell and their application for the fabrication of solid-state lighting device. The spectral overlap between absorptions from both QDs and emission from InGaN-based blue light-emitting diode (LED) chip was excellent. Thus, the efficient down-conversion of blue-to-QD emission for the generation of white light was accomplished by sequentially dispensing InP/ZnS QDs on top of CIS/ZnS ones within epoxy resin. The white QD-LED generated a high-quality white light with a high color rendering index of 90 and a warm color temperature of 3803K under a drive current of 20 mA. Drive current-dependent variations of its primary electroluminescent properties were investigated in details.

Absolute instrument spectral response measurements using angle-resolved parametric fluorescence

The broadband parametric fluorescence from a nonlinear crystal can be used as a compact primary source instead of a blackbody for absolute measurements of instrument spectral efficiency. We describe such a setup for measuring the instrument spectral response function in the wavelength range from 450 to 1000 nm. We perform angle-resolved imaging spectroscopy of conical parametric fluorescence in a beta-barium borate crystal pumped by a 405-nm diode laser. The experimental angle-resolved spectra and the generation efficiency of parametric down-conversion agree with a plane-wave theoretical analysis.

Continuous-wave intra-cavity singly resonant optical parametric oscillator with resonant wave output coupling

We report herein the enhancement in both power and efficiency performance of a continuous-wave intra-cavity singly resonant optical parametric oscillator (ICSRO) by introducing finite resonant wave output coupling. While coupling out the resonant wave to useful output, the output coupling increases the SRO threshold properly thus suppresses the back-conversion under high pump power. Therefore, the down-conversion efficiency is maintained under high pump without having to raise the threshold by defocusing. With a T = 9.6% signal wave output coupler used, the SRO threshold is 2.46 W and the down-conversion efficiency is 72.9% under the maximum pump power of 21.4 W. 1.43 W idler power at 3.66 μm and 5.03 W signal power at 1.5 μm are obtained, corresponding to a total extraction efficiency of 30.2%. The resonant wave out coupling significantly levels up the upper limit for the power range where the ICSRO exhibits high efficiency, without impeding its advantage of low threshold.