Quantum Conversion Efficiency Research Articles

High-efficiency mid-infrared CW-seeded optical parametric generation in PPLN waveguide

Mid-infrared optical sources have been extensively used in a variety of applications, including gas sensing and metrology, by exploiting the fingerprint fundamental vibrational absorption bands of molecules in this spectral region. Parametric frequency conversion techniques, such as optical parametric generation (OPG) and optical parametric oscillator (OPO), represent common methodologies for mid-infrared generation. Notably, continuous-wave (CW)-seeded OPG exhibits superior stability and performance compared to conventional OPG. Here, we present a simple method for mid-infrared source generation based on femtosecond OPG in periodically poled lithium niobate (PPLN) waveguides. In addition to its robustness and simplicity, mid-infrared CW-seeded OPG features a high quantum conversion efficiency of 46.5% and a low threshold. The phase of CW-seeded OPG was passively referenced to the CW laser, making it suitable for dual-comb applications. This system shows great promise in the miniaturization of mid-infrared combs and will be applied to non-laboratory applications, including open-path atmospheric gas sensing and portable environmental monitoring.

An Updated Review for Performance Enhancement of Solar Cells by Spectral Modification

Photovoltaic technology has become one of the major renewable ways to generate electric power. However, the mismatch between the incident solar spectrum and photo-electric response efficiency of solar cells severely constrains their performance. Hence, spectral modification technologies, e.g., up-conversion (UC), down-conversion (DC), and luminescent down-shifting (LDS) technologies have been applied widely in the photovoltaic field to reform the incident spectrum to match the best response band possible. In this paper, we review the latest developments of the three technologies above in terms of material selection, optical characteristics, and photovoltaic performance. It is found that the three most popular materials for conversion are NaYF4: Er3+, Yb3+, and Yb3+. The excitation bands for the three technologies are 800–1550 nm, 250–488 nm, and 250–488 nm, respectively, while the emission bands are 523–669 nm, 520–1031 nm, and 490–1010 nm, respectively. Furthermore, issues hindering the development of spectral modification technologies are pointed out, e.g., low absorption efficiency, poor quantum conversion efficiency, and hurdles in commercialization. Finally, suggestions and solutions to address the above-mentioned issues are provided.

Highly efficient octave-spanning long-wavelength infrared generation with a 74% quantum efficiency in a χ(2) waveguide

The realization of compact and efficient broadband mid-infrared (MIR) lasers has enormous impacts in promoting MIR spectroscopy for various important applications. A number of well-designed waveguide platforms have been demonstrated for MIR supercontinuum and frequency comb generations based on cubic nonlinearities, but unfortunately third-order nonlinear response is inherently weak. Here, we propose and demonstrate for the first time a χ(2) micrometer waveguide platform based on birefringence phase matching for long-wavelength infrared (LWIR) laser generation with a high quantum efficiency. In a ZnGeP2-based waveguide platform, an octave-spanning spectrum covering 5–11 μm is generated through optical parametric generation (OPG). A quantum conversion efficiency of 74% as a new record in LWIR single-pass parametric processes is achieved. The threshold energy is measured as ~616 pJ, reduced by more than 1-order of magnitude as compared to those of MIR OPGs in bulk media. Our prototype micro-waveguide platform could be extended to other χ(2) birefringence crystals and trigger new frontiers of MIR integrated nonlinear photonics.

Generation of terahertz waves based on nonlinear frequency conversion with double rapid adiabatic passage technique

In this paper, a nonlinear optical cascaded difference frequency generation model based on double rapid adiabatic passage technique is established, and a theoretical scheme for generating THz waves based on the above model is proposed. In this model, when the incident signal laser interacts with a pump laser, the signal laser can be completely converted into the output laser by special processing of the coupling wave equation and making reasonable assumptions. Numerical simulation results show that THz waves with a centre frequency of 260 GHz can be obtained. The maximum quantum conversion efficiency of the signal laser to THz waves is about 43.4%. Under the premise of keeping the wavelength of the pump laser unchanged, the tunable THz waves of 0.26–2.94 THz can be obtained when tuning the wavelength of the signal laser to change in the range of 1.054–1.064 μm. Compared with the scheme using stimulated Raman adiabatic passage technique, the scheme can still generate terahertz waves during the application of a pump laser to two simultaneous difference frequency generation processes, and the intensity of the pump laser can be reduced from Gigawatt level to Megawatt level. This scheme is robust to the temperature variation and provides a new method for generating terahertz wave band for high-speed wireless transmission.

High-Performance Planar Field-Emission Photodetector of Monolayer Tungsten Disulfide with Microtips.

Monolayer tungsten disulfide (ML WS2 ) is believed as an ideal photosensitive material due to its small direct bandgap, large exciton/trion binding energy, high carrier mobility, and considerable quantum conversion efficiency. Compared with other photosensitive devices, planar field emission (FE)-type photodetectors with a full-plane structure should simultaneously have rapider switching speed and lower power consumption. In this work, ML WS2 microtips are fabricated by electron beam lithography (EBL) way and used to construct a planar FE-type photodetector. By optimization design, ML WS2 with three microtips can exhibit the maximum current density as high as 52 A cm-2 (@300V µm-1 ), and the largest photoresponsivity is up to 6.8 × 105 A W-1 under green light irradiation, superior to that of many other ML transition metal dichalcogenide (TMDC) detectors. More interestingly, ML WS2 devices with microtips can effectively solve the contradictory problem between large photoresponsivity and rapid switching speed. The excellent photoresponse performances of ML WS2 with microtips should be attributed to their high carrier mobility, sharp emission edge, ultrahigh quantum yield, and unique planar FE device structure. Ourresearch may shed new light on exploring the fabrication technology and photosensitive mechanism of two dimensional (2D) material-based planar FE photodetectors.

Effect of Bismuth Doping on Noise Figure of Er-Doped Silica Fiber

We investigate the spectral and amplification characteristics of Bi/Er co-doped fiber (BEDF) and Er-doped fiber (EDF), prepared by atomic layer deposition (ALD). BEDF features a noise figure (NF) below 5.2 dB, the bandwidth with NF less than 4 dB is approximately 23 nm (1560–1583 nm), and that with a gain exceeding 25 dB is approximately 42 nm (1525–1567 nm). The quantum conversion efficiency (QCE) of BEDF at 1550 nm is 72.1% at a pump power of 300 mW. The saturated output power of BEDF at 1550 nm is 21.4 dBm. Raman spectra indicate that the maximum phonon energy of BEDF is lower than that of EDF. The fluorescence lifetime of the BEDF is 11.64 ms, which is longer than that of the EDF. These results show that co-doping with Bi ions could increase the fluorescence lifetime of Er <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> , reduce the NF, and improve the QCE and saturated output power. Therefore, this type of BEDF is a promising gain medium for optical amplifiers and lasers.

Upconverting Near‐Infrared Light Detection in Lead Halide Perovskite with Core–Shell Lanthanide Nanoparticles

Light detection in the near‐infrared (NIR) region is necessary in a wide range of optical applications, such as optical communication, medical diagnostics, and environmental analysis. Herein, a novel NIR detection method based on energy conversion by upconversion (UC) materials is demonstrated. Nanoparticles (NPs) containing erbium (Er) ions are used as the UC material. Conventional NPs that contain ≈2 mol% Er ions have very weak light absorption ability, which is dramatically improved by incorporating high concentrations of Er ions (≈100 mol%). By coating the NP surfaces with inorganic perovskite containing heavy atoms (CsPbBr3) instead of organic alkyl chains, the thermal vibrations that deactivate the UC emission are suppressed effectively, resulting in high UC emission quantum efficiency of &gt;5%. The perovskite coating process facilitates the introduction of the NPs into CsPbI3 thin films (which absorbs visible light only) and forms highly crystalline and uniform thin films. The resulting CsPbI3 thin film can be excited with NIR light, showing the emission from CsPbI3 via the UC. This enables fabrication of a UC‐based photodetector generating a photocurrent signal through the energy conversion process from NIR to visible light. The photodetector shows a high responsivity to weak NIR light with the quantum conversion efficiency of 75%.

2.76–3.98 μm picosecond mid-infrared optical parametric generation in a muti-grating MgO: PPLN

The high repetition rate, widely tunable, picosecond mid-infrared laser plays an important role in various fields. In this paper, the single-pass optical parametric generator (OPG) pumped by Yb-fiber laser based on MgO: PPLN is proposed. The Yb-fiber laser is mode locked by a nonlinear amplifying loop mirror (NALM) with an all-polarization-maintaining (PM) structure. A maximum power of 6.2 W was obtained with a repetition rate of 15.8 MHz and pulse duration of 18 ps after amplification by cascaded fiber. Based on the principle of nonlinear frequency down-conversion, the tunable mid-infrared output wavelength of 2.76–3.98 μm was realized by varying the crystal grating periods and temperature. The maximum power was 500 mW at 3.22 μm, corresponding to a quantum conversion efficiency of 25%, and the relative power fluctuation was measured as 0.37% over 30 min.

Generation of terahertz waves based on nonlinear frequency conversion with stimulated Raman adiabatic passage.

In recent years, high-power, tunable terahertz (THZ) radiation sources have become the key areas of research in the world. The method of THZ waves by nonlinear optical difference frequency generation (DFG) has the advantages of wide tuning, high power, room temperature operation, and compact structure. However, the conversion efficiency of the current difference frequency method is low, which needs a trade-off between conversion efficiency and tuning range. We apply the nonlinear optical cascade difference frequency conversion theory based on stimulated Raman adiabatic passage (STIRAP) and propose a theoretical scheme to generate THZ waves. Numerical simulation investigates the cascaded difference frequency process of generating THZ waves with the help of the nonlinear medium lithium niobate (LN) crystal. The theoretical analysis shows that the maximum quantum conversion efficiency from signal laser to THZ waves is 43.2 % when the wavelength of the tuned signal laser varies between 1.044 - 1.065 µm with the fixed two pump laser wavelengths constant. The tunable THZ waves of 0.48 - 5.0 THz can be obtained and the maximum output intensity of THZ waves is 2.17 MW/cm2, and the method is robust to temperature variations. It also provides a novel idea for the cascaded difference frequency generation of THZ waves.

Ultrabroad (3.7-17 µm) tunable femtosecond optical parametric amplifier based on BaGa4Se7 crystal.

We demonstrate the first (to the best of our knowledge) tunable femtosecond (fs) mid-infrared (MIR) optical parametric amplifier (OPA) based on BaGa4Se7 (BGSe) crystal with an ultra-broadband spectral range. Benefiting from the broad transparency range, high nonlinearity, and relatively large bandgap of BGSe, the MIR OPA pumped at 1030 nm with a repetition of 50 kHz has an output spectrum that is tunable across an extremely wide spectral range spanning from 3.7 to 17 µm. The maximum output power of the MIR laser source is measured as 10 mW at a center wavelength of 16 µm, corresponding to a quantum conversion efficiency of 5%. Power scaling is straightforwardly achieved by using a stronger pump in BGSe with an available large aperture size. A pulse width of 290 fs centered at 16 µm is supported by the BGSe OPA. Our experimental result indicates that BGSe crystal could serve as a promising nonlinear crystal for fs MIR generation with an ultra-broadband tuning spectral range via parametric downconversion for applications such as MIR ultrafast spectroscopy.

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.

The role and effects of Ru1−xTixO2 solid solution transition layer in Ru/TiO2 composite photocatalyst by DFT calculations

In order to more clearly analyze the effects of Ru cocatalyst on TiO2 photocatalyst, two typical Ru1−xTixO2 solid solutions were selected as the transition layer of Ru/TiO2 hetero-structure for further in-depth analysis and discussion in this work, based on our previous works. The interface mismatch and interface formation energy between Ru1−xTixO2 solid solution transition layer and TiO2 layer or RuO2 layer are lower. Therefore, the introduction and existence of Ru1−xTixO2 solid solution transition layer can reduce the interface stress and imbalance of abrupt-type RuO2/TiO2 hetero-structure and make it more stable. By bridging oxygen atoms on the interface, electron transfer occurs between Ru1−xTixO2 solid solution transition layer and TiO2 layer or RuO2 layer, so as to form a smoother electrostatic potential distribution between them. More importantly, the pseudo-bandgap value of the selected Ru1−xTixO2 solid solution transition layer is between the bandgap values of TiO2 or RuO2, which can form a transition band offset between them. This effect reduces the unfavorable large band offset of abrupt-type RuO2/TiO2 hetero-structure. The above effects illustrate that the introduction of Ru1−xTixO2 solid solution transition layer can ensure the effective synergy of Ru cocatalyst, so as to significantly improve the quantum conversion efficiency of Ru/TiO2 composite photocatalyst.

Significant Fluorescence Enhancement through Rapid Laser Annealing and Nonthermal Coupling Optical Temperature Sensing of Er-Doped Yttria Nanocrystals

Rare-earth-ion-doped nanocrystals have low quantum conversion efficiency due to their high surface-to-volume ratios and many surface defects, which induce fluorescence quenching. We propose a laser annealing strategy that uses a 974.5 nm laser with a high power density to heat Er3+-doped Y2O3 nanocrystals for several seconds. As a result, the upconversion of Er3+-doped Y2O3 nanocrystals is enhanced obviously after laser annealing: the green emission is enhanced by 90 times and red emission by 24 times. The main reason for the upconversion enhancement is the ultrahigh temperature caused by laser annealing, which increases the grain size and reduces the surface defects. It is an encouraging result, and laser annealing is suitable for various rare-earth-ion-doped oxide nanoparticles. Then, the strong upconversion is applied to measure temperature. After laser annealing, the highly improved fluorescence efficiency decreases the required excitation power density. The thermal effect of excitation light is negligible when the excitation power density is less than 0.13 W/mm2. The thermally coupled levels of I538/I563 and nonthermally coupled levels of I661/I800 are studied for thermometry. The experimental results show that I661/I800 has high absolute sensitivity than I538/I563 at a temperature below 400 K. Rate equations and multiphonon transitions are applied to explain the relationship between the nonthermally coupled fluorescence intensity ratio and temperature. It is instructive for the design of new temperature sensors.

Artificial foliage with remarkable quantum conversion efficiency in bicarbonate to formate

An efficient heterogeneous immobilized device for the photoreduction of bicarbonate to formate was achieved by a thin-film semiconductor with metallophthalocyanines.

A strategy for integrating transition metal-complex cocatalyst onto g-C3N4 to enable efficient photocatalytic hydrogen evolution

Achieving efficient and stable solar-to-hydrogen conversion is of great significance to the sustainable development of global energy. Herein, we report a novel strategy for metal functionalization via in-situ construction of metal coordination sites in graphitic carbon nitride (CN), in which pyrimidinone derivative ligands are incorporated onto CN framework through one-step vapor diffusion process, and then transition metal ions (such as Ni, Cu, Co) are covalently immobilized by strong coordination interaction. The coordination-introduced metal ions not only serve as catalytic reaction sites, but also improve the utilization of visible light through the metal ion to ligand charge transfer (MLCT) mechanism. Specially, the strong interaction between metal ion and ligand promotes charge separation by expanding the π-conjugated delocalization system of CN, which makes it possible to continuously generate hydrogen without noble metals. The optimal sample Ni/DBM(0.12%)-CN shows a hydrogen evolution rate of 68.6 μmol h−1 (λ ≥ 420 nm) and a remarkable quantum conversion efficiency of 5.57% under irradiation of the light with wavelength of 450 nm. The mechanism of high-activity hydrogen production was studied through both experiments and theoretical calculations. This study provides a new strategy for designing cost-effective CN photocatalyst with alternative metal coordination sites for efficient visible light utilization.

Cr3+ Activated Garnet Phosphor with Efficient Blue to Far‐Red Conversion for pc‐LED

AbstractFar‐red (FR) phosphor‐converted light‐emitting diodes (pc‐LEDs) driven by blue LED chips are novel light sources for applications in phototherapy, photosynthesis, natural light simulation, and so on but still bear low electricity‐to‐FR light conversion efficiency (≤ 23%) due to deficiency of phosphors that have both high blue light absorptance and high FR internal quantum efficiency (IQE) for the realization of efficient blue to FR conversion. FR Ca3Sc2Si3O12 (CSSG):Cr3+ garnet phosphor has ultra‐high IQE (&gt; 90%) but low blue light absorption. Here, a twofold increase of blue light absorption cross‐section is reported with still high FR IQE via Lu3+−Mg2+ substitution for Ca2+−Sc3+ in CSSG:Cr3+ to form Ca3–xLuxMgxSc2–xSi3O12:Cr3+ garnet solid solution, which is attributed to enhanced mixing of odd‐parity configuration into 3d configuration of Cr3+ induced by the cations substitution. Optical properties, crystal field strengths, and thermal stabilities of the solid solution phosphors are studied as a function of x. After complete replacement of Sc3+, the as‐fabricated FR pc‐LED emits at 750 nm with 65.7% blue‐to‐FR quantum conversion efficiency and consequently offers an electricity‐to‐FR light conversion efficiency over 30%. Moreover, the solid solution phosphors show high quenching temperatures in the range of 580–610 K. The results can advance the development of efficient FR phosphors and pc‐LEDs for various applications.

Thermally stable laser-driven phosphor converted white light source using multilayer structured diffuser system

We report the development of a thermally stable laser-driven phosphor-converted white light source using a multilayer structured diffuser system for general illumination. The developed extended diffuser system comprises an acrylic sheet, two glass plates, and a phosphor layer with the arrangement in a glass-acrylic-phosphor-glass combination. The high-power laser beam was gradually distributed inside the diffuser system for exciting the phosphor layer optimally without burning the resin and thermal quenching. The method is simple and efficient in which both the directionality and scattering properties of laser source are exploited using the transmission properties of diffusers. The high specular transmission of diffusers resulted in the optimum transfer of laser directional photons for exciting the middle section of phosphor layer, and the remaining laser photons were scattered from diffuser surfaces. The glass-acrylic combination results the refractive index inhomogeneity inside the system and helped to expand the scattered photons throughout the phosphor layer. The radiant power distribution of blue laser source inside the system resulted in synchronization between optimal absorption quantum efficiency and fluorescent conversion quantum efficiency of the phosphor layer. As a result, the thermal quenching effect on phosphor inside the white light system was minimized for efficient illumination. With this geometry, a bright and thermally stable white light source was realized due to the proper distribution of high-power laser throughout the system. The luminescence properties of the proposed laser-based white light system were examined and found thermally stable in terms of spectral changes, luminance, and correlated color temperature.

On the Use of Brillouin Scattering to Evaluate Quantum Conversion Efficiency in Yb-doped Optical Fibers

A method, based on Brillouin scattering, to evaluate the quantum conversion efficiency (QCE) in Yb-doped optical fibers is designed and tested. An adapted version of the heterodyne approach to measuring the Brillouin spectrum is first used to precisely quantify the temperature change of the fiber upon pumping. Next, a theoretical model based on the thermal conduction equation is constructed in order to simulate the temperature change assuming a QCE of one. Lastly, the QCE value of the fiber can be determined by comparing the experimental and simulation results. Importantly, the analysis can provide insight into the origin of thermal energy in a fiber. Serving in the proof-of-concept measurement, a commercial, lightly Yb-doped fiber is determined to have a QCE of 99.07%. Finally, this result is compared with slope efficiency measurements from a linear cavity laser using the same fiber as the active medium. Excellent agreement between the two measurements demonstrates the feasibility of the approach, with greater accuracy afforded by the Brillouin-based measurement. The proposed method is particularly useful for optical fiber optimization in high-power fiber laser and amplifier applications. It can also be valuable as a sensitive temperature probe in the study of anti-Stokes fluorescence cooling in optical fibers.

Injection-seeded backward terahertz-wave parametric oscillator

Engineered quasi-phase-matching in a periodically poled nonlinear crystal has enabled backward optical parametric oscillators, where a pump photon is down-converted to counterpropagating signal and idler photons without an optical cavity. Here, we demonstrate an injection-seeded backward terahertz-wave parametric oscillator (BW-TPO) based on a slant-strip-type periodically poled lithium niobate crystal. A continuous-wave single-frequency laser is used in the BW-TPO pumped by sub-nanosecond 1064.4-nm pulses to provide an injection seed beam for the forward-propagating idler wavelength at 1065.7 nm, resulting in over a 1000-fold enhancement in backward-propagating 0.335-THz output energy, a 63% reduction of the oscillation threshold, and a quantum conversion efficiency as high as 47%. Thanks to the unique advantage of cavity-less oscillation, these improvements and stable operation in the BW-TPO are achieved without any stabilization control of either seed or oscillation modes. Our results show that such an injection-seeded BW-TPO is promising as a compact, efficient, and robust THz-wave source for a wide variety of applications.

11 µm, high beam quality idler-resonant CdSe optical parametric oscillator with continuous-wave injection-seeded at 2.58 µm.

We report an idler-resonant, continuous-wave (CW) seed injected, optical parametric oscillator (OPO) based on cadmium selenide (CdSe). The CdSe OPO was pumped by a 2.09 µm ns-pulsed laser and injection-seeded by a 2.58 µm CW laser. The idler-resonant oscillator was designed to maximize the optical-to-optical conversion efficiency and optimize the beam quality. The injected seed laser was designed to reduce the pump threshold. With this setup, the average idler output power of 802 mW was obtained corresponding to a pulse energy of 0.8 mJ at the wavelength of 11.01 µm and linewidth (FWHM) of 0.6 cm-1, optical-to-optical conversion efficiency of 4.4%, quantum conversion efficiency of 23.3%, beam quality of M2x = 1.23, M2y = 1.12, and pulse width of 21 ns. In addition, by turning the angle of the CdSe, wavelength tuning of 10.55-11.98 µm was achieved.