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A novel fluorenone derivative, FO52, is designed and synthesized. The fluorenone group is introduced to provide the central π-conjugated system in the molecule and triphenylamine is substituted at both sides. Intramolecular Charge Transfer (ICT) from the terminal groups to the molecular center is confirmed via DFT calculations. Ultrafast optical nonlinearities are investigated via Z-scan and transient absorption spectroscopy (TAS) studies with a 190 fs laser. Reverse saturable absorption, two-photon induced excited-state absorption, and pure two-photon absorption are observed at 532 nm, 650 nm, and 800 nm, respectively. The different mechanisms at these wavelengths are discussed and interpreted with assistance from the results from TAS. Furthermore, strong excited-state refraction and ultrafast negative refraction from the bound electron response are resolved and discussed in phase object pump probe (POPP) experiments. The results suggest that the ICT-enhanced optical nonlinearities provide FO52 with strong optical limiting capabilities at visible wavelengths and ultrafast refraction with tiny attenuation in the near infrared region. The combination of these properties in one compound could be attractive for applications like laser protection and low-loss all-optical switching.

The third-order nonlinear optical (NLO) materials with stimuli-responsive properties have received extraordinary attention due to their controllable photophysical properties. In this work, two attractive metal complexes third-order NLO switches, which are far superior to congeneric optical switches in terms of their performance conversion, versatility, and fast response, were successfully designed and synthesized. The test of their third-order NLO properties proves that the metal complexes exhibit reverse saturable absorption and self-defocusing refraction. After light irradiation, the third-order NLO behavior turns quickly into self-focusing refraction. The relation between the molecular structures and the third-order NLO properties was investigated via1H nuclear magnetic resonance and ultraviolet-visible absorption. The results show that the metal ions have a significant influence on the NLO behavior and reveal the origin of third-order NLO properties via Z-scan determinations, pump-probe technology, and density functional theory calculations. These metal complexes can be used as third-order NLO switches with excellent fatigue resistance and broaden the application range of third-order NLO materials with adjustable performances.

A comprehensive understanding of excited-state dynamics of semiconductor quantum dots or nanomaterials at the atomic or molecular level is of scientific importance. Pure inorganic (or non-covalently protected) seimiconductor molecular nanoclusters with atomically precise structure are contributive to establish accurate correlation of excited-state dynamics with their composition/structure, however, the related studies are almost blank because of unresolved solvent dispersion issue. Herein, we designedly created the largest discrete chalcogenide seimiconductor molecular nanoclusters (denoted P2-CuMSnS, M = In or/and Ga) with great dispersibility, and revealed an interesting intracluster “core-shell” charge transfer relaxation dynamics. A systematic red shift in absorption spectra with the gradual substitution of In by Ga was experimentally and computationally investigated, and femtosecond transient absorption measurements further manifested there were three ultrafast processes in excited-state dynamics of P2 nanoclusters with the corresponding amplitudes directed by composition variation. Current results hold the great promise of the solution-processible applications of semiconductor-NC-based quantum dots and facilitate the development of atomically precise nano-chemistry.

There is great interest in transition metal-doped InSe because of its high nonlinearity and ultrafast response time at higher light fluence. Herein, Ag-doped InSe nanofilms were precisely manufactured using a direct current-radio frequency sputtering method, and their ultrafast broadband nonlinear optical responses in near-infrared were systematically researched. Ag-doped InSe nanofilm exhibited a broadband nonlinear optical response (800–1100 nm) and ultrafast carrier absorption (

We report the different nonlinear optical mechanisms and defect-related carrier dynamics in Sn-doped β-Ga2O3 crystal by utilizing time-resolved pump-probe technique based on phase object under UV excitation. The obtained nonlinear optical parameters arise from bound electron can be well explained by the theoretical calculation of two-band model and Kramers-Kronig transformation. By tuning the probe wavelength, the carrier nonlinearity can be modulated greatly due to additional absorption of defects within the bandgap. The results reveal that by choosing a proper probe wavelength that matches the defect state to the valence band, the nonlinear absorption and refraction of the carriers can be greatly enhanced, which provides an important reference for the design of gallium oxide-based waveguide materials and all-optical switching materials in the future.

We present an all-optical scheme for the generation of longitudinal magnetization superoscillation based on the vectorial diffraction theory and the inverse Faraday effect. To achieve this, an azimuthally polarized high-order Laguerre–Gaussian vortex mode is firstly focused by a high numerical aperture (NA) objective and then impinges on an isotropic magneto-optical material. It is found that, by judiciously controlling the intrinsic arguments (radial mode index (p) and truncation parameter (β)) of such a configurable vectorial vortex beam, the longitudinal magnetic domain induced in the focal plane can be switched from a peak sub-wavelength magnetization (> 0.36λ/NA), via the fastest Fourier magnetization component (∼0.36λ/NA), to a super-oscillation magnetization hotspot (< 0.36λ/NA). We further examine the dependence of the transverse size, the side lobe, and the energy conversion efficiency within the focal magnetization domain on both the p and β of the initial vortex modes, confirming that the higher-order structured vortex beams are preferable alternatives to trigger robust longitudinal magnetization superoscillation. In addition, the underlying mechanisms behind the well-defined magnetization phenomena are unveiled. The ultra-small-scale longitudinal magnetization demonstrated here may hold massive potential applications in high-density all-optical magnetic recording/storage, super-resolution magnetic resonance imaging, atom trapping and spintronics.

The Er3+/Tm3+/Yb3+ tri-doped Y7O6F9 phosphor is synthesized by a typical hydrothermal method. The Y7O6F9: 0.6%Er3+/0.2%Tm3+/15%Yb3+ phosphor is selected for the study. The up-conversion characteristics and temperature sensing performances of the phosphor are investigated under the excitation of 980 nm laser. The temperature sensing performances are investigated in the temperature range of 306–546 K based on thermal and non-thermal coupled levels by using fluorescence intensity ratio technology. The maximum sensitivity is 54.3 × 10−3 K−1 at 546 K, which is based on 3F2,3 (red emission at 695 nm of Tm3+) and 4S3/2 (green emission at 545 nm of Er3+) levels. After comparison, it is found that the sensing sensitivity of Y7O6F9: 0.6%Er3+/0.2%Tm3+/15%Yb3+ phosphor is relatively high. The excellent temperature sensing performances of the phosphor are verified. What's more, the white luminescent emission of the phosphor is achieved by adjusting excitation power and sample temperature during the experiment. And the luminescent emission color of the phosphor can be modulated widely. The above experimental results not only enable us to have a deeper understanding of tunable upconversion luminescent emission and optical temperature sensing, but also provide a significant step for high sensitivity thermometry, white light illuminating and color displaying.

The light–matter interaction in noble metal nanoparticles is intrinsically determined by the surface plasmon resonance (SPR) and bandgap. To determine the process of electronic transition and the limitation of energy bands on nonlinearity at different excitation wavelengths, we investigate the broadband nanosecond nonlinear absorption of gold nanobipyramids from 532 nm to 850 nm. The experimental results indicate that with the increase of the incident light intensity, gold nanobipyramids have the characteristic of transforming from saturable absorption (SA) to reverse saturable absorption (RSA) at 532 nm to 690 nm, and exhibit a single SA after 690 nm until 850 nm. Based on the wavelength-dependent response, the SA is attributed to the intraband transition of sp-band, whereas sequential two-photon absorption leads to RSA. Unlike other metal nanoparticles, gold nanobipyramids exhibited a significantly higher modulation depth up to 47%. Lastly, building on the excellent SA characteristic, we achieve pulse width compression of nanosecond laser pulses at multi-wavelengths.

The multilayer dielectric (MLD) grating is a critical device for combining multiple laser beams into a single beam in a spectral beam combining (SBC) system. We established a theoretical thermal deformation model of the laser-irradiated MLD grating. Thermal deformation on the surface of the grating is simulated according to a series of parameters including the laser irradiation time, laser power density, and substrate size. To verify the model, we exposed a 960 l/mm, 50×50×1.5 mm3 grating to a laser power density of 3.61 kW/cm2 and observed the temperature change. We used a Twyman-Green interferometer to measure the interference fringes on the grating surface. Based on the Fourier-transform method and a Zernike polynomial fitting method, the real-time grating surface profile is reconstructed. The results show that substrate thickness increase or area decrease can reduce thermal deformation, the average decreases are 18.3% and 19.9%, respectively. The discussion and analysis of the grating thermal deformation are potentially valuable for designing grating to decrease the thermal deformation and improve the combined beam quality of a SBC system.

In this work, a novel single-shot pump-probe technique based on mirror array is proposed, which can be used to investigate the dynamics of nonlinear absorption with a single-shot femtosecond laser. Using this unique mirror array, the dispersion of femtosecond pulse laser caused by time delay can be avoided and the delay time can be increased effectively. Additionally, the energy requirements of the femtosecond laser are relatively low compared with conventional single-shot pump-probe technique. The validity of this technique is demonstrated by investigating the nonlinear absorption dynamics in ZnSe and symmetrical phenoxazinium bromine salt with 220 fs pulses at a wavelength of 515 nm.

Rare earth doped materials with dual-mode luminescence have broad application potential. Among them, the dual-mode luminescence ink has been widely concerned in the field of anti-counterfeiting technology. In this work, the Y2O3: Eu@SiO2 nanomaterials were prepared with Y2O3: Eu as the core nanocrystals, SiO2 as the shell layer. Then, the Y2O3: Er(Tm/Yb) nanocrystals are adsorbed on the surface of the SiO2 layer to form Y2O3: Eu@SiO2/Y2O3: Er(Tm/Yb) composite nanomaterials. The Y2O3: Eu@SiO2/Y2O3: Er(Tm/Yb) composite nanomaterials can simultaneously achieve the dual-mode (up-conversion and down-shifting) luminescence. The Y2O3: Eu@SiO2/Y2O3: Er composite nanomaterials can emit different color luminescence emission under the 405 nm excitation, 980 nm excitation and 980/405 nm co-excitation, respectively. The up-conversion luminescence color under 980 nm excitation is regulated by doping different rare earth ions in the absorption layer. The dual-mode luminescence characteristics of composite nanomaterials make it possible to extract information only when two excitation sources exist at the same time, which increases the illegal forgery difficulty. Moreover, the composite nanomaterials can emit more variable color emission via adjusting the power intensity of 980 nm laser under 980/405 nm co-excitation, which improves the color resolution of multicolor emission. These results indicate that the Y2O3: Eu@SiO2/Y2O3: Er(Tm/Yb) composite nanomaterials are expected to be applied in the fields of color displaying and fluorescence encryption anti-counterfeiting.

Lanthanide-doped up-converting nano-particles reveal significant promise for data storage, biological imaging, and temperature sensing, but have not been extensively used due to their low emission intensity. Here, we investigate the up-conversion emissions of Gd2(MoO4)3:Er/Yb thin films that are sandwiched by Gd2(MoO4)3:Yb thin films. Our results show that the sandwiched thin films possess significant enhancement of emission intensities. The enhancement factors of the green and red emissions, radiated from Gd2(MoO4)3:Er/Yb films, reach 9.5 and 5, respectively. Moreover, the ability of sandwich structured thin films in multi-photon up-conversion enhancement is more excellent than that of laser power, attributed to the suppression of undesired radiative/non-radiative recombination of the intermediate states. In addition, the intensity-based temperature absolutely sensitivity of sandwich structured Gd2(MoO4)3:Er3+/Yb3+ reaches 0.013 K−1 at 498 K, which indicate this sandwich structured thin films have larger potential in thermal sensors. This sandwich structure might extend the applications of Ln3+-doped materials in super-resolution imaging and temperature sensing.

A crucial issue faced by quantum dot solar cells (QDSC) is how to achieve a quantum dot material with high photoluminescence (PL) quantum yield and long excited state lifetime at the same time to maximize charge carrier collection efficiency. Here, we present a new type of Zn0.9Cd0.1S/ZnSe hetero-nanocrystals that have a staggered band alignment, i.e., a type-II structure by which the wave function overlap between electrons and holes is reduced, thereby leading to a significant enhancement in the PL lifetime. The resultant QD also exhibits a QY up to 75.4%, which is, to the best of our knowledge, the highest one for a type-II QD to date. In particular, we explore the underlying relationship between the broad tunability and energy level alignment of these hetero-nanocrystals, and we highlight the properties that have delivered QDSCs and light-emitting diodes.

The nearly pure green and red emissions of Ho3+/Yb3+ co-doped Gd2(MoO4)3 were realized by tri-doping Eu3+ and Ce3+, respectively. The Ho3+/Yb3+ co-doped Gd2(MoO4)3 powder, radiating intense green and red emissions, was prepared by a typical sol-gel method. Under the excitation of 980 nm laser, the red-to-green ratio was tuned from 0.2 to 4 as the concentration of Eu3+ changed from 6 to 0 mol%, attributed to the energy transfer processes between Ho3+ and Eu3+. Meanwhile, the red-to-green ratio altered from 4 to 20 as the concentration of Ce3+ changed from 0 to 20 mol% via the cross relaxing processes between Ho3+ and Ce3+. The prepared phosphors, possessing large tuning range of red-to-green ratio and resisting high temperature, can be potentially applied to colorful display and solid-state lighting.

The methods of modulating emitting color are greatly important for up-conversion phosphors to broaden their applications, ranging from information storage to surface display. In this paper, the Ho3+/Tm3+/Yb3+ tri-doped Gd2(MoO4)3 phosphors are synthesized by typical sol-gel method. It is found that the phosphors can radiate intense blue (475 nm), green (542 nm) and red (660 nm) up-conversion luminescence under 980 nm or 915 nm laser excitation. Especially, the emission color of the phosphors can be tuned from red to lavender by adjusting the excitation power of 980 nm or 915 nm continuous lasers. In addition, the up-conversion luminescence is also investigated under the excitation of 980 nm pulse laser. The emission color varies from green to blue and green to yellow by altering the pulse width and frequency, respectively. Combining the above methods, multi-color and white light are realized, indicating the potential application in dynamic colorful display.

It is very meaningful to implement a tunable transparency window device with a simple metasurface structure in the mid-infrared band. In this paper, we propose a symmetric metasurface structure that consists of thin layers of graphene and a split anti-ring (SAR). This structure can achieve a tunable lattice-induced transparency-like (LIT-like) spectral response in the wide frequency range of 20–40 THz. Our simulation results show that the realization of the LIT-like phenomenon is due to the strong coupling between the lattice diffraction resonance mode (LDRM) and the wide plasmon resonance mode produced by SAR. The LIT-like response can be dynamically shifted by adjusting the incident pitch angle, the single and double LIT-like can also be switched by adjusting the azimuth angle, which provides convenient post-fabrication tunability. The coupling mechanism of the single LIT-like phenomenon is explained via a dual-oscillator model, which agrees well with the simulation results. In addition, the effects of spacer layer thickness, spacer layer refractive index, ambient refractive index on the position and size of the transparency window are analyzed. Lastly, a tunable delay bandwidth product between 0.46 and 0.86 will be produced due to the addition of graphene. Our finding manifests a novel design of the mid-infrared dynamically tuned slow-light device.

The Y2O3: Tm/Yb, Y2O3: Tm/Yb@Y2O3: Yb and Y2O3: Tm/Yb@Y2O3: Yb@Y2O3: Er/Yb thin films are successfully prepared via spin-coating method. The surface grain size of the thin films is changed by multiple calcinations. The up-conversion luminescence emission of the thin films under 980 nm excitation is improved through two methods, including the increase of surface grain size and the structure of Y2O3: Tm/Yb@Y2O3: Yb bilayer. When the emission intensity of the Y2O3: Tm/Yb@Y2O3: Yb thin films are maximized by adjusting the Yb3+ concentration of the coating layer, the bilayer thin film continually undergoes multiple calcinations, thus further increasing the enhancement effect of each band emission. Besides, the Y2O3: Tm/Yb@Y2O3: Yb@Y2O3: Er/Yb multilayer thin films are designed to realize the modulation of up-conversion emission color, meaning that its luminescence emission color can change from blue to white and finally to orange by adjusting the thickness of the top Y2O3: Er/Yb luminescence layer and the number of calcination. With the capabilities of up-conversion emission enhancement and multicolor luminescence modulation, this multilayer thin film can promote the application of rare earth doped materials in the fields of fluorescence detection and color displaying.

An ultrathin composite films with alternated layers containing anionic lithium tetracarboxyl indium phthalocyanine chloride (ClInPc(COOLi)4) and cationic poly(diallydimethyl ammonium chloride) (PDDA) were prepared using layer-by-layer assembly technique. The resulting films were characterized by Ultraviolet-visible and Fourier transform infrared spectroscopy, showing regularly consecutive adsorption. The ultrafast third-order nonlinear optical properties of the films were investigated via a 4f coherent imaging system with phase object with 190 fs laser pulses at 532nm. The results demonstrate that the ultrathin films exhibited excellent self-focusing effect (n2 ∼ 8.86 × 10−17 m2/W) and reverse saturable absorption properties (β ∼ 5.19 × 10−10 m/W), while the nonlinear optical response of the same composite in aqueous solution were found to be negligible. Our results indicate that the ClInPc(COOLi)4/PDDA film is a promising candidate for future nonlinear optical devices.

International audience; Three identical new WEST Ion Cyclotron Resonance Heating (ICRH) antennas have been designed, assembled then commissioned on plasma from 2013 to 2019. The WEST ICRH system is both load-resilient and compatible with long-pulse operations. The three antennas have been successfully operated together on plasma in 2019 and 2020. The load resilience capability has been demonstrated and the antenna feedback controls for phase and matching have been developed. The breakdown detection systems have been validated and successfully protected the antennas. The use of ICRH in combination with Lower Hybrid has triggered the first high confinement mode transitions identified on WEST.

WEST represents the ideal device to test a reactor relevant travelling wave array (TWA) due to its long pulse capability and full metal wall configuration. Moreover, the already installed ICRF high power launchers will allow a direct comparison between a classical in-port antenna and the TWA. The design and exploitation of a high power (2 MW) TWA mock-up to be tested in TITAN represent the first step towards its integration in WEST. This paper presents the progress in the RF characterization of this high power mock-up. An integrated modelling scheme based on the full-wave code HFSS and on Python packages for RF circuit simulations is developed and used to parametrically analyse the response of the mock-up inside TITAN. The sensitivity of the antenna to geometrical deformations, e.g. thermal dilatation or manufacturing tolerances, is analysed. The use of trimmers to compensate those deformations is discussed and results from a manufactured trimmer test-bed are presented. A diagnostic system is proposed to measure electric field and current on the straps. Aspects like sensitivity and calibration are briefly discussed. In conclusion, the next steps of the project are outlined.

Metal nanorod arrays exhibit hyperbolic dispersion and optical nonlocality under certain conditions. Therefore, their optical behaviors can hardly be expressed by incident-angle-independent effective permittivity. Here we extract effective permittivity of silver nanorod arrays with diameters of 4 nm, 12 nm, and 20 nm by polarized transmission method in the visible range. The incident angles are chosen from 20° to 60° to study the influence of optical nonlocality on permittivity. We demonstrate how the diameter of the nanorods can control the effective permittivity beyond the effective medium theory. The results suggest that the effective permittivity gradually loses its accuracy as the diameter increases due to the optical nonlocality. Our experiment verifies that ultrathin nanorod arrays can resist the fluctuations caused by changes in incident angle. We also extract k-dependent effective permittivity of nanorods with larger diameters.

The temperature-dependent up-conversion luminescence of the Yb3+-Er3+ doped NaYF4:Er3+/Yb3+ nanorods, which can emit green, red and blue emissions, was investigated under 980 nm laser excitation. Our results showed that the natural log of I 520/I 550, I 654/I 550, I 520/I 407 and I 654/I 407 had well linear relationship with the reciprocal of temperature, suggesting their potential application in temperature measurement. At 471 K, the maximal sensing sensitivities of the above thermometry methods reached 0.007, 0.032, 0.6, 2.7 K−1, respectively. Moreover, the luminescence color shifted from green to yellow with the temperature increasing from 296 to 471 K. The calculated CIE coordinates varied linearly with the temperature, which also can be used in thermometry. These excellent properties make NaYF4:Er3+/Yb3+ nanorods an excellent candidate as a temperature sensor.

Improving the emission from rare earth ions doped materials is of great importance to broaden their application in bio-imaging, photovoltaics and temperature sensing. The green emissions of Gd2(MoO4)3:Er3+/Yb3+ powder upon co-excitation with 980 and 808 nm lasers were investigated in this paper. Distinct enhancement of green emissions was observed compared with single laser excitation. Based on the energy level structure of Er3+, the enhancement mechanism was discussed. Moreover, the result of temperature-dependent enhancement revealed that the enhancement factor reached its maximum (2.5) as the sample heated to 120°C, which is due to the competition of two major thermal effects acting in the co-excited up-conversion processes. In addition, the same enhancement of green emissions was also observed in Gd2(MoO4)3:Er3+ powder and NaYF4:Er3+/Yb3+ powder.

Compound Dyakonov modes supported by a symmetric uniaxial-isotropic-uniaxial planar structure are investigated, of which the uniaxial layers have the identical principal values of the relative permittivity tensor and optic axis orientation. By solving the canonical boundary-value problem, two branches of compound Dyakonov modes are obtained with symmetric and antisymmetric field distributions respectively. The shapes of the field distributions are proven to be closely related to the reflectance of TE polarized waves at the interfaces of the structure. The relevant characteristics of propagation constants, field distributions, propagation direction ranges and polarization states are numerically calculated. The results show that these characteristics are highly affected by the thickness and the permittivity of the middle isotropic layer, the antisymmetric modes will cutoff when the thickness decreases below critical value and the polarization states of symmetric modes will vary significantly from Dyakonov surface waves due to the strong coupling.

The free-carrier dynamics and nonlinear refractive index in bulk polycrystalline diamond are investigated using a femtosecond ultraviolet-pump-optical-probe technique with a phase object. Normalized transmittances under open-aperture and closed-aperture conditions are simultaneously measured to determine the dynamics of nonlinear absorption and refraction in the sample. In the dynamics of nonlinear absorption, three processes are clearly observed: nondegenerate two-photon absorption, fast and slower decay components of the free-carrier dynamics. The lifetimes of the free carriers are precisely measured. In the dynamics of nonlinear refraction, we obtain the nonlinear refractive coefficient and find that the results obtained under the femtosecond regime differ from those of other semiconducting materials previously measured under the picosecond regime.

Due to the reliability problem and high melting point, the development of lead-free solders is hindered. Alloying is considered to be a promising approach to design new Sn-based solders. Cu 6 Sn 5 plays an important role in the quality of microelectronic packaging that formed at the interface of solder and Cu substrate. In the present study, first principles calculations were employed to study the occupation properties of alloying elements (A = P, Zn, Ag, In, Sb, Ce, Pb, and Bi) and anti-site defects (Cu and Sn) in (0 1 0) surface of Cu 6 Sn 5 and their influence on the electronic structures of Cu 6 Sn 5 (0 1 0) surface. Formation energies were calculated to evaluate the occupation preferences of alloying elements in considered eight sites of Cu 6 Sn 5 (0 1 0) surface. Different alloying elements show different occupation preferences. The occupations of alloying elements in Cu 6 Sn 5 surface show negative formation energies comparing the positive formation energies in bulk Sn. Therefore, the alloying elements will easily occur in the Cu 6 Sn 5 compound during soldering process. Electronic structures were studied to reveal the occupation mechanisms of alloying elements, and the interactions between them and Cu 6 Sn 5 surface. Large values of density of states appear around the Fermi energy level for Ce contained system. In terms of the lowest work function, the electrons in Ce contained systems will be much easier to lose than that in other contained systems. Therefore, alloying of Ce will greatly affect the stability and electrical properties of solder joints in microelectronic packaging.

The authors describe a new phosphor of the composition Gd2(WO4)3:0.3%Ho/0.5%Tm/15%Yb that displays distinctly improved sensitivity to temperature (T) due to non-thermally coupled energy levels of Ho(III) and Tm(III) ions. Under 980 nm excitation, the upconverted blue (peaking at 473 nm), green (540 nm) and red (655 and 700 nm) fluorescences exhibit T-dependent luminescence in the 295 to 595 K range. The T sensing capability was investigated by using a ratiometric (2-wavelength) technique. The maximum sensitivity to T is found to be 26.14 × 10−3 K−1 at 595 K based on the red [Tm(III)] and green [Ho(III)] emissions. The colors of the phosphor are located in the white region of the CIE diagram in the whole T-range which also makes it suitable for use in white displays.

An ultrathin composite films containing cobalt phthalocyanine (CoPc) and poly(diallydimethyl ammonium chloride) (PDDA) were fabricated through layer-by-layer assembly technique. The resulting films were characterized by UV-Vis spectroscopy and atomic force microscopy. Their third-order nonlinear optical properties and photo-excitation dynamics were investigated with 20 ps laser pulses at 532 nm. The results demonstrate that the ultrathin films have notable third order optical nonlinearities (χ(3)∼4.7 × 10−9 (esu)) and fast excited-state relaxation (τ∼67 ± 6 ps), indicating their potential applications in future nonlinear optical devices.

We investigate the impact of carbon defects on carrier trapping and recombination processes in high-quality GaN crystals using transient absorption spectroscopy (TAS). The different wavelengths for probing carrier dynamics enable discrimination of electron- and hole-capture processes by carbon acceptors (CN), and two charge states (−1 and 0) of the CN are demonstrated from TAS by tuning the pump fluence. The hole-capture coefficients for the CN− and CN0 states are at least ten times larger than that for Fe centers. On the other hand, electrons can also recombine efficiently with holes at the 0/+ level of the CN defect. The resulting carrier trapping on carbon in GaN would mediate carrier recombination and significantly reduce the efficiency of optoelectronic devices even for a low defect concentration (∼1015 cm−3).