Lei Meng

Laser frequency conversion in wide bandgap nonlinear optical thin films enables compact coherent ultraviolet (UV) light sources for UV photonics integrated circuits. This work unveils the generation of coherent 350 nm wavelength UVA light in a-axis oriented wurtzite Zn0.67Mg0.33O and Zn0.51Mg0.49O thin films through frequency doubling of a femtosecond laser. The conversion efficiency of the 0.66 μm-thick Zn0.67Mg0.33O and 0.40 μm-thick Zn0.51Mg0.49O thin films reached 3.25 × 10−4 and 1.07 × 10−4, respectively. The single-phase wurtzite Zn0.51Mg0.49O thin film shows an optical bandgap of 4.13 eV, a light absorption coefficient of less than 1 cm−1 at 350 nm wavelength, and a second-order nonlinear susceptibility element χ33(2) of 10.83 pm/V at 700 nm wavelength. Increasing Mg incorporation into wurtzite ZnO causes lattice distortion and deteriorates the non-centrosymmetry of the wurtzite structure, resulting in the reduction in χ33(2). The introduction of a Zn0.79Mg0.21O buffer layer enhances the χ33(2) of the Zn0.51Mg0.49O thin film by 1.3 times through improving film growth quality and relieving the lattice distortion. These results suggest the great prospects of high-efficiency coherent UV light generation in widegap Zn1−xMgxO thin films integrated with reversible polarization.

ZnO-based thin films unfold unique potentials for compact and ultrafast electro-optic (EO) modulation with low power consumption, because they show a strong Pockels effect, low loss of light propagation, and moderate permittivity. This work clarifies the distinct roles of Mg and Li incorporation in affecting the effective Pockels EO coefficients (rc) of wurtzite ZnO thin films by deeply analyzing the lattice distortion and charge density distributions. Mg incorporation significantly enhances internal parameter u and reduces the anion–cation bond lengths along the c-axis direction. This facilitates Zn displacement along the c-axis direction and increases electric susceptibility, enhancing the ionic contribution to rc. Li incorporation produces slight lattice expansion and increases anion–cation bond lengths along the c-axis direction. First-principle calculations show that Li interstitials in oxygen octahedron markedly change the charge density around the nearest-neighbor O. With the evidence of enhancing the second-order nonlinear susceptibility under the optical frequency electrical field, Li incorporation improves rc by enhancing electronic contribution. The Li incorporated Zn0.7Mg0.3O thin film, with a thickness of 400 nm and an optical bandgap of 3.99 eV, exhibits a rc of 2.85 ± 0.01 pm/V. These findings uncover the great potential of widegap ZnMgO:Li thin films for electro-optic modulation in ultraviolet band.

Widegap ZnMgO thin films hold great potential for applications to the high-efficiency generation of deep UV laser via frequency conversion, because of low light propagation loss and reversible polarization. In this work, we studied the effect of Li concentration on the second harmonic generation (SHG) from a-axis oriented ZnMgO thin films with an optical bandgap of about 3.96 eV. The SHG intensity per unit thickness from the ZnMgO thin film with a small Li concentration is approximately 10.7 times larger than the one without Li incorporation. The Li incorporation obviously enlarges interplanar spacing of ( 11 2 ̅ 0 ) plane and reduces the leakage current by three orders of magnitude. X-ray photoelectron spectroscopy measurements suggest the Li incorporation enhances SHG via enhancing electronic polarization. These findings uncover defect complexes based on Li interstitials in oxygen octahedron play a very important role in enhancing the nonlinear polarization of widegap ZnMgO thin films under optical frequency electric field.

The second-order nonlinear-polarization originated from the interaction between thin-film materials with second-order nonlinear susceptibility (χ ⁽²⁾) and high-power laser is essential for integrated optics and photonics. In this work, strong second-order nonlinear-polarization was found in a-axis oriented Zn_1-x Mg _x O (ZnMgO) epitaxial thin-films with Li incorporation, which were deposited by radio-frequency magnetron sputtering. Mg incorporation (x > 0.3) causes a sharp fall in the matrix element χ ₃₃ of χ ⁽²⁾ tensor, although it widens optical bandgap (E _opt). In contrast, moderate Li incorporation significantly improves χ ₃₃ and resistance to high-power laser pulses with a little influence on E _opt. In particular, a Zn_0.67Mg_0.33O:Li [Li/(Zn + Mg + Li) = 0.07] thin-film shows a |χ ₃₃| of 36.1 pm V⁻¹ under a peak power density (E _p) of 81.2 GW cm⁻², a resistance to laser pulses with E _p of up to 124.9 GW cm⁻², and an E _opt of 3.95 eV. Compared to that of ZnO, these parameters increase by 37.8%, 53.4%, and 18.6%, respectively. Specially, the Zn_0.67Mg_0.33O:Li shows higher radiation resistance than a Mg-doped LiNbO₃ crystal with a comparable E _opt. First-principle calculations reveal the Li occupation at octahedral interstitial sites of wurtzite ZnO enhances radiation resistance by improving structural stability. X-ray photoelectron spectroscopy characterizations suggest moderate Li incorporation increases χ ₃₃ via enhancing electronic polarization. These findings uncover the close relationship between the octahedra interstitial defects in wurtzite ZnMgO and its nonlinear-polarization behavior under the optical frequency electric field of high-power laser.

Rare-earth nickelates (RENiO3: RE≠La) exhibit metal–insulator transition (MIT) properties that enable potential applications, such as critical temperature resistance thermistors, optoelectronic switches, and correlated logical devices. Nevertheless, their abrupt structural distortion across MIT results in mechanical stresses and forms microcracks within the bulk RENiO3, and this irreversibly reduces their resistive change during MIT that further impedes their practical applications. Herein, we demonstrate a compositing strategy that simultaneously improves the MIT performances and mechanical strength of RENiO3 by introducing a secondary phase of perovskite oxides with similar lattice parameters and high resistivity. Despite its much higher resistivity compared to RENiO3 (e.g., RE = Sm or Pr), introducing the LaMnO3 compositing phase under high oxygen pressure surprisingly reduces the matrix resistivity. Furthermore, such a compositing process (e.g., 20% LaMnO3) also effectively improves the mechanical strength of RENiO3 by eight times. Such counterintuitive variations are attributed to the similar structure and lattice parameter between RENiO3 and the perovskite composites that modify the grain boundary. As a result, the resistive change is more abrupt across MIT owing to the reduction in the resistivity associated with the grain boundary, while the defect generation and propagation are also suppressed that improves the mechanical properties. This further pave the way to the application of bulk RENiO3 as discrete devices in correlated electronics.

Direct epitaxial growth of group III-V light sources with excellently thermal performance on silicon photonics chips promises low-cost, low-power-consumption, high-performance photonic integrated circuits. Here, we report on the achievement of ultra-high thermal stability 1.3 µm InAs/GaAs quantum dot (QD) lasers directly grown on an on-axis Si (001) with a record-high continuous-wave (CW) operating temperature of 150 °C. A GaAs buffer layer with a low threading dislocation density (TDD) of 4.3 × 10⁶cm^-2 was first deposited using an optimized three-step growth method by molecular beam epitaxy. Then, an eight-layer QD laser structure with p-type modulation doping to enhance the temperature stability of the device was subsequently grown on the low TDD Si-based GaAs buffer layer. It is shown that the QD laser exhibits the ultra-high temperature stability with a characteristic temperature T₀=∞ and T₁=∞ in the wide temperature range of 10-75 °C and 10-140 °C, respectively. Moreover, a maximum CW operating temperature of up to 150 °C and a pulsed operating temperature of up to 160 °C are achieved for the QD laser. In addition, the QD laser shows a high CW saturation power of 50 mW at 85 °C and 19 mW at 125 °C, respectively.

We report the significantly enhanced performance of InAs/GaAs quantum dot (QD) lasers on Si(001) by spatially separated co-doping, including n-doping in the QDs and p-doping in the barrier layers simultaneously. The QD lasers are a ridge waveguide of 6 × 1000 µm² containing five InAs QD layers. Compared with p-doped alone laser, the co-doped laser exhibits a large reduction in threshold current of 30.3% and an increase in maximum output power of 25.5% at room temperature. In the range of 15°C-115°C (under 1% pulse mode), the co-doped laser shows better temperature stability with higher characteristic temperatures of threshold current (T₀) and slope efficiency (T₁). Furthermore, the co-doped laser can maintain stable continuous-wave ground-state lasing up to a high temperature of 115°C. These results prove the great potential of co-doping technique for enhancing silicon-based QD laser performances towards lower power consumption, higher temperature stability, and higher operating temperature, to boost the development of high-performance silicon photonic chips.

a-axis oriented ZnMgO epitaxial thin films with a strong linear electro-optic (EO) effect were developed by radio frequency magnetron sputtering. The Mg incorporation into ZnO thin films not only obviously increases the transmittance at the wavelength range of 400–800 nm but also reduces the leakage current by 3–6 orders of magnitude. Furthermore, with the increase in the Mg content, the linear EO response enhances significantly. In particular, the derived effective EO coefficient r_c of the Zn_0.72Mg_0.28O thin film is (7.6 ± 0.2) pm/V, which is over three times larger than the reported values for ZnO-based thin films and over twice larger than that of ZnO single crystals. The results and discussion conclude that an enhanced intrinsic contribution can be responsible for the increase in r_c with Mg incorporation. These findings open the way for the ZnO-based thin films to EO devices in optical communication and optical interconnects.