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Công nghệ laser tuy được hình thành từ khá sớm (khoảng 1965~1967) nhưng việc ứng dụng trong công nghệ chế tạo (khoan, cắt, khắc, hàn, nhiệt luyện,..) thì chỉ phát triển và trở nên phổ biến ở những năm sau này và có thể xếp vào nhóm các công nghệ gia công đặc biệt (phi truyền thống). Hiện nay, ở Việt Nam, công nghệ laser đang phát triển nhanh và được ứng dụng trong nhiều ngành nghề khác nhau như cơ khí, trang trí nội thất và y học. Tuy nhiên, việc nghiên cứu-chế tạo những thiết bị ứng dụng công nghệ laser vẫn chưa được đầu tư để đáp ứng với nhu cầu thực tế như khắc bề mặt gỗ, mica, dưa hấu và các chi tiết có bề mặt tròn xoay. Vì vậy, máy khắc CNC 4 trục dựa trên ứng dụng của Laser đã được đề xuất nghiên cứu để giải quyết những nhu cầu trên. Bài báo trình bày các kết quả phát triển, hoàn thiện thiết kế-chế tạo máy khắc CNC Laser 4 trục. Với cụm trục A được phát triển để khắc các chi tiết có bề mặt tròn xoay và cụm trục Z để điều chỉnh khoảng cách bàn máy với đầu laser. Thiết bị ứng dụng phần mềm Benbox để điều khiển trong việc khắc bề mặt các sản phẩm lưu niệm trên nền vật liệu mica, gỗ, bề mặt dưa hấu và các chi tiết tròn xoay. Ngoài ra, máy khắc CNC Laser 4 trục sẽ phục vụ cho công việc giảng dạy, nghiên cứu và học tập của giảng viên và sinh viên cho môn máy và hệ thống điều khiển số.

We report net gain measurements at room temperature in Al

International audience; Electron-beam pumped AlGaN lasers are interesting candidates to overcome the technical issues in the development of deep UV laser diodes [1]. However, an improvement of the device design and deeper understanding of the physical processes involved are necessary for the implementation of devices that operate at room temperature. In this presentation, we analyze the optical properties of AlGaN-based separate confinement heterostructures (SCHs) designed for electron beam pumped lasing at 355 nm. We study their photoluminescence under various injection levels and compare the internal quantum efficiency of the spontaneous emission and the lasing threshold power density to determine the limiting factors. At room temperature, carrier localization in the quantum wells leads to an enhancement of the radiative efficiency. However, our results show that the lasing threshold is more sensitive to the injection efficiency than to the radiative efficiency. Therefore, designs including graded alloys (graded index separate confinement heterostructures, GRINSCH [3]) can result in a reduction of the lasing threshold, in spite of their lower internal quantum efficiency.[1] S. Cuesta et al., J. Phys. D: Appl. Phys. (2022). DOI: 10.1088/1361-6463/ac6237[2] S. Cuesta et al., Appl. Phys Lett. 119, 151103 (2021)[3] S. Cuesta et al., Optics Express 29, 13084 (2021)

B-site doping is an emerging strategy for tuning the emission wavelength of cesium lead halide ABX

We report a two-step process to obtain smooth and vertical {10-10} m-plane facets in AlGaN/GaN separate confinement heterostructures designed to fabricate UV lasers emitting at 355 nm. The process consists in a dry etching by RIE-ICP combined with a crystallographic-selective wet etching process using a KOH-based solution. The anisotropy in the wet etching rates between the different crystallographic planes of the AlGaN structure, allows the fabrication of flat and parallel facets without a degradation of the multilayered ensemble. The optical performance of the lasers display a major improved when using the two-step process for the definition of the cavity, in comparison to cavities fabricated by mechanical cleaving, with the lasing threshold under optical pumping being reduced to almost half., 20 pages, 5 figures

International audience

International audience; In this paper, we study the internal quantum efficiency and lasing threshold of AlGaN/GaN separate confinement heterostructures designed for ultraviolet laser emission. We discuss the effect of carrier localization and carrier diffusion on the optical performance. The implementation of graded index separate confinement heterostructures results in an improved carrier collection at the multi-quantum well, which facilitates population inversion and reduces the lasing threshold. However, this improvement is not correlated with the internal quantum efficiency of the spontaneous emission. We show that carrier localization at alloy inhomogeneities results in an enhancement of the radiative efficiency but does not reduce the laser threshold, which is more sensitive to the carrier injection efficiency.

Here, we present the design and fabrication of AlGaN/GaN heterostructures that constitute the active element of electron-beam pumped UV lasers. The design of multi-quantum-well separate confinement heterostructures (SCHs) and graded-index separate confinement heterostructures (GRINSCHs) was adapted based on Monte Carlo simulations of the electron penetration depth. The structures were synthesized by plasma-assisted molecular beam epitaxy on bulk GaN substrates, and validated using transmission electron microscopy and X-ray diffraction. Mirror facets were fabricated by cleaving. Cathodoluminescence studies prove the benefits of the GRINSCH design in terms of carrier collection efficiency. The threshold power density under optical pumping with an Nd-YAG laser (266 nm) was below 200 kW/cm2 at room temperature.

International audience; UV disinfection is receiving renewed attention due to the global pandemic, as an immediately deployable and cost-effective option. The common sources used for this application are low-pressure arc lamps, emitting at 254 nm and containing toxic mercury, which Europe is trying to phase out. Furthermore, this radiation is highly carcinogenic and cataractogenic, which represents a health hazard. Recently different replacements for mercury lamps have been proposed, e.g. excimer lamps that emit at 222 nm, supposedly harmless. However, most of the focus is now on AlGaN-based UV LEDs, with numerous advantages (fast on/off switching, longer lifetime, reduced ozone generation, wider wavelength selection). Yet the wall plug efficiency of commercial LEDs at  260 nm is WPE < 1%, far below that of mercury lamps, and drops dramatically at shorter wavelengths. Therefore, there is still a need and opportunity for alternative UV sources.This work proposes electron-beam pumped UV lamps as an alternative to LEDs to overcome issues related to doping, transport and contacting. This approach obviates the need for p-type doping and efficient and homogeneous carrier injection is achieved without an electron blocking layer. Furthermore, the WPE should not vary much over 210−350 nm. Here, we study the performance of AlGaN/AlN dots-in-a-wire [1] and Stranski-Krastanov quantum dot superlattices [2], whose UV emission can be tailored in the 230-330 nm range. The three-dimensional (3D) carrier confinement in such nanostructures results in high internal quantum efficiency (IQE = 50% on average) and promising external quantum efficiency (EQE up to 5% for as-grown structures). Studies conducted under operation conditions (high injection) show no degradation of the IQE for excitation power densities up to 1 MW/cm2, obtained by pumping with a pulsed Nd-YAG laser. With e-beam excitation, the emission efficiency remains stable up to 10 kV of acceleration voltage and 0.5 mA of injected current (current limit of our setup). E-beam pumping offers also an interesting alternative for the fabrication of UV lasers, highly demanded in the fields of medicine and biotechnology, as well as in 3D printing and non-line-of-sight communication. Today, this spectral range is covered by gas lasers (ArF, KrF, XeF) or lasers based on frequency conversion (Nd:YAG). III-nitride semiconductor laser diodes are promising candidates to provide an efficient semiconductor-based alternative, but current injection is a major problem for wavelengths shorter than 360 nm.For this application, advantages of the e-beam pumping include higher flexibility in the choice of materials for the active medium due to the absence of doping or electrical contacts, as well as higher radiative recombination efficiency since the electrons and holes generated by impact ionization share the same distribution in the active medium. This technology has enabled the fabrication of ZnSe-based pulsed lasers that emit up to 600 W at 535 nm. There are some studies of e-beam pumped UV lasers using AlGaN/GaN separate confinement heterostructures (SCH), but they are limited to pulsed electron beam excitation at cryogenic temperatures [3,4]. New device architectures are required with the prospect of achieving room temperature lasers. Here, we present a study of undoped AlGaN/GaN SCHs designed to operate under e-beam injection with an acceleration voltage  10 kV. We discuss the effect of spontaneous and piezoelectric polarization on the carrier diffusion and demonstrate that the performance is improved using an asymmetric graded-index separate-confinement heterostructure (GRINSCH).[1] H. Arkumar et al., Nanotechnology 31 505205 (2020).[2] I. Dimkou et al., Nanotechnology 31 204001 (2020).[3] T. Wunderer et al., IEEE Photonics Technology Letters 29 1344 (2017).[4] T. Hayashi et al., Scientific Reports 7 2944 (2017).

International audience; We present a study of undoped AlGaN/GaN separate confinement heterostructures designed to operate as electron beam pumped ultraviolet lasers. We discuss the effect of spontaneous and piezoelectric polarization on carrier diffusion, comparing the results of cathodoluminescence with electronic simulations of the band structure and Monte Carlo calculations of the electron trajectories. Carrier collection is significantly improved using an asymmetric graded-index separate confinement heterostructure (GRINSCH). The graded layers avoid potential barriers induced by polarization differences in the heterostructure and serve as strain transition buffers which reduce the mosaicity of the active region and the linewidth of spontaneous emission.

We present net gain measurements at room temperature in Al$_{0.07}$Ga$_{0.93}$N/GaN 10-period multi-quantum well emitting at 367 nm, using the variable stripe length method. Measurements were conducted at two different positions on the sample, where the net gain threshold was attained at 218 kW/cm$^{2}$ and 403 kW/cm$^{2}$. At the position with higher threshold, we observed an anomalous amplification of the photoluminescence intensity that occurs for long stripe lengths (superior to 400 $\mu$m) and high pumping power (superior to 550 kW/cm$^{2}$), leading to an overestimation of the gain value. We attribute such a phenomenon to the feedback provided by the reflection from cracks, which were created during the epitaxial growth due to the strong lattice mismatch between different layers. The highest gain value without anomalous amplification was 131 cm$^{-1}$, obtained at the maximum pumping power density of the experimental setup (743 kW/cm$^{2}$). Using the intrinsic efficiency limit of the cathodoluminescence process, we estimate a lower limit for electron beam pumped laser threshold at room temperature of 390 kW/cm$^{2}$ for this multi-quantum well structure., Comment: 9 pages, 7 figures, 1 table

Rocksalt $\mathrm{Z}{\mathrm{n}}_{x}\mathrm{M}{\mathrm{g}}_{1\ensuremath{-}x}\mathrm{O}$ alloys are theoretically and experimentally investigated for near- and deep-UV optoelectronics with a tunable band gap of 4.2--7.8 eV. Regarding the key question about the composition $x$, at which there is a transition between the direct and indirect gaps, we performed ab initio calculations for various Zn concentrations and all possible atomic arrangements in eight- to 64-atom supercells. We show that, depending on the detailed Zn distribution (clustered, random, or uniform distribution), the alloy band gap can vary by as much as 1.27 eV. The band gap is indirect for clustered and random Zn arrangements in the supercell. For uniform Zn arrangements, the gap is also indirect, except for $xl0.5$ and atom uniform arrangements excluding Zn-O-Zn nearest neighbor bridges, for which the direct gap can be lowered below the indirect gap by about 0.1 eV. The mechanisms of band-gap fluctuation, Zn clustering, and direct-indirect band-gap transitions are analyzed and explained in terms of atomic contributions to band structures by projecting Bloch functions onto localized Wannier functions. Simultaneously, cathodoluminescence measurements were performed on a set of $\mathrm{Z}{\mathrm{n}}_{x}\mathrm{M}{\mathrm{g}}_{1\ensuremath{-}x}\mathrm{O}$ multiquantum wells grown by molecular beam epitaxy on MgO substrates. We observed strong and broad emission bands, redshifting with increasing Zn concentration but featuring no clear-cut evidence for any direct to indirect band-gap crossover. We argue that these alloys are well suited for deep-UV optoelectronics, thanks to the rare combination of strong exciton binding energy, coupling to phonons, and carrier localization, which is favored by the marked flattening of the top valence bands by both short-range and long-range Zn-Zn interactions.

We found that optical Aharonov–Bohm oscillations in a single GaAs/GaAlAs quantum ring can be controlled by excitation intensity. With a weak excitation intensity of 1.2 kW cm–2, the optical Aharonov–Bohm oscillation period of biexcitons was observed to be half that of excitons in accordance with the period expected for a two-exciton Wigner molecule. When the excitation intensity is increased by an order of magnitude (12 kW cm–2), a gradual deviation of the Wigner molecule condition occurs with decreased oscillation periods and diamagnetic coefficients for both excitons and biexcitons along with a spectral shift. These results suggest that the effective orbit radii and rim widths of electrons and holes in a single quantum ring can be modified by light intensity via photoexcited carriers, which are possibly trapped at interface defects resulting in a local electric field.

Cubic ZnxMg1-xO have been proposed as wide bandgap semiconductors for short wavelength optoelectronic applications operating in the deep UV region. By combing MBE growth and HRTEM we were able to determine conditions in which ZnO and ZnxMg1-xO alloys in the rocksalt phase can be grown on MgO substrates. It was found that the maximum ZnxMg1-xO layer thickness strongly depends on Zn concentration, decreasing with x, which reflects the alloy phase instability. The band structures of rocksalt ZnxMg1-xO alloys were calculated in a supercell geometry by density functional theory in the Local Density Approximation (LDA). The atomic coordinates were determined using pseudopotentials implemented in the VASP Simulation Package. Then, the band structures were obtained by a Linear-Muffin-Tin-Orbital method in a full-potential version with a semi-empirical correction (LDA+C) for the band gaps. As MgO in the rocksalt structure has a direct band gap and ZnO has an indirect one, we expected transition: direct to the indirect gap for a certain content, x, of Zn. However, it is shown, that the ZnxMg1-xO band gaps depend strongly on the local arrangement of atoms in a 64 atoms supercell. For each concentration of Zn we obtained a set of the band gap values depending on the arrangement of atoms. Instead of two crossing lines illustrating the dependence of the direct and indirect gaps on composition, we got two crossing bands. The crossing of the two bands covers composition from 10% of Zn up to almost 70% of Zn. The results are compared with the experimental data.

International audience

International audience; We present a set of experimental results showing a combination of various effects, that is, surface recombination velocity, surface charge traps, strain, and structural defects, that govern the carrier dynamics of self-catalyzed GaAs/AlGaAs core-shell nano wires (NWs) grown on a Si(111) substrate by molecular beam epitaxy. Time-resolved photoluminescence of NW ensemble and spatially resolved cathodoluminescence of single NWs reveal that emission intensity, decay time, and carrier diffusion length of the GaAs NW core strongly depend on the AlGaAs shell thickness but in a nonmonotonic fashion. Although 7 nm AlGaAs shell can efficiently suppress the surface recombination velocity of the GaAs NW core, the influence of the surface charge traps and the strain between the core and the shell that redshift the luminescence of the GaAs NW core remain observable in the whole range of the shell thickness. In addition, the band bending effect induced by the surface charge traps can alter the scattering of the excess carriers inside the GaAs NW core at the core/shell interface. If the AlGaAs shell thickness is larger than SO nm, the luminescence efficiency of the GaAs NW cores deteriorates, ascribed to defect formation inside the AIGaAs shell evidenced by transmission electron microscopy.

The optical Aharonov-Bohm effect in a single quantum ring is associated with disorder effects. In the presence of structure anisotropy, localisation, internal electric field, and impurity scattering, optical Aharonov-Bohm oscillations of an electron-hole pair become modulated. Additionally, provided that a strongly correlated exciton pair is formed in a single quantum ring similar to the Wigner molecule, novel oscillations can be observed for increasing magnetic field. In this case, the biexciton emission energy changes abruptly at transition magnetic fields with a fractional oscillation period compared to that of the exciton, the so-called fractional optical Aharonov-Bohm oscillations.

Generally confinement size is considered to determine the dimensionality of nanostructures. While the exciton Bohr radius is used as a criterion to define either weak or strong confinement in optical experiments, the binding energy of confined excitons is difficult to measure experimentally. One alternative is to use the temperature dependence of the radiative recombination time, which has been employed previously in quantum wells and quantum wires. A one-dimensional loop structure is often assumed to model quantum rings, but this approximation ceases to be valid when the rim width becomes comparable to the ring radius. We have evaluated the density of states in a single quantum ring by measuring the temperature dependence of the radiative recombination of excitons, where the photoluminescence decay time as a function of temperature was calibrated by using the low temperature integrated intensity and linewidth. We conclude that the quasi-continuous finely-spaced levels arising from the rotation energy give rise to a quasi-one-dimensional density of states, as long as the confined exciton is allowed to rotate around the opening of the anisotropic ring structure, which has a finite rim width.

We find that the exciton dipole–dipole interaction in a single laterally coupled GaAs/AlGaAs quantum dot structure can be controlled by the linear polarization of a nonresonant optical excitation. When the excitation intensity is increased with the linearly polarized light parallel to the lateral coupling direction [11̅0], excitons (X1 and X2) and local biexcitons (X1X1 and X2X2) of the two separate quantum dots (QD1 and QD2) show a redshift along with coupled biexcitons (X1X2), while neither coupled biexcitons nor a redshift are observed when the polarization of the exciting beam is perpendicular to the coupling direction. The polarization dependence and the redshift are attributed to an optical nonlinearity in the exciton Förster resonant energy transfer interaction, whereby exciton population transfer between the two quantum dots also becomes significant with increasing excitation intensity. We have further distinguished coupled biexcitons from local biexcitons by their large diamagnetic coefficient.

International audience; The Aharonov-Bohm effect in ring structures in the presence of electronic correlation and disorder is an open issue. We report novel oscillations of a strongly correlated exciton pair, similar to a Wigner molecule, in a single nanoquantum ring, where the emission energy changes abruptly at the transition magnetic field with a fractional oscillation period compared to that of the exciton, a so-called fractional optical Aharonov-Bohm oscillation. We have also observed modulated optical Aharonov-Bohm oscillations of an electron–hole pair and an anticrossing of the photoluminescence spectrum at the transition magnetic field, which are associated with disorder effects such as localization, built-in electric field, and impurities.

Strong coupling is demonstrated in monolithic ZnSe-based microcavities. Under nonresonant excitation the polariton dispersion has been investigated in dependence on the photon-exciton detuning for different excitation densities at low temperatures. For zero detuning indications of a polariton lasing threshold are observed like a k-space and energy dispersion narrowing of the lower polariton branch with increasing excitation density. Furthermore, it is observed that this effect is hampered for measurements at negative detunings as a result of the less effective polariton relaxation to the ground state. Latter results in the formation of a discrete polariton distribution at finite k values as known for the polariton bottleneck. In order to investigate the influence of a three-dimensional optical confinement on the polariton relaxation, pillar structured microcavities were fabricated. The formation of discrete polariton states in the k-space distribution is observed. Furthermore, indications for a softening of the k-conservation arising from the structural confinement are found leading to a more effective polariton relaxation. This process would be beneficial for the realization of efficient polariton lasing processes.

Optical properties of ZnO nanorods (NRs) grown by vapour-liquid-solid (VLS) technique on 4H-p-SiC substrates were probed by cathodoluminescence (CL) measurements at room temperature and at 5 K complemented with electroluminescence. At room temperature the CL spectra for defect related emission intensity was enhanced with the electron beam penetration depth. We observed a variation in defect related green emission along the nanorod axis. This indicates a relatively poor structural quality near the interface between ZnO NRs and p-SiC substrate. We associate the green emission with oxygen vacancies. Analysis of the low-temperature (5 K) emission spectra in the UV region suggests that the synthesized nanorods contain shallow donors and acceptors. Original Publication:Nargis Bano, I Hussain, Omer Nour, Magnus Willander, Qamar Ul Wahab, Anne Henry, H S Kwack, D Le Si Dang, Depth-resolved cathodoluminescence study of zinc oxide nanorods catalytically grown on p-type 4H-SiC, 2010, JOURNAL OF LUMINESCENCE, (130), 6, 963-968.http://dx.doi.org/10.1016/j.jlumin.2010.01.006Copyright: Elsevier Science B.V., Amsterdam.http://www.elsevier.com/

High quality vertically aligned ZnO nanorods (NRs) were grown by low-temperature aqueous chemical technique on 4H-n-SiC substrates. Schottky light-emitting diodes (LEDs) were fabricated. The current-voltage (I–V) characteristics of Schottky diodes reveal good rectifying behavior. Optical properties of the ZnO nanorods (NRs) were probed by cathodoluminescence (CL) measurements at room temperature complemented with electroluminescence (EL). The room-temperature CL spectra of the ZnO NRs exhibit near band edge (NBE) emission as well as strong deep level emission (DLE) centered at 690 nm. At room temperature the CL spectra intensity of the DLE was enhanced with the increase of the electron beam penetration depth due to the increase of defect concentration at the interface and due to the conversion of self-absorbed UV emission. We observed a variation in the DLE along the nanorod depth. This indicates a relatively lower structural quality near the interface between ZnO NRs and n-SiC substrate. The room-temperature CL spectra of SiC show very weak emission, which confirms that most of the DLE is originating from the ZnO NRs, and SiC has a minute contribution to the emission.

Growth of GaN nanowires by using a vapor-liquid-solid (VLS) mechanism with Pt catalysts was investigated. Single-crystal GaN nanowires with diameters of ∼100 nm and lengths of ∼5 μm were grown with Pt and with the well-known VLS catalyst of Ni. The growth rate of nanowires with Pt was lower than that with Ni under identical processing conditions. Pt-Ga alloy globules were observed by Transmission electron microscopy at the ends of the nanowires which lead the growth by the VLS mechanism. Electrical data from individual nanowires indicated high conductivity associated with high electron concentration in the Pt-GaN nanowires.

Vertically aligned ZnO nanorods (NRs) with a diameter in the range of 160-200 nm were grown on p-GaN/sapphire substrates by aqueous chemical growth technique and white light emitting I diodes (LEDs ...

The position-controlled growth and structural and optical characteristics of ZnO nanotubes and their coaxial heterostructures are reported. To control both the shape and position of ZnO nanotubes, hole-patterned SiO2 growth-mask layers on Si(111) substrates with GaN/AlN intermediate layers using conventional lithography are prepared. ZnO nanotubes are grown only on the hole patterns at 600 °C by catalyst-free metal–organic vapor-phase epitaxy. Furthermore, the position-controlled nanotube growth method allows the fabrication of artificial arrays of ZnO-based coaxial nanotube single-quantum-well structures (SQWs) on Si substrates. In situ heteroepitaxial growth of ZnO and Zn0.8Mg0.2O layers along the circumference of the ZnO nanotube enable an artificial formation of quantum-well arrays in a designed fashion. The structural and optical characteristics of the ZnO nanotubes and SQW arrays are also investigated using synchrotron radiation X-ray diffractometry and photoluminescence and cathodoluminescence spectroscopy.

We have grown zinc-blende cadmium selenide (CdSe) epilayers on ZnTe-(0 0 1) substrate by molecular beam epitaxy (MBE). By controlling the substrate temperature and beam-equivalent pressure (BEP) ratio, of Se to Cd, we determined the most suitable growth condition based on reflection high-energy electron diffraction (RHEED) pattern. At a substrate temperature of 280 °C and a BEP ratio of 3.6, the RHEED pattern showed a V-like feature, indicating a rough surface with facets. As the substrate temperature was increased to 360 °C at the same BEP ratio, a V-like RHEED pattern moved to a clear streaky pattern. Moreover when the BEP ratio was increased to 4.8 at 360 °C of substrate temperature, a clear (2 × 1) reconstruction of the CdSe layer was observed. A CdSe/CdMgSe single quantum well structure was also grown on ZnTe-(0 0 1) substrate by MBE. The RHEED pattern showed a clear (2 × 1) surface reconstruction during the growth. By photoluminescence measurement, a good optical property of the structure was obtained.

Optical properties of ZnO-based single nanorods are probed by cathodoluminescence (CL) measurements at T = 5 K. We observe a variation of the ZnO near band edge CL by three orders of magnitude along the nanorod axis, accompanied by a spectral blueshift of 10-30 meV. This indicates a rather poor structural quality of the nanorod bottom part, close to the substrate. ZnO/ZnMgO quantum wells grown on top of ZnO nanorods are found to exhibit much stronger confinement effects as compared to their two-dimensional counterparts, suggesting a reduced spontaneous and piezoelectric polarization effects.

We report on the self organized growth of GaN quantum dots deposited on Al x Ga 1-x N layer by plasma-assisted molecular beam epitaxy. It is found that the relaxation of Al x Ga 1-x N layer on AlN depends on Al composition and thickness. The measurement of the variation of lattice parameter indicated that GaN quantum dots do not relax completely on Al x Ga 1-x N layer. Optical properties of undoped and Eu-doped quantum dots on AlGaN layer were studied. Visible light emission from Eu 3+ ions has been observed and showed strong thermal stability.