Jan Suffczynski

We demonstrate room-temperature lasing in the blue spectral range, 440 to 460 nm, where optical gain is provided by InGaN quantum wells coupled to quasi-bound states in the continuum hosted by a monolithic GaN-based subwavelength grating. A low-refractive-index nanoporous GaN layer obtained by electrochemical etching, placed beneath the structure, suppresses the coupling of the optical modes to the substrate. The gratings are designed by theoretical modeling and fabricated with sub-10 nm precision using e-beam lithography and dry etching. The presence of quasi-bound states in the continuum is confirmed by angle-resolved reflectivity and observation of a polarization vortex. The structures ensure a low threshold (0.16 mJ cm^-2 pulse^-1), narrow spectral bandwidth (below 0.04 nm), and strongly linearly polarized lasing emission. The efficient tunability of the lasing wavelength is achieved by adjusting the grating geometry parameters. This work opens new opportunities for compact and tunable monolithic GaN-based photonic devices.

High refractive index (4.4 at 1100 nm), negligibly small absorption in the near-infrared spectral range, and ease of processing make MoSe₂ the perfect material for applications in near-infrared photonics. So far, implementation of MoSe₂-based photonic structures has been hindered by the lack of large-surface MoSe₂ substrates. The use of molecular beam epitaxy allows the production of homogeneous layers of MoSe₂ with a few-inch surface and a thickness controlled at the sub-nm level. In the present work, we design by theoretical calculations and fabricate by a simple lithography process an ultrathin subwavelength grating out of a 42 nm thick, epitaxially grown MoSe₂ layer. Our polarization-resolved reflectivity measurements confirm that the gratings host a peculiar type of a confined optical mode that is a bound state in the continuum. Moreover, the fabricated structures enhance the efficiency of the third-harmonic generation by over 3 orders of magnitude as compared to the unstructured MoSe₂ layer. The presented results are promising for the realization of flat, ultracompact devices for lasing, wavefront control, and higher-order topological states of the light.

Single‐photon emitters (SPEs) in 2D materials are highly promising candidates for quantum technologies. SPEs in hexagonal boron nitride (hBN) are widely investigated, but mostly in exfoliated or powder samples that require an activation process, making it difficult to compare studies and reproduce results. Here, this problem is addressed and a platform based on large‐area metalorganic vapor phase epitaxy (MOVPE)‐grown hBN is proposed, which combines reproducibility and scalability with the ability to readily host SPEs without activation. Through the creation of bubbles via electron‐beam irradiation, additional functionalities are achieved, including an interference‐mediated enhancement of emission by approximately 100–200%, dedicated structures that allow the relocation of individual emitters across different systems, and the opportunity to investigate strain‐induced effects. Moreover, in contrast to other gas‐filled bubbles that deflate at low temperatures, the bubbles remain stable under cryogenic conditions, allowing studies as a function of temperature. To improve the control over the shape and position of bubbles, a mask‐based method is demonstrated that enables deterministic control over bubble formation. The presented hBN bubbles constitute a versatile platform for reproducible studies of hBN‐based emitters, providing a reliable insight into their nature and properties.

The unusual magnetic properties of van der Waals‐type antiferromagnetic semiconductors such as transition metal thiophosphates make them highly attractive for spintronics and optoelectronics. However, a link between the magnetic and optical properties of these materials, required for practical applications, has not yet been established. A combined experimental and theoretical study of magnetic, optical, and structural properties of CrPS ₄ samples is reported. It is found that the magnetic‐field‐dependent circular polarization degree of the photoluminescence is a direct measure of the net magnetization of CrPS ₄ . Complementary Raman scattering measured as a function of magnetic field and temperature enables the determination of the magnetic susceptibility curve of the material. Experimental results are supported by density functional theory calculations that take as input the lattice parameters determined from temperature‐dependent X‐ray diffraction measurements. This allows the impact of spin ordering on the spectral position of Raman transitions in CrPS ₄ to be explained, as well as the anomalous temperature shifts of some of them. The presented method for all‐optical determination of magnetic properties is highly promising for studies of spin ordering and magnetic phase transitions in single‐ or few‐layer samples of magnetic layered materials, for which a poor signal‐to‐noise ratio precludes reliable neutron scattering or magnetometry measurements.

In various material systems, an antiferromagnetic phase was found to coexist with a weak ferromagneticlike signal, while symmetry-based theoretical predictions indicate a possibility of a nonzero anomalous Hall effect (AHE) even in the absence of sample magnetization. This is the case of nominally collinear antiferromagnets, in particular, hexagonal MnTe, where the AHE and no detectable magnetization have been recently reported. To clarify the role of magnetization, we present a study of bulk MnTe samples, combining experiment and theory. We demonstrate that the existence of the AHE in the hexagonal MnTe is accompanied by the presence of a weak but detectable ferromagneticlike signal, vanishing at the Néel temperature. In contrast to thin layer samples, we find that the AHE hysteresis loop shows an opposite sign and Barkhausen-like jumps. We introduce a macrospin model involving the Dzyaloshinskii–Moriya type interaction, which explains the existence of a nonzero magnetic moment in the absence of external field and reproduces well hysteretic behavior of the AHE. Using analysis of Néel-vector-dependent Berry curvature, we show that the intrinsic AHE in hexagonal MnTe can be nonzero even when the magnetization vanishes and, also, that it changes sign depending on the Fermi energy position. Published by the American Physical Society 2024

The integration of photonic elements with nitride optoelectronic structures allows control of emitted light properties, which is advantageous for achieving, e.g., a single wavelength lasing. Positioning of the photonic structures on the top surface of GaN‐based devices is problematic, in particular, for deposition of a metal contact to p‐type top layer. In this work, custom‐shaped submicron air channels arranged periodically 150 nm below the sample surface, forming an air/GaN diffraction grating embedded within a volume of the structure is proposed and fabricated. The fabrication process includes selective area Si ion implantation, GaN regrowth using plasma‐assisted molecular beam epitaxy, ultra‐high‐pressure annealing for efficient electrical activation of implanted Si without diffusion, and electrochemical etching for the removal of selectively doped material. Embedded air/GaN diffraction gratings with periodicity of 460 and 631 nm are shown. Width of air channels ranges from 46 to 320 nm. Angle and polarization resolved reflectivity measurements combined with theoretical modeling confirm the designed optical performance of the embedded diffraction gratings in the GaN volume. The presented design and fabrication of custom‐shaped, fully integrated photonic structures buried below the surface paves the way for novel type constructions of optoelectronic devices, such as compact distributed feedback laser diodes

Self-assembled, highly anisotropic nanostructures are spontaneously formed in the molecular beam epitaxy of antimony triselenide on GaAs substrates.

Newly discovered altermagnets are magnetic materials exhibiting both compensated magnetic order, similar to antiferromagnets, and simultaneous non-relativistic spin-splitting of the bands, akin to ferromagnets. This characteristic arises from specific symmetry operation that connects the spin sublattices. In this report, we show with ab initio calculations that semiconductive MnSe exhibits altermagnetic spin-splitting in the wurtzite phase as well as a critical temperature well above room temperature. It is the first material from such a space group identified to possess altermagnetic properties. Furthermore, we demonstrate experimentally through structural characterization techniques that it is possible to obtain thin films of both the intriguing wurtzite phase of MnSe and more common rock-salt MnSe using molecular beam epitaxy on GaAs substrates. The choice of buffer layers plays a crucial role in determining the resulting phase and consequently extends the array of materials available for the physics of altermagnetism.

Highly reflective mirrors are indispensable components in a variety of state‐of‐the‐art photonic devices. Typically used, bulky, multi‐layered distributed Bragg (DBR) reflectors are limited to lattice‐matched semiconductors or nonconductive dielectrics. Here, we introduce an inverted refractive index‐contrast grating (ICG) as compact, single‐layer alternative to DBR. In the ICG, a subwavelength one‐dimensional grating made of a low‐refractive‐index material is implemented on a high‐refractive‐index cladding. Our numerical simulations show that the ICG provides nearly total optical power reflectance for the light incident from the side of the cladding whenever the refractive index of the grating exceeds 1.75, irrespective of the refractive index of the cladding. Additionally, the ICG enables polarization discrimination and phase tuning of the reflected and transmitted light, the property not achievable with the DBR. We experimentally demonstrate a proof‐of‐concept ICG fabricated according to the proposed design, using the technique of sub‐µm 3D laser lithography in which thin stripes of IP‐Dip photoresist are micro‐printed on a Si cladding. This one‐step method avoids laborious and often destructive etching‐based procedures for grating structuration, making it possible to implement the grating on any arbitrary cladding material.