arXiv subjects

Mijin Kim

Publications and source records attributed to Mijin Kim.

13 recordsLinked to original sources

arXiv2026

Microwave noise downconversion in interband cascade laser frequency combs

Chip-scale semiconductor laser frequency combs offer remarkable prospects for compact and power-efficient optical sensors. For the laser to be suitable for typical comb applications, its degree of coherence must first be assessed from a microwave self-mixing signal. Unfortunately, such measurements require scarcely available high-speed photodetectors with multi-GHz bandwidths and radio-frequency electronics. However, in this work, we demonstrate a simplified approach to comb coherence assessment for interband cascade lasers based on a relationship between easily-accessible MHz-frequency (baseband) noise and the multi-GHz-frequency intermode beat note. The downconversion of microwave noise to near-DC frequencies is found to originate intrinsically from the laser, which simultaneously acts as a frequency mixer due to electrical nonlinearities and a phase-to-amplitude noise converter due to the linewidth enhancement factor. Correlation between the electrical signals is explored in both frequency and time domains. Since this phenomenon is potentially universal in semiconductor lasers, it creates a new opportunity for frequency comb characterization, which may be particularly valuable in wavelength regions where fast photodetectors have limited availability.

physics.optics↗

arXiv2026

Deriving and Validating Requirements Engineering Principles for Large-Scale Agile Development: An Industrial Longitudinal Study

In large scale agile systems development, the lack of a unified requirements engineering (RE) process is a major challenge, exacerbated by the absence of high level guiding principles for effective requirements management. To address this challenge, we conducted a five year longitudinal case study with Grundfos AB, in collaboration with the Software Centre in Sweden. RE principles were first derived through qualitative data collection spanning more than 25 sprints, approximately 320 weekly synchronisation meetings, and seven cross-company, company-specific workshops between 2019 and 2024. These activities engaged practitioners from diverse roles, representing several hundred developers across domains. In late 2024, five in depth focus groups with senior leaders at Grundfos provided retrospective validation of the principles and assessed their strategic impact. We aim to (1) empirically examine RE principles in large scale agile system development, (2) explore their benefits in practice within the case company, and (3) identify a set of transferable RE principles for large scale contexts. Using thematic analysis, six key RE principles architectural context, stakeholder-driven validation and alignment, requirements practices in large-scale agile organisations. evolution with lightweight documentation, delegated requirements management, organisational roles and responsibilities, and a shared understanding of requirements are derived. The study was further validated through crosscompany expert evaluation with three additional multinational organisations (Bosch, Ericsson, and Volvo Cars), which are directly responsible for largescale requirements management. Together, these efforts provide a scalable and adaptable foundation for improving requirements practices in largescale agile organisations.

cs.SE↗

arXiv2024

Deep learning based ECG segmentation for delineation of diverse arrhythmias

Accurate delineation of key waveforms in an ECG is a critical step in extracting relevant features to support the diagnosis and treatment of heart conditions. Although deep learning based methods using segmentation models to locate P, QRS, and T waves have shown promising results, their ability to handle arrhythmias has not been studied in any detail. In this paper we investigate the effect of arrhythmias on delineation quality and develop strategies to improve performance in such cases. We introduce a U-Net-like segmentation model for ECG delineation with a particular focus on diverse arrhythmias. This is followed by a post-processing algorithm which removes noise and automatically determines the boundaries of P, QRS, and T waves. Our model has been trained on a diverse dataset and evaluated against the LUDB and QTDB datasets to show strong performance, with F1-scores exceeding 99% for QRS and T waves, and over 97% for P waves in the LUDB dataset. Furthermore, we assess various models across a wide array of arrhythmias and observe that models with a strong performance on standard benchmarks may still perform poorly on arrhythmias that are underrepresented in these benchmarks, such as tachycardias. We propose solutions to address this discrepancy.

cs.LG↗

arXiv2024

Mapping Phonon Polaritons with Visible Light

Phonon polaritons (PhPs) are hybrid photon-phonon waves which enable strong light-matter interactions and subdiffractional confinement, potentially empowering applications in sensing, nonlinear optics and nanoscale energy manipulation. In this work, we use confocal Raman microscopy to investigate the coupling between bulk phonon modes and localized surface phonon polariton (SPhP) modes in indium phosphide (InP) nanopillars and 4H-silicon carbide (4H-SiC) gratings. The Raman intensity within the nanostructures is described in terms of the SPhP eigenmodes and used to reconstruct the field intensity, providing a method to map SPhP eigenmodes using visible and near-IR light. Our results indicate that, contrary to expectation, all Raman-active bulk phonon modes of InP and 4H-SiC couple to the localized SPhP modes. Further, we confirm that polarizability selection rules form the predominant coupling mechanism between phonons and SPhP modes, with electron-phonon coupling playing a role for certain phonon modes (A1(LO) and E1(TO) in 4H-SiC). These observations provide a method for extending Raman studies of PhP modes to achieve full 3D reconstruction of the PhP eigenmodes and visualize light-matter interactions within nanostructures, thus advancing Raman scattering as a technique for understanding PhP modes.

cond-mat.mtrl-sci↗

arXiv2022

Collective scattering from quantum dots in a photonic crystal waveguide

We demonstrate scattering of laser light from two InAs quantum dots coupled to a photonic crystal waveguide, which is achieved by strain-tuning the optical transitions of the dots into mutual resonance. By performing measurements of the intensity and photon statistics of transmitted laser light before and after tuning the dots into resonance, we show that the nonlinearity is enhanced by collective scattering. In addition to providing a means of manipulating few-photon optical nonlinearities, our approach establishes new opportunities for multi-emitter quantum optics in a solid-state platform.

quant-ph↗

arXiv2020

Zinc Sulphide Overlayer Two-Dimensional Photonic Crystal for Enhanced Extraction of Light from a Micro Cavity Light-Emitting Diode

A two-dimensional (2D) ZnS photonic crystal was deposited on the surface of a one-dimensional (1D) III-nitride micro cavity light-emitting diode (LED), to intermix the light extraction features of both structures (1D+2D). The deposition of an ideal micro-cavity optical thickness of lambda/2 is impractical for III-nitride LEDs, and in realistic multi-mode devices a large fraction of the light is lost to internal refraction as guided light. Therefore, a 2D photonic crystal on the surface of the LED was used to diffract and thus redirect this guided light out of the semiconductor over several hundred microns. Additionally, the employment of a post-epitaxy ZnS 2D photonic crystal avoided the typical etching into the GaN:Mg contact layer, a procedure which can cause damage to the near surface.

physics.app-ph↗

arXiv2020

Scalable in operando strain tuning in nanophotonic waveguides enabling three-quantum dot superradiance

The quest for an integrated quantum optics platform has motivated the field of semiconductor quantum dot research for two decades. Demonstrations of quantum light sources, single photon switches, transistors, and spin-photon interfaces have become very advanced. Yet the fundamental problem that every quantum dot is different prevents integration and scaling beyond a few quantum dots. Here, we address this challenge by patterning strain via local phase transitions to selectively tune individual quantum dots that are embedded in a photonic architecture. The patterning is implemented with in operando laser crystallization of a thin HfO$_{2}$ film "sheath" on the surface of a GaAs waveguide. Using this approach, we tune InAs quantum dot emission energies over the full inhomogeneous distribution with a step size down to the homogeneous linewidth and a spatial resolution better than 1 $μ$m. Using these capabilities, we tune multiple quantum dots into resonance within the same waveguide and demonstrate a quantum interaction via superradiant emission from three quantum dots.

quant-ph↗

arXiv2018

Mid-infrared dual-comb spectroscopy with low drive-power on-chip sources

Two semiconductor optical frequency combs consuming less than 1 W of electrical power are used to demonstrate high-sensitivity mid-infrared dual-comb spectroscopy in the important 3-4 $μ$m spectral region. The devices are 4 millimeters long by 4 microns wide, and each emits 8 mW of average optical power. The spectroscopic sensing performance is demonstrated by measurements of methane and hydrogen chloride with a spectral coverage of 33 cm$^{-1}$ (1 THz), 0.32 cm$^{-1}$ (9.7 GHz) frequency sampling interval, and peak signal-to-noise ratio of ~100 at 100 $μ$s integration time. The monolithic design, low drive power, and direct generation of mid-infrared radiation are highly attractive for portable broadband spectroscopic instrumentation in future terrestrial and space applications.

physics.ins-det↗

arXiv2018

Generation of photoluminescent ultrashort carbon nanotubes through nanoscale exciton localization at sp3 -defect sites

The intrinsic near-infrared photoluminescence observed in long single walled carbon nanotubes is systematically quenched in ultrashort single-walled carbon nanotubes (usCNTs, below 100 nm length) due to their short dimension as compared to the exciton diffusion length. It would however be key for number of applications to have such tiny nanostructure displaying photoluminescence emission to complement their unique physical, chemical and biological properties. Here we demonstrate that intense photoluminescence can be created in usCNTs (~40 nm length) upon incorporation of emissive sp3-defect sites in order to trap excitons. Using super-resolution imaging at <25 nm resolution, we directly reveal the localization of excitons at the defect sites on individual usCNTs. They are found preferentially localized at nanotube ends which can be separated by less than 40 nm and behave as independent emitters. The demonstration and control of bright near-infrared photoluminescence in usCNTs through exciton trapping opens the possibility to engineering tiny carbon nanotubes for applications in various domains of research including quantum optics and bioimaging.

physics.app-ph↗

arXiv2017

Multiheterodyne spectroscopy using interband cascade lasers

While mid-infrared radiation can be used to identify and quantify numerous chemical species, contemporary broadband mid-IR spectroscopic systems are often hindered by large footprints, moving parts and high power consumption. In this work, we demonstrate multiheterodyne spectroscopy using interband cascade lasers, which combines broadband spectral coverage with high spectral resolution and energy-efficient operation. The lasers generate up to 30 mW of continuous wave optical power while consuming less than 0.5 W of electrical power. A computational phase and timing correction algorithm is used to obtain kHz linewidths of the multiheterodyne beat notes and up to 30 dB improvement in signal-to-noise ratio. The versatility of the multiheterodyne technique is demonstrated by performing both rapidly swept absorption and dispersion spectroscopic assessments of low-pressure ethylene (C$_2$H$_4$) acquired by extracting a single beat note from the multiheterodyne signal, as well as broadband multiheterodyne spectroscopy of methane (CH$_4$) acquired with all available beat notes with microsecond temporal resolution and an instantaneous optical bandwidth of 240 GHz. The technology shows excellent potential for portable and high-resolution solid state spectroscopic chemical sensors operating in the mid-infrared.

physics.ins-det↗

arXiv2017

Active Tuning of Surface Phonon Polariton Resonances via Carrier Photoinjection

Surface-phonon polaritons (SPhPs) are attractive alternatives to far-infrared plasmonics for sub-diffractional confinement of light. Localized SPhP resonances in semiconductor nanoresonators are very narrow, but that linewidth and the limited extent of the Reststrahlen band inherently limit spectral coverage. To address this limitation, we report active tuning of SPhP resonances in InP and 4H-SiC by photoinjecting free carriers into the nanoresonators, taking advantage of the coupling between the carrier plasma and optical phonons to blue-shift SPhP resonances. We demonstrate state-of-the-art tuning figures of merit upon continuous-wave (CW) excitation (in InP) or pulsed excitation (in 4H-SiC). Lifetime effects cause the tuning to saturate in InP, and carrier-redistribution leads to rapid (<50 ps) recovery of the tuning in 4H-SiC. This work opens the path toward actively tuned nanophotonic devices, such as modulators and beacons, in the infrared and identifies important implications of coupling between electronic and photonic excitations.

cond-mat.mtrl-sci↗

arXiv2014

Cavity-stimulated Raman emission from a single quantum dot spin

Solid state quantum emitters have shown strong potential for applications in quantum information, but spectral inhomogeneity of these emitters poses a significant challenge. We address this issue in a cavity-quantum dot system by demonstrating cavity-stimulated Raman spin flip emission. This process avoids populating the excited state of the emitter and generates a photon that is Raman shifted from the laser and enhanced by the cavity. The emission is spectrally narrow and tunable over a range of at least 125 GHz, which is two orders of magnitude greater than the natural linewidth. We obtain the regime in which the Raman emission is spin-dependent, which couples the photon to a long-lived electron spin qubit. This process can enable an efficient, tunable source of indistinguishable photons and deterministic entanglement of distant spin qubits in a photonic crystal quantum network.

quant-ph↗

arXiv2012

Quantum control of a spin qubit coupled to a photonic crystal cavity

A key ingredient for a quantum network is an interface between stationary quantum bits and photons, which act as flying qubits for interactions and communication. Photonic crystal architectures are promising platforms for enhancing the coupling of light to solid state qubits. Quantum dots can be integrated into a photonic crystal, with optical transitions coupling to photons and spin states forming a long-lived quantum memory. Many researchers have now succeeded in coupling these emitters to photonic crystal cavities, but there have been no demonstrations of a functional spin qubit and quantum gates in this environment. Here we have developed a coupled cavity-quantum dot system in which the dot is controllably charged with a single electron. We perform the initialization, rotation and measurement of a single electron spin qubit using laser pulses and find that the cavity can significantly improve these processes.

quant-ph↗