Compact Light Source Research Articles

Quasi‐Bound States in the Continuum Enhanced Second‐Harmonic Generation in Thin‐Film Lithium Niobate

AbstractThis work reports the second‐harmonic generation (SHG) in a z‐cut thin‐film lithium niobate (LN) under nearly normal‐incident linearly‐polarized pumping. To utilize the largest nonlinear term which is along the out‐of‐plane direction in the z‐cut LN film, the film is patterned with a 2D periodic array to support symmetry‐protected bound states in the continuum (BICs) bearing strong out‐of‐plane electric field component. Enabled by such optical resonances near the BIC condition at the pumping wavelength, significant enhancement in SHG compared to an unpatterned film is observed. This approach, combining transparent thin‐film nonlinear materials with tailored BICs, offers prospects for engineering compact coherent light sources in a broad wavelength range.

High-Harmonic Generation Using a Single Dielectric Nanostructure

High-harmonic generation (HHG) from solids is a novel method used to emanate coherent extreme-ultraviolet (EUV) pulses. The efficiency of plasmonic HHG can be improved by enhancing the field of nanostructures. However, the nanostructures used for plasmonic HHG have a limitation owing to the damage caused by the amplified field. This study presents a single conical sapphire nanostructure used as a compact HHG emitter that generates high-order harmonics with wavelengths up to approximately 60 nm without causing severe damage. We compare the structure with a gold-layered conical sapphire structure and a bulk sapphire. The conical sapphire structure has a higher damage threshold and reusability for EUV generation even though it has a lower HHG intensity than that of the gold-layered conical sapphire structure because of the lower intensity enhancement. The measured signal intensity of the high-order harmonics in the EUV band from the conical sapphire structure is ten times higher than that of the bulk sapphire. The results confirm the possibility of creating a compact EUV light source for nanoscale applications.

Fully integrated electrically driven optical frequency comb at communication wavelength

Abstract To meet the high demand of data transmission capacity, optical communications systems have been developed. In order to increase the channel numbers for larger communication bandwidth, multi-mode lasers and laser arrays can be used. As an alternative coherent light source, optical frequency comb (OFC) contains multi-wavelength signal, and hence enables communication with data stream of tens of terabit/s. Fully integrated electrically driven OFCs are expected as a compact, robust, and low-cost light source for data communication. In this review article, the recent development progress on fully integrated electrically driven OFC generators are reviewed, with focus on the demonstrations in the past five years. Based on comb generation approaches, the works are categorized into two main types: one is OFC generators based on four-wave mixing in high-Q resonator, and the other is OFC generators based on mode-locked laser. At the end, a summary and future outlook are provided.

From Alignment Laser to 3D Structures: Determining Structures of GPCRs in a Compact X‐ray Light Source

The Compact X‐ray Light Source (CXLS) at Arizona State University produces diffraction patterns with intense X‐ray by Inverse Compton Scattering (ICS). In ICS, a photon acquires energy from an electron to produce an X‐ray. With this system, we can study G‐Protein Coupled Receptors (GPCRs), which are peripheral membrane proteins targeted by 60% of FDA approved drugs. When GPCRs are activated, a series of reaction can occur. For example, opening and closing of ion channels, activation or deactivation of enzymes, and protein hydrolysis. The structural characterization of GPCRs can lead to optimization of drug development, by assessing a better targeting mechanism that will lead to fewer side‐effects and better drug biding. We conducted a laser alignment in the CXLS using a YAG screen, which is a fluorescent screen that helps determine the size and intensity of the beam. Furthermore, we provide the crystallization process of GPCRs using Lipidic Cubic Phase (LCP) and describe its potential for structure determination using the CXLS and CXFEL.

Laboratory system for optical coherence tomography (OCT) using a laser plasma source of soft x-rays and extreme ultraviolet and focusing ellipsoidal optics.

Optical coherence tomography (OCT) with the use of soft x-rays (SXR) and extreme ultraviolet (EUV) has been recently demonstrated [Fuchs et al. Sci. Rep.6, 20658 (2016)10.1038/srep20658; Fuchs et al. Optica4, 903 (2017)10.1364/OPTICA.4.000903]. This new imaging technique, named XCT, makes it possible to obtain cross-sectional and tomographic images of objects with nanometer spatial resolution. The article presents a newly developed laboratory system for XCT using a compact laser plasma light source operating in the SXR and EUV spectral ranges. The source is based on a gas puff target containing Kr gas or a Kr/Xe gas mixture irradiated with nanosecond laser pulses from an Nd:YAG laser. The use of the gas puff target enables efficient emission of SXR and EUV radiation without generating target debris associated with laser ablation when using a solid target. The system is equipped with an ellipsoidal mirror to collect radiation from the source and focus on the imaged object. The XCT measurements are made by processing the spectrum of the radiation reflected from the object recorded with a transmission grating spectrometer equipped with an identical focusing mirror and a CCD camera. The paper presents the characterization and optimization of the new XCT system and its application to the measurements of layered nanostructures.

Ultra‐Efficient GAGG:Cr3+Ceramic Phosphor‐Converted Laser Diode: A Promising High‐Power Compact Near‐Infrared Light Source Enabling Clear Imaging

AbstractNear‐infrared (NIR) light is widely used in fields such as organic components detection and biological imaging, due to its strong tissue penetration and non‐destructive characteristics. To improve the detection sensitivity and spatial resolution of bioimaging, it is important to obtain a new NIR light source with good thermal stability, high efficiency, and ultra‐high optical output power, which currently remains a great challenge. In view of this, a Gd3Al2Ga3O12:Cr3+(GAGG:Cr3+) NIR ceramic phosphor is proposed in this study. By optimizing the preparation process of GAGG:Cr3+ceramic phosphor and the concentration of Cr3+ions, high external quantum efficiency (EQE = 70.1%) and excellent thermal stability (96.8%@150 °C) are achieved. After coupling the GAGG:Cr3+ceramic phosphor with a blue light‐emitting diode (LED), the electro‐optical conversion efficiency of the NIR ceramic phosphor converted LED (NIR‐cpc‐LED) reaches 31%@20 mA, which is higher than the previously reported values. Moreover, when coupled with a blue laser diode (LD), the optical output power of the NIR‐cpc‐LD reaches as high as 1652.6 mW@5.5 W. Therefore, GAGG:Cr3+NIR‐cpc‐LD not only shows a great prospect in practical applications, but also provides a new solution for high‐power compact NIR light sources.

Near-infrared dual-wavelength surface-emitting light source using InAs quantum dots resonant with vertical cavity modes

We developed a compact dual-wavelength surface-emitting light source using InAs quantum dots (QDs) embedded in a vertical cavity (VC). The VC was designed to possess two optical cavity modes that resonate with the discrete emission lines of the QDs. The fabricated light source exhibited significant enhancements in the vertical light emission corresponding to the VC modes. In addition, the light source demonstrated selectivity to the enhanced emission wavelengths with changes in temperature. Compared to conventional dual-wavelength vertical external cavity surface-emitting lasers, these QD-based dual-wavelength emission devices allow for the realization of simple structures because the InAs QDs act as dual-light-emitting materials. These results can be applied to simple dual-wavelength surface-emitting light sources.

Cr3+ and Nd3+ co-activated garnet phosphor for NIR super broadband pc-LED application

Near-infrared phosphor converted light emitting diodes (NIR pc-LEDs), as a compact light source, play a constructive role in wearable optical biosensing devices. In order to detect the properties of different substances, the NIR emission should be as wide as possible. The typical strategy for broadening the band is the superposition emission of Cr3+ and Yb3+, the bandwidth of which is 300–320 nm, and it needs to be further broadened. In this paper, NIR Ca2LuZr2Al3O12 garnet phosphor, co-activated by Cr3+ and Nd3+, is prepared. The emission quantum efficiencies of Nd3+ at 900 nm and 1060 nm are close to 100%, and the emission band is complementary to that of Yb3+ at 1030 nm. An efficient super broadband NIR pc-LEDs (47.6 mW@100 mA), with the bandwidth of 350 nm, are achieved. The bandwidth and the efficiency of the pc-LEDs are improved greatly, compared with other reported ones.

Multiple coherent dispersive waves generation in silicon nitride slot waveguide

A method to generate multiple dispersive waves (DWs) with pumping in a normal dispersion regime is proposed. We show flexibility of producing four DWs in a silicon nitride (Si3N4) slot waveguide and explain the underlying dynamics of a four DWs generation process in detail, i.e., DWs emission via the optical wave-breaking phenomenon in an anomalous dispersion regime. Compression of the first anomalous DW results in the subsequent cascade DW generation. Combined with the soliton induced blue-shifted dispersive wave and red-shifted dispersive wave, the spectrum spans from visible to mid-IR with high coherence. The four DWs generation process has agreement with prediction of the phase-matching condition. Overall, this multiple DWs generation method in a Si3N4 waveguide provides the possibility for having an efficient, compact, and coherent mid-IR light source.

Improved efficiency of photo-induced synthetic reactions enabled by advanced photo flow technologies.

In this article, we discuss how effective photo-induced organic reactions became when applied evolving photo flow technologies through our experiences over these two last decades. We started with the flow update of traditional [2 + 2] cycloaddition using Mikroglas Dwell device as a flow reactor and a compact light source, such as blacklight, instead of a high-pressure mercury lamp. Then we examined Barton nitrite reaction using a photo flow reactor consisting of stainless-steel channels and a quartz glass top provided by DNS. Again the use of blacklight was successful. However, the energy profile of these reactions was improved further by the use of LED lights. We used a photo-flow set-up, consisting of stainless steel engraved microchannels covered by a quartz top (MiChS L-1) and a sodium lamp, for the isomerization of a fulleroid to PCBM. Photo-redox-catalyzed alkene alkylation proceeded within a shortened reaction time when the same photo flow reactor and white LED were used instead of a batch reactor. Photo-induced reductive 5-exo-dig radical cyclization and reduction of alkenyl halides proceeded smoothly, thanks to the combination of a photo flow reactor and low-pressure Hg lamp. We also applied flow technologies for photo-bromination and chlorination of C-H bonds. Photocatalytic oxidation of benzyl alcohol by molecular oxygen became quick when high-power LED irradiation was employed.

Broadband NIR Garnet Phosphors with Improved Thermal Stability via Energy Transfer

Near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) have attracted more and more attention because of their many potential optical applications. However, high-performance broadband NIR phosphors are still rare. Here, we demonstrated that Cr3+-to-Yb3+ energy transfer (ET) is an effective strategy to boost the properties of NIR luminescence materials in Ca2LaZr2Ga2.8Al0.2O12: Cr3+,Yb3+ (CZGG: Cr3+, Yb3+). In addition to the enrichment of short-wave NIR emission in 900–1100 nm, the overall thermal stability of CZGG: Cr3+, Yb3+ can be improved by leveraging high-efficiency ET from Cr3+ to Yb3+, ascribed to the fast ET from Cr3+ to its nearest Yb3+ as well as the superior thermal stability of Yb3+. Additionally, an NIR pc-LED device was packaged via combining CZGG: Cr3+, Yb3+ with a blue LED chip to demonstrate its possible application in compact nonvisible light sources.

Multi-wavelength 128 Gbit s−1 λ −1 PAM4 optical transmission enabled by a 100 GHz quantum dot mode-locked optical frequency comb

Semiconductor mode-locked lasers (MLLs) with extremely high repetition rates are promising optical frequency comb (OFC) sources for their usage as compact, high-efficiency, and low-cost light sources in high-speed dense wavelength-division multiplexing transmissions. The fully exploited conventional C- and L- bands require the research on O-band to fulfil the transmission capacity of the current photonic networks. In this work, we present a passive two-section InAs/InGaAs quantum-dot (QD) MLL-based OFC with a fundamental repetition rate of ∼100 GHz operating at O-band wavelength range. The specially designed device favours the generation of nearly Fourier-transform-limited pulses in the entire test range by only pumping the gain section while with the absorber unbiased. The typical integrated relative intensity noise of the whole spectrum and a single tone are −152 and −137 dB Hz−1 in the range of 100 MHz–10 GHz, respectively. Back-to-back data transmissions for seven selected tones have been realised by employing a 64 Gbaud four-level pulse amplitude modulation format. The demonstrated performance shows the feasibility of the InAs QD MLLs as a simple structure, easy operation, and low power consumption OFC sources for high-speed fibre-optic communications.

Eu2+‐Doped Layered Double Borate Phosphor with Ultrawide Near‐Infrared Spectral Distribution in Response to Ultraviolet–Blue Light Excitation

AbstractUltrabroadband near‐infrared (NIR) luminescent materials are important components for compact light sources used in food testing, medical and biosensing applications. A major long‐term challenge facing NIR luminescent materials is to design ultrabroadband phosphors activated by the rare‐earth ions characterized by the f–d transition. Here, an Eu2+‐doped Ba3Lu(BO3)3 phosphor, showing peculiar luminescence characteristics such as a long emission wavelength ≈720 nm, a broader full width at half maximum of ≈197 nm, a large Stokes shift of ≈8300 cm–1, and a longer decay time ≈5.58 µs, is reported. The smaller Eu2+ occupy the irregular and larger Ba2+ sites further reducing the symmetry, resulting in the increased splitting of the degenerate 5d level, thereby lowering the energy of Eu2+ emission. The longer decay time is also associated with the irregular Ba sites. This work improves the NIR mechanism of Eu2+ and opens up a new idea for exploring promising broadband NIR luminescent materials.

Materials Science Research Using Compact Synchrotron Light Source: Toward the Creation of Facility with Outstanding Feature

Materials Science Research Using Compact Synchrotron Light Source: Toward the Creation of Facility with Outstanding Feature

Optimizing high harmonic generation in hollow-core gas cell considering variation of gas density

We have developed a coherent and compact extreme-ultraviolet light source based on phase-matched high harmonic generation by using a hollow-core gas cell that can be easily aligned and maintain long stability. The gas cell consists of three bodies to minimize gas leakage and maintain stable vacuum. The photon flux was calculated considering the dependence of the gas number density and the pressure on the radii of the hollow core and the hole in the Al plates on both sides of the gas cell, which showed good coincidence with the experimental values.The photon flux is 2 × 1010 photons s−1 near 13.5 nm (91.5 eV) within 10 nm bandwidth. Also, the compact extreme-ultraviolet light source is found to operate stably for hours, which is suitable for practical applications such as metrology and inspection for extreme-ultraviolet mask, spectroscopy, holography, microscopy, etc.

Development of a high-gradient X-band RF gun with replaceable field emission cathodes for RF breakdown studies

The contradiction between people’s infinite pursuit of charged particle beam energy and finite laboratory space has led to the development of high gradient acceleration structure. As X-band high gradient structures are worldwide used in Collider, FELs and compact light sources, a more detailed observation and deeper understanding of the bottleneck of gradient improvement—RF breakdown are strongly demanded. To this end, an X-band electron gun with a replaceable field emission cathode has been developed at Tsinghua university to serve as the observation object. Said gun is composed of 2.37 cavities and a pluggable cathode, with a tapered tip that was machined on the cathode centre to increase the local emission current. To ensure the cathode replaceability while eliminating the leaking field, a novel design of the first cavity working at TM02 mode was adopted. After conditioning of around 107 pulses, the peak electric field of the cathode tip achieved more than 1 GV/m, and a stable current of 3 mA averaged over rf pulse was detected. These results lay a foundation for the observation of RF breakdown in our next stage.

A Compact Accelerator-Based Light Source for High-Power, Full-Bandwidth Tunable Coherent THz Generation

Terahertz (THz) radiation sources are increasingly significant for many scientific frontiers, while the generation of THz radiation with high-power at wide-tunable frequencies is still a limitation for most existing methods. In this paper, a compact accelerator-based light source is proposed to produce coherent THz radiation with high pulse energy and tunable frequency from 0.1 THz to 60 THz. By using a frequency beating laser-modulated electron beam and undulator taper, intense coherent THz radiation can be generated through undulators. Theoretical analysis and numerical simulations demonstrate that the proposed technique can generate narrow-bandwidth THz radiation with a pulse energy up to 6.3 millijoule (mJ) and the three-dimensional effects of beam has limited influence on its performance. The proposed technique will open up new opportunities for THz spectroscopic and time-resolved experiments.

VCSEL Wavelength Setting by Intra-Cavity Phase Tuning — Numerical Analysis and Experimental Verification

Monolithic multi-wavelength VCSEL arrays, with wavelengths of individual VCSELs precisely set in a post-epitaxial growth process, would enable compact multi-color light sources and transmitters for various sensing and datacom applications. Here we report on a numerical study of the requirements of spectral matching and balancing of DBR reflectances, optical confinement factor, and optical gain for uniformity of threshold current and slope efficiency over wavelength with wavelength set by intra-cavity phase tuning. The requirements are verified by an experimental demonstration of intra-cavity phase tuned VCSELs in the spectral range 1043–1067 nm with a wavelength spacing of 8 ± 1 nm enabled by precise Ar ion-beam etching. Small variations of threshold current and slope efficiency are achieved by close to ideal spectral matching and balancing. The VCSELs are GaAs-based with a dielectric SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> top-DBR. We conclude that high uniformity of threshold current and slope efficiency is demanding in terms of spectral matching and balancing and requires not only very precise etching for intra-cavity phase tuning and precise thickness of the layers in the dielectric top-DBR, but also precise thickness and composition of the layers in the epitaxial semiconductor part.

Maximal terahertz emission in high harmonic generation from 3D Dirac semimetals

Unlike conventional semiconductor platforms, 3D Dirac semimetals (DSMs) require relatively low input laser intensities for efficient terahertz (THz) high harmonic generation (HHG), making them promising materials for developing compact THz light sources. Here, we show that 3D DSMs’ high nonlinearity opens up a regime of nonlinear optics where extreme subwavelength current density features develop within nanoscale propagation distances of the driving field. Our results reveal orders-of-magnitude enhancement in HHG intensity with thicker 3D DSM films, and show that these subwavelength features fundamentally limit HHG enhancement beyond an optimal film thickness. This decrease in HHG intensity beyond the optimal thickness constitutes an effective propagation-induced dephasing. Our findings highlight the importance of propagation dynamics in nanofilms of extreme optical nonlinearity.

Ultra-Compact Fourier Transform Near-Infrared MEMS Spectral Sensor for Smart Industry and IoT

Miniaturized spectrometers are being developed in the recent few years leveraging the rapidly-progressing technology and triggering new markets in the field of on-site and mobile spectroscopic analysis. Although some devices were commercialized for the near-infrared spectroscopy (NIRS), their size and cost still don't meet the requirements for the ubiquitous deployment of the spectrometer as a spectral sensor needed for the smart industry and IoT applications. In this work we report an integrated microelectromechanical system (MEMS)-based Fourier Transform Infrared (FT-IR) spectral sensor. A monolithic scanning Michelson interferometer MEMS chip and an extended InGaAs photodetector chip are integrated with a micro-optical system in a tiny package with a footprint of 18 mm, presenting the smallest reported FT-IR solution to date. A compact light source head is designed and implemented targeting diffuse-reflection applications. The reported sensor's spectral resolution is about 66.6 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> across the spectral range of 1350-2550 nm. The signal-to-noise ratio (SNR) achieved is 8000:1 at 2400 nm in 2 seconds, with an order of magnitude improvement with respect to previously reported fiber-coupled solution. The presented solution provides a low cost, low power, wide wavelength range NIR spectral sensor that can be manufactured with high volumes leveraging economies of scale.