Publications by authors named "Fabian Rotermund"

75 Publications

Interlayer Coupling and Ultrafast Hot Electron Transfer Dynamics in Metallic VSe/Graphene van der Waals Heterostructures.

ACS Nano 2021 Apr 24;15(4):7756-7764. Epub 2021 Mar 24.

Department of Physics, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea.

Atomically thin vanadium diselenide (VSe) is a two-dimensional transition metal dichalcogenide exhibiting attractive properties due to its metallic 1T phase. With the recent development of methods to manufacture high-quality monolayer VSe on van der Waals materials, the outstanding properties of VSe-based heterostructures have been widely studied for diverse applications. Dimensional reduction and interlayer coupling with a van der Waals substrate lead to its distinguishable characteristics from its bulk counterparts. However, only a few fundamental studies have investigated the interlayer coupling effects and hot electron transfer dynamics in VSe heterostructures. In this work, we reveal ultrafast and efficient interlayer hot electron transfer and interlayer coupling effects in VSe/graphene heterostructures. Femtosecond time-resolved reflectivity measurements showed that hot electrons in VSe were transferred to graphene within a 100 fs time scale with high efficiency. Besides, coherent acoustic phonon dynamics indicated interlayer coupling in VSe/graphene heterostructures and efficient thermal energy transfer to three-dimensional substrates. Our results provide valuable insights into the intriguing properties of metallic transition metal dichalcogenide heterostructures and motivate designing optoelectronic and photonic devices with tailored properties.
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http://dx.doi.org/10.1021/acsnano.1c01723DOI Listing
April 2021

Efficient perovskite solar cells via improved carrier management.

Nature 2021 02 24;590(7847):587-593. Epub 2021 Feb 24.

Division of Advanced Materials, Korea Research Institute of Chemical Technology, Daejeon, Republic of Korea.

Metal halide perovskite solar cells (PSCs) are an emerging photovoltaic technology with the potential to disrupt the mature silicon solar cell market. Great improvements in device performance over the past few years, thanks to the development of fabrication protocols, chemical compositions and phase stabilization methods, have made PSCs one of the most efficient and low-cost solution-processable photovoltaic technologies. However, the light-harvesting performance of these devices is still limited by excessive charge carrier recombination. Despite much effort, the performance of the best-performing PSCs is capped by relatively low fill factors and high open-circuit voltage deficits (the radiative open-circuit voltage limit minus the high open-circuit voltage). Improvements in charge carrier management, which is closely tied to the fill factor and the open-circuit voltage, thus provide a path towards increasing the device performance of PSCs, and reaching their theoretical efficiency limit. Here we report a holistic approach to improving the performance of PSCs through enhanced charge carrier management. First, we develop an electron transport layer with an ideal film coverage, thickness and composition by tuning the chemical bath deposition of tin dioxide (SnO). Second, we decouple the passivation strategy between the bulk and the interface, leading to improved properties, while minimizing the bandgap penalty. In forward bias, our devices exhibit an electroluminescence external quantum efficiency of up to 17.2 per cent and an electroluminescence energy conversion efficiency of up to 21.6 per cent. As solar cells, they achieve a certified power conversion efficiency of 25.2 per cent, corresponding to 80.5 per cent of the thermodynamic limit of its bandgap.
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http://dx.doi.org/10.1038/s41586-021-03285-wDOI Listing
February 2021

Non-resonant power-efficient directional Nd:YAG ceramic laser using a scattering cavity.

Nat Commun 2021 Jan 4;12(1). Epub 2021 Jan 4.

Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon, 34141, Republic of Korea.

Non-resonant lasers exhibit the potential for stable and consistent narrowband light sources. Furthermore, non-resonant lasers do not require well-defined optics, and thus has considerably diversified the available types of laser gain materials including powders, films, and turbid ceramics. Despite these intrinsic advantages, the practical applications of non-resonant lasers have been limited so far, mainly because of their low power efficiency and omnidirectional emission. To overcome these limitations, here we propose a light trap design for non-resonant lasers based on a spherical scattering cavity with a small entrance. Using a porous Nd:YAG ceramic, directional laser emission could be observed with significant enhancements in the slope efficiency and linewidth (down to 32 pm). A theoretical model is also developed to describe and predict the operation characteristics of proposed non-resonant laser.
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http://dx.doi.org/10.1038/s41467-020-20114-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7782720PMC
January 2021

Single-walled carbon-nanotube saturable absorber assisted Kerr-lens mode-locked Tm:MgWO laser.

Opt Lett 2020 Nov;45(22):6142-6145

We demonstrate sub-100-fs Kerr-lens mode-locking of a : laser emitting at ∼2µ assisted by a single-walled carbon-nanotube saturable absorber. A maximum average output power of 100 mW is achieved with pulse duration of 89 fs at a pulse repetition rate of ∼86. The shortest pulse duration derived from frequency-resolved optical gating amounts to 76 fs at 2037 nm, corresponding to nearly bandwidth-limited pulses. To the best of our knowledge, these are the shortest pulses generated from any Tm-doped tungstate crystal and the first report on saturable absorber assisted Kerr-lens mode-locking of a Tm laser at ∼2µ.
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http://dx.doi.org/10.1364/OL.411288DOI Listing
November 2020

Organic Broadband THz Generators Optimized for Efficient Near-Infrared Optical Pumping.

Adv Sci (Weinh) 2020 Oct 3;7(20):2001738. Epub 2020 Sep 3.

Department of Molecular Science and Technology Ajou University Suwon 443-749 Korea.

New organic THz generators are designed herein by molecular engineering of the refractive index, phonon mode, and spatial asymmetry. These benzothiazolium crystals simultaneously satisfy the crucial requirements for efficient THz wave generation, including having nonlinear optical chromophores with parallel alignment that provide large optical nonlinearity; good phase matching for enhancing the THz generation efficiency in the near-infrared region; strong intermolecular interactions that provide restraining THz self-absorption; high solubility that promotes good crystal growth ability; and a plate-like crystal morphology with excellent optical quality. Consequently, the as-grown benzothiazolium crystals exhibit excellent characteristics for THz wave generation, particularly at near-infrared pump wavelengths around 1100 nm, which is very promising given the availability of femtosecond laser sources at this wavelength, where current conventional THz generators deliver relatively low optical-to-THz conversion efficiencies. Compared to a 1.0-mm-thick ZnTe crystal as an inorganic benchmark, the 0.28-mm-thick benzothiazolium crystal yields a 19 times higher peak-to-peak THz electric field with a broader spectral bandwidth (>6.5 THz) when pumped at 1140 nm. The present work provides a valuable approach toward realizing organic crystals that can be pumped by near-infrared sources for efficient THz wave generation.
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http://dx.doi.org/10.1002/advs.202001738DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7578856PMC
October 2020

Spectroscopy and diode-pumped laser operation of transparent Tm:LuAlO ceramics produced by solid-state sintering.

Opt Express 2020 Sep;28(19):28399-28413

A transparent Tm:LuAlO ceramic is fabricated by solid-state reactive sintering at 1830 °C for 30 h using commercial α-AlO and LuO/TmO powders and sintering aids - MgO and TEOS. The ceramic belongs to the cubic system and exhibits a close-packed structure (mean grain size: 21 µm). The in-line transmission at ∼1 µm is 82.6%, close to the theoretical limit. The spectroscopic properties of the ceramic are studied in detail. The maximum stimulated-emission cross-section is 2.37×10 cm at 1749nm and the radiative lifetime of the F state is about 10 ms. The modified Judd-Ofelt theory accounting for configuration interaction is applied to determine the transition probabilities of Tm, yielding the intensity parameters Ω = 2.507, Ω = 1.236, Ω = 1.340 [10 cm] and α = 0.196×10 cm. The effect of excited configurations on lower-lying interconnected states with the same J quantum number is discussed. First laser operation is achieved under diode-pumping at 792 nm. A 4 at.% Tm:LuAlO ceramic laser generated 3.12 W at 2022-2035nm with a slope efficiency of 60.2%. The ceramic is promising for multi-watt lasers at >2 µm.
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http://dx.doi.org/10.1364/OE.400802DOI Listing
September 2020

Transition of pulsed operation from Q-switching to continuous-wave mode-locking in a Yb:KLuW waveguide laser.

Opt Express 2020 Jun;28(12):18027-18034

We report on the diverse pulsed operation regimes of a femtosecond-laser-written Yb:KLuW channel waveguide laser emitting near 1040 nm. By the precise position tuning of a carbon-nanotube-coated saturable absorber (SA) mirror, the transition of the pulsed operation from Q-switching, Q-switched mode-locking and finally sub-GHz continuous-wave mode-locking are obtained based on the interplay of dispersion and mode area control. The Q-switched pulses exhibit typical fast SA Q-switched pulse characteristics depending on absorbed pump powers. In the Q-switched mode-locking, amplitude modulations of the mode-locked pulses on the Q-switched envelope are observed. The radio-frequency spectrum represents the coexistence of Q-switching and mode-locking signals. In the purely mode-locked operation, the waveguide laser generates 2.05-ps pulses at 0.5 GHz.
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http://dx.doi.org/10.1364/OE.395701DOI Listing
June 2020

Laser-micromachined zebra-patterned graphene as a mode locker with adjustable loss.

Opt Lett 2020 Apr;45(7):1826-1829

In this Letter, we describe a novel, to the best of our knowledge, device based on micro-structured graphene, referred to as zebra-patterned graphene saturable absorber (ZeGSA), which can be used as a saturable absorber with adjustable loss to initiate femtosecond pulse generation. Femtosecond laser micro-machining was employed to ablate monolayer graphene on an infrasil substrate in the form of stripes with a different duty cycle, resulting in the formation of regions with variable insertion loss in the 0.21%-3.12% range. The mode-locking performance of the device was successfully tested using a ${{\rm Cr}^{4 {+} }}{:}\,{\rm forsterite}$Cr:forsterite laser, operating near 1250 nm. In comparison with mode locking using non-ablated graphene, the ZeGSA device with regions of decreasing graphene, enabled improved power performance where the mode-locked output power increased from 68 mW to 114 mW, and the corresponding pulse duration decreased from 62 to 48 fs at the same incident pump power of 6.3 W. These experiments indicate that ZeGSA shows great potential as a laser mode locker with adjustable loss and that it should find applications in the development of femtosecond lasers over a broad spectral range.
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http://dx.doi.org/10.1364/OL.389290DOI Listing
April 2020

MHz-repetition-rate, sub-mW, multi-octave THz wave generation in HMQ-TMS.

Opt Express 2020 Mar;28(7):9631-9641

We demonstrate the first megahertz (MHz) repetition-rate, broadband terahertz (THz) source based on optical rectification in the organic crystal HMQ-TMS driven by a femtosecond Yb:fibre laser. Pumping at 1035 nm with 30 fs pulses, we achieve few-cycle THz emission with a smooth multi-octave spectrum that extends up to 6 THz at -30 dB, with conversion efficiencies reaching 10 and an average output power of up to 0.38 mW. We assess the thermal damage limit of the crystal and conclude a maximum fluence of ∼1.8 mJ·cm at 10 MHz with a 1/e pump beam diameter of 0.10 mm. We compare the performance of HMQ-TMS with the prototypical inorganic crystal gallium phosphide (GaP), yielding a tenfold electric field increase with a peak on-axis field strength of 7 kV·cm and almost double the THz bandwidth. Our results further demonstrate the suitability of organic crystals in combination with fibre lasers for repetition-rate scaling of broadband, high-power THz sources for time-domain spectroscopic applications.
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http://dx.doi.org/10.1364/OE.386604DOI Listing
March 2020

Graphene mode-locked operation of Tm:YLiF and Tm:KYF lasers near 2.3  µm.

Opt Lett 2020 Feb;45(3):656-659

We report experimental demonstration of graphene mode-locked operation of ${{\rm Tm}^{3 + }}\!:\!{{\rm YLiF}_4}$Tm:YLiF (YLF) and ${{\rm Tm}^{3 + }}\!:\!{{\rm KY}_3}{{\rm F}_{10}}$Tm:KYF (KYF) lasers near 2.3 µm. To scale up the intracavity pulse energy, the cavity was extended, and double-end pumping was employed with a continuous-wave, tunable ${{\rm Ti}^{3 + }}\!:\!{\rm sapphire}$Ti:sapphire laser delivering up to 1 W near 780 nm. The extended ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm:KYF laser cavity was purged with dry nitrogen to eliminate pulsing instabilities due to atmospheric absorption lines, but this was not needed in the case of the ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm:YLF laser. Once initiated by graphene, stable uninterrupted mode-locked operation could be maintained with both lasers. With the extended cavity ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm:YLF laser, 921 fs pulses were generated at a repetition rate of 17.2 MHz at 2304 nm. 739 fs pulses were obtained at the repetition rate of 54 MHz from the ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm:KYF laser at 2340 nm. The corresponding pulse energy and peak power were 2.4 nJ and 2.6 kW for the ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm:YLF laser, and 1.2 nJ and 1.6 kW for the ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm:KYF laser. We foresee that it should be possible to generate shorter pulses at higher pump levels.
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http://dx.doi.org/10.1364/OL.385629DOI Listing
February 2020

Measurement of bending-induced birefringence in a hollow-core photonic crystal fiber.

Opt Lett 2019 Dec;44(23):5872-5875

We report the measurement of bending-induced birefringence in the presence of large intrinsic birefringence in a hollow-core photonic crystal fiber (HC-PCF). The fast axis of bending-induced birefringence was found to be normal to the bending plane, in contrast to the conventional fiber case. The dependence of the induced birefringence on the bending radius was also different from the typical inverse square law. Possibilities and design criteria for polarization controllers using bending-induced birefringence in HC-PCFs are presented.
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http://dx.doi.org/10.1364/OL.44.005872DOI Listing
December 2019

21  fs Cr:LiSAF laser mode locked with a single-walled carbon nanotube saturable absorber.

Opt Lett 2019 Oct;44(19):4662-4665

We report the shortest femtosecond pulses directly generated from a solid-state laser that is mode locked by using a single-walled carbon nanotube saturable absorber (SWCNT-SA). In the experiments, we used a 660 nm diode-pumped, low-threshold extended-cavity Cr:LiSAF laser operating around 850 nm with a repetition rate of 47.9 MHz. The SWCNT-SA mode-locked Cr:LiSAF laser produced 21 fs pulses with a time-bandwidth product of 0.56 by using only 210 mW of pump power. Pump-probe spectroscopy measurements showed that the SWCNT-SA exhibited saturable absorption with slow and fast decay times of 2.7 ps and 0.4 ps. The single-pass modulation depth and saturation fluence of the SWCNT-SA were further determined as 0.3% and 45  μJ/cm at the pump wavelength of 850 nm.
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http://dx.doi.org/10.1364/OL.44.004662DOI Listing
October 2019

Electrical control of terahertz frequency conversion from time-varying surfaces.

Opt Express 2019 Apr;27(9):12762-12773

We investigate the electrical control of frequency conversion from a time-varying interdigitated photo-conductive antenna (IPCA) and time-varying metasurface in the terahertz (THz) frequency range. Ultrafast near-infrared (NIR) optical pulses rapidly modify the conductivities of the IPCA and metasurface; however, external voltages can retard this conductivity transition. Thus, external voltages can be used to control the frequency conversion process based on the interaction between the THz waves and the time-varying surfaces. In the IPCA, both frequency up- and down-conversion processes are suppressed by external voltages. However, in the metasurface, the down-conversion is dramatically suppressed by external voltages, whereas the suppression on the up-conversion is less effective.
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http://dx.doi.org/10.1364/OE.27.012762DOI Listing
April 2019

Single-Layer Metasurfaces as Spectrally Tunable Terahertz Half- and Quarter-Waveplates.

ACS Appl Mater Interfaces 2019 Feb 18;11(8):7655-7660. Epub 2019 Feb 18.

Department of Physics , Korea University , Seoul 02841 , Korea.

We propose a single-layer terahertz metasurface that acts as an efficient terahertz waveplate, providing phase retardation of up to 180° with a tunable operation frequency. Designed with the tight coupling of elementary resonators, our metasurface provides extraordinarily strong hyperbolicity that is closely associated with the distance between resonators, enabling both significant phase retardation and spectral tunability through mechanical deformation. The proposed concept of terahertz waveplates based on relatively simple metastructures fabricated on stretchable polydimethylsiloxane is experimentally confirmed using terahertz spectroscopy. It is believed that the proposed design will pave the way for a diverse range of terahertz applications.
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http://dx.doi.org/10.1021/acsami.8b21456DOI Listing
February 2019

67-fs pulse generation from a mode-locked Tm,Ho:CLNGG laser at 2083 nm.

Opt Express 2019 Feb;27(3):1922-1928

We report on a mode-locked Tm,Ho:CLNGG laser emitting in the 2 µm spectral range using single-walled carbon nanotubes (SWCNTs) as a saturable absorber (SA). Pulses with duration of 98 fs are generated at 99.28 MHz repetition rate with an average output power of 123 mW, yielding a pulse energy of 1.24 nJ. Using a 0.5% output coupling, pulses as short as 67 fs, i.e., 10 optical cycles, are produced after extracavity compression with a 3-mm-thick ZnS plate.
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http://dx.doi.org/10.1364/OE.27.001922DOI Listing
February 2019

Comparative study of Yb:KYW planar waveguide lasers Q-switched by direct- and evanescent-field interaction with carbon nanotubes.

Opt Express 2019 Jan;27(2):1488-1496

Both direct- and evanescent-field interactions with carbon nanotubes (CNTs) are applied to achieve stable Q-switched operation of Yb:KYW planar waveguide lasers. The performance characteristics were investigated in a same cavity configuration and analyzed in detail in the following three cases, CNTs deposited onto end mirror (M-coating), output coupler (OC-coating) and top surface of the planar waveguide (WG-coating). Maximum output powers, repetition rates, and minimum pulse durations are 61 mW, 1103 kHz and 215 ns for OC-coating, 39 mW, 1052 kHz and 275 ns for WG-coating, and 26 mW, 1119 kHz and 217 ns for M-coating, respectively. From the calculation of the configuration-dependent stability range, the beam size and the electric field distribution in the Yb:KYW planar waveguide, it is confirmed that the evanescent-field interaction scheme makes stable Q-switching possible with much lower intensities at saturable absorber compared to the direct-field interaction scheme in the presented waveguide laser operation.
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http://dx.doi.org/10.1364/OE.27.001488DOI Listing
January 2019

Graphene and SESAM mode-locked Yb:CNGS lasers with self-frequency doubling properties.

Opt Express 2019 Jan;27(2):590-596

We report on mode-locking of an Yb:CaNbGaSiO laser, which is pumped by a fiber-coupled single-mode laser diode. The shortest pulse duration obtained with a semiconductor saturable absorber mirror is 52 fs, with 75 mW of average output power. Sub-60 fs operation tunable between 1055 and 1074 nm is achieved by employing semiconductor absorbers with different characteristics. We also demonstrate passive mode-locking results with transmissive graphene saturable absorber, reaching an 85 fs pulse duration with 23 mW output power. Moreover, we present the non-phase-matched self-frequency doubling properties of this non-centrosymmetric crystal in the femtosecond regime.
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http://dx.doi.org/10.1364/OE.27.000590DOI Listing
January 2019

Sub-80  fs mode-locked Tm,Ho-codoped disordered garnet crystal oscillator operating at 2081  nm.

Opt Lett 2018 Oct;43(20):5154-5157

We demonstrate a mode-locked (ML) femtosecond laser based on the disordered garnet crystal Tm,Ho:CNGG. Employing a single-walled carbon nanotube saturable absorber, pulses as short as 83 and 76 fs at 2081 nm are achieved without and with external compression, respectively. The latter represents, to the best of our knowledge, the shortest pulse duration obtained from any Ho-doped or Tm,Ho-codoped laser. The average power amounts to 67 mW at a repetition rate of 102 MHz. By analyzing the soliton ML regime, the nonlinear refractive index of Tm,Ho:CNGG is estimated to be ∼1.1×10  m/W.
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http://dx.doi.org/10.1364/OL.43.005154DOI Listing
October 2018

78  fs SWCNT-SA mode-locked Tm:CLNGG disordered garnet crystal laser at 2017  nm.

Opt Lett 2018 Sep;43(17):4268-4271

A passively mode-locked Tm:CLNGG laser using single-walled carbon nanotubes (SWCNT) as a saturable absorber (SA) is demonstrated at 2017 nm. Pulses as short as 78 fs are generated at an 86 MHz repetition rate with an average output power of 54 mW. By increasing the output coupling from 0.5% to 1.5%, a higher power of 100 mW is achieved for slightly longer pulses with a duration of 105 fs at 1996 nm.
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http://dx.doi.org/10.1364/OL.43.004268DOI Listing
September 2018

Anomalous Wavelength Scaling of Tightly Coupled Terahertz Metasurfaces.

ACS Appl Mater Interfaces 2018 Jun 30;10(23):19331-19335. Epub 2018 May 30.

Department of Physics , Korea University , Seoul 02841 , Republic of Korea.

We theoretically and experimentally demonstrate the drastic changes in the wavelength scaling of tightly coupled metasurfaces caused by deep subwavelength variations in the distance between the unit resonators but no change in the length scale of the units themselves. This coupling-dependent wavelength scaling is elucidated by our model metasurfaces of ring resonators arranged with deep subwavelength lattice spacing g, and we show that narrower g results in rapider changes in wavelength scaling. Also, by using terahertz time-domain spectroscopy, we experimentally observed a significant shift of the spectral response arising from very small variations in lattice spacing, confirming our theoretical predictions.
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http://dx.doi.org/10.1021/acsami.8b05806DOI Listing
June 2018

Feature issue introduction: Advanced Solid-State Lasers 2017.

Opt Express 2018 Apr;26(8):11018-11024

The Advanced Solid State Lasers 2017 Conference (ASSL) was held from October 1 to 5, 2017. It was an extraordinary conference at the Nagoya Congress Center in Nagoya, Japan. ASSL 2017 again suggested an impressive platform where miscellaneous perceptions with a variety of approaches to optics, photonics, sensing, laser technology, laser systems, and solid state lasers were presented. This international meeting was highly selective, leading to high level contributions through one plenary conference, 17 invited presentations, 70 regular talks, and 121 posters. The present joint issue of Optics Express and Optical Materials Express features 27 articles written by ASSL 2017 authors and covering the spectrum of solid-state lasers from materials research to sources, and from design innovation to applications.
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http://dx.doi.org/10.1364/OE.26.011018DOI Listing
April 2018

2-GHz carbon nanotube mode-locked Yb:YAG channel waveguide laser.

Opt Express 2018 Mar;26(5):5140-5145

We demonstrate GHz-repetition rate mode-locked operation of a femtosecond-laser-inscribed Yb:YAG channel waveguide laser using single-walled carbon nanotube saturable absorber mirror (SWCNT-SAM). A 6.3-mm-long, type II Yb:YAG waveguide laser with an extended cavity configuration delivers mode-locked picosecond (ps) pulses at GHz repetition rates. The dispersion of the laser cavity is compensated by the combination of a multi-functional output coupler and the Gires-Tournois interferometer (GTI) effect arising from an air-gap between the facet of the waveguide and the output coupler. The incident beam fluence on the SWNCNT-SAM is controlled by adjusting two intracavity lenses to avoid optical damage on the polymer nanocomposite matrix containing the SWCNTs. The average output power of our mode-locked waveguide laser is 322 mW at a pump power of 3.2 W. Nearly Fourier-limited, stable 2-ps-short pulses are generated at a repetition rate of 2.08 GHz.
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http://dx.doi.org/10.1364/OE.26.005140DOI Listing
March 2018

Tm:KY-x-yGdxLuy(WO) planar waveguide laser passively Q-switched by single-walled carbon nanotubes.

Opt Express 2018 Feb;26(4):4961-4966

A Tm monoclinic double tungstate planar waveguide laser is passively Q-switched (PQS) by a saturable absorber (SA) based on single-walled carbon nanotubes (SWCNTs) randomly oriented in a polymer film. The laser is based on a 18 µm-thick 5 at.% Tm:KY-x-yGdxLuy(WO) active layer grown on an undoped (010)-oriented KY(WO) substrate by liquid phase epitaxy with determined propagation losses 0.7 ± 0.2 dB/cm. The PQS laser generated a maximum average output power of 45.6 mW at 1.8354 µm with a slope efficiency of 22.5%. Stable 83-ns-long laser pulses with an energy of 33 nJ were achieved at a repetition rate of 1.39 MHz. The use of SWCNTs as SA is promising for generation of sub-100 ns pulses in such waveguide lasers at ~2 µm.
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http://dx.doi.org/10.1364/OE.26.004961DOI Listing
February 2018

Sub-100  fs Tm:MgWO laser at 2017  nm mode locked by a graphene saturable absorber.

Opt Lett 2017 Aug;42(16):3076-3079

We present the first sub-100 fs bulk solid-state laser in the 2-μm spectral range employing the monoclinic Tm-dopedMgWO crystal as an active medium. By applying a graphene-based saturable absorber and chirped mirrors for dispersion management, stable self-starting mode-locked operation at 2017 nm was achieved. Nearly Fourier-limited pulses as short as 86 fs featuring a bandwidth of 53 nm were generated at a repetition rate of 76 MHz. A pulse energy of 1.1 nJ was achieved at 87 MHz for a pulse duration of 96 fs. The mode-locked Tm:MgWO laser exhibits excellent stability with a fundamental beat note extinction ratio of 80 dBc above noise level.
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http://dx.doi.org/10.1364/OL.42.003076DOI Listing
August 2017

Benzothiazolium Single Crystals: A New Class of Nonlinear Optical Crystals with Efficient THz Wave Generation.

Adv Mater 2017 Aug 7;29(30). Epub 2017 Jun 7.

Department of Molecular Science and Technology, Ajou University, Suwon, 443-749, South Korea.

Highly efficient nonlinear optical organic crystals are very attractive for various photonic applications including terahertz (THz) wave generation. Up to now, only two classes of ionic crystals based on either pyridinium or quinolinium with extremely large macroscopic optical nonlinearity have been developed. This study reports on a new class of organic nonlinear optical crystals introducing electron-accepting benzothiazolium, which exhibit higher electron-withdrawing strength than pyridinium and quinolinium in benchmark crystals. The benzothiazolium crystals consisting of new acentric core HMB (2-(4-hydroxy-3-methoxystyryl)-3-methylbenzo[d]thiazol-3-ium) exhibit extremely large macroscopic optical nonlinearity with optimal molecular ordering for maximizing the diagonal second-order nonlinearity. HMB-based single crystals prepared by simple cleaving method satisfy all required crystal characteristics for intense THz wave generation such as large crystal size with parallel surfaces, moderate thickness and high optical quality with large optical transparency range (580-1620 nm). Optical rectification of 35 fs pulses at the technologically very important wavelength of 800 nm in 0.26 mm thick HMB crystal leads to one order of magnitude higher THz wave generation efficiency with remarkably broader bandwidth compared to standard inorganic 0.5 mm thick ZnTe crystal. Therefore, newly developed HMB crystals introducing benzothiazolium with extremely large macroscopic optical nonlinearity are very promising materials for intense broadband THz wave generation and other nonlinear optical applications.
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http://dx.doi.org/10.1002/adma.201701748DOI Listing
August 2017

Terahertz optical bistability of graphene in thin layers of dielectrics.

Opt Express 2017 Apr;25(8):8484-8490

We theoretically studied in terahertz frequency regime optical bistability of graphene placed at the interface between thin dielectric layers. We solved self-consistently the nonlinear wave equations containing the third-order optical conductivity of graphene in four-layer structures and obtained hysteresis response of the transmitted power as a function of the incident power. We numerically observed that the critical powers for the up and down transitions and the Fermi-energy of graphene required for terahertz optical bistability can be reduced by carefully choosing material properties and the thicknesses of dielectric layers. Furthermore, these values can be substantially decreased when graphene as a randomly stacked multilayer structure is asymmetrically located in thin dielectric layers.
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http://dx.doi.org/10.1364/OE.25.008484DOI Listing
April 2017

Robust, low-noise, polarization-maintaining mode-locked Er-fiber laser with a planar lightwave circuit (PLC) device as a multi-functional element.

Opt Lett 2017 Apr;42(8):1472-1475

We demonstrate a new planar lightwave circuit (PLC)-based device, integrated with a 980/1550 wavelength division multiplexer, an evanescent-field-interaction-based saturable absorber, and an output tap coupler, which can be employed as a multi-functional element in mode-locked fiber lasers. Using this multi-functional PLC device, we demonstrate a simple, robust, low-noise, and polarization-maintaining mode-locked Er-fiber laser. The measured full-width at half-maximum bandwidth is 6 nm centered at 1555 nm, corresponding to 217 fs transform-limited pulse duration. The measured RIN and timing jitter are 0.22% [10 Hz-10 MHz] and 6.6 fs [10 kHz-1 MHz], respectively. Our results show that the non-gain section of mode-locked fiber lasers can be easily implemented as a single PLC chip that can be manufactured by a wafer-scale fabrication process. The use of PLC processes in mode-locked lasers has the potential for higher manufacturability of low-cost and robust fiber and waveguide lasers.
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http://dx.doi.org/10.1364/OL.42.001472DOI Listing
April 2017

Monoclinic Tm3+:MgWO4: a promising crystal for continuous-wave and passively Q-switched lasers at ∼2  μm.

Opt Lett 2017 Mar;42(6):1177-1180

Monoclinic thulium-doped magnesium monotungstate, Tm3+:MgWO4, is promising for efficient power-scalable continuous-wave (CW) and passively Q-switched lasers at >2  μm. Under diode pumping at 802 nm, a compact CW laser based on Z-cut Tm:MgWO4 generated 3.09 W at 2022-2034 nm with a slope efficiency of 50% which represents the highest output power ever achieved with this type of laser host. Stable passive Q-switching of the Tm:MgWO4 laser is demonstrated for the first time, to the best of our knowledge, using single-walled carbon nanotubes, graphene, and Cr2+:ZnS saturable absorbers. Using the latter, the best performance is achieved with 16.1 μJ/13.6 ns pulses at 2017.8 nm with a maximum average output power of 0.87 W and a peak power of 1.18 kW.
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http://dx.doi.org/10.1364/OL.42.001177DOI Listing
March 2017

Femtosecond-laser-written Tm:KLu(WO4)2 waveguide lasers.

Opt Lett 2017 Mar;42(6):1169-1172

Depressed-index channel waveguides with a circular and photonic crystal cladding structures are prepared in a bulk monoclinic Tm:KLu(WO4)2 crystal by 3D direct femtosecond laser writing. The channel waveguide structures are characterized and laser operation is achieved using external mirrors. In the continuous-wave mode, the maximum output power of 46 mW is achieved at 1912 nm corresponding to a slope efficiency of 15.2% and a laser threshold of only 21 mW. Passive Q-switching of a waveguide with a circular cladding is realized using single-walled carbon nanotubes. Stable 7 nJ/50 ns pulses are achieved at a repetition rate of 1.48 MHz. This first demonstration of ∼2  μm fs-laser-written waveguide lasers based on monoclinic double tungstates is promising for further lasers of this type doped with Tm3+ and Ho3+  ions.
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http://dx.doi.org/10.1364/OL.42.001169DOI Listing
March 2017

Control of terahertz nonlinear transmission with electrically gated graphene metadevices.

Sci Rep 2017 02 20;7:42833. Epub 2017 Feb 20.

Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-751, Republic of Korea.

Graphene, which is a two-dimensional crystal of carbon atoms arranged in a hexagonal lattice, has attracted a great amount of attention due to its outstanding mechanical, thermal and electronic properties. Moreover, graphene shows an exceptionally strong tunable light-matter interaction that depends on the Fermi level - a function of chemical doping and external gate voltage - and the electromagnetic resonance provided by intentionally engineered structures. In the optical regime, the nonlinearities of graphene originated from the Pauli blocking have already been exploited for mode-locking device applications in ultrafast laser technology, whereas nonlinearities in the terahertz regime, which arise from a reduction in conductivity due to carrier heating, have only recently been confirmed experimentally. Here, we investigated two key factors for controlling nonlinear interactions of graphene with an intense terahertz field. The induced transparencies of graphene can be controlled effectively by engineering meta-atoms and/or changing the number of charge carriers through electrical gating. Additionally, nonlinear phase changes of the transmitted terahertz field can be observed by introducing the resonances of the meta-atoms.
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http://dx.doi.org/10.1038/srep42833DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5316969PMC
February 2017
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