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International audience; Sub-100 fs pulse generation from a passively mode-locked Tm,Ho-codoped cubic multicomponent disordered garnet laser at ∼2 µm is demonstrated. A single-walled carbon nanotube saturable absorber is implemented to initiate and stabilize the soliton mode-locking. The Tm,Ho:LCLNGG (lanthanum calcium lithium niobium gallium garnet) laser generated pulses as short as 63 fs at a central wavelength of 2072.7 nm with an average output power of 63 mW at a pulse repetition rate of ∼102.5 MHz. Higher average output power of 121 mW was obtained at the expense of longer pulse duration (96 fs) at 2067.6 nm using higher output coupling. To the best of our knowledge, this is the first report on mode-locked operation of the Tm,Ho:LCLNGG crystal.

We report on the first sub-100 fs mode-locked laser operation of a Tm3+-doped disordered calcium lithium tantalum gallium garnet (Tm:CLTGG) crystal. Soliton mode-locking was initiated and stabilized by a transmission-type single-walled carbon nanotube saturable absorber. Pulses as short as 69 fs were achieved at a central wavelength of 2010.4 nm with an average power of 28 mW at a pulse repetition rate of ∼87.7 MHz. In the sub-100 fs regime, the maximum average output power amounted to 103 mW.

We report on a power-scalable sub-100-fs laser in the 2-μm spectral range using a Tm3+-doped ‘mixed’ (Lu,Sc)2O3 sesquioxide ceramic as an active medium. Pulses as short as 58 fs at 2076 nm with an average output power of 114 mW at a pulse repetition rate of approximately 82.9 MHz are generated by employing single-walled carbon nanotubes as a saturable absorber. A higher average power of 350 mW at 2075 nm is obtained at the expense of the pulse duration (65 fs). A maximum average power of 486 mW is achieved for a pulse duration of 98 fs and an optical conversion efficiency of 22.3%, representing the highest value ever reported from sub-100-fs mode-locked Tm lasers.

We present an overview of our recent achievements on fabrication, characterization and CW and passively Q-switched laser operation of Thulium and Holmium depressed-cladding waveguide lasers operating at ~2 μm and inscribed in anisotropic crystals by femtosecond direct laser writing. Variable waveguide geometries (buried and surface guides; circular, rectangular and hexagonal channels; Y-splitters) are described.

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., National Research Foundation of Korea (2018H1A2AA1061480, 2019R1A2C3003504, 2020R1A4A2002828); Spanish Government (FIS2017-87970-R, MAT2016-75716-C2-1-R (AEI/FEDER,UE)); Junta de Castilla y León (SA287P18); Generalitat de Catalunya (2017SGR755).

Tetragonal calcium rare-earth aluminates, CaLnAlO4, combine a structural disorder with good thermo-mechanical properties. We report on efficient continuous-wave (CW) and passively Q-switched (PQS) ~2-μm laser operation of a 4 at.% Tm:CaYAlO4 crystal using a compact (6-mm-long) plane-parallel cavity. The pump source was a 791 nm fibercoupled AlGaAs laser diode. The CW output power reached 5.78 W at ~1970 nm with a slope efficiency of 43.6% and a linear laser polarization. Stable PQS operation was achieved using a single-walled carbon nanotube (SWCNT) based transmission-type saturable absorber. The PQS laser generated 2.15 W at ~1945 nm, a record-high average output power for this type of lasers. The best pulse characteristics (energy/duration) were 9.1 μJ/165 ns at a repetition rate of 235 kHz.

We report experimental demonstration of graphene mode-locked operation of T m 3 + : Y L i F 4 (YLF) and T m 3 + : K Y 3 F 10 (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 T i 3 + : s a p p h i r e laser delivering up to 1 W near 780 nm. The extended T m 3 + : K Y F 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 T m 3 + : Y L F laser. Once initiated by graphene, stable uninterrupted mode-locked operation could be maintained with both lasers. With the extended cavity T m 3 + : Y L F 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 T m 3 + : K Y F laser at 2340 nm. The corresponding pulse energy and peak power were 2.4 nJ and 2.6 kW for the T m 3 + : Y L F laser, and 1.2 nJ and 1.6 kW for the T m 3 + : K Y F laser. We foresee that it should be possible to generate shorter pulses at higher pump levels.

Tm3+-doped cubic potassium yttrium fluoride, KY3F10, is a promising laser crystal for efficient watt-level lasers at ∼1.9 μm because of the relatively easy crystal growth by the Czochralski method, advantageous thermo-optical properties, high available Tm3+ doping levels and very efficient cross-relaxation mechanism. A compact diode-pumped 8 at.% Tm3+:KY3F10 laser generated 1.85 W at 1891 nm with a slope efficiency of 65.2% and a laser threshold of 450 mW. A negative thermal lens was detected in this crystal owing to the negative dn/dT coefficient (−8.9 × 10−6 K−1). Passive Q-switching of the Tm3+:KY3F10 laser by single-walled carbon nanotube saturable absorber was demonstrated yielding 13.2 μJ/490 ns pulses at a repetition rate of 58 kHz.

A SWCNT-SA mode-locked “mixed” sesquioxide Tm:(Lu2/3Sc1/3)2O3 ceramic laser generates nearly transform-limited pulses with a duration of 58 fs at 2076 nm for an average output power of 114 mW at ~82.9 MHz repetition rate.

We describe a laser written, zebra patterned graphene saturable absorber with different ablation duty cycles, which works as a mode-locker with adjustable loss and demonstrate femtosecond pulse generation near 1250 nm with improved power performance.

We report on evanescent-field Q-switched operation of fs-laser-written Yb:YAG surface channel waveguide lasers employing single-walled carbon nanotubes in single- and double-pass pumping geometries.

A transparent Tm:Lu3Al5O12 ceramic is fabricated by solid-state reactive sintering at 1830 °C for 30 h using commercial α-Al2O3 and Lu2O3/Tm2O3 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 2.37×10−21 cm2 at 1749nm and the radiative lifetime of the 3F4 state is about 10 ms. The modified Judd-Ofelt theory accounting for configuration interaction is applied to determine the transition probabilities of Tm3+, yielding the intensity parameters Ω2 = 2.507, Ω4 = 1.236, Ω6 = 1.340 [10−20 cm2] and α = 0.196×10−4 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:Lu3Al5O12 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.

We demonstrate sub-100-fs Kerr-lens mode-locking of a T m : M g W O 4 laser emitting at ∼ 2 µ m 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 M H z . 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 ∼ 2 µ m .

Passive mode locking of a T m : L u Y O 3 ceramic laser in the 2 µm spectral region is demonstrated by employing a single-walled carbon nanotube-saturable absorber. Almost chirp-free pulses as short as 57 fs, i.e., 8 optical cycles are produced at a repetition rate of 72.6 MHz (63 mW average power). The corresponding optical spectrum centered at 2045 nm, exhibits a bandwidth of 80 nm, which corresponds to almost chirp-free pulses. To the best of our knowledge, the results represent the shortest pulses generated by mode-locked Tm solid-state lasers.

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/cm2 at the pump wavelength of 850 nm.

Femtosecond lasers in the spectral region of ∼2 μm are essential for development of pump and seed sources in diverse optical parametric frequency conversion schemes to cover the mid-IR part of the optical spectrum [1]. Passive mode-locking in such solid-state lasers is mainly achieved by saturable absorbers (SAs). To obtain durations corresponding to ten or fewer optical cycles, materials with broadband gain spectrum extending beyond 2 μm are needed so that the structured water vapor atmospheric absorption can be avoided. The newly developed monoclinic tungstate Tm3+:MgWO 4 (Tm:MgW) crystal exhibits such a broad and smooth gain profile due to the low-symmetry structure and the distortion of the crystal field caused by the mismatch of the ionic radii of Mg2+ and Tm3+ [2]. Using a graphene SA, the first sub-100-fs pulses at 2 μιη were obtained [3] while preliminary results with single-walled carbon nanotube (SWCN)-SAs indicated a pulse duration of 168 fs [4]. We report now on further progress with a SWCNT-SA generating nearly bandwidth-limited pulses of 76 fs duration at 2037 nm.

Highly efficient and compact pulsed laser sources have tremendous potential for integrated photonic devices in numerous applications including spectroscopy, metrology and microscopy. Yb3+-doped monoclinic double-tungstate crystals exhibit broad and large absorption and emission cross-sections and extremely low quantum defects. Channel waveguide structures have been actively studied for miniaturization of laser systems and efficient fundamental-mode laser operation with low lasing thresholds. One of the powerful methods to fabricate waveguide structures in a transparent material is the femtosecond direct laser writing (fs-DLW). Recently, pulsed waveguide lasers utilizing saturable absorbers (SAs) have been demonstrated with SESAM and low-dimensional carbon nanostructures. [1–3] Intrinsic characteristics such as ultrafast response, broadband nonlinear absorption and flexibility for integration make single-walled carbon nanotubes (SWCNTs) and graphene unique for various types of pulsed lasers. Placing a SA between the waveguide and the laser mirror results in additional intracavity losses and Q-switching instabilities due to thermal loads through direct-field interaction with the SA. Therefore, the damage-free coupling and the potential of a monolithic design including SAs in compact lasers are highly desired.

In this work, we review our recent results on fabrication, optical characterization and continuous-wave and passively Q-switched 2μm laser operation of active waveguides (WGs) produced by femtosecond direct laser writing (fs-DLW) in thulium and holmium doped low-symmetry (monoclinic) crystals. Fs-DLW was performed by 120-fs / 1-kHz pulses at 795 nm [1]. Various depressed-index photonic micro-structures were fabricated, Fig. 1, such as buried channel WGs with circular and hexagonal (lattice-like) cladding, surface WGs with a half-ring cladding, and straight WGs and Y-branch splitters with a rectangular cross-section. The WG core size was in the range 40–100 μm, comprising both fundamental- and multi-mode WGs.

We report on mode-locking of an Yb:Ca3NbGa3Si2O14 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.

We report, for the first time to our knowledge, continuous-wave, broadly tunable laser operation of the Tm3+:KY3F10 gain medium between 2260 and 2385 nm and graphene mode locking, yielding 976-fs pulses near 2340 nm.

We report, to the best of our knowledge, direct generation of the shortest pulses from a single-walled carbon nanotube mode-locked solid-state laser with a Cr:LiSAF gain medium, yielding 23-fs, nearly transform-limited pulses at 850 nm.

A channel waveguide (WG) buried immediately below the surface of a Yb:KLuW crystal is used as a laser gain medium for passive 𝑄-switching by both evanescent- and direct-field interactions with single-walled carbon nanotubes (SWCNTs) near 1040 nm. The SWCNTs used as saturable absorbers (SAs) are deposited on top of the half-ring-type channel WG fabricated via femtosecond direct laser writing. The 𝑄 -switched WG laser delivers 88.5 ns pulses at a 1.16 MHz repetition rate with a maximum average output power of 680 mW. For the two different interaction schemes with SWCNT-SAs, the pulse characteristics, depending on the output coupling ratio and absorbed pump power, are experimentally investigated and compared to the results of theoretical analyses of the SA 𝑄-switched operation., National Research Foundation of Korea (2017R1A4A1015426, 2018H1A2A1061480); Spanish Government (FIS2017-87970-R, MAT2016-75716-C2-1-R (AEI/FEDER, UE)); Junta de Castilla y León (SA287P18); Generalitat de Catalunya (2017SGR755).

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 C r 4 + : f o r s t e r i t e 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.

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 ∼1.1×10−19 m2/W.

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.

We report, what is to our knowledge, the first demonstration of Kerr-lens mode-locked and graphene mode-locked operations of a 2.3-μm Tm3+:YLF laser, yielding ultrashort pulses of 514 fs and 2 ps, respectively, at 2303 nm.

We report on a mode-locked Tm,Ho:CLNGG laser employing SWCNTs as saturable absorber. Transform-limited 84-fs pulses are generated at ~2080 nm for a repetition rate of ~102 MHz with an average output power of 69 mW.

We report on the crystal growth, spectral properties, and mode-locked laser operation of Tm, Na:CNGG, generating pulses as short as 84 fs at ∼2018 nm using single-walled carbon nanotubes as a saturable absorber.

Bilayer graphene synthesized by chemical vapor deposition is successfully applied as a saturable absorber (SA) for the passive mode-locking of a Tm,Ho:CLNGG laser at 2093nm. Near transform-limited pulses as short as 70 fs, i.e., 10 optical cycles, are produced at a 89 MHz repetition rate with 69 mW average output power. To the best of our knowledge, these are the shortest pulses ever reported from graphene-SA mode-locked Tm, or Ho-lasers in the 2 µm spectral region, including bulk and fiber lasers.

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.

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.

A mode-locked laser based on a Tm:CNNGG disordered crystal as an active medium and a single-walled carbon nanotube saturable absorber is demonstrated, operating at a central wavelength of 2018 nm. Transform-limited 84 fs pulses are generated with an average output power of 22 mW at a repetition rate of ∼90 MHz. A maximum output power of 98 mW is obtained at a slightly longer pulse duration of 114 fs.

A Tm3+ 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:KY1-x-yGdxLuy(WO4)2 active layer grown on an undoped (010)-oriented KY(WO4)2 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.

Penicillin-binding proteins (PBPs), which catalyze peptidoglycan synthesis, have been extensively studied as a well established target of antimicrobial agents, including β-lactam derivatives. However, remarkable resistance to β-lactams has developed among pathogenic bacteria since the clinical use of penicillin began. Recently, the glycosyltransferase (GT) domain of class A PBPs has been proposed as an attractive target for antibiotic development as moenomycin-bound GT-domain structures have been determined. In this study, a class A PBP4 from Listeria monocytogenes was overexpressed, purified and crystallized using the hanging-drop vapour-diffusion method. Diffraction data were collected to 2.1 Å resolution using synchrotron radiation. The crystal belonged to the primitive orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 84.6, b = 127.8, c = 54.9 Å. The structural information will contribute to the further development of moenomycin-derived antibiotics possessing broad-spectrum activity.

Near-field scanning optical microscopy, in which an uncoated fiber tip is used for delivering excitation light and picking up photoluminescence, is applied for diagnosing defects in semiconductor devices. Using this high resolution, potentially subdiffraction limited, and fast acquisition time scanning μ-photoluminescence (PL) technique, we are able to locate nonluminescing defects in a multiple quantum well grown by molecular beam epitaxy. Near-field characteristics of the measured spectral PL intensity are also discussed.

We report on a mode-locked Tm:LuYO3 ceramic laser employing SWCNT as a saturable absorber. Bandwidth-limited 57-fs pulses, i.e., 8 optical cycles, are generated at ~2045 nm at a repetition rate of ~72.6 MHz.

Transparent ceramics are promising materials for solid-state lasers because of their size-scalable synthesis method, high optical quality and similar spectroscopic properties as compared to their single-crystal counterparts. In particular, the Lu 3 Al 5 O 12 (LuAG) garnet is attractive for the fabrication of transparent ceramics due to its excellent thermal properties [1]. Due to the closeness of ionic radii of Lu3+ and Tm3+, LuAG ceramic is attractive for hosting tulium (Tm3+) ions which are known for its eye-safe laser emission at ∼2 μm (the 3F 4 → 3H 6 transition). Moreover, in garnets, the Tm3+ emission typically occurs above 2 μm which is relevant to mode-locking of femtosecond lasers [2].

We report on a Tm:CLNGG laser passively mode-locked by single-walled carbon nanotubes (SWCNTs) at 2017 nm, generating 78-fs pulses with an average output power of 54 mW at a repetition rate of 86 MHz.