Er3+:YLiF4 channeled waveguide laser near 2.7-2.8†µm fabricated by femtosecond laser inscription.

We report, for the first time to our knowledge, a demonstration of robust waveguide lasing near 2.7-2.8 µm in an erbium-doped fluoride host. Femtosecond laser inscription was employed to fabricate 50- and 70-µm diameter channeled waveguides inside an Er3+:YLiF4 crystal. The best power performance was obtained with the 70-µm diameter waveguide and 16% transmitting output coupler. The propagation loss and refractive index contrast were measured as 0.23 dB/cm and 7.1 × 10-4, respectively, for the 70-µm diameter waveguide. Both self-Q-switched (SQS) and continuous-wave (CW) operations could be obtained. During the SQS operation, as short as 240-ns pulses with average power of 51 mW, repetition rate of 368 kHz, and power slope efficiency of 15.2% were generated at the wavelength of 2717 nm with 465 mW of the pump power. During the CW operation, as high as 66 mW of output power was achieved at 2808 nm by using 460 mW of pump power at 798 nm, with a power slope efficiency of 19.6%.

Image-Guided Enhanced PDT/PTT Combination Therapy Using Brominated Hemicyanine-Loaded Folate Receptor-Targeting Ag2S Quantum Dots.

Tumor-targeting nanoparticles and phototherapies are the two major trends in tumor-specific, local cancer therapy with minimal side effects. Organic photosensitizers (PSs) usually offer effective photodynamic therapy (PDT) but require enhanced solubility and tumor-targeting, which may be provided by a nanoparticle. Near-infrared (NIR)-emitting Ag2S quantum dots may act as a delivery vehicle for the PS, NIR tracking agent, and as a phototherapy (PTT) agent. A combination of the two provides luminescent dual-phototherapy agents with tumor-specificity and image-guided and enhanced cytotoxicity as a result of synergistic PDT and PTT. In this study, brominated hemicyanine (Hemi-Br), a photosensitizer, was loaded onto folic acid (FA)-tagged, glutathione (GSH)-coated Ag2S quantum dots (AS-GSH QDs) to provide enhanced phototoxicity via a photodynamic and mild photothermal effect in folate receptor(+) cancer cell lines at clinically relevant 640 nm irradiation. Final particles (AS-GSH-FA/Hemi-Br) had a hydrodynamic size of 75.5 nm, dual emission at both 705 and 910 nm, and a 93% light-to-heat conversion efficiency under 640 nm laser irradiation. In vitro cytotoxicity studies were conducted with folate receptor (FR)-positive HeLa and -negative A549 cell lines to differentiate receptor-mediated uptake. Enhanced phototoxicity on HeLa cells was observed with AS-GSH-FA/Hemi-Br compared to free Hemi-Br and AS-GSH-FA QDs due to increased uptake of the photosensitizer via active targeting and combination therapy, which is especially visible at the safe dose of single agents. Upon irradiation with a 640 nm (300 mW, 0.78 W/cm2) laser for 5 min, the viability of the HeLa cells decreased from 64% to 42 and 25% when treated with free Hemi-Br, AS-GSH-FA, and AS-GSH-FA/Hemi-Br, respectively. Overall, AS-GSH-FA/Hemi-Br provides image-guided enhanced PDT/PTT, which may be adopted for different FR(+) tumors.

Broadly tunable continuous wave 2.3-µm Tm3+:tellurite bulk glass laser.

We report, for the first time to the best of our knowledge, a continuous wave trivalent thulium ion (Tm3+)-doped bulk glass near 2.3 µm. In the experiments, a bulk Tm3+-doped tellurite glass with the stoichiometric composition of 74TeO2-12ZnO-4La2O3-10Na2O (Tm3+:TZLN) was used. Lasing operation was achieved by using an x-fold cavity at the free-running wavelength of 2303 nm. The maximum slope efficiency of 6.2% was obtained with respect to the absorbed pump power with a 1% transmitting output coupler. In this case, as high as 100-mW output power was generated with 2.2 W of absorbed pump power. Continuous, broad tuning was achieved from 2233 nm to 2400 nm. The excitation spectrum of the laser was also investigated and 2.3-µm lasing was obtained by varying the pump wavelength over the 773-809-nm range. The absorption cross section was determined to be 4.4 × 10-21 cm2, based on open-aperture z-scan measurement. By using the laser efficiency data, the emission cross section of the Tm3+:TZLN glass was further determined to be 1.3 × 10-20 cm2 at 2.3 µm.

Development of a cysteine responsive chlorinated hemicyanine for image-guided dual phototherapy.

A cysteine (Cys) activatable chlorinated hemicyanine (Cl-Cys) was introduced as a tumour selective image-guided dual phototherapy agent. Cl-Cys exhibited a significant turn on response in its near-IR emission signal and activated its singlet oxygen generation as well as photothermal conversion potentials upon reacting with Cys. The laser irradiation of Cl-Cys induced significant cell death in cancer cells with high Cys level, while it stayed deactivated and non-emissive in a healthy cell line. A profound synergistic PDT/PTT effect was observed at high doses. Remarkably, Cl-Cys marks the first ever example of Cys-responsive small organic-based therapeutic agent and holds a great promise to develop new activity-based photosensitizers for dual phototherapy action.

Advanced Solid-State Lasers 2019: focus issue introduction.

This joint issue of Optics Express and Optical Materials Express features 17 state-of-the art articles written by authors who participated in the international conference Advanced Solid-State Lasers held in Vienna, Austria, from September 29 to October 3, 2019. This introduction provides a summary of these articles that cover numerous areas of solid-state lasers from materials research to sources and from design to experimental demonstration.

Tunable continuous-wave laser operation of Tm3+:BaY2F8 near 2.3 µm.

We report, for the first time to our knowledge, tunable continuous-wave laser action in the Tm3+:BaY2F8 (BYF) crystal near 2.3 µm. In the experiments, a BYF crystal doped with 3 at. % thulium was end pumped with a narrow-linewidth, tunable Ti3+:sapphire laser with up to 920 mW of incident power. Lasing was achieved for the two pump polarizations of E//x and E//y. The best power performance was obtained in the case of E//x, double-end pumping, where 100 mW of output power was obtained at 2290 nm with 920 mW of pump power and 1% output coupler. The laser could be continuously tuned from 2233 to 2385 nm. Excitation spectra for E//x and E//y pumping were measured in the 760-810 nm range, and the optimum pumping wavelength was determined to be 779 nm for E//x. By using the lifetime and lasing threshold data, the stimulated emission cross section at 2290 nm was further determined to be (0.66±0.06)×10-24m2.

Upconversion pumping of a 2.3 µm Tm3+:KY3F10 laser with a 1064 nm ytterbium fiber laser.

We report efficient lasing of the isotropic ${{\rm Tm}^{3 + }}\!:\!{{\rm KY}_3}{{\rm F}_{10}}$Tm3+:KY3F10 crystal near 2.3 µm via upconversion pumping with a 1064 nm ytterbium fiber laser as the pump source. When pumped at 1064 nm, an x-cavity ${{\rm Tm}^{3 + }}\!:\!{{\rm KY}_3}{{\rm F}_{10}}$Tm3+:KY3F10 laser operated at the free-running wavelength of 2344 nm. Lasing was obtained with output couplers having transmissions in the range of 1-3%, and as high as 124 mW of continuous-wave (cw) output power was generated with 604 mW of absorbed pump power by using the 3% output coupler. Broadly tunable cw lasing could be obtained in the 2268-2373 nm wavelength range. An analysis of the experimental power efficiency data shows that nearly all of the absorbed pump photons were converted to 2.3 µm laser output after accounting for the quantum defect of the laser transition and resonator losses. We expect that higher lasing efficiency should be possible by using longer crystals to increase the pump absorption.

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

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 ${{\rm Cr}^{4 {+} }}{:}\,{\rm forsterite}$Cr4+: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.

Graphene mode-locked operation of Tm3+:YLiF4 and Tm3+:KY3F10 lasers near 2.3 µm.

We report experimental demonstration of graphene mode-locked operation of ${{\rm Tm}^{3 + }}\!:\!{{\rm YLiF}_4}$Tm3+:YLiF4 (YLF) and ${{\rm Tm}^{3 + }}\!:\!{{\rm KY}_3}{{\rm F}_{10}}$Tm3+:KY3F10 (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 ${{\rm Ti}^{3 + }}\!:\!{\rm sapphire}$Ti3+:sapphire laser delivering up to 1 W near 780 nm. The extended ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm3+: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 ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm3+:YLF laser. Once initiated by graphene, stable uninterrupted mode-locked operation could be maintained with both lasers. With the extended cavity ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm3+: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 ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm3+:KYF laser at 2340 nm. The corresponding pulse energy and peak power were 2.4 nJ and 2.6 kW for the ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm3+:YLF laser, and 1.2 nJ and 1.6 kW for the ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm3+:KYF laser. We foresee that it should be possible to generate shorter pulses at higher pump levels.

Continuous-wave mid-infrared laser operation of Tm3+:KY3F10 at 2.3 µm.

We report continuous-wave lasing demonstration of a Tm3+:KY3F10 crystalline gain medium at 2343 nm. A narrow-linewidth, tunable, continuous-wave Ti3+:sapphire laser was used to end pump a Tm3+:KY3F10 crystal with thulium doping of 8 at. %. With 1 W of pump power, the resonator with a 1% output coupler generated output powers of 31 and 122 mW in single-pumping and double-pumping configurations, respectively. Excitation efficiency of the laser was investigated for pump wavelengths in the 765-806 nm range. Four pump bands were identified in this wavelength range, with the two most efficient pump bands centered at 773 and 778 nm. The output of the laser could be tuned smoothly and continuously over a spectral width of 125 nm from 2260 to 2385 nm. The lifetime of the H43 level was further measured to be 16 µs, and the emission cross section was determined based on lasing threshold measurements.

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

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.

Graphene mode-locked femtosecond Alexandrite laser.

We report for the first time, to the best of our knowledge, graphene mode-locked operation of a femtosecond Alexandrite laser at 750 nm. A multipass-cavity configuration was employed to scale the output energy and to eliminate spectral/Q-switching instabilities. By using a monolayer graphene saturable absorber, mode locking could be obtained. With 5 W of pump at 532 nm, nearly transform-limited, 65 fs pulses with a time-bandwidth product of 0.319 were generated. The mode-locked laser operated at a pulse repetition rate of 5.56 MHz and produced 8 mW output power, corresponding to a pulse energy and peak power of 1.4 nJ and 22 kW, respectively. These experiments further show that graphene can be used to initiate mode locking at wavelengths as low as 750 nm.

70 femtosecond Kerr-lens mode-locked multipass-cavity Alexandrite laser.

We report, to the best of our knowledge, the shortest femtosecond pulses generated from a Kerr-lens mode-locked (KLM) Alexandrite laser operating near 750 nm. The Alexandrite gain medium was pumped with a continuous-wave (cw), 532 nm laser, and the performance of both the short and extended resonators was investigated. The use of an extended cavity eliminated the multi-wavelength spectral instabilities observed during the cw operation of the short cavity. Furthermore, since the repetition rate of the Alexandrite laser was reduced from 107 to 5.6 MHz, the resulting increase in the intracavity pulse energy provided enhanced Kerr nonlinearity and eliminated the Q-switching instabilities during mode-locked operation. The KLM MPC Alexandrite laser produced nearly transform-limited, 70 fs pulses at a pulse repetition rate of 5.6 MHz with only 1 W of pump power. The time-bandwidth product was further measured to be 0.331.

1200 nm pumped Tm3+:Lu2O3 ceramic lasers.

We report on an experimental demonstration of a 1200-nm pumped Tm3+:Lu2O3 ceramic laser. By using a gain-switched, tunable Cr4+:forsterite laser, the excitation spectrum was measured, with optimum pumping bands centered near 1198 nm, 1204 nm, and 1211 nm. The highest slope efficiency of 21.5% was obtained at the pump wavelength of 1204 nm. Comparative energy efficiency measurements performed near 1200-nm and 800-nm pumping further showed that nearly 40% improvement was obtained in slope efficiency measured with respect to the incident pump energy for 1200-nm pumping. A transition was further observed from single-wavelength operation at 2066 nm to dual-wavelength operation near 2066 nm and 1967 nm for absorbed pump energies above 50 µJ. In this regime, two consecutive output pulses were observed in the time domain. The shortest temporal duration of the first pulse was 1.1 µs at the incident pulse energy of 105 µJ. The duration and build-up time of the second pulse remained around 5.9 µs and 18.5 µs. We believe that the improved energy efficiency demonstrated for the 1.5% Tm3+:Lu2O3 ceramic with 1200-nm pumping can be used as an alternative scheme for the excitation of Tm3+:Lu2O3 ceramic lasers.

Generation of sub-20-fs pulses from a graphene mode-locked laser.

We demonstrate, what is to our knowledge, the shortest pulses directly generated to date from a solid-state laser, mode locked with a graphene saturable absorber (GSA). In the experiments, a low-threshold diode-pumped Cr3+:LiSAF laser was used near 850 nm. At a pump power of 275 mW provided by two pump diodes, the Cr3+:LiSAF laser produced nearly transform-limited, 19-fs pulses with an average output power of 8.5 mW. The repetition rate was around 107 MHz, corresponding to a pulse energy and peak power of 79 pJ and 4.2 kW, respectively. Once mode locking was initiated with the GSA, stable, uninterrupted femtosecond pulse generation could be obtained. In addition, the femtosecond output of the laser could be tuned from 836 nm to 897 nm with pulse durations in the range of 80-190 fs. We further performed detailed mode locking initiation tests across the full cavity stability range of the laser to verify that pulse generation was indeed started by the GSA and not by Kerr lens mode locking.

Mirrors with designed spherical aberration for multi-pass cavities.

We present a novel multi-pass cavity design based on the use of a rotationally symmetric end mirror having a specifically designed spherical aberration so that its focal length varies inversely as the ray height from the optical axis. We provide a detailed discussion of how ray tracing can be done for this system and show with numerical simulations that a very rich set of exotic spot patterns can be formed on the end mirrors. We further show a specific q-preserving configuration where the q-parameters of the input and output beams remain the same. Finally, we derive the polar form of the mirror surface profile that gives this offset-dependent focal length.

Kerr-lens mode-locked 2.3-µm Tm3+:YLF laser as a source of femtosecond pulses in the mid-infrared.

We report what is to our knowledge a new source of femtosecond pulses in the mid-infrared, based on a Kerr-lens mode-locked (KLM) Tm3+:YLF laser at 2303 nm. An undoped ZnSe substrate was included in the resonator to provide enhanced nonlinear phase modulation during KLM operation. The Tm3+:YLF laser was end-pumped with a continuous-wave Ti3+ : sapphire laser at 780 nm. With 880 mW of pump power, we generated 514-fs pulses at a pulse repetition rate of 41.5 MHz with an average power of 14.4 mW. The spectral width (full width at half-maximum) was measured as 15.4 nm, giving a time-bandwidth product of 0.44. We foresee that the wide availability of this gain medium, as well as the straightforward pumping scheme near 800 nm, will make 2.3-µm, mode-locked Tm3+:YLF lasers versatile sources of ultrashort pulses in the mid-infrared.

2.3-µm Tm3+:YLF laser passively Q-switched with a Cr2+:ZnSe saturable absorber.

We report, what is to our knowledge, the first passively Q-switched operation of a 2.3-µm Tm3+:YLF laser by using a Cr2+:ZnSe saturable absorber. In the experiments, a tunable Ti3+:sapphire laser was used to end pump the Tm3+:YLF gain medium inside an x cavity. A Cr2+:ZnSe saturable absorber was also included in the cavity to initiate passive Q switching. At all pump power levels above lasing threshold, passively Q-switched operation of the Tm3+:YLF laser could be obtained at 2309 nm with pulse durations and repetition frequencies in the ranges of 1.2-1.4 µs and 0.3-2.1 kHz, respectively. Analysis of power dependent repetition rate data further gave an estimated value of 3.1% for the round-trip saturable loss of the Cr2+:ZnSe saturable absorber.

Femtosecond pulse generation from a Ti3+:sapphire laser near 800 nm with voltage reconfigurable graphene saturable absorbers.

We experimentally show that a voltage-controlled graphene-gold supercapacitor saturable absorber (VCG-gold-SA) can be operated as a fast saturable absorber with adjustable linear absorption at wavelengths as low as 795 nm. This was made possible by the use of a novel supercapacitor architecture, consisting of a high-dielectric electrolyte sandwiched between a graphene and a gold electrode. The high-dielectric electrolyte allowed continuous, reversible adjustment of the Fermi level and, hence, the optical loss of the VCG-gold-SA up to the visible wavelengths at low bias voltages of the order of a few volts (0-2 V). The fast saturable absorber action of the VCG-gold-SA and the bias-dependent reduction of its loss were successfully demonstrated inside a femtosecond Ti3+:sapphire laser operating near 800 nm. Dispersion compensation was employed by using dispersion control mirrors and a prism pair. At a bias voltage of 1.2 V, the laser operated with improved power performance in comparison with that at zero bias, and the VCG-gold-SA initiated the generation of nearly transform-limited pulses as short as 48 fs at a pulse repetition rate of 131.7 MHz near 830 nm. To the best of our knowledge, this represents the shortest wavelength where a VCG-gold-SA has been employed as a mode locker with adjustable loss.

Graphene-gold supercapacitor as a voltage controlled saturable absorber for femtosecond pulse generation.

We report, for the first time to the best of our knowledge, use of a graphene-gold supercapacitor as a voltage controlled fast saturable absorber for femtosecond pulse generation. The unique design involving only one graphene electrode lowers the insertion loss of the device, in comparison with capacitor designs with two graphene electrodes. Furthermore, use of the high-dielectric electrolyte allows reversible, adjustable control of the absorption level up to the visible region with low bias voltages of only a few volts (0-2 V). The fast saturable absorber action of the graphene-gold supercapacitor was demonstrated inside a multipass-cavity Cr:forsterite laser to generate nearly transform-limited, sub-100 fs pulses at a pulse repetition rate of 4.51 MHz at 1.24 µm.