Challenges in power scaling of Cr:LiCAF lasers: effect of passive losses.

In this work, we have investigated the continuous-wave (cw) lasing potential of thin slab-shaped Cr:LiCAF crystals with a low chromium doping level of around 1% and various lengths of 1 to 2 cm. These relatively long crystals with low Cr-doping facilitate the distribution of heat load in a larger volume and could enable power scaling of Cr:LiCAF lasers. However, long crystals tend to have larger passive losses, and it is also more challenging to achieve efficient mode-matching to the low-brightness pump mode in a longer gain element, which could hinder laser performance. To explore the issue, we have performed detailed cw lasing experiments in single- and multimode diode-pumped Cr:LiCAF laser systems employing crystals with different doping and length. Our results showed that current state-of-the-art crystal growth methods provide Cr:LiCAF crystals with low enough passive losses to enable cw laser efficiencies of up to 50%, even in these long samples. The pump powers available in this study (5.35 W) limited the cw powers we could achieve experimentally to 2.25 W level; however, our simulations indicate that thin slab-type Cr:LiCAF crystals with low Cr-doping have the potential to achieve cw powers above 10 W level.

100-mJ, 100-W cryogenically cooled Yb:YLF laser.

We present a diode-pumped Yb:YLF laser system generating 100-mJ sub-ps pulses at a 1-kHz repetition rate (100 W average power) by chirped-pulse amplification. The laser consists of a cryogenically cooled 78 K, regenerative, eight-pass booster amplifier seeded by an all-fiber front end. The output pulses are compressed to 980 fs in a single-grating Treacy compressor with a throughput of 89%. The laser will be applied to multi-cycle THz generation and pumping of high average power parametric amplifiers.

Continuous-wave Tm:YLF laser with ultrabroad tuning (1772-2145 nm).

We report detailed experimental data aiming for rigorous investigation of Tm:YLF laser performance, especially with a focus on tuning behavior. Continuous-wave (cw) lasing performance of Tm:YLF crystals with thulium dopings in the 2-6% range is investigated under diode and Ti:Sapphire pumping at 792 nm and 780 nm, respectively. While employing the c-axis, we have achieved cw lasing thresholds below 20 mW, laser output power up to 1.42 W, and laser slope efficiencies as high as 70% with respect to absorbed pump power. The passive loss of the Tm:YLF crystal is estimated to be as low as 0.05% per cm, corresponding to a crystal figure of merit above 10000. Via employing this low-loss crystal and a 2-mm thick off-surface optical axis birefringent filter (BRF) with strong sideband rejection, a record cw tuning range covering the 1772-2145 nm interval is demonstrated (except a small gap between 1801-1815nm region). Detailed lifetime and emission cross section measurements have been performed to explain the observed performance, and strategies for further performance enhancement are discussed.

Parameter sensitivities in tilted-pulse-front based terahertz setups and their implications for high-energy terahertz source design and optimization.

Despite the popularity and ubiquity of the tilted-pulse-front technique for single-cycle terahertz (THz) pulse generation, there is a deficit of experimental studies comprehensively mapping out the dependence of the performance on key setup parameters. The most critical parameters include the pulse-front tilt, the effective length of the pump pulse propagation within the crystal as well as effective length over which the THz beam interacts with the pump before it spatially walks off. Therefore, we investigate the impact of these parameters on the conversion efficiency and the shape of the THz beam via systematically scanning the 5D parameter space spanned by pump fluence, pulse-front-tilt, crystal-position (2D), and the pump size experimentally. We verify predictions so far only made by theory regarding the optimum interaction lengths and map out the impact of cascading on the THz radiation generation process. Furthermore, distortions imposed on the spatial THz beam profile for larger than optimum interaction lengths are observed. Finally, we identify the most sensitive parameters and, based on our findings, propose a robust optimization strategy for tilted-pulse-front THz setups. These findings are relevant for all THz strong-field applications in high demand of robust high-energy table-top single-cycle THz sources such as THz plasmonics, high-harmonic generation in solids as well as novel particle accelerators and beam manipulators.

Broadly tunable (993-1110 nm) Yb:YLF laser.

We have investigated room-temperature continuous-wave (cw) lasing performance of Yb:YLF oscillators in detail using rod-type crystals with low Yb-doping (2%). The laser is pumped by a low-cost, high brightness, 10 W, 960 nm single-emitter multimode diode. Laser performance is acquired in both E//a and E//c configurations, using 12 different output couplers with transmission ranging from 0.015% to 70%. We have estimated the passive loss of the Yb:YLF crystal as 0.06% per cm, corresponding to an impressive crystal figure of merit above 4000. The low-doping level not only reduces the system losses but also minimizes the thermal load as the low doped crystals enable distribution of heat load in a greater volume. Using the advantages of lower loss and improved thermal behavior, we have achieved cw output power above 4 W, cw slope efficiencies up to 78%, and a record cw tuning range covering the 993-1110 nm region (117 nm). The output power performance achieved in this initial work is limited by the available pump power, and future room-temperature Yb:YLF systems have the potential to produce higher output power levels.

Semiconductor saturable absorber mirror mode-locked Yb:YLF laser with pulses of 40 fs.

We have generated pulses as short as 40 fs with an average power of 265 mW from a semiconductor saturable absorber mirror (SESAM) mode-locked Yb:YLF oscillator employing a 1% transmitting output coupler (OC). The room-temperature laser is pumped by a low-cost 960 nm single-emitter multimode diode and dispersion compensation is provided via double chirped mirrors (DCMs). The 40-fs pulses are centered around 1050 nm with a width of 34 nm at a repetition rate of 87.3 MHz. By increasing the output coupling to 5% and by using Gires-Tournois interferometer (GTI) mirrors for dispersion compensation, we have also demonstrated 380-fs pulses with 1.85 W of average power around 1025 nm at a repetition rate of 190.4 MHz. Using an intracavity off-surface optic axis birefringent filter, the central wavelength of the pulses could be tuned in the 1020-1025 nm and 1019-1047 nm ranges for the 5% and 1% transmitting OCs, respectively. To the best of our knowledge, these are the shortest pulses and highest average and peak powers generated from room-temperature Yb:YLF lasers to date.

Broadly tunable two-color lasing of Cr:LiCAF with on-surface and off-surface optical axis birefringent filters: performance comparison.

We studied the two-color lasing performance of a Cr:LiCAF laser using crystal quartz on-surface and off-surface optical axis birefringent filters (BRFs). Four different on-surface optical axis BRFs with thicknesses of 2 mm, 4 mm, 8 mm, and 16 mm, and three different off-surface optical axis BRFs with a diving angle of 25° and thicknesses of 2 mm, 4 mm, and 8 mm have been tested. Two-color lasing operation could be achieved in tens of different pairs of wavelengths using both types of BRFs. Regular on-surface optical axis BRFs provided two-color lasing in the 772-810 nm interval, with a discretely tunable wavelength separation of 1 to 37 nm (0.5 to 17 THz). In comparison, the off-surface optical axis BRFs enabled scanning of two-color lasing spectra in a much broader wavelength range between 745 nm and 850 nm with a discretely tunable wavelength separation of 0.8 to 99 nm (0.4 to 46 THz). The results clearly demonstrate the advantages of using off-surface optical axis BRFs to achieve two-color lasing with broadly tunable wavelength separation.

Mode-locked Cr:LiSAF laser far off the gain peak: tunable sub-200-fs pulses near 1 µm.

We report, to the best of our knowledge, the first mode-locking results of a Cr:LiSAF laser near the 1 µm region. The system is pumped only by a single 1.1 W high-brightness tapered diode laser at 675 nm. A semiconductor saturable absorber mirror (SESAM) with a modulation depth of 1.5% and non-saturable losses below 0.5% was used for mode-locking. Once mode-locked, the Cr:LiSAF laser produced almost-transform-limited sub-200-fs pulses with up to 12.5 mW of average power at a repetition rate of 150 MHz. Using an intracavity birefringent filter, the central wavelength of the pulses could be smoothly tuned in the 1000-1020 nm range. Via careful dispersion optimization, pulse widths could be reduced down to the 110-fs level. The performance in this initial study was limited by the design parameters of the SESAM used, especially its passive losses and could be improved with an optimized SESAM design.

Highly efficient cryogenic Yb:YLF regenerative amplifier with 250 W average power.

We report an efficient diode-pumped high-power cryogenic regenerative amplifier operating at 1019 nm employing the c axis of Yb:YLF. Compared to the usually selected 1017 nm transition of the a axis, the c-axis 1019 nm line has a three-fold higher emission cross section and still possesses a full-width at half-maximum (FWHM) of 6.5 nm at 125 K. The chirped-pulse amplifier system is seeded by a fiber front-end with energy of 30 nJ and a stretched pulse width of 2 ns. In regenerative amplification studies, using the advantage of higher gain in the c axis, we have achieved record average powers up to 370 W with an extraction efficiency of 78% at a 50 kHz repetition rate. The output pulses were centered on 1019.15 nm with a FWHM bandwidth of 1.25 nm, which supports sub-1.5 ps pulse durations. The output beam maintained a TEM00 beam profile at output power levels below 250 W with an M2 below 1.2. Above this power level, the thermally induced lensing in Yb:YLF created a multimode output beam. The thermal lens was rather dynamic and deteriorated the system stability above a 250 W average power level.

High power (>500W) cryogenically cooled Yb:YLF cw-oscillator operating at 995†nm and 1019†nm using E//c axis for lasing.

We present record continuous wave (cw) output power levels from cryogenically cooled Yb:YLiF4 (Yb:YLF) lasers in rod geometry. The laser system is pumped by a state-of-the-art 960 nm diode module, and vertically polarized lasing was employed using the E//c axis of Yb:YLF. Lasing performance was investigated at different output coupling levels in different cavity configurations and the laser crystal temperature was estimated via monitoring the emission spectrum of the gain media. We have obtained a cw output power up to 400 W at a wavelength of 995 nm. The absorbed pump power was around 720 W, and the laser output had a TEM00 beam profile with an M2 of 1.3 in both axes. At higher absorbed pump power levels with increasing laser crystal temperature, we observed a lasing wavelength shift from 995 nm to 1019 nm. In this regime cw output power levels above 500 W have been achieved at an absorbed pump power of 750 W. Further power scaling was limited by the onset of strong thermal lensing. We discuss underlying physical mechanisms for the wavelength shift and present detailed temperature measurements under lasing conditions.

Photodynamic therapy effect of morpholinium containing silicon (IV) phthalocyanine on HCT-116 cells.

In this study, we investigated the in vitro potential of axially 1-morpholiniumpropan-2-ol disubstituted silicon (IV) phthalocyanine (SiPc) which was synthesized previously, on HCT-116 cells as a photodynamic therapy (PDT) agent. The singlet oxygen and photodegradation quantum yields of SiPc were calculated using UV-vis spectrophotometer. The cytotoxic and phototoxic effects of SiPc were evaluated by 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT) assay. Annexin V-FITC/PI double staining kit, cell cycle kit, and mitochondria membrane potential (ΔΨm) assay kit with JC-1 were used to indicate the cell death pathway. Caspase-3 and ß-catenin protein expressions were evaluated by western blotting. The singlet oxygen and photodegradation quantum yields of SiPc were calculated as 0.73 and 3.64 × 10-4 in DMSO. The cell viability assays showed that IC50 value of SiPc did not reach to 100 µM without irradiation. However, excellent phototoxicity was observed in the presence of SiPc upon light irradiation. The cells undergoing early/late apoptosis significantly increased in the presence SiPc at 5 µM upon light irradiation. Besides, the proportion of cells at S and G2/M phase increased. Moreover, mitochondria membrane potentials significantly decreased at 1 and 5 µM of SiPc with light irradiation. While caspase-3 expression increased, ß-catenin expression significantly decreased on HCT-116 in the presence of SiPc (p < 0.01). The results indicated that the PDT could be related to apoptosis and Wnt/ß-catenin signaling pathway. Based on our findings, SiPc exhibited a significant PDT effect on HCT-116 cells therefore, worthy of more detailed study.

High-power passively mode-locked cryogenic Yb:YLF laser.

We report, to the best of our knowledge, the first mode-locked operation of Yb:YLF gain media at cryogenic temperatures. A saturable Bragg reflector was used for initiating and sustaining mode locking. Once aligned, the system was self-starting and quite robust. Using output couplers in the 10-40% range, 3-5 ps long pulses with an average power as high as 28 W were achieved. The repetition rate was 46.45 MHz, and the corresponding pulse energy and peak power were as high as 602 nJ and 126.5 kW, respectively. The central wavelength of the mode-locked pulses could be tuned in the 1013.5-1019 nm range using an intracavity birefringent filter. The achieved output power performance is two to three orders of magnitude higher than previous room-temperature Yb:YLF systems.

20-mJ, sub-ps pulses at up to 70 W average power from a cryogenic Yb:YLF regenerative amplifier.

We report, what is to our knowledge, the highest average power obtained directly from a Yb:YLF regenerative amplifier to date. A fiber front-end provided seed pulses with an energy of 10 nJ and stretched pulsewidth of around 1 ns. The bow-tie type Yb:YLF ring amplifier was pulse pumped by a kW power 960 nm fiber coupled diode-module. By employing a pump spot diameter of 2.1 mm, we could generate 20-mJ pulses at repetition rates between 1 Hz and 3.5 kHz, 10 mJ pulses at 5 kHz, 6.5 mJ pulses at 7.5 kHz and 5 mJ pulses at 10 kHz. The highest average power (70 W) was obtained at 3.5 kHz operation, at an absorbed pump power level of 460 W, corresponding to a conversion efficiency of 15.2%. Despite operating in the unsaturated regime, usage of a very stable seed source limited the power fluctuations below 2% rms in a 5 minute time interval. The output pulses were centered around 1018.6 nm with a FWHM bandwidth of 2.1 nm, and could be compressed to below 1-ps pulse duration. The output beam maintained a TEM00 beam profile at all power levels, and possesses a beam quality factor better than 1.05 in both axis. The relatively narrow bandwidth of the current seed source and the moderate gain available from the single Yb:YLF crystal was the main limiting factor in this initial study.

Efficient, diode-pumped, high-power (>300W) cryogenic Yb:YLF laser with broad-tunability (995-1020.5 nm): investigation of E//a-axis for lasing.

We present, what is to our knowledge, the first detailed lasing investigation of cryogenic Yb:YLF gain media in the E//a-axis. Compared to the usually employed E//c-axis, the a-axis of Yb:YLF provides a much broader and smooth gain profile, but this comes at the expense of reduced gain product. We have shown that, despite the lower gain, which (i) increases susceptibility to cavity losses, (ii) raises lasing threshold, and (iii) inflates thermal load, efficient and high-power lasing could be achieved in the E//a axis as well. A record continuous-wave (cw) powers above 300 W, cw slope efficiencies of 73%, and a tuning range covering the 995-1020.5 nm region were demonstrated. In quasi-cw lasing experiments, via minimization of thermal effects, slope efficiencies can be scaled up to 85%. In gain-switched operation, sub-50-µs long pulses with a peak power exceeding 2.5 kW at multi-kHz repetition rate were attained. We measured a beam quality factor below 1.5 for laser average powers up to 100 W and below 3 for laser average powers up to 300 W. Power scaling limits due to thermal effects, laser dynamics in pulsed pumping, and multicolor lasing operation potential were also investigated. The detailed results presented in this manuscript will pave the way towards development of high-power and high-energy Yb:YLF oscillators and amplifiers with sub-500-fs pulse duration.

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.

Novel water soluble BODIPY compounds: Synthesis, photochemical, DNA interaction, topoisomerases inhibition and photodynamic activity properties.

In this study, BODIPY compounds (2, 3, 5 and 6) bearing 3,4-bis(3-pyridin-3-ylpropoxy)benzyl, 4-(3-pyridin-3-ylpropoxy)benzyl groups were synthesized for the first time and further functionalized in a Knoevenagel condensation reaction with 3,4-bis(3-pyridin-3-ylpropoxy)benzaldehyde and 4-(3-pyridin-3-ylpropoxy)benzaldehyde. The water soluble derivatives of BODIPY compounds (3a and 6a) were synthesized by treating BODIPY compounds 3 and 6 with excess iodomethane in DMF. The photochemical properties and DNA binding modes of 3a and 6a were determined using ct-DNA by UV-Vis spectrophotometer and viscometer. DNA cleavage and topoisomerases inhibition properties were studied DNA using agarose gel electrophoresis. Their topoisomerase inhibition mechanisms were investigated at molecular level and correlations with the in vitro results were searched for using molecular docking method. In addition, cytotoxicity and phototoxicity of both compounds were performed on colorectal cancer cells (HCT-116) using MTT assay for 24 h. Annexin V-FITC/PI test was performed to determine the cell death mechanism of 6a induced by irradiation. Finally, 6a-loaded liposomes (LP6a) and PLGA nanoparticles (NP6a) were prepared and their cytotoxic and phototoxic effects were evaluated by MTT assay. The results claimed that 6a had great potential as photosensitizer agent for colorectal cancer owing to its photochemical, DNA interaction and phototoxic properties.

Efficient Tm:YAG and Tm:LuAG lasers pumped by 681 nm tapered diodes.

In this paper, we present highly efficient continuous-wave (cw) laser operation of Tm:YAG and Tm:LuAG lasers pumped by high-brightness red tapered diodes. The single-emitter tapered diode lasers (TDLs) provide up to 1 W of pump power around 680 nm. By adjusting the operation temperature of the TDL, the pump central wavelength could be matched to the strong absorption peak of Tm3+ ions in this region (H63→F33 excitation). This absorption peak is around threefold stronger than the usually employed 785 nm transition (H63→H34). In the cw laser experiments, we have achieved slope efficiencies exceeding 55% at room temperature, which is far above the Stokes limited slope efficiency (34%), indicating presence of a strong two-for-one cross-relaxation process. Pumping with high-brightness tapered diode lasers further facilitated usage of smaller pump spots (enabling quite low lasing thresholds) and generation of near-diffraction limited output beam profiles from standard z-type cavities. To the best of our knowledge, this is the first report of diode pumping of Tm-doped solid-state lasers around 680 nm as well as the first usage of TDLs as pump sources in Tm-doped laser systems.

Widely tunable dual-wavelength operation of Tm:YLF, Tm:LuAG, and Tm:YAG lasers using off-surface optic axis birefringent filters.

In this work, we have demonstrated dual-wavelength continuous-wave laser operation in diode-end-pumped Tm:YLF, Tm:LuAG, and Tm:YAG lasers. A 3-mm-thick quartz birefringent filter with an optical axis 45° to the surface plane was exploited for achieving broadly tunable two-color laser operation. By using the different orders of the filter with varying filter width and free spectral range values, dual-wavelength operation has been achieved in 11, 12, and 8 different wavelength pairs in Tm:YLF, Tm:LuAG, and Tm:YAG, respectively. Fine tuning of the rotation angle of the birefringent filter enabled control of laser power in each line. To our knowledge, this is the first report of multicolor laser operation in these gain media, and the technique used is applicable to other laser operation regimes including mode-locking.

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.