Showing papers on "Absorption band published in 2019"

Trimetallic FeCoNi@C Nanocomposite Hollow Spheres Derived from Metal–Organic Frameworks with Superior Electromagnetic Wave Absorption Ability

Abstract: Organic ligands and metal ions in the metal-organic frameworks (MOFs, a type of porous magnetic metal/carbon nanocomposites obtained through high-temperature carbonization) have caused widespread concerns in the field of microwave absorption because of the existence of various microwave loss mechanisms in these materials. However, MOF-driven microwave absorbing materials with high absorption intensity and wide absorption band still require further research and development. In this work, hollow sphere trimetallic FeCoNi@C microwave absorbing materials via high-temperature carbonization were obtained using FeCoNi-based MOF-74 (FeCoNi-MOF) as the precursor. The effects of different carbonization conditions on the microwave absorption properties of the materials were studied. FeCoNi-MOF-74 annealed at 700 °C showed superior microwave absorption capacity, where the RL value reached -64.75 dB at 15.44 GHz corresponding to the actual application thickness of the absorber (only 2.1 mm), and the minimum RL values reached -69.03 dB at 5.52 GHz. Furthermore, the as-prepared sample can fully cover the Ku band and X band at only 2.1 and 3.1 mm, respectively. The maximum EAB reached 8.08 GHz (9.92-18 GHz) when the thickness of the absorber was 2.47 mm. Such remarkable absorption performance is attributed to the synergetic effects between the multiple loss mechanisms of the FeCoNi@C, and the improved impedance matching characteristic came from the hollow sphere morphology.

Design of 3-D Multilayer Ferrite-Loaded Frequency-Selective Rasorbers With Wide Absorption Bands

Abstract: A multilayer structure and magnetic ferrite material are employed to expand the absorption bandwidth of 3-D frequency-selective rasorber (FSR). Resonant-like ferrite is used to realize the magnetic loss with high selectivity of absorption over a wide frequency band, while the multilayer structure is proposed to achieve multiple absorption peaks at designated frequencies. Combining the loaded ferrite material with multilayer structure can realize wide absorption bands. Equivalent circuit models are utilized to understand the operating mechanism of the described 3-D structure and to guide the design of multilayer structure with selected ferrite materials. Two broadband 3-D FSRs are designed, fabricated, and measured. Both designs achieve the sextuple bandwidth ratio with reflectivity less than −10 dB, and the absorption band with a bandwidth ratio of >2.6:1 can be achieved at either lower or upper absorption band, respectively.

Thermally Activated Upconversion Near-Infrared Photoluminescence from Carbon Dots Synthesized via Microwave Assisted Exfoliation.

Abstract: Upconversion near-infrared (NIR) fluorescent carbon dots (CDs) are important for imaging applications. Herein, thermally activated upconversion photoluminescence (UCPL) in the NIR region, with an emission peak at 784 nm, which appears under 808 nm continuous-wave laser excitation, are realized in the NIR absorbing/emissive CDs (NIR-CDs). The NIR-CDs are synthesized by microwave-assisted exfoliation of red emissive CDs in dimethylformamide, and feature single or few-layered graphene-like cores. This structure provides an enhanced contact area of the graphene-like plates in the core with the electron-acceptor carbonyl groups in dimethylformamide, which contributes to the main NIR absorption band peaked at 724 nm and a tail band in 800-850 nm. Temperature-dependent photoluminescence spectra and transient absorption spectra confirm that the UCPL of NIR-CDs is due to the thermally activated electron transitions in the excited state, rather than the multiphoton absorption process. Temperature dependent upconversion NIR luminescence imaging is demonstrated for NIR-CDs embedded in a polyvinyl pyrrolidone film, and the NIR upconversion luminescence imaging in vivo using NIR-CDs in a mouse model is accomplished.

Biogenic fabrication of ZnO nanoparticles using Trigonella foenum-graecum (Fenugreek) for proficient photocatalytic degradation of methylene blue under UV irradiation

Abstract: The biosynthesis is an eco-friendly, reliable, sustainable protocol for preparing nanomaterials where use of natural, biodegradable, non-toxic and safe reagents takes place. In the present work, an efficient, facile and eco-friendly approach has been used for the synthesis of zinc oxide nanoparticles (ZnO-NPs) using Trigonella foenum-graecum (Fenugreek) aqueous seed extract as bio-reducing agents and capping agent, thus eradicating the requirement of conventional reducing agents. Different characterization techniques like UV–Vis spectroscopy, UV–Visible diffuse reflectance spectroscopy, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy, photoluminescence (PL) study and energy dispersive X-ray were employed for confirmation of optical properties, shape, size, surface structure, crystalline nature and elemental proportions of the biogenic ZnO-NPs. FTIR analysis confirms the active role of bioactive phytochemical constituents present in the Trigonella foenum-graecum aqueous seed extract. XRD analyses of the as prepared ZnO-NPs are crystalline in nature and have no other impurity phase. UV–Vis spectral data suggested optical band gap energy of 2.97 eV for biosynthesized ZnO-NPs showing their small size owing to quantum confinement. UV–Vis spectra of ZnO-NPs show the characteristic absorption band at 364 nm, which can be assigned to the intrinsic band gap absorption of ZnO-NPs because of the electron transitions from the valence band to the conduction band. In addition, the efficacy of biosynthesized ZnO-NPs to act as highly efficient photocatalyst for methylene blue (MB) dye degradation under UV-light under different experimental conditions was confirmed in this study. The effect of initial dye concentration, ZnO photocatalyst dosage and the reusability tests were investigated. Improved photocatalytic behavior was discussed and influence of active species was further investigated using hydroxyl radical (●OH), superoxide anions (●O2−) and hole (h+) scavengers to explain the possible mechanism of the photocatalytic MB dye degradation under UV light irradiation.

Influence of Fe doping on the structural, optical and thermal properties of α-MnO 2 nanowires

Abstract: We report the synthesis of pure and Fe-doped α-MnO2 nanowires by a facile hydrothermal method. The influence of iron with varying doping concentration on structural, optical and thermal properties of crystalline α-MnO2 nanowires is discussed. X-ray diffraction studies indicate that the synthesized nanowires have tetragonal structure and are in crystalline α-MnO2 phase with average crystallites size of 17.36 nm for pure α-MnO2 while 14.59 nm, 14.93 nm and 13.32 nm for 5 mol %, 10 mol % and 15 mol % Fe-doped α-MnO2, respectively. Scanning electron microscopy images show that the synthesized products have nanowire morphology with an average diameter of 38 nm for pure α-MnO2 and 28 nm for 5 mol % Fe-doped α-MnO2. In Fourier transform infrared spectra, the strong absorption band at 501 cm−1 and 699 cm−1 correspond to the bending and stretching vibration of Mn–O bond. Furthermore, Thermal gravimetric analysis indicates that Fe doping in MnO2 leads to the enhancement of its thermal stability. The broad absorption band of the UV–vis spectra appeared at 250 nm to 600 nm for pure α-MnO2 and at 250 nm to 700 nm for Fe-doped α-MnO2 nanowires is assigned to the d-d transition between lower t2g and higher eg of the metallic ions. For the first time we are reporting an indirect optical bandgap energy of 0.30 eV, 0.29 eV and 0.18 eV for different Fe-doped α-MnO2 samples.

Characterization of borate glasses doped with copper oxide for optical application

Abstract: Borate glasses having the formula x CuO–(50 − x)B2O3–30Na2O–20ZnO where x = 0, 0.2, 0.5, 1, 4 and 7 mol% were investigated. The samples were prepared using the conventional melt-quenching technique. Physical properties were studied such as; X-ray diffraction, FTIR spectra, and UV–visible transmission. The optical basicity, average electro negativity have been estimated. The estimated optical basicity, unlike electron negativity, exhibits an increase with increasing the CuO content. FTIR spectra show that BO4 ratio decreases with increasing CuO content leading to the formation of non-bridging oxygen’s. Some optical parameters such as the optical band gap, band tail width, crystal field strength and the UV and NIR cut off were determined. The transmission spectrum revealed that the glassy samples containing high content of copper oxide behave as bandpass filters at higher content. The characterized parameters of these filters were found to be highly affected by the addition of CuO. In these glasses the broad absorption band observed at the wavelength 550 nm is characterized by the existence of Cu2+ ions. The optical absorption and ESR studies suggest that, the Cu-ions exist in the Cu2+ state and act as modifiers by increasing the degree of disorder in the glass network. Hence, the present system behaves as a bandpass filter in the ultraviolet–visible region.

Ground-State Structural Disorder and Excited-State Symmetry Breaking in a Quadrupolar Molecule.

Abstract: The influence of torsional disorder around the ethynyl π-bridges of a linear D−π–A−π–D molecule on the nature of its S1 excited state was investigated using ultrafast time-resolved infrared spectroscopy. By tuning the pump wavelength throughout the S1 ← S0 absorption band, subpopulations with different extents of asymmetry could be excited. In nonpolar solvents, the equilibrated S1 state is symmetric and quadrupolar independently of the initial degree of distortion. Photoexcitation of distorted molecules is followed by planarization and symmetrization of the S1 state. Excited-state symmetry breaking is only observed in polar environments, where the equilibrated S1 state has a strong dipolar character. However, neither the extent nor the rate of symmetry breaking are enhanced in an initially distorted molecule. They are only determined by the polarity and the dynamic properties of the solvent.

Broadband Band-Absorptive Frequency-Selective Rasorber With a Hybrid 2-D and 3-D Structure

Abstract: A novel frequency-selective rasorber (FSR) with a broad absorption band is proposed in this letter. The proposed FSR is realized by vertically cascading a two-dimensional (2-D) lossy layer with a 3-D band-stop frequency-selective structure (FSS). The 2-D lossy layer is composed of meandered square loops loaded with lumped resistors, and the 3-D band-stop FSS is constructed by parallel-plate waveguide arrays (PPWAs) stacked along the TE polarization direction (linear polarization perpendicular to the PPW). The band-absorptive FSR exhibits an absorption band (| S 11| ≤ –10 dB and | S 21| ≤ –10 dB) from 7.85 to 12.85 GHz and a low-pass band (| S 21| ≥ –1 dB) up to 3.8 GHz. Its transition efficiency of the transition band (FRap) is 2.1. A prototype is fabricated, and its measured results show that the proposed FSR has a low loss and flat low-pass band, a wide absorption band with high selectivity, and a steep transition band between the low-pass band and absorption band.

Effects of Ag doping on the electronic and optical properties of CdSe quantum dots.

Abstract: Cadmium selenide (CdSe) nanocrystals are important photoelectric materials. Doping heterovalent impurities such as silver (Ag) in CdSe nanocrystal quantum dots (QDs) can provide additional charge carriers, which can significantly enhance the performance of CdSe QDs for their potential applications in high-efficiency photovoltaic devices. Using density functional theory (DFT) based calculations with the Heyd-Scuseria-Ernzerhof (HSE06) screened hybrid functional, we demonstrate that Ag doping can affect the structural, electronic and optical properties of CdSe QDs significantly. The location and number of Ag dopant atoms are critical factors for modifying the electronic structure, in particular the change of energy position and shape of the valence and conduction band edges. It is found that doping of Ag atoms into the core region of a CdSe nanoparticle induces metallic-like electronic characteristics with a dense number of electrons emerging at the Fermi level. However, incorporation of Ag dopant into the surface of a CdSe quantum dot introduces some mid-gap states that mainly consist of Se 4p states, and results in a new sub-bandgap electronic transition from mid-gap states to the conduction band. The calculated absorption spectra indicate that doping of just one or two Ag atoms greatly strengthens the absorption in the ultraviolet-visible regime and extends the absorption edges of CdSe QDs into the infrared regime. In particular, the spectra show a high-intensity absorption band between 424 and 600 nm with just 1 Ag atom incorporated into the CdSe QDs. Based on the improved absorption spectra, the present results provide a science-based strategy for designing Ag-doped CdSe QDs with enhanced visible light absorption for their application in high-efficiency photovoltaic devices.

On a blue emitting phosphor Na3RbMg7(PO4)6:Eu2+ showing ultra high thermal stability

Abstract: This work concerns a novel blue emitting LED phosphor activated by Eu2+. It was found that this luminescent material can be efficiently pumped by near UV or UV-A radiation sources while it exhibits a high quantum yield at a doping level between 0.3 and 3.0%. More impressively, this material has a very high quenching temperature T1/2 and will thus be applicable on-chip in high power phosphor converted LEDs or even in laser diodes. The emission and thermal stability are almost identical to that of BAM:Eu2+ while the absorption band is broader and reaches into the near UV range. Therefore, Na3RbMg7(PO4)6:Eu2+ is a promising candidate as a blue emitter in high CRI full conversion LEDs. To investigate the site preference of Eu2+, DFT and Ligand Field Theory based calculations were performed to successfully predict the emission spectrum. An unusual decay behavior was observed at low temperatures and the underlying mechanism involving spin-forbidden transitions of Eu2+ is discussed.

Infrared Spectroscopy of Size-Selected Hydrated Carbon Dioxide Radical Anions CO2.−(H2O)n (n=2–61) in the C−O Stretch Region

Abstract: Understanding the intrinsic properties of the hydrated carbon dioxide radical anions CO2.- (H2 O)n is relevant for electrochemical carbon dioxide functionalization. CO2.- (H2 O)n (n=2-61) is investigated by using infrared action spectroscopy in the 1150-2220 cm-1 region in an ICR (ion cyclotron resonance) cell cooled to T=80 K. The spectra show an absorption band around 1280 cm-1 , which is assigned to the symmetric C-O stretching vibration νs . It blueshifts with increasing cluster size, reaching the bulk value, within the experimental linewidth, for n=20. The antisymmetric C-O vibration νas is strongly coupled with the water bending mode ν2 , causing a broad feature at approximately 1650 cm-1 . For larger clusters, an additional broad and weak band appears above 1900 cm-1 similar to bulk water, which is assigned to a combination band of water bending and libration modes. Quantum chemical calculations provide insight into the interaction of CO2.- with the hydrogen-bonding network.

Femtosecond infrared spectroscopy of chlorophyll f-containing photosystem I.

Abstract: The recent discovery of extremely red-shifted chlorophyll f pigments in both photosystem I (PSI) and photosystem II has led to the conclusion that chlorophyll f plays a role not only in the energy transfer, but also in the charge separation processes [Nurnberg et al., Science, 2018, 360, 1210–1213]. We have employed ultrafast transient infrared absorption spectroscopy to study the contribution of far-red light absorbing chlorophyll f to energy transfer and charge separation processes in far-red light-grown PSI (FRL-PSI) from the cyanobacterium Chroococcidiopsis thermalis PCC 7203. We compare the kinetics and spectra of FRL-grown PSI excited at 670 nm and 740 nm wavelengths to those of white light-grown PSI (WL-PSI) obtained at 675 nm excitation. We report a fast decay of excited state features of chlorophyll a and complete energy transfer from chlorophyll a to chlorophyll f in FRL-PSI upon 670 nm excitation, as indicated by a frequency shift in a carbonyl absorption band occurring within a 1 ps timescale. While the WL-PSI measurements support the assignment of initial charge separation to A−1+˙A0−˙ [Di Donato et al., Biochemistry, 2011, 50, 480–490] from the kinetics of a distinct cation feature at 1710 cm−1, in the case of FRL-PSI, small features at 1715 cm−1 from the chlorophyll cation are present from sub-ps delays instead, supporting the replacement of the A−1 pigment with chlorophyll f. Comparisons of nanosecond spectra show that charge separation proceeds with 740 nm excitation, which selectively excites chlorophyll f, and modifications in specific carbonyl absorption bands assigned to P700+˙ minus P700 and A1−˙ minus A1 indicate dielectric differences of FRL-PSI compared to WL-PSI in one or both of the two electron transfer branches of FRL-PSI.

Stability, electronic and optical properties of the boron nitride cage (B47N53) from quantum mechanical calculations

Abstract: The structural, electronic and optical properties of an isomer proposed for the boron nitride cage (BN; BxNy; x = 47, y = 53) in the gas phase were carried out by means of DFT calculations. The results of the quantum simulations indicate there is not magnetism associated to the stable structure with neutral charge. However, the system exhibits a high cohesion energy (−6.08 eV/atom), this feature is relating to its high chemical stability. The quantum descriptors indicate a moderate polarity, low chemical reactivity as well as low work function for this nano-cage. This system has electronic behavior like-conductor, opposite to others BN nanostructures . On the other hand, the electronic absorption spectrum was obtained using six density functionals under the formalism of the time-dependent DFT and show a good agreement with the experimental data, these calculations reveal a strong absorption band at ≈ 260 nm in the UV region. The natural transition orbitals suggest an intramolecular charge transfer inside of BN cage, and the laplacian of the electron density evaluated in the bond critical points confirms that the cage is stabilized by the presence of homo-nuclear bonds B B and N N.

Resonant Raman Scattering in Undoped and Lanthanide-Doped CeO2

Abstract: CeO2 has a narrow, empty band of Ce 4f states that lies between an O 2p-based valence band and a Ce 5d-based conduction band. The O 2p–Ce 4f optical band gap is positioned at ∼3.2 eV with an absorption band centered at ∼3.8 eV. We investigated the Raman scattering of bulk CeO2 in the excitation energy range of 1.96–3.81 eV. The resonant enhancement profile of the longitudinal optical (LO) phonon at ∼590 cm–1 closely follows that of the 2LO band and both profiles track the optical absorption of the O 2p–Ce 4f electronic transition. Multi-LO phonon bands were found to appear up to the sixth order, pointing to an electron–phonon Frohlich interaction as the source of the resonant enhancement. The ∼600 cm–1 off-resonant D2 band (denoted as MO8-type complex in ceria doped with M aliovalent ions) is overshadowed under resonant conditions by the resonant LO phonon scattering. Hence, spectral analysis of defect bands under resonant conditions has to be distinct from that applied under off-resonant conditions and c...

Photoluminescence of Seven-Coordinate Zirconium and Hafnium Complexes with 2,2'-Pyridylpyrrolide Ligands.

Abstract: Luminescent seven-coordinated zirconium and hafnium complexes bearing three mono-anionic 2,2'-pyridylpyrrolide ligands and one chloride were synthesized. Solid-state structures and the dynamic behaviors in solution were probed by X-ray crystallography and variable temperature 1 H NMR experiments, respectively. Absorption spectroscopy and time-dependent density functional theory (TD-DFT) calculations supported a hybrid of ligand-to-metal charge transfer (LMCT)/ligand-to-ligand charge transfer (LLCT) for the visible light absorption band. The complexes (Me PMPMe )3 MCl (M=Zr, Hf, Me PMPMe =3,5-dimethyl-2-(2-pyridyl)pyrrolide) are emissive in solution at room temperature upon irradiation with visible light due to a combination of phosphorescence and fluorescence characterized by excited state lifetimes in the μs and low to sub-ns timescale, respectively. Electrochemical experiments revealed that the zirconium complex possesses a reversible redox event under highly reducing condition (-2.29 V vs. Fc+/0 ).

Third order nonlinear optical properties and transient dynamics of thiophene-contained pyrene derivatives: Effect of peripheral substituent group

Abstract: In this work, two pyrene derivatives with delocalized π-electron system 1-(pyren-1-yl)−3-(5-Chlorothiophene-2-yl)acrylic ketone (1612) and 1-(pyren-1-yl)−3-(5-Phenylthiophene-2-yl) acrylic ketone (1613) were synthesized. The third-order nonlinear optical (NLO) response of these compounds dissolved in DMSO were investigated using Z-scan technique with 190 fs laser pulses at 532 nm. Large two-photon absorption was found for the two compounds, while the NLO response of 1613 was larger than 1612. The femtosecond transient absorption (TA) results demonstrate that both compounds have broad TA absorption band over the visible regime. Compared to 1612, the TA band of 1613 exhibit a 15-nm blue shift of its peak while its UV–Vis absorption peak have 50 nm red-shift. Meanwhile, the TA decay curve also show that the excited-state lifetime of 1613 is longer, indicating the substituent group functionalization also has a significant impact on the excited-state relaxation processes. Our results show that both the NLO response and photo-physical dynamics of pyrene chromophores are readily tunable via peripheral substituent group, which is essential for the development of novel NLO materials based on this conjugated molecules.

Elucidating the structural properties of gold selenide nanostructures

Abstract: Noble transition metal chalcogenide gold selenide is a relatively unexplored layered material. Herein, we report on the synthesis and characterization of polymorphic mixed-valence AuSe (Au1+Au3+Se2) by varying the sequence of the addition of the precursors in a colloidal synthesis. Despite the variations, all produced materials showed the co-existence of α- and β-AuSe. Although both polymorphs were observed, XRD showed that the addition of the gold precursor at higher temperatures resulted in α-AuSe being the dominant phase while the addition at lower temperatures resulted in β-AuSe being preferred. The crystal structures of both α- and β-AuSe consist of repeating units of a linearly bonded Au1+ ion to two Se atoms and a Au3+ ion bonded to four Se atoms in a square planar geometry. The Au4f core level spectrum of XPS showed only the Au+1 oxidation state, however, using the Se3d core level spectrum, the formation of AuSe (Au1+Au3+Se2) was evident. Using DFT calculations, the Raman spectra of α- and β-AuSe were simulated and only the square planar geometry was found to be Raman active. The square planar geometry (Au3+Se4)− ions belonging to the D4h point group produced three Raman active vibrational modes, namely, a symmetric stretch (A1g), a planar bend (B1g) and an asymmetric stretch (B2g) for α-AuSe as well as A1g and B1g for β-AuSe. Experimentally, all samples showed Raman vibrational lines from both phases. Moreover, Raman spectroscopy confirmed the presence of Au3+ in AuSe which was not detected using XPS. From the TEM and SEM results, it was evident that the morphologies of the predominantly α-AuSe samples were nanobelts while the predominantly β-AuSe samples showed plate-like structures. The predominantly α-AuSe samples showed a broad absorption band with a maximum at 853 nm while the predominantly β-AuSe samples showed evidence of absorption however with no defined excitonic peak.

Optical Study of the Electronic Structure and Lattice Dynamics of NdBaMn2O6 Single Crystals

Abstract: We investigated the electronic structure and lattice dynamics of double perovskite NdBaMn2O6 single crystals through spectroscopic ellipsometry and Raman scattering spectroscopy. The optical absorption band centered at approximately 0.88 eV was assigned to on-site d–d transitions in Mn, whereas the optical feature at approximately 4.10 eV was assigned to charge-transfer transitions between the 2p state of O and 3d state of Mn. Analysis of the temperature dependence of the d-d transition indicated anomalies at 290 and 235 K. The activated phonon mode, which appeared at approximately 440 cm−1 alongside with the enhancement of the 270 cm−1 phonon mode, coupled strongly to the metal–insulator transition at 290 K, which was associated with a charge/orbital ordering. Moreover, the MnO6 octahedral breathing mode at 610 cm−1 exhibited softening at a temperature lower than 235 K (temperature of the antiferromagnetic phase transition), which revealed the strong coupling between the lattice and magnetic degrees of freedom. The spin–phonon coupling constant obtained was λ = 2.5 cm−1. These findings highlight the importance of charge–orbital–spin interactions in establishing NdBaMn2O6 phases with novel properties.

Sub-100-fs bulk solid-state lasers near 2-micron

Abstract: Mode-locked lasers emitting ultrashort pulses in the 2-μm spectral range at high (100-MHz) repetition rates offer unique opportunities for time-resolved molecular spectroscopy and are interesting as pump/seed sources for parametric frequency down-conversion and as seeders of ultrafast regenerative laser amplifiers. Passively mode-locked lasers based on Tm3+- and Ho3+-doped bulk solid-state materials have been under development for about a decade. In 2009 we demonstrated the first steady-state operation of such a Tm:KLu(WO4)2 laser using a single-walled carbon nanotube (SWCNT) saturable absorber (SA), generating 10-ps pulses at 1.95 μm. In 2012 this laser produced 141-fs pulses at 2.037 μm. More recently, the study of numerous active media with different SAs resulted in the generation of sub-100-fs (sub-10-optical-cycle) pulses. Materials with broad and smooth spectral gain profile were selected, naturally emitting above 2 μm to avoid water vapor absorption/dispersion effects, including anisotropic materials, strong crystal-field distortion in hosts that do not contain rare-earths, crystals with structural or compositional (i.e. mixed compounds) disorder that exhibit inhomogeneous line broadening, mixed laser ceramics, and Tm,Ho-codoping of ordered and disordered crystals and ceramics. A broad absorption band in semiconducting SWCNTs spans from 1.6 to 2.1-μm whereas the absorption of graphene extends into the mid-IR and scales for multilayers, increasing the modulation depth. Compared to GaSb-based semiconductor SA mirrors (SESAMs), the carbon nanostructures exhibit broader spectral response and can be fabricated by simpler and inexpensive techniques. Chirped mirrors were implemented for groupvelocity dispersion compensation, to generate the shortest pulses, down to 52 fs at 2.015 μm.