There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

This Review discusses the emerging field of mid-infrared frequency comb generation, including technologies based on novel laser gain media, nonlinear frequency conversion and microresonators, as well as the applications of these combs in precision spectroscopy and direct frequency comb spectroscopy. Laser frequency combs are coherent light sources that emit a broad spectrum of discrete, evenly spaced narrow lines whose absolute frequency can be measured to within the accuracy of an atomic clock. Their development in the near-infrared and visible domains has revolutionized frequency metrology while also providing numerous unexpected opportunities in other fields such as astronomy and attosecond science. Researchers are now exploring how to extend frequency comb techniques to the mid-infrared spectral region. Versatile mid-infrared frequency comb generators based on novel laser gain media, nonlinear frequency conversion or microresonators promise to significantly expand the applications of frequency combs. In particular, novel approaches to molecular spectroscopy in the 'fingerprint region', with dramatically improved precision, sensitivity, recording time and/or spectral bandwidth may lead to new discoveries in the various fields relevant to molecular science.

Mid-infrared frequency combs

We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro-PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high-temperature annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include the following: (1) the oxygen chemical adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of nonradiative recombination of excitons at defect sites, which was verified by low-temperature PL measurements. First-principle calculations reveal a strong binding energy of ∼2.395 eV for an oxygen molecule adsorbed on a S vacancy of MoS2. The chemically adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to physically adsorbed oxygen on an ideal MoS2 surface. We also demonstrate that the defect enginee...

Strong Photoluminescence Enhancement of MoS2 through Defect Engineering and Oxygen Bonding

In the 2015 review paper 'Petawatt Class Lasers Worldwide' a comprehensive overview of the current status of highpower facilities of >200 TW was presented. This was largely based on facility specifications, with some description of their uses, for instance in fundamental ultra-high-intensity interactions, secondary source generation, and inertial confinement fusion (ICF). With the 2018 Nobel Prize in Physics being awarded to Professors Donna Strickland and Gerard Mourou for the development of the technique of chirped pulse amplification (CPA), which made these lasers possible, we celebrate by providing a comprehensive update of the current status of ultra-high-power lasers and demonstrate how the technology has developed. We are now in the era of multi-petawatt facilities coming online, with 100 PW lasers being proposed and even under construction. In addition to this there is a pull towards development of industrial and multidisciplinary applications, which demands much higher repetition rates, delivering high-average powers with higher efficiencies and the use of alternative wavelengths: mid-IR facilities. So apart from a comprehensive update of the current global status, we want to look at what technologies are to be deployed to get to these new regimes, and some of the critical issues facing their development.

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Petawatt and exawatt class lasers worldwide

The absorption and emission properties of transition metal (TM)-doped zinc chalcogenides have been investigated to understand their potential application as room-temperature, mid-infrared tunable laser media. Crystals of ZnS, ZnSe, and ZnTe, individually doped with Cr/sup 2+/, Co/sup 2+/, Ni/sup 2+/, or Fe/sup 2+/ have been evaluated. The absorption and emission properties are presented and discussed in terms of the energy levels from which they arise. The absorption spectra of the crystals studied exhibit strong bands between 1.4 and 2.0 /spl mu/m which overlap with the output of strained-layer InGaAs diodes. The room-temperature emission spectra reveal wide-band emissions from 2-3 /spl mu/m for Cr and from 2.8-4.0 /spl mu/m for Co, Cr luminesces strongly at room temperature; Co exhibits significant losses from nonradiative decay at temperatures above 200 K, and Ni and Fe only luminesce at low temperatures, Cr/sup 2+/ is estimated to have the highest quantum yield at room temperature among the media investigated with values of /spl sim/75-100%. Laser demonstrations of Cr:ZnS and Cr:ZnSe have been performed in a laser-pumped laser cavity with a Co:MgF/sub 2/ pump laser. The output of both lasers were determined to peak at wavelengths near 2.35 /spl mu/m, and both lasers demonstrated a maximum slope efficiency of approximately 20%. Based on these initial results, the Cr/sup 2+/ ion is predicted to be a highly favorable laser ion for the mid-IR when doped into the zinc chalcogenides; Co/sup 2+/ may also serve usefully, but laser demonstrations yet remain to be performed.

Transition metal-doped zinc chalcogenides: Spectroscopy and laser demonstration of a new class of gain media

We report experimental evidence of the lowest exciton state splitting and carrier trapping in PbS quantum dots (QDs) in glass matrix. Our measurements of photoluminescence (PL) of PbS QDs using steady-state and time-resolved PL spectroscopy reveal strong temperature dependences of the PL intensity and decay kinetics. We find that consistent quantitative description of our experimental results can be achieved using a simple model taking into account the lowest 1S-1S exciton state splitting and multiphonon-assisted carrier trapping to states outside a PbS QD. Using our model, we estimate the lowest exciton splitting energy and lifetimes of the dark and bright exciton states of PbS QDs. Consistent with our model, the PL transition energy of PbS QDs deviates at higher temperatures from the one predicted based on the bulk PbS band-gap dependence due to the lowest 1S-1S exciton state splitting. Temperature-induced broadening of the PL spectrum of PbS QDs is explained by the exciton-phonon interaction.

Temperature-dependent photoluminescence of PbS quantum dots in glass: Evidence of exciton state splitting and carrier trapping

We report the ultrashort-pulse Cr:ZnS laser mode-locked by graphene-based saturable absorber mirror. Using the combination of bulk material and a chirped mirror, we demonstrate the shortest reported so far mid-IR pulses of only 5.1 optical cycles (41 fs) centered at 2.4 µm with 190 nm spectral bandwidth. The pulse spectrum almost completely fills the water-free atmospheric window. The output parameters reach 2.3 nJ pulse energy and 250 mW average output power at 108 MHz repetition rate.

Graphene Mode-locked Cr:ZnS Laser with 41 fs Pulse Duration

This article summarizes the latest achievements in the growth and characterization of Er,Yb:YAl3(BO3)4 laser crystal emitting in the 1.5–1.6 μm spectral range. Fundamental spectroscopic parameters relevant to laser performance of Er,Yb:YAB derived from absorption and emission spectra and from excited state dynamics are presented. The laser performance of Er,Yb:YAB in the cw and mode-locked regimes is reported.

Er,Yb:YAl3(BO3)4—efficient 1.5 μm laser crystal

We review the latest breakthroughs in ultrafast fiber laser technology in the mid-IR wavelength range ≥2 μm. In particular, we concentrate on two novel laser systems built around passively mode-locked Tm:fiber lasers and fiber-based Cr:ZnS lasers, generating sub-100 femtosecond pulses and frequency combs with several Watt average output powers, hundreds of kilowatts peak powers and tens of nanojoule pulse energies. The tunability in the broad wavelength range between 2 and 2.5 μm as well as a simple all-silica-fiber design makes the Tm-fiber laser a truly unique broadband light source, particularly relevant for applications in gas sensing, fine material processing of semiconductors, composite materials, glasses and plastics, as well as for brain surgery, breath analysis, remote sensing and stand-off trace gas detection, especially in oil and gas industry. We also review techniques for coherent supercontinuum generation in the mid-IR, including a novel technique of direct generation of the supercontinuum in the fiber. A competing Watt level few-optical cycle Cr:ZnS laser operating at 2.4 μm (Patent pending, ATLA Lasers, Trondheim, Norway) is distinguished by extremely short pulse duration of only 41 fs, reliability and compactness. This unique ultrashort-pulsed laser generates intrinsically coherent frequency combs, which further extends the application range to high-resolution and high-sensitivity spectroscopy and optical clocks.

Mid-IR Ultrashort Pulsed Fiber-Based Lasers

We report the soft-aperture Kerr-lens mode-locked Cr:ZnS laser, generating 550 mW of 69 fs nearly transform-limited pulses at 2.39 μm wavelength. The pulse energy reached 3.8 nJ at 145 MHz repetition rate, limited by the onset of double-pulsing. This corresponds to the shortest-pulse and highest-energy direct femtosecond laser source in the mid-infrared. Dispersion compensation was achieved by a single chirped mirror and a thin sapphire plate, making the laser design simple, compact and very stable, and operating at ambient air and room temperature. The superb thermal and mechanical properties of Cr:ZnS, exceeding those of Cr:ZnSe and many established femtosecond laser crystals, should allow for further scaling of output power.

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Nikolai Tolstik

Papers

Temperature-dependent photoluminescence of PbS quantum dots in glass: Evidence of exciton state splitting and carrier trapping

Graphene Mode-locked Cr:ZnS Laser with 41 fs Pulse Duration

Er,Yb:YAl3(BO3)4—efficient 1.5 μm laser crystal

Mid-IR Ultrashort Pulsed Fiber-Based Lasers

Kerr-lens mode-locked Cr:ZnS laser.