Laser & Photonics Reviews

Cover image for Vol. 10 Issue 4

Editor: Katja Paff

Online ISSN: 1863-8899

Associated Title(s): Advanced Materials, Advanced Optical Materials, Journal of Biophotonics

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October 05, 2015

Quantum Cascade Lasers

Quantum Cascade Lasers

Steady progress turns multi-wavelength quantum cascade laser arrays into a powerful versatile source for next-generation spectroscopy and stand-off detection systems.

Cambridge (MA/USA) – While conventional interband semiconductor lasers emit electromagnetic radiation due to the recombination of electrons and holes across the band gap of the material, laser emission in quantum cascade lasers (QCLs) is based on only one type of carrier and intersubband transitions in a repeated stack of semiconductor multiple quantum well heterostructures. Since the first demonstration in 1994 of such a unipolar semiconductor laser using an AlInAs/GaInAs heterostructure, QCLs have been attracting research interest from both applied and basic research communities. Steady progress has expanded the wavelength range accessible by QCLs, now extending from below 3 μm over the long-wavelength mid-infrared to the far-infrared and terahertz (THz) regime.

Multi-wavelength quantum cascade laser arrays in the mid-infrared are of special interest as they are a powerful, robust and versatile source for next-generation spectroscopy and stand-off detection systems. In a review article published in Laser & Photonics Reviews, Federico Capasso (Harvard University, USA) and Patrick Rauter (Harvard University and Johannes Kepler University Linz, Austria) give an overview of recent progress in the development of multi-wavelength QCL arrays as novel sources of tunable, coherent, monochromatic radiation.

The authors discuss various approaches for the array elements, from conventional distributed-feedback lasers over master-oscillator power-amplifier devices to tapered oscillators. The variety of possible applications is illustrated based on some selected publications on spectroscopy and sensing demonstrations. Another topic are external cavity (EC) systems as a common, commercially available solution for QCL tuning. Moreover, the challenges associated with reliably achieving single-mode operation at deterministic wavelengths for each laser element in combination with a uniform distribution of high output power across the array are discussed. Spectroscopy and hyperspectral imaging demonstrations by quantum cascade laser arrays are reviewed.

Rauter and Capasso report on master-oscillator power-amplifier (MOPA) QCL arrays as a more powerful alternative to distributed-feedback (DFB) devices. They achieve multiwatt power levels for tunable single-mode emission while preserving the spectral purity and high beam quality of narrow DFB ridges.

The authors conclude: “Spectroscopy and detection systems based on multiwavelength QCL arrays as tunable mid-infrared sources have tremendous potential to outperform conventional systems like EC QCL-based platforms or FTIRs in a variety of fields and to open up new applications calling for fast, compact and mechanically robust solutions with high customizability.”
(Text contributed by K. Maedefessel-Herrmann)

See the original publication: Rauter, P., Capasso, F.; Laser Photonics Rev., 9:5, 452-477 (2015); DOI 10.1002/lpor.201500095

May 29, 2015

Short, Ultrashort, Sub-Cycle Short

Short, Ultrashort, Sub-Cycle Short

Making light pulses ever-shorter is one of the goals in ultrafast optics. Sub-cycle optical waveforms may be reached by coherent synthesis of pulses generated by separate sources.

Milan (Italy) – Ultrashort light pulses are a valuable achievement. They make it possible to observe chemical reactions, opening up the field of femtochemistry. Other current and/or future applications include high capacity telecommunications systems, photonic switching devices, optical coherence tomography, and high precision surgical cutting. The damage to surrounding tissues during surgery is greatly reduced as the pulse duration decreases. Of course, this is by far not the only reason why scientists are keen to reduce the pulse duration further and further. Light pulses with the shortest possible duration offer a wealth of benefits both for fundamental research and for technical applications.
Since the invention of the laser and the discovery of mode locking, there has been a constant effort directed at the generation of ever-shorter light pulses, in a quest to approach (and overcome) the limit set by the period of the optical carrier wave. “The two key ingredients for a short light pulse are broad bandwidth, dictated by the Fourier theorem, and accurate control of the dispersion, i.e. of the relative arrival time of the different frequency components, so as to achieve transform-limited pulse widths”, explains Cristian Manzoni in a recently published review article. The transform limit is the lower limit for the pulse duration possible for a given optical spectrum of a pulse. The generation of sub-optical-cycle, carrier-envelope phase-stable light pulses is one of the frontiers of ultrafast optics.
Sub-cycle pulse generation needs bandwidths substantially exceeding one octave and accurate control of the spectral phase. These requirements are very challenging to satisfy with a single laser beam. One promising strategy for shortening the duration of light pulses is coherent combination, or synthesis, of longer pulses from separate sources. Says Manzoni: “Intense research activity is currently devoted to the coherent synthesis of pulses generated by separate sources.”
In their review article, Manzoni and his co-authors from Politecnico di Milano (Milan, Italy), Elektronen-Synchrotron DESY and Hamburg Center for Ultrafast Imaging (Hamburg, Germany), Cornell University (Ithaca, USA), Columbia University (New York, USA), and MIT (Cambridge, USA) discuss the conceptual schemes and experimental tools that can be employed for the generation, amplification, control, and combination of separate light pulses. They give an overview on main conceptual approaches to waveform synthesis and discuss the experimental tools required for coherent synthesis, giving details of various techniques for the control of pulse relative delay and carrier-envelope phase, and the tailoring of their spectral phase.
The main techniques for the spectrotemporal characterization of the synthesized fields are also described. In addition, recent implementations of coherent waveform synthesis are presented: from the first demonstration of a single-cycle optical pulse by the addition of two pulse trains derived from a fiber laser, to the coherent combination of the outputs from optical parametric chirped-pulse amplifiers (OPCPAs).
The authors are convinced that sub-cycle waveform synthesizers working at optical frequencies will be able to overcome the traditional bandwidth limitations of ultrafast amplifiers. In addition, optical parametric amplifier (OPA)- and OPCPA-based synthesizers will in the future overcome energy and average power bottlenecks.
(Text contributed by K. Maedefessel-Herrmann)

See the original publication: Cristian Manzoni, Oliver D. Mücke, Giovanni Cirmi, Shaobo Fang, Jeffrey Moses, Shu-Wei Huang, Kyung-Han Hong, Giulio Cerullo and Franz X. Kärtner, Coherent pulse synthesis: towards sub-cycle optical waveforms, Laser Photonics Rev., 9:2, 129-171 (2015)

April 29, 2015

A new paradigm

A new paradigm

Optical metasurfaces are thin-layer subwavelength-patterned structures that interact strongly with light. Offering a wealth of useful functionalities, they are a logical extension of the field of metamaterials towards their practical applications. Their nonlinear effects can be enhanced with the help of metasurface engineering.

Acton (Australia) – The expression metamaterials is used for artificial materials that exhibit properties which cannot be observed in natural materials. Usually, their special characteristics do not arise from their chemical attributes but from the way their building blocks are arranged. In most cases, they are arranged in repeating patterns at scales smaller than the wavelengths of the phenomena they influence. By precisely adjusting their shape, geometry, size, orientation, and arrangement, materials with tailored properties can be achieved.
Two-dimensional thin-film planar structures composed of resonant metamaterial elements are called metasurfaces. “Metasurfaces have opened up recently our imagination for the realization of a new generation of flat optical elements with unique functionalities and numerous potential applications”, Yuri S. Kivshar points out in a recently published review article. “Metasurfaces have become a new paradigm in the physics of metamaterials, showing many intriguing realizations of the metamaterial concept.” Being a logical extension of the field of metamaterials towards their practical applications, metasurfaces have become the subject of several rapidly growing areas of research.
In their review, Kivshar and his co-authors from The Australian National University, Acton and the Lomonosov Moscow State University, Moscow, Russian Federation introduce the basic concepts, sum up important historical aspects, and give an overview on the most interesting properties of photonic metasurfaces, demonstrating their useful functionalities such as frequency selectivity, wavefront shaping, and polarization control. They discuss the ways to achieve tunability of metasurfaces and also demonstrate that nonlinear effects can be enhanced with the help of metasurface engineering.
One of the recent developments that may lead to a dramatic enhancement of metasurface performance is associated with the use of optically resonant dielectric metamaterial structures such as single-layer arrays of dielectric silicon disks. These resonant dielectric metasurfaces employ both electric and magnetic Mie resonances of high-index dielectric nanoparticles. The most remarkable example of an interplay of electric and magnetic Mie resonances can be found in the recently realized high-efficiency all-dielectric Huygens metasurfaces which can be employed for almost lossless wavefront manipulation and laser pulse compression. Another recent important direction of research on metasurfaces employs graphene and graphene-based structures as metasurface materials. For example, the nanostructuring of a graphene layer into an array of closely packed nanodisks was shown to enhance dramatically light absorption in the infrared region of the spectrum and this enhanced absorption can be tuned efficiently with voltage.
The authors anticipate that future technologies will demand a substantial increase in photonic integration and energy efficiency far surpassing those of bulk optical components and modern silicon photonics: “Such advances can be achieved only by embedding the photonic functionalities at the material level, creating a new paradigm of metadevices.” Tunable photonic metasurfaces can add many new exciting functionalities (like variable focal length). Enhancement and engineering of nonlinear effects by metasurfaces can increase the efficiency of frequency mixing utilized in frequency up- and down-conversion processes as well as lead to novel ways for all-optical subwavelength control of light waves.
(Text contributed by K. Maedefessel-Herrmann)

See the original publication: A. E. Minovich, A. E. Miroshnichenko, A. Y. Bykov, T. V. Murzina, D. N. Neshev, Y. S. Kivshar, Laser Photonics Rev., 9:2, 195-213 (2015); DOI: 10.1002/lpor.201400402

October 05, 2015
Quantum Cascade Lasers

May 29, 2015
Short, Ultrashort, Sub-Cycle Short

April 29, 2015
A new paradigm

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