Wave propagation and group velocity

Have you ever felt that the very title, Progress in Optics, conjured an image in your mind? Don’t you see a row of handsomely printed books, bearing the editorial stamp of one of the most brilliant members of the optics community, and chronicling the field of optics since the invention of the laser? If so, you are certain to move the bookend to make room for Volume 16, the latest of this series. It contains seven chapters on widely diverging topics, written by well-known authorities in their fields. These are: 1) Laser Selective Photophysics and Photochemistry by V. S. Letokhov, 2) Recent Advances in Phase Profiles (sic) Generation by J. J. Clair and C. I. Abitbol, 3 ) Computer-Generated Holograms: Techniques and Applications by W.-H. Lee, 4) Speckle Interferometry by A. E. Ennos, 5 ) Deformation  Invariant, Space-Variant Optical  Pattern Recognition by D. Casasent and D. Psaltis, 6) Light Emission from High-Current Surface-Spark Discharges by R. E. Beverly, and 7) Semiclassical Radiation  Theory within a  QuantumMechanical Framework by I. R. Senitzkt. The  breadth of topic  matter  spanned  by  these  chapters makes it impossible, for this reviewer at least, to pass judgement on the comprehensiveness, relevance, and completeness of every chapter. With an editorial board as prominent as that of Progress in Optics, however, it seems hardly likely that such comments should be necessary. It should certainly be possible to take the authority of each author as credible. The only remaining judgment to be  made on these chapters is their readability. In short, what are they like to read? The first sentence of the first chapter greets the eye with an obvious typographical error: “The creation of coherent laser light source, that have tunable radiation, opened the . . . .” Two pages later we find: “When two types of atoms or molecules of different isotopic composition ( A and B ) have even one spectral line that does not overlap with others, it is pos-

Progress in optics

A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation, photonic crystal fiber

Color as a Science

Hybrid nanostructures composed of graphene or other two-dimensional (2D) nanomaterials and plasmonic metal components have been extensively studied. The unusual properties of 2D materials are associated with their atomically thin thickness and 2D morphology, and many impressive structures enable the metal nanomaterials to establish various interesting hybrid nanostructures with outstanding plasmonic properties. In addition, the hybrid nanostructures display unique optical characteristics that are derived from the close conjunction of plasmonic optical effects and the unique physicochemical properties of 2D materials. More importantly, the hybrid nanostructures show several plasmonic electrical effects including an improved photogeneration rate, efficient carrier transfer, and a plasmon-induced "hot carrier", playing a significant role in enhancing device performance. They have been widely studied for plasmon-enhanced optical signals, photocatalysis, photodetectors (PDs), and solar cells. In this review, the developments in the field of metal/2D hybrid nanostructures are comprehensively described. Preparation of hybrid nanostructures is first presented according to the 2D material type, as well as the metal nanomaterial morphology. The plasmonic properties and the enabled applications of the hybrid nanostructures are then described. Lastly, possible future research in this promising field is discussed.

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Hybrid nanostructures of metal/two-dimensional nanomaterials for plasmon-enhanced applications

Circular dichroism (CD) studies on single nanostructures can yield novel insights into chiroptical physics that are not available from traditional ensemble-based measurements, yet they are challenging because of their weak signals. By introducing an oblique excitation beam, we demonstrate the observation and spectroscopic analysis of a prominent plasmonic chiroptical response from a single v-shaped gold nanorod dimer nanostructure. We show that circular differential scattering from the obliquely excited gold nanorod dimer yields a characteristic bisignate peak-dip spectral shape at hybridized energies of the dimer. This chiroptical response can be ascribed to extrinsic chirality which depends on the geometry configurations of the chiral arrangement. Due to strong near-field coupling, the dipole orientations of the hybridized resonance modes can be in favor of the incident circularly polarized light where a maximum g-factor of ∼0.4 is observed. Promising applications of this chiroptical arrangement as a key component can be in electronics, photonics, or metamaterials.

Circular dichroism from single plasmonic nanostructures with extrinsic chirality

A multiwavelength erbium-doped fiber (EDF) laser based on a nonlinear amplifying loop mirror (NALM) is proposed and experimentally demonstrated. The NALM provides intensity-dependent transmissivity to equalize different-wavelength powers and the transmission can be uniquely optimized by controlling the cavity loss associated with a section of un-pumped EDF, which also enhances the output signal-to-noise ratio (SNR). Through adjusting the polarization controllers (PCs), under only 70 mW pump power, up to 62-wavelength output with channel spacing of 0.45 nm has been achieved. Also, the lasing tunability and stability are verified.

http://www.physics.sjtu.edu.cn/laserlab/sites/www.physics.sjtu.edu.cn.laserlab/files/2012-Multiwavelength%20erbium-doped%20fiber%20laser%20based%20on%20a%20nonlinear%20amplifying%20loop%20mirror%20assisted%20by%20un-pumped%20EDF.pdf

Multiwavelength erbium-doped fiber laser based on a nonlinear amplifying loop mirror assisted by un-pumped EDF

We report superluminal propagation in optical fibers using Brillouin lasing oscillation in a ring cavity. Negative group velocity propagation through a 10-m single mode fiber has been experimentally demonstrated. An advancement of 221.2 ns was observed before the input signal, which was achieved with a very high slope efficiency of $211.3\text{ }\text{ }\mathrm{ns}/\mathrm{dB}$. This indicates that this way is suitable for long-distance low-loss superluminal propagation via optical fibers. Correspondingly, the group velocity is $\ensuremath{-}0.151c$ and the group index is $\ensuremath{-}6.636$---the highest group velocity ever reported for optical fibers.

http://www.physics.sjtu.edu.cn/laserlab/sites/www.physics.sjtu.edu.cn.laserlab/files/2011-ZL-PRL.pdf

Superluminal propagation at negative group velocity in optical fibers based on Brillouin lasing oscillation.

A novel concept of individually switchable and widely tunable erbium-doped multiwavelength fiber laser (MWFL) is proposed and experimentally demonstrated. The key component of the laser is a channel transmissivity individually variable comb filter composed of two cascaded different-length long-period fiber gratings (LPFGs), named cascaded mismatching LPFGs. When inserted into the laser cavity, this polarization-dependent comb filter functions as the wavelength selector and switching filter simultaneously. By properly adjusting the polarization controllers (PCs) in the laser cavity and in the high birefringence Sagnac loop mirror (HiBi-SLM), eleven individually switchable wavelengths with different flexible lasing states, including successively tunable adjacent single-, dual- and triple-wavelength outputs, nonadjacent dual- and triple-wavelength outputs, as well as quadruple- and quintuple-wavelength outputs, have been achieved. This MWFL may be useful in optical fiber sensing or other fields desiring very flexible optical source.

/pdf/individually-switchable-and-widely-tunable-multiwavelength-44e5x6t6i4.pdf

Individually Switchable and Widely Tunable Multiwavelength Erbium-Doped Fiber Laser Based on Cascaded Mismatching Long-Period Fiber Gratings

We systematically investigate the passive harmonic mode locking (HML) of the bound states of two solitons in a fiber laser that has been mode locked by nonlinear polarization rotation (NPR). The experiment shows that the stable HML state of the bound solitons (BSs) with a fixed and discrete separation is obtained. The repetition rate changes by increasing the pump power and slightly altering the polarization state in the cavity. The dynamic is similar to the HML of a single-pulse operation. In our experiment, the repetition rate can be turned from the fundamental mode locking up to ninth-order HML when the pump power is increased from 168 to 476.1 mW. Once the BSs are obtained, their separation is fixed at 1.5 ps, regardless of the HML order. Under the direct BS interaction, the BS trains are very stable and easily reproducible. This HML behavior of BSs confirms that the BS is another intrinsic feature of the laser, except for the single soliton.

/pdf/harmonic-mode-locking-of-bound-solitons-4ghweeabtd.pdf

Li Zhan

Papers

Circular dichroism from single plasmonic nanostructures with extrinsic chirality

Multiwavelength erbium-doped fiber laser based on a nonlinear amplifying loop mirror assisted by un-pumped EDF

Superluminal propagation at negative group velocity in optical fibers based on Brillouin lasing oscillation.

Individually Switchable and Widely Tunable Multiwavelength Erbium-Doped Fiber Laser Based on Cascaded Mismatching Long-Period Fiber Gratings

Harmonic mode locking of bound solitons