Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Bose-Einstein condensation in a gas of sodium atoms

Temperature (T) is probably the most fundamental parameter in all kinds of science. Respective sensors are widely used in daily life. Besides conventional thermometers, optical sensors are considered to be attractive alternatives for sensing and on-line monitoring of T. This Review article focuses on all kinds of luminescent probes and sensors for measurement of T, and summarizes the recent progress in their design and application formats. The introduction covers the importance of optical probes for T, the origin of their T-dependent spectra, and the various detection modes. This is followed by a survey on (a) molecular probes, (b) nanomaterials, and (c) bulk materials for sensing T. This section will be completed by a discussion of (d) polymeric matrices for immobilizing T-sensitive probes and (e) an overview of the various application formats of T-sensors. The review ends with a discussion on the prospects, challenges, and new directions in the design of optical T-sensitive probes and sensors.

Luminescent probes and sensors for temperature.

We show that the Raman scattering technique can give complete structural information for one-dimensional systems, such as carbon nanotubes. Resonant confocal micro-Raman spectroscopy of an (n,m) individual single-wall nanotube makes it possible to assign its chirality uniquely by measuring one radial breathing mode frequency omega(RBM) and using the theory of resonant transitions. A unique chirality assignment can be made for both metallic and semiconducting nanotubes of diameter d(t), using the parameters gamma(0) = 2.9 eV and omega(RBM) = 248/d(t). For example, the strong RBM intensity observed at 156 cm(-1) for 785 nm laser excitation is assigned to the (13,10) metallic chiral nanotube on a Si/SiO2 surface.

Structural (n,m) determination of isolated single wall carbon nanotubes by resonant Raman scattering

Light modulation is an essential operation in photonics and optoelectronics. With existing and emerging technologies increasingly demanding compact, efficient, fast and broadband optical modulators, high-performance light modulation solutions are becoming indispensable. The recent realization that 2D layered materials could modulate light with superior performance has prompted intense research and significant advances, paving the way for realistic applications. In this Review, we cover the state of the art of optical modulators based on 2D materials, including graphene, transition metal dichalcogenides and black phosphorus. We discuss recent advances employing hybrid structures, such as 2D heterostructures, plasmonic structures, and silicon and fibre integrated structures. We also take a look at the future perspectives and discuss the potential of yet relatively unexplored mechanisms, such as magneto-optic and acousto-optic modulation.

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Optical modulators with 2D layered materials

Black phosphorus (BP), an emerging narrow direct band-gap two-dimensional (2D) layered material that can fill the gap between the semi-metallic graphene and the wide-bandgap transition metal dichalcogenides (TMDs), had been experimentally found to exhibit the saturation of optical absorption if under strong light illumination. By taking advantage of this saturable absorption property, we could fabricate a new type of optical saturable absorber (SA) based on mechanically exfoliated BPs, and further demonstrate the applications for ultra-fast laser photonics. Based on the balanced synchronous twin-detector measurement method, we have characterized the saturable absorption property of the fabricated BP-SAs at the telecommunication band. By incorporating the BP-based SAs device into the all-fiber Erbium-doped fiber laser cavities, we are able to obtain either the passive Q-switching (with maximum pulse energy of 94.3 nJ) or the passive mode-locking operation (with pulse duration down to 946 fs). Our results show that BP could also be developed as an effective SA for pulsed fiber or solid-state lasers.

Mechanically exfoliated black phosphorus as a new saturable absorber for both Q-switching and Mode-locking laser operation

We demonstrate a compact Q-switched dual-wavelength erbium-doped fiber (EDF) laser based on graphene as a saturable absorber (SA). By optically driven deposition of graphene on a fiber core, the SA is constructed and inserted into a diode-pumped EDF laser cavity. Also benefiting from the strong third-order optical nonlinearity of graphene to suppress the mode competition of EDF, a stable dual-wavelength Q-switching operation has been achieved using a two-reflection peak fiber Bragg grating as the external cavity mirror. The Q-switched EDF laser has a low pump threshold of 6.5mW at 974nm and a wide range of pulse-repetition rate from 3.3 to 65.9kHz. The pulse duration and the pulse energy have been characterized. This is, to the best of our knowledge, the first demonstration of a graphene-based Q-switched laser.

Graphene-based passively Q-switched dual-wavelength erbium-doped fiber laser.

We propose and demonstrate 1, 1.5, and 2 μm passively Q-switched fiber lasers by exploiting a few-layer Molybdenum sulfide (MoS2) polymer composite as broadband saturable absorber (SA), respectively. The few-layer MoS2 nanosheets are prepared by the liquid-phase exfoliation method, and are composited with polyvinyl alcohol (PVA). The PVA-MoS2 film is sandwiched between two fiber ferrules to form the fiber-compatible SA. The few-layer MoS2 not only shows good transparency from ultraviolet to mid-infrared spectral region, but also possesses the nonlinear saturable absorption. The modulation depth and saturation optical intensity of the PVA-MoS2 film are measured to be 1.6% and 13 MW/cm2 at 1566 nm by the balanced twin-detector technique, respectively. By further inserting the filmy PVA-MoS2 SA into the cavities of Yb-, Er- and Tm-doped fiber lasers, we achieve stable Q-switching operations at 1.06, 1.56, and 2.03 μm, respectively. The output characteristics of the Q-switched pulses at the three wavelengths have been investigated, respectively. The MoS2-based Q-switching enables the large pulse energy of ∼1 μJ with a pulse width of 1.76 μs. This is, to the best of our knowledge, the first demonstration of MoS2-based Q-switched fiber lasers in a wide wavelength range (from 1 to 2 μm). Our results experimentally confirm that the new-type 2-D material, few-layer MoS2, is a promising broadband SA to Q-switch fiber lasers covering all major wavelengths from near- to mid-infrared region.

1-, 1.5-, and 2-μm Fiber Lasers Q-Switched by a Broadband Few-Layer MoS 2 Saturable Absorber

Passive Q-switching of an ytterbium-doped fiber (YDF) laser with few-layer topological insulator (TI) is, to the best of our knowledge, experimentally demonstrated for the first time. The few-layer TI: Bi2Se3 (2–4 layer thickness) is firstly fabricated by the liquid-phase exfoliation method, and has a low saturable optical intensity of 53 MW/cm2 measured by the Z-scan technique. The optical deposition technique is used to induce the few-layer TI in the solution onto a fiber ferrule for successfully constructing the fiber-integrated TI-based saturable absorber (SA). By inserting this SA into the YDF laser cavity, stable Q-switching operation at 1.06 μm is achieved. The Q-switched pulses have the shortest pulse duration of 1.95 μs, the maximum pulse energy of 17.9 nJ and a tunable pulse-repetition-rate from 8.3 to 29.1 kHz. Our results indicate that the TI as a SA is also available at 1 μm waveband, revealing its potential as another broadband SA (like graphene).

1.06 μm Q-switched ytterbium-doped fiber laser using few-layer topological insulator Bi₂Se₃ as a saturable absorber.

Passive Q-switching of an ytterbium-doped fiber (YDF) laser with few-layer topological insulator (TI) is, to the best of our knowledge, experimentally demonstrated for the first time. The few-layer TI: Bi2Se3 (2-4 layer thickness) is fabricated by the liquid-phase exfoliation method, and has a low saturable optical intensity of 53 MW/cm2 measured by the Z-scan technique. The optical deposition technique is used to induce the few-layer TI in the solution onto a fiber ferrule for successfully constructing the fiber-integrated TI-based saturable absorber (SA). By inserting this SA into the YDF laser cavity, stable Q-switching operation at 1.06 {\mu}m is achieved. The Q-switched pulses have the shortest pulse duration of 1.95 {\mu}s, the maximum pulse energy of 17.9 nJ and a tunable pulse-repetition-rate from 8.3 to 29.1 kHz. Our results indicate that the TI as a SA is also available at 1 {\mu}m waveband, revealing its potential as another wavelength-independent SA (like graphene).

1.06 {\mu}m Q-switched ytterbium-doped fiber laser using few-layer topological insulator Bi2Se3 as a saturable absorber

Passive Q-switching or mode-locking by placing a saturable absorber inside the laser cavity is one of the most effective and popular techniques for pulse generation. However, most of the current saturable absorbers cannot work well in the visible spectral region, which seriously impedes the progress of passively Q-switched/mode-locked visible pulsed fibre lasers. Here, we report a kind of visible saturable absorber-two-dimensional transition-metal dichalcogenides (TMDs, e.g. WS2, MoS2, MoSe2), and successfully demonstrate compact red-light Q-switched praseodymium (Pr(3+))-doped all-fibre lasers. The passive Q-switching operation at 635 nm generates stable laser pulses with ∼200 ns pulse duration, 28.7 nJ pulse energy and repetition rate from 232 to 512 kHz. This achievement is attributed to the ultrafast saturable absorption of these layered TMDs in the visible region, as well as the compact and all-fibre laser-cavity design by coating a dielectric mirror on the fibre end facet. This work may open a new route for next-generation high-performance pulsed laser sources in the visible (even ultraviolet) range.

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Zhiping Cai

Papers

Graphene-based passively Q-switched dual-wavelength erbium-doped fiber laser.

1-, 1.5-, and 2-μm Fiber Lasers Q-Switched by a Broadband Few-Layer MoS 2 Saturable Absorber

1.06 μm Q-switched ytterbium-doped fiber laser using few-layer topological insulator Bi₂Se₃ as a saturable absorber.

1.06 {\mu}m Q-switched ytterbium-doped fiber laser using few-layer topological insulator Bi2Se3 as a saturable absorber

Two-dimensional material-based saturable absorbers: towards compact visible-wavelength all-fiber pulsed lasers