Bo Meng

Broadband high photoresponse from pure monolayer graphene photodetector

Graphene has attracted large interest in photonic applications owing to its promising optical properties, especially its ability to absorb light over a broad wavelength range, which has lead to several studies on pure monolayer graphene-based photodetectors. However, the maximum responsivity of these photodetectors is below 10 mA W(-1), which significantly limits their potential for applications. Here we report high photoresponsivity (with high photoconductive gain) of 8.61 A W(-1) in pure monolayer graphene photodetectors, about three orders of magnitude higher than those reported in the literature, by introducing electron trapping centres and by creating a bandgap in graphene through band structure engineering. In addition, broadband photoresponse with high photoresponsivity from the visible to the mid-infrared is experimentally demonstrated. To the best of our knowledge, this work demonstrates the broadest photoresponse with high photoresponsivity from pure monolayer graphene photodetectors, proving the potential of graphene as a promising material for efficient optoelectronic devices.

Electrically pumped mid-infrared random lasers

Electrically pumped random lasers in the mid-infrared regime at λ~ 10 μm have been demonstrated for the first time to our knowledge. The laser is based on a quantum cascade (QC) gain media with designed filling fractions of scatterers.

Broadband Saturable Absorption of Graphene Oxide Thin Film and Its Application in Pulsed Fiber Lasers

Graphene oxide (GO) has been used for optical intensity manipulation and short pulse shaping. Most interestingly, it can be used as a broadband light absorber. Such property has been systematically investigated in this paper, in a broad wavelength range from 1 to 2-μm. We fabricated one type of GO/polyvinyl alcohol (GO/PVA) film and use it as a saturable absorber (SA) in Er-, Yb-, and Tm-doped fiber ring lasers, respectively, to build pulsed fiber lasers. It is demonstrated that broad range of mode-locking in the 1-μm and 1.5-μm regions can be obtained. In addition, Q-switching around wavelength of 1870 nm can be obtained in a Tm-doped fiber ring lasers. To the best our knowledge, this is the broadest wavelength regime in which one type of GO SA film can be used to build all-fiber pulsed ring lasers.

Theoretical investigation of injection-locked high modulation bandwidth quantum cascade lasers

In this study, we report for the first time to our knowledge theoretical investigation of modulation responses of injection-locked mid-infrared quantum cascade lasers (QCLs) at wavelengths of 4.6 μm and 9 μm, respectively. It is shown through a three-level rate equations model that the direct intensity modulation of QCLs gives the maximum modulation bandwidths of ~7 GHz at 4.6 μm and ~20 GHz at 9 μm. By applying the injection locking scheme, we find that the modulation bandwidths of up to ~30 GHz and ~70 GHz can be achieved for QCLs at 4.6 μm and 9 μm, respectively, with an injection ratio of 5 dB. The result also shows that an ultrawide modulation bandwidth of more than 200 GHz is possible with a 10 dB injection ratio for QCLs at 9 μm. An important characteristic of injection-locked QCLs is the nonexistence of unstable locking region in the locking map, in contrast to their diode laser counterparts. We attribute this to the ultra-short upper laser state lifetimes of QCLs.

Single-wall carbon nanotubes and graphene oxide-based saturable absorbers for low phase noise mode-locked fiber lasers

Low phase noise mode-locked fiber laser finds important applications in telecommunication, ultrafast sciences, material science, and biology, etc. In this paper, two types of carbon nano-materials, i.e. single-wall carbon nanotube (SWNT) and graphene oxide (GO), are investigated as efficient saturable absorbers (SAs) to achieve low phase noise mode-locked fiber lasers. Various properties of these wall-paper SAs, such as saturable intensity, optical absorption and degree of purity, are found to be key factors determining the performance of the ultrafast pulses. Reduced-noise femtosecond fiber lasers based on such carbon-based SAs are experimentally demonstrated, for which the phase noise has been reduced by more than 10 dB for SWNT SAs and 8 dB for GO SAs at 10 kHz. To the best of our knowledge, this is the first investigation on the relationship between different carbon material based SAs and the phase noise of mode-locked lasers. This work paves the way to generate high-quality low phase noise ultrashort pulses in passively mode-locked fiber lasers.

Broadly tunable single-mode mid-infrared quantum cascade lasers*

In this paper, we review the recent progress in broadly tunable single-mode mid-infrared quantum cascade lasers (QCLs). With a brief introduction on various applications of broadly tunable single-mode mid-infrared QCLs, we first categorize these lasers based on their configurations and tuning schemes. Then recent progress in each scheme is investigated, together with detailed discussions. Furthermore, the pros and cons of different approaches are compared, and the opportunities and challenges for future developments from our point of view are listed. Last but not least, potential new directions and prospects for obtaining broadly tunable singlemode characteristics are also provided. It is foreseeable that, with the development of high performance broadly tunable single-mode mid-infrared QCLs, more applications could be exploited in the near future with new phenomena to be discovered.

Tunable single-mode slot waveguide quantum cascade lasers

We report experimental demonstration of tunable, monolithic, single-mode quantum cascade lasers (QCLs) at ∼10 μm with a two-section etched slot structure. A single-mode tuning range of 77 cm−1 (785 nm), corresponding to ∼7.8% of the relative tuning range, was realized with a ∼20 dB side mode suppression ratio within the whole tuning range. Compared with integrated distributed feedback QCLs, our devices have the advantages of easy fabrication and a broader tuning range. Further theoretical analyses and numerical simulations show that it is possible to achieve a broad continuous tuning range by optimizing the slot structures. The proposed slot-waveguide design could provide an alternative but simple approach to the existing tuning schemes for realizing broadly continuous tunable single-mode QCLs.

Coherent emission from integrated Talbot-cavity quantum cascade lasers

We report experimental realization of phase-locked quantum cascade laser (QCL) array using a monolithically integrated Talbot cavity. An array with six laser elements at a wavelength of ~4.8 μm shows a maximum peak power of ~4 W which is more than 5 times higher than that of a single ridge laser element and a slope efficiency of 1 W/A at room temperature. Operation of in-phase coherent supermode has been achieved over the whole dynamic range of the Talbot-cavity QCL. The structure was analysed using a straightforward theoretical model, showing quantitatively good agreement with the experimental results. The reduced thermal resistance makes the structure an attractive approach to achieve high beam quality continuous wave QCLs.

Broadly continuously tunable slot waveguide quantum cascade lasers based on a continuum-to-continuum active region design

We report our progress in the development of broadly tunable single-mode slot waveguide quantum cascade lasers based on a continuum-to-continuum active region design. The electroluminescence spectrum of the continuum-to-continuum active region design has a full width at half maximum of 440 cm−1 at center wavelength ∼10 μm at room temperature (300 K). Devices using the optimized slot waveguide structure and the continuum-to-continuum design can be tuned continuously with a lasing emission over 42 cm−1, from 9.74 to 10.16 μm, at room temperature by using only current tuning scheme, together with a side mode suppression ratio of above 15 dB within the whole tuning range.

Relative Intensity Noise of Silicon Hybrid Laser

We theoretically investigate the relative intensity noise (RIN) of a silicon hybrid laser by taking into account two-dimension transverse modes in the hybrid waveguide. It shows that, when only one transverse mode is excited, RIN spectrum of the hybrid laser exhibits a larger peak value at a lower relaxation oscillation (RO) frequency as compared with the case of a conventional multiple-quantum well (MQW) laser with the the same active region design and dimension. In contrast, when two transverse modes are excited, the hybrid laser shows a lower peak value at a higher RO frequency as compared with the MQW laser. The effects of waveguide dimensions, e.g., Si ridge height and IIII-V ridge width on RIN spectra of the hybrid laser are also investigated.