Guozhen Liang

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.

Single-mode surface-emitting concentric-circular-grating terahertz quantum cascade lasers

We demonstrate single-mode surface-emitting terahertz frequency quantum cascade lasers utilising non-uniform second-order distributed feedback concentric-circular-gratings. The grating is designed for single-mode operation and surface emission for efficient and directional optical power out-coupling. The devices exhibit single-mode operation over the entire dynamic range with a side-mode-suppression-ratio of around 30 dB at 78 K, and a six-fold rotationally symmetric far-field pattern. In addition, the devices show a peak output power approximately three times higher than in ridge-waveguide lasers of similar size, whilst maintaining similar threshold current densities for the 3.8 THz emission and without remarkably sacrificing the maximum temperature operation performance. Owing to the high symmetry of the structure and the broad area light emission from surface, the devices are potentially very suitable for use as single-mode, high power emitters for integration into two-dimensional laser arrays.

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.

Low divergence single-mode surface-emitting concentric-circular-grating terahertz quantum cascade lasers

We report the design, fabrication and experimental characterization of surface-emitting terahertz (THz) frequency quantum cascade lasers (QCLs) with distributed feedback concentric-circular-gratings. Single-mode operation is achieved at 3.73 THz with a side-mode suppression ratio as high as ~30 dB. The device emits ~5 times the power of a ridge laser of similar dimensions, with little degradation in the maximum operation temperature. Two lobes are observed in the far-field emission pattern, each of which has a divergence angle as narrow as ~13.5° × 7°. We demonstrate that deformation of the device boundary, caused by anisotropic wet chemical etching is the cause of this double-lobed profile, rather than the expected ring-shaped pattern.

Planar integrated metasurfaces for highly-collimated terahertz quantum cascade lasers

We report planar integration of tapered terahertz (THz) frequency quantum cascade lasers (QCLs) with metasurface waveguides that are designed to be spoof surface plasmon (SSP) out-couplers by introducing periodically arranged SSP scatterers. The resulting surface-emitting THz beam profile is highly collimated with a divergence as narrow as ~4° × 10°, which indicates a good waveguiding property of the metasurface waveguide. In addition, the low background THz power implies a high coupling efficiency for the THz radiation from the laser cavity to the metasurface structure. Furthermore, since all the structures are in-plane, this scheme provides a promising platform where well-established surface plasmon/metasurface techniques can be employed to engineer the emitted beam of THz QCLs controllably and flexibly. More importantly, an integrated active THz photonic circuit for sensing and communication applications could be constructed by incorporating other optoelectronic devices such as Schottky diode THz mixers, and graphene modulators and photodetectors.

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.

OPTICAL RESONATOR ANALOG OF A PHOTONIC TOPOLOGICAL INSULATOR: A FINITE-DIFFERENCE TIME-DOMAIN STUDY

A periodic lattice of optical ring resonators can act as a photonic topological insulator, with the role of spin played by the direction of propagation of light within each ring. Using finite-difference time-domain (FDTD) simulations, we compute the projected band diagram of the system and its transmission spectrum, and demonstrate the existence of the topological edge states. The FDTD results are in good agreement with the predictions of transfer matrix theory.

Recent Developments of Terahertz Quantum Cascade Lasers

Terahertz (THz) quantum cascade lasers (QCLs) are semiconductor-based electrically pumped laser sources that are based on electron transitions between the subbands in the conduction band of a multiple-quantum-well heterostructure. Being such, the wavelength of THz QCL can be tuned by varying the thickness of the quantum wells, covering the frequency range of ~1-5 THz. THz QCLs are also high-power light sources in the THz region, with highest output power in the 100 mW and watt levels for the continuous mode and pulsed mode operation, respectively. However, there are also limitations that hinder their widespread applications, which include low operating temperature, large beam divergence, the intrinsic carrier dynamics, etc. Nevertheless, continuous efforts have been devoted to this area and pushing the frontier further and further. In this paper, we will review both the aforementioned limitations and some of the key recent developments in THz QCLs in the past few years.

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.