Using a 4.5-W average power Cr2+:ZnS laser having a pulse width 43 fs and a spectral bandwidth of 138 nm centred at 2360 nm with a repetition rate of 80 MHz, we have produced femtosecond pulses in yellow wavelength. Using a 1 mm long Type 0 MgO: PPLN crystal in the first stage of our experiment, we have generated a maximum of ~ 2.43 W power of ~ 60 fs pulse width and ~ 39 nm spectral bandwidth centred at 1180 nm with a maximum conversion efficiency as high as ~ 65%. In the second stage, two different crystals, MgO:PPLN and BIBO were used to generate ultrafast coherent yellow source. The 1.18 μm radiation is first frequency-doubled in a multigrating 1 mm long Type 0 MgO:PPLN crystal with grating periods Λ=8.9 - 9.45 μm. A coherent yellow source with wavelength tunability from 577- 589 nm with a spectral bandwidth of ∼ 2 nm and temporal pulsewidth of ∼ 913 fs was achieved. At optimum focussing, we obtained a maximum power of 0.92 W for 2.2 W of pump power having a conversion efficiency of 40%. In order to address the large GVM between 1180 nm and 590 nm wavelength, we used another 1.2 mm long nonlinear crystal BIBO for birefringent phase matching. With BIBO crystal, the near-IR radiation was efficiently frequency doubled into yellow range (~ 591 nm) with average power of ~ 1 W and having a maximum conversion efficiency as high as 47%. The generated beam has a pulse width of ~ 130 fs in Gaussian shaped and ~ 4 nm spectral bandwidth with a time-bandwidth product of 0.45 showing almost no chirp. The output beam is a Gaussian shaped transverse beam profile with measured M2 values of M2x ∼ 1.07 and M2y ∼1.01.
We explore current-driven Dirac plasmon dynamics in monolayer graphene metasurfaces. DC-current-induced complete suppression of the graphene absorption is experimentally observed in a broad frequency range followed by a giant amplification (up to ∼ 9 % gain) of an incoming terahertz radiation at room temperature.
We report on efficient, two stage single-pass second harmonic generation of ultrafast Cr2+:ZnS laser with spectral bandwidth of 138 nm centered at ~2360 nm and pulse width of ~43 fs at a repetition rate of 80 MHz into tunable yellow radiation across 577 - 589 nm in multi-grating MgO:PPLN crystals. A maximum average output power ~940 mW at 589 nm wavelength with a single-pass conversion efficiency as high as 41% was achieved. The yellow radiation has a spectral bandwidth of 2 nm and pulse-width of ~913 fs in absence of any pulse compression with a time-bandwidth product of 1.58.
Current-injection pumping in graphene makes carrier population inversion enabling lasing and/or amplification of terahertz (THz) radiation. We have recently demonstrated single-mode THz lasing at 100K in graphene-channel transistor laser structures. Introduction of a gated double-graphene-layered (G-DGL) van der Waals heterostructure is a promising route to further increase the operation temperature and radiation intensity via plasmon- and/or photon-assisted quantummechanical tunneling. We have proposed a cascading of the G-DGL unit element working as a new type of THz quantumcascade lasers. Numerical analyses demonstrate further increase of the quantum efficiency of THz lasing by orders of magnitude compared to a transistor or single G-DGL structure.
Linear and gapless energy spectrum of graphene carriers enables population inversion under optical and electrical pumping. We first theoretically discovered this phenomenon and demonstrated experimental observation of single-mode THz lasing with rather weak intensity at 100K in current-injection pumped graphene-channel field-effect transistors (GFETs). We introduce graphene surface plasmon polariton (SPP) instability to substantially boost the THz gain. We demonstrate our experimental observation of giant amplification of THz radiation at 300K stimulated by graphene plasmon instabilities in asymmetric dual-grating gate (ADGG) GFETs. Integrating the graphene SPP amplifier into a GFET laser will be a promising solution towards room-temperature intense THz lasing.
This paper reviews recent advances in the terahertz (THz) graphene-based 2D-heterostructure lasers and amplifiers. The linear gapless graphene energy spectrum enables population inversion under optical and electrical pumping giving rise to the negative dynamic conductivity in a wide THz frequency range. We first theoretically discovered these phenomena and recently reported on the experimental observation of the amplified spontaneous THz emission and single-mode THz lasing at 100K in the current-injection pumped graphene-channel field-effect transistors (GFETs) with a distributedfeedback dual-gate structure. We also observed the light amplification of stimulated emission of THz radiation driven by graphene-plasmon instability in the asymmetric dual-grating gate (ADGG) GFETs by using a THz time-domain spectroscopy technique. Integrating the graphene surface plasmon polariton (SPP) oscillator into a current-injection graphene THz laser transistor is the most promising approach towards room-temperature intense THz lasing.
This paper reviews recent advances in the research of graphene-based van der Waals heterostructures for emission and detection of terahertz radiation. A gated double-graphene-layer (DGL) nanocapacitor is the core shell under consideration, in which a thin tunnel barrier layer is sandwiched by outer graphene layers at both sides. The DGL can support symmetric optical and anti-symmetric acoustic coupled plasmon modes in the GLs. The latter mode can modulate the band-offset between the GL, giving rise to modulation of the inter-GL-layer resonant tunneling. This can dramatically enhance the THz gain or responsivity via plasmon-assisted inter-GL resonant tunneling.
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