Pulak Chandra Debnath

A Q-switched, mode-locked fiber laser using a graphene oxide-based polarization sensitive saturable absorber

We demonstrate the use of a graphene oxide (GO)-deposited D-shaped fiber as a polarization sensitive saturable absorber for the implementation of a stable Q-switched, mode-locked fiber laser. Using both features of nonlinear saturable absorption and large polarization dependence loss of GO-deposited D-shaped fiber, stable Q-switched mode-locked pulses are readily obtained from an erbium-doped fiber ring laser through simple intra-cavity polarization control under a fixed pump power. It is shown that bursts of sub-picosecond, mode-locked pulses with a Q-switching envelope of a ~2.63 μs temporal width and a ~71.3 kHz repetition rate can readily be generated from the laser. It is also demonstrated that the operating regime of the pulsed laser can be readily changed between mode-locking, Q-switched mode-locking, and Q-switching simply by changing the polarization state of the oscillated beam within the cavity.

A Mode-Locked 1.91 µm Fiber Laser Based on Interaction between Graphene Oxide and Evanescent Field

The use of a graphene oxide (GO)-deposited D-shaped fiber as a saturable absorber for mode locking of a thulium-doped fiber laser is experimentally demonstrated. By using the evanescent field interaction of an oscillating beam with GO, a passive mode locking operation at a wavelength of ~1.91 µm is shown to be achievable from a ring cavity. Stable picosecond pulses are readily obtained at a repetition rate of 15.9 MHz. This demonstration confirms that GO is a cost-effective saturable absorber applicable for ~2 µm ultrafast pulse generation.

Controllability of threshold voltage in Ag-doped ZnO nanowire field effect transistors by adjusting the diameter of active channel nanowire

Silver doped zinc oxide nanowires (NWs) were synthesized on (0001) sapphire substrate by hot-walled pulsed laser deposition. Both enhancement mode and depletion mode NW field effect transistors (FETs) were fabricated. The shift of threshold voltage of silver doped zinc oxide NW FET was observed from 2.45 to −3.2 V depending on the diameter of NWs without any significant changes of the subthreshold swing, carrier concentration, and on/off ratios. We demonstrate that the transfer characteristics of silver doped zinc oxide NW FETs were closely related with the size of NW diameter and control the shift of threshold voltage.

Effects of silver impurity on the structural, electrical, and optical properties of ZnO nanowires

Abstract1, 3, and 5 wt.% silver-doped ZnO (SZO) nanowires (NWs) are grown by hot-walled pulsed laser deposition. After silver-doping process, SZO NWs show some change behaviors, including structural, electrical, and optical properties. In case of structural property, the primary growth plane of SZO NWs is switched from (002) to (103) plane, and the electrical properties of SZO NWs are variously measured to be about 4.26 × 106, 1.34 × 106, and 3.04 × 105 Ω for 1, 3, and 5 SZO NWs, respectively. In other words, the electrical properties of SZO NWs depend on different Ag ratios resulting in controlling the carrier concentration. Finally, the optical properties of SZO NWs are investigated to confirm p-type semiconductor by observing the exciton bound to a neutral acceptor (A0X). Also, Ag presence in ZnO NWs is directly detected by both X-ray photoelectron spectroscopy and energy dispersive spectroscopy. These results imply that Ag doping facilitates the possibility of changing the properties in ZnO NWs by the atomic substitution of Ag with Zn in the lattice.

Full swing logic inverter with amorphous SiInZnO and GaInZnO thin film transistors

A high-performance n-channel metal-oxide-semiconductor inverter implemented consisting of enhancement mode driving thin-film transistor with amorphous Ga-In-Zn-O (a-GIZO) and depletion mode load with amorphous Si-In-Zn-O (a-SIZO) is demonstrated. The threshold voltage of the post-annealed a-SIZO load thin film transistor (TFT) exhibits negative value while the threshold voltage of the GIZO driving TFT exhibits positive value. The proposed inverter composed of a-SIZO and a-GIZO TFT shows much improved switching characteristics with higher voltage gain.

Temperature stress on pristine ZnO nanowire field effect transistor

We have investigated the effect of the temperature dependency on the device stability of pristine ZnO nanowires (NWs) field effect transistor (FET). Pristine ZnO NW FET shows a large threshold voltage (Vth) shift by 6.5 V after increasing the measured temperature from 323 to 363 K. This large shift in Vth is mainly due to thermally activated process. Thermally activated electrons from the deep level trap site can be free carriers which results in the shift in Vth in negative direction. Also, activation energy of ZnO NW FET is derived to be about 1.432 eV based on thermally activated Arrhenius model.

Nonlinear Black Phosphorus for Ultrafast Optical Switching

The outstanding electronic and optical properties of black phosphorus (BP) in a two-dimensional (2D) but unique single-layer puckered structure have opened intense research interest ranging from fundamental physics to nanoscale applications covering the electronic and optical domains. The direct and controllable electronic bandgap facilitating wide range of tunable optical response coupled with high anisotropic in-plane properties made BP a promising nonlinear optical material for broadband optical applications. Here, we investigate ultrafast optical switching relying on the optical nonlinearity of BP. Wavelength conversion for modulated signals whose frequency reaches up to 20 GHz is realized by four-wave-mixing (FWM) with BP-deposited D-shaped fiber. In the successful demonstration of the FWM based wavelength conversion, performance parameter has been increased up to ~33% after employing BP in the device. It verifies that BP is able to perform efficient optical switching in the evanescent field interaction regime at very high speed. Our results might suggest that BP-based ultra-fast photonics devices could be potentially developed for broadband applications.

Thermal damage suppression of a black phosphorus saturable absorber for high-power operation of pulsed fiber lasers.

Recent studies of black phosphorus (BP) have shown its future potential in the field of photonics. We determined the optical damage threshold of BP at 21.8 dBm in a fiber ring laser cavity, and demonstrated the high-power operation capacity of an evanescent field interaction-based BP saturable absorber. The long-term stability of a passively mode-locked fiber laser with a saturable absorber operating at the optical power of 23.3 dBm was verified for 168 h without any significant performance degradation. The center wavelength, spectral width, and pulse width of the laser output are 1558.8 nm, 14.2 nm, and 805 fs, respectively.

Transfer-free synthesis of multilayer graphene using a single-step process in an evaporator and formation confirmation by laser mode-locking

Multilayer graphene is synthesized by a simplified process employing an evaporator in which a target substrate is deposited with a Ni catalyst layer before being heated to grow graphene directly. Carbon atoms adsorbed onto the surface of the Ni source as impurities from the atmosphere are incorporated into the catalyst layer during the deposition, and diffuse toward the catalyst/substrate interface, where they crystallize as graphene with a thickness of less than 2 nm. The need for a transfer process and external carbon supply is eliminated. The graphene is characterized by conventional analysis approaches, including nano-scale visualization and Raman spectroscopy, and utilizing photonics, graphene-functionalized passive laser mode-locking is demonstrated to confirm the successful synthesis of the graphene layer, resulting in an operating center wavelength of 1569.4 nm, a pulse duration of 1.35 ps, and a repetition rate of 31.6 MHz.

Experimental investigation on a Q-switched, mode-locked fiber laser based on the combination of active mode locking and passive Q switching

We performed an experimental investigation on a Q-switched, mode-locked laser based on the combination of passive Q switching and active mode locking. This study was carried out using an erbium fiber ring laser incorporating an active mode locker employing a fast Si-based variable optical attenuator and a passive Q switch based on a carbon nanotube saturable absorber. It was found that stable Q-switched, mode-locked pulses can be readily generated when using a particular cavity modulation index and pump power. It was also found that our scheme can easily alter the operating regime among the Q switching, Q-switched mode locking, and mode locking by simply controlling the cavity modulation index and/or the pump power. For this particular laser configuration, the cavity modulation index and pump power operating conditions have been experimentally analyzed for three laser operating regimes.