In Hyung Baek

Efficient Mode-Locking of Sub-70-fs Ti:Sapphire Laser by Graphene Saturable Absorber

The efficient passive mode-locking of a Ti:sapphire laser with a monolayer graphene saturable absorber is demonstrated for the first time. High-quality and large-area (1 in.) monolayer graphene, synthesized by chemical vapor deposition, exhibits ultrafast recovery times and excellent nonlinear absorption behavior for bulk solid-state laser mode-locking near 800 nm. The continuous-wave mode-locked Ti:sapphire laser generates 63-fs pulses with output powers up to 480 mW under stable operation at 99.4 MHz.

High-quality, large-area monolayer graphene for efficient bulk laser mode-locking near 1.25 μm

High-quality monolayer graphene as large as 1.2×1.2 cm2 was synthesized by chemical vapor deposition and used as a transmitting saturable absorber for efficient passive mode-locking of a femtosecond bulk solid-state laser. The monolayer graphene mode-locked Cr:forsterite laser was tunable around 1.25 μm and delivered sub-100 fs pulses with output powers up to 230 mW. The nonlinear optical characteristics of the monolayer graphene saturable absorber and the mode-locked operation were then compared with the case of the bilayer graphene saturable absorber.

Single-walled carbon nanotube saturable absorber assisted high-power mode-locking of a Ti:sapphire laser

We report on passive mode-locking of a Ti:sapphire laser employing a single-walled carbon nanotube saturable absorber (SWCNT-SA) specially designed and fabricated for wavelengths near 800 nm. Mode-locked pulses as short as 62 fs were generated at a repetition rate of 99.4 MHz. We achieved output powers from the SWCNT-SA mode-locked laser as high as 600 mW with a slope efficiency of 26%. The characteristics of SWCNT-SA-assisted mode-locking were compared with those of Kerr-lens mode-locking without SWCNT-SA.

Highly nonlinear organic crystal OHQ-T for efficient ultra-broadband terahertz wave generation beyond 10 THz.

We report on efficient generation of ultra-broadband terahertz (THz) waves via optical rectification in a novel nonlinear organic crystal with acentric core structure, i.e. 2-(4-hydroxystyryl)-1-methylquinolinium 4-methylbenzenesulfonate (OHQ-T), which possesses an ideal molecular structure leading to a maximized nonlinear optical response for near-infrared-pumped THz wave generation. By systematic studies on wavelength-dependent phase-matching conditions in OHQ-T crystals of different thicknesses we are able to generate coherent THz waves with a high peak-to-peak electric field amplitude of up to 650 kV/cm and an upper cut-off frequency beyond 10 THz. High optical-to-THz conversion efficiency of 0.31% is achieved by efficient index matching with a selective pumping at 1300 nm.

Monolayer graphene coated Yb:YAG channel waveguides for Q-switched laser operation

We studied operation characteristics of an Yb:YAG channel waveguide laser, Q-switched by employing monolayer graphene. Uniform monolayer graphene grown by thermal chemical vapor deposition is transferred directly onto one end facet of the channel waveguide which simultaneously serves as an output coupling mirror (OC), making a monolithic Q-switched waveguide laser possible. In this cavity configuration, the Q-switched laser delivers a maximum average output power of 85 mW, corresponding to a pulse energy of 64 nJ at 1.33-MHz repetition rate. The laser performance of this device is compared with another cavity configuration, in which the monolayer graphene is coated onto separate OCs. In this case a shorter pulse duration of 79 ns is achieved, but the laser operation performance is worse with respect to efficiency and output power. The proposed monolithic approach demonstrates the potential for developing more compact Q-switched laser devices.

Diffraction-Limited High-Power Single-Cycle Terahertz Pulse Generation in Prism-Cut LiNbO 3 for Precise Terahertz Applications

We report the generation of 3.3-mW single-cycle terahertz (THz) pulses at 1-kHz repetition rate via optical rectification in MgO-doped prism-cut stoichiometric LiNbO3. Efficient pulse-front tilting of 800-nm pulses was realized by an optimized single-lens focusing scheme for radially-symmetric propagation of THz beams. In this geometry, nearly-diffraction-limited THz Gaussian beams with electric field strength as high as 350 kV/cm were generated. The pump-to-THz energy conversion efficiency of

Control of terahertz nonlinear transmission with electrically gated graphene metadevices

1.36{\times}10^{-3}

Femtosecond laser machining: A new technique to fabricate carbon nanotube based emitters

and the extremely high signal-to-noise ratio of ~1:15000 achieved are among the best results for 1-kHz single-cycle terahertz pulse generation ever demonstrated in room temperature operation.

Wavelength and fluence-dependent third-order optical nonlinearity of mono- and multi-layer graphene

Graphene, which is a two-dimensional crystal of carbon atoms arranged in a hexagonal lattice, has attracted a great amount of attention due to its outstanding mechanical, thermal and electronic properties. Moreover, graphene shows an exceptionally strong tunable light-matter interaction that depends on the Fermi level - a function of chemical doping and external gate voltage - and the electromagnetic resonance provided by intentionally engineered structures. In the optical regime, the nonlinearities of graphene originated from the Pauli blocking have already been exploited for mode-locking device applications in ultrafast laser technology, whereas nonlinearities in the terahertz regime, which arise from a reduction in conductivity due to carrier heating, have only recently been confirmed experimentally. Here, we investigated two key factors for controlling nonlinear interactions of graphene with an intense terahertz field. The induced transparencies of graphene can be controlled effectively by engineering meta-atoms and/or changing the number of charge carriers through electrical gating. Additionally, nonlinear phase changes of the transmitted terahertz field can be observed by introducing the resonances of the meta-atoms.

THz-pump and X-ray-probe sources based on an electron linac

A convenient and facile method to fabricate cathodes for efficient field-electron emission, which consists of columns of vertically aligned carbon nanotubes (VACNTs), is presented. The new top-down approach that uses femtosecond laser pulses to make a VACNT-column array starting from a mat of VACNTs is versatile in making diverse cathode architecture because the synthesis and sculpturing processes are completely decoupled. Moreover, this laser-machining process is scalable for the fabrication of large size cathodes. Test of field-electron emission from a typical laser-machined cathode showed a promising result. Emission currents of 0.10 μA and 0.57 mA were measured at the respective applied fields of 0.76 and 1.47 V/μm from a cathode that consisted of a 5×5 array of VACNT columns. The total cathode area occupied by the 5×5 VACNT columns was 0.80×0.80 mm2 while the net area of 25 VACNT columns’ top surfaces was 1.00×10−2 mm2. The stability of the 5×5 VACNT-column array was proved by the continuous operatio...