Research

Ultrafast lasers & amplifiers

 
Coherent light sources emitting ultrashort pulses and high peak powers are needed for investigation of various nonlinear and ultrafast phenomena. We are developing ultrafast solid-state (bulk, fiber and waveguide) lasers & amplifiers operating in different spectral ranges. In addition, novel types of saturable absorbers necessary to achieve stable and self-starting pulse formation (mode-locking), are developed based on low-dimensional carbon nanostructures.

 
 
 
 

 
Low-dimensional carbon nanostructure-based photonic devices

CNTs and graphene are one of the most attractive materials for applications in photonics and optoelectronics. We are working on developing novel ultrafast switching devices based on CNTs and graphene for laser mode-locking and Q-switching. The broadband absorption and nonlinear optical properties of CNTs can be optimized through synthesis and manufacturing process of devices. We recently demonstrated broadband CNT-based saturable absorbers applicable for femtosecond laser mode-locking in a broad spectral range between 0.8 and 2.1 um. We are also developing graphene saturable absorbers with almost 'unlimited' 
                                                                  operation wavelength range due to unique point bandgap structure.


 
 
 
Terahertz nonlinear photonics

We focus on THz nonlinear & time-resolved spectroscopy and THz nonlinear phenomena in various materials including highly nonlinear organic crystals, low-dimensional nanomaterials, metamaterials, hybrid nanostructures and their applications. In addition, we develop ultrafast high-power (> 5 mW), high-field(~1 MV/cm) broadband THz source to investigate THz nonlinearities and their enhancement. Different THz setups based on Ti:sapphire oscillator and regenerative amplifier are established for different applications. 


 
 
Time-resolved & nonlinear spectroscopy

In order to understand ultrafast carrier dynamics and nonlinear responses, we investigate degenerate and non-degenerate pump-probe & nonlinear spectroscopy, whereas different femtosecond excitation sources (Ti:sapphire laser, Yb-doped, Cr-doped and Er-doped lasers, OPO, OPA and THz sources) are used. We are able to measure both transmission- and reflection-type samples in a broad spectral range from UV to THz.


 
 
 
 
 
Nonlinear optical characteristics of novel materials


One of our research interests is the nonlinear optical characterization of various materials with high-resolution. We are able to investigate third-order nonlinearity such as nonlinear absorption. The transmission change an be resolved with a resolution of less than 0.1%.



 
 
 
 

 
Laser interference lithography

Laser interference lithography (LIL) is a technique for large-area patterning regular arrays of fine features by exposure on the surface of substrate, based on interference effect of beams. LIL is a maskless lithography, so it can easily control the pattern size and does not occur the distorted pattern induced by a mask contact. In particular, it is relatively easy to fabricate large-area 1- and 2-dim. nanostructures. This technique can be, for instance, applied for developing highly efficient solar cells and LEDs.  

 
 

 
Femtosecond laser machining

Femtosecond laser machining can be an another approach for surface texturing. As an example, line or hole arrays on semiconductor, metal and dielectric surfaces can be uniformly fabricated by using femtosecond laser pulses with a real-time monitoring. This technique will be also useful for patterning of solar cells and LEDs to enhance efficiency. Depending on materials and application, optimization process and control of depth are required to avoid damages of used layers.