Tsinghua Research Group (THRG) conducts innovative and multi-disciplinary research in the fields of optics, physics, chemistry, material, and biology. We aspire to solve critical problems using non-conventional approaches inspired from novel physical principles to enable revolutionary advances. We are currently conducting the following research topics:
Gain-guided Index-antiguided waveguide laser resonators
Category: laser and resonator, quantum electronics, waveguide and fiber optoelectronic devices, photonic crystals, Bragg fibers.
Application: high-power laser, partial coherent light source.
Gain-guided and Index-antiguided waveguide lasers support leaky modes that exhibit very different resonator characteristics from their conventional index-guided counterparts. For one thing, the large modal differential loss in IAG waveguides enables strong mode discrimination that favors single transverse mode (STM) operation, even for waveguide with very large mode area (LMA). This property lends itself as a promising platform for high-power or high-efficiency lasers. For another, resonator modes in open waveguides such as IAG waveguides in known to exhibit excess quantum noise, a property that could be exploited to yield lasers with controlled coherence properties.
Our group design and fabricate photonic bandgap fibers to achieve STM and LMA for laser radiation while simultaneously strong confinement of pump radiation. We also made the world-first demonstration of continuous-wave GG+IAG planar waveguide laser, fabricated using diffusion bonding by Northrop Grumman Synoptics in Charlotte.
- Tsinghua Her, “Gain-guiding in transverse grating waveguides for large modal area laser amplifiers,” Optics Express 16 (10) 7197-7202 (2008).
- Tsing-Hua Her, Xianyu Ao, and Lee W. Casperson, “Gain saturation in gain-guided slab waveguides with large-index antiguiding,” Optics Letters 34 (16) 2411-2413 (2009).
- Xianyu Ao, Tsing-Hua Her, and Lee W. Casperson, “Gain guiding in large-core Bragg fibers,” Optics Express 17 (25) 22666-22672 (2009).
- Chaofan Wang, Tsing-Hua Her, Lei Zhao, Xianyu Ao, Lee Casperson, Chih-Hsien Lai, Hung-Chun Chang, “Gain Saturation and Output Characteristics of Index-Antiguided Planar Waveguide Amplifiers with Homogeneous Broadening,” Journal of Lightwave Technology 29, p. 1958 (2011).
- Chaofan Wang, Tsing-Hua Her, Lee Casperson, “Power characteristics of homogeneously broadened index-antiguided waveguide lasers,” submitted to Optics Letters (2012).
Metamaterial enabled optical microresonators
Category: metamaterials, microresonators, whispering gallery modes, photonic crystals.Application: micro-lasers, gyroscope, frequency combs, microwave synthesizer, sensors.
Whispering-gallery-mode microresonators (WGMR) in the forms of sphere, cylinders, and disks have optical modes orbit around the peripheral inside the resonators, accompanied by a tail funneling energy into the surrounding continuum. They exhibit very high-Q resonance that enables numerous applications in optical physics, photonics, sensing, metrology, and quantum information processing.
In this research we incorporate metamaterials or photonic crystal components into microresonators to enable advanced mode control in WGMR, and study this new type of resonances, as well as its enabling new applications. For example, strong coupling with quantum dots for quantum information processing, ultrasensitive chembio detection, ultrasensitive gyroscope, ultra-low-noise microwave synthesizer, and microresonator based frequency combs with larger bandwidth or more comb power. Students who are interested in this work please contact THRG for more information.
Femtosecond 2-color ablation
Category: femtosecond & nonlinear optics, arbitrary waveform generation, attosecond pulse, ultrafast phenomena, laser matter interaction, laser material processing, pulse shaping.
Application: laser nanofabrication with higher resolution, more efficient and precise laser ablation tools for industry &LASIK, laser direct energy sources.
Femtosecond laser is known to produce nanoscale features in material removal and modification. Key to this fascinating property is it deposits energy into electrons faster than phonons can take it away so energy is effectively localized to remove bonding among atoms.
In this research we explore methods to shape the electric field of laser beams to create transient field pattern that is distinct from conventional sinusoidal oscillation. This could improve energy coupling from laser to electrons. Applications include improving speed and power budget for laser material processing in drilling/cutting/laser weapon, creating features less than 100 nm for nanofabrication, more efficient laser electron accelerator, etc. Students who are interested in this work please contact THRG for more information.
Dispersive wavelength modulation spectroscopy
Category: laser modulation spectroscopy, femtosecond laser frequency comb, single-particle absorption spectroscopy, balanced detection.
Application: homeland security, early cancer detection, quantum information science, environmental monitoring, combustion diagnostics, atmospheric sensing, analytical chemistry.
Quantum objects and nanostructures, such as quantum dots, molecules, nanowires, proteins, and molecular motors, are building blocks and fundamental machineries for many physical, chemical and biological processes. On the other hand, explosives and biohazard molecules such as TNT and anthrax pose significant thread to homeland security. Sensitive and early detection of these quantum-sized objects is therefore very crucial for many applications in physics, chemistry, biology, medicine, and national defense.
Conventional wavelength modulation spectroscopy (WMS) is a well-established sensitive detection method by modulating the absorption of molecules with laser wavelength. This method, however, is not applicable to molecules at room temperature as they typically have much large absorption bandwidth than traditional laser can afford. In this research, we are investigating a new technique using femtosecond supercontinuum to accomplish the required wavelength sweep. This technique, if successful, can become a new imaging modality for material and biomedicine study. Students who are interested in this work please contact THRG for more information.
Femtosecond-laser-induced periodic self-organized nanostructures
Category: femtosecond optics, directed self-assembly, photochemistry, laser-induced surface periodic structures.
Application: laser based nanofabrication, sub-wavelength optics.
Directed self-organization is to use external stimuli such as temperature, stress, electromagnetic fields, etc, to influence the global organization of constituent components in a deterministic fashion. In this work we have observed light as a driving force for DSO of tungsten atoms during laser chemical vapor deposition to form periodic nanostructures. This could lead to new fabrication schemes of large-area templates for data storage, catalysts, and sensors.
1. H. Zhang, M. Tang, J. McCoy, and T. Her, “Deposition of tungsten nanogratings induced by a single femtosecond laser beam,” Optics Express 15 5937 (2007).
2. Mingzhen Tang, Haitao Zhang, and Tsing-Hua Her, “Self-assembly of tunable and highly-uniform tungsten nanogratings induced by femtosecond laser with nanojoule energy,” Nanotechnology 18 (2007) 485304 (5pp).
3. Tsing-Hua Her, “Femtosecond-Laser-Induced Periodic Self-Organized Nanostructures,” appeared in Comprehensive Nanoscience and Technology, edited by David Andrews, Greg Scholes, and Gary Wiederrecht, published by Elsevier (Dec. 2010).
4. Haitao Zhang, Terry T. Xu, Mingzheng Tang, Tsing-Hua Her, and Shu-you Li, “Selective growth of tungsten oxide nanowires via a vapor-solid process,” J. Vacuum Science and Technology B 28 (2), pp. 310-315 (2010).