This project aims to generate a train of single-cycle sub-femtosecond pulses in the near-to-deep uv regime. The approach we are taking is by molecular modulation in hydrogen gas at room temperature. We have succeeded in generating collinearly propagating Raman sidebands with wavelengths that range from 2216 nm in the infrared to 133 nm in the vacuum ultraviolet. The frequencies covered by these sidebands span over 4 octaves for a total of more than 70600 cm-1 in the optical region of the spectrum. We demonstrated control of the carrier-envelope phase of ultrashort periodic waveforms that are synthesized from a subset of this Raman generated optical frequency comb. We showed that when the comb is generated by adiabatically driving a molecular vibrational coherence with a beam at a fundamental frequency plus its second harmonic, full inter-pulse phase-locking of the comb components is realized. The results set the stage for the synthesis of periodic arbitrary waveforms in the femtosecond and sub-femtosecond regime with full control.
As a part of this process we develop techniques to adjust the phase of each of these sidebands so that they interfere constructively to produce barious waveforms. Once we produce such a pulse train, we shall develop techniques to manipulate the pulses in the train, vary the spacing between the pulses, select a single pulse, and apply these pulses to study electron dynamics and extreme nonlinear optics in the attosecond timescale. First figure shows generated collinear sidebands from order -2 (1203 nm) to order 7 (219 nm). The second figure demonstrates control of the carrier-envelope phase.
We have recently succeeded in constructing a multi-kHz visible-near ir all solid state OPO. This OPO along with our fully-developed mid-ir OPO will open up many novel applications that were not possible in the past. Two developments are in progress. In one case we make use of the high repetition rate of the nanosecond tunable pulse to perform nonlinear microscopy that for many years has been done only with ultrafast lasers. An advantage of the nanosecond lasers is the relatively high spectral resolution. We have demonstrated two-photon fluorescence imaging with our lasers. Our next goal is to obtain CARS images with our near-ir OPO. Once successful the technique can be applied to image biological samples and novel micro-nano structures such as photonic crystals. We shall attempt to take advantage of our spectral resolution. One application is the imaging of nanodiamonds that Huan Chang in our institute is using as biological probes. Another potential application is to study SFG of colloidal systems that Hai Lung Dai is interested in. Another one of our device development is a high power multicolor light source - RGB light source - for industrial applications. We have constructed a 500 mW RGB laser source. (see figure) With further development, this source has the capability to be used in color projection systems ranging from portable laser projectors to large screen TVs.
In collaboration with colleagues in NTU and Nanjing University we are investigating nonlinear photonic crystals and crystal fibers with an emphasis on micron to submicron and nanoscale fabrication, characterization, and device development. Experiments and modeling are being conducted in tandem. Our emphasis is on developing methods that could produce device-scale (>1 mm) material. We have succeeded in fabricating 2D periodically-poled micron size domain structures for efficient SHG and SFG. The next step is to push the dimensions to below 1 micron. This will permit the construction of sub-compact devices for generation to the blue and uv wavelength region. Recently we generated the second to the sixth harmonics covering two and a half octaves in frequency from the infrared to the blue in a single multiply-periodically-poled lithium tantalate crystal by cascaded quasi-phase-matched frequency mixing from the output of our optical parametric oscillator. The frequency comb that is composed of these harmonics permits the synthesis of a periodic train of sub-cycle sub-femtosecond pulses in a compact setting. We also showed that simultaneous generation and phasing of the harmonics can be performed in a monolithic aperiodic optical superlattice (AOS). With this novel approach stable periodic waveforms can be delivered to a predetermined location by simply sending a laser beam through a properly designed and fabricated AOS crystal.
In addition to short wavelength generation, periodically inverted sub-micron domain structures are useful to realize unique backward nonlinear parametric processes and for tunable terahertz generation. Electronic control of sub-micron ferroelectric domains would meet the demand for extremely high-density data storage.
