Jingxin Ding

Laser polarization and phase control of up-conversion fluorescence in rare-earth ions

We theoretically and experimentally demonstrate the up-conversion fluorescence control via resonance-mediated two-photon absorption in rare-earth ions by varying both the laser polarization and phase. We show that both the laser polarization and phase can control the up-conversion fluorescence, and the up-conversion fluorescence intensity is decreased when the laser polarization changes from linear through elliptical to circular. We also show that the laser polarization will affect the control efficiency of the up-conversion fluorescence by varying the laser phase, and the circular polarization will reduce the control efficiency. Furthermore, we suggest that the control efficiency by varying the laser polarization and the effect of the laser polarization on the control efficiency by varying the laser phase can be artificially manipulated by controlling the laser spectral bandwidth. This optical control method opens a new opportunity to control the up-conversion fluorescence of rare-earth ions, which may have significant impact on the related applications of rare-earth ions.

Enhancing up-conversion luminescence of Er3+/Yb3+-codoped glass by two-color laser field excitation

Improving up-conversion luminescence efficiency of rare-earth ions is always a research hotspot because of its important applications in laser source, color display, photoelectric conversion and multiplexed biolabeling. Herein, we first utilize a combined two-color laser field with the laser wavelengths of 800 and 980 nm to further enhance the up-conversion luminescence in an Er3+/Yb3+-codoped glass sample. We show that the green up-conversion luminescence intensity by the combined two-color laser field can be greatly enhanced by comparing it with the sum of that induced by the two individual laser fields. We also show that the luminescence enhancement can be attributed to the cooperative up-conversion excitation process by the energy transfer from Yb3+ to Er3+ ions via the 980 nm laser field excitation and then the excited state absorption via the 800 nm laser field excitation. These studies present a clear physical picture for the up-conversion luminescence generation and enhancement in an Er3+/Yb3+-codoped glass sample, which are very helpful for properly designing the laser field to generate or improve the up-conversion luminescence efficiency in various lanthanide-codoped luminescent materials.

Single and two-photon fluorescence control of Er3+ ions by phase-shaped femtosecond laser pulse

We experimentally demonstrate the control of the single and two-photon fluorescence (SPF and TPF) in Er3+ ions by shaping the femtosecond laser pulse with a π or square phase modulation. With the low laser intensity (8.4 × 1010 W/cm2), SPF keeps a constant while TPF is effectively suppressed by the two control schemes. With the high laser intensity (1.2 × 1013 W/cm2), both SPF and TPF are simultaneously enhanced or suppressed by the π phase modulation, and SPF is enhanced while TPF is effectively suppressed by the square phase modulation. The up/down-conversion fluorescence enhancement, suppression, or tuning by the optical control method can greatly expand its applications in various related fields.

Realizing up-conversion fluorescence tuning in lanthanide-doped nanocrystals by femtosecond pulse shaping method

The ability to tune color output of nanomaterials is very important for their applications in laser, optoelectronic device, color display and multiplexed biolabeling. Here we first propose a femtosecond pulse shaping technique to realize the up-conversion fluorescence tuning in lanthanide-doped nanocrystals dispersed in the glass. The multiple subpulse formation by a square phase modulation can create different excitation pathways for various up-conversion fluorescence generations. By properly controlling these excitation pathways, the multicolor up-conversion fluorescence can be finely tuned. This color tuning by the femtosecond pulse shaping technique is realized in single material by single-color laser field, which is highly desirable for further applications of the lanthanide-doped nanocrystals. This femtosecond pulse shaping technique opens an opportunity to tune the color output in the lanthanide-doped nanocrystals, which may bring a new revolution in the control of luminescence properties of nanomaterials.

Polarization control of intermediate state absorption in resonance-mediated multi- photon absorption process

We theoretically and experimentally demonstrate the control of the intermediate state absorption in an (n + m) resonance-mediated multi-photon absorption process by the polarization-modulated femtosecond laser pulse. An analytical solution of the intermediate state absorption in a resonance-mediated multi-photon absorption process is obtained based on the time-dependent perturbation theory. Our theoretical results show that the control efficiency of the intermediate state absorption by the polarization modulation is independent of the laser intensity when the transition from the intermediate state to the final state is coupled by the single-photon absorption, but will be affected by the laser intensity when this transition is coupled by the non-resonant multi-photon absorption. These theoretical results are experimentally confirmed via a two-photon fluorescence control in (2 + 1) resonance-mediated three-photon absorption of Coumarin 480 dye and a single-photon fluorescence control in (1 + 2) resonance-mediated three-photon absorption of IR 125 dye.

Effect of two-color laser pulse intensity ratio on intense terahertz generation

We theoretically demonstrate the effect of the intensity ratio of the two-color laser field on the terahertz generation based on a transient photocurrent model. We show that the terahertz generation depends on the intensity ratio of the two-color laser field at a given total laser intensity, and the optimal intensity ratio for the maximum terahertz generation will decrease with the increase of the total laser intensity. We also show that the final ionization degree can be used to explain the optimal intensity ratio change at different laser intensities. Furthermore, we utilize the increasing rate of electron density and the electron drift velocity to illustrate the physical mechanism of the maximum terahertz radiation generation.

Effect of laser spectral bandwidth on coherent control of resonance-enhanced multiphoton-ionization photoelectron spectroscopy.

The high-resolution (2 + 1) resonance-enhanced multiphoton-ionization photoelectron spectroscopy (REMPI-PS) can be obtained by measuring the photoelectron intensity at a given kinetic energy and scanning the single π phase step position. In this paper, we further demonstrate that the high-resolution (2 + 1) REMPI-PS cannot be achieved at any measured position of the kinetic energy by this measurement method, which is affected by the laser spectral bandwidth. We propose a double π phase step modulation to eliminate the effect of the laser spectral bandwidth, and show the advantage of the double π phase step modulation on achieving the high-resolution (2 + 1) REMPI-PS by considering the contributions involving on- and near-resonant three-photon excitation pathways.

Enhancing intermediate state absorption of resonance-mediated multiphoton absorption process.

We theoretically and experimentally demonstrate the control of the intermediate state absorption in (1+2) resonance-mediated multiphoton absorption process by shaping the femosecond laser pulse. A theoretical model is proposed to investigate the intermediate state absorption of (1+2) resonance-mediated three-photon absorption process in the molecular system, and an analytical solution is obtained on the basis of time-dependent perturbation theory. Our theoretical results show that the intermediate state absorption can be enhanced by controlling the laser spectral phase due to final state absorption reduction, and this absorption enhancement efficiency increases with the increase of the laser intensity. These theoretical results are experimentally confirmed in IR144 dye by varying the laser spectral phase with a sinusoidal modulation function.