Yongchen Sun

Recent Progress in Developing New Rare Earth Materials for Hole Burning and Coherent Transient Applications

To develop new spectral hole burning materials and optimize known materials for applications such as optical correlator and memory devices, a broad range of experiments, from optical coherent transients to photoelectron spectroscopy, have been used to elucidate fundamental aspects of the rare-earth electronic structure. We report progress in the characterization of Er 3+ doped materials where we have measured an ultra-narrow line width of 50Hz in Er 3+ :Y 2 SiO 5 and a Γ inh /Γ h ratio as high as 10 8 in Er 3+ :LiNbO 3 . Progress is also reported for Nd 3+ :YVO 4 where the high oscillator strength is an advantage over other rare earth ions and excellent coherence properties can be achieved at modest magnetic fields. Finally, we report the advances in the pursuit of photon-gated hole burning materials through the study of the energies of the localized rare earth energy states relative to the host band states, providing the foundation for understanding photoionization in these materials.

Fluorescence Efficiency and Visible Re-emission Spectrum of Tetraphenyl Butadiene Films at Extreme Ultraviolet Wavelengths

A large number of current and future experiments in neutrino and dark matter detection use the scintillation light from noble elements as a mechanism for measuring energy deposition. The scintillation light from these elements is produced in the extreme ultraviolet (EUV) range, from 60{200 nm. Currently, the most practical technique for observing light at these wavelengths is to surround the scintillation volume with a thin lm of Tetraphenyl Butadiene (TPB) to act as a uor. The TPB lm absorbs EUV photons and reemits visible photons,

Yb:YAG absorption at ambient and cryogenic temperatures

We have performed absorption measurements and generated absorption cross sections as a function of wavelength for the laser material YAG doped with ytterbium at 300, 175, and 75 K. This data was generated to enable a direct comparison of the absorption intensity and linewidths at room and cryogenic temperatures, and in particular near the temperature of liquid nitrogen at 77 K. The data have been used to compute universal absorption contour plots that display absorption as a function of the incident light center wavelength and optical thickness (doping density times penetration depth) for a number of bandwidths, and assuming that the spectrum of the incident light can be described as a Gaussian. Curves are presented for both 300 and 75 K, and may be used to optimize the absorption and laser efficiency.

PHOTONIC MATERIALS AND DEVICES Progress in relating rare-earth ion 4f and 5d energy levels to host bands in optical materials for hole burning, quantum information and phosphors

Rare-earth ions play an important role in modern technology as optically active elements in solid-state luminescent materials. In many of these materials, interactions between the electronic band states of the host crystal and the rare-earth ions localized 4f N and 4f N−1 5d states influence the materials optical properties. The importance of these interactions is discussed for material applications in photon-gated hole burning, quantum information and phosphors. Material dependent trends in the relative binding energies of the 4f N states and the host bands have been observed and are summarized. An empirical model for the ion dependence of the 4f electron binding energies is formulated in terms of atomic number and compared with previous models. These models are extended to describe the 4f N−1 5d states with one additional parameter. Improved estimates for the free-ion ionization potentials used in the model are also presented and discussed.

Systematics of 4f electron energies relative to host bands by resonant photoemission of rare earth doped optical materials

Abstract Relative energies of 4f n electronic states and crystal band states are important for a fundamental understanding of rare-earth-doped optical materials and a practical understanding of each materials potential performance in specific applications. With this motivation, the 4f n ground state binding energies of rare earth ions have been studied in the gallium garnets using resonant photoemission spectroscopy and compared with the aluminum and iron garnets. The 4d–4f photoemission resonance was used to separate and identify the 4f n and valence band components of the spectra, and theoretical 4f photoemission spectra were fit to experimental results to accurately determine electron binding energies. A two-parameter empirical model was used to successfully describe the relative energies of the 4f n ground states in these materials. The success of this empirical model indicates that measurements on as few as two different rare earth ions in a host are sufficient to predict the energies of all rare earth ions in that host. This analysis shows that systematic shifts in the relative energies of 4f n states and crystal band states between different garnets arise entirely from shifts of the band states, while each rare earth ion maintains the same absolute binding energy for all garnets studied. These results suggest that further studies of additional host compounds using both photoemission and optical spectroscopy will rapidly lead to a broader picture of the host crystals effect on 4f electron binding energies.

Demonstration of real-time address header decoding for optical data routing at 1536??nm

We have demonstrated real-time decoding of 20-bit biphase-coded address header pulses, using stimulated photon echoes in a phase-matched crossed-beam configuration. This decoding is one of the functions required for coherent transient optical data routing, packet switching, and processing. The active medium used was single-crystal Y(2)SiO(5) doped with Er(3+), which provides an operating wavelength of 1536 nm.

Diode laser frequency stabilization to transient spectral holes and spectral diffusion in Er3+ : Y2SiO5 at 1536 nm

Diode laser frequency stabilization to 500 Hz Allan deviation is demonstrated over 2 ms integration times with drift reduced to 7 kHz/min. This was achieved at 1536 nm in the technologically important communications band by stabilizing external cavity diode lasers to regenerative transient spectral holes in the inhomogeneously broadened 4I15/2(1)→4I13/2(1) optical absorption of Er3+ : Y2SiO5. Spectral diffusion, which currently limits the achievable stabilization performance, has been studied using stimulated photon echoes. Due to spectral diffusion, significant broadening of the homogeneous linewidth at low magnetic fields from a few kHz to tens of kHz develops as the waiting time T between pulses two and three was increased from microseconds up to the T1∼10 ms lifetime of the excited state. This evolution of the homogeneous linewidth has been mapped out as a function of magnetic field. The classic spectral diffusion can be reduced to negligible levels upon application of a magnetic field in a 0.02 atomic percent Er3+ : Y2SiO5 crystal.

Measuring and analyzing excitation-induced decoherence in rare-earth-doped optical materials

A method is introduced for quantitatively analyzing photon echo decay measurements to characterize excitation-induced decoherence resulting from the phenomenon of instantaneous spectral diffusion. Detailed analysis is presented that allows fundamental material properties to be extracted that predict and describe excitation-induced decoherence for a broad range of measurements, applications and experimental conditions. Motivated by the need for a method that enables systematic studies of ultra-low decoherence systems and direct comparison of properties between optical materials, this approach employs simple techniques and analytical expressions that avoid the need for difficult to measure and often unknown material parameters or numerical simulations. This measurement and analysis approach is demonstrated for the 3 H6 to 3 H4 optical transition of three thulium-doped crystals, Tm 3+ :YAG, Tm 3+ :LiNbO3 and Tm 3+ :YGG, that are currently employed in quantum information and classical signal processing demonstrations where minimizing decoherence is essential to achieve high efficiencies and large signal bandwidths. These new results reveal more than two orders of magnitude variation in sensitivity to excitation-induced decoherence among the materials studied and establish that the Tm 3+ :YGG system offers the longest optical coherence lifetimes and the lowest levels of excitation-induced decoherence yet observed for any known thulium-doped material.

Semiconductor lasers stabilized to spectral holes in rare-earth crystals

Single-frequency diode lasers have been frequency stabilized to 200 Hz at 1.5 microns and to 20 Hz at 793 nm with 10-100 ms integration times using narrow spectral holes in the absorption lines of Er3+ and Tm3+ doped cryogenic crystals. The narrow spectral holes are used as frequency references, and this laser performance was obtained without requiring vibrational isolation of either the laser or frequency reference. Kilohertz frequency stability for 100 s integration times is provided by these techniques, and that performance should be improved to the Hertz level and should be extended to longer integration times with further development. Miniaturized external cavity diode lasers and 5 mm-sized reference crystals will provide compact portable packages with a closed cycle cryocooler. The achieved frequency stabilization provides lasers that are ideal for interferometry, high-resolution spectroscopy such as photon echoes, real time optical signal processing based on spectral holography, and other applications requiring ultranarrow-band light sources or coherent detection.

Relating localized electronic states to host band structure in rare-earth-activated optical materials

crystal’s electronic band states relative to the 4f N or 4f N-1 5d 1 states responsible for the ion’s optical transitions is important for understanding the properties and performance of each material since energy and electron transfer between these states influences the material’s efficiency and stability. 1 Little is known about the relationships between these states, but there is growing motivation to explore these properties for developing ultraviolet laser materials, phosphors for applications including plasma displays and mercury-free lamps, scintillator materials for medical imaging, and optical data processing and storage technologies based on photorefractivity or photon-gated photoionization holeburning. Continued advances in optical technologies require knowledge of the systematic trends and behavior of rare-earth energies relative to crystal band states so that the properties of current materials may be fully understood and new materials may be logically developed. We have recently initiated a systematic study of the relative energies of the rare-earth ions’ electronic states and the host band states in optical materials using resonant electron photoemission spectroscopy (RPES). 2,3 RPES directly determines the energies of all occupied electronic states relative to a common energy reference and can unambiguously separate and assign spectral features to a particular electronic state. 4 Figure 1 presents results for yttrium aluminum garnet (YAG), the most important host crystal for solidstate lasers. Circles represent measured binding energies of the rare-earth 4f N ground state relative to the valence band maximum (the host’s highest energy occupied state). These results have led to an empirical model that successfully describes the rareearth binding energies in optical materials with two parameters: one describes a constant shift experienced by all rare-earth ions and the second describes a smaller dependence on the rare earth’s ionic radius. These empirical parameters may be determined from measurements on just two different rare-earth ions, or, in certain cases, simply from measurements on the host crystal itself. With parameters determined from our measured