Rufus L. Cone

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.

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.

Coherent integration of 0.5 GHz spectral holograms at 1536 nm using dynamic biphase codes

Spectral hole-burning-based optical processing devices are proposed for coherent integration of multiple high-bandwidth interference patterns in a spectral hole-burning medium. In this implementation, 0.5 GHz spectral holographic gratings are dynamically accumulated in Er3+:Y2SiO5 at 4.2 K using a 1536 nm laser frequency stabilized to a spectral hole, along with commercial off-the-shelf components. The processed data, representing time delays over 0.5–2.0 μs, were optically read out using a frequency-swept probe; this approach makes possible the use of low-bandwidth, large-dynamic-range detectors and digitizers and enables competitive processing for applications such as radar, lidar, and radio astronomy. Coherent integration dynamics and material advances are reported.

Spatial-spectral holographic correlator at 1536 nm using 30-symbol quadriphase- and binary-phase-shift keyed codes.

Optical 30-symbol quadriphase-shift keyed (QPSK) and binary-phase-shift keyed (BPSK) codes were processed in a spatial-spectral holographic correlator with the Er(3+): Y(2)SiO(5) spectral hole-burning material operating at 1536 nm in the important 1550-nm communications band. The results demonstrate the ability of spatial-spectral holographic correlators to process QPSK codes and BPSK codes with the same apparatus. The high-fidelity correlations produced by this optical coherent transient device exhibit the low sidelobe characteristics expected for the codes used.

Programmable frequency reference for subkilohertz laser stabilization by use of persistent spectral hole burning.

We report what is believed to be the first demonstration of laser frequency stabilization directly to persistent spectral holes in a solid-state material. The frequency reference material was deuterated CaF(2): Tm(3+) prepared with 25-MHz-wide persistent spectral holes on the H(6)(3)?H(4)(3) transition at 798 nm. The beat frequency between two lasers that were independently locked to persistent spectral holes in separate crystal samples showed typical root Allan variances of 780+/-120Hz for 20-50-ms integration times.

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.

Exceptionally narrow homogeneous linewidth in erbium-doped glasses

We show that rare-earth (RE-) doped glasses can have homogeneous linewidths as narrow as 287 kHz at (4)He temperatures. This is far narrower than others reported in glasses in the same temperature range and is suitable for precise spectral hole burning and spatial-spectral holographic applications. It is known that cw spectral hole burning linewidth measurements of RE ions in glasses are hindered by the presence of spectral diffusion but, even in glasses, application of a magnetic field can freeze out RE spin-spin interactions responsible for spectral diffusion and isolate the remaining contribution of two-level systems (TLSs). The Er(3+):2G2S glasses have unusually low TLS contributions, making it possible to study the real homogeneous linewidth using photon echo measurements. The contribution from TLSs is only 170 T(1.3) kHz when subjected to a field of 5 T.