A scheme of left-handed metamaterial (LHM) composed of superconducting quantum interference devices (SQUIDs) and conducting wires is proposed.The permeability of a probe field can be smoothly tuned over a wide range with another electromagnetic (coupling) field due to quantum interference effect. Similar to electromagnetically induced transparency (EIT) of atomic systems, the absorption of the probe field can be strongly suppressed even in the case of negative permeability. There are two passbands of negative refractive index with low loss, which can be tuned with the coupling field.

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We report the operation of a diode-pumped monolithic self-Q-switched Cr,Nd:YAG laser in which the codoped ions create saturable absorption for Nd 3 " laser emission at 1064 nm. With a 70-/um beam diameter in the gain medium, the Q-switched pulse has a duration of 3.5 ns and a peak power of 2 kW. The output is linearly polarized with an extinction ratio of 600:1. The pulse-to-pulse intensity fluctuation is less than the instrument resolution of 0.25%. A 5-mm-long KTP crystal butted against the with a peak power-conversion efficiency of 30%.Recently, chromium-doped host crystals, such as gadolinium scandium gallium garnet, forsterite, and yttrium aluminum garnet (YAG) were found to exhibit saturable absorption in the 900-1200-nm range.`3 The absorption has color-centerlike characteristics and is believed to be associated with the Cr 4 " ions. 4 ' 5 Cr-doped forsterite has been used to realize passive Q switching and mode locking in Nd:YAG lasers. 2When YAG and gadolinium scandium gallium garnet host crystals are codoped with Nd and Cr, the functions of the gain medium and the saturable absorber are combined. This result has led to self-Q-switched operation."3 The output of the Cr,Nd:YAG crystal is linearly polarized without any polarizing optics in the cavity. 3 In this Letter we report the operation of a diodepumped monolithic self-Q-switched solid-state laser that is based on Cr,Nd:YAG. Our purpose is to demonstrate a compact, efficient, highly polarized, and highly stable Q-switched laser. The applications of Q-switched lasers are well known: lidars, remote sensing, pollution detection, nonlinear-optical processes, and material processing. Pulse-to-pulse stability is also important for measurement accuracy.The device configuration is illustrated in Fig. 1. A Cr,Nd:YAG crystal is polished to a planar-concave geometry. The concave mirror has a radius of curvature of 80 mm and is coated for high transmission at 808 nm and total reflection at 1064 nm. The planar surface is coated for 95% reflection as the output coupler. The misalignment of the axes of the two mirrors is measured to be less than 0.3°. The absorption coefficient is measured to be 5.25 cm-' at the pumping wavelength of 808 nm, which is typical of a 1 wt. % Nd:YAG. Because both Cr 3 + and Cr 4 + are present in the absorption spectrum, 3 the concentration of the absorbers is not known, although absorption loss at 1064 nm is 0.32 cm-' at low power. The saturation power density at 1064 nm was measured previously to be 3.6 MW/cm 2 .3 Thus this absorption can easily be saturated with an monolithic cavity produces 2-ns-long pulses at 532 nm intracavity power of 100 W in a diode-end-pumped Nd:YAG laser if the spot diameter at the absorber is 50 /.m. With end pumping, the pumping power is mostly depleted within the 3-mm length in the front end of the crystal facing the pump source, and the saturable loss is evenly distributed throughout the length. A longer crystal is expected to result in shorter pulses and higher threshold. As a compromise between the avail...

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