We present a novel method for three-dimensional optical data storage that has submicrometer size resolution, provides a large contrast in index of refraction, and is applicable to a wide range of transparent materials. Bits are recorded by use of a 0.65-N.A. objective to focus 100-fs laser pulses inside the material. The laser pulse produces a submicrometer-diameter structurally altered region with high contrast in index of refraction. We record binary information by writing such bits in multiple planes and read it out with a microscope objective with a short depth of field. We demonstrate data storage and retrieval with 2-microm in-plane bit spacing and 15-microm interplane spacing (17 Gbits/cm(3)). Scanning electron microscopy and atomic force microscopy show structural changes confined to an area 200 nm in diameter.

We report that silicon surfaces develop an array of sharp conical spikes when irradiated with 500 laser pulses of 100-fs duration, 10-kJ/m2 fluence in 500-Torr SF6 or Cl2. The spikes are up to 40-μm tall, and taper to about 1-μm diam at the tip. Irradiation of silicon surfaces in N2, Ne, or vacuum creates structured surfaces, but does not create sharp conical spikes.

We find that silicon surfaces develop arrays of sharp conical spikes when irradiated with 500-fs laser pulses in SF 6 . The height of the spikes decreases with increasing pulse duration or decreasing laser fluence, and scales nonlinearly with the average separation between spikes. The spikes have the same crystallographic orientation as bulk silicon and always point along the incident direction of laser pulses. The base of the spikes has an asymmetric shape and its orientation is determined by the laser polarization. Our data suggest that both laser ablation and laser-induced chemical etching of silicon are involved in the formation of the spikes. PACS: 61.80.B; 79.20.D; 82.65.JVarious micron-sized surface features have been observed on silicon surfaces after ion beam or pulsed laser irradiation. For example, mounds and columns are formed when silicon is sputtered by energetic Ar ions [1,2]. When irradiated by nanosecond laser pulses with fluence close to the melting threshold, silicon surfaces develop ripples with submicron periodicity [3]. Laser pulses with even higher fluence can induce laser ablation of silicon, leaving a crater on the surface surrounded by irregular cones protruding above the surface [4]. Previously we reported that silicon surfaces spontaneously develop arrays of conical spikes when repeatedly irradiated with high-fluence fs laser pulses in SF 6 or Cl 2 [5]. Similar structures have recently been reported with ns laser irradiation [6]. The spikes we observe have a high aspect ratio, a quasi-periodic spatial distribution, and are only formed in an atmosphere containing a halogen. Besides their scientific interest, the silicon spikes are of interest because of their potential applications as light absorbers [7] for solar cells and photodetectors, and as microneedles for transdermal drug * delivery [8]. In this paper, we characterize the surface morphology and study the effect of laser fluence and pulse duration on the spikes. ExperimentalWe carried out our experiments on n-type (arsenic-doped) Si(100) wafers with resistivity less than 5 × 10 −5 Ω m. Each wafer is cleaned with trichloroethylene, rinsed in acetone, and then rinsed in methanol. The wafer is mounted on a three-axis translation stage in a vacuum chamber with a base pressure of less than 10 −4 Torr. During the experiments, the chamber is backfilled with 500-Torr SF 6 .The laser system, consisting of a Ti:sapphire oscillator and a chirped-pulse-regenerative amplifier, produces a 1-kHz train of 100-fs, 0.5-mJ pulses at 800 nm. Longer pulses are obtained by adjusting the pulse compressor. The laser pulses are focused with a 0.1-m focal-length lens and, except where noted, incident normal to the sample. The spatial profile of the laser pulse is nearly Gaussian, with a fixed beam waist of 200 µm at the sample. The fluence (energy per unit area) varies over the laser spatial profile; values quoted below refer to the fluence at the center of the spatial profile. The fluence is controlled by changing the total incident energy with a halfwav...

No abstract

No abstract