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Physics Today

Short, Intense Laser Pulse Makes Crystal Transparent

Gloria B. Lubkin

Published Aug 1, 1967• 2 citations
Publisher

Abstract

Although ruby is almost opaque to certain light frequencies, it can be made transparent to one of those frequencies according to Erwin Hahn and Samuel McCall of the University of California, Berkeley. The trick is to apply a sharp pulse of intense, coherent light at a frequency that matches a resonant transition in ruby. If the ruby is cold enough the pulse will be transmitted and the light can be slowed down to as little as 1% of its usual velocity. This self-induced transparency, predicted theoretically a year and a half ago by Hahn and McCall, was recently reported by them (Phys. Rev. Letters 18, 908).

DOI
10.1063/1.3034426

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Although ruby is almost opaque to certain light frequencies, it can be made transparent to one of those frequencies according to Erwin Hahn and Samuel McCall of the University of California, Berkeley. The trick is to apply a sharp pulse of intense, coherent light at a frequency that matches a resonant transition in ruby. If the ruby is cold enough the pulse will be transmitted and the light can be slowed down to as little as 1% of its usual velocity. This self-induced transparency, predicted theoretically a year and a half ago by Hahn and McCall, was recently reported by them (Phys. Rev. Letters 18, 908).
By applying a short, intense pulse from a Q-switched carbon dioxide laser to gaseous sulfur hexafluoride, Kumar Patel and Richard Slusher of Bell Telephone Laboratories have made the gas transparent to 10.6-micron radiation (Phys. Rev. Letters 19, 1019, 1967). They have thus verified the self-induced transparency first found with a ruby laser system by Erwin Hahn and Samuel McCall of Berkeley (PHYSICS TODAY, August 1967, page 47). To observe the effect, one must match the exciting frequency to a strongly absorbing transition in the sample.
Self-induced transparency has been studied in detail experimentally for a simple two-level atomic absorption. A Hg II laser pulse is used at resonance with the D1{D}_{1}D1​ line of Rb. This system eliminates many previous experimental problems such as nonuniform wave fronts and degenerate off-resonant absorbers. Nonlinear transmission, pulse delays, pulse breakup, peak amplification, incoherent-decay effects, off-resonance effects, and focusing effects have been observed. Quantitative comparison with theory is obtained by computer solutions which include the incoherent decay and optical-pulse shapes of the Hg-Rb system. Good agreement is obtained with the theory of McCall and Hahn. Pulse compression by an order of magnitude is obtained by focusing in the resonant absorber. The theory predicting preferred pulse widths and velocities proposed by Davidovich and Eberly is discussed and found inappropriate for the Hg-Rb and other similar systems.
Self-induced transparency is reported for circularly polarized light in the R1(−3/2) line of a 30 ppm ruby (α-Al2O3:Cr3+) at 1.7 K in a magnetic field of B‖c=4.5 T. In such a field and temperature, a 30 ppm ruby is in the so-called superhyperfine limit resulting in a long phase memory time, TM=50 μs, and a thousand-fold slower pulse propagation velocity of ∼300 m/s was observed, compared to the ∼300 km/s measured in the first observation of self-induced transparency (SIT) ∼50 years ago, that employed a ruby with a 500 ppm chromium concentration in zero field and at 4.2 K. To date, this is the slowest pulse propagation ever observed in a SIT experiment.
Formation of optical solitons in self-induced transparency (SIT) regime, where light pulses propagate virtually without loss in nominally strongly absorbing medium, is one of the most striking coherent transient phenomena in optics. Here we study experimentally and by numeric simulations how a square shape pulse gradually transforms into a smooth sech shape pulse of well-defined pulse area, depending on the parameters such as the pulse amplitude, duration, propagation distance etc. The SIT experiments for circularly polarized light are performed in the R1−3∕2) line of a 30 ppm ruby (α-Al2O3:Cr3+) at 1.7 K in a magnetic field of BIIc = 4.5 T, which corresponds to effective absorption coefficient, αL =14.5. In such a magnetic field and temperature range, a 30 ppm ruby is in the so-called superhyperfine limit resulting in a very long decoherence (phase memory) time, TM = 50 μs. We show, in good quantitative agreement with the simulations, how SIT soliton is formatted and how this results in extremely slow pulse peak propagation velocity of ∼300 m∕s, which is to date, the slowest pulse propagation ever observed in a SIT experiment. We also show that for accurate quantitative description of the observed SIT and soliton pulse shapes, the simulations need to account for variation of the incident pulse amplitude across the beam spatial profile. Potential implications of the SIT effect on classical- and quantum information storage will be discussed.
Two types of nonlinear transmission of light pulses associated with phase modulation are observed in experiments connected with self-induced transparency (SIT) in ruby. Anomalous pulse-power transmission is observed to be an order of magnitude greater than linear transmission during the condition of zero-area pulse propagation, where the inhomogeneous line spectral width is greater than the applied pulse spectral width. For application of a pulse off-resonance by the order of a linewidth, nonlinear transmission occurs in which a type of "precursor" phase-modulated signal appears ahead of a developing SIT pulse. The precursor pulse velocity is greater than the SIT pulse velocity, but less than the background velocity of light in the medium.
The mechanism by which light is slowed through ruby has been the subject of great debate. To distinguish between the two main proposed mechanisms, we investigate the problem in the time domain by modulating a laser beam with a chopper to create a clean square wave. By exploring the trailing edge of the pulsed laser beam propagating through ruby, we can determine whether energy is delayed beyond the input pulse. The effects of a time-varying absorber alone cannot delay energy into the trailing edge of the pulse, as a time-varying absorber can only attenuate a coherent pulse. Therefore, our observation of an increase in intensity at the trailing edge of the pulse provides evidence for a complicated model of slow light in ruby that requires more than just pulse reshaping. In addition, investigating the Fourier components of the modulated square wave shows that harmonic components with different frequencies are delayed by different amounts, regardless of the intensity of the component itself. Understanding the difference in delays of the individual Fourier components of the modulated beam reveals the cause of the distortion the pulse undergoes as it propagates through the ruby.