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The Journal of Chemical Physics

Slowing down of light by laser induced barrier hopping

Ioannis Thanopulos, Moshe Shapiro

Published Oct 24, 2002• 4 citations
Publisher

Abstract

We suggest that a very significant reduction in the group velocity of light occurs in a laser induced barrier-hopping process, known as “Laser Catalysis.” In this process a system is made to hop over a potential barrier via a virtual transition to an excited bound state. As in electromagnetically induced transparency (EIT), the light absorption is predicted to dip to zero at a certain frequency, thereby also changing the refractive index, inducing in turn the slowing down of the group velocity of the light. In contrast to EIT, a single pulse is predicted to induce its own transparency and in contrast to self-induced transparency the process is predicted to be insensitive to the pulse area (as long as the pulse is intense enough to meet the adiabaticity criterion). Thus, the medium becomes transparent to the light, as the laser induced barrier-hopping process brings about a fundamental change in the medium.

DOI
10.1063/1.1506909

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We suggest that a very significant reduction in the group velocity of light occurs in a laser induced barrier-hopping process, known as “Laser Catalysis.” In this process a system is made to hop over a potential barrier via a virtual transition to an excited bound state. As in electromagnetically induced transparency (EIT), the light absorption is predicted to dip to zero at a certain frequency, thereby also changing the refractive index, inducing in turn the slowing down of the group velocity of the light. In contrast to EIT, a single pulse is predicted to induce its own transparency and in contrast to self-induced transparency the process is predicted to be insensitive to the pulse area (as long as the pulse is intense enough to meet the adiabaticity criterion). Thus, the medium becomes transparent to the light, as the laser induced barrier-hopping process brings about a fundamental change in the medium.
The two-level atom can, for good reason, be considered the basic material structure for the study of quantum optics [1]. The two-level atom has played a fundamental role in studies of physical processes, including self-induced transparency (SIT), photon echoes, optical nutation, saturation spectroscopy, and, most recently, slow- and fast-light research. This latter field has recently been the subject of great activity, as researchers have learned how to utilize various physical processes to exert control over the velocity of propagation of light pulses through material systems [2]. This interest stems both from the intellectual intrigue in situations in which the velocity of light can be speeded up or slowed down by many orders of magnitude and the promise that such techniques might be useful for applications in fields such as telecommunications and optical computing [3‐6]. Most of the recent work in this field has made use of the process of electromagnetically induced transparency (EIT) to induce a narrow transparency region in an otherwise highly absorbing optical material [7, 8]. The rapid variation of the refractive index n associated with this transparency window then leads to a strong modification of the group velocity n g in accordance with the standard relation
Summary form only given. Electromagnetically induced transparency (EIT) has been the subject of many experimental and theoretical studies over recent years. Most recently, substantial experimental and theoretical work in this area has been focused on the use of EIT to give ultraslow group velocities for light in both ultracold atomic gas and hot atomic vapors. Much of this interest is based on the fact that slowing the group velocity of a laser pulse down to the speed of sound in the same material can produce strong coupling between acoustic waves and the electromagnetic field. It has been suggested that a giant nonlinearity obtained by this method might be utilized for efficient multi-wave mixing and quantum nondemolition measurements. Slow light might also allow a very efficient nonlinear interaction between laser pulses of extremely low (down to a single photon) energies. Here, we report the first direct observation of an ultraslow group velocity of light down to 45 m/s in a solid, well below the speed of sound in the medium. This group velocity is fundamentally limited only by the inhomogeneous broadening of the ground state transition. At low coupling beam intensities, the fraction of centers that are pumped into the "dark state" increases with coupling laser power, so that the observed group delay is increasing with coupling beam intensity. At higher intensities, power broadening reduces the sharpness of the dispersive feature and the observed group delay decreases.
We prove that it is possible to freeze a light pulse (i.e., to bring it to a full stop) or even to make its group velocity negative in a coherently driven Doppler broadened atomic medium via electromagnetically induced transparency (EIT). This remarkable phenomenon of the ultraslow EIT polariton is based on the spatial dispersion of the refraction index n(omega,k), i.e., its wave number dependence, which is due to atomic motion and provides a negative contribution to the group velocity. This is related to, but qualitatively different from, the recently observed light slowing caused by large temporal (frequency) dispersion.
This thesis investigates the result of modulation schemes in electromagnetically induced transparency (EIT) in rubidium vapor. Through the process of EIT, a control beam can be used to change the optical properties of a medium experienced by a probe field, generating a transparency window. The corresponding change in index of refraction results in slowed group velocity while the beam propagates through the medium. Modulation is used to transmit a message signal through a medium. In classical systems, this entails converting data into a radio frequency by adding a message to an electronic or optical carrier signal. The properties of EIT, light-slowing, and their impacts on a probe pulse with modulation will be confirmed through a detailed semi-classical analysis and by experimental observation.
As evidenced by this book, there has been a flurry of activity over the last decade on tailoring the dispersive properties of optical materials [1]. What has captured the attention of the research community were some of the early results on creating spectral regions of large normal dispersion [2-4]. Large normal dispersion results in extremely small group velocities, where the group velocity is the approximate speed at which a pulse of light propagates through a material. We denote the group velocity by υg = c/ng, where c is the speed of light in vacuum and ng is known as the group index. In the early experiments, described in greater detail in Chapter 1, a dilute gas of atoms is illuminated by a control or coupling beamwhose frequency is tuned precisely to an optical transition of an atom. This control field modifies the absorption and dispersion properties of another atomic transition that shares a common energy level. A narrow transparencywindow is created on this second transition-a process known as electromagnetically induced transparency (EIT)—and within this window, υg takes on extremely small values. Many experiments have now observed υg ∼ 1 m/s or less, implying ng > 108. This result is remarkable, considering the fact that the refractive index n of a material rarely exceeds 3 in the visible part of the spectrum.What are the implications of such large group indices? What applications are enabled by this basic science discovery?