Distortion and attenuation free gain-assisted Superluminal Propagtion in a Rare-Earth Doped Crystal

Abstract

A light pulse that travels through a narrow spectral window, i.e. a
frequency range of low absorption, within a strongly absorbing frequency
range will be delayed due to strong dispersion. If however, the absorbing
structure is inverted, such that it is amplifying instead, one will see that
a pulse travelling through this window will have its pulse peak come out
of the material before it is sent in. This phenomenon is called superluminal
propagation, light with negative group velocity or fast light. In this
thesis negative group velocity of light through anomalous dispersion of
a Europium-doped YSO crystal was studied. A spectral window of low
absorption was created in the inhomogeneous profile. Within this spectral
window, an absorbing structure is made up of two identical, equidistant
from the centre frequency peaks (ω = ω0±ωshift). The structure was composed
of ions with strong oscillator strengths, collected through reshuffling
ions to different hyperfine levels. This allowed for the collection of a total
absorption that exceeds the natural level near the centre frequency.
Between the two absorption peaks a region of strong dispersion exists,
the magnitude of the dispersion here is far higher in between the peaks
than elsewhere in the created spectral window. This structure is then
inverted to get a gain structure instead of an absorbing structure. This
changes the sign of the dispersion and hence created a region of strong
anomalous dispersion and allowed for the propagation of a Gaussian pulse
with negative group velocity. This method is unique in that it does not
alter the pulse shape or cause significant attenuation. Furthermore, this
method allows for the isolated generation of a fast light pulse without the
simultaneous generation of a slow light pulse. Through Maxwell-Bloch
simulations and using estimates of the experimental conditions an optimal
recipe was determined to create the maximal pulse advancement with
respect to its time-bandwidth product. The simulations for the maximal
width of the pit gave a frequency width of 29 MHz and the optimal width
of the two gain structures was determined to be 1 MHz with a hole of 0.3
MHz in between them. The optimal αL from simulation was 12. However,
due to experimental factors, an absorption of αL = 4.8 had to be
used. This allowed for a Gaussian propagation pulse with tFWHM = 5 μs.
Experimentally a group refractive index of ng ≈-10135 was achieved and
a pulse advancement of 10-14.2% of the tFHWM, probe pulses had little to
no distortion and little to no attenuation/amplification when remaining
within the linear regime. A non linear relationship for the optimal αL -
tFWHM seems to exist.

Popular Abstract

Going beyond the speed of light, undistorted and unattenuated!

The speed of light is absolute, and nothing can exceed it. Or can something? Light pulses travel at their group velocity. This is the velocity at which envelope of the pulse travels. In vacuum all frequencies travel at the same velocity, but in a material there can be a frequency dependence. This is called dispersion and causes some frequency components to travel slower than others, this results in the pulses spreading out in time. Dispersion can be a real issue, for example in telecommunication where pulses travel in fibres and where it is desired to keep the pulse length as short as possible. But for other applications dispersion can be extremely useful. For example for laser stabilisation and for making pulses peaks travel faster than the speed of light! The latter is the subject of this thesis, where we attempted and succeeded in attaining this so called ‘fast light’.

Naturally the laws of physics cannot be violated, and as it turns out pulse peak that travels faster than the speed of light does not break said laws. As a matter of fact the statement “nothing can travel faster than the speed of light in vacuum” is a bit incorrect. One should rather say: no information can travel faster than the speed of light in vacuum. This is a little unfortunate with regards to information transfer, since the ‘fast light’ cannot transfer information and hence it cannot be used for faster data transfer. This has to do with how information is defined, which is a little
beyond the scope of this summary, but the physics are fascinating nonetheless! To actually achieve fast light one can turn to a couple of different methods, but most suffer from one or more drawbacks. Fast light pulses can be attenuated or distorted or have rather minor advancement with respect to their duration.

The method used in this project does not have any of these problems and has a large advancement. This method in broad terms, works as follows, we start out with a crystal that contains the element ‘Europium’. These atoms absorb light at a range of frequencies, and this absorption can be manipulated. But for that to work the crystal must be cooled down. It is put in a cryostat and cooled to only a few Kelvin. At this temperature one can create so called ‘spectral windows’ and ‘spectral structures’ within the range at which the atoms absorb light. A spectral window means that there is a range of frequencies where there is no absorption, surrounded by frequencies where there is absorption. A spectral structure is made by accumulating absorption at certain frequencies. By creating these structures and spectral windows in a smart way we can change the dispersion properties and slow down the light pulses. This structure that is created is a set of strongly absorbing frequency ranges with a narrow transparent region in between them. This is the opposite effect of what we wanted, but that can be changed by some turning the accumulated spectral structure from absorbing to amplifying. This is achieved by exciting the atoms from their ground state to their excited state. This project showed that it is possible to achieve the strong fast light effect without the drawbacks of other methods.

• author: Renders, Antonius Johannes
• supervisor: Lars Rippe; David Gustavsson; Adam Kinos
• organization: Atomic Physics; Department of Physics
• course: PHYM03 20222
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Fast Light, Superluminal Propagation, Negative group velocity, Rare earth
• report number: 591
• language: English
• id: 9133098
• date added to LUP: 2023-09-14 08:58:26
• date last changed: 2023-09-14 08:58:26