excited-state absorption (original) (raw)

Acronym: ESA

Definition: absorption of light by ions or atoms in an excited electronic state, rather than in the electronic ground state

Categories: laser devices and laser physics, physical foundations

• optical effects
  • absorption
    * absorbance
    * absorptance
    * absorption coefficient
    * absorption length
    * excited-state absorption
    * infrared absorption
    * multiphonon absorption
    * multiphoton absorption
    * light-induced absorption
    * pump absorption
    * two-photon absorption
    * (more topics)

Related: absorptionlaser gain mediasaturable absorbersupconversionrate equation modeling

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DOI: 10.61835/e3n Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is Excited-state Absorption?

Excited state absorption (ESA) means that atoms, ions or molecules in already excited states (rather than in their ground state) cause absorption of light. That can happen if two conditions are met:

• There are one or more higher-lying energy levels in a suitable distance above an excited level, i.e., fitting to the photon energy according to the wavelength of the incident light.
• There is some population in the excited starting level. (ESA may be largely suppressed if there cannot be a significant population in the starting level, e.g. because it is very short-lived.)

The second condition is obviously more easily fulfilled if the starting level is a metastable state, i.e., a level with a substantial lifetime.

Strength of Excited-state Absorption

In many cases, one can use rate equation modeling. Here, the transition rate of an ion subject e.g. to some optical pump intensity ($I_{\rm p}$) is ($R_{\rm ESA} = \sigma \; I_{\rm p} / h\nu_{\rm p}$) with the ESA transition cross-section ($\sigma$), which depends on the pump wavelength.

In many cases, it is possible to largely avoid problems related to ESA by choosing suitable wavelengths where the ESA cross-sections are small.

Detrimental Effects of ESA in Lasers and Amplifiers

Figure 1: ESA in an erbium-doped amplifier, pumped at 808 nm

In solid-state laser gain media, for example, it can occur that the population in the upper laser level does not only lead to amplification by stimulated emission, but also to absorption processes for the pump or laser radiation where laser ions are excited to a higher-lying energy level. For example, that happened when erbium-doped fiber amplifiers were pumped with laser diodes emitting at 808 nm (Figure 1). Pumping at that wavelength did not only lead to the population in the upper laser level, but also to the useless excitation of higher-lying levels through ESA. The problem was later solved by pumping with laser diodes which emit around 975 nm, where ESA is largely avoided.

For a laser, such additional losses by ESA can raise the threshold pump power and reduce the slope efficiency. Of course, excited-state absorption may not only occur with pump light, but also with laser or signal light. It may thus degrade the gain and efficiency of an amplifier for certain ranges of signal wavelength, or cause a laser to operate at somewhat different wavelengths where it can largely escape excited-state absorption.

ESA is a common problem particularly for broadband laser gain media such as transition-metal-doped crystals, but less so for rare-earth-doped crystals with their relatively narrow-bandwidth transitions. Of course, ESA is more likely to be relevant for laser ions with multiple electronic levels, such as erbium or thulium, whereas it is not possible for ytterbium.

ESA is also common in various saturable absorber materials such as Cr4+:YAG. Here, the ground state absorption can be fully bleached, but what remains even at rather high optical intensities is the excited-state absorption, which recovers much more rapidly. In effect, ESA causes nonsaturable losses (at least for nanosecond pulses), which may amount to a significant fraction of the saturable losses.

ESA in Upconversion Lasers

Figure 2: Excitation of higher-lying electronic states of thulium (Tm3+) ions in ZBLAN fiber via ESA (red arrows).

This allows the construction of blue (480-nm) upconversion lasers. The short gray arrows indicate multiphonon transitions.

Although excited-state absorption is in most cases a detrimental effect, it can also be useful for upconversion pumping, where the excitation of higher-lying energy levels is required. This is exploited e.g. in some thulium-doped lasers (Figure 2), and also in other upconversion lasers. Rate equation models require the values of ESA cross-sections (see below), in addition to the lifetimes of intermediate energy levels.

Calculating Effects of ESA

In some cases, it is relatively simple to include ESA in a laser model. For example, pump or signal ESA may simply lead to an additional absorption term, if ESA leads ions to levels from where they quickly relax to the upper laser level. In more complicated situations, such as the thulium level scheme discussed above, rate equation modeling may be applied.

Measurement of ESA Cross-sections

The measurement of excited-state absorption is more difficult than that for ground-state absorption. A common technique is based on the use of a modulated pump beam, creating a modulated population in a certain electronic level, and monitoring the transmission of the sample with a monochromator, a photodetector, and a lock-in amplifier. The spectra obtained essentially show the difference in laser gain and ESA, but can also contain contributions from other levels.

Frequently Asked Questions

What is excited-state absorption (ESA)?

Excited-state absorption (ESA) is the absorption of light by atoms, ions, or molecules that are already in an excited electronic state, rather than their ground state. It occurs when a photon has the right energy to excite them to an even higher energy level.

Why is excited-state absorption often a problem in lasers?

In lasers and amplifiers, ESA is often a detrimental loss mechanism. It can absorb pump or laser light, which increases the laser's threshold pump power and reduces its slope efficiency, as the absorbed energy is often wasted.

Can excited-state absorption ever be useful?

Yes, ESA is essential for upconversion pumping in certain lasers. It allows an ion to absorb multiple photons sequentially to reach a high energy level, enabling the emission of light at a shorter wavelength than that of the pump light.

Which laser ions are particularly affected by ESA?

ESA is a common issue for laser ions with complex energy level structures, such as erbium (Er3+) and thulium (Tm3+). In contrast, ytterbium (Yb3+) is not subject to ESA due to its very simple energy level structure.

How does ESA affect saturable absorbers?

In some saturable absorbers, such as Cr4+:YAG, ESA from the saturated (excited) state causes nonsaturable losses. Even at high optical intensities where ground-state absorption is bleached, this residual absorption can remain.

Bibliography

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