A 80 mW laser beam with wavelength λ=600 nm is focused on a Si sample of 80 μm thickness. The

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A 80 mW laser beam with wavelength λ=600 nm is focused on...

A $80 \mathrm{~mW}$ laser beam with wavelength $\lambda=600 \mathrm{~nm}$ is focused on a Si sample of $80 \mu \mathrm{m}$ thickness. The absorption coefficient of the sample is $\alpha=8 \times 10^{3} / \mathrm{cm}$. Find the number of photons emitted per second, assuming perfect quantum efficiency. What power should be delivered to the sample as heat?

Key Concepts

Quantum Efficiency

Quantum efficiency is defined as the ratio of the number of emitted (or useful) photons to the number of absorbed photons. Assuming a perfect (100%) quantum efficiency means that every absorbed photon results in an emitted photon, with no energy lost to non-radiative processes. This idealization is important for determining the ultimate photon output of a device.

Beer–Lambert Law

This law describes how the intensity of light decreases as it travels through an absorbing medium. It states that the transmitted intensity is given by I = I0 exp(–?x), where ? is the absorption coefficient and x is the thickness of the medium. This concept helps determine what fraction of the incident photons are absorbed by a material.

Heat Generation in Photoexcited Materials

In materials that absorb light, not all the absorbed energy will necessarily be re-emitted as light; some can be converted into heat. However, when perfect quantum efficiency is assumed, the energy absorbed is completely re-emitted as photons, implying that no energy is lost as heat. This concept is critical in the analysis of energy flows in optoelectronic devices.

Photon Energy

This concept refers to the energy carried by a single photon, which is given by the relation E = hc/?, where h is Planck’s constant, c is the speed of light and ? is the wavelength. It is fundamental in converting between a beam’s power (energy per unit time) and the number of photons it contains.

Power-to-Photon Conversion

When a light beam has a known power, the number of photons per second can be found by dividing the total power by the energy of a single photon. This conversion is essential for understanding the quantized nature of light and is widely used in photonics and quantum optics.

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