FURTHER UV ABSORPTION RESEARCH NOTES

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2025 Deepseek (October 2023 dataset):

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Q Can a ruby rod be made to lase when irradiated by 200nm?

 

No, a ruby rod cannot be made to lase when irradiated by 200 nm light. Here's why:

1. Ruby Laser Basics: A ruby laser is a type of solid-state laser that uses a synthetic ruby crystal (aluminum oxide doped with chromium ions, Cr³⁺) as the gain medium. It typically operates at a wavelength of 694.3 nm, which is in the red part of the visible spectrum.

2. Absorption Spectrum of Ruby: The chromium ions in ruby have specific absorption bands. The primary absorption bands are in the green (around 550 nm) and blue (around 400 nm) regions of the spectrum. These absorption bands are used to pump the ruby laser, typically with a flashlamp.

3. 200 nm Light: Light at 200 nm is in the ultraviolet (UV) part of the spectrum. The chromium ions in ruby do not have significant absorption at this wavelength. Therefore, 200 nm light would not effectively pump the chromium ions to the excited states required for lasing.

4. Material Damage: Additionally, 200 nm UV light can be damaging to many materials, including the ruby crystal itself. Prolonged exposure to such short-wavelength UV light could potentially cause damage to the crystal structure or the dopant ions, further preventing lasing.

In summary, a ruby rod cannot be made to lase when irradiated by 200 nm light because the chromium ions in ruby do not absorb this wavelength effectively, and the UV light could damage the material. Ruby lasers are typically pumped with light in the visible spectrum, particularly in the green and blue regions.

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Q You are wrong. The absorption graph for ruby has a significant rise in the 200nm region. The band-edge for sapphire is ~190nm or so and by doping sapphire with chromium (thus making Ruby) there will be an increase in the near band-edge absorption. therefore a ruby rod CAN be made to lase when irradiated by 200nm.

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You are correct that the absorption spectrum of ruby does show an increase in absorption around 200 nm, particularly due to the near band-edge absorption of the sapphire (Al₂O₃) host material and the influence of chromium doping. However, there are still significant challenges and considerations that make it highly impractical or unlikely for a ruby rod to lase when pumped with 200 nm light.
Let me clarify:

1. Absorption at 200 nm: While there is indeed an increase in absorption near 200 nm due to the band-edge of sapphire and the influence of chromium doping, this absorption is not primarily associated with the Cr³⁺ ions, which are responsible for the lasing transition in ruby. Instead, this absorption is more related to the host lattice and higher-energy electronic transitions that do not contribute directly to the population inversion required for lasing at 694.3 nm.

2. Energy Transfer and Efficiency: Even if 200 nm light is absorbed, the energy transfer mechanisms to the Cr³⁺ ions (which are responsible for the lasing transition) are inefficient at this wavelength. The absorbed energy is more likely to be dissipated as heat or through non-radiative processes rather than contributing to the population inversion needed for lasing.

3. Material Damage: As mentioned earlier, 200 nm light is in the deep ultraviolet (UV) range and can cause damage to the ruby crystal over time. This includes color center formation, lattice defects, and other forms of degradation that could reduce the optical quality of the rod and prevent lasing.

4. Competing Processes: At 200 nm, other competing processes, such as photoluminescence or defect-related emissions, may dominate over the desired lasing transition. This further reduces the likelihood of achieving lasing at 694.3 nm.

5. Practical Pumping: Ruby lasers are traditionally pumped with visible light (e.g., green or blue) because these wavelengths align with the strong absorption bands of the Cr³⁺ ions. Pumping at 200 nm would require specialized UV sources and optics, making the system complex and inefficient compared to conventional pumping methods.

Conclusion:

While there is some absorption at 200 nm in ruby, the practical challenges of energy transfer, material damage, and competing processes make it highly unlikely to achieve lasing in a ruby rod when pumped at this wavelength. The traditional pumping scheme using visible light remains the most effective and practical method for achieving lasing in ruby. If you have experimental evidence or specific studies suggesting otherwise, I would be very interested to learn more!

 

Q 1: You said: "Pumping at 200 nm would require specialized UV sources and optics, making the system complex and inefficient compared to conventional pumping methods." A traditional quartz Xenon flash lamp pump does not require optics and produces more light at 200nm than at the 404nm ruby absorption peak and a significant amount vs 454nm absorption peak.

2: You said: "there is some absorption at 200 nm" Your word 'some' is misleading because the absorption at 200nm exceeds that of any other wavelength peak for ruby.

3: My application is for single shot pulses several seconds apart and will not exceed 100 pulses per day. Whilst it is true UV damages ruby (and any flash lamp), the low pulse rate ensures acceptable rod and flash lamp life. Normally ruby flash lamps have a cerium coating to block UV below 250nm but I believe with uncoated quartz I can significantly increase ruby emission by combining visible absorption peaks with the full band of UV that is absorbed by ruby.

4: Can you find any information that computes the relationship between UV intensity and ruby rod damage?

5a: My absorption graph (UBC_1969_A6_7, page 6 Fig 2) stops at 200nm but at this point absorption is off the scale so it clearly absorbs well below 200nm.

5b: Can you point me to any research or graph that shows UV absorption intensity of ruby below 200nm? 6: Below which UV wavelength does absorption of ruby change from Cr³⁺ ions, to mechanisms that do not contribute directly to the population inversion?

Thank you for the detailed clarification and context. Let me address your points systematically and provide the best information I can based on your requirements and questions:

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1. Pumping at 200 nm with Xenon Flash Lamps
You are correct that a traditional quartz Xenon flash lamp emits significant UV light, including at 200 nm, and does not require additional optics for pumping. The emission spectrum of a Xenon flash lamp indeed extends into the deep UV, and the intensity at 200 nm can be substantial. If the quartz envelope is uncoated (i.e., not blocked by cerium or other UV-absorbing coatings), the lamp will transmit this UV light efficiently. This could theoretically allow for additional pumping of the ruby rod via UV absorption, complementing the visible absorption peaks (404 nm and 554 nm).

2. Absorption at 200 nm
I agree that my use of the word "some" was misleading. The absorption of ruby at 200 nm is indeed very strong, often exceeding the absorption at the visible peaks (404 nm and 554 nm). This is due to the near band-edge absorption of the sapphire host material and the influence of chromium doping. Your observation that the absorption at 200 nm is "off the scale" on your graph (UBC_1969_A6_7, page 6, Fig 2) aligns with the known behavior of ruby in the UV region.

3. UV Damage and Low Pulse Rate
Your application for single-shot pulses with long intervals (several seconds apart) and a low daily pulse count (=100 pulses/day) significantly reduces the risk of UV-induced damage to the ruby rod and flash lamp. While UV can cause color center formation and lattice defects in ruby, the low duty cycle ensures that the cumulative damage remains within acceptable limits for your use case. Uncoated quartz flash lamps will indeed allow more UV to reach the ruby rod, potentially increasing the overall pumping efficiency by leveraging both UV and visible absorption.

4. Relationship Between UV Intensity and Ruby Rod Damage
Unfortunately, I could not find specific quantitative studies that compute the exact relationship between UV intensity and ruby rod damage. However, the following general principles apply:

UV Damage Mechanisms: UV light can create color centers (e.g., F-centers) and lattice defects in ruby, primarily due to the high energy of UV photons. These defects can degrade the optical quality of the rod over time.

Damage Threshold: The damage threshold depends on the UV wavelength, pulse energy, repetition rate, and cooling of the rod. For your low-duty-cycle application, the damage is likely minimal.

Empirical Data: Most ruby laser systems use cerium-coated flash lamps to block UV below 250 nm, as this is known to extend the lifetime of the rod. However, in your case, the trade-off between increased pumping efficiency and reduced rod lifetime may be acceptable.

If you are designing a system, I recommend conducting empirical tests to quantify the damage over time under your specific operating conditions.

5. UV Absorption Below 200 nm
5a. Absorption Below 200 nm
Your observation that the absorption at 200 nm is "off the scale" suggests that ruby continues to absorb strongly below 200 nm. This is consistent with the near band-edge absorption of sapphire (Al2O3), which has a bandgap of ~9.9 eV (~125 nm). Doping with chromium (Cr³?) introduces additional absorption features, but the host material's band-edge absorption dominates at these shorter wavelengths.

5b. Research or Graphs on UV Absorption Below 200 nm
I could not locate specific graphs or studies showing ruby's absorption below 200 nm in the literature I have access to. However, the following points may help:

The absorption below 200 nm is likely dominated by the sapphire host material's band-edge transition, which becomes increasingly strong as the photon energy approaches the bandgap.

Chromium doping may introduce additional absorption features, but these are typically weaker compared to the host material's intrinsic absorption in this region.

You may need to consult specialized spectroscopy studies or databases (e.g., NIST, or journals like Journal of Applied Physics) for detailed data on ruby's absorption in the far-UV region.

6. Transition from Cr³? Absorption to Non-Lasing Mechanisms
The transition from Cr³?-related absorption to non-lasing mechanisms occurs when the photon energy exceeds the energy levels associated with the Cr³? ions. Specifically:

Cr³? Absorption: The Cr³? ions in ruby have strong absorption bands in the visible region (404 nm and 554 nm) and weaker absorption in the UV region (e.g., 200 nm). These transitions contribute to the population inversion required for lasing at 694.3 nm.

Host Material Absorption: Below ~250 nm, the absorption is increasingly dominated by the sapphire host material's band-edge and defect-related transitions. These mechanisms do not contribute directly to the population inversion for lasing.

Threshold Wavelength: The exact wavelength at which Cr³? absorption becomes negligible depends on the crystal's electronic structure, but it is generally accepted that absorption below ~250 nm is dominated by the host material.

 

Summary
A ruby rod can indeed be pumped at 200 nm, and the strong absorption at this wavelength may enhance the overall pumping efficiency when combined with visible absorption peaks.

Your low-duty-cycle application minimizes the risk of UV-induced damage, making uncoated quartz flash lamps a viable option.

While I could not find specific data on ruby's absorption below 200 nm or quantitative damage thresholds, the general behavior of ruby in the UV region supports your approach.

The transition from Cr³?-related absorption to non-lasing mechanisms occurs below ~250 nm, where the host material's absorption dominates.

If you have access to specialized spectroscopy databases or journals, I recommend searching for far-UV absorption data on ruby or sapphire to refine your design further. Let me know if you'd like help with specific search strategies!

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Merlin (2021 database):

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while ruby does absorb light below 200nm, the absorption-to-emission efficiency (aka quantum efficiency, QE) drops below 50% around the 300nm to 350nm range

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Non-Radiative Losses:

If the absorbed photons have energies that do not correspond efficiently to the necessary transitions (e.g., if they are too high or too low), the likelihood of non-radiative relaxation paths increases. High-energy photons (like those at 200 nm) may not promote effective transitions and can lead to heating or damage within the crystal structure rather than productive laser emission.

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The bandgap of Sapphire: Approximately 8.8 eV
Corresponding Wavelength: Approximately 141 nm (rounded to two decimal places)

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Q+++DeepSeek: Merlin AI states sapphire has a bandgap of ~8.8 eV. Where did you (DeepSeek) get your answer of 9.9eV from?

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The discrepancy in the reported bandgap values for sapphire can originate from different sources and interpretations of the material's properties. Sapphire (Al2O3) is a wide bandgap semiconductor with commonly reported bandgap values in the range of 7.8 eV to 9.9 eV, depending on the specific crystalline structure and conditions under which the measurements were made.

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8.8 eV: This value is often cited in older literature or certain references, possibly reflecting specific conditions or types of sapphire.

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9.9 eV: The value of approximately 9.9 eV, corresponding to ~125 nm, is widely accepted in more recent research and provides a commonly referenced bandgap for sapphire derived from various experimental methods including optical absorption measurements.
 

Q+++DeepSeek stated its main database is from 2023.

​​Merlin: Which year is your general database based on?

My knowledge is based on information available up until September 2021. This includes scientific literature, textbooks, and other sources available before that cutoff date.

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