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Mid-infrared ultra-high- Q resonators based on fluoride crystalline materials

View Article: PubMed Central - PubMed

ABSTRACT

The unavailability of highly transparent materials in the mid-infrared has been the main limitation in the development of ultra-sensitive molecular sensors or cavity-based spectroscopy applications. Whispering gallery mode microresonators have attained ultra-high-quality (Q) factor resonances in the near-infrared and visible. Here we report ultra-high Q factors in the mid-infrared using polished alkaline earth metal fluoride crystals. Using an uncoated chalcogenide tapered fibre as a high-ideality coupler in the mid-infrared, we study via cavity ringdown technique the losses of BaF2, CaF2, MgF2 and SrF2 microresonators. We show that MgF2 is limited by multiphonon absorption by studying the temperature dependence of the Q factor. In contrast, in SrF2 and BaF2 the lower multiphonon absorption leads to ultra-high Q factors at 4.5 μm. These values correspond to an optical finesse of , the highest value achieved for any type of mid-infrared resonator to date.

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Mid-IR multiphonon absorption in crystalline microcavities.(a) Measurements for different fluoride crystals of the XF2 family (where X=Ca, Mg, Ba and Sr) prove the possibility of attaining the ultra-high-Q regime in the mid-IR. Except for MgF2, for which we reach the theoretical limit imposed by multiphonon absorption (orange shaded region) at room temperature, other materials offer Q≥108 around 4.5 μm. The lines represent the theoretical multiphonon absorption limit of Q with respect to the wavelength. The circles represent our experimental values. Despite the clear differences with the near-IR region, mid-IR cavities are able to overcome the high-Q regime achieving Q>108. Measurements around 2 μm show typical level of impurities and defects that limits quality factors when they are not limited by Rayleigh scattering (short wavelengths) or multiphonon absorption (long wavelengths). Measurements around 3 μm highlight that OH absorption can strongly degrade the intrinsic Q factor of crystalline materials and constitutes a lower bound of Q limitation due to impurities and defects. The dashed grey line is a guide to the eye depicting bulk water absorption Q limitation (from ref. 50). Note that 1.5 μm represents the less-affected wavelength by OH absorption (by a few orders of magnitude compared with any other wavelengths). (b) Experimental proof of multiphonon absorption in a microresonator made out of an ionic crystal. Temperature sweeps of the microresonator reveal a typical temperature dependence of intrinsic multiphonon absorption for MgF2 (fitted slopes of ∼4 × 10−2 MHz K−1) and consistently no temperature dependence for extrinsic absorption limited SrF2 (fitted slope of 10−3 MHz K−1). Sweep with increasing (up) and decreasing (down) temperatures corroborate the temperature dependence of the intrinsic losses. From the theoretical slope (see Discussion) we infer that two-phonon (N=2) processes contribute mostly to the temperature dependence.
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f5: Mid-IR multiphonon absorption in crystalline microcavities.(a) Measurements for different fluoride crystals of the XF2 family (where X=Ca, Mg, Ba and Sr) prove the possibility of attaining the ultra-high-Q regime in the mid-IR. Except for MgF2, for which we reach the theoretical limit imposed by multiphonon absorption (orange shaded region) at room temperature, other materials offer Q≥108 around 4.5 μm. The lines represent the theoretical multiphonon absorption limit of Q with respect to the wavelength. The circles represent our experimental values. Despite the clear differences with the near-IR region, mid-IR cavities are able to overcome the high-Q regime achieving Q>108. Measurements around 2 μm show typical level of impurities and defects that limits quality factors when they are not limited by Rayleigh scattering (short wavelengths) or multiphonon absorption (long wavelengths). Measurements around 3 μm highlight that OH absorption can strongly degrade the intrinsic Q factor of crystalline materials and constitutes a lower bound of Q limitation due to impurities and defects. The dashed grey line is a guide to the eye depicting bulk water absorption Q limitation (from ref. 50). Note that 1.5 μm represents the less-affected wavelength by OH absorption (by a few orders of magnitude compared with any other wavelengths). (b) Experimental proof of multiphonon absorption in a microresonator made out of an ionic crystal. Temperature sweeps of the microresonator reveal a typical temperature dependence of intrinsic multiphonon absorption for MgF2 (fitted slopes of ∼4 × 10−2 MHz K−1) and consistently no temperature dependence for extrinsic absorption limited SrF2 (fitted slope of 10−3 MHz K−1). Sweep with increasing (up) and decreasing (down) temperatures corroborate the temperature dependence of the intrinsic losses. From the theoretical slope (see Discussion) we infer that two-phonon (N=2) processes contribute mostly to the temperature dependence.

Mentions: We fabricated our microresonators either from disk or cylinder blanks. The microresonators were first shaped by grinding or diamond-cutting tool and polished in an air-bearing spindle by successive smaller diamond particle slurries to obtain a smooth protrusion as visualized in Fig. 1b. Their final diameters are ∼5 mm. We analysed the microresonators in a scanning electron microscope, and from this we determined the transverse radii of the crystalline MgF2 disk to be of . The intensity profile of the fundamental WGM of the MgF2 microresonator in the mid-IR is displayed in Fig. 1c. From finite element model simulations we obtained an effective mode area of Aeff∼600 μm2 at . To ensure that we are not limited by scattering losses from residual surface roughness, optical quality factors were first measured at , with a tunable, narrow-linewidth (short-term <100 kHz) fibre laser using silica tapered fibres to excite the WGM26. The MgF2 disk features loaded optical factors of Q≥5 × 108; CaF2 and SrF2 exhibit Q factors of ∼2 × 109 and the BaF2 cylinder features Q∼6.4 × 109 (detailed below and in Fig. 5).


Mid-infrared ultra-high- Q resonators based on fluoride crystalline materials
Mid-IR multiphonon absorption in crystalline microcavities.(a) Measurements for different fluoride crystals of the XF2 family (where X=Ca, Mg, Ba and Sr) prove the possibility of attaining the ultra-high-Q regime in the mid-IR. Except for MgF2, for which we reach the theoretical limit imposed by multiphonon absorption (orange shaded region) at room temperature, other materials offer Q≥108 around 4.5 μm. The lines represent the theoretical multiphonon absorption limit of Q with respect to the wavelength. The circles represent our experimental values. Despite the clear differences with the near-IR region, mid-IR cavities are able to overcome the high-Q regime achieving Q>108. Measurements around 2 μm show typical level of impurities and defects that limits quality factors when they are not limited by Rayleigh scattering (short wavelengths) or multiphonon absorption (long wavelengths). Measurements around 3 μm highlight that OH absorption can strongly degrade the intrinsic Q factor of crystalline materials and constitutes a lower bound of Q limitation due to impurities and defects. The dashed grey line is a guide to the eye depicting bulk water absorption Q limitation (from ref. 50). Note that 1.5 μm represents the less-affected wavelength by OH absorption (by a few orders of magnitude compared with any other wavelengths). (b) Experimental proof of multiphonon absorption in a microresonator made out of an ionic crystal. Temperature sweeps of the microresonator reveal a typical temperature dependence of intrinsic multiphonon absorption for MgF2 (fitted slopes of ∼4 × 10−2 MHz K−1) and consistently no temperature dependence for extrinsic absorption limited SrF2 (fitted slope of 10−3 MHz K−1). Sweep with increasing (up) and decreasing (down) temperatures corroborate the temperature dependence of the intrinsic losses. From the theoretical slope (see Discussion) we infer that two-phonon (N=2) processes contribute mostly to the temperature dependence.
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5121327&req=5

f5: Mid-IR multiphonon absorption in crystalline microcavities.(a) Measurements for different fluoride crystals of the XF2 family (where X=Ca, Mg, Ba and Sr) prove the possibility of attaining the ultra-high-Q regime in the mid-IR. Except for MgF2, for which we reach the theoretical limit imposed by multiphonon absorption (orange shaded region) at room temperature, other materials offer Q≥108 around 4.5 μm. The lines represent the theoretical multiphonon absorption limit of Q with respect to the wavelength. The circles represent our experimental values. Despite the clear differences with the near-IR region, mid-IR cavities are able to overcome the high-Q regime achieving Q>108. Measurements around 2 μm show typical level of impurities and defects that limits quality factors when they are not limited by Rayleigh scattering (short wavelengths) or multiphonon absorption (long wavelengths). Measurements around 3 μm highlight that OH absorption can strongly degrade the intrinsic Q factor of crystalline materials and constitutes a lower bound of Q limitation due to impurities and defects. The dashed grey line is a guide to the eye depicting bulk water absorption Q limitation (from ref. 50). Note that 1.5 μm represents the less-affected wavelength by OH absorption (by a few orders of magnitude compared with any other wavelengths). (b) Experimental proof of multiphonon absorption in a microresonator made out of an ionic crystal. Temperature sweeps of the microresonator reveal a typical temperature dependence of intrinsic multiphonon absorption for MgF2 (fitted slopes of ∼4 × 10−2 MHz K−1) and consistently no temperature dependence for extrinsic absorption limited SrF2 (fitted slope of 10−3 MHz K−1). Sweep with increasing (up) and decreasing (down) temperatures corroborate the temperature dependence of the intrinsic losses. From the theoretical slope (see Discussion) we infer that two-phonon (N=2) processes contribute mostly to the temperature dependence.
Mentions: We fabricated our microresonators either from disk or cylinder blanks. The microresonators were first shaped by grinding or diamond-cutting tool and polished in an air-bearing spindle by successive smaller diamond particle slurries to obtain a smooth protrusion as visualized in Fig. 1b. Their final diameters are ∼5 mm. We analysed the microresonators in a scanning electron microscope, and from this we determined the transverse radii of the crystalline MgF2 disk to be of . The intensity profile of the fundamental WGM of the MgF2 microresonator in the mid-IR is displayed in Fig. 1c. From finite element model simulations we obtained an effective mode area of Aeff∼600 μm2 at . To ensure that we are not limited by scattering losses from residual surface roughness, optical quality factors were first measured at , with a tunable, narrow-linewidth (short-term <100 kHz) fibre laser using silica tapered fibres to excite the WGM26. The MgF2 disk features loaded optical factors of Q≥5 × 108; CaF2 and SrF2 exhibit Q factors of ∼2 × 109 and the BaF2 cylinder features Q∼6.4 × 109 (detailed below and in Fig. 5).

View Article: PubMed Central - PubMed

ABSTRACT

The unavailability of highly transparent materials in the mid-infrared has been the main limitation in the development of ultra-sensitive molecular sensors or cavity-based spectroscopy applications. Whispering gallery mode microresonators have attained ultra-high-quality (Q) factor resonances in the near-infrared and visible. Here we report ultra-high Q factors in the mid-infrared using polished alkaline earth metal fluoride crystals. Using an uncoated chalcogenide tapered fibre as a high-ideality coupler in the mid-infrared, we study via cavity ringdown technique the losses of BaF2, CaF2, MgF2 and SrF2 microresonators. We show that MgF2 is limited by multiphonon absorption by studying the temperature dependence of the Q factor. In contrast, in SrF2 and BaF2 the lower multiphonon absorption leads to ultra-high Q factors at 4.5&thinsp;&mu;m. These values correspond to an optical finesse of , the highest value achieved for any type of mid-infrared resonator to date.

No MeSH data available.


Related in: MedlinePlus