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

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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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Chalcogenide taper coupling efficiency around 4.5 μm(a) Representative transmission spectrum composed of several resonance families over a wide mid-IR range when taper–resonator coupling is achieved with the MgF2 microresonator and set-up described in Fig. 1. (b) Measurement (blue curve) of a resonance linewidth at critical coupling with frequency calibration provided by a Pound–Drever–Hall (PDH) signal (green curve) and Lorentzian fit (red curve). The typical full-width at half maximum width of  corresponds to a critically coupled quality factor of Qc∼1.0 × 107. (c) Transmission as a function of the coupling parameter  for varying taper waist radius. The dashed line marks the critical coupling point . The experimental data (blue circles) are consistent with the theoretical model  (red curve) demonstrating that the ChG taper behaves as a nearly ideal coupler in mid-IR with close to unity ideality.
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f2: Chalcogenide taper coupling efficiency around 4.5 μm(a) Representative transmission spectrum composed of several resonance families over a wide mid-IR range when taper–resonator coupling is achieved with the MgF2 microresonator and set-up described in Fig. 1. (b) Measurement (blue curve) of a resonance linewidth at critical coupling with frequency calibration provided by a Pound–Drever–Hall (PDH) signal (green curve) and Lorentzian fit (red curve). The typical full-width at half maximum width of corresponds to a critically coupled quality factor of Qc∼1.0 × 107. (c) Transmission as a function of the coupling parameter for varying taper waist radius. The dashed line marks the critical coupling point . The experimental data (blue circles) are consistent with the theoretical model (red curve) demonstrating that the ChG taper behaves as a nearly ideal coupler in mid-IR with close to unity ideality.

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
Chalcogenide taper coupling efficiency around 4.5 μm(a) Representative transmission spectrum composed of several resonance families over a wide mid-IR range when taper–resonator coupling is achieved with the MgF2 microresonator and set-up described in Fig. 1. (b) Measurement (blue curve) of a resonance linewidth at critical coupling with frequency calibration provided by a Pound–Drever–Hall (PDH) signal (green curve) and Lorentzian fit (red curve). The typical full-width at half maximum width of  corresponds to a critically coupled quality factor of Qc∼1.0 × 107. (c) Transmission as a function of the coupling parameter  for varying taper waist radius. The dashed line marks the critical coupling point . The experimental data (blue circles) are consistent with the theoretical model  (red curve) demonstrating that the ChG taper behaves as a nearly ideal coupler in mid-IR with close to unity ideality.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC5121327&req=5

f2: Chalcogenide taper coupling efficiency around 4.5 μm(a) Representative transmission spectrum composed of several resonance families over a wide mid-IR range when taper–resonator coupling is achieved with the MgF2 microresonator and set-up described in Fig. 1. (b) Measurement (blue curve) of a resonance linewidth at critical coupling with frequency calibration provided by a Pound–Drever–Hall (PDH) signal (green curve) and Lorentzian fit (red curve). The typical full-width at half maximum width of corresponds to a critically coupled quality factor of Qc∼1.0 × 107. (c) Transmission as a function of the coupling parameter for varying taper waist radius. The dashed line marks the critical coupling point . The experimental data (blue circles) are consistent with the theoretical model (red curve) demonstrating that the ChG taper behaves as a nearly ideal coupler in mid-IR with close to unity ideality.
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