Oliver H. Heckl

Continuous probing of cold complex molecules with infrared frequency comb spectroscopy

For more than half a century, high-resolution infrared spectroscopy has played a crucial role in probing molecular structure and dynamics. Such studies have so far been largely restricted to relatively small and simple systems, because at room temperature even molecules of modest size already occupy many millions of rotational/vibrational states, yielding highly congested spectra that are difficult to assign. Targeting more complex molecules requires methods that can record broadband infrared spectra (that is, spanning multiple vibrational bands) with both high resolution and high sensitivity. However, infrared spectroscopic techniques have hitherto been limited either by narrow bandwidth and long acquisition time, or by low sensitivity and resolution. Cavity-enhanced direct frequency comb spectroscopy (CE-DFCS) combines the inherent broad bandwidth and high resolution of an optical frequency comb with the high detection sensitivity provided by a high-finesse enhancement cavity, but it still suffers from spectral congestion. Here we show that this problem can be overcome by using buffer gas cooling to produce continuous, cold samples of molecules that are then subjected to CE-DFCS. This integration allows us to acquire a rotationally resolved direct absorption spectrum in the C–H stretching region of nitromethane, a model system that challenges our understanding of large-amplitude vibrational motion. We have also used this technique on several large organic molecules that are of fundamental spectroscopic and astrochemical relevance, including naphthalene, adamantane and hexamethylenetetramine. These findings establish the value of our approach for studying much larger and more complex molecules than have been probed so far, enabling complex molecules and their kinetics to be studied with orders-of-magnitude improvements in efficiency, spectral resolution and specificity.

High-performance near- and mid-infrared crystalline coatings

Substrate-transferred crystalline coatings have recently emerged as a groundbreaking new concept in optical interference coatings. Building upon our initial demonstration of this technology, we have now realized significant improvements in the limiting optical performance of these novel single-crystal

Direct frequency comb measurement of OD + CO → DOCO kinetics

GaAs/Al_{x}Ga_{1-x}As

Three-photon absorption in optical parametric oscillators based on OP-GaAs

multilayers. In the near-infrared (NIR), for coating center wavelengths spanning 1064 to 1560 nm, we have reduced the excess optical losses (scatter + absorption) to levels as low as 3 parts per million, enabling the realization of a cavity finesse exceeding

Phase-stabilized 100 mW frequency comb near 10 μm

3times 10^{5}

Mid-infrared crystalline mirrors with ultralow optical losses

at the telecom-relevant wavelength range near 1550 nm. Moreover, we demonstrate the direct measurement of sub-ppm optical absorption at 1064 nm. Concurrently, we investigate the mid-IR (MIR) properties of these coatings and observe exceptional performance for first attempts in this important wavelength region. Specifically, we verify excess losses at the hundred ppm level for wavelengths of 3300 and 3700 nm. Taken together, our NIR optical losses are now fully competitive with ion beam sputtered multilayer coatings, while our first prototype MIR optics have already reached state-of-the-art performance levels for reflectors covering this portion of the fingerprint region for optical gas sensing. Mirrors fabricated with our crystalline coating technique exhibit the lowest mechanical loss, and thus the lowest Brownian noise, the highest thermal conductivity, and, potentially, the widest spectral coverage of any supermirror technology in a single material platform. Looking ahead, we see a bright future for crystalline coatings in applications requiring the ultimate levels of optical, thermal, and optomechanical performance

Advancements in substrate-transferred crystalline coatings

Combing through CO oxidation kinetics Carbon monoxide reacts with OH radicals to produce CO2. This process is central to combustion and atmospheric oxidation chemistry. The reaction sequence is widely assumed to involve the intermediacy of a HOCO adduct that has eluded direct monitoring under thermal conditions. Bjork et al. successfully observed the formation of the deuterated analog of this intermediate, DOCO, while simultaneously monitoring OD by using a multifrequency infrared comb. The results confirm the termolecular nature of the formation mechanism and its sensitivity to the ambient bath gas. Science, this issue p. 444 A broadband frequency comb tracks the kinetics of a previously elusive intermediate in the oxidation of carbon monoxide to carbon dioxide The kinetics of the hydroxyl radical (OH) + carbon monoxide (CO) reaction, which is fundamental to both atmospheric and combustion chemistry, are complex because of the formation of the hydrocarboxyl radical (HOCO) intermediate. Despite extensive studies of this reaction, HOCO has not been observed under thermal reaction conditions. Exploiting the sensitive, broadband, and high-resolution capabilities of time-resolved cavity-enhanced direct frequency comb spectroscopy, we observed deuteroxyl radical (OD) + CO reaction kinetics and detected stabilized trans-DOCO, the deuterated analog of trans-HOCO. By simultaneously measuring the time-dependent concentrations of the trans-DOCO and OD species, we observed unambiguous low-pressure termolecular dependence of the reaction rate coefficients for N2 and CO bath gases. These results confirm the HOCO formation mechanism and quantify its yield.

Time-resolved frequency comb spectroscopy for studying the kinetics and branching ratio of OD+CO

We report on, to the best of our knowledge, the first singly resonant (SR), synchronously pumped optical parametric oscillator (OPO) based on orientation-patterned gallium arsenide (OP-GaAs). Together with a doubly resonant (DR) degenerate OPO based on the same OP-GaAs material, the output spectra cover 3 to 6 μm within ∼3u2009u2009dB of relative power. The DR-OPO has the highest output power reported to date from a femtosecond, synchronously pumped OPO based on OP-GaAs. We observed strong three-photon absorption with a coefficient of 0.35±0.08u2009u2009cm3/GW2 for our OP-GaAs sample, which limits the output power of these OPOs as mid-IR light sources. We present a detailed study of the three-photon loss on the performance of both the SR- and DR-OPOs, and compare them to those without this loss mechanism.

High resolution rovibrational spectroscopy of large molecules using infrared frequency combs and buffer gas cooling

Long-wavelength mid-infrared (MIR) frequency combs with high power and flexible tunability are highly desired for molecular spectroscopy, including investigation of large molecules such as C60. We present a high power, phase-stabilized frequency comb near 10xa0μm, generated by a synchronously pumped, singly resonant optical parametric oscillator (OPO) based on AgGaSe2. The OPO can be continuously tuned from 8.4 to 9.5xa0μm, with a maximum average idler power of 100xa0mW at the center wavelength of 8.5xa0μm. Both the repetition rate (frep) and the carrier-envelope offset frequency (fceo) of the idler wave are phase-locked to microwave signals referenced to a Cs clock. We describe the detailed design and construction of the frequency comb, and discuss potential applications for precise and sensitive direct frequency comb spectroscopy.

OD + CO → D + CO 2 branching kinetics probed with time-resolved frequency comb spectroscopy

Substrate-transferred crystalline coatings are a groundbreaking new concept for the fabrication of ultralow-loss mirrors. The low defect density single-crystal nature of these semiconductor supermirrors enables the lowest mechanical losses and hence unmatched Brownian noise performance [1], which currently limits the stability of precision optical interferometers. Another outstanding feature of these coatings is the wide spectral coverage of the GaAs/AlGaAs material platform. Limited by interband absorption at short wavelengths and the reststrahlenband at long wavelengths [2], crystalline coatings can be employed as low-loss multilayers from approximately 900 nm up to 5 μm and beyond. Excellent optical performance has been demonstrated in the near-infrared with excess optical losses (scatter + absorption) as low as 3 parts per million (ppm) [3], enabling cavity finesse values up to 360, 000 at 1.55 μm at room temperature and as high as 400, 000 at cryogenic temperatures for the same wavelength. Our first attempts at applying crystalline coatings in the mid-infrared has resulted in mirrors with excess optical losses (scatter + absorption) of 159 and 242 ppm at 3.3 and 3.7 μm, respectively. Remarkably, these results are already on par with current state-of-the-art amorphous mirror coatings, with the first use of mid-infrared crystalline mirrors in a cavity-enhanced spectroscopy setup enabling a detailed investigation of the reaction kinetics of the deuterated hydroxyl molecule, OD, and carbon monoxide, CO [4]. Absorption measurements based on photothermal common-path interferometry (PCI) [5] reveal that the optical losses are largely dominated by optical scatter. Via, PCI, we have confirmed absorption losses below 10 ppm at 3.7 μm, showing the enormous potential of GaAs/AlGaAs Bragg mirrors at mid-infrared wavelengths. An optimized fabrication process, which is currently under development, can efficiently suppress optical scatter due to accumulated growth defects on the surface. Ultimately, we foresee excess losses significantly less than 50 ppm in the mid-infrared spectral region.