Publications and Research Highlights
Below is a list of our academic journal publications that provides details on the semiconductor coating and bonding technology as well as application results when using our groundbreaking technology.
Near-infrared scanning cavity ringdown for optical loss characterization of supermirrors
G. W. Truong, G. Winkler, T. Zederbauer, D. Bachmann, P. Heu, D. Follman, M. E. White, O. H. Heckl, and G. D. Cole
"Near-infrared scanning cavity ringdown for optical loss characterization of supermirrors", Optics Express, vol. 27, pp. 19141-19149, 8 July 2019. https://doi.org/10.1364/OE.27.019141
A cavity ringdown system for probing the spatial variation of optical loss across high-reflectivity mirrors is described. This system is employed to examine substrate-transferred crystalline supermirrors and to quantify the effect of manufacturing process imperfections. Excellent agreement is observed between the ringdown-generated spatial measurements and differential interference contrast microscopy images. A 2-mm diameter ringdown scan in the center of a crystalline supermirror reveals highly uniform coating properties with excess loss variations below 1 ppm.
Measurement of quantum back action in the audio band at room temperature
J. Cripe, N. Aggarwal, R. Lanza, A. Libson, R. Singh, P. Heu, D. Follman, G. D. Cole, N. Mavalvala, and T. Corbitt
"Measurement of quantum back action in the audio band at room temperature," Nature, online 25 March 2019. doi.org/10.1038/s41586-019-1051-4
Quantum mechanics places a fundamental limit on the precision of continuous measurements. The Heisenberg uncertainty principle dictates that as the precision of a measurement of an observable (for example, position) increases, back action creates increased uncertainty in the conjugate variable (for example, momentum). In interferometric gravitational-wave detectors, higher laser powers reduce the position uncertainty created by shot noise (the photon-counting error caused by the quantum nature of the laser) but necessarily do so at the expense of back action in the form of quantum radiation pressure noise (QRPN). Once at design sensitivity, the gravitational-wave detectors Advanced LIGO, VIRGO and KAGRA will be limited by QRPN at frequencies between 10 hertz and 100 hertz. There exist several proposals to improve the sensitivity of gravitational-wave detectors by mitigating QRPN, but until now no platform has allowed for experimental tests of these ideas. Here we present a broadband measurement of QRPN at room temperature at frequencies relevant to gravitational-wave detectors. The noise spectrum obtained shows effects due to QRPN between about 2 kilohertz and 100 kilohertz, and the measured magnitude of QRPN agrees with our model. We now have a testbed for studying techniques with which to mitigate quantum back action, such as variational readout and squeezed light injection, with the aim of improving the sensitivity of future gravitational-wave detectors.
Mechanical Ringdown Studies of Large-Area Substrate-Transferred GaAs/AlGaAs Crystalline Coatings
S. D. Penn, M. M. Kinley-Hanlon, I. A. O. MacMillan, P. Heu, D. Follman, C. Deutsch, G. D. Cole, and G. M. Harry
Journal of the Optical Society of America B (JOSA B), vol. 36, no. 4, C15-C21, 1 April 2019.
We investigated elastic loss in GaAs/AlGaAs multilayers to help determine the suitability of these coatings for future gravitational wave detectors. We measured large (≈70-mm diameter) substrate-transferred crystalline coating samples with an improved substrate polish and bonding method. The elastic loss, when decomposed into bulk and shear contributions, was shown to arise entirely from the bulk loss, φBulk=(5.33±0.03)×10^−4, with φShear=(0.0 +5.2/-0.0)×10^−7. These results predict the coating loss of an 8-mm diameter coating in a 35-mm long cavity with a 250-μm spot size (radius) to be φcoating=(4.78±0.05)×10^−5, in agreement with the published result from direct thermal noise measurement of φcoating=(4±4)×10^−5. Bonding defects were shown to have little impact on the overall elastic loss.
© 2019 Optical Society of America
The Macek and Davis experiment revisited: A large ring laser interferometer operating on the 2s² -> 2p⁴ transition of neon
C. L. Anyi, R. J. Thirkettle, D. Zou, D. Follman, G. D. Cole, K. U. Schreiber, and J.-P. R. Wells
Applied Optics, vol. 58, no. 2, pp. 302-307, 10 January 2019.
We operate a large helium–neon-based ring laser interferometer with single-crystal GaAs/AlGaAs optical coatings on the 2s2→2p4 transition of neon at a wavelength of 1.152276 μm. For either single longitudinal- or phase-locked multi-mode operation, the preferable gas composition for gyroscopic operation is 0.2 and 0.3 mbar of 50:50 neon with total pressures between 6–12 mbar. The Earth rotation bias is sufficient to unlock the device, yielding a Sagnac frequency of approximately 60 Hz.
© 2019 Optical Society of America
Reduction of absorption losses in MOVPE-grown AlGaAs Bragg mirrors
J. Pohl, G. D. Cole, U. Zeimer, M. Aspelmeyer, and M. Weyers.
"Reduction of absorption losses in MOVPE-grown AlGaAs Bragg mirrors," Optics Letters, vol. 43, no. 15, pp. 3522-3525, 1 August 2018. https://doi.org/10.1364/OL.43.003522
Residual p-type doping from carbon has been identified as the root cause of excess absorption losses in (Al)GaAs/AlGaAs Bragg mirrors for high-finesse optical cavities when grown by metalorganic vapor phase epitaxy (MOVPE). Through optimization of the growth parameters with the aim of realizing low carbon uptake, we have shown a path for decreasing the parasitic background absorption in these mirrors from 100 to the 10 ppm range near 1064 nm. This significant reduction is realized via compensation of the carbon acceptors by intentional doping with the donor silicon in the uppermost layer pairs of 40-period GaAs/AlGaAs Bragg mirrors. Thus, we find that such compensation enables MOVPE-derived multilayer mirrors with the potential for a high cavity finesse (>100,000 in the near infrared) approaching the performance levels found with Bragg mirrors grown by molecular beam epitaxy (MBE).
Optical performance of large-area crystalline coatings
M. Marchiò, R. Flaminio, L. Pinard, D. Forest, C. Deutsch, P. Heu, D. Follman, and G. D. Cole.
"Optical performance of large-area crystalline coatings," Optics Express, vol. 26, no. 5, pp. 6114-6125, 5 March 2018. https://doi.org/10.1364/OE.26.006114
Given their excellent optical and mechanical properties, substrate-transferred crystalline coatings are an exciting alternative to amorphous multilayers for applications in precision interferometry. The high mechanical quality factor of these single-crystal interference coatings reduces the limiting thermal noise in precision optical instruments such as reference cavities for narrow-linewidth laser systems and interferometric gravitational wave detectors. In this manuscript, we explore the optical performance of GaAs/AlGaAs crystalline coatings transferred to 50.8-mm (2-inch) diameter fused silica and sapphire substrates. We present results for the transmission, scattering, absorption, and surface quality of these prototype samples including the defect density and micro-roughness. These novel coatings exhibit optical performance on par with state-of-the-art dielectric structures, encouraging further work focused on the fabrication of larger optics using this technique.
16 W DBR-free membrane semiconductor disk laser with dual-SiC heatspreader
Z. Yang, D. Follman, A. R. Albrecht, P. Heu, N. Giannini, G. D. Cole, and M. Sheik-Bahae.
"16 W DBR-free membrane semiconductor disk laser with dual-SiC heatspreader," Electronics Letters, vol. 54, no. 7, pp. 430-432, 5 April 2018. http://dx.doi.org/10.1049/el.2018.0101
A record output power of 16.1 W with a direct-bonded dual-SiC-heatspreader distributed Bragg reflector (DBR)-free active region at 10.5 °C coolant temperature is reported. A comparison in laser performance confirms the dual-heatspreader DBR-free configuration dissipates heat more effectively than the single-heatspreader geometry.
Direct frequency comb measurement on OD + CD -> DOCO kinetics
B. J. Bjork, T. Q. Bui, O. H. Heckl, P. B Changala, B. Spaun, P. Heu, D. Follman, C. Deutsch, G. D. Cole, M. Aspelmeyer, M. Okumura, and J. Ye.
"Direct frequency comb measurement on OD + CD -> DOCO kinetics," Science, vol. 354, no. 6311, pp. 444-448, 28 Oct 2016. DOI: 10.1126/science.aag1862
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.
High-performance near- and mid-infrared crystalline coatings
G. D. Cole, W. Zhang, B. J. Bjork, D. Follman, P. Heu, C. Deutsch, L. Sonderhouse, J. Robinson, C. Franz, A. Alexandrovski, M. Notcutt, O. H. Heckl, J. Ye, and M. Aspelmeyer
"High-performance near- and mid-infrared crystalline coatings," Optica, Vol. 3, No. 6, 647-656, June 2016. https://doi.org/10.1364/OPTICA.3.000647
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 GaAs/Al𝑥Ga1−𝑥As multilayers. In the near-infrared (NIR), for coating center wavelengths spanning 1064–1560 nm, we have reduced the excess optical losses (scatter + absorption) to levels as low as 3 parts per million (ppm), enabling the realization of a cavity finesse exceeding 3×10^5 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-infrared (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.
Optimized SESAMs for kilowatt-level ultrafast lasers
A. Diebold, T. Zengerle, C. G. E. Alfieri, C. Schriber, F. Emaury, M. Mangold, M. Hoffmann, C. J. Saraceno, M. Golling, D. Follman, G. D. Cole, M. Aspelmeyer, T. Südmeyer, and U. Keller.
"Optimized SESAMs for kilowatt-level ultrafast lasers,” Opt. Express 24, issue 10, 10512-10526, May 2016. https://doi.org/10.1364/OE.24.010512
We present a thorough investigation of surface deformation and thermal properties of high-damage threshold large-area semiconductor saturable absorber mirrors (SESAMs) designed for kilowatt average power laser oscillators. We compare temperature rise, thermal lensing, and surface deformation of standard SESAM samples and substrate-removed SESAMs contacted using different techniques. We demonstrate that for all cases the thermal effects scale linearly with the absorbed power, but the contacting technique critically affects the strength of the temperature rise and the thermal lens of the SESAMs (i.e. the slope of the linear change). Our best SESAMs are fabricated using a novel substrate-transfer direct bonding technique and show excellent surface flatness (with non-measureable radii of curvature (ROC), compared to astigmatic ROCs of up to 10 m for standard SESAMs), order-of-magnitude improved heat removal, and negligible deformation with absorbed power. This is achieved without altering the saturation behavior or the recovery parameters of the samples. These SESAMs will be a key enabling component for the next generation of kilowatt-level ultrafast oscillators.
Coherent cancellation of photothermal noise in GaAs/Al0.92Ga0.08As Bragg mirrors
T. Chalermsongsak, E. D. Hall, G. D. Cole, D. Follman, F. Seifert, K. Arai, E. K. Gustafson, J. R. Smith, M. Aspelmeyer, and R. X. Adhikari,
"Coherent cancellation of photothermal noise in GaAs/Al0.92Ga0.08As Bragg mirrors," Metrologia, vol. 53, no. 2, pp. 860-868, 9 March 2016. https://doi.org/10.1088/0026-1394/53/2/860
Thermal noise is a limiting factor in many high-precision optical experiments. A search is underway for novel optical materials with reduced thermal noise. One such pair of materials, gallium arsenide and aluminum-alloyed gallium arsenide (collectively referred to as AlGaAs), shows promise for its low Brownian noise when compared to conventional materials such as silica and tantala. However, AlGaAs has the potential to produce a high level of thermo-optic noise. We have fabricated a set of AlGaAs crystalline coatings, transferred to fused silica substrates, whose layer structure has been optimized to reduce thermo-optic noise by inducing coherent cancellation of the thermoelastic and thermorefractive effects. By measuring the photothermal transfer function of these mirrors, we find evidence that this optimization has been successful.
Technology for the next gravitational wave detectors
V. P. Mitrofanov, S. Chao, H.-W. Pan, L.-C. Kuo, G. Cole, J. Degallaix, and B. Willke
"Technology for the next gravitational wave detectors," Science China Physics, Mechanics & Astronomy, vol. 58, 120404, December 2015. https://doi.org/10.1007/s11433-015-5738-8
This paper reviews some of the key enabling technologies for advanced and future laser interferometer gravitational wave detectors, which must combine test masses with the lowest possible optical and acoustic losses, with high stability lasers and various techniques for suppressing noise. Sect. 1 of this paper presents a review of the acoustic properties of test masses. Sect. 2 reviews the technology of the amorphous dielectric coatings which are currently universally used for the mirrors in advanced laser interferometers, but for which lower acoustic loss would be very advantageous. In sect. 3 a new generation of crystalline optical coatings that offer a substantial reduction in thermal noise is reviewed. The optical properties of test masses are reviewed in sect. 4, with special focus on the properties of silicon, an important candidate material for future detectors. Sect. 5 of this paper presents the very low noise, high stability laser technology that underpins all advanced and next generation laser interferometers.
Mapping the optical absorption of a substrate-transferred crystalline AlGaAs coating at 1.5 µm
J. Steinlechner, I. W. Martin, A. Bell, G. D. Cole, J. Hough, S. Penn, S. Rowan, and S. Steinlechner
"Mapping the optical absorption of a substrate-transferred crystalline AlGaAs coating at 1.5 µm," Classical and Quantum Gravity, vol. 32, no. 10, 105008, 21 May 2015. https://doi.org/10.1088/0264-9381/32/10/105008
The sensitivity of second and third generations of interferometric gravitational wave (GW) detectors will be limited by the thermal noise of the test-mass mirrors and highly reflective coatings. Recently developed crystalline coatings show a promising thermal noise reduction compared to presently used amorphous coatings. However, stringent requirements apply to the optical properties of the coatings as well. We have mapped the optical absorption of a crystalline AlGaAs coating that is optimized for high reflectivity for a wavelength of 1064 nm. The absorption was measured at 1530 nm, where the coating stack transmits approximately 70% of the laser light. The measured absorption was lower than (30.2 +/- 11.1) ppm which is equivalent to (3.6 +/- 1.3) ppm for a coating stack that is highly reflective at 1530 nm. While this is a very promising low absorption result for alternative low-loss coating materials, further work will be necessary to reach the requirements of < 1 ppm for future GW detectors.
Sensing Earth rotation with a helium-neon ring laser operating at 1.15 µm
K. U. Schreiber, R. J. Thirkettle, R. B. Hurst, D. Follman, G. D. Cole, M. Aspelmeyer, and J.-P. R. Wells
“Sensing Earth rotation with a helium-neon ring laser operating at 1.15 µm,” Optics Letters, vol. 40, no. 8, pp. 1705-1708, 15 April 2015. https://doi.org/10.1364/OL.40.001705
We report on the operation of a 2.56 m^2 helium–neon based ring laser interferometer at a wavelength of 1.152276 μm using crystalline coated intracavity supermirrors. This work represents the first implementation of crystalline coatings in an active laser system and expands the core application area of these low-thermal-noise cavity end mirrors to inertial sensing systems. Stable gyroscopic behavior can only be obtained with the addition of helium to the gain medium as this quenches the 1.152502 μm (2s4→2p7) transition of the neon doublet which otherwise gives rise to mode competition. For the first time at this wavelength, the ring laser is observed to readily unlock on the bias provided by the earth’s rotation alone, yielding a Sagnac frequency of approximately 59 Hz.
Tenfold reduction of Brownian noise in high-reflectivity optical coatings
G. D. Cole, W. Zhang, M. J. Martin, J. Ye, and M. Aspelmeyer
“Tenfold reduction of Brownian noise in high-reflectivity optical coatings,” Nature Photonics, vol. 7, no. 8, pp. 644-650, August 2013. https://doi.org/10.1038/nphoton.2013.174
Thermally induced fluctuations impose a fundamental limit on precision measurement. In optical interferometry, the current bounds of stability and sensitivity are dictated by the excess mechanical damping of the high-reflectivity coatings that comprise the cavity end mirrors. Over the last decade, the dissipation of these amorphous multilayer reflectors has at best been reduced by a factor of two. Here, we demonstrate a new paradigm in optical coating technology based on direct-bonded monocrystalline multilayers, which exhibit both intrinsically low mechanical loss and high optical quality. Employing these ‘crystalline coatings’ as end mirrors in a Fabry–Pérot cavity, we obtain a finesse of 150,000. More importantly, at room temperature, we observe a thermally limited noise floor consistent with a tenfold reduction in mechanical damping when compared with the best dielectric multilayers. These results pave the way for the next generation of ultra-sensitive interferometers, as well as for new levels of laser stability.
Cavity optomechanics with low-noise crystalline mirrors
G. D. Cole
"Cavity optomechanics with low-noise crystalline mirrors," SPIE Optics & Photonics, Optical Trapping and Optical Micromanipulation IX, San Diego, CA, USA, 8458-07, 12–16 August 2012. https://doi.org/10.1117/12.931226
Cavity optomechanics is a rapidly evolving field operating at the intersection of solid-state physics and modern optics. The fundamental process at the heart of this interdisciplinary endeavor is the enhancement of radiation pressure within a high-finesse optical cavity. Isolating this weak interaction, i.e. the momentum transfer of photons onto the cavity boundaries, requires the development of mechanical resonators that simultaneously exhibit high reflectivity (requiring low absorption and scatter loss) and low mechanical dissipation. In a Fabry-Pérot implementation, this is realized by fabricating suspended micrometer-scale mechanical resonators directly from high-reflectivity multilayers. Thus, the properties of the mirror material—particularly the loss angle and optical absorption—drive the ultimate performance of the devices. Interestingly, similar requirements are found in a broad spectrum of applications, ranging from gravitational wave interferometers to stabilized lasers for optical atomic clocks. This overlap leads to an intimate link between advances in the seemingly disparate areas of macroscopic interferometry (e.g. precision measurement and spectroscopy) and micro- and nanoscale optomechanical systems. In this manuscript, I will outline the fascinating implications of cavity optomechanics and present proof-of-concept experiments including MHz-frequency resonators aimed at the demonstration of quantum states of mechanical systems, as well as low-frequency (<1 kHz) devices for the observation of quantum radiation pressure noise. Additionally, I will discuss off-shoot technologies developed in the course of this work, such as a numerical solver for the determination of support-mediated losses in mechanical resonators, as well as a new strategy for the realization of ultra-high-stability optical reference cavities based on transferred crystalline multilayers.
Phonon tunnelling dissipation in mechanical resonators
G. D. Cole, I. Wilson-Rae, K. Werbach, M. R. Vanner, and M. Aspelmeyer
"Phonon tunnelling dissipation in mechanical resonators,” Nature Communications, vol. 2, article 231, 8 March 2011. DOI: 10.1038/ncomms1212
Microscale and nanoscale mechanical resonators have recently emerged as ubiquitous devices for use in advanced technological applications, for example, in mobile communications and inertial sensors, and as novel tools for fundamental scientific endeavours. Their performance is in many cases limited by the deleterious effects of mechanical damping. In this study, we report a significant advancement towards understanding and controlling support-induced losses in generic mechanical resonators. We begin by introducing an efficient numerical solver, based on the 'phonon-tunnelling' approach, capable of predicting the design-limited damping of high-quality mechanical resonators. Further, through careful device engineering, we isolate support-induced losses and perform a rigorous experimental test of the strong geometric dependence of this loss mechanism. Our results are in excellent agreement with the theory, demonstrating the predictive power of our approach. In combination with recent progress on complementary dissipation mechanisms, our phonon-tunnelling solver represents a major step towards accurate prediction of the mechanical quality factor.
Free-standing AlxGa1-xAs heterostructures by gas-phase etching of germanium
G. D. Cole, Y. Bai, M. Aspelmeyer, and E. A. Fitzgerald
"Free-standing AlxGa1-xAs heterostructures by gas-phase etching of germanium,” Applied Physics Letters, vol. 96, no. 26, 261102, 28 June 2010. http://dx.doi.org/10.1063/1.3455104
We outline a facile fabrication technique for the realization of free-standing AlxGa1−xAs heterostructures of arbitrary aluminum content. Utilizing xenon difluoride XeF2 we rapidly and selectively remove a sacrificial germanium Ge underlayer in a room temperature gas-phase etching procedure. We demonstrate two possibilities for exploiting this unique process: 1 bulk micromachining of a suspended high-frequency low-dissipation micro-optomechanical resonator consisting of an epitaxial GaAs/AlAs multilayer grown on a Ge substrate and 2 epitaxial lift-off of a GaAs film via removal of an embedded Ge sacrificial layer, resulting in lateral etch rates up to 3 mm/h and a conservative selectivity of 106.
Megahertz monocrystalline optomechnaical resonators with minimal dissipation
G. D. Cole, I. Wilson-Rae, M. R. Vanner, S. Gröblacher, J. Pohl, M. Zorn, M. Weyers, A. Peters, and M. Aspelmeyer
"Megahertz monocrystalline optomechanical resonators with minimal dissipation,” 23rd IEEE International Conference on MEMS, Hong Kong, China, TP133, pp. 847-850, 24–28 January 2010. https://doi.org/10.1109/MEMSYS.2010.5442339
We present detailed experimental and theoretical results for novel micro-optomechanical resonators, representing a significant improvement in the performance of such structures. These devices exhibit eigenfrequencies (fr) approaching 4 MHz, reflectivities exceeding 99.98% at 1064 nm, and mechanical quality factors (Q) of 0.8 × 10^5 (measured at 20 K and 2.5 × 10^-7 millibar pressure); yielding a Q·fr product of 3.1 × 10^11 Hz, while enabling a finesse of approximately 20,000 when used as an end mirror in an impedance-matched Fabry-Perot optical cavity. These results represent a breakthrough in the development of optomechanical devices applicable to the emerging field of quantum optomechanics.
Monocrystalline AlxGa1-xAs heterostructures for high-reflectivity high-Q micromechanical resonators in the megahertz regime
G. D. Cole, S. Gröblacher, K. Gugler, S. Gigan, and M. Aspelmeyer
“Monocrystalline AlxGa1-xAs heterostructures for high-reflectivity high-Q micromechanical resonators in the megahertz regime,” Applied Physics Letters, vol. 92, no. 26, 261108, 30 June 2008. https://doi.org/10.1063/1.2952512
We present high-performance megahertz micromechanical oscillators based on freestanding epitaxial AlxGa1−xAs distributed Bragg reflectors. Compared with dielectric reflectors, the low mechanical loss of the monocrystalline heterostructure gives rise to significant improvements in the achievable mechanical quality factor Q while simultaneously exhibiting near unity reflectivity. Experimental characterization yields an optical reflectivity exceeding 99.98% and mechanical quality factors up to 20 000 at 4K. This materials system is not only an interesting candidate for optical coatings with ultralow thermal noise, but also provides a promising path toward quantum optical control of massive micromechanical mirrors.