Semiconductor Supermirrors: Advancing High Performance Laser Optics with Substrate-Transferred Crystalline Coatings
This substrate-transferred crystalline coating technology enables the integration of single-crystal semiconductor films onto arbitrary optical substrates. The unique properties of monocrystalline semiconductor materials in precision laser optics are exploited, including the potential for ultralow Brownian noise, ultralow optical losses and high thermal conductivity can be exploited. Applications include trace gas analysis, references for communications, and inertial navigation.
High-reflectivity optical coatings are key components for a wide variety of laser-based applications. With the continuing progress in laser development, however, state-of-the-art optical coatings have now become a major limitation for various applications in the domain of precision measurement and instrumentation.
Substrate-transferred crystalline coatings represent a game-changing technology that enables the integration of single-crystal films onto arbitrary, including curved, optical substrates, exploiting, for the first time, the unique properties of monocrystalline semiconductor materials for high-performance laser optics. This technological breakthrough was long overdue and semiconductor supermirrors promise to increase performance levels in existing applications and the development of new applications relevant to optical precision measurement.
Innovations & Advantages
GaAs/AlGaAs-based distributed Bragg reflectors (DBRs) have been applied for the fabrication of optical interference coatings since the 1970s. However, only the development of the proprietary substrate-transfer coating technology, allows to transfer semiconductor DBRs onto arbitrary substrates, and achieving the following advantages:
- low thermal noise,
Mirror assemblies produced with crystalline coating technology offer an up to 10x reduction in thermo-mechanical (Brownian) noise compared to current state-of the-art technology.
- low optical losses in the mid-infrared
Compared to dielectric materials, arsenide-based DBRs can achieve much lower optical losses in the important mid-IR spectral region between 2 and 5 μm, with absorption losses below 10 ppm. In the first preliminary tests on mirrors performed in the Ye group at JILA/NIST, a cavity with a finesse exceeding 10,000 was demonstrated, while simultaneously exhibiting a cavity reflection contrast of 71%.
- high thermal conductivity
Currently, in high-power lasers the low thermal conductivity of ion-beam sputtered coatings drastically limits the effectiveness of any heat sink solution. Semiconductor supermirrors show a thermal conductivity up to 30 times higher than in traditionally sputtered coatings. This property makes substrate-transferred crystalline mirrors an ideal optical system for superior thermal management in harsh environments. In addition, the intrinsic bonding process allows integration of such supermirrors onto arbitrary substrates including excellent thermal conductors such as SiC or diamond.
< 5 ppm excess optical loss (scattering plus absorption) for the wavelength range 900-2000 nm. Center wavelength and transmission loss is selectable for the wavelength range 900 – 6000 nm. Thermal conductivity > 30 W/cm2. Ability to prequalify multilayer before coating final substrates. Ability to bond to surfaces as rough as ~1 nm RMS, while maintaining a 0.1 nm RMS surface roughness.
Current and Potential Domains of Application
Substrate-transferred crystalline coatings are a radically new technology in the domain of laser-based precision measurement and instrumentation. This technology impacts both developments in fundamental research (via advancements in optical atomic clocks, gravitational wave detectors, gyroscopes, etc.) and industrial applications (including mid-IR cavity-enhanced trace gas analysis, navigation, frequency and timing references in microwave and fiber-optic communications, high-power lasers, etc.).
Ultra-low thermal noise performance enables the next critical step in the development of high-performance ultra-stable laser systems. Noise reduction is immediately relevant for improved performance of optical clocks and laser-based precision measurements. In addition, the advancement in the sensitivity of optical precision measurement systems impacts consumer-facing applications including optical and wireless networking (e.g., low-noise microwave generation enabling high speed analog to digital conversion, remote synchronization, and processing of ultra-broadband radiofrequency signals), satellite-based navigation (GNSS), as well as sensing applications (e.g. emerging applications in geodesy and hydrology).
Low optical losses in the mid-IR spectral regime open the way to completely new capabilities of mid-IR laser systems and applications, including medical and environmental monitoring. The latter is of significant interest as many large molecules for atmospheric science, medicine, and national security have fundamental vibrational transitions in this region, making it ideal for trace gas detection efforts.
The high-thermal conductivity of monocrystalline semiconductor coatings, together with the ability to bond them to arbitrary optical substrates and heat sinks, make the supermirror technology a promising candidate for novel high-power laser applications. A recent example is the use of crystalline coating technology to construct an optimized large-area Semiconductor Saturable Absorber Mirror (SESAM), which demonstrated an order-of-magnitude improvement in both surface flatness and heat removal under optical load.