Germanium-based integrated photonics from near- to mid-infrared applications

Abstract

Germanium (Ge) has played a key role in silicon photonics as an enabling material for datacom applications. Indeed, the unique properties of Ge have been leveraged to develop high performance integrated photodectors, which are now mature devices. Ge is also very useful for the achievement of compact modulators and monolithically integrated laser sources on silicon. Interestingly, research efforts in these domains also put forward the current revolution of mid-IR photonics. Ge and Ge-based alloys also present strong advantages for mid-infrared photonic platform such as the extension of the transparency window for these materials, which can operate at wavelengths beyond 8 μm. Different platforms have been proposed to take benefit from the broad transparency of Ge up to 15 μm, and the main passive building blocks are now being developed. In this review, we will present the most relevant Ge-based platforms reported so far that have led to the demonstration of several passive and active building blocks for mid-IR photonics. Seminal works on mid-IR optical sensing using integrated platforms will also be reviewed.

III-V on CaF2: a possible waveguiding platform for mid-IR photonic devices

“…Germanium-on-SiO2 [12,32,33] and Germanium-on-CaF2 [34] for example. In most cases, materials are processed to achieve an intimate, direct adhesion between the active semiconductor region and the host substrate, mimicking the ideal case which would come from a direct epitaxial growth.…”

28 Gbps silicon-germanium hetero-structure avalanche photodetectors

“…Ge thus provides singular advantages and complementarities over pure Si platforms. Group-IV nanophotonics therefore enable complex on-chip functionalities [6][7][8][9][10] , from light generation and modulation to waveguiding and light detection [11][12][13][14] .…”

Ultra-wideband Ge-rich silicon germanium mid-infrared polarization rotator with mode hybridization flattening

“…Commercially available mid-IR spectroscopic systems are bulky and expensive and integrated solutions are being investigated to develop miniaturized, low cost, mass produced and broadband systems. In this context, numerous platforms have been investigated: III-V materials [5,6], chalcogenide glasses [7,8] and a family of Si compatible platforms such as silicon on insulator (SOI) [9], suspended Si [10], Si on sapphire [11,12], Germanium (Ge) on Si [13][14][15][16], Silicon Germanium (SiGe) [17,18] and Ge-rich SiGe on graded buffer [19][20][21][22][23][24][25][26]. A plethora of photonic components has been implemented on these platforms: waveguides, cavities [7], Mach-Zehnder interferometers and spectrometers [21,24], (de)multiplexers [14], reconfigurable circuits [15] and modulators [27], to name a few.…”