Midinfrared Plasmon-Enhanced Spectroscopy with Germanium Antennas on Silicon Substrates

Abstract

Midinfrared plasmonic sensing allows the direct targeting of unique vibrational fingerprints of molecules. While gold has been used almost exclusively so far, recent research has focused on semiconductors with the potential to revolutionize plasmonic devices. We fabricate antennas out of heavily doped Ge films epitaxially grown on Si wafers and demonstrate up to 2 orders of magnitude signal enhancement for the molecules located in the antenna hot spots compared to those located on a bare silicon substrate. Our results set a new path toward integration of plasmonic sensors with the ubiquitous CMOS platform.

Highly doped semiconductor plasmonic nanoantenna arrays for polarization selective broadband surface-enhanced infrared absorption spectroscopy of vanillin

“…Enhancement factors from 2900 to 14,000 are obtained, as indicated in Table 1. This is one to two orders of magnitude higher than what has been reported for Ge nanoantennas, exploiting additionally a hotspot in a gap between nanoantennas [24] and is in the range of enhancement factors for noble metal nanoantennas found in other studies which range from 1000 to 100,000 depending on the antenna shape, material and arrangement [48]. As already mentioned, the proposed semiconductor nanoantenna array in the investigated geometrical configuration does not exploit confinement in gaps or the lightning rod effect which is stronger at high aspect ratio nanoantennas, so that there is potential to reach even higher enhancement factors when these more sophisticated geometries are employed.…”

“…While numerous studies have been carried out on metallic nanoantennas, either as individual objects [12][13][14][15][16][17], or organized in arrays [1,10,[18][19][20][21], semiconductor plasmonic nanoantenna surfaces have rarely been investigated [22], especially in ordered array arrangements [23][24][25]. Highly doped semiconductors (HDSC) have been introduced as engineered metals or "designed metals" for plasmonics [22,23] as they offer the possibility of tuning the plasma frequency, an intrinsic parameter for traditional metals, via the doping level.…”

n-Ge on Si for mid-infrared plasmonic sensors

“…1 demonstrates the increase in the wavenumber (decrease in wavelength) of the plasmon edge as the doping density increased for n-Ge epilayers of thicknesses between 600 nm and 1000 nm [2][4] [5]. The lowest wavelength of ∼ 3.1 µm was achieved for an activated doping density of 2.1×10 20 cm −3 at 300 K. This is sufficient to enable electron beam lithography and reactive ion etching from the 1000 nm thick n-Ge material [4] into arrays over a 5 × 5 mm 2 array. The n-Ge thickness is comparable to the skin depth in the 10 to 20 µm wavelength region.…”

“…2 provides a scanning electron microscope (SEM) image of a set of gap antennas. Antennas with lengths from 1 to 4 µm were produced since the substrate side plasmonic resonance can be tuned through varying the antenna length allowing the targeting of specific molecular absorption lines for identification [4].…”

Germanium nanoantennas for plasmon-enhanced third harmonic generation in the mid infrared

“…This technology paves the way for mid-infrared nanoplasmonics with application in integrated telecommunication systems and enhanced molecular sensing in the so-called vibrational fingerprint spectral region [1].…”