Resonant cavities for efficient LT-GaAs photoconductors operating at λ = 1550 nm

Abstract

We show that photoconductors based on low-temperature-grown GaAs (LT-GaAs) can be efficiently operated by 1.55 μm telecom wavelength by using metallic mirror based optical cavities. Two different semi-transparent front mirrors are compared: the first one is a thin gold layer, whereas the second one consists of a gold grating. Light absorption in grating mirror based optical cavities is numerically, analytically, and experimentally investigated allowing for an appropriate optical design. We show a 3 times improvement of the LT-GaAs photoconductor photoresponse by using, as front mirror, the gold grating once compared with the thin gold layer. It reaches around 0.5 mA/W under continuous wave, whereas a transient photoresistivity (Ron) as low as 5 Ω is deduced from dc photocurrents measured under femtosecond pulsed laser excitation. This work paves the way to efficient and reliable optoelectronics systems for GHz or THz waves sampling driven by 1.55 μm pulsed lasers widely available.

Low‐temperature‐grown gallium arsenide photoconductors with subpicosecond carrier lifetime and photoresponse reaching 25 mA/W under 1550 nm CW excitation

“…This was designed using a rigorous coupled‐wave analysis (RCWA) in order to excite a Fabry–Pérot (FP) resonance for t = 450 nm under 1550 nm illumination (polarisation parallel to the grating direction as shown Fig. 1 a ) [7].…”

“…1 a was obtained by transferring the LT‐GaAs epitaxial layers onto 2‐in.‐diameter silicon wafer. This is done by using an Au–Au thermocompression layer bonding technique [7]. The gold grating was patterned on the LT‐GaAs layer by using electron lithography, electron beam evaporation, and lift‐off techniques.…”

“…The photoresponse experiment was conducted using a fibre coupled external cavity laser emitting at λ = 1550 nm and focused through a cleaved fibre (for S 2 , S 4 ) or a lensed fibre (for S 1 , S 3 ) into a Gaussian spot of mode field diameter, w ≈10.5 µm and w ≈2.5 µm, respectively. It is worth noting that the spot width variation is unlikely to affect the results of this study since it has been previously shown that under CW illumination, in linear absorption regime, the spot diameter does not influence the photoresponse of the optical cavity photoconductors as far it is smaller than the active area [7]. The polarisation is adjusted by means of a fibre polarisation controller in order to align the electric field orthogonally to the grating electrodes.…”

“…We have recently shown that LT‐GaAs ultrafast photoconductors can operate at λ = 1550 nm, despite a photon energy E ph ≈ 0.75 eV, i.e. lower than the energy gap of GaAs ( E g = 1.42 eV), by placing the LT‐GaAs layer inside an optical resonant cavity [7]. This photoconductor has been then successfully used to sub‐sample continuous waves at frequencies up to 300 GHz, demonstrating a sub‐picosecond response time of photocurrents generated by 1550 nm illumination [8].…”

“…This photoconductor has been then successfully used to sub-sample continuous waves at frequencies up to 300 GHz, demonstrating a sub-picosecond response time of photocurrents generated by 1550 nm illumination [8]. However, the much higher dark resistivity of LT-GaAs material (>10 3 kΩ cm) in comparison with LT-InGaAs/InAlAs multilayers material [9] or Fe-doped InGaAs layers [10] (<2 kΩ cm) employed in 1550-nm ultrafast photoconductors is far from compensating the low photoresponse despite the use of an optical cavity (≈ 1 mA/W under CW illumination [7]).…”