High-efficiency Detection of Single Photons at Picosecond Resolution for use at Telecommunications Wavelengths

Abstract

summary lime-correlated single photon counting (TCSPC) is now the procedure of choice for many applications that depend on the detection of low level andlor fast optical signals. For example. use of photon counting in picosecond time-resolved photoluminescence (TRF'L) measurements on semiconductor material and devices has led to the study of carrier dynamics at low photogenerated carrier densities (e.g. ilOcmJ) [I], Aim. quantum cryptography [21. which enables the secure distribution of cryplographic keys across telecnmmunication networks relies on sending the key at the photon per bit level (or less an average). 'These Syslems rrqulre detectors to possess picosecond time resolulion and high detection eiiiciency. AS a consequence, single photon avalanche diodes (SPALYs) are now widely usd for single photon detection in place of photomultipiier tubes at wavelengths below lvm. A SPAD is a reversed biased diode operated at fields above breakdown. such that a single carrier can iniliale a selkustaining avalanche. Silicon SPAD's are now commercially available with good mom temperature performance at wavelengths below 1.1 pm. The detection of photons between 1.1 vm and 1.6pm with high efticiency is not possible with silicon detwtors. Thii wavelength range is of panicular imponance for both quantum cryptographic demonsueton and TapL systems. as well as several other applications. The transmission range of a quantum cryplography system across a telecommunication network can te subrtanlklly increased by using photons of wavelength 1.3 pm and 1.55 pm because ofthe low atsorption within silica-based optical fibre at these specific wavelengths. To date. wc have primarily used Ge-based SPAD's over the wavelenglh range 1.1 pm to 1.50 pm. albeit at 77K to reduce thermally generated dark events Ill. The necessary cryogenic operation of such Ge SPAD's has prnduaed a signkant shift in the absorption edge, such that only very low quantum efficiencies can be obtained at the important 1.5.5 pm wavelength (i.c.c<l%). This has led to investigations of alternative long wavelength SPAD detectors. fabricated from InGaAsnnP. which show greater potential for operation at wavelengths of 1.55vm or above. Results to be presented include a comprehensive characterisation of InGaAs/InP SAGM SPAD's operated as photon counling dewtors and TRPL measurements of semiconductor marcrials using these devices. A discussion of the design. fabrication and characterisation of InCidAsLlnP deviccs specif~ally for photon-counting use will also be presented.