Measuring photon bunching at ultrashort timescale by two-photon absorption in semiconductors

Abstract

Photon beams exhibit temporal correlations that are characteristics of their emission mechanism. For instance, photons issued from incoherent sources tend to be detected in bunches. This striking 'bunching' behaviour has been observed in the seminal experiment by Hanbury-Brown and Twiss (HBT) in the fifties, who measured the time of arrival of partially coherent photons on two separate photon-counting modules 1. Since then, HBT interferometry has become a widespread technique to study photon correlations down to only the nanosecond range, because of the detector-limited bandwidth, preventing the observation of bunching for real thermal sources. It has been suggested later that two-photon absorption (TPA) could measure the photon temporal correlations at a much shorter timescale 2,3 , as it involves an almost simultaneous absorption of two photons, within a maximum delay given by the Heisenberg principle. Here, for the first time, this prediction is experimentally demonstrated using TPA in a GaAs photon-counting module. We have observed photon bunching in the femtosecond range for real blackbody sources (an enhancement of six orders of magnitude in the time resolution of present techniques), opening the way to monitor optical quantum statistics at the ultrashort timescale. The Hanbury-Brown and Twiss (HBT) experiment actually measures the degree of second-order coherence (DSOC) g (2) (τ) = I (t) I (t + τ) / I (t) 2 of optical fields, where I (t) is the intensity of the fields, τ is the optical path time difference between the two detectors and designates statistical averaging. A straightforward calculation, in which the fields are treated in terms of classical fluctuating electromagnetic waves, shows that g (2) (0) is always larger than 1 and is equal to 2 for chaotic sources 3. The experiment was first carried out in the radiofrequency domain, using radiotelescopes, where the undulatory nature of light is unquestionable. It was later extended by the two authors to the optical domain, where they measured a value of g (2) (0) = 1.93 using photocounters and a narrowband partially coherent light source (a 546.1 nm Hg discharge lamp followed by an interference filter) 4. Although this result can still be interpreted classically in wave-like terms, its interpretation in terms of light corpuscles-more suited to photon counters-is more subtle, as it implies that photons emitted by incoherent sources are correlated: this is the so-called 'photon bunching effect'. Actually, the HBT experiment was a great surprise at that time and fostered a lot of controversy in the literature 5-8. Later, Glauber 7 developed a unifying quantum theory of optical coherence. Using a quantum approach of the LETTERS NATURE PHYSICS

Two-photon interference with non-identical photons

“…These photons are distinguishable for the detection system based on photoelectric effect and no second-order interference pattern can be observed. The time measurement uncertainty of two-photon absorption is at femtosecond range [46]. Photons with frequency difference less than 10 6 GHz are indistinguishable for the detection system.…”

Shaping the Ultrafast Temporal Correlations of Thermal-Like Photons

“…They showed that the coherence area was related to the angular size of the radiation source [7]. Interestingly, a similar effect also appears in the temporal domain [8], i.e., for thermal-like sources, the photons appear correlated for time instants shorter than the inverse of the bandwidth of the source. On the contrary, for fermions [9] and photons showing sub-Poisson statistics [10], the effect is just the opposite.…”

“…However, the measurement of second-and higher-order coherence functions of broadband light sources is especially challenging since, due to their large bandwidths, ultrafast (typically in the femtosecond regime) detection schemes are needed [13]. In fact, it was not until very recently that the second-order coherence properties of true thermal photons were experimentally observed [8].…”

Photon-Statistics Excitation Spectroscopy of a Quantum-Dot Micropillar Laser

“…While usually the spectral and temporal properties of light fields are considered to be of highest interest, especially for studies of coherence properties, the quantum statistics of light fields matter as well [1][2][3]. During the last few years, experimental techniques aimed at studies of photon number statistics have improved significantly [4][5][6][7][8][9][10][11], as has the fundamental understanding of photon number statistics [12]. Recently, some emphasis has also been placed upon using the quantum statistics of the excitation light field as a degree of freedom in spectroscopy [13][14][15][16] and quantum-optical spectroscopy has recently been applied to reveal significant physical effects and new quasiparticles, such as dropletons [17,18].…”