Michael Ware

Single-photon detector characterization using correlated photons: The march from feasibility to metrology

Abstract Correlated photons can be used to directly measure the detection efficiency of photon counting detectors without any ties to externally calibrated standards. An overview of the history of this technique is given and the paper reviews how to implement it in a practical lab setting. Some of the sources of uncertainty in the technique and how they can be minimized and quantified are discussed. The intent is to provide the information necessary to encourage the movement of this technique from the metrology lab into the general photon-counting detector community.

Direct observation of laser filamentation in high-order harmonic generation

We investigate the spatial evolution of a laser pulse used to generate high-order harmonics (orders ranging from 45 to 91) in a semi-infinite helium-filled gas cell. The 5 mJ, 30 fs laser pulses experience elongated focusing with two distinct waists when focused with f/125 optics in 80 Torr of helium. Extended phase matching for the generation of harmonics occurs in the region between the double foci of the laser, where the laser beam changes from diverging to converging.

On the measurement of two-photon single-mode coupling efficiency in parametric down-conversion photon sources

Photon-based quantum information schemes have increased the need for light sources that produce individual photons, with many such schemes relying on optical parametric down-conversion (PDC). Practical realizations of this technology require that the PDC light be collected into a single spatial mode defined by an optical fibre. We present two possible models to describe single-mode fibres coupling with PDC light fields in a non-collinear configuration, that lead to two different results. These approaches include factors such as crystal length and walk-off, non-collinear phase-matching and transverse pump field distribution. We propose an experimental test to distinguish between the two. The goal is to help clarify open issues, such as how to extend the theory beyond the simplest experimental arrangements and, more importantly, to suggest ways to improve the collection efficiency.

Role of group velocity in tracking field energy in linear dielectrics.

A new context for the group delay function (valid for pulses of arbitrary bandwidth) is presented for electromagnetic pulses propagating in a uniform linear dielectric medium. The traditional formulation of group velocity is recovered by taking a narrowband limit of this generalized context. The arrival time of a light pulse at a point in space is defined using a time expectation integral over the Poynting vector. The delay between pulse arrival times at two distinct points consists of two parts: a spectral superposition of group delays and a delay due to spectral reshaping via absorption or amplification. The use of the new context is illustrated for pulses propagating both superluminally and subluminally. The inevitable transition to subluminal behavior for any initially superluminal pulse is also demonstrated.

Calibrating photon-counting detectors to high accuracy: background and deadtime issues

When photon-counting detectors are calibrated in the presence of a background signal, deadtime effects can be significant and must be carefully accounted for to achieve high accuracy. We present a method for separating the correlated signal from the background signal that appropriately handles deadtime effects. This method includes consideration of pulse timing and afterpulsing issues that arise in typical avalanche photodiode (APD) detectors. We illustrate how these effects should be accounted for in the calibration process. We also discuss detector timing issues that should be considered in detector calibration.

Energy transport in linear dielectrics

We examine the energy exchanged between an electromagnetic pulse and a linear dielectric medium in which it propagates. While group velocity indicates the presence of field energy (the locus of which can move with arbitrary speed), the velocity of energy transport maintains strict luminality. This indicates that the medium treats the leading and trailing portions of the pulse differently. The principle of causality requires the medium to respond to the instantaneous spectrum, the spectrum of the pulse truncated at each new instant as a given locale in the medium experiences the pulse.

Role of the instantaneous spectrum on pulse propagation in causal linear dielectrics.

A model-independent theorem demonstrates how a causal linear dielectric medium responds to the instantaneous spectrum, that is, the spectrum of the electric field pulse that is truncated at each new instant (as a given locale in the medium experiences the pulse). This process leads the medium to exchange energy with the front of a pulse differently than with the back as the instantaneous spectrum laps onto or off of nearby resonances. So-called superluminal pulse propagation in either absorbing or amplifying media as well as highly subluminal pulse propagation are understood qualitatively and quantitatively within this context.

Measured laser-beam evolution during high-order harmonic generation in a semi-infinite gas cell.

We report on direct measurements of self-guiding of 800 nm, 30 fs, 5 mJ laser pulses used to generate high-order harmonics in 80 torr helium. We track the spatial evolution of the laser pulses as they propagate several centimeters near the focus under conditions suitable for harmonic generation. The laser is observed to focus, diverge, and refocus. This behavior is accompanied by a flattop beam profile. Both of these features are absent when the laser is focused in vacuum. We also observed a 4 nm spectral blue shift in the center of the laser beam near the focus in contrast with no spectral shift at wider radii.

Measured Optical Constants of Copper from 10 nm to 35 nm

We use laser high-order harmonics and a polarization-ratio-reflectance technique to determine the optical constants of copper and oxidized copper in the wavelength range 10-35 nm. This measurement resolves previously conflicting data sets, where disagreement on optical constants of copper in the extreme ultraviolet most likely arises from inadvertent oxidation of samples before measurement.

High-accuracy calibration of photon-counting detectors

We discuss a practical implementation of a photon-counting detector calibration using correlated photon pairs produced by parametric down-conversion. In this calibration scheme, the detection of a first photon triggers the measurement sequence aimed at detection of a second photon by a detector under test (DUT). We also describe measurements of radiant power with a photon-counting detector, which is important for implementation of a conventional calibration technique based on detector substitution. In the experiment, we obtain a time-delay histogram of DUT detection events consisting of a correlated signal and a background. We present a method for separating the correlated signal from the background signal that appropriately handles complex properties of typical avalanche photodiode (APD) detectors. Also discussed are measurements of relevant APD properties, including count-rate-dependent afterpulsing, delayed (by up to 10 ns) electronic detections and deadtime effects. We show that understanding of these is essential to perform an accurate calibration.