Techniques

Time Resolved Photo-Luminescence (TRPL) is an experimental technique that provides the spectral and temporal evolution of the emission of a sample following its illumination by a short pulse of light. More precisely, the short pulse of light generates electron-hole pairs that decay to lower energy levels of the sample. These electron-hole pairs can subsequently recombine and emit light. The emitted light is composed of a set of wavelengths corresponding to transition energies of the sample and, as a result, the measurement of the optical spectrum as a function of time provides a means to measure the transition energies and their lifetimes. Since these decay times are on the order of picoseconds or nanoseconds, and the intensity of light emitted can be very weak, a conventional spectrum analyser cannot provide the resolution required. Instead, it is necessary to use a device known as a streak camera.

The streak camera is an ultra-high-speed detector that captures light emission phenomena occurring in extremely short time periods. The light pulse to be measured is projected onto the slit and is focused by a lens into an optical image on the photocathode, able to cover a wavelength range between 300 nm and 1500 nm. Here, the photons are converted into a stream of electrons proportional to the intensity of the incident light. As the electron stream created from the light pulse passes between a pair of sweep electrodes, a time-varying voltage is applied to the electrodes, resulting in a high-speed sweep. This means that the early part of the pulse is deflected less than the later part of the pulse, so that different parts of the pulse strike the micro channel plate (MCP) at different positions. Thus, the temporal structure of the pulse is converted into a spatial distribution, or ‘streak’, pattern. As the electrons pass the MCP, they are multiplied several thousands of times and are then bombarded against the phosphor screen, where they are converted back into light. The fluorescence image corresponding to the early part of the incident light pulse is positioned at the top of the phosphor screen, with later parts positioned in descending order; in other words, the axis in the perpendicular direction on the phosphor screen serves as the temporal axis. The brightness of the fluorescence image is proportional to the intensity of the corresponding incident light pulses and the position in the horizontal direction on the phosphor screen corresponds to the wavelength of the incident light.

To perform TRPL experiments, the sample under investigation is mounted on a cold finger in the vacuum cell of a closed cycle helium cryostat (operating from 7 K to 300 K), and is illuminated with very fast (picosecond or femtosecond) laser pulses. The optical response is then collected by a microscope objective and is sent to the entrance slit of an imaging spectrograph, directly connected with the input optic of a streak camera.

The typical spectrum collected by a TRPL experiment is called a streak image. It depicts the intensity of the optical response of the sample (colour scale) as a function of the wavelength (horizontal axis) and the time (vertical axis). One such example concerns the emission dynamics of an InGaAs/GaAs quantum well (QW) at 8 K. A 780 nm laser pulse excites the InGaAs QW above the band gap of the material (~890 nm at this temperature), generating photo-carriers in the GaAs barrier. From here, they flow into the well, and recombine to emit light. As the number of carriers is depleted, the intensity of the emitted light decays. The carrier dynamics occurring during these processes provides not only information about the position of the peak emission, as in a normal PL experiment, but also the lifetime of the exactions involved in the recombination. In fact, considering vertical or horizontal cuts of the streak image, the trends of the intensity of the emission as a function of time or wavelength, respectively, can be extracted. In this particular example, for InGaAs/GaAs QW, the peak position is measured at 888 nm and the decay time, after a single exponential fitting, is estimated to be 0.7 ns.

The Frequency Resolved Electro-Absorption Gating (FREAG) technique gives complete recovery of spectral and temporal information on ultrafast laser pulses in real time, and is similar to, but far more sensitive than, Frequency Resolved Optical Gating (FROG). FREAG pulse analysers work on the same principle as a Second Harmonic Generation (SHG) auto correlator, but use linear transmission through a modulated gate, contrasting with the non-linear crystal required for SHG auto correlators. The result is the same, accurate recovery of real and imaginary parts of the electric field but without requiring high intensity input.

FREAG is very sensitive and can resolve sub-picosecond pulses since it is not limited by the response time of the detector. In addition to this, at each delay position of the gate a complete spectrum is measured, so that both the spectral and temporal characteristics of the pulse are captured simultaneously. Therefore, while a 10Gb/s data stream may look perfectly normal on a conventional oscilloscope, the FREAG will show all the changes in the underlying carrier phase in real-time, allowing complete and rapid optimisation of the system.

Semiconductor mode-locked lasers can spontaneously emit picosecond light pulses at high rates, making them valuable candidates for applications such as optical time division multiplexing. Traditional methods of pulse characterisation such as autocorrelation require high laser intensities and often lose critical pulse information. For example, autocorrelation always returns symmetric traces, regardless of the true pulse shape. With FREAG, a trigger is derived from the laser pulses and used to open and shut an optical gate. The remaining laser light is passed through this gate and a set of laser spectra gathered as a function of pulse delay with respect to the gate. This spectrogram can then be inverted and the full laser intensity and phase information recovered. With this tool, several pulse stabilisation techniques such as dual-tone optical injection have been proven and outstanding pulse jitter performance achieved.

Photonic sensors are key elements in a wide range of applications: security, safety, space, defence, healthcare, multimedia, scientific instruments and others. Photonic sensors provide the means to evaluate samples in-situ obtaining direct molecular specific information through the measurement of rotational-vibrational transitions in the target analytes enabling thus direct qualitative and quantitative analysis without the need to chemically prepare or damage the sample in question. Such sensors are rapid, they can be configured in a portable format and avoid the use of consumables required by most established modern analytical techniques.

Optical sensors thus provide ideal solutions for Process Analytical Technology (PAT) applications as increasingly requested by the chemical and diverse food industries Quantum Photonics. The efficient generation of quantum states of light is a vital task in Quantum Photonics. Current approaches are bulky and expensive with low generation rates and the few commercial single photon sources are either not compatible with telecoms standards, require cryogenic temperatures or are bulky benchtop devices.