Pump-probe System Research Articles

Broadly tunable ultrafast pump-probe system operating at multi-kHz repetition rate

Femtosecond systems based on ytterbium as active medium are ideal for driving ultrafast optical parametric amplifiers in a broad frequency range. The excellent stability of the source and the repetition rate tunable to up to hundreds of kHz allow for the implementation of an advanced two-color pump probe setup with the capability to achieve excellent signal-to-noise performances with sub-10 fs temporal resolution.

Laser switching and characterisation of chalcogenides: systems, measurements, and applicability to photonics [Invited

The objective of this paper is to review the characterisation methods and procedures used to laser switch phase change materials, and then assess their applicability for characterising phase change materials for active photonics devices. Specifically we characterise the performance of our pump-probe laser system and compare it with other ‘static’ and ‘dynamic’ testers. Our pump-probe system was developed to measure the phase transformation kinetics of chalcogenide films by simultaneously measuring the transmission and reflection of a probe laser with a temporal resolution of 1 ns. We also use the system to measure the second order nonlinear refractive index of chalcogenide thin films. Laser switching of chalcogenides are efficient methods to screen new materials but the switching time seems to have a strong dependence on the measurement method and procedure. Therefore in this article we recommend bespoke methods and procedures for assessing the performance of new chalcogenide compositions for specific photonic devices.

Developments and enhancements to the HELIOS pump probe system

In this progress report we describe several design improvements that have been implemented at the HELIOS laboratory, as well as presenting the output characteristics that have been measured as a result. The main focus will be on the redesign of the gas cell, which has enhanced the photon flux of the XUV probe beam. Also, a frequency trippler utilizing sum frequency generation has been installed at the end of the pump line, which increases the photon flux available for both 3.1eV (400nm) and 4.66eV (266nm) applications without significant increment in the pulse width of the pump.

Multi-level storage and ultra-high speed of superlattice-like Ge50Te50/Ge8Sb92 thin film for phase-change memory application

Superlattice-like Ge50Te50/Ge8Sb92 (SLL GT/GS) thin film was systematically investigated for multi-level storage and ultra-fast switching phase-change memory application. In situ resistance measurement indicates that SLL GT/GS thin film exhibits two distinct resistance steps with elevated temperature. The thermal stability of the amorphous state and intermediate state were evaluated with the Kissinger and Arrhenius plots. The phase-structure evolution revealed that the amorphous SLL GT/GS thin film crystallized into rhombohedral Sb phase first, then the rhombohedral GeTe phase. The microstructure, layered structure, and interface stability of SLL GT/GS thin film was confirmed by using transmission electron microscopy. The transition speed of crystallization and amorphization was measured by the picosecond laser pump-probe system. The volume variation during the crystallization was obtained from x-ray reflectivity. Phase-change memory (PCM) cells based on SLL GT/GS thin film were fabricated to verify the multi-level switching under an electrical pulse as short as 30 ns. These results illustrate that the SLL GT/GS thin film has great potentiality in high-density and high-speed PCM applications.

Three-dimensional wide-field pump-probe structured illumination microscopy.

We propose a new structured illumination scheme for achieving depth resolved wide-field pump-probe microscopy with sub-diffraction limit resolution. By acquiring coherent pump-probe images using a set of 3D structured light illumination patterns, a 3D super-resolution pump-probe image can be reconstructed. We derive the theoretical framework to describe the coherent image formation and reconstruction scheme for this structured illumination pump-probe imaging system and carry out numerical simulations to investigate its imaging performance. The results demonstrate a lateral resolution improvement by a factor of three and providing 0.5 µm level axial optical sectioning.

The ultrafast tunable saturable absorption of metal complexes containing redox-active 1-(2-pyridylazo)-2-acenaphthequinol ligands

The extraordinary ultrafast tunable saturable absorption properties of metal complexes M(PAAL)2 (M = Co, Ni, Cu, Zn) containing redox-active 1-(2-pyridylazo)-2-acenaphthequinol ligands (PAAL) were investigated by Z-scan and pump-probe system. The obtained third-order nonlinear optical (NLO) absorption and refraction coefficient of each metal complex reach ∼10−2 cm/GW and ∼10−6 cm2/GW at 510 nm wavelength, respectively. Meanwhile, an ultrafast carrier relaxation process of approximately (200 ± 50) fs is observed for all of them. Furthermore, we found that the third-order nonlinear absorption coefficient of M(PAAL)2 is proportional to the central metal proton number. All the NLO properties indicate that those metal complexes posses potential applications for fabricating photonic devices.

Photoelectron detection from transient species in organic semiconducting thin films by dual laser pulse irradiation

An Nd3+:YAG pulsed laser was employed as a light source for two-photon photoemission from organic semiconducting thin films in low vacuum and air. Photoionization by the two-photon process was confirmed in both the environments by measuring photoemission current. By constructing a pump–probe system, photoemissions from transient species formed by the pump light irradiation were detected by probe light irradiation as a result of a linear increase in the photocurrent with the pump power via a one-photon process. Thus, we propose a novel method called two-photon photoelectron yield spectroscopy to determine the excited-state energy levels in ambient environments.

Wide-bandgap nonlinear crystal LiGaS2 for femtosecond mid-infrared spectroscopy with chirped-pulse upconversion.

Femtosecond time-resolved mid-infrared (MIR) spectroscopy based on chirped-pulse upconversion is a promising method for observing molecular vibrational dynamics. A quantitative study on nonlinear media for upconversion is still essential for wide applications, particularly at the frequencies below 2000 cm-1. We evaluate wide-bandgap nonlinear crystals of Li-containing ternary chalcogenides based on their performance as the upconversion medium for femtosecond MIR spectroscopy. The upconversion efficiency is measured as a function of the MIR pulse frequency and the chirped pulse energy. LiGaS2 is found to be an efficient crystal for the upconversion of MIR pulses in a wide frequency range of 1100-2700 cm-1, especially below 2000 cm-1. By using LiGaS2 as an efficient upconversion crystal, we develop a MIR pump-probe spectroscopy system with a spectral resolution of 2.5 cm-1, a time resolution of 0.2 ps, and a probe window of 120 cm-1. Vibrational relaxation dynamics of CO stretching modes of Mn2(CO)10 in cyclohexane and bovine serum albumin in D2O are demonstrated with a high signal-to-noise ratio.

Improvement of phase change properties of stacked Ge2Sb2Te5/ZnSb thin films for phase change memory application

Ge2Sb2Te5/ZnSb (GST/ZS) stacked thin films were proposed for high density phase change memories (PCM). Electrical and structural properties were studied by in-situ resistance measurements and X-ray diffraction (XRD), respectively. The films exhibited good thermal stability and two resistance steps during heating process. A picosecond laser pump-probe system was used to measure phase change speed. Phase change memory cells based on [GST(35nm)/ZS(15nm)]1 thin film were fabricated to test and verify multi-level switch between set and reset states.

Broadband pump-probe spectroscopy at 20-MHz modulation frequency.

We introduce an innovative high-sensitivity broadband pump-probe spectroscopy system, based on Fourier-transform detection, operating at 20-MHz modulation frequency. A common-mode interferometer employing birefringent wedges creates two phase-locked delayed replicas of the broadband probe pulse, interfering at a single photodetector. A single-channel lock-in amplifier demodulates the interferogram, whose Fourier transform provides the differential transmission spectrum. Our approach combines broad spectral coverage with high sensitivity, due to high-frequency modulation and detection. We demonstrate its performances by measuring two-dimensional differential transmission maps of a carbon nanotubes sample, simultaneously acquiring the signal over the entire 950-1350 nm range with 2.7·10-6 rms noise over 1.5 s integration time.

Heat damage evaluation in carbon fiber-reinforced composites with a kHz A-scan rate fiber-optic pump-probe laser-ultrasound system

Composites are increasingly replacing metals for a number of applications where their reduced weight and equivalent strength compared to metals is extremely important. As composite structures become more routine, non-destructive tools are needed to monitor their structural integrity when subjected to different damage mechanisms, such as impact and thermal insults. Here we present a non-contact laser-ultrasound (LU) method to quantify and image structural changes in carbon fiber reinforced polymer (CFRP) composites induced by thermal insult. The method uses ultra-wideband acoustic signals generated at the surface of a composite material with a laser pulse (i.e., pump), and non-contact, point-like detection of backscattered transients (i.e., probe). A comparison of results from the LU system with X-ray images of thermally insulted samples shows that high-resolution LU images (better than 1 ply) and spectral analysis of backscattered signals can identify small structural changes in composites long before delamination.

Spectral Features and Charge Dynamics of Lead Halide Perovskites: Origins and Interpretations.

Lead halide perovskite solar cells are presently the forerunner among the third generation solution-processed photovoltaic technologies. With efficiencies exceeding 20% and low production costs, they are prime candidates for commercialization. Critical insights into their light harvesting, charge transport, and loss mechanisms have been gained through time-resolved optical probes such as femtosecond transient absorption spectroscopy (fs-TAS), transient photoluminescence spectroscopy, and time-resolved terahertz spectroscopy. Specifically, the discoveries of long balanced electron-hole diffusion lengths and gain properties in halide perovskites underpin their significant roles in uncovering structure-function relations and providing essential feedback for materials development and device optimization. In particular, fs-TAS is becoming increasingly popular in perovskite characterization studies, with commercial one-box pump-probe systems readily available as part of a researcher's toolkit. Although TAS is a powerful probe in the study of charge dynamics and recombination mechanisms, its instrumentation and data interpretation can be daunting even for experienced researchers. This issue is exacerbated by the sensitive nature of halide perovskites where the kinetics are especially susceptible to pump fluence, sample preparation and handling and even degradation effects that could lead to disparate conclusions. Nonetheless, with end-users having a clear understanding of TAS's capabilities, subtleties, and limitations, cutting-edge work with deep insights can still be performed using commercial setups as has been the trend for ubiquitous spectroscopy instruments like absorption, fluorescence, and transient photoluminescence spectrometers. Herein, we will first briefly examine the photophysical processes in lead halide perovskites, highlighting their novel properties. Next, we proceed to give a succinct overview of the fundamentals of pump-probe spectroscopy in relation to the spectral features of halide perovskites and their origins. In the process, we emphasize some key findings of seminal photophysical studies and draw attention to the interpretations that remain divergent and the open questions. This is followed by a general description into how we prepare and conduct the TAS characterization of CH3NH3PbI3 thin films in our laboratory with specific discussions into the potential pitfalls and the influence of thin film processing on the kinetics. Lastly, we conclude with our views on the challenges and opportunities from the photophysical perspective for the field and our expectations for systems beyond lead halide perovskites.

Gain-assisted superluminal light propagation through a Bose-Einstein condensate cavity system

The propagation of a probe laser field in a cavity optomechanical system with a Bose-Einstein condensate is studied. The transmission properties of the system are investigated and it is shown that the group velocity of the probe pulse field can be controlled by Rabi frequency of the pump laser field. The effect of the decay rate of the cavity photons on the group velocity is studied and it is demonstrated that for small values of the decay rates, the slope of phase dispersion switches from positive to negative just by changing the Rabi frequency of the pump field. Then, the gain-assisted superluminal light propagation due to the cross-Kerr nonlinearity is established in cavity optomechanical system with a Bose-Einstein condensate. Such behavior can not appear in the pump-probe two-level atomic systems in the normal phase. We also find that the amplification is achieved without inversion in the population of the quantum energy levels.

First Terahertz-range Experiments on Pump – Probe Setup at Novosibirsk free Electron Laser

A single-color pump-probe system has been commissioned at the Novosibirsk free electron laser. The laser emits a tunable monochromatic terahertz radiation. To prove the proper system operation, we investigated the time-resolved absorption of a sample of n-type germanium doped with antimony, which was previously investigated at the FELBE facility, in the temperature range from 5 to 40 K. The measured relaxation time amounted to about 1.7 ns, which agreed with the results obtained at the FELBE. The results of pump-probe measurements of non-equilibrium dynamics of hot electrons in the germanium crystal at cryogenic temperatures are presented for wavelengths of 105, 141 and 150 μm.

Superlattice-like SnSb4/Ga3Sb7 thin films for ultrafast switching phase-change memory application

The carrier concentration of Sb-rich phase SnSb4, Ga3Sb7 and superlattice-like [SnSb4(3.5 nm)/Ga3Sb7(4 nm)]7 (SLL-7) thin films as a function of annealing temperature was investigated to explain the reason of resistance change. The activation energy for crystallization was calculated with a Kissinger equation to estimate the thermal stability. In order to illuminate the transition mechanisms, the crystallization kinetics of SLL-7 were explored by using Johnson–Mehl–Avrami theory. The obtained values of Avrami indexes indicate that a one-dimensional growth-dominated mechanism is responsible for the set transition of SLL-7 thin film. X-ray diffractometer and Raman scattering spectra were recorded to investigate the change of crystalline structure. The measurement of atomic force microscopy indicated that SLL-7 thin film has a good smooth surface. A picosecond laser pump-probe system was used to test and verify phase-change speed of the SLL-7 thin film.

High speed and high reliability in Ge8Sb92/Ga30Sb70 stacked thin films for phase change memory applications

Stacked Ge8Sb92/Ga30Sb70 films exhibited better thermal stability. Smaller volume change (2.21%) before and after phase change than Ge2Sb2Te5 (GST). Crystallization process of the [Ge8Sb92(25 nm)/Ga30Sb70(25 nm)]1 thin film was within 11 ns, which was measured by a picosecond laser pump-probe system. Reversible transition of the phase change memory cell based on the [Ge8Sb92(25 nm)/Ga30Sb70(25 nm) ]1 can be achieved by using an electrical pulse of as short as 50 ns.

A 1 kHz A-scan rate pump-probe laser-ultrasound system for robust inspection of composites.

We recently built a fiber-optic laser-ultrasound (LU) scanner for nondestructive evaluation (NDE) of aircraft composites and demonstrated its greatly improved sensitivity and stability compared with current noncontact systems. It is also very attractive in terms of cost, stability to environmental noise and surface roughness, simplicity in adjustment, footprint, and flexibility. A new type of a balanced fiber-optic Sagnac interferometer is a key component of this all-optical LU pump-probe system. Very high A-scan rates can be achieved because no reference arm or stabilization feedback are needed. Here, we demonstrate LU system performance at 1000 A-scans/s combined with a fast 2-D translator operating at a scanning speed of 100 mm/s with a peak acceleration of 10 m/s(2) in both lateral directions to produce parallel B-scans at high rates. The fast scanning strategy is described in detail. The sensitivity of this system, in terms of noise equivalent pressure, was further improved to be only 8.3 dB above the Nyquist thermal noise limit. To our knowledge, this is the best reported sensitivity for a noncontact ultrasonic detector of this dimension used to inspect aircraft composites.

Ultrabroadband Midinfrared Pump-Probe Spectroscopy Using Chirped-Pulse Up-conversion in Gases

Ultrafast dynamics in molecules, nanostructures, and interfaces are commonly studied using pump-probe spectroscopy. The bandwidth for this approach has been limited to about 1000 cm${}^{-1}$ in the midinfrared region. The authors develop an upconversion technique for femtosecond-scale detection, and demonstrate an ultrabroadband pump-probe system with a spectral range of 200---5000 cm${}^{-1}$. Their system dramatically improves access to the energy regions of interest for investigating the relaxation of optically excited systems as diverse as proteins, liquid water, semiconductors, and topological insulators.

Fiber-coupled high-speed asynchronous optical sampling with sub-50 fs time resolution.

We present a fiber-coupled pump-probe system with a sub-50 fs time resolution and a nanosecond time window, based on high-speed asynchronous optical sampling. By use of a transmission grism pulse compressor, we achieve pump pulses with a pulse duration of 42 fs, an average power of 300 mW and a peak power exceeding 5 kW at a pulse repetition rate of 1 GHz after 6 m of optical fiber. With this system we demonstrate thickness mapping of soft X-ray mirrors at a sub-nm thickness resolution on a cm(2) scan area. In addition, terahertz field generation with resolved spectral components of up to 3.5 THz at a GHz frequency resolution is demonstrated.

Upconversion luminescence mechanisms of Er(3+) ions under excitation of an 800 nm laser.

The fundamental processes of upconversion (UC) emissions from trivalent lanthanide ions are essential for designing and improving the performance of UC materials. However, UC mechanisms involve multiple processes in one system because of the abundant energy states, which make it a challenge to exclusively verify and quantitatively evaluate the dominant process. In this paper, the mechanisms of green and red emissions in Er(3+) doped NaYF4-glass ceramics under 800 nm continuous wave laser excitation are studied via excitation power-dependence, polarization-dependence, and the transient evolution. These UC emission properties are compared with those under single-/two-wavelength excitation in a pump-probe system of 800 nm and 1500 nm femtosecond lasers. These results indicate that the excited state absorption pathway of (4)I15/2 → (4)I9/2 → (4)I13/2 → (2)H11/2/(4)S3/2 plays the dominant role in the emission of the green band, and the cross relaxation pathway of (4)I11/2 + (4)I13/2 → (4)F9/2 + (4)I15/2 plays the main role in the emission of the red band.