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2021 ◽  
Vol 11 (1) ◽  
Author(s):  
D. Bajek ◽  
M. A. Cataluna

AbstractWe demonstrate, for the first time, optical sampling by repetition-rate tuning (OSBERT) at record megahertz scan rates. A low-cost, tunable and extremely compact 2-section passively mode-locked laser diode (MLLD) is used as the pulsed laser source, whose repetition rate can be modulated electronically through biasing of the saturable absorber section. The pulsed output is split into two arms comparable to an imbalanced Michelson interferometer, where one arm is significantly longer than the other (a passive delay line, or PDL). The resulting electronic detuning of the repetition rate gives rise to a temporal delay between pulse pairs at a detector; the basis for time-resolved spectroscopy. Through impedance-matching, we developed a new system whereby a sinusoidal electrical bias could be applied to the absorber section of the MLLD via a signal generator, whose frequency could be instantly increased from sub-hertz through to megahertz modulation frequencies, corresponding to a ground-breaking megahertz optical sampling scan rate, which was experimentally demonstrated by the real-time acquisition of a cross-correlation trace of two ultrashort optical pulses within just 1 microsecond of real time. This represents scan rates which are three orders of magnitude greater than the recorded demonstrations of OSBERT to date, and paves the way for highly competitive scan rates across the field of time-resolved spectroscopy and applications therein which range from pump probe spectroscopy to metrology.


2021 ◽  
Vol 188 (12) ◽  
Author(s):  
Lifang Gao ◽  
Xu Zhang ◽  
Runlin Yang ◽  
Zhongwei Lv ◽  
Wenge Yang ◽  
...  

Author(s):  
Jongchan Pyeon ◽  
Joseph Aroh ◽  
Runbo Jiang ◽  
Amit K. Verma ◽  
Benjamin Gould ◽  
...  

2021 ◽  
Author(s):  
◽  
Aleksa Djorović

<p>Optoelectronic materials and devices, such as LEDs and solar cells, are ubiquitous in the modern, technologically driven world. Understanding the fundamental physical process in optoelectronic materials is essential for the design and development of new devices which are more efficient, cheaper, printable, as well as environmentally friendly. Two particularly important material properties for device performance are charge mobility and photoconductivity, as they increase charge separation and extraction efficiencies, and thus give specific insight into device efficiency. The best suited technique for measuring mobility and conductivity on ultrafast timescales is Terahertz spectroscopy. Terahertz spectroscopy is a non-invasive, contact-free probe of the mobility of charges in optoelectronic materials. Terahertz time-domain spectroscopy allows for the direct determination of the entire complex-valued conductivity. As a result, important optical properties such as the complex refractive index and dielectric function of a material can be measured directly. The short duration of THz pulses, on the order of 1 ps, also allows for time-resolved studies of the transient photoconductivity in optically-excited materials with sub-picosecond time resolution, i.e. Time-Resolved Teraherz Spectroscopy (TRTS). Traditionally, only the peak of the THz pulse signal is measured with TRTS, due to the time constraints of a two-delay experiment. This does not allow for frequency-resolved THz spectra. As a result, it discards a lot of the information Terahertz-TDS spectroscopy contains, as well as its advantages over other spectroscopic techniques. Frequency-resolved TRTS would allow for the calculation of transient conductivity at each pump-probe delay time and can differentiate between signals of excitons and free charge carriers. This would allow for robust interpretations of charge mobility in novel materials. However, frequency-resolved TRTS is not practically feasible in a dual-delay configuration. We develop in this thesis a novel single-shot method based on angle-to-time mapping of a rotating probe. This method is applied to build a single-shot Terahertz-TDS spectrometer. A transmissive grating applies pulse front tilt which allows for the measurement of the entire THz transient (over a 5.7 ps window) in a single laser shot on a CMOS multichannel detector, thus alleviating the need for delay stage sampling of the THz transient, and leading to a reduction of experimental time by several orders of magnitude. An optical pump excitation is incorporated to allow a time-resolved measurement (TRTS) of the entire terahertz time-domain spectrum, and thus frequency-resolved TRTS. We show qualitative agreement between the THz time domain spectra obtained with the single shot technique and the standard free-space electro-optic (EO) sampling with balanced photodiodes, with an order of magnitude increased signal sensitivity. A proof-of-concept single shot TRTS study of a Si semiconductor sample is also given, showing we are able to resolve the TRTS signal of the entire THz pulse in a single shot, in time. This technique allows us to obtain significantly more information than traditional TRTS methods without any compromise in experiment time. However we find that the implemented single shot technique seems to suffer at higher frequencies (above 2 THz), which must be addressed to confirm the viability of a full spectrum single shot TRTS experiment. Further improvements, such as tighter focusing of the THz radiation, must be made to both the single-shot spectrometer as well as to the optical pump, for a quantitative single shot measurement. However, the proof-of-concept results in this thesis prove frequency-resolved TRTS is viable by using the developed single-shot detection method. As such it directly allows a novel spectroscopic tool which can lead to new insights into charge mobilities in optoelectronic materials, and may encourage wider application of TRTS.</p>


Author(s):  
Dennis Mayer ◽  
Fabiano Lever ◽  
Markus Gühr

Abstract The random nature of self-amplified spontaneous emission (SASE) is a well-known challenge for x-ray core level spectroscopy at SASE free-electron lasers (FELs). Especially in time-resolved experiments that require a combination of good temporal and spectral resolution the jitter and drifts in the spectral characteristics, relative arrival time as well as power fluctuations can smear out spectral-temporal features. We present a combination of methods for the analysis of time-resolved photoelectron spectra based on power and time corrections as well as self-referencing of a strong photoelectron line. Based on sulfur 2p photoelectron spectra of 2-thiouracil taken at the SASE FEL FLASH2, we show that it is possible to correct for some of the photon energy drift and jitter even when reliable shot-to-shot photon energy data is not available. The quality of pump-probe difference spectra improves as random jumps in energy between delay points reduce significantly. The data analysis allows to identify coherent oscillations of 1 eV shift on the mean photoelectron line of 4 eV width with an error of less than 0.1 eV.


2021 ◽  
Vol 54 (6) ◽  
Author(s):  
Ranjana Rathore ◽  
Himanshu Singhal ◽  
Ajmal Ansari ◽  
Juzer Ali Chakera

Ultra-short laser-pulse-induced strain propagation in a Ge crystal is studied in the [111] and [100] directions using time-resolved X-ray diffraction (TXRD). The strain propagation velocity is derived by analysis of the TXRD signal from the strained crystal planes. Numerical integration of the Takagi–Taupin equations is performed using open source code, which provides a very simple approach to estimate the strain propagation velocity. The present method will be particularly useful for relatively broad spectral bandwidths and weak X-ray sources, where temporal oscillations in the diffracted X-ray intensity at the relevant phonon frequencies would not be visible. The two Bragg reflections of the Ge sample, viz. 111 and 400, give information on the propagation of strain for two different depths, as the X-ray extinction depths are different for these two reflections. The strain induced by femtosecond laser excitation has a propagation velocity comparable to the longitudinal acoustic velocity. The strain propagation velocity increases with increasing laser excitation fluence. This fluence dependence of the strain propagation velocity can be attributed to crystal heating by ambipolar carrier diffusion. Ge is a promising candidate for silicon-based optoelectronics, and this study will enhance the understanding of heat transport by carrier diffusion in Ge induced by ultra-fast laser pulses, which will assist in the design of optoelectronic devices.


2021 ◽  
Author(s):  
◽  
Aleksa Djorović

<p>Optoelectronic materials and devices, such as LEDs and solar cells, are ubiquitous in the modern, technologically driven world. Understanding the fundamental physical process in optoelectronic materials is essential for the design and development of new devices which are more efficient, cheaper, printable, as well as environmentally friendly. Two particularly important material properties for device performance are charge mobility and photoconductivity, as they increase charge separation and extraction efficiencies, and thus give specific insight into device efficiency. The best suited technique for measuring mobility and conductivity on ultrafast timescales is Terahertz spectroscopy. Terahertz spectroscopy is a non-invasive, contact-free probe of the mobility of charges in optoelectronic materials. Terahertz time-domain spectroscopy allows for the direct determination of the entire complex-valued conductivity. As a result, important optical properties such as the complex refractive index and dielectric function of a material can be measured directly. The short duration of THz pulses, on the order of 1 ps, also allows for time-resolved studies of the transient photoconductivity in optically-excited materials with sub-picosecond time resolution, i.e. Time-Resolved Teraherz Spectroscopy (TRTS). Traditionally, only the peak of the THz pulse signal is measured with TRTS, due to the time constraints of a two-delay experiment. This does not allow for frequency-resolved THz spectra. As a result, it discards a lot of the information Terahertz-TDS spectroscopy contains, as well as its advantages over other spectroscopic techniques. Frequency-resolved TRTS would allow for the calculation of transient conductivity at each pump-probe delay time and can differentiate between signals of excitons and free charge carriers. This would allow for robust interpretations of charge mobility in novel materials. However, frequency-resolved TRTS is not practically feasible in a dual-delay configuration. We develop in this thesis a novel single-shot method based on angle-to-time mapping of a rotating probe. This method is applied to build a single-shot Terahertz-TDS spectrometer. A transmissive grating applies pulse front tilt which allows for the measurement of the entire THz transient (over a 5.7 ps window) in a single laser shot on a CMOS multichannel detector, thus alleviating the need for delay stage sampling of the THz transient, and leading to a reduction of experimental time by several orders of magnitude. An optical pump excitation is incorporated to allow a time-resolved measurement (TRTS) of the entire terahertz time-domain spectrum, and thus frequency-resolved TRTS. We show qualitative agreement between the THz time domain spectra obtained with the single shot technique and the standard free-space electro-optic (EO) sampling with balanced photodiodes, with an order of magnitude increased signal sensitivity. A proof-of-concept single shot TRTS study of a Si semiconductor sample is also given, showing we are able to resolve the TRTS signal of the entire THz pulse in a single shot, in time. This technique allows us to obtain significantly more information than traditional TRTS methods without any compromise in experiment time. However we find that the implemented single shot technique seems to suffer at higher frequencies (above 2 THz), which must be addressed to confirm the viability of a full spectrum single shot TRTS experiment. Further improvements, such as tighter focusing of the THz radiation, must be made to both the single-shot spectrometer as well as to the optical pump, for a quantitative single shot measurement. However, the proof-of-concept results in this thesis prove frequency-resolved TRTS is viable by using the developed single-shot detection method. As such it directly allows a novel spectroscopic tool which can lead to new insights into charge mobilities in optoelectronic materials, and may encourage wider application of TRTS.</p>


2021 ◽  
Vol 104 (5) ◽  
Author(s):  
Sucharita Giri ◽  
Jean Christophe Tremblay ◽  
Gopal Dixit

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