A low-cost scheme of high-speed asynchronous optical sampling based on Yb:KYW oscillators is reported. Two GHz diode-pumped oscillators with a slight pulse repetition rate offset serve as pump and probe source, respectively. The temporal resolution of this system is limited to 500 fs mainly by the pulse duration of the oscillators and also by relative timing jitter between the oscillators. A near-shot-noise noise floor around 10−6 (∆R/R) is obtained within a data acquisition time of a few seconds. The performance of the system is demonstrated by measurements of coherent acoustic phonons in a semiconductor sample that resembles a semiconductor saturable absorber mirror or an optically pumped semiconductor chip.
© 2017 Optical Society of America
With the advent of ultrafast laser technology, time-domain spectroscopy has been extensively used to explore ultrafast phenomena in a wide variety of materials, e.g., crystals , metals [2,3], semiconductors [4–6] and gases [7,8]. Ultrafast dynamics is of great importance to a better understanding of phenomena such as heat transfer and phase transition in semiconductors [9,10], electronic excitation in crystals and electron-phonon coupling in graphene [11,12], chemical reactions and solvation processes in chemistry [13,14], or revealing cell properties in biology and medical science [15,16]. In time-domain spectroscopy, ultrafast processes in the sample can be excited and monitored by ultrashort femtosecond laser pulses. The photo-induced properties changes such as reflectivity and transmission are measured in dependency of the time delay between pump and probe pulses. In conventional pump-probe spectroscopy [4,6,17], pump and probe pulses are split off from a single laser source and the time delay between them is accomplished by a mechanical translation stage which leads to limited scan rates, usually a few tens of Hertz for time windows larger than 100 ps, spot size variations and beam-pointing stability issues. Asynchronous optical sampling (ASOPS) uses two ultrafast lasers with a small repetition rate offset, which overcomes the above shortcomings mainly due to the absence of a mechanical delay line. In early picosecond pump-probe experiments based on ASOPS, timing jitter did not have a critical effect on temporal resolution [18,19]. However, for sub-picosecond resolved experiments, timing jitter induced by the laser repetition rate drift is not negligible and active stabilization of the repetition rate offset is required [15,20–22]. For sub-gigahertz repetition rate ASOPS systems [15,20,21], the theoretical high temporal resolution limited only by the pulse duration is achieved at the cost of low scan rates. A 1-GHz ASOPS system based on Ti:sapphire lasers has been developed to allow laser-pulse-duration-limited temporal resolution at a scan rate of a few kilohertz by means of an offset-locking method working at the tenth harmonic of the repetition rate .
Concerning ultrafast time-resolved spectroscopy, Ti:sapphire lasers have been favorable due to their broad spectral bandwidth, wavelength tunability and high output power [4,23,24]. However, since high-power single-mode semiconductor laser diodes emitting at around 980 nm became commercially available as pump source, lasers based on ytterbium-doped gain media such as Yb:KYW, Yb:KGW, Yb:CALGO have been extensively investigated [25–27] for multiple reasons. First of all, they bring considerable cost reduction. Compared to the immense costs of suitable pump sources such as Nd:YVO laser, the total costs of the diode pump laser (including diode, temperature controller and driver, etc.) are only 1/10 of that. Secondly, the system can be built quite compactly. The pump sources and cooling system for Ti:sapphire laser roughly take a few times as much space as the pump sources for ytterbium-doped lasers take. Thirdly, they are scalable in terms of output power and pulse energy by available ytterbium-based amplifiers . These lasers could be promising alternatives for the bulky, power-consuming and expensive green-pumped Ti:sapphire lasers which have been used in ASOPS pump-probe measurements until now. In addition, due to low quantum defects and high thermal conductivities of the available host material, no active crystal cooling is needed and high optical-to-optical conversion efficiencies can be achieved. One drawback of Yb:KYW based systems, however, is the smaller spectral bandwidth compared to Ti:sapphire based systems, which limits the achievable pulse durations. In our previous research , a combination of a Ti:sapphire oscillator and an Yb:KYW oscillator was used to investigate coherent acoustic phonons in a semiconductor superlattice with a temporal resolution of less than 350 fs. Two identical Yb:KYW lasers with slight repetition rate offset have attracted much attention in ASOPS and dual-frequency-comb spectroscopy. Time-resolved pump-probe spectroscopy based on Yb:KYW oscillators with 50-MHz repetition rate has been applied to high-resolution imaging of a single cell via picosecond ultrasonic . A pair of Yb:KYW oscillators with 100-MHz repetition rate has also been developed and could be applied to incoherent ASOPS and dual comb spectroscopy in mid-infrared regime by using an optical parametric oscillator . Kerr-lens mode-locked Yb:KYW oscillators [25,28] are preferred rather than semiconductor saturable absorber mirror (SESAM) mode-locked oscillators [26,29] in our ASOPS experiments due to the simple ring design and the possibility to scale the repetition rate to the 10 GHz level .
Here, we report on a high-speed ASOPS system that employs two nearly identical Kerr-lens mode-locked diode-pumped Yb:KYW oscillators with 1-GHz repetition rate and multi-kilohertz repetition rate offset. This system allows pump-probe spectroscopy over a 1-ns temporal window with a temporal resolution of 500 fs in a cost-efficient and compact configuration. An acoustic phonon measurement on a semiconductor sample verifies the capability of our system by precise non-destructive structure inspection.
2. High-speed Yb:KYW ASOPS system
The high-speed ASOPS system based on Yb:KYW oscillators is schematically illustrated in Fig. 1. Two home-made diode-pumped Kerr-lens mode-locked Yb:KYW lasers with 1-GHz repetition rate serve as pump laser and probe laser, respectively. The repetition rate offset of the oscillators is locked by a stabilization unit (TL-1000-ASOPS, Laser Quantum GmbH) working at the 10th harmonic of repetition rates which are detected by two 12.5-GHz photodiodes. Both lasers are pumped by fiber-Bragg grating stabilized single-mode fiber-coupled diode lasers with a maximum output power of 750 mW and a central wavelength of 980 nm (3SPGroup). The resonators of both oscillators are based on the same ring cavity consisting of four mirrors and an Yb:KYW crystal (EKSMA OPTICS) with a doping concentration of 10 at. % and a thickness of 1 mm as described in . In order to achieve pure mode-locking, we opt for two Gires-Tournois-Interferometer (GTI) mirrors with a combined group delay dispersion of −2200 fs2 and an output coupler mirror with a transmission of 0.6% for the pump Yb:KYW laser. In order to send feedback to the cavity for the repetition rate offset stabilization, a slow piezoelectric actuator (4-μm amplitude) and a fast piezoelectric actuator (0.2-µm amplitude) are attached to the output coupler and the plane GTI mirror in the pump Yb:KYW laser, respectively. Both the pump and probe Yb:KYW lasers emit at a central wavelength of around 1050 nm with a maximum output power of 260 mW and 310 mW, respectively. The measured pulse duration under the assumption of a squared hyperbolic secant shape by interferometric auto-correlation is 210 fs for the pump laser and 280 fs for the probe laser.
A small amount of power (8 mW) is split from both lasers for photodiodes PD1 and PD2 by pellicles. The remaining power is distributed into the trigger branch (the lower part in Fig. 1) and the measurement branch (the middle part in Fig. 1). To generate the trigger signal, the transmitted beams from both the pump laser and probe laser are split by half wave plates (HWPs) and polarizing beam splitters (PBSs) and then focused by lenses L1 and L2 both with a focal length of 100 mm onto a 1-mm BBO crystal. The trigger signal is based on sum frequency generation by using around 50-mW power of each beam. Compared to using a two photon absorption (TPA) cross-correlation signal for triggering which leads to only 40% of the total power for measurements , this method preserves considerably more power (around 80% of the total power) for measurements. The trigger signal is then detected by an AC-coupled high-speed photodiode (PR130, Laser Quantum GmbH) after a short-pass dielectric filter. Both the rest pump power and probe power are sent to the measurement branch. The power-tuned pump beam and probe beam are focused by lens L3 with a focal length of 30 mm and lens L4 with a focal length of 50 mm, respectively, and then non-collinearly overlapped on the sample. The probe beam at around 30° incident angle is detected by a 125-MHz AC-coupled photodiode (FS-1811, NewFocus). The detected transient reflectivity change of the sample is acquired by a 100-MHz A/D converter.
3. High-speed Yb:KYW ASOPS characterization
Important parameters for the time-resolved pump-probe spectroscopy system are temporal resolution and detection sensitivity. Firstly, the temporal resolution of the Yb:KYW ASOPS system is measured by TPA. A 200-µm GaP crystal is inserted at the position of the sample, so that the transient transmission of the crystal versus time delay can be shown as a TPA cross-correlation pulse, which represents a convolution of the pump pulse and the probe pulse. The pulse width of the TPA signal is measured over the whole 1-ns time window by inserting a retro reflector mounted on a variable translation stage in one arm of the trigger branch. The obtained TPA signals at a repetition rate offset ∆f = 5 kHz are displayed in Fig. 2(a). The first pulse indicates a temporal resolution of 405 fs regarding a cross-correlation signal with a squared-hyperbolic secant fit. For ∆f = 5 kHz, as shown in Fig. 2(b), the temporal resolution is increased by around 100 fs over the whole 1-ns time window due to relative timing jitter between two oscillators. The calculated pulse duration of convoluted probe pulse and pump pulse is 380 fs according to the measured pulse durations in section 2, which is in rough agreement with the pulse width of the first convoluted TPA pulse. The gap between the TPA signal pulse duration and the calculated convoluted pulse duration can be explained by the different group delay dispersion in auto-correlation measurements and TPA cross-correlation measurements. Less temporal resolution broadening resulting from timing jitter can be realized by locking the probe laser to a reference optical source. The temporal resolution around 325 fs limited by bandwidth-limited pulse widths could be achieved if dispersion compensation process is conducted. The further improvement of temporal resolution can be achieved by replacing the Yb:KYW crystals with Yb:CALGO crystals which have an emission bandwidth of around 80 nm .
It is important to examine the noise performance for our Yb:KYW lasers, because any noise above shot noise is detrimental for the measurement result. In the noise characterization, an AC-coupled photodiode with a bandwidth of 125 MHz and a cut-off frequency of 25 kHz is used to measure the noise floor of the probe laser. The repetition rate offset ∆f is set to 5 kHz. The noise floor of the probe laser is measured without pump beam incident on the sample and the incident probe power is 4.5 mW. The noise floor is obtained from the standard deviation of time traces. The data acquisition time of a single trace takes around 0.4 ms. The noise floor of the probe laser is slightly above shot noise by a factor of 1.2 as illustrated in Fig. 3(a). By increasing the number N of averaged time traces, the noise floor decreases by a factor of N-1/2. The noise floor is below 10−6 (∆R/R) after around 104 averages which takes a few seconds. The noise power spectral density obtained from time traces is also presented to identify the noise components in the frequency domain. In the detection range from 25 kHz to 100 MHz, as shown in Fig. 3(b), the noise level is reduced by averaging but is still slightly above shot noise. No evident spectral spikes are observed, which can be explained by the low-pass filter effect of Yb:KYW crystal with a long lifetime of the upper laser level (~0.3 ms). As can be seen, pump-induced noise is not present in the time and spectral domain, therefore, our system is capable of conducting ASOPS measurements at the level of 10−6 (∆R/R) in a few seconds which is limited by shot-noise.
In the following we will show the capabilities of the system by measurements of coherent acoustic phonons in semiconductor structures.
4. High-speed Yb:KYW ASOPS experiment
Coherent acoustic phonons have been extensively investigated in quantum wells (QWs) [32,33], semiconductor superlattices [4,24] and thin films [2,34] by means of ultrafast pump-probe spectroscopy. Furthermore, they prove to be very useful in nanoscale applications such as nanostructure imaging , non-destructive inspection , phonon reflectors  and acoustic resonators . A SESAM is a device typically consisting of an embedded QW layer, a semiconductor cap layer and a distributed Bragg reflector (DBR). Folded coherent phonons in the part of DBR induced by impulsive stimulated Raman scattering [4,37] have been detected by an ASOPS system based on Ti:sapphire lasers . Due to strong absorption of near-infrared light in QWs such as InGaAs, Yb:KYW lasers are well suited for the direct excitation of phonons in QWs that mainly result from deformation potential  and piezoelectric screening [39,40].
In our Yb:KYW ASOPS measurement, 100-mW pump power and 4.5-mW probe power are incident on the semiconductor multilayer structure in a reflection geometry and the repetition rate offset is set to 5 kHz. The semiconductor sample that resembles a SESAM or an optically pumped semiconductor (OPS) chip under test [41,42], shown in Fig. 4, is composed of a GaAs cap layer followed by three QW-stacks of QW1, QW2, and QW3 (each one consists of three In0.27Ga0.73As layers alternated with GaAs layers) which are separated by GaAs layers, a DBR consisting of 22 pairs of Al0.95Ga0.05As/GaAs and a GaAs substrate. The bandgap of bulk In0.27Ga0.73As (1.05 eV) is below the pump photon energy (1.18 eV), which causes pump beam absorption mainly in the QWs and thus acoustic waves are excited in every QW it reaches. The absorption edge is shifted to higher energies with respect to the bulk bandgap due to the quantization energies of electrons and holes by about 0.125 eV for the strained QW (calculated by nextnano GmbH), with a small reduction of a few meV through excitonic effects. Hence the on-set of absorption is slightly below the pump energy. In order to find out which parts of the sample act as sources for acoustic phonons, we use an absorption coefficient of 3.4 × 103 cm−1 for bulk In0.27Ga0.73As at 1050 nm , which results in a corresponding optical penetration depth of around 2.9 µm. Since the GaAs layers between the QWs are transparent at the pump wavelength, the pump beam excites all QWs and the detected signal is the superposition of acoustic phonons from all QWs in the sample. Considering the bandgaps of GaAs (1.42 eV) and Al0.95Ga0.05As (2.15 eV) are far above the photon energy of the pump beam, acoustic phonons are not expected from the DBR superlattice.
As shown in the inset of Fig. 5(a), the absorption of the pump pulse leads to a transient relative reflectivity change as high as 10−3. The prominent peak at zero time delay stems from the ultrafast excitation of the electrons in the QW. Subsequent relaxation by electron-electron and electron-phonon scattering leads to thermalization and thus a slow decaying behaviour. The electron excitation causes an impulsive stress generation by the deformation potential mechanism and thus excites coherent acoustic phonons [6,44]. After subtracting the electronic and thermal background, it is possible to resolve the superimposed acoustic phonon oscillations which have an amplitude on the order of 10−6 as presented in Fig. 5(a). The corresponding FFTs of the extracted signal and the second wave packet are depicted in Fig. 5(b). We observe several wavepackets in the first 160 ps, as presented in Fig. 5(c). Each wavepacket contains several oscillations with an equal delay of τ = 2.70 ps and neighbouring wavepackets are equally delayed by T = 30.25 ps. The first wavepacket only shows two peaks rather than three peaks, because the transient signal in the first 3 ps is discarded during background subtraction to avoid fitting artefacts. The FFT of the subtracted signal in Fig. 5(b) indicates a phonon feature centred at 365 GHz which is composed of discrete combs spaced by ∆fphonon = 33 GHz and exhibits an envelope that is mainly given by the FFT of the second wavepacket. The phonon bandwidth is around 120 GHz limited by the thickness of the QW-stack.
We will give a qualitative explanation of the observed signal in the following which already yields detailed information about the sample structure. An acoustic pulse is initiated in each QW by the pump pulse, as depicted in Fig. 6(a), which can be explained by a loaded-string model . Thus each QW-stack consisting of three QWs launches a pulse burst consisting of three pulses in backward (red line) and forward (green line) directions as depicted in Fig. 6(b). The acoustic reflection on the interface of GaAs and In0.27Ga0.73As is neglected due to the small reflection coefficient of 1.8% induced by the small acoustic impedance mismatch. The detection of these pulses is then mainly limited to the QW regions themselves due to the strong interaction of the probe light with the sample in these areas, which means the transient reflectivity of probe pulses is modulated by acoustic phonons in QWs . Because the QW-stacks are equidistant from each other, after propagating away from their respective source QW-stacks, the acoustic pulse bursts will arrive at the same time at the neighbouring QW-stacks. The observed wavepacket-like structure can then be explained by the convolution of the incoming strain pulses with the QW-stacks themselves taking into account the effect of the strain pulses on the barrier layers between the QWs. This simple model allows us to extract structural information about the sample.
Given acoustic sound velocities at 300 K in GaAs in the orientation (100) vGaAs = 4730 m/s  and in In0.27Ga0.73As vInGaAs = 4487 m/s , the distance between neighbouring quantum wells in a QW stack is given by dqw = veff × τ = 12.4 nm, which agrees well with the nominal value 13 nm for the structure. The delay T between wavepackets indicates the distance of neighbouring QW-stacks. Together with the longitudinal sound velocity vGaAs, it enables us to obtain the neighbouring QW-stacks distance as dstack = vGaAs × T = 143 nm, which shows excellent agreement with the nominal stack period of 145 nm.
In conclusion, we have presented a high-speed ASOPS system based on two femtosecond Yb:KYW oscillators with 1-GHz repetition rate. Compared to the system using Ti:sapphire oscillators, this system has benefits in cost-efficiency and compactness due to employment of fiber-coupled pump diodes. The measured temporal resolution for an offset of 5 kHz is around 500 fs limited mainly by the pulse duration of the two oscillators, which could be further improved by implementing dispersion compensation. The measured noise floor is below 10−6 (∆R/R) after an acquisition time of a few seconds, which is limited by shot noise. The first pump-probe experiment based on the high-speed GHz Yb:KYW ASOPS system is performed on a sample that resembles a semiconductor saturable absorber mirror in order to demonstrate the capabilities of the system. Coherent acoustic phonons are generated as a series of wavepackets at a central frequency of 365 GHz with combs spaced by 33 GHz, which agrees well with the structure of the sample. The 1050-nm central wavelength of Yb:KYW oscillators allows us to specifically address InGaAs quantum wells in these multi-layer structures, which holds the potential to tailor acoustic pulses by appropriate structure design.
This research is financially supported by the Center for Applied Photonics at the University of Konstanz (CAP-02). It is partially supported by the Deutsche Forschungsgemeinschaft (DFG) through SFB 767. It is also supported by Chinese Scholarship Council (CSC).
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