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A new ultrafast all-optical solid-state framing camera (UASFC) capable of single-shot ultrafast imaging is proposed and experimentally demonstrated. It is composed of an ultrafast semiconductor chip (USC), an optical time-series system (TSS), and a spatial mapping device (SMD) with an USC to transform signal beam information to the probe beam, a TSS to convert the time axis to wavelength-polarization, and a SMD to map wavelength-polarization image to different spatial positions. In our recent proof-of-principle experiment, better performance than ever of this technique is confirmed by giving six frames with ~3 ps temporal resolution and ~30 lp/mm spatial resolution.
© 2017 Optical Society of America
1. Introduction
To investigate the process of the ultrafast transient dynamics at temporal resolution from picoseconds to nanoseconds is of great importance in many areas such as high energy density physics [1,2], laser fusion [3], plasma radiation [4], and so on [5–8]. To satisfy these demands, various ultrafast imaging equipment [9–12] have been developed and commercialized over recent decades. The most typical representatives of commercialized ultrafast cameras are streak cameras [13,14] and framing cameras [15,16]. The streak camera, which transforms the temporal profile of an incident signal into a spatial profile on an image sensor, enables single-shot imaging with a temporal resolution of ~200 fs, but its obtained images are limited to 1D. On the other hand, current microstrip traveling-wave gating-framing cameras demonstrate 2D ultrafast imaging capabilities. However, their temporal and spatial resolutions are limited to ~30 ps and ~25 μm, respectively, due to transit time, transit time spread, and spatial dispersion of electrons through the MCP [17,18]. In scientific applications, the most popular method is the time-resolved pump-probe technique [19]. The key is the construction of a time-resolved motion picture from repetitive measurements, with different time delays between the pump and probe beam. This technology can capture the dynamics of fast events with a much shorter frame interval, ~100 fs, than any conventional camera, requiring that the event is repetitive. This method is unable to capture difficult-to-reproduce or non-repetitive random events. Several unique optical imaging methods for high-speed imaging have been reported, for instance, serial time-encoded amplified microscopy (STEAM) [10,20], sequentially timed all-optical mapping photography (STAMP) [9,21], compressed ultrafast photography (CUP) [22], and lossless-encoding compressed ultrafast photography (LLE-CUP) [23], that achieve a frame rate and exposure time far beyond what can be reached with conventional imaging sensors. STEAM enables continuous image acquisition at a high frame rate of ~100 Mfps with high sensitivity. STAMP is an all-optical method, and can achieve an ultrahigh frame rate of ~1 Tfps. CUP achieves two-dimensional (2D) receive-only image acquisition at a frame rate of ~100 Gfps. These high-speed optical imaging methods represent breathtaking innovations and have been used to capture plasma dynamics, lattice vibrational waves, microfluidic flow, and phase-explosion effects. However, STEAM and STAMP are active imaging technologies and they cannot measure an ultrafast self-emission process. LLE-CUP has advanced the CUP performance to 100 billion frames per second with single camera exposure, the spatiotemporal sparsity may still be the limiting factor.
In addition to these technologies, a novel solid-state framing camera was reported by K. L. Baker’s group [24–26] and tested in the X-ray regime [27]. In this technique, the imaged X-ray source illuminates the semiconductor sensor chip and produces a transient refractive index change in the semiconductor. Concurrently, a birefringent time-encoding probe beam samples the transient index change of the semiconductor; by using polarization division multiplexing (PDM) with a beam displacer, the two orthogonally polarized probe beams are spatially separated and relay imaged onto different positions of a charge coupled device (CCD) detector. In their experiment, the temporal resolution of the camera reached up to ~5 ps, which is almost the fastest reported for X-ray ultrafast cameras. However, the total number of imaging frames for this method was limited to two, and the recording time was similarly limited. At the same time, because of the ~ns response time of the “integrating” CdSe detector used in this camera, the measured results are a cumulative integral of the X-ray signal. Thus, the signal must be differentiated in time to extract the real-time X-ray signal.
In this letter, an ultrafast all-optical solid-state framing camera (UASFC) is proposed and experimentally demonstrated. In an UASFC, a low-temperature-grown (LT) GaAs/AlGaAs multiple quantum wells (MQWs) is used as the semiconductor sensor. The response time of the semiconductor determined by the electron-hole pair creation and relaxation time is as short as 2.5 ps, so our framing camera is an impulsive detector rather than an integrating detector [28]. The real-time X-ray signal is proportional to the square root of the instantaneously measured intensity [28]. The input probe beam is time-encoded by TSS combined with linearly chirped and birefringent time-delay technologies; meanwhile, the output probe beam is mapped from the temporal region into the spatial region using a SMD consisting of wavelength division multiplexing (WDM) and polarization division multiplex (PDM) technologies. Therefore, the UASFC can record more frames—about 100—and for much longer time—about hundreds of picoseconds. To experimentally demonstrate the principles of the UASFC, TSS is consisted of a SF10 disperser glass and an A-cut TiO2 crystal, accordingly, SMD is combined using three different narrow-band optical filters, a beam splitter, and a beam displacer. An experimental result of six frames with ~3 ps temporal resolution and ~30 lp/mm spatial resolution is demonstrated in the visible range.
2. Principle and experiment
2.1 Principle of UASFC
The structure of the UASFC is shown in Fig. 1. A signal beam from an external source impinges upon the ultrafast semiconductor chip (USC) and generates a transient phase grating in the USC. Meanwhile, a femtosecond laser pulse with broadband spectrum is time-encoded with a time-series system (TSS) and is incident on the USC. Thus, the probe beam is modulated by the USC. The image-encoded probe beam is spatially filtered to transform the phase variations of the probe beam to amplitude changes using a 4f Fourier filter (F-F) system. Then the output probe beam is optically separated using the spatial mapping device (SMD) and directed towards different CCD cameras or different positions on one CCD camera. The details for the TSS are shown in Fig. 1(b). It consists of two components: the temporal disperser and the delay crystal. The temporal disperser—which could be disperse glass, grating pairs, prism pairs, fiber Bragg gratings, or any combination of these, depending on the time duration of the observed event—is used to generate a linearly chirped pulse. The delay crystal—such as an A-cut TiO2 crystal or a YVO4 crystal, depending on the central wavelength of the femtosecond laser—is used to produce a relative delay between the two orthogonal polarizations. The temporal resolution and the recording time of the UASFC can be tuned by adjusting the TSS settings. The specifics of the USC are shown in the lower left inset of Fig. 1(a). The USC consists of a metallic grating, a high-reflection film (HRF) for the probe beam, an ultrafast semiconductor, and an anti-reflection film (ARF) for the probe beam. The HRF is added to the ultrafast semiconductor to reflect the probe beam and a metallic binary grating is etched onto the HRF to modulate the incident signal beam. Then, the transmitted signal pattern absorbed in the USC creates electron-hole pairs, which change the index of refraction of USC in proportion to the intensity of the illumination. This generates a transient phase grating in the USC [25]. Two SMD types are shown in Fig. 1(c). The first consists of a beam displacer, which is used to spatially separate the two orthogonally polarized probe beams. The other is composed of beam splitters and narrow bandwidth filters to image different wavelengths onto different CCD cameras or different areas of a single CCD camera. The UASFC frames can be increased by adding CCD cameras and filters. However, the bandwidth of the optical filter directly affects the framing number of the UASFC. For example, when the probe beam has a 30 nm bandwidth and 2 nm narrow bandwidth filter is used, the UASFC can record 30 frames at most. To acquire more frames, the SF-STAMP [21] method can be used in conjunction with UASFC. With an appropriate design, the number of frames can reach as high as 100.
Fig. 1 Functionality of the UASFC. (a) Principle of the UASFC (b) Details of the UASFC’s TTS system. The time axis can be encoded into wavelength or polarization by chirped and birefringent technology (c) Details of the SMD system. The time axis can be decoded using WDM and PDM technology. Labels in the Fig. 1 are: time-series system (TSS), beam splitter (BS), F-F (Fourier Filter), spatial mapping device (SMD), ultrafast semiconductor chip (USC), binary grating (BG), high-reflection film (HRF), anti-reflection film (ARF), mirror (M), delay crystal(DC), filter(F), and beam displacer (BD).
2.2 Experiment
The UASFC experimental setup is shown in Fig. 2. An amplified Ti:Sapphire laser system running at a 10 Hz repetition rate, producing over 2 mJ/pulse in 50 fs pulses with a central wavelength of 800 nm and a bandwidth of 30 nm. The output laser was split to two-way laser pulses by beamsplitter1. Beamsplitter1 was coated for a 90:10 (R:T) split ratio over 700 nm-900 nm. One pulse was frequency doubled with a single BBO crystal and used for the signal beam centered at 400 nm; the other pulse was used as the probe beam. The pulse probe beam—linearly chirped to 6 ps by traversing an 18-cm length SF10 glass twice—passed through a Blackman apodizer, a half wave plate, and a delay crystal. The delay crystal is an A-cut TiO2 crystal, which was used to introduce a time delay between the two orthogonal polarizations of the chirped probe beam of or ~9 ps for the L = 10 mm long crystal. Thus, the probe beams were time-encoded to wavelength and polarization. The time-encoded probe beams were then relay imaged onto the semiconductor (LT GaAs/AlGaAs MQWs) which band gap energy, 1.65 eV, was close proximity of the probe beam energy, 1.55 eV. Meanwhile, the signal beams impinging upon the USC were spatially modulated by the binary amplitude grating and created a transient phase grating in the semiconductor sensor. The amplitude grating was placed on the back surface to spatially modulate the signal beam. The grating was constructed with an overall pitch of 8.3 μm, with 8.3 μm wide and 1 μm thick aluminum bars. A portion of the grating is shown in the top right of Fig. 2; note the letter “T” used to determine the modulation transfer function (MTF) of the camera. The time-encoded probe beams were incident on the ARF front surface of the semiconductor sensor, passed through the semiconductor, and were reflected back on themselves off the HRF surface. Thus the probe beams were optically modified by the phase grating and diffracted into the various orders of the grating. The diffracted probe beams from the semiconductor sensor were relayed imaged from an achromatic lens formed by the Fourier plane where the different orders of the phase grating were spatially separated and the zeroth order of the diffracted light was filtered. Another lens was used to image the diffracted beams onto three different CCD cameras through a beam displacer—which was used to spatially separate the two orthogonally polarized probe beams and three different interferometer narrow band filters with bandwidth of 5 nm centered at 790 nm, 800 nm, and 810 nm, respectively. Finally, six frames with 3 ps temporal separation were obtained; each CCD has two images corresponding to different polarizations at a particular wavelength.
Fig. 2 Experimental geometry used to demonstrate the basic performance of the UASFC. Labels in the figure are: beamsplitter1 (BS1 90:10), optical delay line (ODL), mirror (M), 400nm filter (F0), half waveplate (HP), delay crystal (DC), beam displace (BD), beam splitter (BS 50:50), 810 nm filter (F1), 800 nm filter (F2), and 790 nm filter (F3).
3. Results and discussions
3.1 Temporal resolution
The persistence time of the phase grating produced by the incident signal beams in the USC is dependent on the response time of the semiconductor, which can be improved by introducing trapping centers into the semiconductor or by LT molecular-beam-epitaxy (MBE) growing technology. The response time of the LT GaAs/AlGaAs MQWs used in the USC was tested using a pump-probe technique with 40 fs step intervals; the response time was about 2.5 ps in Fig. 3(a). Therefore, the semiconductor sensor acted like an “impulsive” sensor in which the time-dependent signal intensity was approximately proportional to the square root of the intensity of diffracted signal. At the same time—by adjusting the ODL step-by-step—an experimental result from the first CCD camera of UASFC is shown in Fig. 3(b); this reveals the two-dimension response character of the semiconductor sensor. Figure 3(b) represents the signal “T” from appearance to disappearance with different delayed time between the signal beam and the probe beam (Visualization 1). Note that the zeroth order was blocked on the Fourier plane filter and the scattered signal had been subtracted. The response time is about 2.5 ps, which agrees with our measurement result in Fig. 3(a). Therefore, the UASFC temporal resolution can be as fast as 2.5 ps—which is much faster than a traditional fast camera.
Fig. 3 (a) Experimental result of the semiconductor’s response time using pump-probe technique with 40 fs step intervals. Reflection at 800 nm versus time after the 400 nm pump pulse. The response time is about 2.5 ps (b) Evolution of the two-dimensional images on the CCD1 with the incident signal and the zeroth order blocked on the Fourier plane filter. The duration of the detected image from appearance to disappearance is about 2.5 ps (Visualization 1), which is in good agreement with the test result from Fig. 3(a).
3.2 Imaging results of UASFC
The functions of the TSS and the SMD are shown in Fig. 4(a), the time axis of the signal beams is converted to wavelength-polarization by the TSS, and images with different wavelength-polarization are spatially mapped to different locations by the SMD. In our test setup, the pulse width of the signal beam, ~50 fs, is too short to test the time resolution of the UASFC. To experimentally demonstrate the principle of the camera, the delay time between the signal beam and the probe beam is precisely adjusted by ODL and the signal—at different delay times—is imaged in different areas of the different CCD cameras. A time sequence composed of six shots with equal time interval of 3 ps is shown in Fig. 4(b). In fact, for each shot, the UASFC obtained six frames separated in time by 3 ps. For demonstration purposes, only images on the CCDs with peak signal are shown. The scattered background has been removed from these images and the real-time signal is proportional to the square root of the instantaneous measured intensity. From this time series, we obtained a 3 ps temporal resolution. The dynamic process of the signal on UASFC was shown by supplementary movie. By introducing trapping centers into the semiconductor to decrease the decay time, sub-picosecond temporal resolution can be acquired. The recording time can be extended from 15 ps to hundreds of picoseconds using a TSS consisting of a Bragged grating fiber. Moreover, the frame number of the UASFC can be increased to 100 by properly designing the SMD such that it mimics SF-STAMP technology [21]. In this experiment, the imaging uniformity was not good enough due to inhomogeneous signal and uneven surface of the USC. The imaging uniformity can be improved through the use of a highly homogenous experimental signal and by polishing the USC to achieve a higher surface quality.
Fig. 4 The experimental result of the UASFC’s six framing images with 3 ps inter-frame time. (a) Results of the TSS and the SMD, the time axis of signal is converted to wavelength-polarization (b) Sequence of two-dimensional images recorded of the signal light with the probe beam incident on the semiconductor as the signal beam incident on the semiconductor sensor at six different times. For demonstration purposes, only images on the CCDs with peak signal are shown. The time axis information is shown at the bottom. The detail of dynamic process on UASFC is shown by Visualization 1.
3.3 Spatial resolution
The spatial resolution of the UASFC was analyzed by computing the MTF of the camera. The edge spread function [27,29], ESF, was calculated in this article. The ESF, at the upper left corner of the letter “T” in the second image of Fig. 4(b), was used to determine the MTF. The ESF in this case was a horizontal lineout across the left edge of the “T” that was averaged in the vertical direction. A curve of the MTF computed from the ESF is denoted by the red line in Fig. 5. This figure shows that 30 lp/mm spatial resolution was obtained in the framing camera. Moreover, the spatial resolution can be improved beyond 100 lp/mm by either increasing the filtered diameter of the circles in the Fourier plane or by decreasing the pitch of the grating.
Fig. 5 Modulation transfer function (MTF) produced from the ESF at the upper left corner of the letter “T” in the second picture of Fig. 4(b).
4. Conclusions
In this letter, we proposed and experimentally demonstrated an ultrafast all-optical solid-state framing camera (UASFC). The UASFC consists of an ultrafast semiconductor chip (USC), a time-serialized system (TSS), and a spatial mapping device (SMD). By using an USC with a 2.5 ps response time, the limited temporal resolution of the UASFC can reach as high as 2.5 ps. Linearly chirped technologies (i.e. dispersing glass, two-grating chirped method, or fiber Bragg-grating chirped method) and polarization time-delayed technology enable the TSS to increase the measured recording time to hundreds of picoseconds. Meanwhile, wavelength division multiplex (WDM) and polarization division multiplex (PDM) form a SMD. By properly designing the SMD, the total frame number can reach ~100. In our proof-of-principle demonstration, an experimental result of six frames with a 3 ps temporal resolution, 30 lp/mm spatial resolution, and 15 ps recording time was obtained in the visible regime. It is worth mentioning that the operation of the framing camera is not limited to visible wavelengths, but can be extended to other electromagnetic wavelengths such as X-ray and ultraviolet. In our further experiment, the UASFC will be demonstrated in the X-ray regime. The spatial resolution can also be advanced over 100 lp/mm by either increasing the filtered diameter of the circles in the Fourier plane or by decreasing the pitch of the grating. In summary, we believe that with the rapid development of UASFCs, this technology may open the door to precision measurement of single-shot ultrafast phenomena with both high temporal resolution and 2D spatial resolution.
Funding
Major Program of the National Natural Science Foundation of China (NSFC) (Grant No. 41530422); National Natural Science Foundation of China (NSFC) (Grant No. 11274377, 11675258); CAS “Light of West China” (Grant No. XAB2015B21).
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