Room temperature operation of germanium-silicon single-photon avalanche diode.

The ability to detect single photons has led to the advancement of numerous research fields1-11. Although various types of single-photon detector have been developed12, because of two main factors-that is, (1) the need for operating at cryogenic temperature13,14 and (2) the incompatibility with complementary metal-oxide-semiconductor (CMOS) fabrication processes15,16-so far, to our knowledge, only Si-based single-photon avalanche diode (SPAD)17,18 has gained mainstream success and has been used in consumer electronics. With the growing demand to shift the operation wavelength from near-infrared to short-wavelength infrared (SWIR) for better safety and performance19-21, an alternative solution is required because Si has negligible optical absorption for wavelengths beyond 1 µm. Here we report a CMOS-compatible, high-performing germanium-silicon SPAD operated at room temperature, featuring a noise-equivalent power improvement over the previous Ge-based SPADs22-28 by 2-3.5 orders of magnitude. Key parameters such as dark count rate, single-photon detection probability at 1,310 nm, timing jitter, after-pulsing characteristic time and after-pulsing probability are, respectively, measured as 19 kHz µm-2, 12%, 188 ps, ~90 ns and <1%, with a low breakdown voltage of 10.26 V and a small excess bias of 0.75 V. Three-dimensional point-cloud images are captured with direct time-of-flight technique as proof of concept. This work paves the way towards using single-photon-sensitive SWIR sensors, imagers and photonic integrated circuits in everyday life.

Experimental generation and analysis of chaos-modulated pulses for pulsed chaos lidar applications based on gain-switched semiconductor lasers subject to optical feedback.

We experimentally generate and analyze chaos-modulated pulses for pulsed chaos lidar applications based on gain-switched semiconductor lasers subject to optical feedback. While conventional pulsed lidars emit repetitive short pulses without specificity making them vulnerable to interference and range ambiguity, chaos lidars possess the advantages of having no range ambiguity and being immune to interference and jamming, which are benefits of the aperiodic and uncorrelated waveforms we use. Compared to the cw chaos lidars originally proposed, the pulsed chaos lidars can have significantly higher peak power under the class-1 eye-safe regulation that is essential for long-range low-reflectivity target detection. We investigate the temporal, spectral, and cross-correlation characteristics of the modulated pulses obtained with different feedback strengths and modulation currents. Induced by the transient response and evolving with the delayed feedback, modulated pulses exhibiting periodic oscillations and complex dynamics such as chaos are observed. Under a weakly damped condition with large modulation current and moderate feedback strength, we successfully generate uncorrelated chaos-modulated pulses suitable for the pulsed chaos lidar applications. With the current configuration, for cross-correlations comparable to the benchmark of 0.19 set by the cross-correlation of the intensity fluctuation on the sole gain-switched pulses without feedback, uncorrelated waveforms with durations up to 218 ns in a 500 ns modulated pulse can be effectively utilized.

3D pulsed chaos lidar system.

We develop an unprecedented 3D pulsed chaos lidar system for potential intelligent machinery applications. Benefited from the random nature of the chaos, conventional CW chaos lidars already possess excellent anti-jamming and anti-interference capabilities and have no range ambiguity. In our system, we further employ self-homodyning and time gating to generate a pulsed homodyned chaos to boost the energy-utilization efficiency. Compared to the original chaos, we show that the pulsed homodyned chaos improves the detection SNR by more than 20 dB. With a sampling rate of just 1.25 GS/s that has a native sampling spacing of 12 cm, we successfully achieve millimeter-level accuracy and precision in ranging. Compared with two commercial lidars tested side-by-side, namely the pulsed Spectroscan and the random-modulation continuous-wave Lidar-lite, the pulsed chaos lidar that is in compliance with the class-1 eye-safe regulation shows significantly better precision and a much longer detection range up to 100 m. Moreover, by employing a 2-axis MEMS mirror for active laser scanning, we also demonstrate real-time 3D imaging with errors of less than 4 mm in depth.

Single-sideband photonic microwave generation with an optically injected quantum-dot semiconductor laser.

We studied single-sideband (SSB) photonic microwave generation with a high sideband rejection ratio (SRR) based on the period-one dynamical states of an optically injected quantum-dot (QD) semiconductor laser and demonstrated that the SSB signals have SRRs of approximately 15 dB higher than those generated with a conventional quantum-well semiconductor laser under conditions of optimal microwave power. The enhancement of SRR in the QD laser, which is important in mitigating the power penalty effect in applications such as radio-over-fiber optical communications, could be primarily attributed to a lower carrier decay rate in the dots, smaller linewidth enhancement factor, and reduced photon decay rate.

Chaos time delay signature suppression and bandwidth enhancement by electrical heterodyning.

We numerically investigated the chaos time delay signature (TDS) suppression and bandwidth enhancement by electrical heterodyning. Chaos signals generated with a semiconductor laser subject to optical feedback typically have distinct loop frequency peaks in their power spectra corresponding to the reciprocals of the time delays, which deteriorates the performance in applications including chaos radar/lidar and fast random bit generation. By electrically heterodyning the chaos signal with a single frequency local oscillator, we show that the power in the chaos spectrum can be redistributed and a smoother spectrum with a broader effective bandwidth can be obtained. Compared with the chaos directly generated from a semiconductor laser subject to optical feedback, the amplitudes of the TDS (ρ(TDS)) measured under different feedback strengths can be suppressed up to 63% and the effective bandwidths can be enhanced up to 46% in average after the electrical heterodyning is applied.

Self-mixing dual-frequency laser Doppler velocimeter.

A self-mixing (SM) dual-frequency (DF) laser Doppler velocimeter (LDV) (SM DF-LDV) is proposed and studied, which integrates the advantages of both the SM-LDV and the DF-LDV. An optically injected semiconductor laser operated in a dual-frequency period-one (P1) dynamical state is used as the light source. By probing the target with the light-carried microwave generated from the beat of the two optical frequency components, the spectral broadening in the Doppler signal due to the speckle noise can be significantly reduced. Together with an SM configuration, the SM DF-LDV has the advantages of direction discriminability, self-alignment, high sensitivity, and compact setup. In this study, speckle noise reduction and direction discriminability with an SM DF-LDV are demonstrated. The signal-to-noise ratios (SNRs) at different feedback powers are investigated. Benefiting from the high sensitivity of the SM configuration, an SNR of 23 dB is achieved without employing an avalanched photodetector or photomultiplier tube. The velocity resolution and the SNR under different speckle noise conditions are studied. Average velocity resolution of 0.42 mm/s and SNR of 22.1 dB are achieved when a piece of paper is rotating at a transverse velocity of 5 m/s. Compared with a conventional single-frequency LDV (SF-LDV), the SM DF-LDV shows improvements of 20-fold in the velocity resolution and 8 dB in the SNR.

Dual-frequency laser Doppler velocimeter for speckle noise reduction and coherence enhancement.

We study the characteristics of a dual-frequency laser Doppler velocimeter (DF-LDV) based on an optically injected semiconductor laser. The laser operated in a period-one (P1) dynamical state with two optical frequencies separated by 11.25 GHz is used as the dual-frequency light source. With a microwave beat signal carried by the light, the DF-LDV possesses both the advantages of good directionality, high intensity, and high spatial resolution from the light and low speckle noise and good coherence from the microwave, respectively. By phase-locking the two frequency components with a microwave signal, the coherence of the dual-frequency light source can be further improved and the detection range can be much extended. In this paper, velocity resolutions of the DF-LDV with different amounts of speckle noise and at different detection ranges are experimentally measured and analyzed. Compared with the conventional single-frequency LDV (SF-LDV), the velocity resolution of the DF-LDV is improved by 8 × 10(3) times from 2.5 m/s to 0.31 mm/s for a target with a longitudinal velocity vz = 4 cm/s, a transverse velocity vt = 5 m/s, and at a detection range of 108 m.

Revisiting La0.5Sr1.5MnO4 lattice distortion and charge ordering with multi-beam resonant diffraction.

Sinusoidal wave type distortions of La0.5Sr1.5MnO4 in the low-temperature orthorhombic phase were observed using multi-beam resonant X-ray diffraction (MRXD) with (7/4 7/4 0) fractional primary diffraction. Two four-beam diffractions with opposite asymmetry were measured at 6.5545 keV and compared with the curves simulated by the dynamical X-ray diffraction theory. This approach provides the possibility of resolving the distortion modes which are perpendicular to the momentum transfer by a single azimuthal scan. The paper also demonstrates the sensitivity of MRXD profiles versus incident X-ray energy in the vicinity of the Mn K edge to the charge disproportion between the two manganese sites, reconfirming the small charge disproportion feature.

Temperature- and energy-dependent phase shifts of resonant multiple-beam X-ray diffraction in germanium crystals.

This paper reports temperature- and energy-dependent phase shifts of resonant multiple-beam X-ray diffraction in germanium crystals, involving forbidden (002) and weak (222) reflections. Phase determination based on multiple-beam diffraction is employed to estimate phase shifts from (002)-based {(002)(375)(373̅)} four-beam cases and (222)-based { (222)(5̅33̅)} three-beam cases in the vicinity of the Ge K edge for temperatures from 20 K up to 300 K. The forbidden/weak reflections enhance the sensitivity of measuring phases at resonance. At room temperature, the resonance triplet phases reach a maximum of 8° for the four-beam cases and -19° for the three-beam cases. It is found that the peak intensities and triplet phases obtained from the (002) four-beam diffraction are related to thermal motion induced anisotropy and anomalous dispersion, while the (222) three-beam diffraction depends on the aspherical covalent electron distribution and anomalous dispersion. However, the electron-phonon interaction usually affects the forbidden reflections with increasing temperatures and seems to have less effect on the resonance triplet phase shifts measured from the (002) four-beam diffraction. The resonance triplet phase shifts of the (222) three-beam diffraction versus temperature are also small.

Skin-scanning technique for superficial blood flow imaging using a high-frequency ultrasound system.

In this paper we propose a skin-scanning technique with a high-frequency ultrasound imaging system that enables images to be acquired at the fixed depth of field of a single-element focused transducer along the profile of an object contour by simultaneously moving the transducer in the horizontal and vertical directions. The scanning path, which closely parallels the profile of the object contour, was determined from the intensity difference between an object and the background in a brightness-mode image. The transducer moved along the profile of the object contour while maintaining a constant distance interval between adjacent pairs of ultrasonic signals in the horizontal direction. The image was then reconstructed by applying an alignment process to eliminate the distortion. The performance of skin-scanning technique was verified in vitro experiment using an arc-shaped phantom and the results showed a percentage error of 0.55% for the volumetric blood flow estimates. Moreover, in vivo experiment on a subcutaneous tumor was also performed. The results indicated that the proposed technique can accurately estimate the blood flow information along the profile of the object contour and avoid distortion of the morphology of blood vessels. The skin-scanning technique has potential for assessing superficial blood flows and prognoses in the oncology and dermatology fields.

The lifetime and attenuation properties measurements of a US/MR multimodality molecular probe.

In our previous studies we explored the potential of using a combined US/magnetic resonance (MR) multimodality contrast agent, albumin-gadolinium-diethylenetriaminepentacetate (Gd-DTPA) MBs, to induce BBB opening and for distinguishing between FUS-induced BBB opening and intracerebral hemorrhage in MR T1-weighted contrast imaging. According to the previous study in the literature, 1-2 µm bubbles have more pronounced acoustic activity at frequencies above 10 MHz. The present study developed a new targeted US/MR multimodality MB and the acoustic properties were compared with two commercial MBs, SonoVue and Targestar SA. The acoustic activities of these 1.15-2.78 µm MBs with different shells at 10 MHz were investigated. The feasibility of designing a new targeted US/MR multimodality MB was investigated. The lifetime (survival of MBs in the liquid suspension) and attenuation properties of lipid MBs (SonoVue and Targestar SA), albumin-(Gd-DTPA) MBs, and avidin-conjugated albumin (avidin-albumin)-(Gd-DTPA) MBs at 10 MHz were investigated with the pulse-echo substitution method. It was found that incorporating avidin into the albumin MBs and avidin-albumin-(Gd-DTPA) MBs affects the size distribution but does not affect the concentration of MBs produced. The avidin-albumin-shelled MBs had more significant nonlinear activity at 4-18 MHz (p=0.025), while the nonlinear activity of the other MBs peaked at 6-24 MHz (p=0.003-0.044). Moreover, the incorporation of paramagnetic metal ions into the MB shells increased their attenuation coefficients. With regard to the lifetime of these agents, the attenuations of the SonoVue and Targestar SA lipid MBs were 87.96% and 8.74%, respectively, while those of albumin MBs, avidin-albumin MBs, albumin-(Gd-DTPA) MBs, and avidin-albumin-(Gd-DTPA) MBs were 49.52%, 41.38%, 74.69%, and 100%, respectively. Avidin conjugation decreased the lifetime of the albumin MBs, but not that of the lipid MBs. The incorporation of paramagnetic metal ions into the shells of albumin MBs did not decrease the lifetime.

Phantom investigation of phase-inversion-based dual-frequency excitation imaging for improved contrast display.

OBJECTIVE: The goal of this work is to examine the effects of pulse-inversion (PI) technique in combination with dual-frequency (DF) excitation method to separate the high-order nonlinear responses from microbubble contrast agents for improvement of image contrast. DF excitation method has been previously developed to induce the low-frequency ultrasound nonlinear responses from bubbles by using the composition of two high-frequency sinusoids (f(1) and f(2)). MOTIVATION: Although the simple filtering was conventionally utilized to provide signal separation, the PI approach is better in the sense that it minimizes the mutual interferences among these high-order nonlinear responses in the presence of spectral overlap. The novelty of the work is that, in addition to the common PI summation, the PI subtraction was also applied in DF excitation method. METHODS: DF excitation pulses having an envelope frequency of 3MHz (i.e., f(1)=8.5MHz and f(2)=11.5MHz) with pulse lengths of 3-10µs and the pressure amplitudes from 0.5 to 1.5MPa were used to interrogate the nonlinear responses of SonoVue™ microbubbles in the phantom experiments. The high-order nonlinear responses in the DF excitation were extracted for contrast imaging using PI summation for even-order nonlinear components or PI subtraction for odd-order nonlinear ones. RESULTS: Our results indicated that, as compared to the conventional filtering technique, the PI processing effectively increases the contrast-to-tissue ratio (CTR) of the third-order nonlinear response at 5.5MHz and the fourth-order nonlinear response at 6MHz by 2-5dB. For these high-order nonlinear components, the CTR increase varies with the transmission pressures from 0.5 to 1.5MPa due to the microbubbles' displacement induced by the radiation force of DF excitation. CONCLUSIONS: For DF excitation technique, the PI processing can help to extract either the odd-order or the even-order nonlinear components for higher CTR estimates.

Phase-dependent dual-frequency contrast imaging at sub-harmonic frequency.

Sub-harmonic imaging techniques have been shown to provide a higher contrast-to-tissue ratio (CTR) at the cost of relatively low signal intensity from ultrasound contrast agents (UCAs). In this study, we propose a method of dual-frequency excitation to further enhance the CTR of subharmonic imaging. A dual-frequency excitation pulse is an amplitude-modulated waveform which consists of two sinusoids with frequencies of f1 (e.g., 9 MHz) and f2 (e.g., 6 MHz) and the resulting envelope component at (f1 - f2) (e.g., 3 MHz) can serve as a driving force to excite the nonlinear response of UCAs. In this study, the f2, at twice of the resonance frequency of UCAs, is adopted to efficiently generate a sub-harmonic component at half of the f2 frequency, and f1 is included to enhance the high-order nonlinear response of UCAs at the sub-harmonic frequency. The second- and third-order nonlinear components resulting from the envelope component would spectrally overlap at the sub-harmonic frequency when f1 and f2 are properly selected. We further optimize the generation of the sub-harmonic component by tuning the phase terms between second- and third-order nonlinear components. The results show that, with dual-frequency excitation, the CTR at sub-harmonic frequency improves compared with the conventional tone-burst method. Moreover, the CTR changes periodically with the relative phase of the separate frequency component in the dual-frequency excitation, leading to a difference of as much as 9.1 dB between the maximal and minimal CTR at 300 kPa acoustic pressure. The echo produced from the envelope component appears to be specific for UCAs, and thus the proposed method has the potential to improve both SNR and CTR in sub-harmonic imaging. Nevertheless, the dual-frequency waveform may suffer from frequency-dependent attenuation that degrades the generation of the envelope component. The deviation of the microbubble's resonance characteristics from the selection of dual-frequency transmission may also decrease the CTR improvement.

Dual-frequency chirp imaging for contrast detection.

The method of dual-frequency (DF) difference excitation is capable of generating a low-frequency envelope component as the driving force of commercial contrast microbubbles by using a high-frequency pulse. Although the DF difference excitation method provides good lateral resolution in high-frequency contrast imaging, it suffers from degraded axial resolution because a longer-than-usual envelope component is required to induce the oscillation of microbubbles. In this study, a coded excitation technique (i.e. chirp waveform) is combined with the DF difference excitation method (also referred to as the DF chirp excitation method) to improve the axial resolution of contrast imaging while maintaining the impinging insonation energy. B-mode images were constructed to compare the performance of the DF chirp excitation method with the conventional tone-burst pulse method. Results indicate that the proposed DF chirp excitation method can provide better axial resolution after pulse compression. Moreover, as compared to the tone-burst pulse with the same pulse duration, the pulse compression results in a higher signal-to-noise ratio because of the temporal concentration of the received energy. Nevertheless, images with the DF chirp excitation method demonstrated noticeable image artefacts resulting from the range sidelobes. The DF chirp excitation method also produced obvious tissue harmonic generation that could degrade the contrast-to-tissue ratio at higher acoustic pressures.

Dual-high-frequency ultrasound excitation on microbubble destruction volume.

OBJECTIVE AND MOTIVATION: The goal of this work was to test experimentally that exposing air bubbles or ultrasound contrast agents in water to amplitude modulated wave allows control of inertial cavitation affected volume and hence could limit the undesirable bioeffects. METHODS: Focused transducer operating at the center frequency of 10 MHz and having about 65% fractional bandwidth was excited by 3 micros 8.5 and 11.5 MHz tone-bursts to produce 3 MHz envelope signal. The 3 MHz frequency was selected because it corresponds to the resonance frequency of the microbubbles used in the experiment. Another 5 MHz transducer was used as a receiver to produce B-mode image. Peak negative acoustic pressure was adjusted in the range from 0.5 to 3.5 MPa. The spectrum amplitudes obtained from the imaging of SonoVue contrast agent when using the envelope and a separate 3 MHz transducer were compared to determine their cross-section at the -6 dB level. RESULTS: The conventional 3 MHz tone-burst excitation resulted in the region of interest (ROI) cross-section of 2.47 mm while amplitude modulated, dual-frequency excitation with difference frequency of 3 MHz produced cross-section equal to 1.2mm. CONCLUSION: These results corroborate our hypothesis that, in addition to the considerably higher penetration depth of dual-frequency excitation due to the lower attenuation at 3 MHz than that at 8.5 and 11.5 MHz, the sample volume of dual-frequency excitation is also smaller than that of linear 3-MHz method for more spatially confined destruction of microbubbles.