Two-photon Detector Research Articles

Multi-Color Two-Photon Microscopic Imaging Based on a Single-Wavelength Excitation.

Two-photon probes with broad absorption spectra are beneficial for multi-color two-photon microscopy imaging, which is one of the most powerful tools to study the dynamic processes of living cells. To achieve multi-color two-photon imaging, multiple lasers and detectors are usually required for excitation and signal collection, respectively. However, one makes the imaging system more complicated and costly. Here, we demonstrate a multi-color two-photon imaging method with a single-wavelength excitation by using a signal separation strategy. The method can effectively solve the problem of spectral crosstalk by selecting a suitable filter combination and applying image subtraction. The experimental results show that the two-color and three-color two-photon imaging are achieved with a single femtosecond laser. Furthermore, this method can also be combined with multi-photon imaging technology to reveal more information and interaction in thick biological tissues.

Detecting two photons with one molecule

We apply input-output theory with quantum pulses [AH Kiilerich, K M\o lmer, Phys. Rev. Lett. {\bf 123}, 123604 (2019)] to a model of a new type of two-photon detector consisting of one molecule that can detect two photons arriving sequentially in time. The underlying process is distinct from the usual two-photon absorption process where two photons arriving simultaneously and with frequencies adding up to the resonance frequency are absorbed by a single molecule in one quantum jump. Our detector model includes a Hamiltonian description of the amplification process necessary to convert the microscopic change in the single molecule to a macroscopic signal.

Imaging through dynamical scattering media by two-photon absorption detectors.

Imaging through a dynamical opaque scattering medium is an almost impossible task, where strong multiple light scattering from moving scatters dynamically prevents imaging formations even with state-of-art techniques like correlation imaging or adaptive optics. Meanwhile, a small number of ballistic photons can still penetrate through but require demanding detection in terms of a ultrashort time gate and high sensitivity. However, visible light is strongly scattered for most of scattering media. Here we experimentally demonstrate a non-invasive coherent imaging scheme based on two-photon absorption capable of imaging through dynamical scattering media with a length equivalent to 28 times mean free paths for single photon transport, where two-photon absorption in a conventional semiconductor photodetector when phase matching is not required works over a wide bandwidth so it can support a fast time gate down to femtosecond level, short enough to distinguish ballistic photons from scattering background, and allows accessing longer wavelengths for deeper penetration. This technique combined with successful optical coherence tomography may pave a new way for imaging through fog, storm, and rain as well as biomedical imaging applications.

Two-photon superbunching effect of broadband chaotic light at the femtosecond timescale based on a cascaded Michelson interferometer

It is challenging to observe the superbunching effect with true chaotic light. Here we propose and demonstrate a method to achieve the superbunching effect of ${g}^{(2)}(0)=2.42\ifmmode\pm\else\textpm\fi{}0.02$ with broadband stationary chaotic light based on a cascaded Michelson interferometer (CMI), exceeding the theoretical upper limit of 2 for the two-photon bunching effect of chaotic light. The superbunching correlation peak is measured with an ultrafast two-photon absorption detector whose full width at half-maximum reaches about 95 fs. Two-photon superbunching theory in a CMI is developed to interpret the effect and is in agreement with experimental results. The theory also predicts that the degree of second-order coherence can be much greater than 2 if chaotic light propagates $N$ times in a CMI. Finally, a type of weak signal detection setup that employs broadband chaotic light circulating in a CMI is proposed. Theoretically, it can increase the detection sensitivity of weak signals 79 times after the chaotic light circulates 100 times in the CMI.

Upconversion single-microbelt photodetector via two-photon absorption simultaneous

Single microbelt (MB) photodetectors with metal–semiconductor-metal structure have been demonstrated and characterized comprehensively. For single-photon absorption, the maximum responsivity of ZnO-MB photodetector can reach as high as 1.4 × 105 A W−1 at 20 V bias. The results about photoresponse of MB-detector reveals that two relaxation mechanisms contribute to the carrier decay time. Moreover, the two-photon absorption upconversion photoresponsivity in the single-MB detector has also been realized, which is the first report about the two-photon absorption detector to the best of our knowledge. The excellent two-photon absorption photoresponsivity characteristic of the MB device can be available not only for detector but also for solar cell and biomedical imaging. The above results present a significant step towards future fabrication of single micro/nano-structure based multiphoton excitation optoelectronic devices.

Mid-infrared two photon absorption sensitivity of commercial detectors

We report on broad-band two-photon absorption (TPA) in several commercially available MIR inter-band bulk semiconductor photodetectors with the spectral cutoff in the range of 4.5–6 μm. The highest TPA responsivity of 2 × 10−5 A·mm2/W2 is measured for a nitrogen-cooled InSb photovoltaic detector. Its performance as a two-photon detector is validated by measuring the second-order interferometric autocorrelation function of a multimode quantum cascade laser emitting at the wavelength of 8 μm.

A time to polarize the unpolarized.

Optics Wavelength, intensity, and polarization are several parameters that define a light field. Light from most natural and many artificial light sources is unpolarized. However, an instantaneous snapshot of a propagating light field shows that it may be completely polarized. Shevchenko et al. propose and demonstrate experimentally that there is a finite window of time for that snapshot in which the unpolarized light is polarized. Using a Michelson interferometer with polarization- sensitive components combined with two-photon absorption detectors, they show for two different unpolarized light sources that the polarization time window varies from 10 to 100 fs. The effect of a finite polarization time could be exploited in a number of applications such as communication and sensing. Optica 4 , 64 (2017).

Chaotic chirped-pulse oscillators

We present results of experimental investigation of the chaotic and quasi-periodic regime in the chirped-pulsed (dissipative soliton) Cr:ZnS and Cr:ZnSe mid-IR oscillators with significant third-order dispersion. The instability develops when the spectrum edge approaches resonance with a linear wave either due to power increase or by dispersion adjustment. In practice, this occurs when the spectrum edge reaches zero dispersion wavelength. The analysis suggests a three-oscillator chaos model, which is confirmed by numerical simulations. The regime is long-term stable and can be easily overlooked in similar systems. We show that chaotic regime is accompanied by a characteristic spectral shape and can be reliably recognized by using wavelength-skewed filters and by second-harmonic or two-photon absorption detectors.

Sum-frequency generation as a detector of high-power two-mode squeezing

We introduce sum-frequency generation (SFG) as an effective physical two-photon detector for high power two-mode squeezed coherent states with arbitrary frequency separation, as produced by parametric oscillators well above the threshold. Using a formalism of "collective modes", we describe both two-mode squeezing and degenerate squeezing on equal footing and derive simple relations between the input degree of squeezing and the measured SFG quadrature noise. We compare the proposed SFG detection to standard homodyne measurement, and show advantages in robustness to detection inefficiency (loss of SFG photons) and acceptance bandwidth.

Ultrashort laser pulse characterization techniques

There are several competing techniques for the characterization of ultrashort laser pulses. Here we report the studies that we have done in the laboratory. These techniques use the autocorrelation or cross-correlation of the signals. The spectral properties of almost-Gaussian functions were applied for the second-order approximation of the coefficients of Hermite-Gaussian functions that described almost perfect pulses. Other techniques are discussed such as frequency-resolved optical-gating (FROG), which is a time-dependent intensity and phase of the laser pulse, the sonogram of an ultrashort pulse using a two-photon detector which is sensitive and robust, SPIDER which measures the interference between two pulses separated in time which are identical except for their central frequencies. Finally, the ambiguity-Wigner function properties were applied for the characterization of ultrashort pulses.

Nondestructive two-photon parity detector with near unity efficiency

It has been shown that two-photon parity check has widespread applications in optical quantum information processing. In recent years much effort has been made to implement nondestructive two-photon parity detection using cross-Kerr nonlinearity (XKNL) effect. We here put forward an XKNL-based scheme for nearly deterministic two-photon parity detection without destroying the photons, in which the required cross-Kerr phase shift could be much (even several orders of magnitude) smaller than that required for previous schemes. The scheme utilizes a ring cavity fed by a coherent state beam as a quantum information bus which interacts with one of the two path modes of two polarized photons through a cross-Kerr medium. The measurement outcome of the bus mode reveals the polarization-parity of the two photons.

Second-order optical coherence tomography: deeper and turbulence-free imaging

The superior resolution of optical coherence tomography (OCT) with respect to alternative imaging modalities makes it highly attractive, and some of its applications are already in extensive clinical use. However, one of the major limitations of OCT is that the tomographic picture it generates is depth-limited to approximately 1 mm in most biological tissues. This is mainly due to the spatially turbulent nature of the tissue, which leads to scattering. Moreover, this technique is extremely sensitive to temporal variations in the medium. We show that insensitivity to temporal and spatial turbulence may be gained by replacing the linear detector with an ultrasensitive two-photon detector. These results have striking implications on the attainable penetration depth of optical imaging and on its sensitivity to sample motion.

Subharmonic Synchronization of Picosecond Yb Fiber Laser to Picosecond Ti:Sapphire Laser for Stimulated Raman Scattering Microscopy

We demonstrate a subharmonically synchronized picosecond laser system and its application to stimulated Raman scattering (SRS) microscopy for label-free biological imaging with high sensitivity and high spectral resolution. Experimentally, 5.5-ps Yb fiber laser pulses at a repetition rate of 38 MHz were successfully synchronized to 4.8-ps Ti:Sapphire laser pulses at a repetition rate of 76 MHz, by using a two-photon detector, an intracavity electro-optic modulator, and a piezoelectric transducer. The temporal jitter was measured to be 120 fs. These pulses were applied to SRS microscopy. We confirmed the shot-noise-limited sensitivity, and succeeded in SRS spectroscopy of a polymer bead as well as three-dimensional SRS imaging of a HeLa cell.

Quantum lithography under imperfect conditions: effects of loss and dephasing on two-photon interference fringes

We propose a method to simulate a two-photon interference fringe by using a density matrix in order to describe various types of imperfections in the input state. Using this method, we numerically discuss the influence of various imperfections in an input state, such as dephasing and misalignment, on the quality (visibility and period) of the two-photon interference fringes. Applying this method to experimental data, we succeeded in numerically reproducing a two-photon interference fringe using the experimentally obtained density matrix, in which almost no free fitting parameters are required. From the results, because the main cause of the degradation of an interference fringe was found to be the limited aperture size of a two-photon detector, we can observe a two-photon interference fringe with a visibility of up to 94% in the experiments if an efficient two-photon absorbing material or a two-photon detector with a sufficiently high spatial resolution can be used.

Pulse Width Stabilization for Ultrafast Laser Systems

In this report, we describe a method and the resultant performance data for precise and efficient laser pulse width stabilization in the ultrafast laser regime. The pulse quality feedback mechanism is based on a two-photon absorption detector, and the adjustments to system dispersion are executed by embedded software tuning the inter-grating distance in the pulse compressor. Relative pulse width variability of only 4% is demonstrated over 8hours despite changing ambient conditions. To our knowledge, this is the first ever ultrafast laser pulse autonomous stabilization scheme integrated into an industrial platform.

Stimulated Raman scattering microscope with shot noise limited sensitivity using subharmonically synchronized laser pulses

We propose and demonstrate the use of subharmonically synchronized laser pulses for low-noise lock-in detection in stimulated Raman scattering (SRS) microscopy. In the experiment, Yb-fiber laser pulses at a repetition rate of 38 MHz are successfully synchronized to Ti:sapphire laser pulses at a repetition rate of 76 MHz with a jitter of <8 fs by a two-photon detector and an intra-cavity electro-optic modulator. By using these pulses, high-frequency lock-in detection of SRS signal is accomplished without high-speed optical modulation. The noise level of the lock-in signal is found to be higher than the shot noise limit only by 1.6 dB. We also demonstrate high-contrast, 3D imaging of unlabeled living cells.

Enhancement of Molecular Ions in Mass Spectrometry Using an Ultrashort Optical Pulse in Multiphoton Ionization

The spectral domain of an ultraviolet femtosecond laser was expanded by stimulated Raman scattering/four-wave Raman mixing, and the resulting laser pulse was compressed using a pair of gratings. The pulse width was then measured using an autocorrelator comprised of a Michelson interferometer equipped with a multiphoton ionization/mass spectrometer which was used as a two-photon detector. A gas chromatograph/mass spectrometer was employed to analyze triacetone triperoxide (TATP), and the molecular ion induced by multiphoton ionization was substantially enhanced by decreasing the laser pulse width.

Coherence properties of spontaneous parametric down-conversion pumped by a multi-mode cw diode laser

Coherence properties of the photon pair generated via spontaneous parametric down-conversion pumped by a multi-mode cw diode laser are studied with a Mach-Zehnder interferometer. Each photon of the pair enters a different input port of the interferometer and the biphoton coherence properties are studied with a two-photon detector placed at one output port. When the photon pair simultaneously enters the interferometer, periodic recurrence of the biphoton de Broglie wave packet is observed, closely resembling the coherence properties of the pump diode laser. With non-zero delays between the photons at the input ports, biphoton interference exhibits the same periodic recurrence but the wave packet shapes are shown to be dependent on both the input delay as well as the interferometer delay. These properties could be useful for building engineered entangled photon sources based on diode laser-pumped spontaneous parametric down-conversion.

Terahertz two-photon quantum well infrared photodetector

A two-photon detector based on intersubband transitions in GaAs/AlGaAs quantum wells operating in the Terahertz regime below the Reststrahlenband is reported. Resonantly enhanced optical nonlinearities enables sensitive quadratic detection at pJ pulse energies. We demonstrate its use in a quadratic autocorrelator for far-infrared picosecond pulses at around 7 THz.

Quadratic detection with two-photon quantum well infrared photodetectors

Two-photon quantum well infrared photodetectors (QWIPs) involving three equidistant subbands take advantage of a resonantly enhanced optical nonlinearity, which is six orders of magnitude stronger than in a bulk semiconductor. This approach results in a sensitive device to measure quadratic autocorrelation of mid-infrared optical pulses from modelocked quantum cascade lasers, nonlinear optical conversion, and free-electron lasers (FEL). We report on autocorrelation measurements at wavelengths in the mid-infrared and Terahertz regimes using ps optical pulses from the FEL at the Forschungszentrum Dresden Rossendorf. In particular, quadratic detection at wavelengths around 5.5 μm is still possible at room-temperature, which is crucial for applications in practical systems. We also report on a two-photon detector which works below the Reststrahlen band at 42 μm (7.1 THz).