Mr. Keegan Hernandez Profile

Keegan Hernandez

at Miftek Inc

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Publications (3)

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Proceedings Article | 13 March 2024 Presentation + Paper

Successive molecular decay measurement by time-correlated multiphoton counting (TCMPC)

Masanobu Yamamoto, John Jaiber González Murillo, Keegan Hernandez, J. Paul Robinson

Proceedings Volume 12863, 128630D (2024) https://doi.org/10.1117/12.3002452

KEYWORDS: Photodetectors, Fluorescence, Sensors, Single photon detectors, Quantum detection, Molecules, Quantum measurement, Signal detection, Quantum sensors, Time correlated single photon counting

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Fluorescence spectrum, intensity and decay measurements are powerful outputs to analyze molecular structure and energy transition in cellular biology and cytometry. Frequently, fluorescence measurements are made by photocurrent detection using Photomultiplier Tubes (PMTs) and recently using Avalanche Photo Diodes (APDs). To address the challenge of measuring nanosecond order fluorescence decay times on single cells, single photon detection technology has been developed using pixel-coupled Silicon Photo Multiplier (SiPM), GHz electronics and waveform analysis. As a result, photoelectron (PE) pulse widths of 500ps and saturation count of 1Gcps with 7 LOG dynamic range have been achieved. This capability enables detection of multiple fluorescence PE pulses during one START-STOP time interval measurement. Combined with high-speed pulsed excitation, it is possible to measure fluorescence intensity during the pulse excitation 10MHz repetition, 80ns ON time, and fluorescence decay during the 20ns OFF time. Statistically, measured decay time is different from currently defined decay time by impulse excitation. We named these techniques “time-correlated multiphoton counting (TCMPC)” and “successive molecular decay (SMD)” to distinguish from conventional definitions and methods. To confirm the techniques, the decay time of Rhodamine in polymer and other fluorescence materials was measured. To apply single photon detection to quantum flow cytometry, which involves several microseconds per event, full spectral detection with 42 channels from UV to IR wavelength and 10Gsps(100ps) data acquisition electronics is under development. SMD and TCMPC are innovative techniques to analyze single molecule behavior and structure and will be powerful tools to understand the quantum nature of biology.

Proceedings Article | 30 May 2022 Presentation + Paper

Deadtime-less single photon detection and potential applications

Masanobu Yamamoto, John Jaiber Gonzalez Murillo, Keegan Hernandez, Valery Patsekin, J. Paul Robinson

Proceedings Volume 12089, 120890A (2022) https://doi.org/10.1117/12.2623627

KEYWORDS: Sensors, Photodetectors, Single photon, Photon counting, Electronics, Statistical analysis, Signal processing, Capacitance, Analog electronics, Luminescence

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Single photon counting is the most sensitive optical measurement method available. The counting range is limited by photoelectron (PE) pulse width and dark count operating in Geiger-mode typical in SPAD and SiPM sensors. The PE width is determined by the recharge process after typical picoseconds avalanche and the sensor time constant by its capacitance. We achieved sub-ns PE pulses using pF range capacitance coupled with each arrayed pixel and GHz electronics. Dark count was reduced by thermoelectric cooling. Current photon counting performance shows 580ps average PE width, saturation count 500Mcps and dark count <100cps/mm². Counting electronics can perform up to 1Gcps with ECL logic after 200ps resolution comparator. Time correlated single photon counting (TCSPC) is an important single photon application. Due to sensor deadtime issues, START-STOP period per excitation impulse becomes the determining factor for a measurement. Deadtime-less photon detection enables the counting of multiple photons within an excitation pulse, enabling simultaneous measure of fluorescence intensity and fluorescence lifetime. The PE pulse stream is captured by a digital oscilloscope and analyzed by MATLAB script, avoiding pulse pair resolution limitations using peak detection and statistical analysis. Time resolution is decided by the sampling rate even in overlapped PE signals. Experiments were performed using a commercial oscilloscope with 8M sampling/2ms at 4Gs=250ps, showing that higher bandwidth and sampling rate instruments improve the measurements. This approach is termed Time Correlated Multi-Photon Counting (TCMPC). When combined with a wide dynamic range photon counting sensor, it is a powerful tool for fluorescence analysis, laser induced photon spectroscopy (LIPS), photon flow cytometry and potentially photon communications in deep and free-space or even underwater.

Proceedings Article | 20 February 2018 Presentation + Paper

Photon spectroscopy by picoseconds differential Geiger-mode Si photomultiplier

Masanobu Yamamoto, Keegan Hernandez, J. Paul Robinson

Proceedings Volume 10500, 1050002 (2018) https://doi.org/10.1117/12.2286743

KEYWORDS: Sensors, Luminescence, Spectroscopy, Photodetectors, Raman spectroscopy, Silicon photomultipliers, Picosecond phenomena, Signal processing, Photon counting

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The pixel array silicon photomultiplier (SiPM) is known as an excellent photon sensor with picoseconds avalanche process with the capacity for millions amplification of photoelectrons. In addition, a higher quantum efficiency(QE), small size, low bias voltage, light durability are attractive features for biological applications. The primary disadvantage is the limited dynamic range due to the 50ns recharge process and a high dark count which is an additional hurdle. We have developed a wide dynamic Si photon detection system applying ultra-fast differentiation signal processing, temperature control by thermoelectric device and Giga photon counter with 9 decimal digits dynamic range.

The tested performance is six orders of magnitude with 600ps pulse width and sub-fW sensitivity. Combined with 405nm laser illumination and motored monochromator, Laser Induced Fluorescence Photon Spectrometry (LIPS) has been developed with a scan range from 200~900nm at maximum of 500nm/sec and 1nm FWHM. Based on the Planck equation E=hν, this photon counting spectrum provides a fundamental advance in spectral analysis by digital processing. Advantages include its ultimate sensitivity, theoretical linearity, as well as quantitative and logarithmic analysis without use of arbitrary units. Laser excitation is also useful for evaluation of photobleaching or oxidation in materials by higher energy illumination. Traditional typical photocurrent detection limit is about 1pW which includes millions of photons, however using our system it is possible to evaluate the photon spectrum and determine background noise and auto fluorescence(AFL) in optics in any cytometry or imaging system component. In addition, the photon-stream digital signal opens up a new approach for picosecond time-domain analysis. Photon spectroscopy is a powerful method for analysis of fluorescence and optical properties in biology.

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