A Compact Aerosol Sensor and Spectroscopic Sorting with UV LEDs
We employ a linear array of UV LEDs whose individual elements may be fired in rapid sequence so as to continuously illuminate a particle during its time-of-flight. A linear array of separate UV LEDs offers several advantages over a single element device; an increase in the total energy delivered to the particle, which is achieved both by extending the excitation time as well as enabling the use of higher injection currents, and a reduction in the background signal.
The array consists of 32 individually addressable elements, each with an optical aperture 200 um wide and 50 um high. The total array height is 3.2 mm and with 1:1 imaging optics the particle is tracked for this distance during its trajectory. In results presented later in this paper, both 340 nm and 275 nm LED arrays are used. The maximum CW output power before thermal rollover from a single 275 nm (340 nm) LED element is approximately 600 uW (1 mW). This occurs at a current injection level of 40 mA (100 mA) where the power is measured directly off the sapphire backside of the die with no integrating sphere. This represent state-of-the-art at both wavelengths, and the inclusion of a 275 nm LED array is the most recent addition to the system presented here.
1. Compact spectroscopic optics:
Schematic of the optical apparatus for acquiring single-particle spectra employing a UV LED array. (a) Side-view of the system illustrating the particle jetstream and placement of the aerodynamic deflector and (b) top-view showing the compact spectroscopic optics and simultaneous scatter channels.
A pair of simple lenses focuses the array to the particle jetstream and a bandpass excitation filter is used to remove the well-known long-wavelength tail associated with LEDs. Scatter from the red LD focused onto the jetstream prior to the LED array, serves as the trigger to synchronize the subsequent operations. Scatter at the UV wavelength is collected at a right angle to the incident light and is detected by a simple photomultiplier tube (PMT) equipped with an appropriate bandpass emission filter. This serves to normalize the fluorescence signal. Simultaneously, the fluorescence spectrum is collected using a single lens, UV-grade transmission grating and a 32-anode PMT. Downstream from the UV LED array and fluorescence collection, we employ a solenoid-based aerodynamic deflector as a compact method to separate particles10. When energized by a current pulse an electromagnetic plunger opens the valve, causing a burst of nitrogen gas which lasts for approximately 250 usec.
2. Electronic Control
To achieve the required timing and control sequence, 3 electronics boards were custom built by Vtech Engineering Corporation. The central PhotoniQ-3G iQSP480 board is a commercial product whose primary function is to acquire and process the real-time fluorescence data from the 32-anode PMT and trigger the particle deflector based on a spectral algorithm applied to the fluorescence data.
Fluorescence collected by the 32-anode PMT is evaluated by a spectral algorithm running on the DSP (digital signal processor) in the iQSP480. Owing to the finite time that the particle is in the jetstream, signal collection and evaluation of the algorithm must be done in real time.Currently a set 8 spectral flags can be defined, each corresponding to a class of particle for instance, and for each particle the software records which, if any, of the flags matches the recorded spectrum. A trigger is generated to the deflector when any of a predefined subset of the spectral flags is recorded.
3. Demonstration of Detection Capability