Based on the nature of ultra-fast carrier life time in semiconductor quantum well, optical modulation of quantum cascade laser offers an unique way to control intersubband transition through interband transition. This method circumvents the problem of parasitic effects associated with electrical modulation, resulting in a high modulation bandwidth. In addition it allows for fast wavelength modulation on standard type quantum cascade lasers by directly injecting charge carriers to laser active region with near-infrared optical excitation. Here, we demonstrate the first infrared spectroscopic measurement conducted with this all-optical modulation approach. Using wavelength modulation spectroscopy, a 1st order derivative spectrum of methanol vapor gas is observed. Optically based wavelength modulation up to 200 MHz is purely induced by pumping the front facet of quantum cascade laser with an intensity-modulated 1550 nm DFB laser. Compared with conventional direct absorption approach, the noise equivalent sensitivity is improved by a factor of 10 by adding optical modulation in a non-optimized system.
In the past two decades, there is an increasing interest in developing new infrared photodetectors based on novel
nanostructures, such as quantum well infrared photodetector (QWIP) and quantum dot infrared photodetector (QDIP).
However, the commonly used electrical read-out approach limits the resolution of QWIP/QDIP infrared imaging to
around 1 mega pixel. In this paper, we reported our theoretical study on an all-optical readout based on quantum dot
phase modulation, which provides a new way for the intersubband infrared detection by measuring the phase change in
the transmitted interband near infrared (NIR) and allows a high-resolution middle infrared (MIR) or far infrared (FIR)
imaging. Utilizing the long life time in the quantum dots, the intersubband infrared resonant light is used to control the
interband NIR resonant light phase. An infrared image can be converted into a visible or near infrared image, which can
be easily captured with a high resolution CCD camera. It provides a new way to obtain a high resolution infrared image.