Terahertz sources based on Čerenkov difference-frequency generation in quantum cascade lasers

Abstract

We report room-temperature terahertz sources based on Č erenkov difference-frequency generation in dual-wavelength mid-infrared quantum cascade lasers with giant resonant optical nonlinearities originating from intersubband transitions. A Č erenkov difference-frequency generation scheme allows for extraction of THz radiation along the whole length of the laser waveguide and provides directional terahertz emission. Experimentally, our sources demonstrate a conversion efficiency of up to 70 lW/W 2 approximately an order of magnitude improvement over the previous reports.

Broadly tunable terahertz generation in mid-infrared quantum cascade lasers

“…This approach circumvents the problem of high THz absorption and inefficient THz outcoupling to free space intrinsic to THz DFG-QCLs based on collinear modal phase matching 10,11,14 . The existence of Cherenkov emission in DFG-QCLs was confirmed by us using proof-ofprinciple devices 12 and multimode THz generation over a 1.2 to 4.5 THz range was observed. However, these proof-of-principle devices demonstrated only a 45 mW W À 2 mid-infrared-to-THz conversion efficiency, an improvement of only 5-10 times over devices based on collinear modal phase matching 10,11,14 .…”

“…1a. In the case of DFG-QCLs, we can write an expression for the non-linear polarization wave at o THz ¼ o 1 À o 2 in the slab-waveguide approximation as 12,19 :…”

“…12. First, the new devices use a 200-nm-thick InGaAs current extraction layer n-doped to 7 Â 10 17 cm À 3 and are separated from the active region by a 3-mm-thick layer of InP n-doped to 1.5 Â 10 16 cm À 3 , as opposed to a 3-mm-thick InP lower cladding and current injection layer n-doped to 10 17 cm À 1 used for devices in ref.…”

“…An alternative approach to generate THz radiation in QCLs are sources based on intracavity difference-frequency generation (DFG) in dual-wavelength mid-infrared (l ¼ 3-15 mm) QCLs designed to have giant optical non-linearity in the active region [10][11][12][13][14] . Not only can these sources operate at room temperature [11][12][13][14] , but they are also uniquely suited to provide THz output over a wide range of frequencies, as the mid-infrared frequencies in a QCL can be tuned well over 5 THz 15,16 and the optical non-linearity for intracavity THz DFG is not expected to change significantly over several THz of tuning 17 . The current challenge that must be addressed is their limited THz power output due to low mid-infrared-to-THz conversion efficiency.…”

“…To improve THz generation efficiency, we recently proposed using a Cherenkov phase-matching scheme in THz DFG-QCLs to generate broadband difference-frequency output at an angle to the mid-infrared pumps 12 . This approach circumvents the problem of high THz absorption and inefficient THz outcoupling to free space intrinsic to THz DFG-QCLs based on collinear modal phase matching 10,11,14 .…”