Quantum cascade (QC) lasers operating at terahertz frequencies were demonstrated two and a half years ago, and, since then, their development has proceeded at a very rapid pace. Most of the research has focused on new concepts for the quantum design of the gain medium as well as on improving device structure by optimizing fabrication and waveguide technology.
These efforts by various groups have led to maximum operating temperatures of about 140 K in pulsed mode, output powers of up to 50 mW, and lasing in continuous wave up to 93 K.
Such advances are making THz QC lasers more and more appealing for applications in various fields like chemical sensing, astronomy, spectroscopy, and imaging.
For their successful implementation, specific requirements have to be addressed, particularly concerning the spectral properties of the emission.
Here we report some latest developments in this direction. We demonstrate the perfect control of laser design for the realization of devices with precisely defined emission frequency in the whole range from 2.3 THz to 4.8 THz. Additionally, single-mode THz lasers with distributed feedback resonators have been achieved and a new technique involving surface plasmon gratings has been developed to improve performances. The latter offers also the possibility of constructing distributed Bragg gratings as a replacement for high-reflection coatings. Finally, solutions allowing broad tuneability are examined, with preliminary results illustrating the viability of external cavity set-ups.
We present an experimental module on electromagnetically induced transparency (EIT) in a Hanle configuration aimed to application for precise metrology. The EIT line width of the order of hundred hertz equivalent to nano-Tesla magnetic flux density has been achieved in rubidium atomic vapors with Ne buffer gases at room temperature. It can be interest for build of EIT based all-optical compact magnetometers, competitive to the most sensitive magnetometers based on superconducting quantum interference devices.
We applied a pumping field to couple the middle level of two- photon absorption with the fourth level. Two-photon absorption minimum was observed. This is the first observation and experimental analysis of such phenomenon after it was predicted four years ago. In sodium vapor, two-photon absorption strength of 3S<SUB>1/2</SUB>(Fequals2)-3P<SUB>3/2</SUB>-4D<SUB>3/2</SUB> was measured by detecting the fluorescence at 568.3 nm (4D<SUB>3/2</SUB>-3P<SUB>1/2</SUB>). Levels 3P<SUB>3/2</SUB> and 5S<SUB>1/2</SUB> were connected by pumping field. Experimental data of two-photon absorption strength vs detuning from middle level (TADM) shows that the two-photon absorption was reduced by 60% at line center in the presence of resonant pumping field. In rubidium vapor, two-photon absorption of 5S<SUB>1/2</SUB>(Fequals3)-5P<SUB>3/2</SUB>-8S<SUB>1/2</SUB>(Fequals3) was measured by detecting the fluorescence at 607 nm (8S<SUB>1/2</SUB>-5P<SUB>1/2</SUB>). Levels 5P<SUB>3/2</SUB> and 5D<SUB>5/2</SUB>(Fequals5) were connected by pumping field. The TADM profiles under different experimental condition indicate that the maximum reduction of two-photon absorption can be 80%. And the reduction effect reduced with lower strength, higher detuning frequency and wider line width of pumping field. At the same time, the position of minimum absorption depends on the detuning of pumping field. All the experimental profiles were fitted well with theoretical calculation results.
An experimental study of electromagnetically induced two- photon transparency (EITT) in rubidium atomic vapor at room temperature is presented. A four level system is considered, involving the two-photon absorption process 5S<SUB>1/2</SUB> to 8S<SUB>1/2</SUB> via an intermediate state 5P<SUB>3/2</SUB>, and the single- photon control process 5P<SUB>3/2</SUB> to 5D<SUB>5/2</SUB>. A controlling pump laser beam is employed to coherently couple the 5P<SUB>3/2</SUB> and 5D<SUB>5/2</SUB> states, thus producing two dressed intermediate states, which give rise to destructive interference in the two-photon transition. Fluorescence from 8S<SUB>1/2</SUB> to 5P<SUB>1/2</SUB> is used to monitor the 5S<SUB>1/2</SUB>(Fequals3) to 8S<SUB>1/2</SUB>(Fequals3) two-photon absorption. An induced two-photon transparency of about 80% has been obtained at resonance; the experimental results are in good agreement with the general theory of Agarwal, when the appropriate spectroscopic parameters are employed.
Dark States for Rubidium 85 have been obtained in glass cells containing saturated vapors of natural Rubidium at room temperature. The two radiation, which are required to induce the dark states, have been generated by modulating the light emission of a laser diode, tuned at the S-P optical transition. A measurement of the energy difference between the hyperfine states F equals 2 and F equals 3 of 5S<SUB>1/2</SUB> is performed by an optical way. By using simultaneously two laser diodes, one tuned at the D<SUB>1</SUB> transition and the other at the D<SUB>2</SUB>, we have generated two different Dark States. Interference between the two dark states prepared on the same place has been studied by changing the phase of the D<SUB>1</SUB> and D<SUB>2</SUB> excitations. A new way to modulate laser beam intensity is shown.