Quantum Physics

Engineers use colloidal quantum dot technology to create a dual-purpose laser and LED gadget.

A Los Alamos Public Research Center group has overcome key difficulties toward mechanically suitable focused energy light producers in view of colloidal quantum speck innovation, bringing about double-capability gadgets that work as both an optically energized laser and a high-brilliance electrically determined light-discharging diode (Drove).

An electrically pumped colloidal quantum dot laser or laser diode, a novel type of device whose impact would span numerous technologies such as integrated electronics and photonics, optical interconnects, lab-on-a-chip platforms, wearable devices, and medical diagnostics, is the focus of this development, which was described in the journal Advanced Materials.

Victor Klimov, a scientist in Los Alamos’ Chemistry Division and the team leader on the research, stated, “A quest for colloidal quantum dot laser diodes represents part of a worldwide effort aimed at realizing electrically pumped lasers and amplifiers based on solution-processable materials.” Scalability, compatibility, and ease of integration with on-chip electronics and photonics, including conventional silicon-based circuits, have sparked interest in these devices.

“The search for colloidal quantum dot laser diodes is part of a global effort to realize electrically pumped lasers and amplifiers based on solution-processable materials.”

Victor Klimov, a scientist in Los Alamos’s Chemistry division.

As in a standard drive, in the group’s new gadgets, the quantum spot layer went about as an electrically induced light producer. However, the devices demonstrated unprecedented brightness levels of more than a million candela per square meter due to extremely high current densities of more than 500 amperes per square centimeter (a candela measures luminous power emitted in a specific direction). They are ideal for daylight displays, projectors, and traffic lights due to their brightness.

With a significant net optical gain, the quantum dot layer also functioned as an effective waveguide amplifier. With a fully operational LED-type device stack that contained all of the charge transport layers and other components necessary for electrical pumping, the Los Alamos team was able to achieve narrow-band lasing. The much-anticipated demonstration of lasing with electrical pumping, which will enable the full implementation of the colloidal quantum dot lasing technology, is now possible thanks to this development.

Colloidal quantum dots

Semiconductor nanocrystals, or colloidal quantum dots, are appealing materials for the construction of laser devices such as laser diodes. Chemical processes at a moderate temperature can be used to make them with atomic precision.

Additionally, quantum dots exhibit discrete atomic-like electronic states whose energies are directly related to particle size due to their small dimensions, which are comparable to the natural extent of electronic wave functions. The so-called “quantum-size” effect has a result that can be used to adjust the wavelength of the lasing line or create a multi-color gain medium that allows lasing at multiple wavelengths. Low optical gain thresholds and reduced sensitivity of lasing characteristics to changes in device temperature are two additional advantages derived from an unusual atomic-like spectrum of quantum dot electronic states.

The majority of quantum dot lasing research has utilized short optical pulses to excite an optical gain medium, providing an innovative design solution to the problems associated with electrical pumping. It is much more difficult to implement lasing using electrically driven quantum dots. The Los Alamos research team made significant progress toward this goal with their new tools.

Designing for innovation to address electrical pumping difficulties

Namyoung Ahn, a lead device expert on the quantum dot team and a postdoctoral fellow at the Laboratory Director’s Office, stated, “One challenge lies in the area of electrical and optical device designs.” Particularly, the charge injection architecture of the device needs to be able to generate and maintain extremely high current densities, which are necessary for laser action. In order to prevent loss of gain in a thin quantum dot active medium, the same device must also have low optical losses.”

The team created new nanocrystals they called “compact compositionally graded quantum dots” to improve optical gain.

Clément Livache, a postdoc on the quantum dot team who conducted spectroscopic studies of the fabricated devices, stated, “These novel quantum dots feature suppressed Auger recombination due to a built-in compositional gradient and simultaneously exhibit a large gain coefficient when assembled in a close-packed solid used as an optical gain medium.” In a complex electroluminescent structure, a thin quantum dot layer combined with multiple charge-conducting layers that absorb light helps achieve net optical gain.

The researchers also reduced the optical losses in their devices to make light amplification easier. They replaced optically lossy metal-like materials with properly optimized low-absorptivity organic layers in the charge injection architecture, specifically. In addition, they designed a device cross-section profile to enhance optical field intensity in the quantum dot gain medium while simultaneously decreasing it in highly absorptive charge transport layers.

Finally, an optical cavity made of a periodic grating was incorporated into one of the device electrodes in order to make laser oscillations possible. Multi-pass amplification was made possible by this grating’s function as a so-called distributed feedback resonator, which allowed light to circulate in the quantum dot layer’s lateral plane.

The final obstacle

The final difficulty was achieving the lasing effect through optical excitation. Due to the degradation of device performance brought on by excessive heat generated by a passing current, lasing using electrical pumping was not observed. The final obstacle that must be overcome in order to demonstrate electrically driven laser oscillations is this one.

Only a couple of years prior, electrically siphoned colloidal quantum spot lasers were broadly considered unimaginable because of issues such as ultrafast drill rot, deficient flow densities in quantum dot LEDs, and troubles in consolidating electroluminescent and lasing capabilities in a similar gadget. The findings of the Los Alamos quantum dot team show that most of these issues can be solved in a practical way, indicating that a working quantum dot laser diode is not far off.

More information: Namyoung Ahn et al, Optically Excited Lasing in a Cavity‐Based, High‐Current‐Density Quantum Dot Electroluminescent Device, Advanced Materials (2022). DOI: 10.1002/adma.202206613

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