第20回サイエンスカフェ講演会を以下の通り開催いたします。

院生・学部生の参加も歓迎します。皆様のご参加をお待ちしております。

--------------------
第一部

講師：　小川哲生　教授　（大阪大学　大学院理学研究科）

（http://wwwacty.phys.sci.osaka-u.ac.jp/~ogawa/cover.html）

日時：　６月２０日（水）　１５：００～１６：３０
場所：　Ｔ－１号館８階　サイエンスカフェ

講演題目：　Cooperative phenomena in electron-hole and electron-hole-photon systems

講演概要：　
We have been focusing in recent years on interacting electron-hole-photon (ehp)
systems realized in semiconductor quantum wells in microcavities, which show
two different types of coherent light emission: lasing and polariton Bose-Einstein
condensation (BEC). The main interests are in their many-body aspects, arising
from the strong light-matter coupling and Coulomb interaction, which can play a
role both in the nonequilibrium (with large cavity loss) [1] and quasi-thermal-equilibrium
(under fast thermalization) [2] situations. Here we report our recent progress in
the theory for the ehp systems.

A many-body feature in semiconductor lasers is recently found when the BCS-type
(Bardeen-Cooper-Schrieffer-type) electron-hole (eh) pairing instability occurs during
low-temperature operation. In this situation, the Fano resonance is found in the gain
spectra due to the interference of the optical transitions via an eh Cooper pair
(discrete level) and unbound eh pairs (continuum). Since their interference occurs
only if the dephasing of eh pairs is present, we understood the Fano　resonance gain
is one of nonequilibrium characteristics found near the BCS phase transition. As a
result of the Fano-resonance gain, the single-mode laser operation is strongly modified
relative to the conventional laser operation with the Lorentzian-type eh plasma gain.
We have studied the stationary state of lasing using the Maxwell-semiconductor-Bloch
equations (MSBE), and found a strong reduction in the carrier density at the lasing
threshold and an enhancement in the stability of the continuous-wave operation.

Macroscopic numbers of the cavity polaritons in ehp systems can be condensed
into a single energy level and exhibit polariton BEC in a quasi-thermal equilibrium
situation. Here, the stationary state of the system is determined by the minimization
of Free energy [2]. Whereas the polariton BEC is a coherent light source similar to
semiconductor lasers, the condition for the stationary states to be sustained in
quasi-thermal equilibrium and nonequilibrium situations are quite different. Thus, their
description had been treated with different theories (BCS-like theory for polariton
BEC, and MSBE for semiconductor lasers). Recently, we successfully unify these
two theories for the Coulomb-correlated ehp systems [3] based on the Keldysh Green's
functions in full range of temperature and nonequilibrium parameters. We found the
crossover of the stationary states from those of the polariton BEC to those of
lasers. By using the unified theory, it becomes now possible to describe the lasing
from the nonequilibrium BCS phase, which will be discussed with some numerical results.

[1] K. Kamide, and T. Ogawa, Phys. Stat. Sol. (c) 4, 1250 (2011).
[2] K. Kamide, and T. Ogawa, Phys. Rev. Lett. 105, 056401 (2010);
Phys. Rev. B 83, 165319 (2011).
[3] M. Yamaguchi, K. Kamide, Y. Yamamoto, and T. Ogawa, New J. Phys. (in print).

--------------------
第二部

講師：　田中耕一郎　教授　（京都大学　物質－細胞統合システム拠点）

（http://www.hikari.scphys.kyoto-u.ac.jp/jp/index.php）

日時：　６月２０日（水）　１６：４５～１８：１５
場所：　Ｔ－１号館８階　サイエンスカフェ

講演題目：　高強度テラヘルツ光発生とその応用

講演概要：
フェムト秒レーザー技術の進展により、ピーク電場強度が1MV/cmを超えるテラヘルツ光の
生成が可能になってきた。
講演では、光源技術の概略を述べたあと、この光源をもちいておこなわれた、
テラヘルツ領域の非線型光学や実時間で動作するテラヘルツ近接場顕微鏡について述べる。
特に、テラヘルツ領域のメタマテリアルをもちいた電場増強効果について触れる予定である。

--------------------
問合せ先：　
先端領域若手研究リーダー育成拠点　石川　陽　（内線８３４９）